linuxnet.pdf

Linux Networking
Paul Cobbaut
Paul Cobbaut
Publication date 2015-05-24 CEST
Abstract
This book is meant to be used in an instructor-led training. For self-study, the intent is to read
this book next to a working Linux computer so you can immediately do every subject, practicing
each command.
This book is aimed at novice Linux system administrators (and might be interesting and useful
for home users that want to know a bit more about their Linux system). However, this book
is not meant as an introduction to Linux desktop applications like text editors, browsers, mail
clients, multimedia or office applications.
More information and free .pdf available at http://linux-training.be .
Feel free to contact the author:
• Paul Cobbaut: paul.cobbaut@gmail.com, http://www.linkedin.com/in/cobbaut
Contributors to the Linux Training project are:
• Serge van Ginderachter: serge@ginsys.be, build scripts; infrastructure setup; minor stuff
• Hendrik De Vloed: hendrik.devloed@ugent.be, buildheader.pl script
We'd also like to thank our reviewers:
• Wouter Verhelst: wouter@grep.be, http://grep.be
• Geert Goossens: mail.goossens.geert@gmail.com, http://www.linkedin.com/in/
geertgoossens
• Elie De Brauwer: elie@de-brauwer.be, http://www.de-brauwer.be
• Christophe Vandeplas: christophe@vandeplas.com, http://christophe.vandeplas.com
• Bert Desmet: bert@devnox.be, http://bdesmet.be
• Rich Yonts: richyonts@gmail.com,
Copyright 2007-2015 Paul Cobbaut
Permission is granted to copy, distribute and/or modify this document under the terms of the
GNU Free Documentation License, Version 1.3 or any later version published by the Free
Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled 'GNU Free Documentation
License'.

iii
Table of Contents
I. network management ................................................................................................................................... 1
1. general networking .......................................................................................................................... 4
1.1. network layers ....................................................................................................................... 5
1.2. unicast, multicast, broadcast, anycast ................................................................................... 8
1.3. lan-wan-man ........................................................................................................................ 10
1.4. internet - intranet - extranet ................................................................................................ 12
1.5. tcp/ip .................................................................................................................................... 13
2. interface configuration .................................................................................................................. 14
2.1. to gui or not to gui .............................................................................................................. 15
2.2. Debian nic configuration ..................................................................................................... 16
2.3. RHEL nic configuration ...................................................................................................... 18
2.4. ifconfig ................................................................................................................................. 20
2.5. ip .......................................................................................................................................... 22
2.6. dhclient ................................................................................................................................ 23
2.7. hostname .............................................................................................................................. 23
2.8. arp ........................................................................................................................................ 24
2.9. route ..................................................................................................................................... 25
2.10. ping .................................................................................................................................... 25
2.11. optional: ethtool ................................................................................................................. 26
2.12. practice: interface configuration ........................................................................................ 27
2.13. solution: interface configuration ....................................................................................... 28
3. network sniffing ............................................................................................................................. 30
3.1. wireshark .............................................................................................................................. 31
3.2. tcpdump ............................................................................................................................... 35
3.3. practice: network sniffing ................................................................................................... 36
3.4. solution: network sniffing ................................................................................................... 37
4. binding and bonding ...................................................................................................................... 38
4.1. binding on Redhat/Fedora ................................................................................................... 39
4.2. binding on Debian/Ubuntu .................................................................................................. 40
4.3. bonding on Redhat/Fedora .................................................................................................. 41
4.4. bonding on Debian/Ubuntu ................................................................................................. 43
4.5. practice: binding and bonding ............................................................................................. 45
4.6. solution: binding and bonding ............................................................................................ 46
5. ssh client and server ...................................................................................................................... 47
5.1. about ssh .............................................................................................................................. 48
5.2. log on to a remote server .................................................................................................... 50
5.3. executing a command in remote ......................................................................................... 50
5.4. scp ........................................................................................................................................ 51
5.5. setting up passwordless ssh ................................................................................................. 52
5.6. X forwarding via ssh ........................................................................................................... 53
5.7. troubleshooting ssh .............................................................................................................. 54
5.8. sshd ...................................................................................................................................... 55
5.9. sshd keys .............................................................................................................................. 55
5.10. ssh-agent ............................................................................................................................ 55
5.11. practice: ssh ....................................................................................................................... 56
5.12. solution: ssh ....................................................................................................................... 57
6. introduction to nfs ......................................................................................................................... 59
6.1. nfs protocol versions ........................................................................................................... 60
6.2. rpcinfo .................................................................................................................................. 60
6.3. server configuration ............................................................................................................. 61
6.4. /etc/exports ........................................................................................................................... 61
6.5. exportfs ................................................................................................................................ 61
6.6. client configuration .............................................................................................................. 62
6.7. practice: introduction to nfs ................................................................................................ 63
7. introduction to networking ........................................................................................................... 64

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7.1. introduction to iptables ........................................................................................................ 65
7.2. practice : iptables ................................................................................................................. 66
7.3. solution : iptables ................................................................................................................. 67
7.4. xinetd and inetd ................................................................................................................... 68
7.5. practice : inetd and xinetd ................................................................................................... 70
7.6. network file system ............................................................................................................. 71
7.7. practice : network file system ............................................................................................. 73
II. apache and squid ....................................................................................................................................... 74
8. apache web server .......................................................................................................................... 76
8.1. introduction to apache ......................................................................................................... 77
8.2. port virtual hosts on Debian ................................................................................................ 84
8.3. named virtual hosts on Debian ........................................................................................... 88
8.4. password protected website on Debian ............................................................................... 90
8.5. port virtual hosts on CentOS .............................................................................................. 91
8.6. named virtual hosts on CentOS .......................................................................................... 95
8.7. password protected website on CentOS .............................................................................. 97
8.8. troubleshooting apache ........................................................................................................ 99
8.9. virtual hosts example ......................................................................................................... 100
8.10. aliases and redirects ......................................................................................................... 100
8.11. more on .htaccess ............................................................................................................. 100
8.12. traffic ................................................................................................................................ 100
8.13. self signed cert on Debian .............................................................................................. 101
8.14. self signed cert on RHEL/CentOS .................................................................................. 103
8.15. practice: apache ............................................................................................................... 105
9. introduction to squid ................................................................................................................... 106
9.1. about proxy servers ........................................................................................................... 106
9.2. installing squid .................................................................................................................. 107
9.3. port 3128 ............................................................................................................................ 107
9.4. starting and stopping ......................................................................................................... 107
9.5. client proxy settings .......................................................................................................... 108
9.6. upside down images .......................................................................................................... 110
9.7. /var/log/squid ...................................................................................................................... 112
9.8. access control .................................................................................................................... 112
9.9. testing squid ....................................................................................................................... 112
9.10. name resolution ............................................................................................................... 112
III. dns server ............................................................................................................................................... 114
10. introduction to DNS ................................................................................................................... 116
10.1. about dns .......................................................................................................................... 117
10.2. dns namespace ................................................................................................................. 120
10.3. caching only servers ........................................................................................................ 125
10.4. authoritative dns servers .................................................................................................. 128
10.5. primary and secondary .................................................................................................... 128
10.6. zone transfers ................................................................................................................... 128
10.7. master and slave .............................................................................................................. 130
10.8. SOA record ...................................................................................................................... 130
10.9. full or incremental zone transfers ................................................................................... 131
10.10. DNS cache ..................................................................................................................... 132
10.11. forward lookup zone example ....................................................................................... 133
10.12. example: caching only DNS server ............................................................................... 134
10.13. example: caching only with forwarder ......................................................................... 136
10.14. example: primary authoritative server .......................................................................... 138
10.15. example: a DNS slave server ........................................................................................ 142
10.16. practice: dns ................................................................................................................... 144
10.17. solution: dns .................................................................................................................. 145
11. advanced DNS ............................................................................................................................ 146
11.1. example: DNS round robin ............................................................................................. 147
11.2. DNS delegation ............................................................................................................... 148
11.3. example: DNS delegation ................................................................................................ 149

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11.4. example: split-horizon dns .............................................................................................. 151
11.5. old dns topics .................................................................................................................. 153
IV. dhcp server ............................................................................................................................................ 157
12. introduction to dhcp .................................................................................................................. 159
12.1. four broadcasts ................................................................................................................ 160
12.2. picturing dhcp .................................................................................................................. 161
12.3. installing a dhcp server ................................................................................................... 162
12.4. dhcp server for RHEL/CentOS ....................................................................................... 162
12.5. client reservations ............................................................................................................ 163
12.6. example config files ........................................................................................................ 163
12.7. older example config files ............................................................................................... 164
12.8. advanced dhcp ................................................................................................................. 166
12.9. Practice: dhcp .................................................................................................................. 167
V. iptables firewall ...................................................................................................................................... 168
13. introduction to routers .............................................................................................................. 170
13.1. router or firewall ............................................................................................................. 171
13.2. packet forwarding ............................................................................................................ 171
13.3. packet filtering ................................................................................................................. 171
13.4. stateful .............................................................................................................................. 171
13.5. nat (network address translation) .................................................................................... 172
13.6. pat (port address translation) ........................................................................................... 172
13.7. snat (source nat) .............................................................................................................. 172
13.8. masquerading ................................................................................................................... 172
13.9. dnat (destination nat) ....................................................................................................... 172
13.10. port forwarding .............................................................................................................. 172
13.11. /proc/sys/net/ipv4/ip_forward ........................................................................................ 173
13.12. /etc/sysctl.conf ................................................................................................................ 173
13.13. sysctl .............................................................................................................................. 173
13.14. practice: packet forwarding ........................................................................................... 174
13.15. solution: packet forwarding ........................................................................................... 176
14. iptables firewall .......................................................................................................................... 179
14.1. iptables tables .................................................................................................................. 180
14.2. starting and stopping iptables .......................................................................................... 180
14.3. the filter table .................................................................................................................. 181
14.4. practice: packet filtering .................................................................................................. 186
14.5. solution: packet filtering ................................................................................................. 187
14.6. network address translation ............................................................................................. 188
VI. Introduction to Samba ........................................................................................................................... 191
15. introduction to samba ................................................................................................................ 194
15.1. verify installed version .................................................................................................... 195
15.2. installing samba ............................................................................................................... 196
15.3. documentation .................................................................................................................. 197
15.4. starting and stopping samba ............................................................................................ 198
15.5. samba daemons ................................................................................................................ 199
15.6. the SMB protocol ............................................................................................................ 200
15.7. practice: introduction to samba ....................................................................................... 201
16. getting started with samba ........................................................................................................ 202
16.1. /etc/samba/smb.conf ......................................................................................................... 203
16.2. /usr/bin/testparm ............................................................................................................... 204
16.3. /usr/bin/smbclient ............................................................................................................. 205
16.4. /usr/bin/smbtree ................................................................................................................ 207
16.5. server string ..................................................................................................................... 208
16.6. Samba Web Administration Tool (SWAT) .................................................................... 209
16.7. practice: getting started with samba ................................................................................ 210
16.8. solution: getting started with samba ............................................................................... 211
17. a read only file server ................................................................................................................ 213
17.1. Setting up a directory to share ........................................................................................ 214
17.2. configure the share .......................................................................................................... 214

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17.3. restart the server .............................................................................................................. 215
17.4. verify the share ................................................................................................................ 215
17.5. a note on netcat ............................................................................................................... 217
17.6. practice: read only file server ......................................................................................... 218
17.7. solution: read only file server ......................................................................................... 219
18. a writable file server .................................................................................................................. 220
18.1. set up a directory to share ............................................................................................... 221
18.2. share section in smb.conf ................................................................................................ 221
18.3. configure the share .......................................................................................................... 221
18.4. test connection with windows ......................................................................................... 221
18.5. test writing with windows ............................................................................................... 222
18.6. How is this possible ? ..................................................................................................... 222
18.7. practice: writable file server ............................................................................................ 223
18.8. solution: writable file server ........................................................................................... 224
19. samba first user account ........................................................................................................... 225
19.1. creating a samba user ...................................................................................................... 226
19.2. ownership of files ............................................................................................................ 226
19.3. /usr/bin/smbpasswd .......................................................................................................... 226
19.4. /etc/samba/smbpasswd ..................................................................................................... 226
19.5. passdb backend ................................................................................................................ 227
19.6. forcing this user ............................................................................................................... 227
19.7. practice: first samba user account ................................................................................... 228
19.8. solution: first samba user account ................................................................................... 229
20. samba authentication ................................................................................................................. 230
20.1. creating the users on Linux ............................................................................................. 231
20.2. creating the users on samba ............................................................................................ 231
20.3. security = user ................................................................................................................. 231
20.4. configuring the share ....................................................................................................... 232
20.5. testing access with net use .............................................................................................. 232
20.6. testing access with smbclient .......................................................................................... 232
20.7. verify ownership .............................................................................................................. 233
20.8. common problems ........................................................................................................... 233
20.9. practice : samba authentication ....................................................................................... 235
20.10. solution: samba authentication ...................................................................................... 236
21. samba securing shares ............................................................................................................... 237
21.1. security based on user name ........................................................................................... 238
21.2. security based on ip-address ........................................................................................... 238
21.3. security through obscurity ............................................................................................... 239
21.4. file system security .......................................................................................................... 239
21.5. practice: securing shares ................................................................................................. 241
21.6. solution: securing shares ................................................................................................. 242
22. samba domain member ............................................................................................................. 244
22.1. changes in smb.conf ........................................................................................................ 245
22.2. joining an Active Directory domain ............................................................................... 246
22.3. winbind ............................................................................................................................ 247
22.4. wbinfo .............................................................................................................................. 247
22.5. getent ................................................................................................................................ 248
22.6. file ownership .................................................................................................................. 249
22.7. practice : samba domain member .................................................................................... 250
23. samba domain controller ........................................................................................................... 251
23.1. about Domain Controllers ............................................................................................... 252
23.2. About security modes ...................................................................................................... 252
23.3. About password backends ............................................................................................... 253
23.4. [global] section in smb.conf ............................................................................................ 253
23.5. netlogon share .................................................................................................................. 254
23.6. other [share] sections ....................................................................................................... 254
23.7. Users and Groups ............................................................................................................ 255
23.8. tdbsam .............................................................................................................................. 255

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23.9. about computer accounts ................................................................................................. 256
23.10. local or roaming profiles ............................................................................................... 256
23.11. Groups in NTFS acls ..................................................................................................... 257
23.12. logon scripts .................................................................................................................. 258
23.13. practice: samba domain controller ................................................................................ 259
24. a brief look at samba 4 ............................................................................................................. 260
24.1. Samba 4 alpha 6 .............................................................................................................. 262
VII. ipv6 ....................................................................................................................................................... 264
25. Introduction to ipv6 ................................................................................................................... 266
25.1. about ipv6 ........................................................................................................................ 267
25.2. network id and host id .................................................................................................... 267
25.3. host part generation ......................................................................................................... 267
25.4. ipv4 mapped ipv6 address ............................................................................................... 268
25.5. link local addresses ......................................................................................................... 268
25.6. unique local addresses ..................................................................................................... 268
25.7. globally unique unicast addresses ................................................................................... 268
25.8. 6to4 .................................................................................................................................. 268
25.9. ISP .................................................................................................................................... 269
25.10. non routable addresses .................................................................................................. 269
25.11. ping6 .............................................................................................................................. 269
25.12. Belgium and ipv6 .......................................................................................................... 270
25.13. other websites ................................................................................................................ 270
25.14. 6to4 gateways ................................................................................................................ 272
25.15. ping6 and dns ................................................................................................................ 272
25.16. ipv6 and tcp/http ............................................................................................................ 272
25.17. ipv6 PTR record ............................................................................................................ 272
25.18. 6to4 setup on Linux ...................................................................................................... 272
VIII. Appendix ............................................................................................................................................. 275
A. License .......................................................................................................................................... 277
Index ............................................................................................................................................................. 284

viii
List of Tables
10.1. the first top level domains .................................................................................................................. 122
10.2. new general purpose tld's ................................................................................................................... 122
13.1. Packet Forwarding Exercise ............................................................................................................... 174
13.2. Packet Forwarding Solution ............................................................................................................... 176

2
Table of Contents
1. general networking .................................................................................................................................... 4
1.1. network layers ................................................................................................................................. 5
1.2. unicast, multicast, broadcast, anycast ............................................................................................. 8
1.3. lan-wan-man .................................................................................................................................. 10
1.4. internet - intranet - extranet .......................................................................................................... 12
1.5. tcp/ip .............................................................................................................................................. 13
2. interface configuration ............................................................................................................................ 14
2.1. to gui or not to gui ....................................................................................................................... 15
2.2. Debian nic configuration .............................................................................................................. 16
2.3. RHEL nic configuration ............................................................................................................... 18
2.4. ifconfig .......................................................................................................................................... 20
2.5. ip .................................................................................................................................................... 22
2.6. dhclient .......................................................................................................................................... 23
2.7. hostname ........................................................................................................................................ 23
2.8. arp .................................................................................................................................................. 24
2.9. route ............................................................................................................................................... 25
2.10. ping .............................................................................................................................................. 25
2.11. optional: ethtool .......................................................................................................................... 26
2.12. practice: interface configuration ................................................................................................. 27
2.13. solution: interface configuration ................................................................................................. 28
3. network sniffing ....................................................................................................................................... 30
3.1. wireshark ....................................................................................................................................... 31
3.2. tcpdump ......................................................................................................................................... 35
3.3. practice: network sniffing ............................................................................................................. 36
3.4. solution: network sniffing ............................................................................................................. 37
4. binding and bonding ............................................................................................................................... 38
4.1. binding on Redhat/Fedora ............................................................................................................. 39
4.2. binding on Debian/Ubuntu ........................................................................................................... 40
4.3. bonding on Redhat/Fedora ............................................................................................................ 41
4.4. bonding on Debian/Ubuntu ........................................................................................................... 43
4.5. practice: binding and bonding ...................................................................................................... 45
4.6. solution: binding and bonding ...................................................................................................... 46
5. ssh client and server ................................................................................................................................ 47
5.1. about ssh ........................................................................................................................................ 48
5.2. log on to a remote server ............................................................................................................. 50
5.3. executing a command in remote ................................................................................................... 50
5.4. scp .................................................................................................................................................. 51
5.5. setting up passwordless ssh .......................................................................................................... 52
5.6. X forwarding via ssh .................................................................................................................... 53
5.7. troubleshooting ssh ....................................................................................................................... 54
5.8. sshd ................................................................................................................................................ 55
5.9. sshd keys ....................................................................................................................................... 55
5.10. ssh-agent ...................................................................................................................................... 55
5.11. practice: ssh ................................................................................................................................. 56
5.12. solution: ssh ................................................................................................................................ 57
6. introduction to nfs ................................................................................................................................... 59
6.1. nfs protocol versions ..................................................................................................................... 60
6.2. rpcinfo ........................................................................................................................................... 60
6.3. server configuration ...................................................................................................................... 61
6.4. /etc/exports ..................................................................................................................................... 61
6.5. exportfs .......................................................................................................................................... 61
6.6. client configuration ....................................................................................................................... 62
6.7. practice: introduction to nfs .......................................................................................................... 63
7. introduction to networking ..................................................................................................................... 64
7.1. introduction to iptables ................................................................................................................. 65

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7.2. practice : iptables .......................................................................................................................... 66
7.3. solution : iptables .......................................................................................................................... 67
7.4. xinetd and inetd ............................................................................................................................ 68
7.5. practice : inetd and xinetd ............................................................................................................ 70
7.6. network file system ....................................................................................................................... 71
7.7. practice : network file system ....................................................................................................... 73

4
Chapter 1. general networking
While this chapter is not directly about Linux, it does contain general networking concepts
that will help you in troubleshooting networks on Linux.

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1.1. network layers
1.1.1. seven OSI layers
When talking about protocol layers, people usually mention the seven layers of the osi
protocol (Application, Presentation, Session, Transport, Network, Data Link and Physical).
We will discuss layers 2 and 3 in depth, and focus less on the other layers. The reason is
that these layers are important for understanding networks. You will hear administrators use
words like "this is a layer 2 device" or "this is a layer 3 broadcast", and you should be able
to understand what they are talking about.
1.1.2. four DoD layers
The DoD (or tcp/ip) model has only four layers, roughly mapping its network access layer
to OSI layers 1 and 2 (Physical and Datalink), its internet (IP) layer to the OSI network
layer, its host-to-host (tcp, udp) layer to OSI layer 4 (transport) and its application layer
to OSI layers 5, 6 and 7.
Below an attempt to put OSI and DoD layers next to some protocols and devices.

general networking
6
1.1.3. short introduction to the physical layer
The physical layer, or layer 1, is all about voltage, electrical signals and mechanical
connections. Some networks might still use coax cables, but most will have migrated to utp
(cat 5 or better) with rj45 connectors.
Devices like repeaters and hubs are part of this layer. You cannot use software to 'see'
a repeater or hub on the network. The only thing these devices are doing is amplifying
electrical signals on cables. Passive hubs are multiport amplifiers that amplify an incoming
electrical signal on all other connections. Active hubs do this by reading and retransmitting
bits, without interpreting any meaning in those bits.
Network technologies like csma/cd and token ring are defined on this layer.
This is all we have to say about layer 1 in this book.
1.1.4. short introduction to the data link layer
The data link layer, or layer 2 is about frames. A frame has a crc (cyclic redundancy check).
In the case of ethernet (802.3), each network card is identifiable by a unique 48-bit mac
address (media access control address).
On this layer we find devices like bridges and switches. A bridge is more intelligent than a
hub because a bridge can make decisions based on the mac address of computers. A switch
also understands mac addresses.
In this book we will discuss commands like arp and ifconfig to explore this layer.
1.1.5. short introduction to the network layer
Layer 3 is about ip packets. This layer gives every host a unique 32-bit ip address. But ip
is not the only protocol on this layer, there is also icmp, igmp, ipv6 and more. A complete
list can be found in the /etc/protocols file.
On this layer we find devices like routers and layer 3 switches, devices that know (and
have) an ip address.
In tcp/ip this layer is commonly referred to as the internet layer.
1.1.6. short introduction to the transport layer
We will discuss the tcp and udp protocols in the context of layer 4. The DoD model calls
this the host-to-host layer.
1.1.7. layers 5, 6 and 7
The tcp/ip application layer includes layers 5, 6 and 7. Details on the difference between
these layers are out of scope of this course.

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1.1.8. network layers in this book
Stacking of layers in this book is based on the Protocols in Frame explanation in the
wireshark sniffer. When sniffing a dhcp packet, we notice the following in the sniffer.
[Protocols in Frame: eth:ip:udp:bootp]
Sniffing for ntp (Network Time Protocol) packets gives us this line, which makes us
conclude to put ntp next to bootp in the protocol chart below.
[Protocols in Frame: eth:ip:udp:ntp]
Sniffing an arp broadcast makes us put arp next to ip. All these protocols are explained
later in this chapter.
[Protocols in Frame: eth:arp]
Below is a protocol chart based on wireshark's knowledge. It contains some very common
protocols that are discussed in this book. The chart does not contain all protocols.

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1.2. unicast, multicast, broadcast, anycast
1.2.1. unicast
A unicast communication originates from one computer and is destined for exactly one other
computer (or host). It is common for computers to have many unicast communications.
1.2.2. multicast
A multicast is destined for a group (of computers).
Some examples of multicast are Realplayer (.sdp files) and ripv2 (a routing protocol).

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1.2.3. broadcast
A broadcast is meant for everyone.
Typical example here is the BBC (British Broadcasting Corporation) broadcasting to
everyone. In datacommunications a broadcast is most common confined to the lan.
Careful, a layer 2 broadcast is very different from a layer 3 broadcast. A layer two
broadcast is received by all network cards on the same segment (it does not pass any router),
whereas a layer 3 broadcast is received by all hosts in the same ip subnet.
1.2.4. anycast
The root name servers of the internet use anycast. An anycast signal goes the the
(geographically) nearest of a well defined group.
With thanks to the nice anonymous wikipedia contributor to put these pictures in the public
domain.

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1.3. lan-wan-man
The term lan is used for local area networks, as opposed to a wan for wide area networks.
The difference between the two is determined by the distance between the computers, and
not by the number of computers in a network. Some protocols like atm are designed for use
in a wan, others like ethernet are designed for use in a lan.
1.3.1. lan
A lan (Local Area Network) is a local network. This can be one room, or one floor, or even
one big building. We say lan as long as computers are close to each other. You can also
define a lan when all computers are ethernet connected.
A lan can contain multiple smaller lan's. The picture below shows three lan's that together
make up one lan.
1.3.2. man
A man (Metropolitan Area Network) is something inbetween a lan and a wan, often
comprising several buildings on the same campus or in the same city. A man can use fddi
or ethernet or other protocols for connectivity.

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1.3.3. wan
A wan (Wide Area Network) is a network with a lot of distance between the computers (or
hosts). These hosts are often connected by leased lines. A wan does not use ethernet, but
protocols like fddi, frame relay, ATM or X.25 to connect computers (and networks).
The picture below shows a branch office that is connected through Frame Relay with
headquarters.
The acronym wan is also used for large surface area networks like the internet.
Cisco is known for their wan technology. They make routers that connect many lan
networks using wan protocols.
1.3.4. pan-wpan
Your home network is called a pan (Personal Area Network). A wireless pan is a wpan.

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1.4. internet - intranet - extranet
The internet is a global network. It connects many networks using the tcp/ip protocol stack.
The origin of the internet is the arpanet. The arpanet was created in 1969, that year only
four computers were connected in the network. In 1971 the first e-mail was sent over the
arpanet. E-mail took 75 percent of all arpanet traffic in 1973. 1973 was also the year ftp
was introduced, and saw the connection of the first European countries (Norway and UK). In
2009 the internet was available to 25 percent of the world population. In 2011 it is estimated
that only a quarter of internet webpages are in English.
An intranet is a private tcp/ip network. An intranet uses the same protocols as the internet,
but is only accessible to people from within one organization.
An extranet is similar to an intranet, but some trusted organizations (partners/clients/
suppliers/...) also get access.

general networking
13
1.5. tcp/ip
1.5.1. history of tcp/ip
In the Sixties development of the tcp/ip protocol stack was started by the US Department of
Defense. In the Eighties a lot of commercial enterprises developed their own protocol stack:
IBM created sna, Novell had ipx/spx, Microsoft completed netbeui and Apple worked with
appletalk. All the efforts from the Eighties failed to survive the Nineties. By the end of the
Nineties, almost all computers in the world were able to speak tcp/ip.
In my humble opinion, the main reason for the survival of tcp/ip over all the other protocols
is its openness. Everyone is free to develop and use the tcp/ip protocol suite.
1.5.2. rfc (request for comment)
The protocols that are used on the internet are defined in rfc's. An rfc or request
for comment describes the inner working of all internet protocols. The IETF (Internet
Engineering Task Force) is the sole publisher of these protocols since 1986.
The official website for the rfc's is http://www.rfc-editor.org. This website contains all
rfc's in plain text, for example rfc2132 (which defines dhcp and bootp) is accessible at http://
www.rfc-editor.org/rfc/rfc2132.txt.
1.5.3. many protocols
For reliable connections, you use tcp, whereas udp is connectionless but faster. The icmp
error messages are used by ping, multicast groups are managed by igmp.
These protocols are visible in the protocol field of the ip header, and are listed in the /etc/
protocols file.
paul@debian5:~$ grep tcp /etc/protocols
tcp 6 TCP # transmission control protocol
1.5.4. many services
Network cards are uniquely identified by their mac address, hosts by their ip address and
applications by their port number.
Common application level protocols like smtp, http, ssh, telnet and ftp have fixed port
numbers. There is a list of port numbers in /etc/services.
paul@ubu1010:~$ grep ssh /etc/services
ssh 22/tcp # SSH Remote Login Protocol
ssh 22/udp

14
Chapter 2. interface configuration
This chapter explains how to configure network interface cards to work with tcp/ip.

interface configuration
15
2.1. to gui or not to gui
Recent Linux distributions often include a graphical application to configure the network.
Some people complain that these applications mess networking configurations up when
used simultaneously with command line configurations. Notably Network Manager (often
replaced by wicd) and yast are known to not care about configuration changes via the
command line.
Since the goal of this course is server administration, we will assume our Linux servers are
always administered through the command line.
This chapter only focuses on using the command line for network interface configuration!
Unfortunately there is no single combination of Linux commands and /etc files that works on
all Linux distributions. We discuss networking on two (large but distinct) Linux distribution
families.
We start with Debian (this should also work on Ubuntu and Mint), then continue with RHEL
(which is identical to CentOS and Fedora).

16
2.2. Debian nic configuration
2.2.1. /etc/network/interfaces
The /etc/network/interfaces file is a core network interface card configuration file on
debian.
dhcp client
The screenshot below shows that our computer is configured for dhcp on eth0 (the first
network interface card or nic).
paul@debian8:~$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
Configuring network cards for dhcp is good practice for clients, but servers usually require
a fixed ip address.
fixed ip
The screenshot below shows /etc/network/interfaces configured with a fixed ip address.
root@debian7~# cat /etc/network/interfaces
auto lo
auto eth0
iface eth0 inet static
address 10.42.189.198
broadcast 10.42.189.207
netmask 255.255.255.240
gateway 10.42.189.193
The screenshot above also shows that you can provide more configuration than just the
ip address. See interfaces(5) for help on setting a gateway, netmask or any of the other
options.

17
2.2.2. /sbin/ifdown
It is adviced (but not mandatory) to down an interface before changing its configuration.
This can be done with the ifdown command.
The command will not give any output when downing an interface with a fixed ip address.
However ifconfig will no longer show the interface.
root@ubu1104srv:~# ifdown eth0
root@ubu1104srv:~# ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:106 errors:0 dropped:0 overruns:0 frame:0
TX packets:106 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:11162 (11.1 KB) TX bytes:11162 (11.1 KB)
An interface that is down cannot be used to connect to the network.
2.2.3. /sbin/ifup
Below a screenshot of ifup bringing the eth0 ethernet interface up using dhcp. (Note that
this is a Ubuntu 10.10 screenshot, Ubuntu 11.04 omits ifup output by default.)
root@ubu1010srv:/etc/network# ifup eth0
Internet Systems Consortium DHCP Client V3.1.3
Copyright 2004-2009 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/eth0/08:00:27:cd:7f:fc
Sending on LPF/eth0/08:00:27:cd:7f:fc
Sending on Socket/fallback
DHCPREQUEST of 192.168.1.34 on eth0 to 255.255.255.255 port 67
DHCPNAK from 192.168.33.100
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3
DHCPOFFER of 192.168.33.77 from 192.168.33.100
DHCPREQUEST of 192.168.33.77 on eth0 to 255.255.255.255 port 67
DHCPACK of 192.168.33.77 from 192.168.33.100
bound to 192.168.33.77 -- renewal in 95 seconds.
ssh stop/waiting
ssh start/running, process 1301
root@ubu1010srv:/etc/network#
The details of dhcp are covered in a separate chapter in the Linux Servers course.

18
2.3. RHEL nic configuration
2.3.1. /etc/sysconfig/network
The /etc/sysconfig/network file is a global (across all network cards) configuration file.
It allows us to define whether we want networking (NETWORKING=yes|no), what the
hostname should be (HOSTNAME=) and which gateway to use (GATEWAY=).
[root@rhel6 ~]# cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=rhel6
GATEWAY=192.168.1.1
There are a dozen more options settable in this file, details can be found in /usr/share/doc/
initscripts-*/sysconfig.txt.
Note that this file contains no settings at all in a default RHEL7 install (with networking
enabled).
[root@rhel71 ~]# cat /etc/sysconfig/network
# Created by anaconda
2.3.2. /etc/sysconfig/network-scripts/ifcfg-
Each network card can be configured individually using the /etc/sysconfig/network-scripts/
ifcfg-* files. When you have only one network card, then this will probably be /etc/
sysconfig/network-scripts/ifcfg-eth0.
dhcp client
Below a screenshot of /etc/sysconfig/network-scripts/ifcfg-eth0 configured for dhcp
(BOOTPROTO="dhcp"). Note also the NM_CONTROLLED paramater to disable control
of this nic by Network Manager. This parameter is not explained (not even mentioned) in
/usr/share/doc/initscripts-*/sysconfig.txt, but many others are.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="08:00:27:DD:0D:5C"
NM_CONTROLLED="no"
BOOTPROTO="dhcp"
ONBOOT="yes"
The BOOTPROTO variable can be set to either dhcp or bootp, anything else will be
considered static meaning there should be no protocol used at boot time to set the interface
values.
RHEL7 adds ipv6 variables to this file.
[root@rhel71 network-scripts]# cat ifcfg-enp0s3
TYPE="Ethernet"
BOOTPROTO="dhcp"
DEFROUTE="yes"
PEERDNS="yes"
PEERROUTES="yes"
IPV4_FAILURE_FATAL="no"

19
IPV6INIT="yes"
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_PEERDNS="yes"
IPV6_PEERROUTES="yes"
IPV6_FAILURE_FATAL="no"
NAME="enp0s3"
UUID="9fa6a83a-2f8e-4ecc-962c-5f614605f4ee"
DEVICE="enp0s3"
ONBOOT="yes"
[root@rhel71 network-scripts]#
fixed ip
Below a screenshot of a fixed ip configuration in /etc/sysconfig/network-scripts/ifcfg-
eth0.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="08:00:27:DD:0D:5C"
NM_CONTROLLED="no"
BOOTPROTO="none"
IPADDR="192.168.1.99"
NETMASK="255.255.255.0"
GATEWAY="192.168.1.1"
ONBOOT="yes"
The HWADDR can be used to make sure that each network card gets the correct name
when multiple network cards are present in the computer. It can not be used to assign a mac
address to a network card. For this, you need to specify the MACADDR variable. Do not
use HWADDR and MACADDR in the same ifcfg-ethx file.
The BROADCAST= and NETWORK= parameters from previous RHEL/Fedora versions
are obsoleted.
2.3.3. nmcli
On RHEL7 you should run nmcli connection reload if you changed configuration files in
/etc/sysconfig/ to enable your changes.
The nmcli tool has many options to configure networking on the command line in RHEL7/
CentOS7
man nmcli
2.3.4. nmtui
Another recommendation for RHEL7/CentOS7 is to use nmtui. This tool will use a
'windowed' interface in command line to manage network interfaces.
nmtui

20
2.3.5. /sbin/ifup and /sbin/ifdown
The ifup and ifdown commands will set an interface up or down, using the configuration
discussed above. This is identical to their behaviour in Debian and Ubuntu.
[root@rhel6 ~]# ifdown eth0 && ifup eth0
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX bytes:257036 (251.0 KiB) TX bytes:184767 (180.4 KiB)
2.4. ifconfig
The use of /sbin/ifconfig without any arguments will present you with a list of all active
network interface cards, including wireless and the loopback interface. In the screenshot
below eth0 has no ip address.
root@ubu1010:~# ifconfig
eth0 Link encap:Ethernet HWaddr 00:26:bb:5d:2e:52
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
Interrupt:43 Base address:0xe000
eth1 Link encap:Ethernet HWaddr 00:26:bb:12:7a:5e
inet6 addr: fe80::226:bbff:fe12:7a5e/64 Scope:Link
RX bytes:3476531617 (3.4 GB) TX bytes:2114919475 (2.1 GB)
Interrupt:23
inet addr:127.0.0.1 Mask:255.0.0.0
You can also use ifconfig to obtain information about just one network card.

