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DHCP Server Configuration for Linux

Author: Edward BuckVersion: .2Last edited: December 21, 2002

Summary

This guide will help you setup a dhcp server to provide network configuration information toclients on the network. These instructions were written with Red Hat 7.x systems in mind but thebasic concepts provided here can be applied to other distributions as well.

Instructions

1. Download dhcp rpm package from Red Hat and install:

# rpm -ivh dhcp-2.0pl5-8.i386.rpm

2. Open file /etc/sysconfig/dhcpd and edit the first line as follows:

DHCPDARGS=eth1

Replace 'eth1' above with the network interface that you want to use for dhcp; this shouldbe an internal network interface; denial of service attacks are possible if dhcp is runningon an external interface.

3. Copy /usr/share/doc/dhcp-2.0pl5/dhcpd.conf.sample to /etc

# cp /usr/share/doc/dhcp-2.0pl5/dhcpd.conf.sample /etc/dhcpd.conf

This sample file is a good starting point for our /etc/dhcpd.conf file, which by default is notinstalled. Alternatively, copy the file from a working server.

4. Edit /etc/dhcpd.conf to suit your needs. An example file is included below for reference:5. #################file begin######################

6. subnet 10.0.0.0 netmask 255.255.255.0 {

7. # --- default gateway

8. option routers 10.0.0.1;

9. option subnet-mask 255.255.255.0;

10.

11.# option nis-domain "mydomain.com";

12. option domain-name "mydomain.com";

13. option domain-name-servers 216.227.56.120,

64.34.4.36;14.

15. option time-offset -28800; # Pacific

Standard Time

16.# option ntp-servers 192.168.1.1;

17.# option netbios-name-servers 192.168.1.1;

18.# --- Selects point-to-point node (default is hybrid). Don't

change this unless

19.# -- you understand Netbios very well

20.# option netbios-node-type 2;

mailto:ebuck@linuxselfhelp.commailto:ebuck@linuxselfhelp.com

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21.

22. range 10.0.0.50 10.0.0.254;

23. default-lease-time 604800;

24. max-lease-time 604800;

25.

26. host test {

27.# option dhcp-client-identifier "test";

28. hardware ethernet 00:e0:18:90:28:b2;

29. fixed-address 10.0.0.10;

30. }

31. # we want the nameserver to appear at a fixed address

32.# host ns {

33.# next-server marvin.redhat.com;

34.# hardware ethernet 12:34:56:78:AB:CD;

35.# fixed-address 207.175.42.254;

36.# }

37.}

########################file end##########################

Notes: specific settings always override global settings; in the above, the range 10.0.0.50

to 10.0.0.254 have been set side for dynamic hosts; this allows anything between10.0.0.1 and 10.0.0.49 to be set aside as static ips. In the example, host 'test' is given astatic ip using its mac address. The option 'dhcp-client-identifier' may work as analternative to mac address, but may require some additional configuration on the client.The max lease time of 604800 translates to 7 days. Lease times are automaticallyrenewed by clients once 50% of the expiration date is reached. Because of this, very longlease times should be unnecessary. If a very long one is required, provide the client astatic ip using the host declaration. Also, the option time-offset setting is in secondsaccording to the manual page; Red Hat's configuration document erroneously lists thissetting in hours. Use option host-name "apex.example.com" in a host declaration toprovide hostnames to clients.

38. Check that the lease database has been created; the rpm should create this file

automatically; if not, create the file:

# touch /var/lib/dhcp/dhcpd.leases

The lease database is recreated from time to time so that it is not too large. First, allknown leases are saved in a temporary lease database. The dhcpd.leases file isrenamed dhcpd.leases~, and the temporary lease database is written to dhcpd.leases.

The DHCP daemon could be killed or the system could crash after the lease databasehas been renamed to the backup file but before the new file has been written. If thishappens, there is no dhcpd.leases file that is required to start the service. Do not create anew lease file if this occurs. If you do, all the old leases will be lost and cause manyproblems. The correct solution is to rename the dhcpd.leases~ backup file to

dhcpd.leases and then start the daemon.

39. Run 'setup' and check dhcpd to have it load at system boot

40. Start/restart the server

# service dhcpd start (restart)

Changes to the file /etc/dhcpd.conf require the dhcp server to be restarted

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41. Test to make sure it works.