Host Configuration

Before making any changes, back up the existing /etc/sysconfig/dhcpd and /etc/dhcpd.conf files.

Configuring a single system for multiple networks

The following /etc/dhcpd.conf example creates two subnets, and configures an IP address for the same system, depending on which network it connects to: 
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;
}
host example0 {
 hardware ethernet 00:1A:6B:6A:2E:0B;
 fixed-address 10.0.0.20;
}
host example1 {
 hardware ethernet 00:1A:6B:6A:2E:0B;
 fixed-address 172.16.0.20;
}
host example0

The host declaration defines specific parameters for a single system, such as an IP address. To configure specific parameters for multiple hosts, use multiple host declarations.

Most DHCP clients ignore the name in host declarations, and as such, this name can anything, as long as it is unique to other host declarations. To configure the same system for multiple networks, use a different name for each host declaration, otherwise the DHCP daemon fails to start. Systems are identified by the hardware ethernet option, not the name in the host declaration.

hardware ethernet 00:1A:6B:6A:2E:0B;

The hardware ethernet option identifies the system. To find this address, run the ifconfig command on the desired system, and look for the HWaddr address.

fixed-address 10.0.0.20;

The fixed-address option assigns a valid IP address to the system specified by the hardware ethernet option. This address must be outside the IP address pool specified with the range option.

If option statements do not end with a semicolon, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages: 

/etc/dhcpd.conf line 20: semicolon expected.
dhcpd: }
dhcpd: ^
dhcpd: /etc/dhcpd.conf line 38: unexpected end of file
dhcpd:
dhcpd: ^
dhcpd: Configuration file errors encountered -- exiting

Configuring systems with multiple network interfaces

The following host declarations configure a single system, that has multiple network interfaces, so that each interface receives the same IP address. This configuration will not work if both network interfaces are connected to the same network at the same time: 
host interface0 {
 hardware ethernet 00:1a:6b:6a:2e:0b;
 fixed-address 10.0.0.18;
}
host interface1 {
 hardware ethernet 00:1A:6B:6A:27:3A;
 fixed-address 10.0.0.18;
}

For this example, interface0 is the first network interface, and interface1 is the second interface. The different hardware ethernet options identify each interface.

If such a system connects to another network, add more host declarations, remembering to:

When a name given in a host declaration is not unique, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages: 

dhcpd: /etc/dhcpd.conf line 31: host interface0: already exists
dhcpd: }
dhcpd: ^
dhcpd: Configuration file errors encountered -- exiting

This error was caused by having multiple host interface0 declarations defined in /etc/dhcpd.conf.