21
When /sbin is not in the $PATH of a normal user you will have to type the full path, as
seen here on Debian.
paul@debian5:~$ /sbin/ifconfig eth3
eth3 Link encap:Ethernet HWaddr 08:00:27:ab:67:30
inet6 addr: fe80::a00:27ff:feab:6730/64 Scope:Link
RX bytes:13095386 (12.4 MiB) TX bytes:25767221 (24.5 MiB)
2.4.1. up and down
You can also use ifconfig to bring an interface up or down. The difference with ifup is that
ifconfig eth0 up will re-activate the nic keeping its existing (current) configuration, whereas
ifup will read the correct file that contains a (possibly new) configuration and use this config
file to bring the interface up.
[root@rhel6 ~]# ifconfig eth0 down
[root@rhel6 ~]# ifconfig eth0 up
2.4.2. setting ip address
You can temporary set an ip address with ifconfig. This ip address is only valid until the
next ifup/ifdown cycle or until the next reboot.
[root@rhel6 ~]# ifconfig eth0 | grep 192
[root@rhel6 ~]# ifconfig eth0 192.168.33.42 netmask 255.255.0.0
[root@rhel6 ~]# ifdown eth0 && ifup eth0
2.4.3. setting mac address
You can also use ifconfig to set another mac address than the one hard coded in the network
card. This screenshot shows you how.
[root@rhel6 ~]# ifconfig eth0 | grep HWaddr
[root@rhel6 ~]# ifconfig eth0 hw ether 00:42:42:42:42:42
[root@rhel6 ~]# ifconfig eth0 | grep HWaddr
eth0 Link encap:Ethernet HWaddr 00:42:42:42:42:42

22
2.5. ip
The ifconfig tool is deprecated on some systems. Use the ip tool instead.
To see ip addresses on RHEL7 for example, use this command:
[root@rhel71 ~]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 08:00:27:89:22:33 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.135/24 brd 192.168.1.255 scope global dynamic enp0s3
valid_lft 6173sec preferred_lft 6173sec
inet6 fe80::a00:27ff:fe89:2233/64 scope link
[root@rhel71 ~]#

23
2.6. dhclient
Home and client Linux desktops often have /sbin/dhclient running. This is a daemon that
enables a network interface to lease an ip configuration from a dhcp server. When your
adapter is configured for dhcp or bootp, then /sbin/ifup will start the dhclient daemon.
When a lease is renewed, dhclient will override your ifconfig set ip address!
2.7. hostname
Every host receives a hostname, often placed in a DNS name space forming the fqdn or
Fully Qualified Domain Name.
This screenshot shows the hostname command and the configuration of the hostname on
Red Hat/Fedora.
[root@rhel6 ~]# grep HOSTNAME /etc/sysconfig/network
HOSTNAME=rhel6
[root@rhel6 ~]# hostname
rhel6
Starting with RHEL7/CentOS7 this file is empty. The hostname is configured in the standard
/etc/hostname file.
[root@rhel71 ~]# cat /etc/hostname
rhel71.linux-training.be
[root@rhel71 ~]#
Ubuntu/Debian uses the /etc/hostname file to configure the hostname.
paul@debian8:~$ cat /etc/hostname
server42
paul@debian8:~$ hostname
server42
On all Linux distributions you can change the hostname using the hostname $newname
command. This is not a permanent change.
[root@rhel6 ~]# hostname server42
server42
On any Linux you can use sysctl to display and set the hostname.
[root@rhel6 ~]# sysctl kernel.hostname
kernel.hostname = server42
[root@rhel6 ~]# sysctl kernel.hostname=rhel6
kernel.hostname = rhel6
[root@rhel6 ~]# sysctl kernel.hostname
kernel.hostname = rhel6
rhel6

24
2.8. arp
The ip to mac resolution is handled by the layer two broadcast protocol arp. The arp table
can be displayed with the arp tool. The screenshot below shows the list of computers that
this computer recently communicated with.
root@barry:~# arp -a
? (192.168.1.191) at 00:0C:29:3B:15:80 [ether] on eth1
agapi (192.168.1.73) at 00:03:BA:09:7F:D2 [ether] on eth1
anya (192.168.1.1) at 00:12:01:E2:87:FB [ether] on eth1
faith (192.168.1.41) at 00:0E:7F:41:0D:EB [ether] on eth1
kiss (192.168.1.49) at 00:D0:E0:91:79:95 [ether] on eth1
laika (192.168.1.40) at 00:90:F5:4E:AE:17 [ether] on eth1
pasha (192.168.1.71) at 00:03:BA:02:C3:82 [ether] on eth1
shaka (192.168.1.72) at 00:03:BA:09:7C:F9 [ether] on eth1
root@barry:~#
Anya is a Cisco Firewall, faith is a laser printer, kiss is a Kiss DP600, laika is a laptop and
Agapi, Shaka and Pasha are SPARC servers. The question mark is a Red Hat Enterprise
Linux server running on a virtual machine.
You can use arp -d to remove an entry from the arp table.
[root@rhel6 ~]# arp
Address HWtype HWaddress Flags Mask Iface
ubu1010 ether 00:26:bb:12:7a:5e C eth0
anya ether 00:02:cf:aa:68:f0 C eth0
[root@rhel6 ~]# arp -d anya
[root@rhel6 ~]# arp
anya (incomplete) eth0
[root@rhel6 ~]# ping anya
PING anya (192.168.1.1) 56(84) bytes of data.
64 bytes from anya (192.168.1.1): icmp_seq=1 ttl=254 time=10.2 ms
...
[root@rhel6 ~]# arp
anya ether 00:02:cf:aa:68:f0 C eth0

25
2.9. route
You can see the computer's local routing table with the /sbin/route command (and also with
netstat -r ).
root@RHEL4b ~]# netstat -r
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0
[root@RHEL4b ~]# route
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0
[root@RHEL4b ~]#
It appears this computer does not have a gateway configured, so we use route add default
gw to add a default gateway on the fly.
[root@RHEL4b ~]# route add default gw 192.168.1.1
[root@RHEL4b ~]# route
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0
default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
[root@RHEL4b ~]#
Unless you configure the gateway in one of the /etc/ file from the start of this chapter, your
computer will forget this gateway after a reboot.
2.10. ping
If you can ping to another host, then tcp/ip is configured.
[root@RHEL4b ~]# ping 192.168.1.5
PING 192.168.1.5 (192.168.1.5) 56(84) bytes of data.
64 bytes from 192.168.1.5: icmp_seq=0 ttl=64 time=1004 ms
64 bytes from 192.168.1.5: icmp_seq=1 ttl=64 time=1.19 ms
--- 192.168.1.5 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3009ms
rtt min/avg/max/mdev = 0.419/251.574/1004.186/434.520 ms, pipe 2
[root@RHEL4b ~]#

26
2.11. optional: ethtool
To display or change network card settings, use ethtool. The results depend on the
capabilities of your network card. The example shows a network that auto-negotiates it's
bandwidth.
root@laika:~# ethtool eth0
Settings for eth0:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000033 (51)
Link detected: yes
This example shows how to use ethtool to switch the bandwidth from 1000Mbit to 100Mbit
and back. Note that some time passes before the nic is back to 1000Mbit.
root@laika:~# ethtool eth0 | grep Speed
Speed: 1000Mb/s
root@laika:~# ethtool -s eth0 speed 100
Speed: 100Mb/s
root@laika:~# ethtool -s eth0 speed 1000
Speed: 1000Mb/s

27
2.12. practice: interface configuration
1. Verify whether dhclient is running.
2. Display your current ip address(es).
3. Display the configuration file where this ip address is defined.
4. Follow the nic configuration in the book to change your ip address from dhcp client to
fixed. Keep the same ip address to avoid conflicts!
5. Did you also configure the correct gateway in the previous question ? If not, then do
this now.
6. Verify that you have a gateway.
7. Verify that you can connect to the gateway, that it is alive.
8. Change the last two digits of your mac address.
9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ?
10. Explain why e-mail and websites are sent over tcp and not udp.
11. Display the hostname of your computer.
12. Which ip-addresses did your computer recently have contact with ?

28
2.13. solution: interface configuration
1. Verify whether dhclient is running.
paul@debian5:~$ ps fax | grep dhclient
2. Display your current ip address(es).
paul@debian5:~$ /sbin/ifconfig | grep 'inet '
inet addr:127.0.0.1 Mask:255.0.0.0
3. Display the configuration file where this ip address is defined.
Ubuntu/Debian: cat /etc/network/interfaces
Redhat/Fedora: cat /etc/sysconfig/network-scripts/ifcfg-eth*
4. Follow the nic configuration in the book to change your ip address from dhcp client to
fixed. Keep the same ip address to avoid conflicts!
Ubuntu/Debian:
ifdown eth0
vi /etc/network/interfaces
ifup eth0
Redhat/Fedora:
ifdown eth0
vi /etc/sysconfig/network-scripts/ifcfg-eth0
ifup eth0
5. Did you also configure the correct gateway in the previous question ? If not, then do
this now.
6. Verify that you have a gateway.
paul@debian5:~$ /sbin/route
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0
default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
7. Verify that you can connect to the gateway, that it is alive.
paul@debian5:~$ ping -c3 192.168.1.1
--- 192.168.1.1 ping statistics ---
rtt min/avg/max/mdev = 2.283/2.524/2.941/0.296 ms
8. Change the last two digits of your mac address.
[root@rhel6 ~]# ifconfig eth0 hw ether 08:00:27:ab:67:XX
9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ?
root@rhel6 ~# grep ^'http ' /etc/services

29
http 80/tcp www www-http # WorldWideWeb HTTP
http 80/udp www www-http # HyperText Transfer Protocol
root@rhel6 ~# grep ^'smtp ' /etc/services
smtp 25/tcp mail
smtp 25/udp mail
root@rhel6 ~# grep ^'ssh ' /etc/services
ssh 22/tcp # The Secure Shell (SSH) Protocol
ssh 22/udp # The Secure Shell (SSH) Protocol
root@rhel6 ~# grep ^'telnet ' /etc/services
telnet 23/tcp
telnet 23/udp
root@rhel6 ~# grep ^'nntp ' /etc/services
nntp 119/tcp readnews untp # USENET News Transfer Protocol
nntp 119/udp readnews untp # USENET News Transfer Protocol
root@rhel6 ~# grep ^'ftp ' /etc/services
ftp 21/tcp
ftp 21/udp fsp fspd
10. Explain why e-mail and websites are sent over tcp and not udp.
Because tcp is reliable and udp is not.
11. Display the hostname of your computer.
paul@debian5:~$ hostname
debian5
12. Which ip-addresses did your computer recently have contact with ?
root@rhel6 ~# arp -a
? (192.168.1.1) at 00:02:cf:aa:68:f0 [ether] on eth2
? (192.168.1.30) at 00:26:bb:12:7a:5e [ether] on eth2
? (192.168.1.31) at 08:00:27:8e:8a:a8 [ether] on eth2

30
Chapter 3. network sniffing
A network administrator should be able to use a sniffer like wireshark or tcpdump to
troubleshoot network problems.
A student should often use a sniffer to learn about networking. This chapter introduces you
to network sniffing.

network sniffing
31
3.1. wireshark
3.1.1. installing wireshark
This example shows how to install wireshark on .deb based distributions (including Debian,
Mint, Xubuntu, and others).
root@debian8:~# apt-get install wireshark
Reading package lists... Done
Building dependency tree
Reading state information... Done
... (output truncated)
On .rpm based distributions like CentOS, RHEL and Fedora you can use yum to install
wireshark.
[root@centos7 ~]# yum install wireshark
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
... (output truncated)
3.1.2. selecting interface
When you start wireshark for the first time, you will need to select an interface. You will
see a dialog box that looks similar to this one.
It is possible that there are no interfaces available because some distributions only allow
root to sniff the network. You may need to use sudo wireshark.
Or you can follow the general advice to sniff using tcpdump or any other tool, and save the
capture to a file. Any saved capture can be analyzed using wireshark at a later time.

network sniffing
32
3.1.3. minimize traffic
Sniffing a network can generate many thousands of packets in a very short time. This can
be overwhelming. Try to mitigate by isolating your sniffer on the network. Preferably sniff
an isolated virtual network interface over which you control all traffic.
If you are at home to learn sniffing, then it could help to close all network programs on
your computer, and disconnect other computers and devices like smartphones and tablets
to minimize the traffic.
Even more important than this is the use of filters which will be discussed in this chapter.
3.1.4. sniffing ping
I started the sniffer and captured all packets while doing these three ping commands (there
is no need for root to do this):
root@debian7:~# ping -c2 ns1.paul.local
PING ns1.paul.local (10.104.33.30) 56(84) bytes of data.
64 bytes from 10.104.33.30: icmp_req=1 ttl=64 time=0.010 ms
--- ns1.paul.local ping statistics ---
root@debian7:~# ping -c3 linux-training.be
PING linux-training.be (188.93.155.87) 56(84) bytes of data.
64 bytes from antares.ginsys.net (188.93.155.87): icmp_req=1 ttl=56 time=15.6 ms
--- linux-training.be ping statistics ---
root@debian7:~# ping -c1 centos7.paul.local
PING centos7.paul.local (10.104.33.31) 56(84) bytes of data.
--- centos7.paul.local ping statistics ---
In total more than 200 packets were sniffed from the network. Things become clearer when
you enter icmp in the filter field and press the apply button.

network sniffing
33
3.1.5. sniffing ping and dns
Using the same capture as before, but now with a different filter. We want to see both dns
and icmp traffic, so we enter both in the filter field.
We put dns or icmp in the filter to achieve this. Putting dns and icmp would render nothing
because there is no packet that matches both protocols.
In the screenshot above you can see that packets 25 and 26 both have 10.104.33.30 as source
and destination ip address. That is because the dns client is the same computer as the dns
server.
The same is true for packets 31 and 32, since the machine is actually pinging itself.
3.1.6. specific ip address
This is a screenshot that filters for dns packets that contain a certain ip address. The filter
in use is ip.addr==10.104.33.30 and dns. The and directive forces each displayed packet
to match both conditions.
Packet 93 is the dns query for the A record of linux-training.be. Packet 98 is the response
from the dns server. What do you think happened in the packets between 93 and 98 ? Try
to answer this before reading on (it always helps to try to predict what you will see, and
then checking your prediction).
3.1.7. filtering by frame
The correct technical term for a packet as sniffed is a frame (because we sniff on layer two).
So to display packets with certain numbers, we use frame.number in the filter.

network sniffing
34
3.1.8. looking inside packets
The middle pane can be expanded. When selecting a line in this pane, you can see the
corresponding bytes in the frame in the bottom panel.
This screenshot shows the middle pane with the source address of my laptop selected.
Note that the above works fine when sniffing one interface. When sniffing with for example
tcpdump -i any you will end up with Linux cooked at this level.
3.1.9. other filter examples
You can combine two protocols with a logical or between them. The example below shows
how to filter only arp and bootp (or dhcp) packets.
This example shows how to filter for dns traffic containing a certain ip address.

network sniffing
35
3.2. tcpdump
Sniffing on the command line can be done with tcpdump. Here are some examples.
Using the tcpdump host $ip command displays all traffic with one host (192.168.1.38 in
this example).
root@ubuntu910:~# tcpdump host 192.168.1.38
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
Capturing only ssh (tcp port 22) traffic can be done with tcpdump tcp port $port. This
screenshot is cropped to 76 characters for readability in the pdf.
root@deb503:~# tcpdump tcp port 22
14:22:20.716313 IP deb503.local.37973 > rhel53.local.ssh: P 666050963:66605
14:22:20.719936 IP rhel53.local.ssh > deb503.local.37973: P 1:49(48) ack 48
14:22:20.720922 IP rhel53.local.ssh > deb503.local.37973: P 49:113(64) ack
14:22:20.721820 IP deb503.local.37973 > rhel53.local.ssh: . ack 161 win 200
14:22:20.760602 IP deb503.local.37973 > rhel53.local.ssh: . ack 225 win 200
14:22:23.108106 IP deb503.local.54424 > ubuntu910.local.ssh: P 467252637:46
14:22:23.116804 IP ubuntu910.local.ssh > deb503.local.54424: P 1:81(80) ack
14:22:23.116844 IP deb503.local.54424 > ubuntu910.local.ssh: . ack 81 win 2
^C
10 packets captured
10 packets received by filter
0 packets dropped by kernel
Same as above, but write the output to a file with the tcpdump -w $filename command.
root@ubuntu910:~# tcpdump -w sshdump.tcpdump tcp port 22
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
^C
17 packets captured
17 packets received by filter
0 packets dropped by kernel
With tcpdump -r $filename the file created above can be displayed.
root@ubuntu910:~# tcpdump -r sshdump.tcpdump
Many more examples can be found in the manual page of tcpdump.

network sniffing
36
3.3. practice: network sniffing
1. Install wireshark on your computer (not inside a virtual machine).
2. Start a ping between your computer and another computer.
3. Start sniffing the network.
4. Display only the ping echo's in the top pane using a filter.
5. Now ping to a name (like www.linux-training.be) and try to sniff the DNS query and
response. Which DNS server was used ? Was it a tcp or udp query and response ?
6. Find an amateur/hobby/club website that features a login prompt. Attempt to login with
user 'paul' and password 'hunter2' while your sniffer is running. Now find this information
in the sniffer.

network sniffing
37
3.4. solution: network sniffing
1. Install wireshark on your computer (not inside a virtual machine).
Debian/Ubuntu: aptitude install wireshark
Red Hat/Mandriva/Fedora: yum install wireshark
2. Start a ping between your computer and another computer.
ping $ip_address
3. Start sniffing the network.
(sudo) wireshark
select an interface (probably eth0)
4. Display only the ping echo's in the top pane using a filter.
type 'icmp' (without quotes) in the filter box, and then click 'apply'
5. Now ping to a name (like www.linux-training.be) and try to sniff the DNS query and
response. Which DNS server was used ? Was it a tcp or udp query and response ?
First start the sniffer.
Enter 'dns' in the filter box and click apply.
root@ubuntu910:~# ping www.linux-training.be
PING www.linux-training.be (88.151.243.8) 56(84) bytes of data.
64 bytes from fosfor.openminds.be (88.151.243.8): icmp_seq=1 ttl=58 time=14.9 ms
64 bytes from fosfor.openminds.be (88.151.243.8): icmp_seq=2 ttl=58 time=16.0 ms
^C
--- www.linux-training.be ping statistics ---
The wireshark screen should look something like this.
The details in wireshark will say the DNS query was inside a udp packet.
6. Find an amateur/hobby/club website that features a login prompt. Attempt to login with
user 'paul' and password 'hunter2' while your sniffer is running. Now find this information
in the sniffer.

38
Chapter 4. binding and bonding
Sometimes a server needs more than one ip address on the same network card, we call this
binding ip addresses.
Linux can also activate multiple network cards behind the same ip address, this is called
bonding.
This chapter will teach you how to configure binding and bonding on the most common
Linux distributions.

binding and bonding
39
4.1. binding on Redhat/Fedora
4.1.1. binding extra ip addresses
To bind more than one ip address to the same interface, use ifcfg-eth0:0, where the last
zero can be anything else. Only two directives are required in the files.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:0
DEVICE="eth0:0"
IPADDR="192.168.1.133"
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:1
DEVICE="eth0:0"
IPADDR="192.168.1.142"
4.1.2. enabling extra ip-addresses
To activate a virtual network interface, use ifup, to deactivate it, use ifdown.
[root@rhel6 ~]# ifup eth0:0
[root@rhel6 ~]# ifconfig | grep 'inet '
inet addr:127.0.0.1 Mask:255.0.0.0
[root@rhel6 ~]# ifup eth0:1
[root@rhel6 ~]# ifconfig | grep 'inet '
inet addr:127.0.0.1 Mask:255.0.0.0
4.1.3. verifying extra ip-addresses
Use ping from another computer to check the activation, or use ifconfig like in this
screenshot.
[root@rhel6 ~]# ifconfig
eth0:0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
eth0:1 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C

binding and bonding
40
4.2. binding on Debian/Ubuntu
4.2.1. binding extra ip addresses
The configuration of multiple ip addresses on the same network card is done in /etc/network/
interfaces by adding eth0:x devices. Adding the netmask is mandatory.
debian5:~# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
# The primary network interface
iface eth0 inet static
address 192.168.1.34
network 192.168.1.0
netmask 255.255.255.0
gateway 192.168.1.1
auto eth0
auto eth0:0
iface eth0:0 inet static
address 192.168.1.233
netmask 255.255.255.0
auto eth0:1
iface eth0:1 inet static
address 192.168.1.242
netmask 255.255.255.0
4.2.2. enabling extra ip-addresses
Use ifup to enable the extra addresses.
debian5:~# ifup eth0:0
debian5:~# ifup eth0:1
4.2.3. verifying extra ip-addresses
Use ping from another computer to check the activation, or use ifconfig like in this
screenshot.
debian5:~# ifconfig | grep 'inet '
inet addr:127.0.0.1 Mask:255.0.0.0

binding and bonding
41
4.3. bonding on Redhat/Fedora
We start with ifconfig -a to get a list of all the network cards on our system.
[root@rhel6 network-scripts]# ifconfig -a | grep Ethernet
eth1 Link encap:Ethernet HWaddr 08:00:27:DA:C1:49
eth2 Link encap:Ethernet HWaddr 08:00:27:40:03:3B
In this demo we decide to bond eth1 and eth2.
We will name our bond bond0 and add this entry to modprobe so the kernel can load the
bonding module when we bring the interface up.
[root@rhel6 network-scripts]# cat /etc/modprobe.d/bonding.conf
alias bond0 bonding
Then we create /etc/sysconfig/network-scripts/ifcfg-bond0 to configure our bond0
interface.
[root@rhel6 network-scripts]# pwd
/etc/sysconfig/network-scripts
[root@rhel6 network-scripts]# cat ifcfg-bond0
DEVICE=bond0
IPADDR=192.168.1.199
NETMASK=255.255.255.0
ONBOOT=yes
BOOTPROTO=none
USERCTL=no
Next we create two files, one for each network card that we will use as slave in bond0.
[root@rhel6 network-scripts]# cat ifcfg-eth1
DEVICE=eth1
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no
[root@rhel6 network-scripts]# cat ifcfg-eth2
DEVICE=eth2
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no
Finally we bring the interface up with ifup bond0.
[root@rhel6 network-scripts]# ifup bond0
[root@rhel6 network-scripts]# ifconfig bond0
bond0 Link encap:Ethernet HWaddr 08:00:27:DA:C1:49
inet6 addr: fe80::a00:27ff:feda:c149/64 Scope:Link
UP BROADCAST RUNNING MASTER MULTICAST MTU:1500 Metric:1
The bond should also be visible in /proc/net/bonding.

binding and bonding
42
[root@rhel6 network-scripts]# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.5.0 (November 4, 2008)
Bonding Mode: load balancing (round-robin)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0
Slave Interface: eth1
MII Status: up
Link Failure Count: 0
Permanent HW addr: 08:00:27:da:c1:49
MII Status: up
Permanent HW addr: 08:00:27:40:03:3b

binding and bonding
43
4.4. bonding on Debian/Ubuntu
We start with ifconfig -a to get a list of all the network cards on our system.
debian5:~# ifconfig -a | grep Ethernet
eth0 Link encap:Ethernet HWaddr 08:00:27:bb:18:a4
eth1 Link encap:Ethernet HWaddr 08:00:27:63:9a:95
eth2 Link encap:Ethernet HWaddr 08:00:27:27:a4:92
In this demo we decide to bond eth1 and eth2.
We also need to install the ifenslave package.
debian5:~# aptitude search ifenslave
p ifenslave - Attach and detach slave interfaces to a bonding device
p ifenslave-2.6 - Attach and detach slave interfaces to a bonding device
debian5:~# aptitude install ifenslave
...
Next we update the /etc/network/interfaces file with information about the bond0 interface.
debian5:~# tail -7 /etc/network/interfaces
iface bond0 inet static
address 192.168.1.42
netmask 255.255.255.0
gateway 192.168.1.1
slaves eth1 eth2
bond-mode active-backup
bond_primary eth1
On older version of Debian/Ubuntu you needed to modprobe bonding, but this is no longer
required. Use ifup to bring the interface up, then test that it works.
debian5:~# ifup bond0
debian5:~# ifconfig bond0
bond0 Link encap:Ethernet HWaddr 08:00:27:63:9a:95
inet6 addr: fe80::a00:27ff:fe63:9a95/64 Scope:Link
UP BROADCAST RUNNING MASTER MULTICAST MTU:1500 Metric:1
The bond should also be visible in /proc/net/bonding.
debian5:~# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.2.5 (March 21, 2008)
Bonding Mode: fault-tolerance (active-backup)
Primary Slave: eth1
Currently Active Slave: eth1
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0
MII Status: up

binding and bonding
44
Permanent HW addr: 08:00:27:63:9a:95
MII Status: up
Permanent HW addr: 08:00:27:27:a4:92

binding and bonding
45
4.5. practice: binding and bonding
1. Add an extra ip address to one of your network cards. Test that it works (have your
neighbour ssh to it)!
2. Use ifdown to disable this extra ip address.
3. Make sure your neighbour also succeeded in binding an extra ip address before you
continue.
4. Add an extra network card (or two) to your virtual machine and use the theory to bond
two network cards.

binding and bonding
46
4.6. solution: binding and bonding
1. Add an extra ip address to one of your network cards. Test that it works (have your
neighbour ssh to it)!
Redhat/Fedora:
add an /etc/sysconfig/network-scripts/ifcfg-ethX:X file
as shown in the theory
Debian/Ubuntu:
expand the /etc/network/interfaces file
2. Use ifdown to disable this extra ip address.
ifdown eth0:0
3. Make sure your neighbour also succeeded in binding an extra ip address before you
continue.
ping $extra_ip_neighbour
or
ssh $extra_ip_neighbour
4. Add an extra network card (or two) to your virtual machine and use the theory to bond
two network cards.
Redhat/Fedora:
add ifcfg-ethX and ifcfg-bondX files in /etc/sysconfig/network-scripts
and don't forget the modprobe.conf
Debian/Ubuntu:
expand the /etc/network/interfaces file
and don't forget to install the ifenslave package

47
Chapter 5. ssh client and server
The secure shell or ssh is a collection of tools using a secure protocol for communications
with remote Linux computers.
This chapter gives an overview of the most common commands related to the use of the
sshd server and the ssh client.

ssh client and server
48
5.1. about ssh
5.1.1. secure shell
Avoid using telnet, rlogin and rsh to remotely connect to your servers. These older protocols
do not encrypt the login session, which means your user id and password can be sniffed by
tools like wireshark or tcpdump. To securely connect to your servers, use ssh.
The ssh protocol is secure in two ways. Firstly the connection is encrypted and secondly
the connection is authenticated both ways.
An ssh connection always starts with a cryptographic handshake, followed by encryption of
the transport layer using a symmetric cypher. In other words, the tunnel is encrypted before
you start typing anything.
Then authentication takes place (using user id/password or public/private keys) and
communication can begin over the encrypted connection.
The ssh protocol will remember the servers it connected to (and warn you in case something
suspicious happened).
The openssh package is maintained by the OpenBSD people and is distributed with a lot of
operating systems (it may even be the most popular package in the world).
5.1.2. /etc/ssh/
Configuration of ssh client and server is done in the /etc/ssh directory. In the next sections
we will discuss most of the files found in /etc/ssh/.
5.1.3. ssh protocol versions
The ssh protocol has two versions (1 and 2). Avoid using version 1 anywhere, since it
contains some known vulnerabilities. You can control the protocol version via /etc/ssh/
ssh_config for the client side and /etc/ssh/sshd_config for the openssh-server daemon.
paul@ubu1204:/etc/ssh$ grep Protocol ssh_config
# Protocol 2,1
paul@ubu1204:/etc/ssh$ grep Protocol sshd_config
Protocol 2

49
5.1.4. public and private keys
The ssh protocol uses the well known system of public and private keys. The below
explanation is succinct, more information can be found on wikipedia.
http://en.wikipedia.org/wiki/Public-key_cryptography
Imagine Alice and Bob, two people that like to communicate with each other. Using public
and private keys they can communicate with encryption and with authentication.
When Alice wants to send an encrypted message to Bob, she uses the public key of Bob.
Bob shares his public key with Alice, but keeps his private key private! Since Bob is the
only one to have Bob's private key, Alice is sure that Bob is the only one that can read the
encrypted message.
When Bob wants to verify that the message came from Alice, Bob uses the public key of
Alice to verify that Alice signed the message with her private key. Since Alice is the only
one to have Alice's private key, Bob is sure the message came from Alice.
5.1.5. rsa and dsa algorithms
This chapter does not explain the technical implementation of cryptographic algorithms,
it only explains how to use the ssh tools with rsa and dsa. More information about these
algorithms can be found here:
http://en.wikipedia.org/wiki/RSA_(algorithm)
http://en.wikipedia.org/wiki/Digital_Signature_Algorithm

50
5.2. log on to a remote server
The following screenshot shows how to use ssh to log on to a remote computer running
Linux. The local user is named paul and he is logging on as user admin42 on the remote
system.
paul@ubu1204:~$ ssh admin42@192.168.1.30
The authenticity of host '192.168.1.30 (192.168.1.30)' can't be established.
RSA key fingerprint is b5:fb:3c:53:50:b4:ab:81:f3:cd:2e:bb:ba:44:d3:75.
Are you sure you want to continue connecting (yes/no)?
As you can see, the user paul is presented with an rsa authentication fingerprint from the
remote system. The user can accepts this bu typing yes. We will see later that an entry will
be added to the ~/.ssh/known_hosts file.
paul@ubu1204:~$ ssh admin42@192.168.1.30
The authenticity of host '192.168.1.30 (192.168.1.30)' can't be established.
RSA key fingerprint is b5:fb:3c:53:50:b4:ab:81:f3:cd:2e:bb:ba:44:d3:75.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.30' (RSA) to the list of known hosts.
admin42@192.168.1.30's password:
Welcome to Ubuntu 12.04 LTS (GNU/Linux 3.2.0-26-generic-pae i686)
* Documentation: https://help.ubuntu.com/
1 package can be updated.
0 updates are security updates.
Last login: Wed Jun 6 19:25:57 2012 from 172.28.0.131
admin42@ubuserver:~$
The user can get log out of the remote server by typing exit or by using Ctrl-d.
admin42@ubuserver:~$ exit
logout
Connection to 192.168.1.30 closed.
paul@ubu1204:~$
5.3. executing a command in remote
This screenshot shows how to execute the pwd command on the remote server. There is no
need to exit the server manually.
paul@ubu1204:~$ ssh admin42@192.168.1.30 pwd
/home/admin42
paul@ubu1204:~$

51
5.4. scp
The scp command works just like cp, but allows the source and destination of the copy to
be behind ssh. Here is an example where we copy the /etc/hosts file from the remote server
to the home directory of user paul.
paul@ubu1204:~$ scp admin42@192.168.1.30:/etc/hosts /home/paul/serverhosts
hosts 100% 809 0.8KB/s 00:00
Here is an example of the reverse, copying a local file to a remote server.
paul@ubu1204:~$ scp ~/serverhosts admin42@192.168.1.30:/etc/hosts.new
serverhosts 100% 809 0.8KB/s 00:00

52
5.5. setting up passwordless ssh
To set up passwordless ssh authentication through public/private keys, use ssh-keygen to
generate a key pair without a passphrase, and then copy your public key to the destination
server. Let's do this step by step.
In the example that follows, we will set up ssh without password between Alice and Bob.
Alice has an account on a Red Hat Enterprise Linux server, Bob is using Ubuntu on his
laptop. Bob wants to give Alice access using ssh and the public and private key system. This
means that even if Bob changes his password on his laptop, Alice will still have access.
5.5.1. ssh-keygen
The example below shows how Alice uses ssh-keygen to generate a key pair. Alice does
not enter a passphrase.
[alice@RHEL5 ~]$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/alice/.ssh/id_rsa):
Created directory '/home/alice/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/alice/.ssh/id_rsa.
Your public key has been saved in /home/alice/.ssh/id_rsa.pub.
The key fingerprint is:
9b:ac:ac:56:c2:98:e5:d9:18:c4:2a:51:72:bb:45:eb alice@RHEL5
[alice@RHEL5 ~]$
You can use ssh-keygen -t dsa in the same way.
5.5.2. ~/.ssh
While ssh-keygen generates a public and a private key, it will also create a hidden .ssh
directory with proper permissions. If you create the .ssh directory manually, then you need
to chmod 700 it! Otherwise ssh will refuse to use the keys (world readable private keys are
not secure!).
As you can see, the .ssh directory is secure in Alice's home directory.
[alice@RHEL5 ~]$ ls -ld .ssh
drwx------ 2 alice alice 4096 May 1 07:38 .ssh
[alice@RHEL5 ~]$
Bob is using Ubuntu at home. He decides to manually create the .ssh directory, so he needs
to manually secure it.
bob@laika:~$ mkdir .ssh
bob@laika:~$ ls -ld .ssh
drwxr-xr-x 2 bob bob 4096 2008-05-14 16:53 .ssh
bob@laika:~$ chmod 700 .ssh/
bob@laika:~$
5.5.3. id_rsa and id_rsa.pub
The ssh-keygen command generate two keys in .ssh. The public key is named ~/.ssh/
id_rsa.pub. The private key is named ~/.ssh/id_rsa.

53
[alice@RHEL5 ~]$ ls -l .ssh/
total 16
-rw------- 1 alice alice 1671 May 1 07:38 id_rsa
-rw-r--r-- 1 alice alice 393 May 1 07:38 id_rsa.pub
The files will be named id_dsa and id_dsa.pub when using dsa instead of rsa.
5.5.4. copy the public key to the other computer
To copy the public key from Alice's server tot Bob's laptop, Alice decides to use scp.
[alice@RHEL5 .ssh]$ scp id_rsa.pub bob@192.168.48.92:~/.ssh/authorized_keys
bob@192.168.48.92's password:
id_rsa.pub 100% 393 0.4KB/s 00:00
Be careful when copying a second key! Do not overwrite the first key, instead append the
key to the same ~/.ssh/authorized_keys file!
cat id_rsa.pub >> ~/.ssh/authorized_keys
Alice could also have used ssh-copy-id like in this example.
ssh-copy-id -i .ssh/id_rsa.pub bob@192.168.48.92
5.5.5. authorized_keys
In your ~/.ssh directory, you can create a file called authorized_keys. This file can contain
one or more public keys from people you trust. Those trusted people can use their private
keys to prove their identity and gain access to your account via ssh (without password). The
example shows Bob's authorized_keys file containing the public key of Alice.
bob@laika:~$ cat .ssh/authorized_keys
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEApCQ9xzyLzJes1sR+hPyqW2vyzt1D4zTLqk
MDWBR4mMFuUZD/O583I3Lg/Q+JIq0RSksNzaL/BNLDou1jMpBe2Dmf/u22u4KmqlJBfDhe
yTmGSBzeNYCYRSMq78CT9l9a+y6x/shucwhaILsy8A2XfJ9VCggkVtu7XlWFDL2cum08/0
mRFwVrfc/uPsAn5XkkTscl4g21mQbnp9wJC40pGSJXXMuFOk8MgCb5ieSnpKFniAKM+tEo
/vjDGSi3F/bxu691jscrU0VUdIoOSo98HUfEf7jKBRikxGAC7I4HLa+/zX73OIvRFAb2hv
tUhn6RHrBtUJUjbSGiYeFTLDfcTQ== alice@RHEL5
5.5.6. passwordless ssh
Alice can now use ssh to connect passwordless to Bob's laptop. In combination with ssh's
capability to execute commands on the remote host, this can be useful in pipes across
different machines.
[alice@RHEL5 ~]$ ssh bob@192.168.48.92 "ls -l .ssh"
total 4
-rw-r--r-- 1 bob bob 393 2008-05-14 17:03 authorized_keys
[alice@RHEL5 ~]$
5.6. X forwarding via ssh
Another popular feature of ssh is called X11 forwarding and is implemented with ssh -X.
Below an example of X forwarding: user paul logs in as user greet on her computer to start the
graphical application mozilla-thunderbird. Although the application will run on the remote
computer from greet, it will be displayed on the screen attached locally to paul's computer.

54
paul@debian5:~/PDF$ ssh -X greet@greet.dyndns.org -p 55555
Warning: Permanently added the RSA host key for IP address
'81.240.174.161' to the list of known hosts.
Password:
Linux raika 2.6.8-2-686 #1 Tue Aug 16 13:22:48 UTC 2005 i686 GNU/Linux
Last login: Thu Jan 18 12:35:56 2007
greet@raika:~$ ps fax | grep thun
greet@raika:~$ mozilla-thunderbird &
[1] 30336
5.7. troubleshooting ssh
Use ssh -v to get debug information about the ssh connection attempt.
paul@debian5:~$ ssh -v bert@192.168.1.192
OpenSSH_4.3p2 Debian-8ubuntu1, OpenSSL 0.9.8c 05 Sep 2006
debug1: Reading configuration data /home/paul/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 192.168.1.192 [192.168.1.192] port 22.
debug1: Connection established.
debug1: identity file /home/paul/.ssh/identity type -1
debug1: identity file /home/paul/.ssh/id_rsa type 1
debug1: identity file /home/paul/.ssh/id_dsa type -1
debug1: Remote protocol version 1.99, remote software version OpenSSH_3
debug1: match: OpenSSH_3.9p1 pat OpenSSH_3.*
debug1: Enabling compatibility mode for protocol 2.0
...

55
5.8. sshd
The ssh server is called sshd and is provided by the openssh-server package.
root@ubu1204~# dpkg -l openssh-server | tail -1
ii openssh-server 1:5.9p1-5ubuntu1 secure shell (SSH) server,...
5.9. sshd keys
The public keys used by the sshd server are located in /etc/ssh and are world readable. The
private keys are only readable by root.
root@ubu1204~# ls -l /etc/ssh/ssh_host_*
-rw------- 1 root root 668 Jun 7 2011 /etc/ssh/ssh_host_dsa_key
-rw-r--r-- 1 root root 598 Jun 7 2011 /etc/ssh/ssh_host_dsa_key.pub
-rw------- 1 root root 1679 Jun 7 2011 /etc/ssh/ssh_host_rsa_key
-rw-r--r-- 1 root root 390 Jun 7 2011 /etc/ssh/ssh_host_rsa_key.pub
5.10. ssh-agent
When generating keys with ssh-keygen, you have the option to enter a passphrase to protect
access to the keys. To avoid having to type this passphrase every time, you can add the key
to ssh-agent using ssh-add.
Most Linux distributions will start the ssh-agent automatically when you log on.
root@ubu1204~# ps -ef | grep ssh-agent
paul 2405 2365 0 08:13 ? 00:00:00 /usr/bin/ssh-agent...
This clipped screenshot shows how to use ssh-add to list the keys that are currently added
to the ssh-agent
paul@debian5:~$ ssh-add -L
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAvgI+Vx5UrIsusZPl8da8URHGsxG7yivv3/
...
wMGqa48Kelwom8TGb4Sgcwpp/VO/ldA5m+BGCw== paul@deb503

56
5.11. practice: ssh
0. Make sure that you have access to two Linux computers, or work together with a partner
for this exercise. For this practice, we will name one of the machines the server.
1. Install sshd on the server
2. Verify in the ssh configuration files that only protocol version 2 is allowed.
3. Use ssh to log on to the server, show your current directory and then exit the server.
4. Use scp to copy a file from your computer to the server.
5. Use scp to copy a file from the server to your computer.
6. (optional, only works when you have a graphical install of Linux) Install the xeyes package
on the server and use ssh to run xeyes on the server, but display it on your client.
7. (optional, same as previous) Create a bookmark in firefox, then quit firefox on client and
server. Use ssh -X to run firefox on your display, but on your neighbour's computer. Do you
see your neighbour's bookmark ?
8. Use ssh-keygen to create a key pair without passphrase. Setup passwordless ssh between
you and your neighbour. (or between your client and your server)
9.Verify that the permissions on the server key files are correct; world readable for the public
keys and only root access for the private keys.
10. Verify that the ssh-agent is running.
11. (optional) Protect your keypair with a passphrase, then add this key to the ssh-agent
and test your passwordless ssh to the server.

