Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual page 326

Chapter 21. Dynamic Host Configuration Protocol (DHCP)
Before making any changes, back up the existing /etc/sysconfig/dhcpd and /etc/dhcpd.conf
files.
The DHCP daemon listens on all network interfaces unless otherwise specified. Use the /etc/
sysconfig/dhcpd file to specify which network interfaces the DHCP daemon listens on. The
following /etc/sysconfig/dhcpd example specifies that the DHCP daemon listens on the eth0
and eth1 interfaces:
DHCPDARGS="eth0 eth1";
If a system has three network interfaces cards -- eth0, eth1, and eth2 -- and it is only desired that
the DHCP daemon listens on eth0, then only specify eth0 in /etc/sysconfig/dhcpd:
DHCPDARGS="eth0";
The following is a basic /etc/dhcpd.conf file, for a server that has two network interfaces, eth0
in a 10.0.0.0/24 network, and eth1 in a 172.16.0.0/24 network. Multiple subnet declarations allow
different settings to be defined for multiple networks:
ddns-update-style interim;
default-lease-time 600;
max-lease-time 7200;
subnet 10.0.0.0 netmask 255.255.255.0 {
option subnet-mask 255.255.255.0;
option routers 10.0.0.1;
range 10.0.0.5 10.0.0.15;
}
subnet 172.16.0.0 netmask 255.255.255.0 {
option subnet-mask 255.255.255.0;
option routers 172.16.0.1;
range 172.16.0.5 172.16.0.15;
}
subnet 10.0.0.0 netmask 255.255.255.0
A subnet declaration is required for every network your DHCP server is serving. Multiple subnets
require multiple subnet declarations. If the DHCP server does not have a network interface in a
range of a subnet declaration, the DHCP server does not serve that network.
If there is only one subnet declaration, and no network interfaces are in the range of that subnet,
the DHCP daemon fails to start, and an error such as the following is logged to /var/log/
messages:
dhcpd: No subnet declaration for eth0 (0.0.0.0).
dhcpd: ** Ignoring requests on eth0.
dhcpd: you want, please write a subnet declaration
dhcpd: in your dhcpd.conf file for the network segment
dhcpd: to which interface eth1 is attached. **
dhcpd:
dhcpd:
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