6.4. Configuring a Multihomed DHCP Server

A multihomed DHCP server serves multiple networks, that is, multiple subnets. The examples in these sections detail how to configure a DHCP server to serve multiple networks, select which network interfaces to listen on, and how to define network settings for systems that move networks.

Before making any changes, back up the existing /etc/sysconfig/dhcpd and /etc/dhcp/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/dhcp/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:

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.  If this is not what
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:
dhcpd: Not configured to listen on any interfaces!

option subnet-mask 255.255.255.0;

The option subnet-mask option defines a subnet mask, and overrides the netmask value in the subnet declaration. In simple cases, the subnet and netmask values are the same.

option routers 10.0.0.1;

The option routers option defines the default gateway for the subnet. This is required for systems to reach internal networks on a different subnet, as well as external networks.

range 10.0.0.5 10.0.0.15;

The range option specifies the pool of available IP addresses. Systems are assigned an address from the range of specified IP addresses.

For further information, refer to the dhcpd.conf(5) man page.

Alias Interfaces

Alias interfaces are not supported by DHCP. If an alias interface is the only interface, in the only subnet specified in /etc/dhcp/dhcpd.conf, the DHCP daemon fails to start.

6.4.1. Host Configuration

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

Configuring a single system for multiple networks

The following /etc/dhcp/dhcpd.conf example creates two subnets, and configures an IP address for the same system, depending on which network it connects to:

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 ip link command.
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/dhcp/dhcpd.conf line 20: semicolon expected.
dhcpd: }
dhcpd: ^
dhcpd: /etc/dhcp/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:

assign a valid fixed-address for the network the host is connecting to.
make the name in the host declaration unique.

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/dhcp/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/dhcp/dhcpd.conf.