57
5.12. solution: ssh
0. Make sure that you have access to two Linux computers, or work together with a partner
for this exercise. For this practice, we will name one of the machines the server.
1. Install sshd on the server
apt-get install openssh-server (on Ubuntu/Debian)
yum -y install openssh-server (on Centos/Fedora/Red Hat)
2. Verify in the ssh configuration files that only protocol version 2 is allowed.
grep Protocol /etc/ssh/ssh*_config
3. Use ssh to log on to the server, show your current directory and then exit the server.
user@client$ ssh user@server-ip-address
user@server$ pwd
/home/user
user@server$ exit
4. Use scp to copy a file from your computer to the server.
scp localfile user@server:~
5. Use scp to copy a file from the server to your computer.
scp user@server:~/serverfile .
6. (optional, only works when you have a graphical install of Linux) Install the xeyes package
on the server and use ssh to run xeyes on the server, but display it on your client.
on the server:
apt-get install xeyes
on the client:
ssh -X user@server-ip
xeyes
7. (optional, same as previous) Create a bookmark in firefox, then quit firefox on client and
server. Use ssh -X to run firefox on your display, but on your neighbour's computer. Do you
see your neighbour's bookmark ?
8. Use ssh-keygen to create a key pair without passphrase. Setup passwordless ssh between
you and your neighbour. (or between your client and your server)
See solution in book "setting up passwordless ssh"
9. Verify that the permissions on the server key files are correct; world readable for the
public keys and only root access for the private keys.
ls -l /etc/ssh/ssh_host_*
10. Verify that the ssh-agent is running.
ps fax | grep ssh-agent
11. (optional) Protect your keypair with a passphrase, then add this key to the ssh-agent
and test your passwordless ssh to the server.

58
man ssh-keygen
man ssh-agent
man ssh-add

59
Chapter 6. introduction to nfs
The network file system (or simply nfs) enables us since the Eighties to share a directory
with other computers on the network.
In this chapter we see how to setup an nfs server and an nfs client computer.

introduction to nfs
60
6.1. nfs protocol versions
The older nfs versions 2 and 3 are stateless (udp) by default (but they can use tcp). The more
recent nfs version 4 brings a stateful protocol with better performance and stronger security.
NFS version 4 was defined in rfc 3010 in 2000 and rfc 3530 in 2003 and requires tcp (port
2049). It also supports Kerberos user authentication as an option when mounting a share.
NFS versions 2 and 3 authenticate only the host.
6.2. rpcinfo
Clients connect to the server using rpc (on Linux this can be managed by the portmap
daemon). Look at rpcinfo to verify that nfs and its related services are running.
root@RHELv4u2:~# /etc/init.d/portmap status
portmap (pid 1920) is running...
root@RHELv4u2:~# rpcinfo -p
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100024 1 udp 32768 status
100024 1 tcp 32769 status
root@RHELv4u2:~# service nfs start
Starting NFS services: [ OK ]
Starting NFS quotas: [ OK ]
Starting NFS daemon: [ OK ]
Starting NFS mountd: [ OK ]
The same rpcinfo command when nfs is started.
100024 1 udp 32768 status
100024 1 tcp 32769 status
100011 1 udp 985 rquotad
100011 1 tcp 988 rquotad
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 4 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
100003 4 tcp 2049 nfs
100021 1 udp 32770 nlockmgr
100021 1 tcp 32789 nlockmgr
100005 1 udp 1004 mountd
100005 1 tcp 1007 mountd
100005 2 udp 1004 mountd
100005 2 tcp 1007 mountd
100005 3 udp 1004 mountd
100005 3 tcp 1007 mountd

introduction to nfs
61
6.3. server configuration
nfs is configured in /etc/exports. You might want some way (ldap?) to synchronize userid's
across computers when using nfs a lot.
The rootsquash option will change UID 0 to the UID of a nobody (or similar) user account.
The sync option will write writes to disk before completing the client request.
6.4. /etc/exports
Here is a sample /etc/exports to explain the syntax:
paul@laika:~$ cat /etc/exports
# Everyone can read this share
/mnt/data/iso *(ro)
# Only the computers named pasha and barry can readwrite this one
/var/www pasha(rw) barry(rw)
# same, but without root squashing for barry
/var/ftp pasha(rw) barry(rw,no_root_squash)
# everyone from the netsec.local domain gets access
/var/backup *.netsec.local(rw)
# ro for one network, rw for the other
/var/upload 192.168.1.0/24(ro) 192.168.5.0/24(rw)
More recent incarnations of nfs require the subtree_check option to be explicitly set (or
unset with no_subtree_check). The /etc/exports file then looks like this:
root@debian6 ~# cat /etc/exports
/srv/iso *(ro,no_subtree_check)
# Only the computers named pasha and barry can readwrite this one
/var/www pasha(rw,no_subtree_check) barry(rw,no_subtree_check)
/var/ftp pasha(rw,no_subtree_check) barry(rw,no_root_squash,no_subtree_check)
6.5. exportfs
You don't need to restart the nfs server to start exporting your newly created exports. You
can use the exportfs -va command to do this. It will write the exported directories to /var/
lib/nfs/etab, where they are immediately applied.
root@debian6 ~# exportfs -va
exporting pasha:/var/ftp
exporting barry:/var/ftp
exporting pasha:/var/www
exporting barry:/var/www
exporting *:/srv/iso

introduction to nfs
62
6.6. client configuration
We have seen the mount command and the /etc/fstab file before.
root@RHELv4u2:~# mount -t nfs barry:/mnt/data/iso /home/project55/
root@RHELv4u2:~# cat /etc/fstab | grep nfs
barry:/mnt/data/iso /home/iso nfs defaults 0 0
root@RHELv4u2:~#
Here is another simple example. Suppose the project55 people tell you they only need a
couple of CD-ROM images, and you already have them available on an nfs server. You
could issue the following command to mount this storage on their /home/project55 mount
point.
root@RHELv4u2:~# mount -t nfs 192.168.1.40:/mnt/data/iso /home/project55/
root@RHELv4u2:~# ls -lh /home/project55/
total 3.6G
drwxr-xr-x 2 1000 1000 4.0K Jan 16 17:55 RHELv4u1
-rw-r--r-- 1 root root 1.6G Oct 13 15:22 sled10-vmwarews5-vm.zip
root@RHELv4u2:~#

introduction to nfs
63
6.7. practice: introduction to nfs
1. Create two directories with some files. Use nfs to share one of them as read only, the other
must be writable. Have your neighbour connect to them to test.
2. Investigate the user owner of the files created by your neighbour.
3. Protect a share by ip-address or hostname, so only your neighbour can connect.

64
Chapter 7. introduction to networking

introduction to networking
65
7.1. introduction to iptables
7.1.1. iptables firewall
The Linux kernel has a built-in stateful firewall named iptables. To stop the iptables firewall
on Red Hat, use the service command.
root@RHELv4u4:~# service iptables stop
Flushing firewall rules: [ OK ]
Setting chains to policy ACCEPT: filter [ OK ]
Unloading iptables modules: [ OK ]
root@RHELv4u4:~#
The easy way to configure iptables, is to use a graphical tool like KDE's kmyfirewall
or Security Level Configuration Tool. You can find the latter in the graphical menu,
somewhere in System Tools - Security, or you can start it by typing system-config-
securitylevel in bash. These tools allow for some basic firewall configuration. You can
decide whether to enable or disable the firewall, and what typical standard ports are allowed
when the firewall is active. You can even add some custom ports. When you are done, the
configuration is written to /etc/sysconfig/iptables on Red Hat.
root@RHELv4u4:~# cat /etc/sysconfig/iptables
# Firewall configuration written by system-config-securitylevel
# Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:RH-Firewall-1-INPUT - [0:0]
-A INPUT -j RH-Firewall-1-INPUT
-A FORWARD -j RH-Firewall-1-INPUT
-A RH-Firewall-1-INPUT -i lo -j ACCEPT
-A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT
-A RH-Firewall-1-INPUT -p 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -j ACCEPT
-A RH-Firewall-1-INPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT
-A RH-Firewall-1-INPUT -p udp -m udp --dport 631 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited
COMMIT
root@RHELv4u4:~#
To start the service, issue the service iptables start command. You can configure iptables
to start at boot time with chkconfig.
root@RHELv4u4:~# service iptables start
Applying iptables firewall rules: [ OK ]
root@RHELv4u4:~# chkconfig iptables on
root@RHELv4u4:~#

66
One of the nice features of iptables is that it displays extensive status information when
queried with the service iptables status command.
root@RHELv4u4:~# service iptables status
Table: filter
Chain INPUT (policy ACCEPT)
target prot opt source destination
RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0
Chain FORWARD (policy ACCEPT)
RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0
Chain OUTPUT (policy ACCEPT)
Chain RH-Firewall-1-INPUT (2 references)
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 icmp type 255
ACCEPT esp -- 0.0.0.0/0 0.0.0.0/0
ACCEPT ah -- 0.0.0.0/0 0.0.0.0/0
ACCEPT udp -- 0.0.0.0/0 224.0.0.251 udp dpt:5353
ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:631
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22
REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited
root@RHELv4u4:~#
Mastering firewall configuration requires a decent knowledge of tcp/ip. Good iptables
tutorials can be found online here http://iptables-tutorial.frozentux.net/iptables-tutorial.html
and here http://tldp.org/HOWTO/IP-Masquerade-HOWTO/.
7.2. practice : iptables
1. Verify whether the firewall is running.
2. Stop the running firewall.

67
7.3. solution : iptables
1. Verify whether the firewall is running.
root@rhel55 ~# service iptables status | head
Table: filter
num target prot opt source destination
1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0
num target prot opt source destination
1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0
2. Stop the running firewall.
root@rhel55 ~# service iptables stop
Flushing firewall rules: [ OK ]
Setting chains to policy ACCEPT: filter [ OK ]
Unloading iptables modules: [ OK ]
root@rhel55 ~# service iptables status
Firewall is stopped.

68
7.4. xinetd and inetd
7.4.1. the superdaemon
Back when resources like RAM memory were limited, a super-server was devised to listen
to all sockets and start the appropriate daemon only when needed. Services like swat, telnet
and ftp are typically served by such a super-server. The xinetd superdaemon is more recent
than inetd. We will discuss the configuration both daemons.
Recent Linux distributions like RHEL5 and Ubuntu10.04 do not activate inetd or xinetd by
default, unless an application requires it.
7.4.2. inetd or xinetd
First verify whether your computer is running inetd or xinetd. This Debian 4.0 Etch is
running inetd.
root@barry:~# ps fax | grep inet
3870 ? Ss 0:00 /usr/sbin/inetd
This Red Hat Enterprise Linux 4 update 4 is running xinetd.
[root@RHEL4b ~]# ps fax | grep inet
3003 ? Ss 0:00 xinetd -stayalive -pidfile /var/run/xinetd.pid
Both daemons have the same functionality (listening to many ports, starting other daemons
when they are needed), but they have different configuration files.
7.4.3. xinetd superdaemon
The xinetd daemon is often called a superdaemon because it listens to a lot of incoming
connections, and starts other daemons when they are needed. When a connection request
is received, xinetd will first check TCP wrappers (/etc/hosts.allow and /etc/hosts.deny) and
then give control of the connection to the other daemon. This superdaemon is configured
through /etc/xinetd.conf and the files in the directory /etc/xinetd.d. Let's first take a look
at /etc/xinetd.conf.
paul@RHELv4u2:~$ cat /etc/xinetd.conf
#
# Simple configuration file for xinetd
#
# Some defaults, and include /etc/xinetd.d/
defaults
{
instances = 60
log_type = SYSLOG authpriv
log_on_success = HOST PID
log_on_failure = HOST
cps = 25 30

69
}
includedir /etc/xinetd.d
paul@RHELv4u2:~$
According to the settings in this file, xinetd can handle 60 client requests at once. It uses the
authpriv facility to log the host ip-address and pid of successful daemon spawns. When a
service (aka protocol linked to daemon) gets more than 25 cps (connections per second), it
holds subsequent requests for 30 seconds.
The directory /etc/xinetd.d contains more specific configuration files. Let's also take a look
at one of them.
paul@RHELv4u2:~$ ls /etc/xinetd.d
amanda chargen-udp echo klogin rexec talk
amandaidx cups-lpd echo-udp krb5-telnet rlogin telnet
amidxtape daytime eklogin kshell rsh tftp
auth daytime-udp finger ktalk rsync time
chargen dbskkd-cdb gssftp ntalk swat time-udp
paul@RHELv4u2:~$ cat /etc/xinetd.d/swat
# default: off
# description: SWAT is the Samba Web Admin Tool. Use swat
# to configure your Samba server. To use SWAT,
# connect to port 901 with your favorite web browser.
service swat
{
port = 901
socket_type = stream
wait = no
only_from = 127.0.0.1
user = root
server = /usr/sbin/swat
log_on_failure += USERID
disable = yes
}
paul@RHELv4u2:~$
The services should be listed in the /etc/services file. Port determines the service port, and
must be the same as the port specified in /etc/services. The socket_type should be set to
stream for tcp services (and to dgram for udp). The log_on_failure += concats the userid
to the log message formatted in /etc/xinetd.conf. The last setting disable can be set to yes
or no. Setting this to no means the service is enabled!
Check the xinetd and xinetd.conf manual pages for many more configuration options.
7.4.4. inetd superdaemon
This superdaemon has only one configuration file /etc/inetd.conf. Every protocol or daemon
that it is listening for, gets one line in this file.
root@barry:~# grep ftp /etc/inetd.conf
tftp dgram udp wait nobody /usr/sbin/tcpd /usr/sbin/in.tftpd /boot/tftp
root@barry:~#

70
You can disable a service in inetd.conf above by putting a # at the start of that line. Here an
example of the disabled vmware web interface (listening on tcp port 902).
paul@laika:~$ grep vmware /etc/inetd.conf
#902 stream tcp nowait root /usr/sbin/vmware-authd vmware-authd
7.5. practice : inetd and xinetd
1. Verify on all systems whether they are using xinetd or inetd.
2. Look at the configuration files.
3. (If telnet is installable, then replace swat in these questions with telnet) Is swat installed ?
If not, then install swat and look at the changes in the (x)inetd configuration. Is swat enabled
or disabled ?
4. Disable swat, test it. Enable swat, test it.

71
7.6. network file system
7.6.1. protocol versions
The older nfs versions 2 and 3 are stateless (udp) by default, but they can use tcp. Clients
connect to the server using rpc (on Linux this is controlled by the portmap daemon. Look
at rpcinfo to verify that nfs and its related services are running.
root@RHELv4u2:~# /etc/init.d/portmap status
portmap (pid 1920) is running...
100024 1 udp 32768 status
100024 1 tcp 32769 status
root@RHELv4u2:~# service nfs start
Starting NFS services: [ OK ]
Starting NFS quotas: [ OK ]
Starting NFS daemon: [ OK ]
Starting NFS mountd: [ OK ]
The same rpcinfo command when nfs is started.
100024 1 udp 32768 status
100024 1 tcp 32769 status
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 4 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
100003 4 tcp 2049 nfs
100005 1 udp 1004 mountd
100005 1 tcp 1007 mountd
100005 2 udp 1004 mountd
100005 2 tcp 1007 mountd
100005 3 udp 1004 mountd
100005 3 tcp 1007 mountd
root@RHELv4u2:~#
nfs version 4 requires tcp (port 2049) and supports Kerberos user authentication as an
option. nfs authentication only takes place when mounting the share. nfs versions 2 and 3
authenticate only the host.

72
7.6.2. server configuration
nfs is configured in /etc/exports. Here is a sample /etc/exports to explain the syntax. You
need some way (NIS domain or LDAP) to synchronize userid's across computers when
using nfs a lot. The rootsquash option will change UID 0 to the UID of the nfsnobody user
account. The sync option will write writes to disk before completing the client request.
paul@laika:~$ cat /etc/exports
/mnt/data/iso *(ro)
# Only the computers barry and pasha can readwrite this one
/var/www pasha(rw) barry(rw)
/var/ftp pasha(rw) barry(rw,no_root_squash)
# everyone from the netsec.lan domain gets access
/var/backup *.netsec.lan(rw)
# ro for one network, rw for the other
/var/upload 192.168.1.0/24(ro) 192.168.5.0/24(rw)
You don't need to restart the nfs server to start exporting your newly created exports. You
can use the exportfs -va command to do this. It will write the exported directories to /var/
lib/nfs/etab, where they are immediately applied.
7.6.3. client configuration
We have seen the mount command and the /etc/fstab file before.
root@RHELv4u2:~# mount -t nfs barry:/mnt/data/iso /home/project55/
root@RHELv4u2:~# cat /etc/fstab | grep nfs
barry:/mnt/data/iso /home/iso nfs defaults 0 0
root@RHELv4u2:~#
Here is another simple example. Suppose the project55 people tell you they only need a
couple of CD-ROM images, and you already have them available on an nfs server. You
could issue the following command to mount this storage on their /home/project55 mount
point.
root@RHELv4u2:~# mount -t nfs 192.168.1.40:/mnt/data/iso /home/project55/
root@RHELv4u2:~# ls -lh /home/project55/
total 3.6G
-rw-r--r-- 1 root root 1.6G Oct 13 15:22 sled10-vmwarews5-vm.zip
root@RHELv4u2:~#

73
7.7. practice : network file system
1. Create two directories with some files. Use nfs to share one of them as read only, the other
must be writable. Have your neighbour connect to them to test.
2. Investigate the user owner of the files created by your neighbour.
3. Protect a share by ip-address or hostname, so only your neighbour can connect.

75
Table of Contents
8. apache web server ................................................................................................................................... 76
8.1. introduction to apache ................................................................................................................... 77
8.2. port virtual hosts on Debian ......................................................................................................... 84
8.3. named virtual hosts on Debian ..................................................................................................... 88
8.4. password protected website on Debian ........................................................................................ 90
8.5. port virtual hosts on CentOS ........................................................................................................ 91
8.6. named virtual hosts on CentOS .................................................................................................... 95
8.7. password protected website on CentOS ....................................................................................... 97
8.8. troubleshooting apache ................................................................................................................. 99
8.9. virtual hosts example .................................................................................................................. 100
8.10. aliases and redirects .................................................................................................................. 100
8.11. more on .htaccess ...................................................................................................................... 100
8.12. traffic ......................................................................................................................................... 100
8.13. self signed cert on Debian ........................................................................................................ 101
8.14. self signed cert on RHEL/CentOS ............................................................................................ 103
8.15. practice: apache ......................................................................................................................... 105
9. introduction to squid ............................................................................................................................. 106
9.1. about proxy servers ..................................................................................................................... 106
9.2. installing squid ............................................................................................................................ 107
9.3. port 3128 ..................................................................................................................................... 107
9.4. starting and stopping ................................................................................................................... 107
9.5. client proxy settings .................................................................................................................... 108
9.6. upside down images .................................................................................................................... 110
9.7. /var/log/squid ............................................................................................................................... 112
9.8. access control .............................................................................................................................. 112
9.9. testing squid ................................................................................................................................ 112
9.10. name resolution ......................................................................................................................... 112

76
Chapter 8. apache web server
In this chapter we learn how to setup a web server with the apache software.
According to NetCraft (http://news.netcraft.com/archives/web_server_survey.html) about
seventy percent of all web servers are running on Apache. The name is derived from a
patchy web server, because of all the patches people wrote for the NCSA httpd server.
Later chapters will expand this web server into a LAMP stack (Linux, Apache, Mysql, Perl/
PHP/Python).

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8.1. introduction to apache
8.1.1. installing on Debian
This screenshot shows that there is no apache server installed, nor does the /var/www
directory exist.
root@debian7:~# ls -l /var/www
ls: cannot access /var/www: No such file or directory
root@debian7:~# dpkg -l | grep apache
To install apache on Debian:
root@debian7:~# aptitude install apache2
The following NEW packages will be installed:
apache2 apache2-mpm-worker{a} apache2-utils{a} apache2.2-bin{a} apache2.2-com
mon{a} libapr1{a} libaprutil1{a} libaprutil1-dbd-sqlite3{a} libaprutil1-ldap{a}
ssl-cert{a}
0 packages upgraded, 10 newly installed, 0 to remove and 0 not upgraded.
Need to get 1,487 kB of archives. After unpacking 5,673 kB will be used.
Do you want to continue? [Y/n/?]
After installation, the same two commands as above will yield a different result:
root@debian7:~# ls -l /var/www
total 4
-rw-r--r-- 1 root root 177 Apr 29 11:55 index.html
root@debian7:~# dpkg -l | grep apache | tr -s ' '
ii apache2 2.2.22-13+deb7u1 amd64 Apache HTTP Server metapackage
ii apache2-mpm-worker 2.2.22-13+deb7u1 amd64 Apache HTTP Server - high speed th
readed model
ii apache2-utils 2.2.22-13+deb7u1 amd64 utility programs for webservers
ii apache2.2-bin 2.2.22-13+deb7u1 amd64 Apache HTTP Server common binary files
ii apache2.2-common 2.2.22-13+deb7u1 amd64 Apache HTTP Server common files

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8.1.2. installing on RHEL/CentOS
Note that Red Hat derived distributions use httpd as package and process name instead of
apache.
To verify whether apache is installed in CentOS/RHEL:
[root@centos65 ~]# rpm -q httpd
package httpd is not installed
[root@centos65 ~]# ls -l /var/www
ls: cannot access /var/www: No such file or directory
To install apache on CentOS:
[root@centos65 ~]# yum install httpd
After running the yum install httpd command, the Centos 6.5 server has apache installed
and the /var/www directory exists.
[root@centos65 ~]# rpm -q httpd
httpd-2.2.15-30.el6.centos.x86_64
[root@centos65 ~]# ls -l /var/www
total 16
drwxr-xr-x. 2 root root 4096 Apr 3 23:57 cgi-bin
drwxr-xr-x. 3 root root 4096 May 6 13:08 error
drwxr-xr-x. 2 root root 4096 Apr 3 23:57 html
drwxr-xr-x. 3 root root 4096 May 6 13:08 icons
[root@centos65 ~]#

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8.1.3. running apache on Debian
This is how you start apache2 on Debian.
root@debian7:~# service apache2 status
Apache2 is NOT running.
root@debian7:~# service apache2 start
Starting web server: apache2apache2: Could not reliably determine the server's
fully qualified domain name, using 127.0.1.1 for ServerName
.
To verify, run the service apache2 status command again or use ps.
root@debian7:~# service apache2 status
Apache2 is running (pid 3680).
root@debian7:~# ps -C apache2
PID TTY TIME CMD
3680 ? 00:00:00 apache2
3683 ? 00:00:00 apache2
3684 ? 00:00:00 apache2
3685 ? 00:00:00 apache2
root@debian7:~#
Or use wget and file to verify that your web server serves an html document.
root@debian7:~# wget 127.0.0.1
--2014-05-06 13:27:02-- http://127.0.0.1/
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 177 [text/html]
Saving to: `index.html'
100%[==================================================>] 177 --.-K/s in 0s
2014-05-06 13:27:02 (15.8 MB/s) - `index.html' saved [177/177]
root@debian7:~# file index.html
index.html: HTML document, ASCII text
root@debian7:~#
Or verify that apache is running by opening a web browser, and browse to the ip-address of
your server. An Apache test page should be shown.
You can do the following to quickly avoid the 'could not reliably determine the fqdn' message
when restarting apache.
root@debian7:~# echo ServerName Debian7 >> /etc/apache2/apache2.conf
root@debian7:~# service apache2 restart
Restarting web server: apache2 ... waiting .
root@debian7:~#

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8.1.4. running apache on CentOS
Starting the httpd on RHEL/CentOS is done with the service command.
[root@centos65 ~]# service httpd status
httpd is stopped
[root@centos65 ~]# service httpd start
Starting httpd: httpd: Could not reliably determine the server's fully qualifie
d domain name, using 127.0.0.1 for ServerName
[ OK ]
[root@centos65 ~]#
To verify that apache is running, use ps or issue the service httpd status command again.
[root@centos65 ~]# service httpd status
httpd (pid 2410) is running...
[root@centos65 ~]# ps -C httpd
PID TTY TIME CMD
2410 ? 00:00:00 httpd
2412 ? 00:00:00 httpd
2413 ? 00:00:00 httpd
2414 ? 00:00:00 httpd
2415 ? 00:00:00 httpd
2416 ? 00:00:00 httpd
2417 ? 00:00:00 httpd
2418 ? 00:00:00 httpd
2419 ? 00:00:00 httpd
[root@centos65 ~]#
To prevent the 'Could not reliably determine the fqdn' message, issue the following
command.
[root@centos65 ~]# echo ServerName Centos65 >> /etc/httpd/conf/httpd.conf
[root@centos65 ~]# service httpd restart
Stopping httpd: [ OK ]
Starting httpd: [ OK ]
[root@centos65 ~]#

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8.1.5. index file on CentOS
CentOS does not provide a standard index.html or index.php file. A simple wget gives an
error.
[root@centos65 ~]# wget 127.0.0.1
HTTP request sent, awaiting response... 403 Forbidden
2014-05-06 15:10:22 ERROR 403: Forbidden.
Instead when visiting the ip-address of your server in a web browser you get a noindex.html
page. You can verify this using wget.
[root@centos65 ~]# wget http://127.0.0.1/error/noindex.html
--2014-05-06 15:16:05-- http://127.0.0.1/error/noindex.html
Length: 5039 (4.9K) [text/html]
Saving to: “noindex.html”
100%[=============================================>] 5,039 --.-K/s in 0s
2014-05-06 15:16:05 (289 MB/s) - “noindex.html” saved [5039/5039]
[root@centos65 ~]# file noindex.html
noindex.html: HTML document text
[root@centos65 ~]#
Any custom index.html file in /var/www/html will immediately serve as an index for this
web server.
[root@centos65 ~]# echo 'Welcome to my website' > /var/www/html/index.html
[root@centos65 ~]# wget http://127.0.0.1
Saving to: “index.html”
100%[=============================================>] 22 --.-K/s in 0s
2014-05-06 15:19:16 (1.95 MB/s) - “index.html” saved [22/22]
[root@centos65 ~]# cat index.html
Welcome to my website

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8.1.6. default website
Changing the default website of a freshly installed apache web server is easy. All you need
to do is create (or change) an index.html file in the DocumentRoot directory.
To locate the DocumentRoot directory on Debian:
root@debian7:~# grep DocumentRoot /etc/apache2/sites-available/default
DocumentRoot /var/www
This means that /var/www/index.html is the default web site.
root@debian7:~# cat /var/www/index.html
<html><body><h1>It works!</h1>
<p>This is the default web page for this server.</p>
<p>The web server software is running but no content has been added, yet.</p>
</body></html>
root@debian7:~#
This screenshot shows how to locate the DocumentRoot directory on RHEL/CentOS.
[root@centos65 ~]# grep ^DocumentRoot /etc/httpd/conf/httpd.conf
DocumentRoot "/var/www/html"
RHEL/CentOS have no default web page (only the noindex.html error page mentioned
before). But an index.html file created in /var/www/html/ will automatically be used as
default page.
[root@centos65 ~]# echo '<html><head><title>Default website</title></head><body
><p>A new web page</p></body></html>' > /var/www/html/index.html
[root@centos65 ~]# cat /var/www/html/index.html
<html><head><title>Default website</title></head><body><p>A new web page</p></b
ody></html>
[root@centos65 ~]#

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8.1.7. apache configuration
There are many similarities, but also a couple of differences when configuring apache on
Debian or on CentOS. Both Linux families will get their own chapters with examples.
All configuration on RHEL/CentOS is done in /etc/httpd.
[root@centos65 ~]# ls -l /etc/httpd/
total 8
drwxr-xr-x. 2 root root 4096 May 6 13:08 conf
drwxr-xr-x. 2 root root 4096 May 6 13:08 conf.d
lrwxrwxrwx. 1 root root 19 May 6 13:08 logs -> ../../var/log/httpd
lrwxrwxrwx. 1 root root 29 May 6 13:08 modules -> ../../usr/lib64/httpd/modu
les
lrwxrwxrwx. 1 root root 19 May 6 13:08 run -> ../../var/run/httpd
[root@centos65 ~]#
Debian (and ubuntu/mint/...) use /etc/apache2.
root@debian7:~# ls -l /etc/apache2/
total 72
-rw-r--r-- 1 root root 9659 May 6 14:23 apache2.conf
drwxr-xr-x 2 root root 4096 May 6 13:19 conf.d
-rw-r--r-- 1 root root 1465 Jan 31 18:35 envvars
-rw-r--r-- 1 root root 31063 Jul 20 2013 magic
drwxr-xr-x 2 root root 4096 May 6 13:19 mods-available
drwxr-xr-x 2 root root 4096 May 6 13:19 mods-enabled
-rw-r--r-- 1 root root 750 Jan 26 12:13 ports.conf
drwxr-xr-x 2 root root 4096 May 6 13:19 sites-available
drwxr-xr-x 2 root root 4096 May 6 13:19 sites-enabled
root@debian7:~#

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8.2. port virtual hosts on Debian
8.2.1. default virtual host
Debian has a virtualhost configuration file for its default website in /etc/apache2/sites-
available/default.
root@debian7:~# head -2 /etc/apache2/sites-available/default
<VirtualHost *:80>
ServerAdmin webmaster@localhost
8.2.2. three extra virtual hosts
In this scenario we create three additional websites for three customers that share a clubhouse
and want to jointly hire you. They are a model train club named Choo Choo, a chess club
named Chess Club 42 and a hackerspace named hunter2.
One way to put three websites on one web server, is to put each website on a different port.
This screenshot shows three newly created virtual hosts, one for each customer.
root@debian7:~# vi /etc/apache2/sites-available/choochoo
root@debian7:~# cat /etc/apache2/sites-available/choochoo
<VirtualHost *:7000>
DocumentRoot /var/www/choochoo
</VirtualHost>
root@debian7:~# vi /etc/apache2/sites-available/chessclub42
root@debian7:~# cat /etc/apache2/sites-available/chessclub42
DocumentRoot /var/www/chessclub42
</VirtualHost>
root@debian7:~# vi /etc/apache2/sites-available/hunter2
root@debian7:~# cat /etc/apache2/sites-available/hunter2
DocumentRoot /var/www/hunter2
</VirtualHost>
Notice the different port numbers 7000, 8000 and 9000. Notice also that we specified a
unique DocumentRoot for each website.
Are you using Ubuntu or Mint, then these configfiles need to end in .conf.

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8.2.3. three extra ports
We need to enable these three ports on apache in the ports.conf file. Open this file with vi
and add three lines to listen on three extra ports.
root@debian7:~# vi /etc/apache2/ports.conf
Verify with grep that the Listen directives are added correctly.
root@debian7:~# grep ^Listen /etc/apache2/ports.conf
Listen 80
Listen 7000
Listen 8000
Listen 9000
8.2.4. three extra websites
Next we need to create three DocumentRoot directories.
root@debian7:~# mkdir /var/www/choochoo
root@debian7:~# mkdir /var/www/chessclub42
root@debian7:~# mkdir /var/www/hunter2
And we have to put some really simple website in those directories.
root@debian7:~# echo 'Choo Choo model train Choo Choo' > /var/www/choochoo/inde
x.html
root@debian7:~# echo 'Welcome to chess club 42' > /var/www/chessclub42/index.ht
ml
root@debian7:~# echo 'HaCkInG iS fUn At HuNtEr2' > /var/www/hunter2/index.html

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8.2.5. enabling extra websites
The last step is to enable the websites with the a2ensite command. This command will create
links in sites-enabled.
The links are not there yet...
root@debian7:~# cd /etc/apache2/
root@debian7:/etc/apache2# ls sites-available/
chessclub42 choochoo default default-ssl hunter2
root@debian7:/etc/apache2# ls sites-enabled/
000-default
So we run the a2ensite command for all websites.
root@debian7:/etc/apache2# a2ensite choochoo
Enabling site choochoo.
To activate the new configuration, you need to run:
service apache2 reload
root@debian7:/etc/apache2# a2ensite chessclub42
Enabling site chessclub42.
root@debian7:/etc/apache2# a2ensite hunter2
Enabling site hunter2.
The links are created, so we can tell apache.
root@debian7:/etc/apache2# ls sites-enabled/
000-default chessclub42 choochoo hunter2
root@debian7:/etc/apache2# service apache2 reload
Reloading web server config: apache2.
root@debian7:/etc/apache2#

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87
8.2.6. testing the three websites
Testing the model train club named Choo Choo on port 7000.
root@debian7:/etc/apache2# wget 127.0.0.1:7000
--2014-05-06 21:16:03-- http://127.0.0.1:7000/
100%[============================================>] 32 --.-K/s in 0s
root@debian7:/etc/apache2# cat index.html
Choo Choo model train Choo Choo
Testing the chess club named Chess Club 42 on port 8000.
Saving to: `index.html.1'
100%[===========================================>] 25 --.-K/s in 0s
2014-05-06 21:16:20 (2.16 MB/s) - `index.html.1' saved [25/25]
root@debian7:/etc/apache2# cat index.html.1
Welcome to chess club 42
Testing the hacker club named hunter2 on port 9000.
100%[===========================================>] 26 --.-K/s in 0s
HaCkInG iS fUn At HuNtEr2
Cleaning up the temporary files.
root@debian7:/etc/apache2# rm index.html index.html.1 index.html.2
Try testing from another computer using the ip-address of your server.

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8.3. named virtual hosts on Debian
8.3.1. named virtual hosts
The chess club and the model train club find the port numbers too hard to remember. They
would prefere to have their website accessible by name.
We continue work on the same server that has three websites on three ports. We need to
make sure those websites are accesible using the names choochoo.local, chessclub42.local
and hunter2.local.
We start by creating three new virtualhosts.
root@debian7:/etc/apache2/sites-available# vi choochoo.local
root@debian7:/etc/apache2/sites-available# vi chessclub42.local
root@debian7:/etc/apache2/sites-available# vi hunter2.local
root@debian7:/etc/apache2/sites-available# cat choochoo.local
<VirtualHost *:80>
ServerName choochoo.local
DocumentRoot /var/www/choochoo
</VirtualHost>
root@debian7:/etc/apache2/sites-available# cat chessclub42.local
<VirtualHost *:80>
ServerName chessclub42.local
</VirtualHost>
root@debian7:/etc/apache2/sites-available# cat hunter2.local
<VirtualHost *:80>
ServerName hunter2.local
DocumentRoot /var/www/hunter2
</VirtualHost>
root@debian7:/etc/apache2/sites-available#
Notice that they all listen on port 80 and have an extra ServerName directive.
8.3.2. name resolution
We need some way to resolve names. This can be done with DNS, which is discussed in
another chapter. For this demo it is also possible to quickly add the three names to the /etc/
hosts file.
root@debian7:/etc/apache2/sites-available# grep ^192 /etc/hosts
192.168.42.50 choochoo.local
192.168.42.50 chessclub42.local
192.168.42.50 hunter2.local
Note that you may have another ip address...

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8.3.3. enabling virtual hosts
Next we enable them with a2ensite.
root@debian7:/etc/apache2/sites-available# a2ensite choochoo.local
Enabling site choochoo.local.
root@debian7:/etc/apache2/sites-available# a2ensite chessclub42.local
Enabling site chessclub42.local.
root@debian7:/etc/apache2/sites-available# a2ensite hunter2.local
Enabling site hunter2.local.
8.3.4. reload and verify
After a service apache2 reload the websites should be available by name.
root@debian7:/etc/apache2/sites-available# service apache2 reload
Reloading web server config: apache2.
root@debian7:/etc/apache2/sites-available# wget chessclub42.local
--2014-05-06 21:37:13-- http://chessclub42.local/
Resolving chessclub42.local (chessclub42.local)... 192.168.42.50
Connecting to chessclub42.local (chessclub42.local)|192.168.42.50|:80... conne
cted.
100%[=============================================>] 25 --.-K/s in 0s
root@debian7:/etc/apache2/sites-available# cat index.html

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8.4. password protected website on Debian
You can secure files and directories in your website with a .htaccess file that refers to a
.htpasswd file. The htpasswd command can create a .htpasswd file that contains a userid
and an (encrypted) password.
This screenshot creates a user and password for the hacker named cliff and uses the -c flag
to create the .htpasswd file.
root@debian7:~# htpasswd -c /var/www/.htpasswd cliff
New password:
Re-type new password:
Adding password for user cliff
root@debian7:~# cat /var/www/.htpasswd
cliff:$apr1$vujll0KL$./SZ4w9q0swhX93pQ0PVp.
Hacker rob also wants access, this screenshot shows how to add a second user and password
to .htpasswd.
root@debian7:~# htpasswd /var/www/.htpasswd rob
New password:
Adding password for user rob
root@debian7:~# cat /var/www/.htpasswd
cliff:$apr1$vujll0KL$./SZ4w9q0swhX93pQ0PVp.
rob:$apr1$HNln1FFt$nRlpF0H.IW11/1DRq4lQo0
Both Cliff and Rob chose the same password (hunter2), but that is not visible in the
.htpasswd file because of the different salts.
Next we need to create a .htaccess file in the DocumentRoot of the website we want to
protect. This screenshot shows an example.
root@debian7:~# cd /var/www/hunter2/
root@debian7:/var/www/hunter2# cat .htaccess
AuthUserFile /var/www/.htpasswd
AuthName "Members only!"
AuthType Basic
require valid-user
Note that we are protecting the website on port 9000 that we created earlier.
And because we put the website for the Hackerspace named hunter2 in a subdirectory of the
default website, we will need to adjust the AllowOvveride parameter in /etc/apache2/sites-
available/default as this screenshot shows (with line numbers on Debian7, your may vary).
9 <Directory /var/www/>
10 Options Indexes FollowSymLinks MultiViews
11 AllowOverride Authconfig
12 Order allow,deny
13 allow from all
14 </Directory
Now restart the apache2 server and test that it works!

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8.5. port virtual hosts on CentOS
8.5.1. default virtual host
Unlike Debian, CentOS has no virtualHost configuration file for its default website. Instead
the default configuration will throw a standard error page when no index file can be found
in the default location (/var/www/html).
8.5.2. three extra virtual hosts
In this scenario we create three additional websites for three customers that share a clubhouse
and want to jointly hire you. They are a model train club named Choo Choo, a chess club
named Chess Club 42 and a hackerspace named hunter2.
One way to put three websites on one web server, is to put each website on a different port.
This screenshot shows three newly created virtual hosts, one for each customer.
[root@CentOS65 ~]# vi /etc/httpd/conf.d/choochoo.conf
[root@CentOS65 ~]# cat /etc/httpd/conf.d/choochoo.conf
DocumentRoot /var/www/html/choochoo
</VirtualHost>
[root@CentOS65 ~]# vi /etc/httpd/conf.d/chessclub42.conf
[root@CentOS65 ~]# cat /etc/httpd/conf.d/chessclub42.conf
DocumentRoot /var/www/html/chessclub42
</VirtualHost>
[root@CentOS65 ~]# vi /etc/httpd/conf.d/hunter2.conf
[root@CentOS65 ~]# cat /etc/httpd/conf.d/hunter2.conf
DocumentRoot /var/www/html/hunter2
</VirtualHost>
Notice the different port numbers 7000, 8000 and 9000. Notice also that we specified a
unique DocumentRoot for each website.
8.5.3. three extra ports
We need to enable these three ports on apache in the httpd.conf file.
[root@CentOS65 ~]# vi /etc/httpd/conf/httpd.conf
root@debian7:~# grep ^Listen /etc/httpd/conf/httpd.conf
Listen 80
Listen 7000
Listen 8000
Listen 9000

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8.5.4. SELinux guards our ports
If we try to restart our server, we will notice the following error:
[root@CentOS65 ~]# service httpd restart
Starting httpd:
(13)Permission denied: make_sock: could not bind to address 0.0.0.0:7000
no listening sockets available, shutting down
[FAILED]
This is due to SELinux reserving ports 7000 and 8000 for other uses. We need to tell SELinux
we want to use these ports for http traffic
[root@CentOS65 ~]# semanage port -m -t http_port_t -p tcp 7000
[root@CentOS65 ~]# semanage port -m -t http_port_t -p tcp 8000
[root@CentOS65 ~]# service httpd restart
8.5.5. three extra websites
Next we need to create three DocumentRoot directories.
[root@CentOS65 ~]# mkdir /var/www/html/choochoo
[root@CentOS65 ~]# mkdir /var/www/html/chessclub42
[root@CentOS65 ~]# mkdir /var/www/html/hunter2
And we have to put some really simple website in those directories.
[root@CentOS65 ~]# echo 'Choo Choo model train Choo Choo' > /var/www/html/chooc
hoo/index.html
[root@CentOS65 ~]# echo 'Welcome to chess club 42' > /var/www/html/chessclub42/
index.html
[root@CentOS65 ~]# echo 'HaCkInG iS fUn At HuNtEr2' > /var/www/html/hunter2/ind
ex.html
8.5.6. enabling extra websites
The only way to enable or disable configurations in RHEL/CentOS is by renaming or
moving the configuration files. Any file in /etc/httpd/conf.d ending on .conf will be loaded
by Apache. To disable a site we can either rename the file or move it to another directory.
The files are created, so we can tell apache.
[root@CentOS65 ~]# ls /etc/httpd/conf.d/
chessclub42.conf choochoo.conf hunter2.conf README welcome.conf
[root@CentOS65 ~]# service httpd reload
Reloading httpd:

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8.5.7. testing the three websites
Testing the model train club named Choo Choo on port 7000.
[root@CentOS65 ~]# wget 127.0.0.1:7000
100%[===========================================>] 32 --.-K/s in 0s
[root@CentOS65 ~]# cat index.html
Choo Choo model train Choo Choo
Testing the chess club named Chess Club 42 on port 8000.
[root@CentOS65 ~]# wget 127.0.0.1:8000
100%[===========================================>] 25 --.-K/s in 0s
Testing the hacker club named hunter2 on port 9000.
[root@CentOS65 ~]# wget 127.0.0.1:9000
100%[===========================================>] 26 --.-K/s in 0s
HaCkInG iS fUn At HuNtEr2
Cleaning up the temporary files.
[root@CentOS65 ~]# rm index.html index.html.1 index.html.2

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8.5.8. firewall rules
If we attempt to access the site from another machine however, we will not be able to view the
website yet. The firewall is blocking incoming connections. We need to open these incoming
ports first
[root@CentOS65 ~]# iptables -I INPUT -p tcp --dport 80 -j ACCEPT
And if we want these rules to remain active after a reboot, we need to save them
[root@CentOS65 ~]# service iptables save
iptables: Saving firewall rules to /etc/sysconfig/iptables:[ OK ]

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8.6. named virtual hosts on CentOS
8.6.1. named virtual hosts
The chess club and the model train club find the port numbers too hard to remember. They
would prefere to have their website accessible by name.
We continue work on the same server that has three websites on three ports. We need to
make sure those websites are accesible using the names choochoo.local, chessclub42.local
and hunter2.local.
First, we need to enable named virtual hosts in the configuration
[root@CentOS65 ~]# grep ^NameVirtualHost /etc/httpd/conf/httpd.conf
NameVirtualHost *:80
[root@CentOS65 ~]#
Next we need to create three new virtualhosts.
[root@CentOS65 ~]# vi /etc/httpd/conf.d/choochoo.local.conf
[root@CentOS65 ~]# vi /etc/httpd/conf.d/chessclub42.local.conf
[root@CentOS65 ~]# vi /etc/httpd/conf.d/hunter2.local.conf
[root@CentOS65 ~]# cat /etc/httpd/conf.d/choochoo.local.conf
<VirtualHost *:80>
ServerName choochoo.local
DocumentRoot /var/www/html/choochoo
</VirtualHost>
[root@CentOS65 ~]# cat /etc/httpd/conf.d/chessclub42.local.conf
<VirtualHost *:80>
ServerName chessclub42.local
DocumentRoot /var/www/html/chessclub42
</VirtualHost>
[root@CentOS65 ~]# cat /etc/httpd/conf.d/hunter2.local.conf
<VirtualHost *:80>
ServerName hunter2.local
DocumentRoot /var/www/html/hunter2
</VirtualHost>
[root@CentOS65 ~]#
Notice that they all listen on port 80 and have an extra ServerName directive.
8.6.2. name resolution
We need some way to resolve names. This can be done with DNS, which is discussed in
another chapter. For this demo it is also possible to quickly add the three names to the /etc/
hosts file.
[root@CentOS65 ~]# grep ^192 /etc/hosts
192.168.1.225 choochoo.local
192.168.1.225 chessclub42.local
192.168.1.225 hunter2.local
Note that you may have another ip address...

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8.6.3. reload and verify
After a service httpd reload the websites should be available by name.
[root@CentOS65 ~]# service httpd reload
Reloading httpd:
[root@CentOS65 ~]# wget chessclub42.local
--2014-05-25 16:59:14-- http://chessclub42.local/
Resolving chessclub42.local... 192.168.1.225
Connecting to chessclub42.local|192.168.1.225|:80... connected.
Saving to: âindex.htmlâ
100%[=============================================>] 25 --.-K/s in 0s
2014-05-25 16:59:15 (1014 KB/s) - `index.html' saved [25/25]
[root@CentOS65 ~]# cat index.html

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8.7. password protected website on CentOS
You can secure files and directories in your website with a .htaccess file that refers to a
.htpasswd file. The htpasswd command can create a .htpasswd file that contains a userid
and an (encrypted) password.
This screenshot creates a user and password for the hacker named cliff and uses the -c flag
to create the .htpasswd file.
[root@CentOS65 ~]# htpasswd -c /var/www/.htpasswd cliff
New password:
Adding password for user cliff
[root@CentOS65 ~]# cat /var/www/.htpasswd
cliff:QNwTrymMLBctU
Hacker rob also wants access, this screenshot shows how to add a second user and password
to .htpasswd.
[root@CentOS65 ~]# htpasswd /var/www/.htpasswd rob
New password:
Adding password for user rob
[root@CentOS65 ~]# cat /var/www/.htpasswd
cliff:QNwTrymMLBctU
rob:EC2vOCcrMXDoM
[root@CentOS65 ~]#
Both Cliff and Rob chose the same password (hunter2), but that is not visible in the
.htpasswd file because of the different salts.
Next we need to create a .htaccess file in the DocumentRoot of the website we want to
protect. This screenshot shows an example.
[root@CentOS65 ~]# cat /var/www/html/hunter2/.htaccess
AuthUserFile /var/www/.htpasswd
AuthName "Members only!"
AuthType Basic
require valid-user
Note that we are protecting the website on port 9000 that we created earlier.
And because we put the website for the Hackerspace named hunter2 in a subdirectory of the
default website, we will need to adjust the AllowOvveride parameter in /etc/httpd/conf/
httpd.conf under the <Directory "/var/www/html"> directive as this screenshot shows.

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<Directory "/var/www/html">
#
# Possible values for the Options directive are "None", "All",
# or any combination of:
# Indexes Includes FollowSymLinks SymLinksifOwnerMatch ExecCGI MultiViews
#
# Note that "MultiViews" must be named *explicitly* --- "Options All"
# doesn't give it to you.
#
# The Options directive is both complicated and important. Please see
# http://httpd.apache.org/docs/2.2/mod/core.html#options
# for more information.
#
Options Indexes FollowSymLinks
#
# AllowOverride controls what directives may be placed in .htaccess files.
# It can be "All", "None", or any combination of the keywords:
# Options FileInfo AuthConfig Limit
#
AllowOverride Authconfig
#
# Controls who can get stuff from this server.
#
Order allow,deny
Allow from all
</Directory>
Now restart the apache2 server and test that it works!

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8.8. troubleshooting apache
When apache restarts, it will verify the syntax of files in the configuration folder /etc/
apache2 on debian or /etc/httpd on CentOS and it will tell you the name of the faulty file,
the line number and an explanation of the error.
root@debian7:~# service apache2 restart
apache2: Syntax error on line 268 of /etc/apache2/apache2.conf: Syntax error o
n line 1 of /etc/apache2/sites-enabled/chessclub42: /etc/apache2/sites-enabled
/chessclub42:4: <VirtualHost> was not closed.n/etc/apache2/sites-enabled/ches
sclub42:1: <VirtualHost> was not closed.
Action 'configtest' failed.
The Apache error log may have more information.
failed!
Below you see the problem... a missing / before on line 4.
root@debian7:~# cat /etc/apache2/sites-available/chessclub42
<VirtualHost>
Let us force another error by renaming the directory of one of our websites:
root@debian7:~# mv /var/www/choochoo/ /var/www/chooshoo
root@debian7:~# !ser
service apache2 restart
Restarting web server: apache2Warning: DocumentRoot [/var/www/choochoo] does n
ot exist
Warning: DocumentRoot [/var/www/choochoo] does not exist
... waiting Warning: DocumentRoot [/var/www/choochoo] does not exist
Warning: DocumentRoot [/var/www/choochoo] does not exist
.
As you can see, apache will tell you exactly what is wrong.
You can also troubleshoot by connecting to the website via a browser and then checking the
apache log files in /var/log/apache.

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8.9. virtual hosts example
Below is a sample virtual host configuration. This virtual hosts overrules the default Apache
ErrorDocument directive.
<VirtualHost 83.217.76.245:80>
ServerName cobbaut.be
ServerAlias www.cobbaut.be
DocumentRoot /home/paul/public_html
ErrorLog /home/paul/logs/error_log
CustomLog /home/paul/logs/access_log common
ScriptAlias /cgi-bin/ /home/paul/cgi-bin/
<Directory /home/paul/public_html>
Options Indexes IncludesNOEXEC FollowSymLinks
allow from all
</Directory>
ErrorDocument 404 http://www.cobbaut.be/cobbaut.php
</VirtualHost>
8.10. aliases and redirects
Apache supports aliases for directories, like this example shows.
Alias /paul/ "/home/paul/public_html/"
Similarly, content can be redirected to another website or web server.
Redirect permanent /foo http://www.foo.com/bar
8.11. more on .htaccess
You can do much more with .htaccess. One example is to use .htaccess to prevent people
from certain domains to access your website. Like in this case, where a number of referer
spammers are blocked from the website.
paul@lounge:~/cobbaut.be$ cat .htaccess
# Options +FollowSymlinks
RewriteEngine On
RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-adipex.fw.nu.*$ [OR]
RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-levitra.asso.ws.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-tramadol.fw.nu.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-viagra.lookin.at.*$ [NC,OR]
...
RewriteCond %{HTTP_REFERER} ^http://(www.)?www.healthinsurancehelp.net.*$ [NC]
RewriteRule .* - [F,L]
paul@lounge:~/cobbaut.be$
8.12. traffic
Apache keeps a log of all visitors. The webalizer is often used to parse this log into nice
html statistics.

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8.13. self signed cert on Debian
Below is a very quick guide on setting up Apache2 on Debian 7 with a self-signed certificate.
Chances are these packages are already installed.
root@debian7:~# aptitude install apache2 openssl
No packages will be installed, upgraded, or removed.
Need to get 0 B of archives. After unpacking 0 B will be used.
Create a directory to store the certs, and use openssl to create a self signed cert that is valid
for 999 days.
root@debian7:~# mkdir /etc/ssl/localcerts
root@debian7:~# openssl req -new -x509 -days 999 -nodes -out /etc/ssl/local
certs/apache.pem -keyout /etc/ssl/localcerts/apache.key
Generating a 2048 bit RSA private key
...
...
writing new private key to '/etc/ssl/localcerts/apache.key'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:BE
State or Province Name (full name) [Some-State]:Antwerp
Locality Name (eg, city) []:Antwerp
Organization Name (eg, company) [Internet Widgits Pty Ltd]:linux-training.be
Organizational Unit Name (eg, section) []:
Common Name (e.g. server FQDN or YOUR name) []:Paul
Email Address []:
A little security never hurt anyone.
root@debian7:~# ls -l /etc/ssl/localcerts/
total 8
-rw-r--r-- 1 root root 1704 Sep 16 18:24 apache.key
-rw-r--r-- 1 root root 1302 Sep 16 18:24 apache.pem
root@debian7:~# chmod 600 /etc/ssl/localcerts/*
root@debian7:~# ls -l /etc/ssl/localcerts/
total 8
-rw------- 1 root root 1704 Sep 16 18:24 apache.key
-rw------- 1 root root 1302 Sep 16 18:24 apache.pem
Enable the apache ssl mod.
root@debian7:~# a2enmod ssl
Enabling module ssl.
See /usr/share/doc/apache2.2-common/README.Debian.gz on how to configure SSL
and create self-signed certificates.
service apache2 restart
Create the website configuration.
root@debian7:~# vi /etc/apache2/sites-available/choochoos

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root@debian7:~# cat /etc/apache2/sites-available/choochoos
DocumentRoot /var/www/choochoos
SSLEngine On
SSLCertificateFile /etc/ssl/localcerts/apache.pem
SSLCertificateKeyFile /etc/ssl/localcerts/apache.key
</VirtualHost>
root@debian7:~#
And create the website itself.
root@debian7:/var/www/choochoos# vi index.html
root@debian7:/var/www/choochoos# cat index.html
Choo Choo HTTPS secured model train Choo Choo
Enable the website and restart (or reload) apache2.
root@debian7:/var/www/choochoos# a2ensite choochoos
Enabling site choochoos.
root@debian7:/var/www/choochoos# service apache2 restart
Restarting web server: apache2 ... waiting .
Chances are your browser will warn you about the self signed certificate.

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8.14. self signed cert on RHEL/CentOS
Below is a quick way to create a self signed cert for https on RHEL/CentOS. You may need
these packages:
[root@paulserver ~]# yum install httpd openssl mod_ssl
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: ftp.belnet.be
* extras: ftp.belnet.be
* updates: mirrors.vooservers.com
base | 3.7 kB 00:00
Setting up Install Process
Package httpd-2.2.15-31.el6.centos.x86_64 already installed and latest version
Package openssl-1.0.1e-16.el6_5.15.x86_64 already installed and latest version
Package 1:mod_ssl-2.2.15-31.el6.centos.x86_64 already ins... and latest version
Nothing to do
We use openssl to create the certificate.
[root@paulserver ~]# mkdir certs
[root@paulserver ~]# cd certs
[root@paulserver certs]# openssl genrsa -out ca.key 2048
Generating RSA private key, 2048 bit long modulus
.........+++
.........................................................+++
e is 65537 (0x10001)
[root@paulserver certs]# openssl req -new -key ca.key -out ca.csr
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [XX]:BE
State or Province Name (full name) []:antwerp
Locality Name (eg, city) [Default City]:antwerp
Organization Name (eg, company) [Default Company Ltd]:antwerp
Organizational Unit Name (eg, section) []:
Common Name (eg, your name or your server's hostname) []:paulserver
Email Address []:
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:
[root@paulserver certs]# openssl x509 -req -days 365 -in ca.csr -signkey ca.ke
y -out ca.crt
Signature ok
subject=/C=BE/ST=antwerp/L=antwerp/O=antwerp/CN=paulserver
Getting Private key
We copy the keys to the right location (You may be missing SELinux info here).
[root@paulserver certs]# cp ca.crt /etc/pki/tls/certs/
[root@paulserver certs]# cp ca.key ca.csr /etc/pki/tls/private/
We add the location of our keys to this file, and also add the NameVirtualHost *:443
directive.
[root@paulserver certs]# vi /etc/httpd/conf.d/ssl.conf

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[root@paulserver certs]# grep ^SSLCerti /etc/httpd/conf.d/ssl.conf
SSLCertificateFile /etc/pki/tls/certs/ca.crt
SSLCertificateKeyFile /etc/pki/tls/private/ca.key
Create a website configuration.
[root@paulserver certs]# vi /etc/httpd/conf.d/choochoos.conf
[root@paulserver certs]# cat /etc/httpd/conf.d/choochoos.conf
<VirtualHost *:443>
SSLEngine on
SSLCertificateFile /etc/pki/tls/certs/ca.crt
SSLCertificateKeyFile /etc/pki/tls/private/ca.key
DocumentRoot /var/www/choochoos
ServerName paulserver
</VirtualHost>
[root@paulserver certs]#
Create a simple website and restart apache.
[root@paulserver certs]# mkdir /var/www/choochoos
[root@paulserver certs]# echo HTTPS model train choochoos > /var/www/choochoos/
index.html
[root@paulserver httpd]# service httpd restart
And your browser will probably warn you that this certificate is self signed.

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8.15. practice: apache
1. Verify that Apache is installed and running.
2. Browse to the Apache HTML manual.
3. Create three virtual hosts that listen on ports 8472, 31337 and 1201. Test that it all works.
4. Create three named virtual hosts startrek.local, starwars.local and stargate.local. Test that
it all works.
5. Create a virtual hosts that listens on another ip-address.
6. Protect one of your websites with a user/password combo.

106
Chapter 9. introduction to squid
9.1. about proxy servers
9.1.1. usage
A proxy server is a server that caches the internet. Clients connect to the proxy server with
a request for an internet server. The proxy server will connect to the internet server on behalf
of the client. The proxy server will also cache the pages retrieved from the internet server.
A proxy server may provide pages from his cache to a client, instead of connecting to the
internet server to retrieve the (same) pages.
A proxy server has two main advantages. It improves web surfing speed when returning
cached data to clients, and it reduces the required bandwidth (cost) to the internet.
Smaller organizations sometimes put the proxy server on the same physical computer that
serves as a NAT to the internet. In larger organizations, the proxy server is one of many
servers in the DMZ.
When web traffic passes via a proxy server, it is common practice to configure the proxy
with extra settings for access control. Access control in a proxy server can mean user account
access, but also website(url), ip-address or dns restrictions.
9.1.2. open proxy servers
You can find lists of open proxy servers on the internet that enable you to surf anonymously.
This works when the proxy server connects on your behalf to a website, without logging
your ip-address. But be careful, these (listed) open proxy servers could be created in order
to eavesdrop upon their users.
9.1.3. squid
This module is an introduction to the squid proxy server (http://www.squid-cache.org). We
will first configure squid as a normal proxy server.

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107
9.2. installing squid
This screenshot shows how to install squid on Debian with aptitude. Use yum if you are
on Red Hat/CentOS.
root@debian7:~# aptitude install squid
squid squid-common{a} squid-langpack{a}
Need to get 1,513 kB of archives. After unpacking 4,540 kB will be used.
...output truncated...
Setting up squid-langpack (20120616-1) ...
Setting up squid-common (2.7.STABLE9-4.1) ...
Setting up squid (2.7.STABLE9-4.1) ...
Creating squid spool directory structure
2014/08/01 15:19:31| Creating Swap Directories
Restarting Squid HTTP proxy: squid.
squid's main configuration file is /etc/squid/squid.conf. The file explains every parameter
in great detail.
root@debian7:~# wc -l /etc/squid/squid.conf
4948 /etc/squid/squid.conf
9.3. port 3128
By default the squid proxy server will lsiten to port 3128.
root@debian7:~# grep ^http_port /etc/squid/squid.conf
http_port 3128
root@debian7:~#
9.4. starting and stopping
You can manage squid with the standard service command as shown in this screenshot.
root@debian7:~# service squid start
Starting Squid HTTP proxy: squid.
root@debian7:~# service squid restart
Restarting Squid HTTP proxy: squid.
root@debian7:~# service squid status
squid is running.
root@debian7:~# service squid stop
Stopping Squid HTTP proxy: squid.
root@debian7:~#

108
9.5. client proxy settings
To enable a proxy server in Firefox or Iceweasel go to Edit Preferences and configure as
shown in this screenshot (replace 192.168.1.60 with the ip address of your proxy server).
Test that your internet works with the proxy enabled. Also test that after a service squid
stop command on your proxy server that you get a message similar to this schreenshot.

109
To enable a proxy server with Google Chrome (or Debian Chromium) start the program
from the command line like this:
paul@debian7:~$ chromium --proxy-server='192.168.1.60:3128'
Disabling the proxy with service squid stop should result in an error message similar to
this screenshot.

110
9.6. upside down images
A proxy server sits inbetween your browser and the internet. So besides caching of internet
data (the original function of a proxy server) and besides firewall like restrictions based on
www content, a proxy server is in the perfect position to alter the webpages that you visit.
You could for instance change the advertising on a webpage (or remove certain advertisers),
or like we do in this example; change all images so they are upside down.
The server needs command line tools to manipulate images and a perl script that uses these
tools (and wget to download the images locally and serve them with apache2). In this
example we use imagemagick (which provides tools like convert and mogrify).
root@debian7:~# aptitude install imagemagick wget perl apache2
...output truncated...
root@debian7:~# dpkg -S $(readlink -f $(which mogrify))
imagemagick: /usr/bin/mogrify.im6
root@debian7:~#
The perl script that is shown in the screenshot below can be found on several websites, yet
I have not found the original author. It is however a very simple script that uses wget and
mogrify to download images (.jpg .gif and .png), flip them and store them in /var/www/
images.
root@debian7:~# cat /usr/local/bin/flip.pl
#!/usr/bin/perl
$|=1;
$count = 0;
$pid = $$;
while (<>) {
chomp $_;
if ($_ =~ /(.*.jpg)/i) {
$url = $1;
system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.jpg", "$url");
system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.jpg");
print "http://127.0.0.1/images/$pid-$count.jpgn";
}
elsif ($_ =~ /(.*.gif)/i) {
$url = $1;
system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.gif", "$url");
system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.gif");
print "http://127.0.0.1/images/$pid-$count.gifn";
}
elsif ($_ =~ /(.*.png)/i) {
$url = $1;
system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.png", "$url");
system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.png");
print "http://127.0.0.1/images/$pid-$count.pngn";
}
else {
print "$_n";;
}
$count++;
}
Change (or enable) also the following line in /etc/squid/suiqd.conf.
http_access allow localnet
http_port 3128 transparent

111
url_rwwrite_program /usr/local/bin/flip.pl
The directory this script uses is /var/www/images and should be accessible by both the
squid server (which uses the user named proxy and by the apache2 webserver (which
uses the user www-data. The screenshot below shows how to create this directory, set the
permissions and make the users a member of the other groups.
root@debian7:~# mkdir /var/www/images
root@debian7:~# chown www-data:www-data /var/www/images
root@debian7:~# chmod 755 /var/www/images
root@debian7:~# usermod -aG www-data proxy
root@debian7:~# usermod -aG proxy www-data
Test that it works after restarting squid and apache2.

112
9.7. /var/log/squid
The standard log file location for squid is /var/log/squid.
[root@RHEL4 ~]# grep "/var/log" /etc/squid/squid.conf
# cache_access_log /var/log/squid/access.log
# cache_log /var/log/squid/cache.log
# cache_store_log /var/log/squid/store.log
9.8. access control
The default squid setup only allows localhost access. To enable access for a private network
range, look for the "INSERT YOUR OWN RULE(S) HERE..." sentence in squid.conf and
add two lines similar to the screenshot below.
# INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS
acl company_network src 192.168.1.0/24
http_access allow company_network
9.9. testing squid
First, make sure that the server running squid has access to the internet.
[root@RHEL4 ~]# wget -q http://linux-training.be/index.html
[root@RHEL4 ~]# ls -l index.html
-rw-r--r-- 1 root root 2269 Sep 18 13:18 index.html
[root@RHEL4 ~]#
Then configure a browser on a client to use the proxy server, or you could set the
HTTP_PROXY (sometimes http_proxy) variable to point command line programs to the
proxy.
[root@fedora ~]# export HTTP_PROXY=http://192.168.1.39:8080
[root@ubuntu ~]# export http_proxy=http://192.168.1.39:8080
Testing a client machine can then be done with wget (wget -q is used to simplify the
screenshot).
[root@RHEL5 ~]# > /etc/resolv.conf
[root@RHEL5 ~]# wget -q http://www.linux-training.be/index.html
[root@RHEL5 ~]# ls -l index.html
-rw-r--r-- 1 root root 2269 Sep 18 2008 index.html
[root@RHEL5 ~]#
9.10. name resolution
You need name resolution working on the squid server, but you don't need name resolution
on the clients.
[paul@RHEL5 ~]$ wget http://grep.be
--14:35:44-- http://grep.be
Resolving grep.be... failed: Temporary failure in name resolution.
[paul@RHEL5 ~]$ export http_proxy=http://192.168.1.39:8080
[paul@RHEL5 ~]$ wget http://grep.be
--14:35:49-- http://grep.be/

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Proxy request sent, awaiting response... 200 OK
Length: 5390 (5.3K) [text/html]
100%[================================>] 5,390 --.-K/s in 0.1s
14:38:29 (54.8 KB/s) - `index.html' saved [5390/5390]
[paul@RHEL5 ~]$

115
Table of Contents
10. introduction to DNS ............................................................................................................................ 116
10.1. about dns ................................................................................................................................... 117
10.2. dns namespace ........................................................................................................................... 120
10.3. caching only servers ................................................................................................................. 125
10.4. authoritative dns servers ........................................................................................................... 128
10.5. primary and secondary .............................................................................................................. 128
10.6. zone transfers ............................................................................................................................ 128
10.7. master and slave ........................................................................................................................ 130
10.8. SOA record ............................................................................................................................... 130
10.9. full or incremental zone transfers ............................................................................................. 131
10.10. DNS cache .............................................................................................................................. 132
10.11. forward lookup zone example ................................................................................................ 133
10.12. example: caching only DNS server ........................................................................................ 134
10.13. example: caching only with forwarder ................................................................................... 136
10.14. example: primary authoritative server .................................................................................... 138
10.15. example: a DNS slave server ................................................................................................. 142
10.16. practice: dns ............................................................................................................................ 144
10.17. solution: dns ............................................................................................................................ 145
11. advanced DNS ...................................................................................................................................... 146
11.1. example: DNS round robin ....................................................................................................... 147
11.2. DNS delegation ......................................................................................................................... 148
11.3. example: DNS delegation ......................................................................................................... 149
11.4. example: split-horizon dns ........................................................................................................ 151
11.5. old dns topics ............................................................................................................................ 153

116
Chapter 10. introduction to DNS
dns is a fundamental part of every large computer network. dns is used by many network
services to translate names into network addresses and to locate services on the network
(by name).
Whenever you visit a web site, send an e-mail, log on to Active Directory, play Minecraft,
chat, or use VoIP, there will be one or (many) more queries to dns services.
Should dns fail at your organization, then the whole network will grind to a halt (unless you
hardcoded the network addresses).
You will notice that even the largest of organizations benefit greatly from having one dns
infrastructure. Thus dns requires all business units to work together.
Even at home, most home modems and routers have builtin dns functionality.
This module will explain what dns actually is and how to set it up using Linux and bind9.

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10.1. about dns
10.1.1. name to ip address resolution
The domain name system or dns is a service on a tcp/ip network that enables clients to
translate names into ip addresses. Actually dns is much more than that, but let's keep it
simple for now.
When you use a browser to go to a website, then you type the name of that website in the
url bar. But for your computer to actually communicate with the web server hosting said
website, your computer needs the ip address of that web server. That is where dns comes in.
In wireshark you can use the dns filter to see this traffic.
10.1.2. history
In the Seventies, only a few hundred computers were connected to the internet. To resolve
names, computers had a flat file that contained a table to resolve hostnames to ip addresses.
This local file was downloaded from hosts.txt on an ftp server in Stanford.
In 1984 Paul Mockapetris created dns, a distributed treelike hierarchical database that will
be explained in detail in these chapters.
Today, dns or domain name system is a worldwide distributed hierarchical database
controlled by ICANN. Its primary function is to resolve names to ip addresses, and to point
to internet servers providing smtp or ldap services.
The old hosts.txt file is still active today on most computer systems under the name /etc/
hosts (or C:/Windows/System32/Drivers/etc/hosts). We will discuss this file later, as it can
influence name resolution.

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10.1.3. forward and reverse lookup queries
The question a client asks a dns server is called a query. When a client queries for an ip
address, this is called a forward lookup query (as seen in the previous drawing).
The reverse, a query for the name of a host, is called a reverse lookup query.
Below a picture of a reverse lookup query.
Here is a screenshot of a reverse lookup query in nslookup.
root@debian7:~# nslookup
> set type=PTR
> 188.93.155.87
Server: 192.168.1.42
Address: 192.168.1.42#53
Non-authoritative answer:
87.155.93.188.in-addr.arpa name = antares.ginsys.net.
This is what a reverse lookup looks like when sniffing with tcpdump.
root@debian7:~# tcpdump udp port 53
11:01:29.357685 IP 192.168.1.103.42041 > 192.168.1.42.domain: 14763+ PT
R? 87.155.93.188.in-addr.arpa. (44)
11:01:29.640093 IP 192.168.1.42.domain > 192.168.1.103.42041: 14763 1/0
/0 PTR antares.ginsys.net. (76)
And here is what it looks like in wireshark (note this is an older screenshot).

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10.1.4. /etc/resolv.conf
A client computer needs to know the ip address of the dns server to be able to send queries
to it. This is either provided by a dhcp server or manually entered.
Linux clients keep this information in the /etc/resolv.conf file.
root@debian7:~# cat /etc/resolv.conf
domain linux-training.be
search linux-training.be
nameserver 192.168.1.42
root@debian7:~#
You can manually change the ip address in this file to use another dns server. For example
Google provides a public name server at 8.8.8.8 and 8.8.4.4.
nameserver 8.8.8.8
root@debian7:~#
Please note that on dhcp clients this value can be overwritten when the dhcp lease is
renewed.

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10.2. dns namespace
10.2.1. hierarchy
The dns namespace is hierarchical tree structure, with the root servers (aka dot-servers) at
the top. The root servers are usually represented by a dot.
Below the root-servers are the Top Level Domains or tld's.
There are more tld's than shown in the picture. Currently about 200 countries have a tld. And
there are several general tld's like .com, .edu, .org, .gov, .net, .mil, .int and more recently
also .aero, .info, .museum, ...
10.2.2. root servers
There are thirteen root servers on the internet, they are named A to M. Journalists often
refer to these servers as the master servers of the internet, because if these servers go
down, then nobody can (use names to) connect to websites.
The root servers are not thirteen physical machines, they are many more. For example the
F root server consists of 46 physical machines that all behave as one (using anycast).
http://root-servers.org
http://f.root-servers.org
http://en.wikipedia.org/wiki/Root_nameserver.

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10.2.3. root hints
Every dns server software will come with a list of root hints to locate the root servers.
This screenshot shows a small portion of the root hints file that comes with bind 9.8.4.
root@debian7:~# grep -w 'A ' /etc/bind/db.root
A.ROOT-SERVERS.NET. 3600000 A 198.41.0.4
B.ROOT-SERVERS.NET. 3600000 A 192.228.79.201
C.ROOT-SERVERS.NET. 3600000 A 192.33.4.12
D.ROOT-SERVERS.NET. 3600000 A 199.7.91.13
E.ROOT-SERVERS.NET. 3600000 A 192.203.230.10
F.ROOT-SERVERS.NET. 3600000 A 192.5.5.241
G.ROOT-SERVERS.NET. 3600000 A 192.112.36.4
H.ROOT-SERVERS.NET. 3600000 A 128.63.2.53
I.ROOT-SERVERS.NET. 3600000 A 192.36.148.17
J.ROOT-SERVERS.NET. 3600000 A 192.58.128.30
K.ROOT-SERVERS.NET. 3600000 A 193.0.14.129
L.ROOT-SERVERS.NET. 3600000 A 199.7.83.42
M.ROOT-SERVERS.NET. 3600000 A 202.12.27.33
root@debian7:~#
10.2.4. domains
One level below the top level domains are the domains. Domains can have subdomains
(also called child domains).
This picture shows dns domains like google.com, chess.com, linux-training.be (there are
millions more).
DNS domains are registered at the tld servers, the tld servers are registered at the dot
servers.

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10.2.5. top level domains
Below the root level are the top level domains or tld's. Originally there were only seven
defined:
Table 10.1. the first top level domains
year TLD purpose
1985 .arpa Reverse lookup via in-addr.arpa
1985 .com Commercial Organizations
1985 .edu US Educational Institutions
1985 .gov US Government Institutions
1985 .mil US Military
1985 .net Internet Service Providers, Internet Infrastructure
1985 .org Non profit Organizations
1988 .int International Treaties like nato.int
Country tld's were defined for individual countries, like .uk in 1985 for Great Britain (yes
really), .be for Belgium in 1988 and .fr for France in 1986. See RFC 1591 for more info.
In 1998 seven new general purpose tld's where chosen, they became active in the 21st
century.
Table 10.2. new general purpose tld's
year TLD purpose
2002 .aero aviation related
2001 .biz businesses
2001 .coop for co-operatives
2001 .info informative internet resources
2001 .museum for museums
2001 .name for all kinds of names, pseudonyms and labels...
2004 .pro for professionals
Many people were surprised by the choices, claiming not much use for them and wanting
a separate .xxx domain (introduced in 2011) for adult content, and .kidz a save haven for
children. In the meantime more useless tld's were create like .travel (for travel agents) and
.tel (for internet communications) and .jobs (for jobs sites).
In 2012 ICANN released a list of 2000 new tld's that would gradually become available.

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10.2.6. fully qualified domain name
The fully qualified domain name or fqdn is the combination of the hostname of a machine
appended with its domain name.
If for example a system is called gwen and it is in the domain linux-training.be, then the
fqdn of this system is gwen.linux-training.be.
On Linux systems you can use the hostname and dnsdomainname commands to verify
this information.
root@gwen:~# hostname
gwen
root@gwen:~# dnsdomainname
linux-training.be
root@gwen:~# hostname --fqdn
gwen.linux-training.be
root@gwen:~# cat /etc/debian_version
6.0.10
10.2.7. dns zones
A zone (aka a zone of authority) is a portion of the DNS tree that covers one domain name
or child domain name. The picture below represents zones as blue ovals. Some zones will
contain delegate authority over a child domain to another zone.
A dns server can be authoritative over 0, 1 or more dns zones. We will see more details
later on the relation between a dns server and a dns zone.
A dns zone consists of records, also called resource records. We will list some of those
resource records on the next page.

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10.2.8. dns records
A record
The A record, which is also called a host record contains the ipv4-address of a computer.
When a DNS client queries a DNS server for an A record, then the DNS server will resolve
the hostname in the query to an ip address. An AAAA record is similar but contains an ipv6
address instead of ipv4.
PTR record
A PTR record is the reverse of an A record. It contains the name of a computer and can be
used to resolve an ip address to a hostname.
NS record
A NS record or nameserver record is a record that points to a DNS name server (in this
zone). You can list all your name servers for your DNS zone in distinct NS records.
glue A record
An A record that maps the name of an NS record to an ip address is said to be a glue record.
SOA record
The SOA record of a zone contains meta information about the zone itself. The contents of
the SOA record is explained in detail in the section about zone transfers. There is exactly
one SOA record for each zone.
CNAME record
A CNAME record maps a hostname to a hostname, creating effectively an alias for an
existing hostname. The name of the mail server is often aliased to mail or smtp, and the
name of a web server to www.
MX record
The MX record points to an smtp server. When you send an email to another domain, then
your mail server will need the MX record of the target domain's mail server.

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10.3. caching only servers
A dns server that is set up without authority over a zone, but that is connected to other
name servers and caches the queries is called a caching only name server. Caching only
name servers do not have a zone database with resource records. Instead they connect to
other name servers and cache that information.
There are two kinds of caching only name servers. Those with a forwarder, and those that
use the root servers.

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10.3.1. caching only server without forwarder
A caching only server without forwarder will have to get information elsewhere. When it
receives a query from a client, then it will consult one of the root servers. The root server
will refer it to a tld server, which will refer it to another dns server. That last server might
know the answer to the query, or may refer to yet another server. In the end, our hard working
dns server will find an answer and report this back to the client.
In the picture below, the clients asks for the ip address of linux-training.be. Our caching only
server will contact the root server, and be refered to the .be server. It will then contact the .be
server and be refered to one of the name servers of Openminds. One of these name servers
(in this cas ns1.openminds.be) will answer the query with the ip address of linux-training.be.
When our caching only server reports this to the client, then the client can connect to this
website.
Sniffing with tcpdump will give you this (the first 20 characters of each line are cut).
192.168.1.103.41251 > M.ROOT-SERVERS.NET.domain: 37279% [1au] A? linux-tr
aining.be. (46)
M.ROOT-SERVERS.NET.domain > 192.168.1.103.41251: 37279- 0/11/13 (740)
192.168.1.103.65268 > d.ns.dns.be.domain: 38555% [1au] A? linux-training.
be. (46)
d.ns.dns.be.domain > 192.168.1.103.65268: 38555- 0/7/5 (737)
192.168.1.103.7514 > ns2.openminds.be.domain: 60888% [1au] A? linux-train
ing.be. (46)
ns2.openminds.be.domain > 192.168.1.103.7514: 60888*- 1/0/1 A 188.93.155.
87 (62)

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10.3.2. caching only server with forwarder
A caching only server with a forwarder is a DNS server that will get all its information
from the forwarder. The forwarder must be a dns server for example the dns server of
an internet service provider.
This picture shows a dns server on the company LAN that has set the dns server from their
isp as a forwarder. If the ip address of the isp dns server is 212.71.8.10, then the following
lines would occur in the named.conf file of the company dns server:
forwarders {
212.71.8.10;
};
You can also configure your dns server to work with conditional forwarder(s). The
definition of a conditional forwarder looks like this.
zone "someotherdomain.local" {
type forward;
forward only;
forwarders { 10.104.42.1; };
};
10.3.3. iterative or recursive query
A recursive query is a DNS query where the client that is submitting the query expects a
complete answer (Like the fat red arrow above going from the Macbook to the DNS server).
An iterative query is a DNS query where the client does not expect a complete answer (the
three black arrows originating from the DNS server in the picture above). Iterative queries
usually take place between name servers. The root name servers do not respond to recursive
queries.

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10.4. authoritative dns servers
A DNS server that is controlling a zone, is said to be the authoritative DNS server for that
zone. Remember that a zone is a collection of resource records.
10.5. primary and secondary
When you set up the first authoritative dns server for a zone, then this is called the primary
dns server. This server will have a readable and writable copy of the zone database. For
reasons of fault tolerance, performance or load balancing you may decide to set up another
dns server with authority over that zone. This is called a secondary dns server.
10.6. zone transfers
The slave server receives a copy of the zone database from the master server using a
zone transfer. Zone transfers are requested by the slave servers at regular intervals. Those
intervals are defined in the soa record.
You can force a refresh from a zone with rndc. The example below force a transfer of the
fred.local zone, and shows the log from /var/log/syslog.
root@debian7:/etc/bind# rndc refresh fred.local

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root@debian7:/etc/bind# grep fred /var/log/syslog | tail -7 | cut -c38-
zone fred.local/IN: sending notifies (serial 1)
received control channel command 'refresh fred.local'
zone fred.local/IN: Transfer started.
transfer of 'fred.local/IN' from 10.104.109.1#53: connected using 10.104.33.30#57367
zone fred.local/IN: transferred serial 2
transfer of 'fred.local/IN' from 10.104.109.1#53: Transfer completed: 1 messages, 10 records, 2
zone fred.local/IN: sending notifies (serial 2)
root@debian7:/etc/bind#

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10.7. master and slave
When adding a secondary dns server to a zone, then you will configure this server as a
slave server to the primary server. The primary server then becomes the master server
of the slave server.
Often the primary dns server is the master server of all slaves. Sometimes a slave server
is master server for a second line slave server. In the picture below ns1 is the primary dns
server and ns2, ns3 and ns4 are secondaries. The master for slaves ns2 and ns3 is ns1, but
the master for ns4 is ns2.
10.8. SOA record
The soa record contains a refresh value. If this is set to 30 minutes, then the slave server
will request a copy of the zone file every 30 minutes. There is also a retry value. The retry
value is used when the master server did not reply to the last zone transfer request. The value
for expiry time says how long the slave server will answer to queries, without receiving
a zone update.
Below an example of how to use nslookup to query the soa record of a zone (linux-
training.be).
> set type=SOA
> server ns1.openminds.be
> linux-training.be
Server: ns1.openminds.be
Address: 195.47.215.14#53
linux-training.be
origin = ns1.openminds.be
mail addr = hostmaster.openminds.be
serial = 2321001133
refresh = 14400
retry = 3600
expire = 604800
minimum = 3600
Zone transfers only occur when the zone database was updated (meaning when one or more
resource records were added, removed or changed on the master server). The slave server

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will compare the serial number of its own copy of the SOA record with the serial number
of its master's SOA record. When both serial numbers are the same, then no update is needed
(because no records were added, removed or deleted). When the slave has a lower serial
number than its master, then a zone transfer is requested.
Below a zone transfer captured in wireshark.
10.9. full or incremental zone transfers
When a zone tranfer occurs, this can be either a full zone transfer or an incremental zone
transfer. The decision depends on the size of the transfer that is needed to completely update
the zone on the slave server. An incremental zone transfer is prefered when the total size
of changes is smaller than the size of the zone database. Full zone transfers use the axfr
protocol, incremental zone transfer use the ixfr protocol.

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10.10. DNS cache
DNS is a caching protocol.
When a client queries its local DNS server, and the local DNS server is not authoritative
for the query, then this server will go looking for an authoritative name server in the DNS
tree. The local name server will first query a root server, then a tld server and then a domain
server. When the local name server resolves the query, then it will relay this information to
the client that submitted the query, and it will also keep a copy of these queries in its cache.
So when a(nother) client submits the same query to this name server, then it will retrieve
this information form its cache.
For example, a client queries for the A record on www.linux-training.be to its local server.
This is the first query ever received by this local server. The local server checks that it is
not authoritative for the linux-training.be domain, nor for the .be tld, and it is also not a root
server. So the local server will use the root hints to send an iterative query to a root server.
The root server will reply with a reference to the server that is authoritative for the .be
domain (root DNS servers do not resolve fqdn's, and root servers do not respond to recursive
queries).
The local server will then sent an iterative query to the authoritative server for the .be tld.
This server will respond with a reference to the name server that is authoritative for the
linux-training.be domain.
The local server will then sent the query for www.linux-training.be to the authoritative server
(or one of its slave servers) for the linux-training.be domain. When the local server receives
the ip address for www.linux-training.be, then it will provide this information to the client
that submitted this query.
Besides caching the A record for www.linux-training.be, the local server will also cache the
NS and A record for the linux-training.be name server and the .be name server.

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10.11. forward lookup zone example
The way to set up zones in /etc/bind/named.conf.local is to create a zone entry with a
reference to another file (this other file contains the zone database).
Here is an example of such an entry in /etc/bind/named.conf.local:
root@debian7:~# cat /etc/bind/named.conf.local
//
// Do any local configuration here
//
// Consider adding the 1918 zones here, if they are not used in your
// organization
//include "/etc/bind/zones.rfc1918";
zone "paul.local" IN {
type master;
file "/etc/bind/db.paul.local";
allow-update { none; };
};
root@debian7:~#
To create the zone file, the easy method is to copy an existing zone file (this is easier than
writing from scratch).
root@debian7:/etc/bind# cp db.empty db.paul.local
root@debian7:/etc/bind# vi db.paul.local
Here is an example of a zone file.
root@debian7:/etc/bind# cat db.paul.local
; zone for classroom teaching
$TTL 86400
@ IN SOA debianpaul.paul.local. root.paul.local (
2014100100 ; Serial
1h ; Refresh
1h ; Retry
2h ; Expire
86400 ) ; Negative Cache TTL
;
; name servers
;
IN NS ns1
IN NS debianpaul
IN NS debian7
;
; servers
;
debianpaul IN A 10.104.33.30
debian7 IN A 10.104.33.30
ns1 IN A 10.104.33.30
;www IN A 10.104.33.30

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10.12. example: caching only DNS server
1. installing DNS software on Debian
root@debian7:~# aptitude update && aptitude upgrade
...
root@debian7:~# aptitude install bind9
...
root@debian7:~# dpkg -l | grep bind9 | tr -s ' '
ii bind9 1:9.8.4.dfsg.P1-6+nmu2+deb7u2 amd64 Internet Domain Name Server
ii bind9-host 1:9.8.4.dfsg.P1-6+nmu2+deb7u2 amd64 Version of 'host' bundled...
ii bind9utils 1:9.8.4.dfsg.P1-6+nmu2+deb7u2 amd64 Utilities for BIND
ii libbind9-80 1:9.8.4.dfsg.P1-6+nmu2+deb7u2 amd64 BIND9 Shared Library use...
root@debian7:~#
2. Discover the default configuration files. Can you define the purpose of each file ?
root@debian7:~# ls -l /etc/bind
total 52
-rw-r--r-- 1 root root 2389 Sep 5 20:25 bind.keys
-rw-r--r-- 1 root root 237 Sep 5 20:25 db.0
-rw-r--r-- 1 root root 353 Sep 5 20:25 db.empty
-rw-r--r-- 1 root root 270 Sep 5 20:25 db.local
-rw-r--r-- 1 root root 3048 Sep 5 20:25 db.root
-rw-r--r-- 1 root bind 463 Sep 5 20:25 named.conf
-rw-r--r-- 1 root bind 490 Sep 5 20:25 named.conf.default-zones
-rw-r--r-- 1 root bind 374 Oct 1 20:01 named.conf.local
-rw-r--r-- 1 root bind 913 Oct 1 13:24 named.conf.options
-rw-r----- 1 bind bind 77 Oct 1 11:14 rndc.key
-rw-r--r-- 1 root root 1317 Sep 5 20:25 zones.rfc191
3. Setup caching only dns server. This is normally the default setup. A caching-only name
server will look up names for you and cache them. Many tutorials will tell you to add a
forwarder, but we first try without this!
Hey this seems to work without a forwarder. Using a sniffer you can find out what really
happens. Your freshly install dns server is not using a cache, and it is not using your local
dns server (from /etc/resolv.conf). So where is this information coming from ? And what
can you learn from sniffing this dns traffic ?

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4. Explain in detail what happens when you enable a caching only dns server without
forwarder. This wireshark screenshot can help, but you learn more by sniffing the traffic
yourself.
You should see traffic to a root name server whenever you try a new tld for the first time.
Remember that dns is a caching protocol, which means that repeating a query will generate
a lot less traffic since your dns server will still have the answer in its memory.

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10.13. example: caching only with forwarder
5. Add the public Google dns server as a forwarder. The ip address of this server is 8.8.8.8 .
Before the change:
root@debian7:~# grep -A2 'forwarders {' /etc/bind/named.conf.options
// forwarders {
// 0.0.0.0;
// };
changing:
root@debian7:~# vi /etc/bind/named.conf.options
After the change:
root@debian7:~# grep -A2 'forwarders {' /etc/bind/named.conf.options
forwarders {
8.8.8.8;
};
Restart the server:
root@debian7:~# service bind9 restart
Stopping domain name service...: bind9.
Starting domain name service...: bind9.
6. Explain the purpose of adding the forwarder. What is our dns server doing when it
receives a query ?
> server
Default server: 10.104.33.30
Address: 10.104.33.30#53
> linux-training.be
Server: 10.104.33.30
Address: 10.104.33.30#53
Non-authoritative answer:
Name: linux-training.be
Address: 188.93.155.87
>
This is the output of tcpdump udp port 53 while executing the above query for linux-
training.be in nslookup.
root@debian7:~# tcpdump udp port 53
You should find the following two lines in the output of tcpdump:
10.104.33.30.19381 > google-public-dns-a.google.com.domain: 18237+% [1au] A?
linux-training.be. (46)
google-public-dns-a.google.com.domain > 10.104.33.30.19381: 18237 1/0/1 A 188
.93.155.87 (62)

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Below is an (old) wireshark screenshot that can help, you should see something similar (but
with different ip addresses).
7. What happens when you query for the same domain name more than once ?
8. Why does it say "non-authoritative answer" ? When is a dns server authoritative ?
9. You can also use dig instead of nslookup.
root@debian7:~# dig @10.104.33.30 linux-training.be +short
188.93.155.87
root@debian7:~#
10. How can we avoid having to set the server in dig or nslookup ?
Change this:
root@debian7:~#
into this:
root@debian7:~#
11. When you use dig for the first time for a domain, where is the answer coming from ?
And the second time ? How can you tell ?

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10.14. example: primary authoritative server
1. Instead of only cachng the information from other servers, we will now make our server
authoritative for our own domain.
2. I choose the top level domain .local and the domain paul.local and put the information
in /etc/bind/named.conf.local.
root@debian7:~# cat /etc/bind/named.conf.local
//
//
// organization
type master;
};
3. Also add a zone database file, similar to this one (add some A records for testing). Set
the Refresh and Retry values not too high so you can sniff this traffic (this example makes
the slave server contact the master every hour).
root@debian7:~# cat /etc/bind/db.paul.local
; zone for classroom teaching
$TTL 86400
@ IN SOA debianpaul.paul.local. root.paul.local (
2014100101 ; Serial
1h ; Refresh
1h ; Retry
2h ; Expire
;
; name servers
;
IN NS ns1
IN NS debianpaul
IN NS debian7
;
; servers
;
debianpaul IN A 10.104.33.30
debian7 IN A 10.104.33.30
ns1 IN A 10.104.33.30
;www IN A 10.104.33.30
root@debian7:~#
Note that the www record is commented out, so it will not resolve.

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10.14.1. using your own DNS server
If you are confident that your dns server works, then set it as default and only dns server
in /etc/resolv.conf.
root@debian7:~#
In case you also use dhclient, you will need to add your dns server to /etc/dhcp/
dhclient.conf.
root@debian7:~# diff /etc/dhcp/dhclient.conf /etc/dhcp/dhclient.conf.original
21c21
< prepend domain-name-servers 10.104.33.30;
---
> #prepend domain-name-servers 127.0.0.1;
23,24c23
< # domain-name, domain-name-servers, domain-search, host-name,
< domain-name, domain-search, host-name,
---
> domain-name, domain-name-servers, domain-search, host-name,
root@debian7:~#
The above screenshot shows that 10.104.33.30 is now a default option that the dhcp client
should no longer request from the dhcp server.
Adjust /etc/hosts to reflect your domain name and verify with hostname and
dnsdomainname.
root@debian7:~# grep debian7 /etc/hosts
127.0.1.1 debian7.paul.local debian7
root@debian7:~# hostname
debian7
root@debian7:~# hostname --fqdn
debian7.paul.local
root@debian7:~# dnsdomainname
paul.local

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10.14.2. using your own domain
Consider the following screenshot:
root@debian7b:~# cat /etc/resolv.conf
root@debian7b:~# ping -c1 www
ping: unknown host www
root@debian7b:~# vi /etc/resolv.conf
root@debian7b:~# cat /etc/resolv.conf
domain paul.local
root@debian7b:~# ping -c1 www
PING www.paul.local (10.104.33.31) 56(84) bytes of data.
--- www.paul.local ping statistics ---
root@debian7b:~#
Adding the domain paul.local directive to /etc/resolv.conf allows omitting the domain
when using hostnames.
You can accomplish this feature automatically by adjusting dhclient.conf.
root@debian7:~# grep paul.local /etc/dhcp/dhclient.conf
prepend domain-name "paul.local";
prepend domain-search "paul.local";
root@debian7:~#

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4. Restart the DNS server and check your zone in the error log.
root@debian7:~# grep paul.local /var/log/syslog
Oct 6 09:22:18 debian7 named[2707]: zone paul.local/IN: loaded seria
l 2014100101
Oct 6 09:22:18 debian7 named[2707]: zone paul.local/IN: sending noti
fies (serial 2014100101)
5. Use dig or nslookup (or even ping) to test your A records.
root@debian7:~# ping -c1 ns1.paul.local
PING ns1.paul.local (10.104.33.30) 56(84) bytes of data.
--- ns1.paul.local ping statistics ---
root@debian7:~# ping -c1 www.paul.local
ping: unknown host www.paul.local
Note that the www record was commented out, so it should fail.
root@debian7:~# dig debian7.paul.local
; <<>> DiG 9.8.4-rpz2+rl005.12-P1 <<>> debian7.paul.local
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50491
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 2
;; QUESTION SECTION:
;debian7.paul.local. IN A
;; ANSWER SECTION:
debian7.paul.local. 86400 IN A 10.104.33.30
;; AUTHORITY SECTION:
paul.local. 86400 IN NS ns1.paul.local.
paul.local. 86400 IN NS debian7.paul.local.
paul.local. 86400 IN NS debianpaul.paul.local.
;; ADDITIONAL SECTION:
ns1.paul.local. 86400 IN A 10.104.33.30
debianpaul.paul.local. 86400 IN A 10.104.33.30
;; Query time: 4 msec
;; SERVER: 10.104.33.30#53(10.104.33.30)
;; WHEN: Mon Oct 6 09:35:25 2014
;; MSG SIZE rcvd: 141
root@debian7:~#
6. Our primary server appears to be up and running. Note the information here:
server os : Debian 7
ip address : 10.104.33.30
domain name: paul.local
server name: ns1.paul.local

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10.15. example: a DNS slave server
1. A slave server transfers zone information over the network from a master server (a slave
can also be a master). A primary server maintains zone records in its local file system. As
an exercise, and to verify the work of all students, set up a slave server of all the master
servers in the classroom.
2. Before configuring the slave server, we may have to allow transfers from our zone to this
server. Remember that this is not very secure since transfers are in clear text and limited to
an ip address. This example follows our demo from above.
Imagine a student named Jesse having completed the setup as shown before, with the domain
name jesse.local and the ip address 10.104.15.20. The goal is to have a slave server of
paul.local on Jesse's computer and a slave zone of jesse.local on my computer.
Below is an example of an allow-transfer statement. Careful, maybe the default allows
transfer to any.
root@debian7:/etc/bind# cat named.conf.local
//
//
// organization
type master;
allow-transfer { 10.104.15.20; };
};

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3. With the configuration below I can make my server a slave for the jesse.local zone.
root@debian7:/etc/bind# tail -6 named.conf.local
zone "jesse.local" IN {
type slave;
file "/var/cache/named/db.jesse.local";
masters { 10.104.15.20; };
};
root@debian7:/etc/bind# mkdir /var/cache/named/
root@debian7:/etc/bind# chown bind:bind /var/cache/named/
root@debian7:/etc/bind# ls -ld /var/cache/named/
drwxr-xr-x 2 bind bind 4096 Oct 1 20:01 /var/cache/named/
Note that we put the slave zones in /var/cache/named and not in /etc/bind.
4. Restarting bind on the slave server should transfer the zone database file. Verify this in /
var/log/syslog. (time and date are truncated from the screenshot, and Jesse did not use the
current date in the serial number...)
root@debian7:/etc/bind# grep jesse /var/log/syslog
named[2731]: zone jesse.local/IN: Transfer started.
named[2731]: transfer of 'jesse.local/IN' from 10.104.15.20#53: connected u
sing 10.104.33.30#44719
named[2731]: zone jesse.local/IN: transferred serial 20110516
named[2731]: transfer of 'jesse.local/IN' from 10.104.15.20#53: Transfer co
mpleted: 1 messages, 8 records, 239 bytes, 0.001 secs (239000 bytes/sec)
And the contents of the slave zone:
root@debian7:/etc/bind# cat /var/cache/named/db.jesse.local
$ORIGIN .
$TTL 604800 ; 1 week
jesse.local IN SOA ns.jesse.local. root.jesse.local.jesse.local. (
20110516 ; serial
300 ; refresh (5 minutes)
200 ; retry (3 minutes 20 seconds)
2419200 ; expire (4 weeks)
604800 ; minimum (1 week)
)
NS ns.jesse.local.
$ORIGIN jesse.local.
anya A 10.104.15.1
mac A 10.104.15.30
ns A 10.104.15.20
ubu1010srv A 10.104.15.20
www A 10.104.15.25
root@debian7:/etc/bind#

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10.16. practice: dns
1. Install bind9 and verify with a sniffer how it works.
2. Add a forwarder and verify that it works.
3. Create a primary forward lookup zone named yourname.local with at least two NS
records and four A records.
4. Use dig and nslookup to verify your NS and A records.
5. Create a slave of your primary zone (on another server) and verify the zone transfer.
6. Set up two primary zones on two servers and implement a conditional forwarder (you
can use the two servers from before).

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10.17. solution: dns
1. Install bind9 and verify with a sniffer how it works.
You should see queries to the root name servers with tcpdump or wireshark.
2. Add a forwarder and verify that it works.
The forwarder van be added in named.conf.options as seen in the theory.
3. Create a primary forward lookup zone named yourname.local with at least two NS
records and four A records.
This is literally explained in the theory.
4. Use dig and nslookup to verify your NS and A records.
5. Create a slave of your primary zone (on another server) and verify the zone transfer.
6. Set up two primary zones on two servers and implement a conditional forwarder (you
can use the two servers from before).
A conditional forwarder is set in named.conf.local as a zone.
(see the theory on forwarder)

146
Chapter 11. advanced DNS
This chapter expands your DNS server with topics like round robin dns for load balancing
servers, dns delegation to delegate child domains to another team and split horizon dns so
you can provide local service locations to clients.
There is more to dns, content will be added rsn.

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11.1. example: DNS round robin
When you create multiple A records for the same name, then bind will do a round robin of
the order in which the records are returned. This allows the use of DNS as a load balancer
between hosts, since clients will usually take the first ip-address offered.
Consider this example from the /etc/bind/db.paul.local zone configuration file. There are
two A records for www pointing to two distinct ip addresses.
root@debian7:~# grep www /etc/bind/db.paul.local
www IN A 10.104.33.30
www IN A 10.104.33.31
Below a screenshot of nslookup querying a load balanced A record. Notice the order of ip
addresses returned.
root@debian7:~# nslookup www.paul.local 10.104.33.30
Server: 10.104.33.30
Address: 10.104.33.30#53
Name: www.paul.local
Address: 10.104.33.31
Address: 10.104.33.30
root@debian7:~# nslookup www.paul.local 10.104.33.30
Server: 10.104.33.30
Address: 10.104.33.30#53
Address: 10.104.33.30
Address: 10.104.33.31
Try to set up a website on two web servers (with a small difference so you can distinguish
the websites) and test the round robin.

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11.2. DNS delegation
You can delegate a child domain to another DNS server. The child domain then becomes
a new zone, with authority at the new dns server.
When delegation is properly set up, then clients that query your parent zone will also be
able to resolve the delegated child zones.

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11.3. example: DNS delegation
We have another Linux server named debian7b and we want to make it responsible for the
child domain test42.paul.local.
Note the name of the servers in the screenshots are either debian7 (hosting the parent
domain) or debian7b (hosting the child domain).
We start by adjusting the /etc/bind/named.comf.local file (on the server hosting the parent
domain) to make sure that no forwarder will be used when resolving authoritative names.
root@debian7:~# grep -A4 paul.local /etc/bind/named.conf.local
type master;
allow-transfer { 10.104.15.20; };
forwarders { };
};
root@debian7:~#
Technically, you could also set allow-transfer to { any; }; while troubleshooting and then
refine it later, but this is not needed for delegation.
Then we add the delegation to our zone database:
root@debian7:~# tail -3 /etc/bind/db.paul.local
$ORIGIN test42.paul.local.
@ IN NS ns2.test42.paul.local.
ns2 IN A 10.104.33.31 ; the glue record
root@debian7:~#
Don't forget to restart bind and verify /var/log/syslog.
root@debian7:~# grep paul.local /var/log/syslog | cut -c28- | tail -2
named[3202]: zone paul.local/IN: loaded serial 2014100801
named[3202]: zone paul.local/IN: sending notifies (serial 2014100801)
root@debian7:~#
Note that on your terminal you can type tail -40 /var/log/syslog because the only reason I
use grep, cut and tail -2 is to limit the size of the screenshots in this book.

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Next we create a zone database file on the second server, as seen in this screenshot:
root@debian7b:~# cat /etc/bind/db.test42.paul.local
; child zone for classroom teaching
$TTL 86400
$ORIGIN test42.paul.local.
@ IN SOA ns2.test42.paul.local. root.test42.paul.local. (
2014100802 ; Serial
1h ; Refresh
1h ; Retry
2h ; Expire
;
; name servers
;
IN NS ns2.test42.paul.local.
IN NS debian7b.test42.paul.local.
;
; servers
;
ns2 IN A 10.104.33.31
debian7b IN A 10.104.33.31
testsrv IN A 10.104.33.31
root@debian7b:~#
The second server also needs a zone definition in named.conf.local, followed by a restart
of bind.
root@debian7b:~# cat /etc/bind/named.conf.local
//
//
// organization
zone "test42.paul.local" IN {
type master;
file "/etc/bind/db.test42.paul.local";
allow-transfer { any; };
};
root@debian7b:~#
Testing on the parent server:
root@debian7:~# dig ns1.paul.local +short
10.104.33.30
root@debian7:~# dig ns2.test42.paul.local +short
10.104.33.31
root@debian7:~# dig debian7b.test42.paul.local +short
10.104.33.31

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11.4. example: split-horizon dns
Suppose you want to answer dns queries depending on who is asking. For example
when someone from the 10.104.15.0/24 network (managed by Jesse) asks for the A
record www.paul.local, then dns answers with 10.104.33.30. But when someone from the
10.104.42.0/24 network (managed by Keith) asks for the same A record of www.paul.local,
he will get 10.104.33.31 as an answer.
A split-horizon setup can be used to redirect people to local copies of certain services.
In this example we want to decide on specific answers for two networks (Jesse's and Keith's)
and prevent them from using our dns server for recursion, while maintaining the capability
to resolve the internet and our paul.local zone from our own network.
We start by creating three view clauses in named.conf.local.
root@debian7:/etc/bind# cat named.conf.local
view "paul" {
match-clients { 10.104.33.0; localhost; };
include "/etc/bind/named.conf.default-zones";
type master;
};
}; // end view internal
view "jesse" {
match-clients { 10.104.15/24; };
type master;
file "/etc/bind/db.paul.local.jesse";
};
}; // end view jesse
view "keith" {
match-clients { 10.104.42/24; };
type master;
file "/etc/bind/db.paul.local.keith";
};
}; // end view keith
Note that we included the default-zones in the internal zone. It is mandatory to put all zones
inside views when using a view.
The zone files are identical copies, except for the www record. You can see that the
round robin is still active for internal users, computers from 10.104.15.0/24 (Jesse) will
always receive 10.104.33.30 while computers from 10.104.42.0/24 (Keith) will receive
10.104.33.31.
root@debian7:/etc/bind# grep www db.paul.local db.paul.local.[jk]*
db.paul.local:www IN A 10.104.33.30
db.paul.local:www IN A 10.104.33.31
db.paul.local.jesse:www IN A 10.104.33.30
db.paul.local.keith:www IN A 10.104.33.31

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11.5. old dns topics
All the dns things below this paragraph are old and in urgent need of review.
11.5.1. old example: reverse DNS
1. We can add ip to name resolution to our dns-server using a reverse dns zone.
2. Start by adding a .arpa zone to /etc/bind/named.conf.local like this (we set notify to no to
avoid sending of notify messages to other name servers):
root@ubu1010srv:/etc/bind# grep -A4 arpa named.conf.local
zone "1.168.192.in-addr.arpa" {
type master;
notify no;
file "/etc/bind/db.192";
};
3. Also create a zone database file for this reverse lookup zone.
root@ubu1010srv:/etc/bind# cat db.192
;
; BIND reverse data file for 192.168.1.0/24 network
;
$TTL 604800
@ IN SOA ns.cobbaut.paul root.cobbaut.paul. (
20110516 ; Serial
604800 ; Refresh
86400 ; Retry
2419200 ; Expire
;
@ IN NS ns.
37 IN PTR ns.cobbaut.paul.
1 IN PTR anya.cobbaut.paul.
30 IN PTR mac.cobbaut.paul.
root@ubu1010srv:/etc/bind#
4. Test with nslookup or dig:
root@ubu1010srv:/etc/bind# dig 1.168.192.in-addr.arpa AXFR

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11.5.2. old DNS load balancing
Not as above. When you have more than one DNS server authoritative for a zone, you can
spread queries amongst all server. One way to do this is by creating NS records for all servers
that participate in the load balancing of external queries.
You could also configure different name servers on internal clients.
11.5.3. old DNS notify
The original design of DNS in rfc 1034 and rfc 1035 implemented a refresh time in the
SOA record to configure a time loop for slaves to query their master server. This can result
in a lot of useless pull requests, or in a significant lag between updates.
For this reason dns notify (rfc 1996) was designed. The server will now notify slaves
whenever there is an update. By default this feature is activated in bind.
Notify can be disabled as in this screenshot.
zone "1.168.192.in-addr.arpa" {
type master;
notify no;
file "/etc/bind/db.192";
};
11.5.4. old testing IXFR and AXFR
Full zone transfers (AXFR) are initiated when you restart the bind server, or when you
manually update the zone database file directly. With nsupdate you can update a zone
database and initiate an incremental zone transfer.
You need DDNS allowed for nsupdate to work.
root@ubu1010srv:/etc/bind# nsupdate
> server 127.0.0.1
> update add mac14.linux-training.be 86400 A 192.168.1.23
> send
update failed: REFUSED
11.5.5. old DDNS integration with DHCP
Some organizations like to have all their client computers in DNS. This can be cumbersome
to maintain. Luckily rfc 2136 describes integration of DHCP servers with a DNS server.
Whenever DHCP acknowledges a client ip configuration, it can notify DNS with this clients
ip-address and name. This is called dynamic updates or DDNS.
11.5.6. old reverse is forward in-addr.arpa
Reverse lookup is actually iomplemented as a forward lookup in the in-addr.arpa domain.
This domain has 256 child domains (from 0.in-addr.arpa to 255.in-addr.arpa), with each
child domain having again 256 child domains. And this twice more to a structure of over
four billion (2 to the power 32) domains.

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11.5.7. old ipv6
With rfc 3596 came ipv6 extensions for DNS. There is the AAAA record for ipv6 hosts on
the network, and there is the ip6.int domain for reverse lookup (having 16 child domains
from 0.ip6.int to f.ip6.int, each of those having again 16 child domains...and this 16 times.
11.5.8. old DNS security: file corruption
To mitigate file corruption on the zone files and the bind configuration files protect them
with Unix permissions and take regular backups.
11.5.9. old DNS security: zone transfers
Limit zone transfers to certain ip addresses instead of to any. Nevermind that ip-addresses
can be spoofed, still use this.
11.5.10. old DNS security: zone transfers, ip spoofing
You could setup DNSSEC (which is not the easiest to maintain) and with rfc 2845(tsig?) and
with rfc 2930(tkey, but this is open to brute force), or you could disable all zone transfers
and use a script with ssh to copy them manually.
11.5.11. old DNS security: queries
Allow recursion only from the local network, and iterative queries from outside only when
necessary. This can be configured on master and slave servers.
view "internal" {
match-clients { 192.168.42/24; };
recursion yes;
...
};
view "external" {
match-clients { any; };
recursion no;
...
};
Or allow only queries from the local network.
options {
allow-query { 192.168.42.0/24; localhost; };
};
zone "cobbaut.paul" {
allow-query { any; };
};
Or only allow recursive queries from internal clients.
options {
allow-recursion { 192.168.42.0/24; localhost; };

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156
};
11.5.12. old DNS security: chrooted bind
Most Linux distributions allow an easy setup of bind in a chrooted environment.
11.5.13. old DNS security: DNSSEC
DNSSEC uses public/private keys to secure communications, this is described in rfc's 4033,
4034 and 4035.
11.5.14. old DNS security: root
Do not run bind as root. Do not run any application daemon as root.

158
Table of Contents
12. introduction to dhcp ............................................................................................................................ 159
12.1. four broadcasts .......................................................................................................................... 160
12.2. picturing dhcp ........................................................................................................................... 161
12.3. installing a dhcp server ............................................................................................................. 162
12.4. dhcp server for RHEL/CentOS ................................................................................................. 162
12.5. client reservations ..................................................................................................................... 163
12.6. example config files .................................................................................................................. 163
12.7. older example config files ........................................................................................................ 164
12.8. advanced dhcp ........................................................................................................................... 166
12.9. Practice: dhcp ............................................................................................................................ 167

159
Chapter 12. introduction to dhcp
Dynamic Host Configuration Protocol (or short dhcp) is a standard tcp/ip protocol that
distributes ip configurations to clients. dhcp is defined in rfc 2131 (before that it was defined
as an update to bootp in rfc 1531/1541.
The alternative to dhcp is manually entering the ip configuration on each client computer.

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160
12.1. four broadcasts
dhcp works with layer 2 broadcasts. A dhcp client that starts, will send a dhcp discover
on the network. All dhcp servers (that have a lease available) will respond with a dhcp
offer. The client will choose one of those offers and will send a dhcp request containing
the chosen offer. The dhcp server usually responds with a dhcp ack(knowledge).
In wireshark it looks like this.
When this procedure is finished, then the client is allowed to use that ip-configuration until
the end of its lease time.

161
12.2. picturing dhcp
Here we have a small network with two dhcp servers named DHCP-SRV1 and DHCP-
SRV2 and two clients (SunWS1 and Mac42). All computers are connected by a hub or switch
(pictured in the middle). All four computers have a cable to the hub (cables not pictured).
1. The client SunWS1 sends a dhcp discover on the network. All computers receive this
broadcast.
2. Both dhcp servers answer with a dhcp offer. DHCP-SRV1 is a dedicated dhcp server
and is faster in sending a dhcp offer than DHCP-SRV2 (who happens to also be a file server).
3. The client chooses the offer from DHCP-SRV1 and sends a dhcp request on the network.
4. DHCP-SRV1 answers with a dhcp ack (short for acknowledge).
All four broadcasts (or five when you count both offers) can be layer 2 ethernet broadcast
to mac address ff:ff:ff:ff:ff:ff and a layer 3 ip broadcast to 255.255.255.255.
The same story can be read in rfc 2131.

162
12.3. installing a dhcp server
dhcp server for Debian/Mint
debian5:~# aptitude install dhcp3-server
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done
dhcp3-server
You get a configuration file with many examples.
debian5:~# ls -l /etc/dhcp3/dhcpd.conf
-rw-r--r-- 1 root root 3551 2011-04-10 21:23 /etc/dhcp3/dhcpd.conf
12.4. dhcp server for RHEL/CentOS
Installing is easy with yum.
[root@rhel71 ~]# yum install dhcp
Loaded plugins: product-id, subscription-manager
Resolving Dependencies
--> Running transaction check
---> Package dhcp.x86_64 12:4.2.5-36.el7 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
================================================================================
Package Arch Version Repository Size
================================================================================
Installing:
dhcp x86_64 12:4.2.5-36.el7 rhel-7-server-rpms 510 k
Transaction Summary
================================================================================
Install 1 Package
Total download size: 510 k
Installed size: 1.4 M
Is this ok [y/d/N]: y
Downloading packages:
dhcp-4.2.5-36.el7.x86_64.rpm | 510 kB 00:01
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
Installing : 12:dhcp-4.2.5-36.el7.x86_64 1/1
Verifying : 12:dhcp-4.2.5-36.el7.x86_64 1/1
Installed:
dhcp.x86_64 12:4.2.5-36.el7
Complete!
[root@rhel71 ~]#
After installing we get a /etc/dhcp/dhcpd.conf that points us to an example file named
dhcpd.conf.sample.

163
[root@rhel71 ~]# cat /etc/dhcp/dhcpd.conf
#
# DHCP Server Configuration file.
# see /usr/share/doc/dhcp*/dhcpd.conf.example
# see dhcpd.conf(5) man page
#
[root@rhel71 ~]#
So we copy the sample and adjust it for our real situation. We name the copy /etc/dhcp/
dhcpd.conf.
[root@rhel71 ~]# cp /usr/share/doc/dhcp-4.2.5/dhcpd.conf.example /etc/dhcp/dhcp
d.conf
[root@rhel71 ~]# vi /etc/dhcp/dhcpd.conf
[root@rhel71 ~]# cat /etc/dhcp/dhcpd.conf
option domain-name "linux-training.be";
option domain-name-servers 10.42.42.42;
default-lease-time 600;
max-lease-time 7200;
log-facility local7;
subnet 10.42.0.0 netmask 255.255.0.0 {
range 10.42.200.11 10.42.200.120;
option routers 10.42.200.1;
}
[root@rhel71 ~]#
The 'routers' option is valid for the subnet alone, whereas the 'domain-name' option is global
(for all subnets).
Time to start the server. Remember to use systemctl start dhcpd on RHEL7/CentOS7 and
service dhcpd start on previous versions of RHEL/CentOS.
[root@rhel71 ~]# systemctl start dhcpd
[root@rhel71 ~]#
12.5. client reservations
You can reserve an ip configuration for a client using the mac address.
host pc42 {
hardware ethernet 11:22:33:44:55:66;
fixed-address 192.168.42.42;
}
You can add individual options to this reservation.
host pc42 {
hardware ethernet 11:22:33:44:55:66;
fixed-address 192.168.42.42;
option domain-name "linux-training.be";
}
12.6. example config files
Below you see several sections of /etc/dhcp/dhcpd.conf on a Debian 6 server.
# NetSec Antwerp Network

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subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.20 192.168.1.199;
option domain-name-servers ns1.netsec.local;
option domain-name "netsec.local";
option broadcast-address 192.168.1.255;
}
Above the general configuration for the network, with a pool of 180 addresses.
Below two client reservations:
#
# laptops
#
host mac {
hardware ethernet 00:26:bb:xx:xx:xx;
fixed-address mac.netsec.local;
}
host vmac {
hardware ethernet 8c:7b:9d:xx:xx:xx;
fixed-address vmac.netsec.local;
}
12.7. older example config files
For dhcpd.conf on Fedora with dynamic updates for a DNS domain.
[root@fedora14 ~]# cat /etc/dhcp/dhcpd.conf
authoritative;
include "/etc/rndc.key";
log-facility local6;
server-identifier fedora14;
ddns-domainname "office.linux-training.be";
ddns-update-style interim;
ddns-updates on;
update-static-leases on;
option domain-name "office.linux-training.be";
option domain-name-servers 192.168.42.100;
option ip-forwarding off;
zone office.linux-training.be {
primary 192.168.42.100;
}
subnet 192.168.4.0 netmask 255.255.255.0 {
range 192.168.4.24 192.168.4.40;
}
Allowing any updates in the zone database (part of the named.conf configuration)
zone "office.linux-training.be" {

165
type master;
file "/var/named/db.office.linux-training.be";
allow-update { any; };
};
Allowing secure key updates in the zone database (part of the named.conf configuration)
zone "office.linux-training.be" {
type master;
file "/var/named/db.office.linux-training.be";
allow-update { key mykey; };
};
Sample key file contents:
[root@fedora14 ~]# cat /etc/rndc.key
key "rndc-key" {
algorithm hmac-md5;
secret "4Ykd58uIeUr3Ve6ad1qTfQ==";
};
Generate your own keys with dnssec-keygen.
How to include a key in a config file:
include "/etc/bind/rndc.key";
Also make sure that bind can write to your db.zone file (using chmod/chown). For Ubuntu
this can be in /etc/bind, for Fedora in /var/named.

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12.8. advanced dhcp
12.8.1. 80/20 rule
DHCP servers should not be a single point of failure. Let us discuss redundant dhcp server
setups.
12.8.2. relay agent
To avoid having to place a dhcp server on every segment, we can use dhcp relay agents.
12.8.3. rogue dhcp servers
Rogue dhcp servers are a problem without a solution. For example accidental connection of
a (believed to be simple) hub/switch to a network with an internal dhcp server.
12.8.4. dhcp and ddns
DHCP can dynamically update DNS when it configures a client computer. DDNS can be
used with or without secure keys.
When set up properly records can be added automaticall to the zone file:
root@fedora14~# tail -2 /var/named/db.office.linux-training.be
ubu1010srv A 192.168.42.151
TXT "00dfbb15e144a273c3cf2d6ae933885782"

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12.9. Practice: dhcp
1. Make sure you have a unique fixed ip address for your DNS and DHCP server (easier
on the same machine).
2. Install DHCP and browse the explanation in the default configuration file /etc/dhcp/
dhcpd.conf or /etc/dhcp3/dhcpd.conf.
3. Decide on a valid scope and activate it.
4. Test with a client that your DHCP server works.
5. Use wireshark to capture the four broadcasts when a client receives an ip (for the first
time).
6. Use wireshark to capture a DHCPNAK and a DHCPrelease.
7. Reserve a configuration for a particular client (using mac address).
8. Configure your DHCP/DNS server(s) with a proper hostname and domainname (/etc/
hosts, /etc/hostname, /etc/sysconfig/network on Fedora/RHEL, /etc/resolv.conf ...). You
may need to disable NetworkManager on *buntu-desktops.
9. Make sure your DNS server still works, and is master over (at least) one domain.
There are several ways to do steps 10-11-12. Google is your friend in exploring DDNS with
keys, with key-files or without keys.
10. Configure your DNS server to allow dynamic updates from your DHCP server.
11. Configure your DHCP server to send dynamic updates to your DNS server.
12. Test the working of Dynamic DNS.

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Table of Contents
13. introduction to routers ........................................................................................................................ 170
13.1. router or firewall ....................................................................................................................... 171
13.2. packet forwarding ..................................................................................................................... 171
13.3. packet filtering .......................................................................................................................... 171
13.4. stateful ....................................................................................................................................... 171
13.5. nat (network address translation) .............................................................................................. 172
13.6. pat (port address translation) .................................................................................................... 172
13.7. snat (source nat) ........................................................................................................................ 172
13.8. masquerading ............................................................................................................................. 172
13.9. dnat (destination nat) ................................................................................................................ 172
13.10. port forwarding ....................................................................................................................... 172
13.11. /proc/sys/net/ipv4/ip_forward .................................................................................................. 173
13.12. /etc/sysctl.conf ......................................................................................................................... 173
13.13. sysctl ........................................................................................................................................ 173
13.14. practice: packet forwarding .................................................................................................... 174
13.15. solution: packet forwarding .................................................................................................... 176
14. iptables firewall .................................................................................................................................... 179
14.1. iptables tables ............................................................................................................................ 180
14.2. starting and stopping iptables ................................................................................................... 180
14.3. the filter table ............................................................................................................................ 181
14.4. practice: packet filtering ........................................................................................................... 186
14.5. solution: packet filtering ........................................................................................................... 187
14.6. network address translation ...................................................................................................... 188

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Chapter 13. introduction to routers
What follows is a very brief introduction to using Linux as a router.

introduction to routers
171
13.1. router or firewall
A router is a device that connects two networks. A firewall is a device that besides acting
as a router, also contains (and implements) rules to determine whether packets are allowed
to travel from one network to another. A firewall can be configured to block access based
on networks, hosts, protocols and ports. Firewalls can also change the contents of packets
while forwarding them.
13.2. packet forwarding
Packet forwarding means allowing packets to go from one network to another. When a
multihomed host is connected to two different networks, and it allows packets to travel from
one network to another through its two network interfaces, it is said to have enabled packet
forwarding.
13.3. packet filtering
Packet filtering is very similar to packet forwarding, but every packet is individually tested
against rules that decide on allowing or dropping the packet. The rules are stored by iptables.
13.4. stateful
A stateful firewall is an advancement over stateless firewalls that inspect every individual
packet. A stateful firewall will keep a table of active connections, and is knowledgeable
enough to recognise when new connections are part of an active session. Linux iptables is
a stateful firewall.

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13.5. nat (network address translation)
A nat device is a router that is also changing the source and/or target ip-address in packets.
It is typically used to connect multiple computers in a private address range (rfc 1918) with
the (public) internet. A nat can hide private addresses from the internet.
It is important to understand that people and vendors do not always use the right term when
referring to a certain type of nat. Be sure you talk about the same thing. We can distuinguish
several types of nat.
13.6. pat (port address translation)
nat often includes pat. A pat device is a router that is also changing the source and/or target
tcp/udp port in packets. pat is Cisco terminology and is used by snat, dnat, masquerading
and port forwarding in Linux. RFC 3022 calls it NAPT and defines the nat/pat combo as
"traditional nat". A device sold to you as a nat-device will probably do nat and pat.
13.7. snat (source nat)
A snat device is changing the source ip-address when a packet passes our nat. snat
configuration with iptables includes a fixed target source address.
13.8. masquerading
Masquerading is a form of snat that will hide the (private) source ip-addresses of your
private network using a public ip-address. Masquerading is common on dynamic internet
interfaces (broadband modem/routers). Masquerade configuration with iptables uses a
dynamic target source address.
13.9. dnat (destination nat)
A dnat device is changing the destination ip-address when a packet passes our nat.
13.10. port forwarding
When static dnat is set up in a way that allows outside connections to enter our private
network, then we call it port forwarding.

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13.11. /proc/sys/net/ipv4/ip_forward
Whether a host is forwarding packets is defined in /proc/sys/net/ipv4/ip_forward. The
following screenshot shows how to enable packet forwarding on Linux.
root@router~# echo 1 > /proc/sys/net/ipv4/ip_forward
The next command shows how to disable packet forwarding.
root@router~# echo 0 > /proc/sys/net/ipv4/ip_forward
Use cat to check if packet forwarding is enabled.
root@router~# cat /proc/sys/net/ipv4/ip_forward
13.12. /etc/sysctl.conf
By default, most Linux computers are not configured for automatic packet forwarding.
To enable packet forwarding whenever the system starts, change the net.ipv4.ip_forward
variable in /etc/sysctl.conf to the value 1.
root@router~# grep ip_forward /etc/sysctl.conf
net.ipv4.ip_forward = 0
13.13. sysctl
For more information, take a look at the man page of sysctl.
root@debian6~# man sysctl
root@debian6~# sysctl -a 2>/dev/null | grep ip_forward

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13.14. practice: packet forwarding
0. You have the option to select (or create) an internal network when adding a network card
in VirtualBox or VMWare. Use this option to create two internal networks. I named them
leftnet and rightnet, but you can choose any other name.
1. Set up two Linux machines, one on leftnet, the other on rightnet. Make sure they both
get an ip-address in the correct subnet. These two machines will be 'left' and 'right' from
the 'router'.
2. Set up a third Linux computer with three network cards, one on leftnet, the other on
rightnet. This computer will be the 'router'. Complete the table below with the relevant
names, ip-addresses and mac-addresses.
Table 13.1. Packet Forwarding Exercise
leftnet computer the router rightnet computer
MAC
IP
3. How can you verify whether the router will allow packet forwarding by default or not ?
Test that you can ping from the router to the two other machines, and from those two
machines to the router. Use arp -a to make sure you are connected with the correct mac
addresses.

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4. Ping from the leftnet computer to the rightnet computer. Enable and/or disable packet
forwarding on the router and verify what happens to the ping between the two networks. If
you do not succeed in pinging between the two networks (on different subnets), then use a
sniffer like wireshark or tcpdump to discover the problem.
5. Use wireshark or tcpdump -xx to answer the following questions. Does the source MAC
change when a packet passes through the filter ? And the destination MAC ? What about
source and destination IP-addresses ?
6. Remember the third network card on the router ? Connect this card to a LAN with internet
connection. On many LAN's the command dhclient eth0 just works (replace eth0 with the
correct interface).
root@router~# dhclient eth0
You now have a setup similar to this picture. What needs to be done to give internet access
to leftnet and rightnet.

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13.15. solution: packet forwarding
1. Set up two Linux machines, one on leftnet, the other on rightnet. Make sure they both
get an ip-address in the correct subnet. These two machines will be 'left' and 'right' from
the 'router'.
The ip configuration on your computers should be similar to the following two screenshots.
Both machines must be in a different subnet (here 192.168.60.0/24 and 192.168.70.0/24). I
created a little script on both machines to configure the interfaces.
root@left~# cat leftnet.sh
pkill dhclient
ifconfig eth0 192.168.60.8 netmask 255.255.255.0
root@right~# cat rightnet.sh
pkill dhclient
2. Set up a third Linux computer with three network cards, one on leftnet, the other on
rightnet. This computer will be the 'router'. Complete the table below with the relevant
names, ip-addresses and mac-addresses.
root@router~# cat router.sh
#echo 1 > /proc/sys/net/ipv4/ip_forward
Your setup may use different ip and mac addresses than the ones in the table below.
Table 13.2. Packet Forwarding Solution
leftnet computer the router rightnet computer
08:00:27:f6:ab:b9 08:00:27:43:1f:5a 08:00:27:be:4a:6b 08:00:27:14:8b:17
192.168.60.8 192.168.60.1 192.168.70.1 192.168.70.9

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3. How can you verify whether the router will allow packet forwarding by default or not ?
Test that you can ping from the router to the two other machines, and from those two
machines to the router. Use arp -a to make sure you are connected with the correct mac
addresses.
This can be done with "grep ip_forward /etc/sysctl.conf" (1 is enabled, 0 is disabled) or
with sysctl -a | grep ip_for.
root@router~# grep ip_for /etc/sysctl.conf
4. Ping from the leftnet computer to the rightnet computer. Enable and/or disable packet
forwarding on the router and verify what happens to the ping between the two networks. If
you do not succeed in pinging between the two networks (on different subnets), then use a
sniffer like wireshark or tcpdump to discover the problem.
Did you forget to add a default gateway to the LAN machines ? Use route add default
gw 'ip-address'.
root@left~# route add default gw 192.168.60.1
root@right~# route add default gw 192.168.70.1
You should be able to ping when packet forwarding is enabled (and both default gateways
are properly configured). The ping will not work when packet forwarding is disabled or
when gateways are not configured correctly.
5. Use wireshark or tcpdump -xx to answer the following questions. Does the source MAC
change when a packet passes through the filter ? And the destination MAC ? What about
source and destination IP-addresses ?
Both MAC addresses are changed when passing the router. Use tcpdump -xx like this:
root@router~# tcpdump -xx -i eth1
root@router~# tcpdump -xx -i eth2

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6. Remember the third network card on the router ? Connect this card to a LAN with internet
connection. On many LAN's the command dhclient eth0 just works (replace eth0 with the
correct interface.
root@router~# dhclient eth0
You now have a setup similar to this picture. What needs to be done to give internet access
to leftnet and rightnet.
The clients on leftnet and rightnet need a working dns server. We use one of Google's
dns servers here.
echo nameserver 8.8.8.8 > /etc/resolv.conf

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Chapter 14. iptables firewall
This chapter introduces some simple firewall rules and how to configure them with iptables.
iptables is an application that allows a user to configure the firewall functionality built into
the Linux kernel.

iptables firewall
180
14.1. iptables tables
By default there are three tables in the kernel that contain sets of rules.
The filter table is used for packet filtering.
root@debian6~# iptables -t filter -L
The nat table is used for address translation.
root@debian6~# iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
Chain POSTROUTING (policy ACCEPT)
The mangle table can be used for special-purpose processing of packets.
Series of rules in each table are called a chain. We will discuss chains and the nat table
later in this chapter.
14.2. starting and stopping iptables
The following screenshot shows how to stop and start iptables on Red Hat/Fedora/CentOS
and compatible distributions.
[root@centos6 ~]# service iptables stop
[root@centos6 ~]# service iptables start
iptables: Applying firewall rules [ ok ]
[root@centos6 ~]#
Debian and *buntu distributions do not have this script, but allow for an uninstall.
root@debian6~# aptitude purge iptables

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14.3. the filter table
14.3.1. about packet filtering
Packet filtering is a bit more than packet forwarding. While packet forwarding uses only
a routing table to make decisions, packet filtering also uses a list of rules. The kernel will
inspect packets and decide based on these rules what to do with each packet.
14.3.2. filter table
The filter table in iptables has three chains (sets of rules). The INPUT chain is used for any
packet coming into the system. The OUTPUT chain is for any packet leaving the system.
And the FORWARD chain is for packets that are forwarded (routed) through the system.
The screenshot below shows how to list the filter table and all its rules.
[root@RHEL5 ~]# iptables -t filter -nL
[root@RHEL5 ~]#
As you can see, all three chains in the filter table are set to ACCEPT everything. ACCEPT
is the default behaviour.

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14.3.3. setting default rules
The default for the default rule is indeed to ACCEPT everything. This is not the most secure
firewall.
A more secure setup would be to DROP everything. A package that is dropped will not
continue in any chain, and no warning or error will be sent anywhere.
The below commands lock down a computer. Do not execute these commands inside a
remote ssh shell.
root@debianpaul~# iptables -P INPUT DROP
root@debianpaul~# iptables -P OUTPUT DROP
root@debianpaul~# iptables -P FORWARD DROP
root@debianpaul~# iptables -L
Chain INPUT (policy DROP)
Chain FORWARD (policy DROP)
Chain OUTPUT (policy DROP)
14.3.4. changing policy rules
To start, let's set the default policy for all three chains to drop everything. Note that you
might lose your connection when typing this over ssh ;-).
[root@RHEL5 ~]# iptables -P INPUT DROP
[root@RHEL5 ~]# iptables -P FORWARD DROP
[root@RHEL5 ~]# iptables -P OUTPUT DROP
Next, we allow the server to use its own loopback device (this allows the server to access
its services running on localhost). We first append a rule to the INPUT chain to allow
(ACCEPT) traffic from the lo (loopback) interface, then we do the same to allow packets to
leave the system through the loopback interface.
[root@RHEL5 ~]# iptables -A INPUT -i lo -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o lo -j ACCEPT
Looking at the filter table again (omitting -t filter because it is the default table).
[root@RHEL5 ~]# iptables -nL
Chain INPUT (policy DROP)
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
Chain FORWARD (policy DROP)
Chain OUTPUT (policy DROP)
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0

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14.3.5. Allowing ssh over eth0
This example show how to add two rules to allow ssh access to your system from outside.
[root@RHEL5 ~]# iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT
The filter table will look something like this screenshot (note that -v is added for more
verbose output).
[root@RHEL5 ~]# iptables -nvL
Chain INPUT (policy DROP 7 packets, 609 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
Chain FORWARD (policy DROP 0 packets, 0 bytes)
Chain OUTPUT (policy DROP 3 packets, 228 bytes)
0 0 ACCEPT all -- * lo 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- * eth0 0.0.0.0/0 0.0.0.0/0 tcp spt:22
[root@RHEL5 ~]#
14.3.6. Allowing access from a subnet
This example shows how to allow access from any computer in the 10.1.1.0/24 network, but
only through eth1. There is no port (application) limitation here.
[root@RHEL5 ~]# iptables -A INPUT -i eth1 -s 10.1.1.0/24 -p tcp -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o eth1 -d 10.1.1.0/24 -p tcp -j ACCEPT
Together with the previous examples, the policy is expanding.
[root@RHEL5 ~]# iptables -nvL
Chain INPUT (policy DROP 7 packets, 609 bytes)
0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
0 0 ACCEPT tcp -- eth1 * 10.1.1.0/24 0.0.0.0/0
Chain FORWARD (policy DROP 0 packets, 0 bytes)
Chain OUTPUT (policy DROP 3 packets, 228 bytes)
0 0 ACCEPT all -- * lo 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- * eth0 0.0.0.0/0 0.0.0.0/0 tcp spt:22
0 0 ACCEPT tcp -- * eth1 0.0.0.0/0 10.1.1.0/24

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14.3.7. iptables save
Use iptables save to automatically implement these rules when the firewall is (re)started.
[root@RHEL5 ~]# /etc/init.d/iptables save
Saving firewall rules to /etc/sysconfig/iptables: [ OK ]
[root@RHEL5 ~]#
14.3.8. scripting example
You can write a simple script for these rules. Below is an example script that implements
the firewall rules that you saw before in this chapter.
#!/bin/bash
# first cleanup everything
iptables -t filter -F
iptables -t filter -X
iptables -t nat -F
iptables -t nat -X
# default drop
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP
# allow loopback device
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
# allow ssh over eth0 from outside to system
iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT
# allow any traffic from 10.1.1.0/24 to system
iptables -A INPUT -i eth1 -s 10.1.1.0/24 -p tcp -j ACCEPT
iptables -A OUTPUT -o eth1 -d 10.1.1.0/24 -p tcp -j ACCEPT

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14.3.9. Allowing ICMP(ping)
When you enable iptables, you will get an 'Operation not permitted' message when trying
to ping other hosts.
[root@RHEL5 ~# ping 192.168.187.130
ping: sendmsg: Operation not permitted
ping: sendmsg: Operation not permitted
The screenshot below shows you how to setup iptables to allow a ping from or to your
machine.
[root@RHEL5 ~]# iptables -A INPUT -p icmp --icmp-type any -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -p icmp --icmp-type any -j ACCEPT
The previous two lines do not allow other computers to route ping messages through your
router, because it only handles INPUT and OUTPUT. For routing of ping, you will need
to enable it on the FORWARD chain. The following command enables routing of icmp
messages between networks.
[root@RHEL5 ~]# iptables -A FORWARD -p icmp --icmp-type any -j ACCEPT

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14.4. practice: packet filtering
1. Make sure you can ssh to your router-system when iptables is active.
2. Make sure you can ping to your router-system when iptables is active.
3. Define one of your networks as 'internal' and the other as 'external'. Configure the router
to allow visits to a website (http) to go from the internal network to the external network
(but not in the other direction).
4. Make sure the internal network can ssh to the external, but not the other way around.

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14.5. solution: packet filtering
A possible solution, where leftnet is the internal and rightnet is the external network.
#!/bin/bash
# first cleanup everything
iptables -t filter -F
iptables -t filter -X
iptables -t nat -F
iptables -t nat -X
# default drop
iptables -P INPUT DROP
iptables -P OUTPUT DROP
# allow loopback device
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
# question 1: allow ssh over eth0
iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT
# question 2: Allow icmp(ping) anywhere
iptables -A INPUT -p icmp --icmp-type any -j ACCEPT
iptables -A FORWARD -p icmp --icmp-type any -j ACCEPT
iptables -A OUTPUT -p icmp --icmp-type any -j ACCEPT
# question 3: allow http from internal(leftnet) to external(rightnet)
iptables -A FORWARD -i eth1 -o eth2 -p tcp --dport 80 -j ACCEPT
iptables -A FORWARD -i eth2 -o eth1 -p tcp --sport 80 -j ACCEPT
# question 4: allow ssh from internal(leftnet) to external(rightnet)
iptables -A FORWARD -i eth2 -o eth1 -p tcp --sport 22 -j ACCEPT
# allow http from external(rightnet) to internal(leftnet)
# iptables -A FORWARD -i eth2 -o eth1 -p tcp --dport 80 -j ACCEPT
# iptables -A FORWARD -i eth1 -o eth2 -p tcp --sport 80 -j ACCEPT
# allow rpcinfo over eth0 from outside to system
# iptables -A INPUT -i eth2 -p tcp --dport 111 -j ACCEPT
# iptables -A OUTPUT -o eth2 -p tcp --sport 111 -j ACCEPT

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14.6. network address translation
14.6.1. about NAT
A NAT device is a router that is also changing the source and/or target ip-address in packets.
It is typically used to connect multiple computers in a private address range with the (public)
internet. A NAT can hide private addresses from the internet.
NAT was developed to mitigate the use of real ip addresses, to allow private address ranges
to reach the internet and back, and to not disclose details about internal networks to the
outside.
The nat table in iptables adds two new chains. PREROUTING allows altering of packets
before they reach the INPUT chain. POSTROUTING allows altering packets after they exit
the OUTPUT chain.
Use iptables -t nat -nvL to look at the NAT table. The screenshot below shows an empty
NAT table.
[root@RHEL5 ~]# iptables -t nat -nL
Chain PREROUTING (policy ACCEPT)
Chain POSTROUTING (policy ACCEPT)
[root@RHEL5 ~]#

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14.6.2. SNAT (Source NAT)
The goal of source nat is to change the source address inside a packet before it leaves the
system (e.g. to the internet). The destination will return the packet to the NAT-device. This
means our NAT-device will need to keep a table in memory of all the packets it changed, so
it can deliver the packet to the original source (e.g. in the private network).
Because SNAT is about packets leaving the system, it uses the POSTROUTING chain.
Here is an example SNAT rule. The rule says that packets coming from 10.1.1.0/24 network
and exiting via eth1 will get the source ip-address set to 11.12.13.14. (Note that this is a
one line command!)
iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j SNAT
--to-source 11.12.13.14
Of course there must exist a proper iptables filter setup to allow the packet to traverse from
one network to the other.
14.6.3. SNAT example setup
This example script uses a typical nat setup. The internal (eth0) network has access via
SNAT to external (eth1) webservers (port 80).
#!/bin/bash
#
# iptables script for simple classic nat websurfing
# eth0 is internal network, eth1 is internet
#
echo 0 > /proc/sys/net/ipv4/ip_forward
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -A FORWARD -i eth0 -o eth1 -s 10.1.1.0/24 -p tcp
--dport 80 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -d 10.1.1.0/24 -p tcp
--sport 80 -j ACCEPT
iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j SNAT
--to-source 11.12.13.14

iptables firewall
190
14.6.4. IP masquerading
IP masquerading is very similar to SNAT, but is meant for dynamic interfaces. Typical
example are broadband 'router/modems' connected to the internet and receiving a different
ip-address from the isp, each time they are cold-booted.
The only change needed to convert the SNAT script to a masquerading is one line.
iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j MASQUERADE
14.6.5. DNAT (Destination NAT)
DNAT is typically used to allow packets from the internet to be redirected to an internal
server (in your DMZ) and in a private address range that is inaccessible directly form the
internet.
This example script allows internet users to reach your internal (192.168.1.99) server via
ssh (port 22).
#!/bin/bash
#
# iptables script for DNAT
# eth0 is internal network, eth1 is internet
#
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -A FORWARD -i eth0 -o eth1 -s 10.1.1.0/24 -j ACCEPT
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 22
-j DNAT --to-destination 10.1.1.99

Part VI. Introduction to Samba

192
Table of Contents
15. introduction to samba ......................................................................................................................... 194
15.1. verify installed version ............................................................................................................. 195
15.2. installing samba ......................................................................................................................... 196
15.3. documentation ........................................................................................................................... 197
15.4. starting and stopping samba ..................................................................................................... 198
15.5. samba daemons ......................................................................................................................... 199
15.6. the SMB protocol ...................................................................................................................... 200
15.7. practice: introduction to samba ................................................................................................. 201
16. getting started with samba ................................................................................................................. 202
16.1. /etc/samba/smb.conf .................................................................................................................. 203
16.2. /usr/bin/testparm ........................................................................................................................ 204
16.3. /usr/bin/smbclient ....................................................................................................................... 205
16.4. /usr/bin/smbtree ......................................................................................................................... 207
16.5. server string ............................................................................................................................... 208
16.6. Samba Web Administration Tool (SWAT) .............................................................................. 209
16.7. practice: getting started with samba ......................................................................................... 210
16.8. solution: getting started with samba ......................................................................................... 211
17. a read only file server ......................................................................................................................... 213
17.1. Setting up a directory to share ................................................................................................. 214
17.2. configure the share .................................................................................................................... 214
17.3. restart the server ........................................................................................................................ 215
17.4. verify the share ......................................................................................................................... 215
17.5. a note on netcat ......................................................................................................................... 217
17.6. practice: read only file server ................................................................................................... 218
17.7. solution: read only file server ................................................................................................... 219
18. a writable file server ........................................................................................................................... 220
18.1. set up a directory to share ........................................................................................................ 221
18.2. share section in smb.conf ......................................................................................................... 221
18.3. configure the share .................................................................................................................... 221
18.4. test connection with windows .................................................................................................. 221
18.5. test writing with windows ........................................................................................................ 222
18.6. How is this possible ? ............................................................................................................... 222
18.7. practice: writable file server ..................................................................................................... 223
18.8. solution: writable file server ..................................................................................................... 224
19. samba first user account ..................................................................................................................... 225
19.1. creating a samba user ............................................................................................................... 226
19.2. ownership of files ..................................................................................................................... 226
19.3. /usr/bin/smbpasswd .................................................................................................................... 226
19.4. /etc/samba/smbpasswd ............................................................................................................... 226
19.5. passdb backend ......................................................................................................................... 227
19.6. forcing this user ........................................................................................................................ 227
19.7. practice: first samba user account ............................................................................................ 228
19.8. solution: first samba user account ............................................................................................ 229
20. samba authentication .......................................................................................................................... 230
20.1. creating the users on Linux ...................................................................................................... 231
20.2. creating the users on samba ...................................................................................................... 231
20.3. security = user ........................................................................................................................... 231
20.4. configuring the share ................................................................................................................ 232
20.5. testing access with net use ........................................................................................................ 232
20.6. testing access with smbclient .................................................................................................... 232
20.7. verify ownership ....................................................................................................................... 233
20.8. common problems ..................................................................................................................... 233
20.9. practice : samba authentication ................................................................................................. 235
20.10. solution: samba authentication ................................................................................................ 236
21. samba securing shares ........................................................................................................................ 237

Introduction to Samba
193
21.1. security based on user name ..................................................................................................... 238
21.2. security based on ip-address ..................................................................................................... 238
21.3. security through obscurity ........................................................................................................ 239
21.4. file system security ................................................................................................................... 239
21.5. practice: securing shares ........................................................................................................... 241
21.6. solution: securing shares ........................................................................................................... 242
22. samba domain member ....................................................................................................................... 244
22.1. changes in smb.conf .................................................................................................................. 245
22.2. joining an Active Directory domain ......................................................................................... 246
22.3. winbind ...................................................................................................................................... 247
22.4. wbinfo ........................................................................................................................................ 247
22.5. getent ......................................................................................................................................... 248
22.6. file ownership ............................................................................................................................ 249
22.7. practice : samba domain member ............................................................................................. 250
23. samba domain controller .................................................................................................................... 251
23.1. about Domain Controllers ......................................................................................................... 252
23.2. About security modes ............................................................................................................... 252
23.3. About password backends ........................................................................................................ 253
23.4. [global] section in smb.conf ..................................................................................................... 253
23.5. netlogon share ........................................................................................................................... 254
23.6. other [share] sections ................................................................................................................ 254
23.7. Users and Groups ...................................................................................................................... 255
23.8. tdbsam ....................................................................................................................................... 255
23.9. about computer accounts .......................................................................................................... 256
23.10. local or roaming profiles ........................................................................................................ 256
23.11. Groups in NTFS acls .............................................................................................................. 257
23.12. logon scripts ............................................................................................................................ 258
23.13. practice: samba domain controller .......................................................................................... 259
24. a brief look at samba 4 ....................................................................................................................... 260
24.1. Samba 4 alpha 6 ....................................................................................................................... 262

194
Chapter 15. introduction to samba
This introduction to the Samba server simply explains how to install Samba 3 and briefly
mentions the SMB protocol.

introduction to samba
195
15.1. verify installed version
15.1.1. .rpm based distributions
To see the version of samba installed on Red Hat, Fedora or CentOS use rpm -q samba.
[root@RHEL52 ~]# rpm -q samba
samba-3.0.28-1.el5_2.1
The screenshot above shows that RHEL5 has Samba version 3.0 installed. The last number
in the Samba version counts the number of updates or patches.
Below the same command on a more recent version of CentOS with Samba version 3.5
installed.
[root@centos6 ~]# rpm -q samba
samba-3.5.10-116.el6_2.i686
15.1.2. .deb based distributions
Use dpkg -l or aptitide show on Debian or Ubuntu. Both Debian 7.0 (Wheezy) and Ubuntu
12.04 (Precise) use version 3.6.3 of the Samba server.
root@debian7~# aptitude show samba | grep Version
Version: 2:3.6.3-1
Ubuntu 12.04 is currently at Samba version 3.6.3.
root@ubu1204:~# dpkg -l samba | tail -1
ii samba 2:3.6.3-2ubuntu2.1 SMB/CIFS file, print, and login server for Unix

196
15.2. installing samba
15.2.1. .rpm based distributions
Samba is installed by default on Red Hat Enterprise Linux. If Samba is not yet installed,
then you can use the graphical menu (Applications -- System Settings -- Add/Remove
Applications) and select "Windows File Server" in the Server section. The non-graphical
way is to use rpm or yum.
When you downloaded the .rpm file, you can install Samba like this.
[paul@RHEL52 ~]$ rpm -i samba-3.0.28-1.el5_2.1.rpm
When you have a subscription to RHN (Red Hat Network), then yum is an easy tool to use.
This yum command works by default on Fedora and CentOS.
[root@centos6 ~]# yum install samba
15.2.2. .deb based distributions
Ubuntu and Debian users can use the aptitude program (or use a graphical tool like
Synaptic).
root@debian7~# aptitude install samba
samba samba-common{a} samba-common-bin{a} tdb-tools{a}
Need to get 15.1 MB of archives. After unpacking 42.9 MB will be used.
...

197
15.3. documentation
15.3.1. samba howto
Samba comes with excellent documentation in html and pdf format (and also as a free
download from samba.org and it is for sale as a printed book).
The documentation is a separate package, so install it if you want it on the server itself.
[root@centos6 ~]# yum install samba-doc
...
[root@centos6 ~]# ls -l /usr/share/doc/samba-doc-3.5.10/
total 10916
drwxr-xr-x. 6 root root 4096 May 6 15:50 htmldocs
-rw-r--r--. 1 root root 4605496 Jun 14 2011 Samba3-ByExample.pdf
-rw-r--r--. 1 root root 608260 Jun 14 2011 Samba3-Developers-Guide.pdf
-rw-r--r--. 1 root root 5954602 Jun 14 2011 Samba3-HOWTO.pdf
This action is very similar on Ubuntu and Debian except that the pdf files are in a separate
package named samba-doc-pdf.
root@ubu1204:~# aptitude install samba-doc-pdf
samba-doc-pdf
...
15.3.2. samba by example
Besides the howto, there is also an excellent book called Samba By Example (again
available as printed edition in shops, and as a free pdf and html).

198
15.4. starting and stopping samba
You can start the daemons by invoking /etc/init.d/smb start (some systems use /etc/init.d/
samba) on any linux.
root@laika:~# /etc/init.d/samba stop
* Stopping Samba daemons [ OK ]
root@laika:~# /etc/init.d/samba start
* Starting Samba daemons [ OK ]
root@laika:~# /etc/init.d/samba restart
* Stopping Samba daemons [ OK ]
* Starting Samba daemons [ OK ]
root@laika:~# /etc/init.d/samba status
* SMBD is running [ OK ]
Red Hat derived systems are happy with service smb start.
[root@RHEL4b ~]# /etc/init.d/smb start
Starting SMB services: [ OK ]
Starting NMB services: [ OK ]
[root@RHEL4b ~]# service smb restart
Shutting down SMB services: [ OK ]
Shutting down NMB services: [ OK ]
[root@RHEL4b ~]#

199
15.5. samba daemons
Samba 3 consists of three daemons, they are named nmbd, smbd and winbindd.
15.5.1. nmbd
The nmbd daemon takes care of all the names and naming. It registers and resolves names,
and handles browsing. According to the Samba documentation, it should be the first daemon
to start.
[root@RHEL52 ~]# ps -C nmbd
PID TTY TIME CMD
5681 ? 00:00:00 nmbd
15.5.2. smbd
The smbd daemon manages file transfers and authentication.
[root@RHEL52 ~]# ps -C smbd
PID TTY TIME CMD
5678 ? 00:00:00 smbd
5683 ? 00:00:00 smbd
15.5.3. winbindd
The winbind daemon (winbindd) is only started to handle Microsoft Windows domain
membership.
Note that winbindd is started by the /etc/init.d/winbind script (two dd's for the daemon and
only one d for the script).
[root@RHEL52 ~]# /etc/init.d/winbind start
Starting Winbind services: [ OK ]
[root@RHEL52 ~]# ps -C winbindd
PID TTY TIME CMD
5752 ? 00:00:00 winbindd
5754 ? 00:00:00 winbindd
On Debian and Ubuntu, the winbindd daemon is installed via a separate package called
winbind.

200
15.6. the SMB protocol
15.6.1. brief history
Development of this protocol was started by IBM in the early eighties. By the end of the
eighties, most develpment was done by Microsoft. SMB is an application level protocol
designed to run on top of NetBIOS/NetBEUI, but can also be run on top of tcp/ip.
In 1996 Microsoft was asked to document the protocol. They submitted CIFS (Common
Internet File System) as an internet draft, but it never got final rfc status.
In 2004 the European Union decided Microsoft should document the protocol to enable
other developers to write compatible software. December 20th 2007 Microsoft came to an
agreement. The Samba team now has access to SMB/CIFS, Windows for Workgroups and
Active Directory documentation.
15.6.2. broadcasting protocol
SMB uses the NetBIOS service location protocol, which is a broadcasting protocol. This
means that NetBIOS names have to be unique on the network (even when you have
different IP-addresses). Having duplicate names on an SMB network can seriously harm
communications.
15.6.3. NetBIOS names
NetBIOS names are similar to hostnames, but are always uppercase and only 15 characters
in length. Microsoft Windows computers and Samba servers will broadcast this name on
the network.
15.6.4. network bandwidth
Having many broadcasting SMB/CIFS computers on your network can cause bandwidth
issues. A solution can be the use of a NetBIOS name server (NBNS) like WINS (Windows
Internet Naming Service).

201
15.7. practice: introduction to samba
0. !! Make sure you know your student number, anything *ANYTHING* you name must
include your student number!
1. Verify that you can logon to a Linux/Unix computer. Write down the name and ip address
of this computer.
2. Do the same for all the other (virtual) machines available to you.
3. Verify networking by pinging the computer, edit the appropriate hosts files so you can
use names. Test the names by pinging them.
4. Make sure Samba is installed, write down the version of Samba.
5. Open the Official Samba-3 howto pdf file that is installed on your computer. How many
A4 pages is this file ? Then look at the same pdf on samba.org, it is updated regularly.
6. Stop the Samba server.

202
Chapter 16. getting started with
samba

getting started with samba
203
16.1. /etc/samba/smb.conf
16.1.1. smbd -b
Samba configuration is done in the smb.conf file. The file can be edited manually, or you
can use a web based interface like webmin or swat to manage it. The file is usually located
in /etc/samba. You can find the exact location with smbd -b.
[root@RHEL4b ~]# smbd -b | grep CONFIGFILE
CONFIGFILE: /etc/samba/smb.conf
16.1.2. the default smb.conf
The default smb.conf file contains a lot of examples with explanations.
[paul@RHEL4b ~]$ ls -l /etc/samba/smb.conf
-rw-r--r-- 1 root root 10836 May 30 23:08 /etc/samba/smb.conf
Also on Ubuntu and Debian, smb.conf is packed with samples and explanations.
paul@laika:~$ ls -l /etc/samba/smb.conf
-rw-r--r-- 1 root root 10515 2007-05-24 00:21 /etc/samba/smb.conf
16.1.3. minimal smb.conf
Below is an example of a very minimalistic smb.conf. It allows samba to start, and to be
visible to other computers (Microsoft shows computers in Network Neighborhood or My
Network Places).
[paul@RHEL4b ~]$ cat /etc/samba/smb.conf
[global]
workgroup = WORKGROUP
[firstshare]
path = /srv/samba/public
16.1.4. net view
Below is a screenshot of the net view command on Microsoft Windows Server 2003 sp2.
It shows how a Red Hat Enterprise Linux 5.3 and a Ubuntu 9.04 Samba server, both with a
minimalistic smb.conf, are visible to Microsoft computers nearby.
C:Documents and SettingsAdministrator>net view
Server Name Remark
----------------------------------------------------------------------
LAIKA Samba 3.3.2
RHEL53 Samba 3.0.33-3.7.el5
W2003
The command completed successfully.
16.1.5. long lines in smb.conf
Some parameters in smb.conf can get a long list of values behind them. You can continue a
line (for clarity) on the next by ending the line with a backslash.
valid users = Serena, Venus, Lindsay

204
Kim, Justine, Sabine
Amelie, Marie, Suzanne
16.1.6. curious smb.conf
Curious but true: smb.conf accepts synonyms like create mode and create mask, and
(sometimes) minor spelling errors like browsable and browseable. And on occasion you
can even switch words, the guest only parameter is identical to only guest. And writable
= yes is the same as readonly = no.
16.1.7. man smb.conf
You can access a lot of documentation when typing man smb.conf.
[root@RHEL4b samba]# apropos samba
cupsaddsmb (8) - export printers to samba for windows clients
lmhosts (5) - The Samba NetBIOS hosts file
net (8) - Tool for administration of Samba and remote CIFS servers
pdbedit (8) - manage the SAM database (Database of Samba Users)
samba (7) - A Windows SMB/CIFS fileserver for UNIX
smb.conf [smb] (5) - The configuration file for the Samba suite
smbpasswd (5) - The Samba encrypted password file
smbstatus (1) - report on current Samba connections
swat (8) - Samba Web Administration Tool
tdbbackup (8) - tool for backing up and ... of samba .tdb files
[root@RHEL4b samba]#
16.2. /usr/bin/testparm
16.2.1. syntax check smb.conf
To verify the syntax of the smb.conf file, you can use testparm.
[paul@RHEL4b ~]$ testparm
Load smb config files from /etc/samba/smb.conf
Processing section "[firstshare]"
Loaded services file OK.
Server role: ROLE_STANDALONE
Press enter to see a dump of your service definitions
16.2.2. testparm -v
An interesting option is testparm -v, which will output all the global options with their
default value.
[root@RHEL52 ~]# testparm -v | head
Processing section "[pub0]"
Processing section "[global$]"
[global]
dos charset = CP850
unix charset = UTF-8
display charset = LOCALE

205
realm =
netbios name = TEACHER0
netbios aliases =
netbios scope =
server string = Samba 3.0.28-1.el5_2.1
...
There were about 350 default values for smb.conf parameters in Samba 3.0.x. This number
grew to almost 400 in Samba 3.5.x.
16.2.3. testparm -s
The samba daemons are constantly (once every 60 seconds) checking the smb.conf file, so it
is good practice to keep this file small. But it is also good practice to document your samba
configuration, and to explicitly set options that have the same default values. The testparm
-s option allows you to do both. It will output the smallest possible samba configuration file,
while retaining all your settings. The idea is to have your samba configuration in another
file (like smb.conf.full) and let testparm parse this for you. The screenshot below shows you
how. First the smb.conf.full file with the explicitly set option workgroup to WORKGROUP.
[root@RHEL4b samba]# cat smb.conf.full
[global]
# This is a demo of a documented smb.conf
# These two lines are removed by testparm -s
server string = Public Test Server
[firstshare]
Next, we execute testparm with the -s option, and redirect stdout to the real smb.conf file.
[root@RHEL4b samba]# testparm -s smb.conf.full > smb.conf
Load smb config files from smb.conf.full
Processing section "[firstshare]"
And below is the end result. The two comment lines and the default option are no longer
there.
[root@RHEL4b samba]# cat smb.conf
# Global parameters
[global]
server string = Public Test Server
[firstshare]
16.3. /usr/bin/smbclient
16.3.1. smbclient looking at Samba
With smbclient you can see browsing and share information from your smb server. It will
display all your shares, your workgroup, and the name of the Master Browser. The -N switch

206
is added to avoid having to enter an empty password. The -L switch is followed by the name
of the host to check.
[root@RHEL4b init.d]# smbclient -NL rhel4b
Anonymous login successful
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.10-1.4E.9]
Sharename Type Comment
--------- ---- -------
firstshare Disk
IPC$ IPC IPC Service (Public Test Server)
ADMIN$ IPC IPC Service (Public Test Server)
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.10-1.4E.9]
Server Comment
--------- -------
RHEL4B Public Test Server
WINXP
Workgroup Master
--------- -------
WORKGROUP WINXP
16.3.2. smbclient anonymous
The screenshot below uses smbclient to display information about a remote smb server (in
this case a computer with Ubuntu 11.10).
root@ubu1110:/etc/samba# testparm smbclient -NL 127.0.0.1
Domain=[LINUXTR] OS=[Unix] Server=[Samba 3.5.11]
--------- ---- -------
share1 Disk
IPC$ IPC IPC Service (Samba 3.5.11)
Domain=[LINUXTR] OS=[Unix] Server=[Samba 3.5.11]
Server Comment
--------- -------
Workgroup Master
--------- -------
LINUXTR DEBIAN6
WORKGROUP UBU1110
16.3.3. smbclient with credentials
Windows versions after xp sp2 and 2003 sp1 do not accept guest access (the
NT_STATUS_ACCESS_DENIED error). This example shows how to provide credentials
with smbclient.
[paul@RHEL53 ~]$ smbclient -L w2003 -U administrator%stargate
Domain=[W2003] OS=[Windows Server 2003 3790 Service Pack 2] Server=...
--------- ---- -------
C$ Disk Default share

207
IPC$ IPC Remote IPC
ADMIN$ Disk Remote Admin
...
16.4. /usr/bin/smbtree
Another useful tool to troubleshoot Samba or simply to browse the SMB network is smbtree.
In its simplest form, smbtree will do an anonymous browsing on the local subnet. displaying
all SMB computers and (if authorized) their shares.
Let's take a look at two screenshots of smbtree in action (with blank password). The first
one is taken immediately after booting four different computers (one MS Windows 2000,
one MS Windows xp, one MS Windows 2003 and one RHEL 4 with Samba 3.0.10).
[paul@RHEL4b ~]$ smbtree
Password:
WORKGROUP
PEGASUS
WINXP
RHEL4B Pegasus Domain Member Server
Error connecting to 127.0.0.1 (Connection refused)
cli_full_connection: failed to connect to RHEL4B<20> (127.0.0.1)
HM2003
[paul@RHEL4b ~]$
The information displayed in the previous screenshot looks incomplete. The browsing
elections are still ongoing, the browse list is not yet distributed to all clients by the (to be
elected) browser master. The next screenshot was taken about one minute later. And it shows
even less.
Password:
WORKGROUP
W2000
[paul@RHEL4b ~]$
So we wait a while, and then run smbtree again, this time it looks a lot nicer.
Password:
WORKGROUP
W2000
PEGASUS
WINXP
RHEL4BADMIN$ IPC Service (Pegasus Domain Member Server)
RHEL4BIPC$ IPC Service (Pegasus Domain Member Server)
RHEL4Bdomaindata Active Directory users only
HM2003
[paul@RHEL4b ~]$ smbtree --version
Version 3.0.10-1.4E.9
[paul@RHEL4b ~]$
I added the version number of smbtree in the previous screenshot, to show you the difference
when using the latest version of smbtree (below a screenshot taken from Ubuntu Feisty
Fawn). The latest version shows a more complete overview of machines and shares.
paul@laika:~$ smbtree --version
Version 3.0.24

208
paul@laika:~$ smbtree
Password:
WORKGROUP
W2000
W2000firstshare
W2000C$ Default share
W2000ADMIN$ Remote Admin
W2000IPC$ Remote IPC
PEGASUS
WINXP
cli_rpc_pipe_open: cli_nt_create failed on pipe srvsvc to machine WINXP.
Error was NT_STATUS_ACCESS_DENIED
HM2003
cli_rpc_pipe_open: cli_nt_create failed on pipe srvsvc to machine HM2003.
Error was NT_STATUS_ACCESS_DENIED
paul@laika:~$
The previous screenshot also provides useful errors on why we cannot see shared info on
computers winxp and w2003. Let us try the old smbtree version on our RHEL server, but
this time with Administrator credentials (which are the same on all computers).
[paul@RHEL4b ~]$ smbtree -UAdministrator%Stargate1
WORKGROUP
W2000
PEGASUS
WINXP
WINXPC$ Default share
WINXPADMIN$ Remote Admin
WINXPshare55
WINXPIPC$ Remote IPC
HM2003
HM2003NETLOGON Logon server share
HM2003SYSVOL Logon server share
HM2003WSUSTemp A network share used by Local Publishing ...
HM2003ADMIN$ Remote Admin
HM2003tools
HM2003IPC$ Remote IPC
HM2003WsusContent A network share to be used by Local ...
HM2003C$ Default share
[paul@RHEL4b ~]$
As you can see, this gives a very nice overview of all SMB computers and their shares.
16.5. server string
The comment seen by the net view and the smbclient commands is the default value for
the server string option. Simply adding this value to the global section in smb.conf and
restarting samba will change the option.
[root@RHEL53 samba]# testparm -s 2>/dev/null | grep server
server string = Red Hat Server in Paris
After a short while, the changed option is visible on the Microsoft computers.

209
C:Documents and SettingsAdministrator>net view
Server Name Remark
-------------------------------------------------------------------------------
LAIKA Ubuntu 9.04 server in Antwerp
RHEL53 Red Hat Server in Paris
W2003
16.6. Samba Web Administration Tool
(SWAT)
Samba comes with a web based tool to manage your samba configuration file. SWAT is
accessible with a web browser on port 901 of the host system. To enable the tool, first find
out whether your system is using the inetd or the xinetd superdaemon.
[root@RHEL4b samba]# ps fax | grep inet
15026 pts/0 S+ 0:00 _ grep inet
2771 ? Ss 0:00 xinetd -stayalive -pidfile /var/run/xinetd.pid
Then edit the inetd.conf or change the disable = yes line in /etc/xinetd.d/swat to disable
= no.
[root@RHEL4b samba]# cat /etc/xinetd.d/swat
# default: off
# description: SWAT is the Samba Web Admin Tool. Use swat
# to configure your Samba server. To use SWAT,
# connect to port 901 with your favorite web browser.
service swat
{
port = 901
socket_type = stream
wait = no
only_from = 127.0.0.1
user = root
server = /usr/sbin/swat
log_on_failure += USERID
disable = no
}
[root@RHEL4b samba]# /etc/init.d/xinetd restart
Stopping xinetd: [ OK ]
Starting xinetd: [ OK ]
Change the only from value to enable swat from remote computers. This examples shows
how to provide swat access to all computers in a /24 subnet.
[root@RHEL53 xinetd.d]# grep only /etc/xinetd.d/swat
only_from = 192.168.1.0/24
Be careful when using SWAT, it erases all your manually edited comments in smb.conf.

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16.7. practice: getting started with samba
1. Take a backup copy of the original smb.conf, name it smb.conf.orig
2. Enable SWAT and take a look at it.
4. Create a minimalistic smb.conf.minimal and test it with testparm.
5. Use tesparm -s to create /etc/samba/smb.conf from your smb.conf.minimal .
6. Start Samba with your minimal smb.conf.
7. Verify with smbclient that your Samba server works.
8. Verify that another (Microsoft) computer can see your Samba server.
9. Browse the network with net view, smbtree and with Windows Explorer.
10. Change the "Server String" parameter in smb.conf. How long does it take before you see
the change (net view, smbclient, My Network Places,...) ?
11. Will restarting Samba after a change to smb.conf speed up the change ?
12. Which computer is the master browser master in your workgroup ? What is the master
browser ?
13. If time permits (or if you are waiting for other students to finish this practice), then install
a sniffer (wireshark) and watch the browser elections.

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16.8. solution: getting started with samba
1. Take a backup copy of the original smb.conf, name it smb.conf.orig
cd /etc/samba ; cp smb.conf smb.conf.orig
2. Enable SWAT and take a look at it.
on Debian/Ubuntu: vi /etc/inetd.conf (remove # before swat)
on RHEL/Fedora: vi /etc/xinetd.d/swat (set disable to no)
/etc/init.d/smb stop (Red Hat)
/etc/init.d/samba stop (Debian)
4. Create a minimalistic smb.conf.minimal and test it with testparm.
cd /etc/samba ; mkdir my_smb_confs ; cd my_smb_confs
vi smb.conf.minimal
testparm smb.conf.minimal
5. Use tesparm -s to create /etc/samba/smb.conf from your smb.conf.minimal .
testparm -s smb.conf.minimal > ../smb.conf
6. Start Samba with your minimal smb.conf.
/etc/init.d/smb restart (Red Hat)
/etc/init.d/samba restart (Debian)
7. Verify with smbclient that your Samba server works.
smbclient -NL 127.0.0.1
8. Verify that another computer can see your Samba server.
smbclient -NL 'ip-address' (on a Linux)
9. Browse the network with net view, smbtree and with Windows Explorer.
on Linux: smbtree
on Windows: net view (and WindowsKey + e)
10. Change the "Server String" parameter in smb.conf. How long does it take before you see
the change (net view, smbclient, My Network Places,...) ?
vi /etc/samba/smb.conf
(should take only seconds when restarting samba)
11. Will restarting Samba after a change to smb.conf speed up the change ?
yes

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12. Which computer is the master browser master in your workgroup ? What is the master
browser ?
The computer that won the elections.
This machine will make the list of computers in the network
13. If time permits (or if you are waiting for other students to finish this practice), then install
a sniffer (wireshark) and watch the browser elections.
On ubuntu: sudo aptitude install wireshark
then: sudo wireshark, select interface

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Chapter 17. a read only file server

a read only file server
214
17.1. Setting up a directory to share
Let's start with setting up a very simple read only file server with Samba. Everyone (even
anonymous guests) will receive read access.
The first step is to create a directory and put some test files in it.
[root@RHEL52 ~]# mkdir -p /srv/samba/readonly
[root@RHEL52 ~]# cd /srv/samba/readonly/
[root@RHEL52 readonly]# echo "It is cold today." > winter.txt
[root@RHEL52 readonly]# echo "It is hot today." > summer.txt
[root@RHEL52 readonly]# ls -l
total 8
-rw-r--r-- 1 root root 17 Jan 21 05:49 summer.txt
-rw-r--r-- 1 root root 18 Jan 21 05:49 winter.txt
[root@RHEL52 readonly]#
17.2. configure the share
17.2.1. smb.conf [global] section
In this example the samba server is a member of WORKGROUP (the default workgroup).
We also set a descriptive server string, this string is visible to users browsing the network
with net view, windows explorer or smbclient.
[root@RHEL52 samba]# head -5 smb.conf
[global]
server string = Public Anonymous File Server
security = share
You might have noticed the line with security = share. This line sets the default security
mode for our samba server. Setting the security mode to share will allow clients (smbclient,
any windows, another Samba server, ...) to provide a password for each share. This is one
way of using the SMB/CIFS protocol. The other way (called user mode) will allow the
client to provide a username/password combination, before the server knows which share
the client wants to access.
17.2.2. smb.conf [share] section
The share is called pubread and the path is set to our newly created directory. Everyone is
allowed access (guest ok = yes) and security is set to read only.
[pubread]
path = /srv/samba/readonly
comment = files to read
read only = yes
guest ok = yes
Here is a very similar configuration on Ubuntu 11.10.

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root@ubu1110:~# cat /etc/samba/smb.conf
[global]
workgroup = LINUXTR
netbios name = UBU1110
security = share
[roshare1]
read only = yes
guest ok = yes
It doesn't really matter which Linux distribution you use. Below the same config on Debian
6, as good as identical.
root@debian6:~# cat /etc/samba/smb.conf
[global]
workgroup = LINUXTR
netbios name = DEBIAN6
security = share
[roshare1]
read only = yes
guest ok = yes
17.3. restart the server
After testing with testparm, restart the samba server (so you don't have to wait).
[root@RHEL4b readonly]# service smb restart
17.4. verify the share
17.4.1. verify with smbclient
You can now verify the existence of the share with smbclient. Our pubread is listed as the
fourth share.
[root@RHEL52 samba]# smbclient -NL 127.0.0.1
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
--------- ---- -------
IPC$ IPC IPC Service (Public Anonymous File Server)
global$ Disk
pub0 Disk
pubread Disk files to read
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
Server Comment
--------- -------
TEACHER0 Samba 3.0.33-3.7.el5
W2003EE
Workgroup Master
--------- -------
WORKGROUP W2003EE

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17.4.2. verify on windows
The final test is to go to a Microsoft windows computer and read a file on the Samba server.
First we use the net use command to mount the pubread share on the driveletter k.
C:>net use K: teacher0pubread
Then we test looking at the contents of the share, and reading the files.
C:>dir k:
Volume in drive K is pubread
Volume Serial Number is 0C82-11F2
Directory of K:
21/01/2009 05:49 <DIR> .
21/01/2009 05:49 <DIR> ..
21/01/2009 05:49 17 summer.txt
21/01/2009 05:49 18 winter.txt
2 File(s) 35 bytes
2 Dir(s) 13.496.242.176 bytes free
Just to be on the safe side, let us try writing.
K:>echo very cold > winter.txt
Access is denied.
K:>
Or you can use windows explorer...

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17.5. a note on netcat
The Windows command line screenshot is made in a Linux console, using netcat as a pipe
to a Windows command shell.
The way this works, is by enabling netcat to listen on the windows computer to a certain
port, executing cmd.exe when a connection is received. Netcat is similar to cat, in the way
that cat does nothing, only netcat does nothing over the network.
To enable this connection, type the following on the windows computer (after downloading
netcat for windows).
nc -l -p 23 -t -e cmd.exe
And then connect to this machine with netcat from any Linux computer. You end up with
a cmd.exe prompt inside your Linux shell.
paul@laika:~$ nc 192.168.1.38 23
Microsoft Windows [Version 5.2.3790]
(C) Copyright 1985-2003 Microsoft Corp.
C:>net use k: /delete
net use k: /delete
k: was deleted successfully.

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17.6. practice: read only file server
1. Create a directory in a good location (FHS) to share files for everyone to read.
2. Make sure the directory is owned properly and is world accessible.
3. Put a textfile in this directory.
4. Share the directory with Samba.
5. Verify from your own and from another computer (smbclient, net use, ...) that the share
is accessible for reading.
6. Make a backup copy of your smb.conf, name it smb.conf.ReadOnlyFileServer.

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17.7. solution: read only file server
1. Create a directory in a good location (FHS) to share files for everyone to read.
choose one of these...
mkdir -p /srv/samba/readonly
mkdir -p /home/samba/readonly
/home/paul/readonly is wrong!!
/etc/samba/readonly is wrong!!
/readonly is wrong!!
2. Make sure the directory is owned properly and is world accessible.
chown root:root /srv/samba/readonly
chmod 755 /srv/samba/readonly
3. Put a textfile in this directory.
echo Hello World > hello.txt
4. Share the directory with Samba.
You smb.conf.readonly could look like this:
[global]
server string = Read Only File Server
netbios name = STUDENTx
security = share
[readonlyX]
comment = read only file share
read only = yes
guest ok = yes
test with testparm before going in production!
5. Verify from your own and from another computer (smbclient, net use, ...) that the share
is accessible for reading.
On Linux: smbclient -NL 127.0.0.1
On Windows Explorer: browse to My Network Places
On Windows cmd.exe: net use L: //studentx/readonly
6. Make a backup copy of your smb.conf, name it smb.conf.ReadOnlyFileServer.
cp smb.conf smb.conf.ReadOnlyFileServer

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Chapter 18. a writable file server

a writable file server
221
18.1. set up a directory to share
In this second example, we will create a share where everyone can create files and write to
files. Again, we start by creating a directory
[root@RHEL52 samba]# mkdir -p /srv/samba/writable
[root@RHEL52 samba]# chmod 777 /srv/samba/writable/
18.2. share section in smb.conf
There are two parameters to make a share writable. We can use read only or writable. This
example shows how to use writable to give write access to a share.
writable = yes
And this is an example of using the read only parameter to give write access to a share.
read only = no
18.3. configure the share
Then we simply add a share to our file server by editing smb.conf. Below the check with
testparm. (We could have changed the description of the server...)
[root@RHEL52 samba]# testparm
Processing section "[pubwrite]"
Processing section "[pubread]"
[global]
server string = Public Anonymous File Server
security = SHARE
[pubwrite]
comment = files to write
path = /srv/samba/writable
read only = No
guest ok = Yes
[pubread]
guest ok = Yes
18.4. test connection with windows
We can now test the connection on a windows 2003 computer. We use the net use for this.
C:>net use L: teacher0pubwrite
net use L: teacher0pubwrite

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18.5. test writing with windows
We mounted the pubwrite share on the L: drive in windows. Below we test that we can
write to this share.
L:>echo hoi > hoi.txt
L:>dir
Volume in drive L is pubwrite
Volume Serial Number is 0C82-272A
Directory of L:
21/01/2009 06:11 <DIR> .
21/01/2009 06:11 <DIR> ..
21/01/2009 06:16 6 hoi.txt
1 File(s) 6 bytes
2 Dir(s) 13.496.238.080 bytes free
18.6. How is this possible ?
Linux (or any Unix) always needs a user account to gain access to a system. The windows
computer did not provide the samba server with a user account or a password. Instead,
the Linux owner of the files created through this writable share is the Linux guest account
(usually named nobody).
[root@RHEL52 samba]# ls -l /srv/samba/writable/
total 4
-rwxr--r-- 1 nobody nobody 6 Jan 21 06:16 hoi.txt
So this is not the cleanest solution. We will need to improve this.

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18.7. practice: writable file server
1. Create a directory and share it with Samba.
2. Make sure everyone can read and write files, test writing with smbclient and from a
Microsoft computer.
3. Verify the ownership of files created by (various) users.

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18.8. solution: writable file server
1. Create a directory and share it with Samba.
mkdir /srv/samba/writable
chmod 777 /srv/samba/writable
the share section in smb.conf can look like this:
[pubwrite]
read only = no
guest ok = yes
2. Make sure everyone can read and write files, test writing with smbclient and from a
Microsoft computer.
to test writing with smbclient:
echo one > count.txt
echo two >> count.txt
echo three >> count.txt
smbclient //localhost/pubwrite
Password:
smb: > put count.txt
3. Verify the ownership of files created by (various) users.
ls -l /srv/samba/writable

225
Chapter 19. samba first user account

samba first user account
226
19.1. creating a samba user
We will create a user for our samba file server and make this user the owner of the directory
and all of its files. This anonymous user gets a clear description, but does not get a login shell.
[root@RHEL52 samba]# useradd -s /bin/false sambanobody
[root@RHEL52 samba]# usermod -c "Anonymous Samba Access" sambanobody
[root@RHEL52 samba]# passwd sambanobody
Changing password for user sambanobody.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
19.2. ownership of files
We can use this user as owner of files and directories, instead of using the root account. This
approach is clear and more secure.
[root@RHEL52 samba]# chown -R sambanobody:sambanobody /srv/samba/
[root@RHEL52 samba]# ls -al /srv/samba/writable/
total 12
drwxrwxrwx 2 sambanobody sambanobody 4096 Jan 21 06:11 .
drwxr-xr-x 6 sambanobody sambanobody 4096 Jan 21 06:11 ..
-rwxr--r-- 1 sambanobody sambanobody 6 Jan 21 06:16 hoi.txt
19.3. /usr/bin/smbpasswd
The sambanobody user account that we created in the previous examples is not yet used
by samba. It just owns the files and directories that we created for our shares. The goal of
this section is to force ownership of files created through the samba share to belong to our
sambanobody user. Remember, our server is still accessible to everyone, nobody needs to
know this user account or password. We just want a clean Linux server.
To accomplish this, we first have to tell Samba about this user. We can do this by adding
the account to smbpasswd.
[root@RHEL52 samba]# smbpasswd -a sambanobody
New SMB password:
Retype new SMB password:
Added user sambanobody.
19.4. /etc/samba/smbpasswd
To find out where Samba keeps this information (for now), use smbd -b. The
PRIVATE_DIR variable will show you where the smbpasswd database is located.
[root@RHEL52 samba]# smbd -b | grep PRIVATE
PRIVATE_DIR: /etc/samba
[root@RHEL52 samba]# ls -l smbpasswd
-rw------- 1 root root 110 Jan 21 06:19 smbpasswd
You can use a simple cat to see the contents of the smbpasswd database. The sambanobody
user does have a password (it is secret).
[root@RHEL52 samba]# cat smbpasswd

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sambanobody:503:AE9 ... 9DB309C528E540978:[U ]:LCT-4976B05B:
19.5. passdb backend
Note that recent versions of Samba have tdbsam as default for the passdb backend
paramater.
root@ubu1110:~# testparm -v 2>/dev/null| grep 'passdb backend'
passdb backend = tdbsam
19.6. forcing this user
Now that Samba knows about this user, we can adjust our writable share to force the
ownership of files created through it. For this we use the force user and force group options.
Now we can be sure that all files in the Samba writable share are owned by the same
sambanobody user.
Below is the renewed definition of our share in smb.conf.
[pubwrite]
force user = sambanobody
force group = sambanobody
read only = no
guest ok = yes
When you reconnect to the share and write a file, then this sambanobody user will own the
newly created file (and nobody needs to know the password).

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19.7. practice: first samba user account
1. Create a user account for use with samba.
2. Add this user to samba's user database.
3. Create a writable shared directory and use the "force user" and "force group" directives
to force ownership of files.
4. Test the working of force user with smbclient, net use and Windows Explorer.

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19.8. solution: first samba user account
1. Create a user account for use with samba.
useradd -s /bin/false smbguest
usermod -c 'samba guest'
passwd smbguest
2. Add this user to samba's user database.
smbpasswd -a smbguest
3. Create a writable shared directory and use the "force user" and "force group" directives
to force ownership of files.
[userwrite]
path = /srv/samba/userwrite
comment = everyone writes files owned by smbguest
read only = no
guest ok = yes
force user = smbguest
force group = smbguest
4. Test the working of force user with smbclient, net use and Windows Explorer.
ls -l /srv/samba/userwrite (and verify ownership)

230
Chapter 20. samba authentication

samba authentication
231
20.1. creating the users on Linux
The goal of this example is to set up a file share accessible to a number of different users.
The users will need to authenticate with their password before access to this share is granted.
We will first create three randomly named users, each with their own password. First we
add these users to Linux.
[root@RHEL52 ~]# useradd -c "Serena Williams" serena
[root@RHEL52 ~]# useradd -c "Justine Henin" justine
[root@RHEL52 ~]# useradd -c "Martina Hingis" martina
[root@RHEL52 ~]# passwd serena
Changing password for user serena.
New UNIX password:
[root@RHEL52 ~]# passwd justine
Changing password for user justine.
New UNIX password:
[root@RHEL52 ~]# passwd martina
Changing password for user martina.
New UNIX password:
20.2. creating the users on samba
Then we add them to the smbpasswd file, with the same password.
[root@RHEL52 ~]# smbpasswd -a serena
New SMB password:
Added user serena.
[root@RHEL52 ~]# smbpasswd -a justine
New SMB password:
Added user justine.
[root@RHEL52 ~]# smbpasswd -a martina
New SMB password:
Added user martina.
20.3. security = user
Remember that we set samba's security mode to share with the security = share directive in
the [global] section ? Since we now require users to always provide a userid and password
for access to our samba server, we will need to change this. Setting security = user will
require the client to provide samba with a valid userid and password before giving access
to a share.
Our [global] section now looks like this.

232
[global]
server string = Samba File Server
security = user
20.4. configuring the share
We add the following [share] section to our smb.conf (and we do not forget to create the
directory /srv/samba/authwrite).
[authwrite]
path = /srv/samba/authwrite
comment = authenticated users only
read only = no
guest ok = no
20.5. testing access with net use
After restarting samba, we test with different users from within Microsoft computers. The
screenshots use the net useFirst serena from Windows XP.
C:>net use m: teacher0authwrite stargate /user:serena
C:>m:
M:>echo greetings from Serena > serena.txt
The next screenshot is martina on a Windows 2000 computer, she succeeds in writing her
files, but fails to overwrite the file from serena.
C:>net use k: teacher0authwrite stargate /user:martina
C:>k:
K:>echo greetings from martina > Martina.txt
K:>echo test overwrite > serena.txt
Access is denied.
20.6. testing access with smbclient
You can also test connecting with authentication with smbclient. First we test with a wrong
password.
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina wrongpass
session setup failed: NT_STATUS_LOGON_FAILURE

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Then we test with the correct password, and verify that we can access a file on the share.
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina stargate
Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
smb: > more serena.txt
getting file serena.txt of size 14 as /tmp/smbmore.QQfmSN (6.8 kb/s)
one
two
three
smb: > q
20.7. verify ownership
We now have a simple standalone samba file server with authenticated access. And the files
in the shares belong to their proper owners.
[root@RHEL52 samba]# ls -l /srv/samba/authwrite/
total 8
-rwxr--r-- 1 martina martina 0 Jan 21 20:06 martina.txt
-rwxr--r-- 1 serena serena 14 Jan 21 20:06 serena.txt
-rwxr--r-- 1 serena serena 6 Jan 21 20:09 ser.txt
20.8. common problems
20.8.1. NT_STATUS_BAD_NETWORK_NAME
You can get NT_STATUS_BAD_NETWORK_NAME when you forget to create the
target directory.
[root@RHEL52 samba]# rm -rf /srv/samba/authwrite/
tree connect failed: NT_STATUS_BAD_NETWORK_NAME
20.8.2. NT_STATUS_LOGON_FAILURE
You can get NT_STATUS_LOGON_FAILURE when you type the wrong password or
when you type an unexisting username.
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina STARGATE
session setup failed: NT_STATUS_LOGON_FAILURE
20.8.3. usernames are (not) case sensitive
Remember that usernames om Linux are case sensitive.
[root@RHEL52 samba]# su - MARTINA
su: user MARTINA does not exist

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[root@RHEL52 samba]# su - martina
[martina@RHEL52 ~]$
But usernames on Microsoft computers are not case sensitive.
smb: > q
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U MARTINA stargate
smb: > q

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20.9. practice : samba authentication
0. Make sure you have properly named backups of your smb.conf of the previous practices.
1. Create three users (on the Linux and on the samba), remember their passwords!
2. Set up a shared directory that is only accessible to authenticated users.
3. Use smbclient and a windows computer to access your share, use more than one user
account (windows requires a logoff/logon for this).
4. Verify that files created by these users belong to them.
5. Try to change or delete a file from another user.

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20.10. solution: samba authentication
1. Create three users (on the Linux and on the samba), remember their passwords!
useradd -c 'SMB user1' userx
passwd userx
2. Set up a shared directory that is only accessible to authenticated users.
The shared section in smb.conf could look like this:
[authwrite]
read only = no
guest ok = no
3. Use smbclient and a windows computer to access your share, use more than one user
account (windows requires a logoff/logon for this).
on Linux: smbclient //studentX/authwrite -U user1 password
on windows net use p: studentXauthwrite password /user:user2
4. Verify that files created by these users belong to them.
ls -l /srv/samba/authwrite
5. Try to change or delete a file from another user.
you should not be able to change or overwrite files from others.

237
Chapter 21. samba securing shares

samba securing shares
238
21.1. security based on user name
21.1.1. valid users
To restrict users per share, you can use the valid users parameter. In the example below,
only the users listed as valid will be able to access the tennis share.
[tennis]
path = /srv/samba/tennis
comment = authenticated and valid users only
read only = No
guest ok = No
valid users = serena, kim, venus, justine
21.1.2. invalid users
If you are paranoia, you can also use invalid users to explicitely deny the listed users access.
When a user is in both lists, the user has no access!
[tennis]
read only = No
guest ok = No
valid users = kim, serena, venus, justine
invalid users = venus
21.1.3. read list
On a writable share, you can set a list of read only users with the read list parameter.
[football]
path = /srv/samba/football
read only = No
guest ok = No
read list = martina, roberto
21.1.4. write list
Even on a read only share, you can set a list of users that can write. Use the write list
parameter.
[football]
path = /srv/samba/golf
read only = Yes
guest ok = No
write list = eddy, jan
21.2. security based on ip-address
21.2.1. hosts allow
The hosts allow or allow hosts parameter is one of the key advantages of Samba. It allows
access control of shares on the ip-address level. To allow only specific hosts to access a
share, list the hosts, separated by comma's.

239
allow hosts = 192.168.1.5, 192.168.1.40
Allowing entire subnets is done by ending the range with a dot.
allow hosts = 192.168.1.
Subnet masks can be added in the classical way.
allow hosts = 10.0.0.0/255.0.0.0
You can also allow an entire subnet with exceptions.
hosts allow = 10. except 10.0.0.12
21.2.2. hosts deny
The hosts deny or deny hosts parameter is the logical counterpart of the previous. The
syntax is the same as for hosts allow.
hosts deny = 192.168.1.55, 192.168.1.56
21.3. security through obscurity
21.3.1. hide unreadable
Setting hide unreadable to yes will prevent users from seeing files that cannot be read by
them.
hide unreadable = yes
21.3.2. browsable
Setting the browseable = no directive will hide shares from My Network Places. But it will
not prevent someone from accessing the share (when the name of the share is known).
Note that browsable and browseable are both correct syntax.
[pubread]
read only = yes
guest ok = yes
browseable = no
21.4. file system security
21.4.1. create mask
You can use create mask and directory mask to set the maximum allowed permissions for
newly created files and directories. The mask you set is an AND mask (it takes permissions
away).
[tennis]
read only = No

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guest ok = No
create mask = 640
directory mask = 750
21.4.2. force create mode
Similar to create mask, but different. Where the mask from above was a logical AND, the
mode you set here is a logical OR (so it adds permissions). You can use the force create
mode and force directory mode to set the minimal required permissions for newly created
files and directories.
[tennis]
read only = No
guest ok = No
force create mode = 444
force directory mode = 550
21.4.3. security mask
The security mask and directory security mask work in the same way as create mask
and directory mask, but apply only when a windows user is changing permissions using
the windows security dialog box.
21.4.4. force security mode
The force security mode and force directory security mode work in the same way as force
create mode and force directory mode, but apply only when a windows user is changing
permissions using the windows security dialog box.
21.4.5. inherit permissions
With inherit permissions = yes you can force newly created files and directories to inherit
permissions from their parent directory, overriding the create mask and directory mask
settings.
[authwrite]
read only = no
guest ok = no
create mask = 600
directory mask = 555
inherit permissions = yes

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21.5. practice: securing shares
1. Create a writable share called sales, and a readonly share called budget. Test that it works.
2. Limit access to the sales share to ann, sandra and veronique.
3. Make sure that roberto cannot access the sales share.
4. Even though the sales share is writable, ann should only have read access.
5. Even though the budget share is read only, sandra should also have write access.
6. Limit one shared directory to the 192.168.1.0/24 subnet, and another share to the two
computers with ip-addresses 192.168.1.33 and 172.17.18.19.
7. Make sure the computer with ip 192.168.1.203 cannot access the budget share.
8. Make sure (on the budget share) that users can see only files and directories to which
they have access.
9. Make sure the sales share is not visible when browsing the network.
10. All files created in the sales share should have 640 permissions or less.
11. All directories created in the budget share should have 750 permissions or more.
12. Permissions for files on the sales share should never be set more than 664.
13. Permissions for files on the budget share should never be set less than 500.
14. If time permits (or if you are waiting for other students to finish this practice), then
combine the "read only" and "writable" statements to check which one has priority.
15. If time permits then combine "read list", "write list", "hosts allow" and "hosts deny".
Which of these has priority ?

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21.6. solution: securing shares
1. Create a writable share called sales, and a readonly share called budget. Test that it works.
see previous solutions on how to do this...
2. Limit access to the sales share to ann, sandra and veronique.
valid users = ann, sandra, veronique
3. Make sure that roberto cannot access the sales share.
invalid users = roberto
4. Even though the sales share is writable, ann should only have read access.
read list = ann
5. Even though the budget share is read only, sandra should also have write access.
write list = sandra
6. Limit one shared directory to the 192.168.1.0/24 subnet, and another share to the two
computers with ip-addresses 192.168.1.33 and 172.17.18.19.
hosts allow = 192.168.1.
hosts allow = 192.168.1.33, 172.17.18.19
7. Make sure the computer with ip 192.168.1.203 cannot access the budget share.
hosts deny = 192.168.1.203
8. Make sure (on the budget share) that users can see only files and directories to which
they have access.
hide unreadable = yes
9. Make sure the sales share is not visible when browsing the network.
browsable = no
10. All files created in the sales share should have 640 permissions or less.
create mask = 640
11. All directories created in the budget share should have 750 permissions or more.
force directory mode = 750
12. Permissions for files on the sales share should never be set more than 664.
security mask = 750
13. Permissions for files on the budget share should never be set less than 500.
force security directory mask = 500
combine the "read only" and "writable" statements to check which one has priority.

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15. If time permits then combine "read list", "write list", "hosts allow" and "hosts deny".
Which of these has priority ?

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Chapter 22. samba domain member

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22.1. changes in smb.conf
22.1.1. workgroup
The workgroup option in the global section should match the netbios name of the Active
Directory domain.
workgroup = STARGATE
22.1.2. security mode
Authentication will not be handled by samba now, but by the Active Directory domain
controllers, so we set the security option to domain.
security = Domain
22.1.3. Linux uid's
Linux requires a user account for every user accessing its file system, we need to provide
Samba with a range of uid's and gid's that it can use to create these user accounts. The range
is determined with the idmap uid and the idmap gid parameters. The first Active Directory
user to connect will receive Linux uid 20000.
idmap uid = 20000-22000
idmap gid = 20000-22000
22.1.4. winbind use default domain
The winbind use default domain parameter makes sure winbind also operates on users
without a domain component in their name.
winbind use default domain = yes
22.1.5. [global] section in smb.conf
Below is our new global section in smb.conf.
[global]
workgroup = STARGATE
security = Domain
server string = Stargate Domain Member Server
idmap uid = 20000-22000
idmap gid = 20000-22000
winbind use default domain = yes

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22.1.6. realm in /etc/krb5.conf
To connect to a Windows 2003 sp2 (or later) you will need to adjust the kerberos realm in
/etc/krb5.conf and set both lookup statements to true.
[libdefaults]
default_realm = STARGATE.LOCAL
dns_lookup_realm = true
dns_lookup_kdc = true
22.1.7. [share] section in smb.conf
Nothing special is required for the share section in smb.conf. Remember that we do not
manually create users in smbpasswd or on the Linux (/etc/passwd). Only Active Directory
users are allowed access.
[domaindata]
path = /srv/samba/domaindata
comment = Active Directory users only
read only = No
22.2. joining an Active Directory domain
While the Samba server is stopped, you can use net rpc join to join the Active Directory
domain.
[root@RHEL52 samba]# service smb stop
[root@RHEL52 samba]# net rpc join -U Administrator
Password:
Joined domain STARGATE.
We can verify in the aduc (Active Directory Users and Computers) that a computer account
is created for this samba server.

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22.3. winbind
22.3.1. adding winbind to nsswitch.conf
The winbind daemon is talking with the Active Directory domain.
We need to update the /etc/nsswitch.conf file now, so user group and host names can be
resolved against the winbind daemon.
[root@RHEL52 samba]# vi /etc/nsswitch.conf
[root@RHEL52 samba]# grep winbind /etc/nsswitch.conf
passwd: files winbind
group: files winbind
hosts: files dns winbind
22.3.2. starting samba and winbindd
Time to start Samba followed by winbindd.
[root@RHEL4b samba]# service smb start
[root@RHEL4b samba]# service winbind start
Starting winbindd services: [ OK ]
22.4. wbinfo
22.4.1. verify the trust
You can use wbinfo -t to verify the trust between your samba server and Active Directory.

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[root@RHEL52 ~]# wbinfo -t
checking the trust secret via RPC calls succeeded
22.4.2. list all users
We can obtain a list of all user with the wbinfo -u command. The domain is not shown when
the winbind use default domain parameter is set.
[root@RHEL52 ~]# wbinfo -u
TEACHER0serena
TEACHER0justine
TEACHER0martina
STARGATEadministrator
STARGATEguest
STARGATEsupport_388945a0
STARGATEpol
STARGATEkrbtgt
STARGATEarthur
STARGATEharry
22.4.3. list all groups
We can obtain a list of all domain groups with the wbinfo -g command. The domain is not
shown when the winbind use default domain parameter is set.
[root@RHEL52 ~]# wbinfo -g
BUILTINadministrators
BUILTINusers
BATMANdomain computers
BATMANdomain controllers
BATMANschema admins
BATMANenterprise admins
BATMANdomain admins
BATMANdomain users
BATMANdomain guests
BATMANgroup policy creator owners
BATMANdnsupdateproxy
22.4.4. query a user
We can use wbinfo -a to verify authentication of a user against Active Directory. Assuming
a user account harry with password stargate is just created on the Active Directory, we get
the following screenshot.
[root@RHEL52 ~]# wbinfo -a harry%stargate
plaintext password authentication succeeded
challenge/response password authentication succeeded
22.5. getent
We can use getent to verify that winbindd is working and actually adding the Active
directory users to /etc/passwd.

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[root@RHEL52 ~]# getent passwd harry
harry:*:20000:20008:harry potter:/home/BATMAN/harry:/bin/false
[root@RHEL52 ~]# getent passwd arthur
arthur:*:20001:20008:arthur dent:/home/BATMAN/arthur:/bin/false
[root@RHEL52 ~]# getent passwd bilbo
bilbo:*:20002:20008:bilbo baggins:/home/BATMAN/bilbo:/bin/false
If the user already exists locally, then the local user account is shown. This is because
winbind is configured in /etc/nsswitch.conf after files.
[root@RHEL52 ~]# getent passwd paul
paul:x:500:500:Paul Cobbaut:/home/paul:/bin/bash
All the Active Directory users can now easily connect to the Samba share. Files created by
them, belong to them.
22.6. file ownership
[root@RHEL4b samba]# ll /srv/samba/domaindata/
total 0
-rwxr--r-- 1 justine 20000 0 Jun 22 19:54 create_by_justine_on_winxp.txt
-rwxr--r-- 1 venus 20000 0 Jun 22 19:55 create_by_venus.txt
-rwxr--r-- 1 maria 20000 0 Jun 22 19:57 Maria.txt

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22.7. practice : samba domain member
1. Verify that you have a working Active Directory (AD) domain.
2. Add the domain name and domain controller to /etc/hosts. Set the AD-DNS in /etc/
resolv.conf.
3. Setup Samba as a member server in the domain.
4. Verify the creation of a computer account in AD for your Samba server.
5. Verify the automatic creation of AD users in /etc/passwd with wbinfo and getent.
6. Connect to Samba shares with AD users, and verify ownership of their files.

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Chapter 23. samba domain controller

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23.1. about Domain Controllers
23.1.1. Windows NT4
Windows NT4 works with single master replication domain controllers. There is exactly one
PDC (Primary Domain Controller) in the domain, and zero or more BDC's (Backup Domain
Controllers). Samba 3 has all features found in Windows NT4 PDC and BDC, and more.
This includes file and print serving, domain control with single logon, logon scripts, home
directories and roaming profiles.
23.1.2. Windows 200x
With Windows 2000 came Active Directory. AD includes multimaster replication and group
policies. Samba 3 can only be a member server in Active Directory, it cannot manage group
policies. Samba 4 can do this (in beta).
23.1.3. Samba 3
Samba 3 can act as a domain controller in its own domain. In a Windows NT4 domain, with
one Windows NT4 PDC and zero or more BDC's, Samba 3 can only be a member server.
The same is valid for Samba 3 in an Active Directory Domain. In short, a Samba 3 domain
controller can not share domain control with Windows domain controllers.
23.1.4. Samba 4
Samba 4 can be a domain controller in an Active Directory domain, including managing
group policies. As of this writing, Samba 4 is not released for production!
23.2. About security modes
23.2.1. security = share
The 'Windows for Workgroups' way of working, a client requests connection to a share
and provides a password for that connection. Aanyone who knows a password for a share
can access that share. This security model was common in Windows 3.11, Windows 95,
Windows 98 and Windows ME.
23.2.2. security = user
The client will send a userid + password before the server knows which share the client
wants to access. This mode should be used whenever the samba server is in control of the
user database. Both for standalone and samba domain controllers.
23.2.3. security = domain
This mode will allow samba to verify user credentials using NTLM in Windows NT4 and
in all Active Directory domains. This is similar to Windows NT4 BDC's joining a native
Windows 2000/3 Active Directory domain.

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23.2.4. security = ads
This mode will make samba use Kerberos to connect to the Active Directory domain.
23.2.5. security = server
This mode is obsolete, it can be used to forward authentication to another server.
23.3. About password backends
The previous chapters all used the smbpasswd user database. For domain control we opt
for the tdbsam password backend. Another option would be to use LDAP. Larger domains
will benefit from using LDAP instead of the not so scalable tdbsam. When you need more
than one Domain Controller, then the Samba team advises to not use tdbsam.
23.4. [global] section in smb.conf
Now is a good time to start adding comments in your smb.conf. First we will take a look at
the naming of our domain and server in the [global] section, and at the domain controlling
parameters.
23.4.1. security
The security must be set to user (which is the default). This mode will make samba control
the user accounts, so it will allow samba to act as a domain controller.
security = user
23.4.2. os level
A samba server is the most stable computer in the network, so it should win all browser
elections (os level above 32) to become the browser master
os level = 33
23.4.3. passdb backend
The passdb backend parameter will determine whether samba uses smbpasswd, tdbsam
or ldap.
passdb backend = tdbsam
23.4.4. preferred master
Setting the preferred master parameter to yes will make the nmbd daemon force an election
on startup.
preferred master = yes
23.4.5. domain logons
Setting the domain logons parameter will make this samba server a domain controller.

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domain logons = yes
23.4.6. domain master
Setting the domain master parameter can cause samba to claim the domain master
browser role for its workgroup. Don't use this parameter in a workgroup with an active
NT4 PDC.
domain master = yes
23.4.7. [global] section
The screenshot below shows a sample [global] section for a samba domain controller.
[global]
# names
workgroup = SPORTS
netbios name = DCSPORTS
server string = Sports Domain Controller
# domain control parameters
security = user
os level = 33
preferred master = Yes
domain master = Yes
domain logons = Yes
23.5. netlogon share
Part of the microsoft definition for a domain controller is that it should have a netlogon
share. This is the relevant part of smb.conf to create this netlogon share on Samba.
[netlogon]
comment = Network Logon Service
path = /srv/samba/netlogon
admin users = root
guest ok = Yes
browseable = No
23.6. other [share] sections
We create some sections for file shares, to test the samba server. Users can all access the
general sports file share, but only group members can access their own sports share.
[sports]
comment = Information about all sports
path = /srv/samba/sports
valid users = @ntsports
read only = No
[tennis]
comment = Information about tennis
valid users = @nttennis
read only = No

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[football]
comment = Information about football
path = /srv/samba/football
valid users = @ntfootball
read only = No
23.7. Users and Groups
To be able to use users and groups in the samba domain controller, we can first set up some
groups on the Linux computer.
[root@RHEL52 samba]# groupadd ntadmins
[root@RHEL52 samba]# groupadd ntsports
[root@RHEL52 samba]# groupadd ntfootball
[root@RHEL52 samba]# groupadd nttennis
This enables us to add group membership info to some new users for our samba domain.
Don't forget to give them a password.
[root@RHEL52 samba]# useradd -m -G ntadmins Administrator
[root@RHEL52 samba]# useradd -m -G ntsports,nttennis venus
[root@RHEL52 samba]# useradd -m -G ntsports,nttennis kim
[root@RHEL52 samba]# useradd -m -G ntsports,nttennis jelena
[root@RHEL52 samba]# useradd -m -G ntsports,ntfootball figo
[root@RHEL52 samba]# useradd -m -G ntsports,ntfootball ronaldo
[root@RHEL52 samba]# useradd -m -G ntsports,ntfootball pfaff
It is always safe to verify creation of users, groups and passwords in /etc/passwd, /etc/shadow
and /etc/group.
[root@RHEL52 samba]# tail -11 /etc/group
ntadmins:x:507:Administrator
ntsports:x:508:venus,kim,jelena,figo,ronaldo,pfaff
ntfootball:x:509:figo,ronaldo,pfaff
nttennis:x:510:venus,kim,jelena
Administrator:x:511:
venus:x:512:
kim:x:513:
jelena:x:514:
figo:x:515:
ronaldo:x:516:
pfaff:x:517:
23.8. tdbsam
Next we must make these users known to samba with the smbpasswd tool. When you add
the first user to tdbsam, the file /etc/samba/passdb.tdb will be created.
[root@RHEL52 samba]# smbpasswd -a root
New SMB password:

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tdbsam_open: Converting version 0 database to version 3.
Added user root.
Adding all the other users generates less output, because tdbsam is already created.
[root@RHEL4b samba]# smbpasswd -a root
New SMB password:
Added user root.
23.9. about computer accounts
Every NT computer (Windows NT, 2000, XP, Vista) can become a member of a domain.
Joining the domain (by right-clicking on My Computer) means that a computer account will
be created in the domain. This computer account also has a password (but you cannot know
it) to prevent other computers with the same name from accidentally becoming member of
the domain. The computer account created by Samba is visible in the /etc/passwd file on
Linux. Computer accounts appear as a normal user account, but end their name with a dollar
sign. Below a screenshot of the windows 2003 computer account, created by Samba 3.
[root@RHEL52 samba]# tail -5 /etc/passwd
jelena:x:510:514::/home/jelena:/bin/bash
figo:x:511:515::/home/figo:/bin/bash
ronaldo:x:512:516::/home/ronaldo:/bin/bash
pfaff:x:513:517::/home/pfaff:/bin/bash
w2003ee$:x:514:518::/home/nobody:/bin/false
To be able to create the account, you will need to provide credentials of an account with
the permission to create accounts (by default only root can do this on Linux). And we will
have to tell Samba how to to this, by adding an add machine script to the global section
of smb.conf.
add machine script = /usr/sbin/useradd -s /bin/false -d /home/nobody %u
You can now join a Microsoft computer to the sports domain (with the root user). After
reboot of the Microsoft computer, you will be able to logon with Administrator (password
Stargate1), but you will get an error about your roaming profile. We will fix this in the next
section.
When joining the samba domain, you have to enter the credentials of a Linux account that
can create users (usually only root can do this). If the Microsoft computer complains with
The parameter is incorrect, then you possibly forgot to add the add machine script.
23.10. local or roaming profiles
For your information, if you want to force local profiles instead of roaming profiles, then
simply add the following two lines to the global section in smb.conf.

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logon home =
logon path =
Microsoft computers store a lot of User Metadata and application data in a user profile.
Making this profile available on the network will enable users to keep their Desktop and
Application settings across computers. User profiles on the network are called roaming
profiles or roving profiles. The Samba domain controller can manage these profiles. First
we need to add the relevant section in smb.conf.
[Profiles]
comment = User Profiles
path = /srv/samba/profiles
readonly = No
profile acls = Yes
Besides the share section, we also need to set the location of the profiles share (this can be
another Samba server) in the global section.
logon path = %LProfiles%U
The %L variable is the name of this Samba server, the %U variable translates to the
username. After adding a user to smbpasswd and letting the user log on and off, the profile
of the user will look like this.
[root@RHEL4b samba]# ll /srv/samba/profiles/Venus/
total 568
drwxr-xr-x 4 Venus Venus 4096 Jul 5 10:03 Application Data
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Cookies
drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Desktop
drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Favorites
drwxr-xr-x 4 Venus Venus 4096 Jul 5 10:03 My Documents
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 NetHood
-rwxr--r-- 1 Venus Venus 524288 Jul 5 2007 NTUSER.DAT
-rwxr--r-- 1 Venus Venus 1024 Jul 5 2007 NTUSER.DAT.LOG
-rw-r--r-- 1 Venus Venus 268 Jul 5 10:03 ntuser.ini
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 PrintHood
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Recent
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 SendTo
drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Start Menu
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Templates
23.11. Groups in NTFS acls
We have users on Unix, we have groups on Unix that contain those users.
[root@RHEL4b samba]# grep nt /etc/group
...
ntadmins:x:506:Administrator
ntsports:x:507:Venus,Serena,Kim,Figo,Pfaff
nttennis:x:508:Venus,Serena,Kim
ntfootball:x:509:Figo,Pfaff

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We already added Venus to the tdbsam with smbpasswd.
smbpasswd -a Venus
Does this mean that Venus can access the tennis and the sports shares ? Yes, all access
works fine on the Samba server. But the nttennis group is not available on the windows
machines. To make the groups available on windows (like in the ntfs security tab of files
and folders), we have to map unix groups to windows groups. To do this, we use the net
groupmap command.
[root@RHEL4b samba]# net groupmap add ntgroup="tennis" unixgroup=nttennis type=d
No rid or sid specified, choosing algorithmic mapping
Successully added group tennis to the mapping db
[root@RHEL4b samba]# net groupmap add ntgroup="football" unixgroup=ntfootball type=d
Successully added group football to the mapping db
[root@RHEL4b samba]# net groupmap add ntgroup="sports" unixgroup=ntsports type=d
Successully added group sports to the mapping db
Now you can use the Samba groups on all NTFS volumes on members of the domain.
23.12. logon scripts
Before testing a logon script, make sure it has the proper carriage returns that DOS files have.
[root@RHEL4b netlogon]# cat start.bat
net use Z: DCSPORTS0SPORTS
[root@RHEL4b netlogon]# unix2dos start.bat
unix2dos: converting file start.bat to DOS format ...
[root@RHEL4b netlogon]#
Then copy the scripts to the netlogon share, and add the following parameter to smb.conf.
logon script = start.bat

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23.13. practice: samba domain controller
1. Setup Samba as a domain controller.
2. Create the shares salesdata, salespresentations and meetings. Salesdata must be accessible
to all sales people and to all managers. SalesPresentations is only for all sales people.
Meetings is only accessible to all managers. Use groups to accomplish this.
3. Join a Microsoft computer to your domain. Verify the creation of a computer account
in /etc/passwd.
4. Setup and verify the proper working of roaming profiles.
5. Find information about home directories for users, set them up and verify that users receive
their home directory mapped under the H:-drive in MS Windows Explorer.
6. Use a couple of samba domain groups with members to set acls on ntfs. Verify that it
works!
7. Knowing that the %m variable contains the computername, create a separate log file for
every computer(account).
8. Knowing that %s contains the client operating system, include a smb.%s.conf file that
contains a share. (The share will only be visible to clients with that OS).
combine "valid users" and "invalid users" with groups and usernames with "hosts allow"
and "hosts deny" and make a table of which get priority over which.

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Chapter 24. a brief look at samba 4

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24.1. Samba 4 alpha 6
A quick view on Samba 4 alpha 6 (January 2009). You can also follow this guide http://
wiki.samba.org/index.php/Samba4/HOWTO
Remove old Samba from Red Hat
yum remove samba
set a fix ip address (Red Hat has an easy GUI)
download and untar
samba.org, click 'download info', choose mirror, dl samba4 latest alpha
once untarred, enter the directory and read the howto4.txt
cd samba-4.0.0alpha6/
more howto4.txt
first we have to configure, compile and install samba4
cd source4/
./configure
make
make install
Then we can use the provision script to setup our realm. I used booi.schot as domain name
(instead of example.com).
./setup/provision --realm=BOOI.SCHOT --domain=BOOI --adminpass=stargate
--server-role='domain controller'
i added a simple share for testing
vi /usr/local/samba/etc/smb.conf
then i started samba
cd /usr/local/samba/sbin/
./samba
I tested with smbclient, it works
smbclient //localhost/test -Uadministrator%stargate
I checked that bind (and bind-chroot) were installed (yes), so copied the srv records
cp booi.schot.zone /var/named/chroot/etc/
then appended to named.conf
cat named.conf >> /var/named/chroot/etc/named.conf

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I followed these steps in the howto4.txt
vi /etc/init.d/named [added two export lines right after start()]
chmod a+r /usr/local/samba/private/dns.keytab
cp krb5.conf /etc/
vi /var/named/chroot/etc/named.conf
--> remove a lot, but keep allow-update { any; };
restart bind (named!), then tested dns with dig, this works (stripped screenshot!)
[root@RHEL52 private]# dig _ldap._tcp.dc._msdcs.booi.schot SRV @localhost
; (1 server found)
;; global options: printcmd
;; Got answer:
;; -HEADER- opcode: QUERY, status: NXDOMAIN, id: 58186
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;_ldap._tcp.dc._msdcs.booi.schot. IN SRV
;; AUTHORITY SECTION:
. 10800 IN SOA A.ROOT-SERVERS.NET....
;; Query time: 54 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Jan 27 20:57:05 2009
;; MSG SIZE rcvd: 124
[root@RHEL52 private]#
made sure /etc/resolv.conf points to himself
[root@RHEL52 private]# cat /etc/resolv.conf
search booi.schot
start windows 2003 server, enter the samba4 as DNS!
ping the domain, if it doesn't work, then add your redhats hostname and your realm to
windows/system32/drivers/etc/hosts
join the windows computer to the domain
reboot the windows
log on with administrator stargate
start run dsa.msc to manage samba4
create an OU, a user and a GPO, test that it works

265
Table of Contents
25. Introduction to ipv6 ............................................................................................................................ 266
25.1. about ipv6 .................................................................................................................................. 267
25.2. network id and host id .............................................................................................................. 267
25.3. host part generation ................................................................................................................... 267
25.4. ipv4 mapped ipv6 address ........................................................................................................ 268
25.5. link local addresses ................................................................................................................... 268
25.6. unique local addresses .............................................................................................................. 268
25.7. globally unique unicast addresses ............................................................................................. 268
25.8. 6to4 ............................................................................................................................................ 268
25.9. ISP ............................................................................................................................................. 269
25.10. non routable addresses ............................................................................................................ 269
25.11. ping6 ........................................................................................................................................ 269
25.12. Belgium and ipv6 .................................................................................................................... 270
25.13. other websites .......................................................................................................................... 270
25.14. 6to4 gateways .......................................................................................................................... 272
25.15. ping6 and dns .......................................................................................................................... 272
25.16. ipv6 and tcp/http ..................................................................................................................... 272
25.17. ipv6 PTR record ...................................................................................................................... 272
25.18. 6to4 setup on Linux ................................................................................................................ 272

266
Chapter 25. Introduction to ipv6

Introduction to ipv6
267
25.1. about ipv6
The ipv6 protocol is designed to replace ipv4. Where ip version 4 supports a maximum
of four billion unique addresses, ip version 6 expands this to four billion times four
billion times four billion times four billion unique addresses. This is more than
100.000.000.000.000.000.000 ipv6 addresses per square cm on our planet. That should be
enough, even if every cell phone, every coffee machine and every pair of socks gets an
address.
Technically speaking ipv6 uses 128-bit addresses (instead of the 32-bit from ipv4). 128-bit
addresses are huge numbers. In decimal it would amount up to 39 digits, in hexadecimal
it looks like this:
fe80:0000:0000:0000:0a00:27ff:fe8e:8aa8
Luckily ipv6 allows us to omit leading zeroes. Our address from above then becomes:
fe80:0:0:0:a00:27ff:fe8e:8aa8
When a 16-bit block is zero, it can be written as ::. Consecutive 16-bit blocks that are zero
can also be written as ::. So our address can from above can be shortened to:
fe80::a00:27ff:fe8e:8aa8
This :: can only occur once! The following is not a valid ipv6 address:
fe80::20:2e4f::39ac
The ipv6 localhost address is 0000:0000:0000:0000:0000:0000:0000:0001, which can be
abbreviated to ::1.
paul@debian5:~/github/lt/images$ /sbin/ifconfig lo | grep inet6
25.2. network id and host id
One of the few similarities between ipv4 and ipv6 is that addresses have a host part and a
network part determined by a subnet mask. Using the cidr notation this looks like this:
fe80::a00:27ff:fe8e:8aa8/64
The above address has 64 bits for the host id, theoretically allowing for 4 billion times four
billion hosts.
The localhost address looks like this with cidr:
::1/128
25.3. host part generation
The host part of an automatically generated (stateless) ipv6 address contains part of the hosts
mac address:
paul@debian5:~$ /sbin/ifconfig | head -3

268
eth3 Link encap:Ethernet HWaddr 08:00:27:ab:67:30
inet6 addr: fe80::a00:27ff:feab:6730/64 Scope:Link
Some people are concerned about privacy here...
25.4. ipv4 mapped ipv6 address
Some applications use ipv4 addresses embedded in an ipv6 address. (Yes there will be an
era of migration with both ipv4 and ipv6 in use.) The ipv6 address then looks like this:
::ffff:192.168.1.42/96
Indeed a mix of decimal and hexadecimal characters...
25.5. link local addresses
ipv6 addresses starting with fe8. can only be used on the local segment (replace the dot with
an hexadecimal digit). This is the reason you see Scope:Link behind the address in this
screenshot. This address serves only the local link.
paul@deb503:~$ /sbin/ifconfig | grep inet6
inet6 addr: fe80::a00:27ff:fe8e:8aa8/64 Scope:Link
These link local addresses all begin with fe8..
Every ipv6 enabled nic will get an address in this range.
25.6. unique local addresses
The now obsolete system of site local addresses similar to ipv4 private ranges is replaced
with a system of globally unique local ipv6 addresses. This to prevent duplicates when
joining of networks within site local ranges.
All unique local addresses strat with fd...
25.7. globally unique unicast addresses
Since ipv6 was designed to have multiple ip addresses per interface, the global ipv6 address
can be used next to the link local address.
These globally unique addresses all begin with 2... or 3... as the first 16-bits.
25.8. 6to4
6to4 is defined in rfc's 2893 and 3056 as one possible way to transition between ipv4 and
ipv6 by creating an ipv6 tunnel.
It encodes an ipv4 address in an ipv6 address that starts with 2002. For example
192.168.1.42/24 will be encoded as:

269
2002:c0a8:12a:18::1
You can use the command below to convert any ipv4 address to this range.
paul@ubu1010:~$ printf "2002:%02x%02x:%02x%02x:%04x::1n" `echo 192.168.1.42/24
|tr "./" " "`
2002:c0a8:012a:0018::1
25.9. ISP
Should you be so lucky to get an ipv6 address from an isp, then it will start with 2001:.
25.10. non routable addresses
Comparable to example.com for DNS, the following ipv6 address ranges are reserved for
examples, and not routable on the internet.
3fff:ffff::/32
2001:0db8::/32
25.11. ping6
Use ping6 to test connectivity between ipv6 hosts. You need to specify the interface (there
is no routing table for 'random' generated ipv6 link local addresses).
[root@fedora14 ~]# ping6 -I eth0 fe80::a00:27ff:fecd:7ffc
PING fe80::a00:27ff:fecd:7ffc(fe80::a00:27ff:fecd:7ffc) from fe80::a00:27ff:fe3c:4346 eth0: 56
64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=1 ttl=64 time=0.586 ms
Below a multicast ping6 that recieves replies from three ip6 hosts on the same network.
[root@fedora14 ~]# ping6 -I eth0 ff02::1
PING ff02::1(ff02::1) from fe80::a00:27ff:fe3c:4346 eth0: 56 data bytes
64 bytes from fe80::a00:27ff:fe3c:4346: icmp_seq=1 ttl=64 time=0.598 ms
64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=1 ttl=64 time=1.87 ms (DUP!)
64 bytes from fe80::8e7b:9dff:fed6:dff2: icmp_seq=1 ttl=64 time=535 ms (DUP!)
64 bytes from fe80::a00:27ff:fe3c:4346: icmp_seq=2 ttl=64 time=0.106 ms
64 bytes from fe80::8e7b:9dff:fed6:dff2: icmp_seq=2 ttl=64 time=1.79 ms (DUP!)
64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=2 ttl=64 time=2.48 ms (DUP!)

270
25.12. Belgium and ipv6
A lot of information on ipv6 in Belgium can be found at www.ipv6council.be.
Sites like ipv6.belgium.be, www.bipt.be and www.bricozone.be are enabled for ipv6. Some
Universities also: fundp.ac.be (Namur) and ulg.ac.be (Liege).
25.13. other websites
Other useful websites for testing ipv6 are:
test-ipv6.com
ipv6-test.com
Going to the ipv6-test.com website will test whether you have a valid accessible ipv6
address.
Going to the test-ipv6.com website will also test whether you have a valid accessible ipv6
address.

272
25.14. 6to4 gateways
To access ipv4 only websites when on ipv6 you can use sixxs.net (more specifically http://
www.sixxs.net/tools/gateway/) as a gatway.
For example use http://www.slashdot.org.sixxs.org/ instead of http://slashdot.org
25.15. ping6 and dns
Below a screenshot of a ping6 from behind a 6to4 connection.
25.16. ipv6 and tcp/http
Below a screenshot of a tcp handshake and http connection over ipv6.
25.17. ipv6 PTR record
As seen in the DNS chapter, ipv6 PTR records are in the ip6.net domain, and have 32
generations of child domains.
25.18. 6to4 setup on Linux
Below a transcript of a 6to4 setup on Linux.
Thanks to http://www.anyweb.co.nz/tutorial/v6Linux6to4 and http://mirrors.bieringer.de/
Linux+IPv6-HOWTO/ and tldp.org!
root@mac:~# ifconfig

273
inet6 addr: fe80::226:bbff:fe5d:2e52/64 Scope:Link
RX bytes:4274849823 (4.2 GB) TX bytes:237002019 (237.0 MB)
Interrupt:43 Base address:0x8000
inet addr:127.0.0.1 Mask:255.0.0.0
root@mac:~# sysctl -w net.ipv6.conf.default.forwarding=1
net.ipv6.conf.default.forwarding = 1
root@mac:~# ip tunnel add tun6to4 mode sit remote any local 81.165.101.125
root@mac:~# ip link set dev tun6to4 mtu 1472 up
root@mac:~# ip link show dev tun6to4
10: tun6to4: <NOARP,UP,LOWER_UP> mtu 1472 qdisc noqueue state UNKNOWN
link/sit 81.165.101.125 brd 0.0.0.0
root@mac:~# ip -6 addr add dev tun6to4 2002:51a5:657d:0::1/64
root@mac:~# ip -6 addr add dev eth0 2002:51a5:657d:1::1/64
root@mac:~# ip -6 addr add dev eth0 fdcb:43c1:9c18:1::1/64
root@mac:~# ifconfig
inet6 addr: fe80::226:bbff:fe5d:2e52/64 Scope:Link
inet6 addr: fdcb:43c1:9c18:1::1/64 Scope:Global
inet6 addr: 2002:51a5:657d:1::1/64 Scope:Global
RX bytes:4274948430 (4.2 GB) TX bytes:237014619 (237.0 MB)
Interrupt:43 Base address:0x8000
inet addr:127.0.0.1 Mask:255.0.0.0
tun6to4 Link encap:IPv6-in-IPv4
inet6 addr: ::81.165.101.125/128 Scope:Compat
inet6 addr: 2002:51a5:657d::1/64 Scope:Global
UP RUNNING NOARP MTU:1472 Metric:1
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
root@mac:~# ip -6 route add 2002::/16 dev tun6to4
root@mac:~# ip -6 route add ::/0 via ::192.88.99.1 dev tun6to4 metric 1
root@mac:~# ip -6 route show
::/96 via :: dev tun6to4 metric 256 mtu 1472 advmss 1412 hoplimit 0
2002:51a5:657d::/64 dev tun6to4 proto kernel metric 256 mtu 1472 advmss 1412 hoplimit 0
2002:51a5:657d:1::/64 dev eth0 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit 0

274
2002::/16 dev tun6to4 metric 1024 mtu 1472 advmss 1412 hoplimit 0
fdcb:43c1:9c18:1::/64 dev eth0 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit 0
fe80::/64 dev eth0 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit 0
fe80::/64 dev tun6to4 proto kernel metric 256 mtu 1472 advmss 1412 hoplimit 0
default via ::192.88.99.1 dev tun6to4 metric 1 mtu 1472 advmss 1412 hoplimit 0
root@mac:~# ping6 ipv6-test.com
PING ipv6-test.com(ipv6-test.com) 56 data bytes
64 bytes from ipv6-test.com: icmp_seq=1 ttl=57 time=42.4 ms
^C
--- ipv6-test.com ping statistics ---

276
Table of Contents
A. License .................................................................................................................................................... 277

277
Appendix A. License
GNU Free Documentation License
Version 1.3, 3 November 2008
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284
Index
Symbols
/etc/apache2, 83
/etc/bind/named.conf.local, 133
/etc/exports, 61, 72
/etc/fstab, 62, 72
/etc/hostname, 23
/etc/httpd, 83
/etc/inetd.conf, 69, 209
/etc/init.d/samba, 198
/etc/init.d/smb, 198
/etc/init.d/winbind, 199
/etc/network/interfaces, 16, 40, 43
/etc/nsswitch.conf, 247, 249
/etc/passwd, 256
/etc/protocols, 13
/etc/resolv.conf, 119
/etc/samba/passdb.tdb, 255
/etc/samba/smb.conf, 203, 204, 205, 221, 245
/etc/samba/smbpasswd, 226, 253
/etc/services, 13, 69
/etc/squid/squid.conf, 107
/etc/ssh, 48
/etc/ssh/ssh_config, 48
/etc/ssh/sshd_config, 48
/etc/sysconfig/iptables, 65
/etc/sysconfig/network, 18
/etc/sysconfig/network-scripts/, 18
/etc/sysconfig/network-scripts/ifcfg-bond0, 41
/etc/sysctl.conf, 173
/etc/xinetd.conf, 68
/etc/xinetd.d, 68
/etc/xinetd.d/swat, 209
/proc/net/bonding, 41, 43
/proc/sys/net/ipv4/ip_forward, 173
/sbin, 21
/usr/share/doc, 18
/var/lib/nfs/etab, 61, 72
/var/log/squid, 112
.htaccess, 100
.htpasswd, 90, 97
.ssh, 52
~/.ssh/authorized_keys, 53
A
A (DNS record), 124
AAAA (DNS record), 124
Alica and Bob, 49
allow hosts (Samba), 238
anycast, 9
apache2, 79
aptitude, 195, 196
arp(1), 24
arp table, 24
atm, 11
authoritative (dns), 128
authoritative zone, 123
axfr, 131
B
bind, 121
bind(DNS), 147
binding, 39
binding(ip), 38
bonding(ip), 38
bootp, 18, 34
broadcast, 9
browsable (Samba), 239
browseable (Samba), 239
browser master, 253
C
cahing only name server, 125
chain (iptables), 180
CIFS, 200
Cisco, 11
CNAME (DNS record), 124
create mask (Samba), 239
D
default gateway, 25
deny hosts (Samba), 239
dhclient, 175
dhclient(1), 23
dhcp, 18, 34
dhcp client, 16, 23
dhcp server, 119
directory mask (Samba), 239
directory security mask(samba), 240
DNAT, 172
dns, 34, 117, 117
dnsdomainname, 123
dns namespace, 120
dns server, 119
domain (dns), 121
domain name system, 117, 117
dpkg, 195
dsa, 49
E
eth0, 16
ethtool(1), 26
exportfs(1), 61, 72
F
fddi, 11
filter table (iptables), 180
firewall, 171
fixed ip, 19
fixed ip address, 16
force create mode(samba), 240

Index
285
force directory mode(samba), 240
force directory security mode(samba), 240
force group(samba), 227
force security mode(samba), 240
force user(samba), 227
forwarder (dns), 127
forward lookup query, 118
FQDN, 23
fqdn, 123
frame relay, 11
ftp, 68
fully qualified domain name, 123
G
gateway, 25
getent(1), 248
glue record (dns), 124
guest ok (Samba), 214
H
hide unreadable (Samba), 239
host (DNS record), 124
hostname, 23, 123, 200
hostname(1), 23
hosts.txt, 117
hosts allow (Samba), 238
hosts deny (Samba), 239
htpasswd(1), 90, 97
httpd, 80
I
IBM, 200
icmp, 13
id_dsa, 53
id_dsa.pub, 53
id_rsa, 52
id_rsa.pub, 52
idmap gid(samba), 245
idmap uid(samba), 245
ifcfg(1), 39
ifcfg-eth0, 19
ifconfig(1), 20, 21, 39, 40, 41, 43
ifdown(1), 17, 20, 21, 39
ifenslave, 43
ifup(1), 17, 20, 21, 39, 40, 41
igmp, 13
inetd, 68
inetd(8), 209
invalid users (Samba), 238
iptables, 65, 179, 180
iptables save, 184
iterative query, 127
ixfr, 131
K
Kerberos, 60, 71
kmyfirewall, 65
L
LAN, 10
ldap, 61
M
mac address, 21, 174
MAN, 10
mangle table (iptables), 180
masquerading, 172
master server (DNS), 130
mount(1), 62, 72
multicast, 8
MX (DNS record), 124
N
NAPT, 172
NAT, 172
nat table (iptables), 180
NetBIOS names, 200
netcat, 217
net groupmap, 258
net rpc join(samba), 246
netstat(1), 25
net use(microsoft), 216, 221, 232
net view(microsoft), 203, 208
network file system, 59
nfs, 59, 60
NFS, 71
nmbd(8), 199
no_subtree_check(nfs), 61
NS (DNS record), 124
nslookup, 118
NT_STATUS_BAD_NETWORK_NAME, 233
NT_STATUS_LOGON_FAILURE, 233
O
OpenBSD, 48
openssh, 48
openssh-server, 55
P
packet filtering, 171, 181
packet forwarding, 171
PAN, 11
passdb backend (Samba), 227
PAT, 172
Paul Mockapetris, 117
ping, 13, 25, 174, 175
port forwarding, 172
portmap, 60, 71
primary dns server, 128
primary server (DNS), 130
private key, 49
proxy server, 106
PTR (DNS record), 124
public key, 49

Index
286
Q
query (dns), 118
R
read list (Samba), 238
read only (Samba), 221
recursive query, 127
reverse lookup query, 118
rfc 3010, 60
rfc 3530, 60
rlogin, 48
roaming profiles(samba), 257
root(DNS), 120
root hints, 121
root server (dns), 126
root servers(DNS), 9
root servers (dns), 120
rootsquash, 61, 72
route(1), 25, 25
router, 11, 171
rpc, 60
RPC, 71
rpcinfo(1), 60, 71
rpm, 195
rpm(8), 196
rsa, 49
rsh, 48
S
samba, 195
scp(1), 53
secondary dns server, 128
secondary server (DNS), 130
security(Samba), 214
security mask(samba), 240
security mode(samba), 231
service(1), 65
service(8), 198
slave server (DNS), 130
SMB, 200
smbclient, 206, 215
smbclient(1), 205, 232
smbd(8), 199, 203, 226
smbpasswd(1), 258
smbpasswd(8), 226, 231
smbtree, 208
smbtree(1), 207
smtp, 124
SNAT, 172
soa (dns record), 128
squid, 106
ssh, 48
ssh_host_dsa_key, 55
ssh_host_dsa_key.pub, 55
ssh_host_rsa_key, 55
ssh_host_rsa_key.pub, 55
sshd, 55
ssh-keygen, 52
ssh-keygen(1), 52
ssh -X, 53
stateful firewall, 171
subtree_check(nfs), 61
swat, 68
swat(8), 209
sysctl, 173
sysctl(1), 23
system-config-securitylevel, 65
T
tcp, 13, 60
tcpdump, 30, 35, 35, 118, 175
tdbsam, 227, 253, 255
telnet, 48, 68
testparm(1), 204, 204, 205
tld, 122
TLD (dns), 122
top level domain, 122
U
udp, 13, 60
V
valid users (Samba), 238
virtualbox, 174
vmware, 174
W
WAN, 11
wbinfo(1), 247, 248
webalizer, 100
winbind(8), 247
winbind(samba), 245
winbindd(8), 199, 199, 247
wireshark, 30, 48, 175
workgroup, 214
WPAN, 11
writable (Samba), 221
write list (Samba), 238
X
X.25, 11
xinetd, 68, 68
xinetd(8), 209
Y
yum, 196
Z
zone (dns), 123, 128
zone transfer (dns), 128

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