Essential System Administration, 3rd Edition

Configuring a DHCP Server

In Section 5.2, we considered the process of configuring a client system to obtain an IP address from a DHCP server. In this section, we complete the picture by discussing DHCP server configuration and management.

DHCP servers vary quite a bit from system to system, but the basic concepts that apply to all of them do not. We’ll consider these concepts before diving into the specifics of server configuration on the various Unix systems.

DHCP servers draw the IP addresses that they provide from lists of addresses that they have been given to manage. These lists are known as scopes . Unix DHCP servers can manage one or more scopes. For example, suppose we have a network of two subnets, 10.10.1.0/24 and 10.10.20.0/24, and we place a DHCP server on each one. We want to use 101 IP addresses from each subnet for dynamic assignment, hosts 100 through 200. We might divide up the addresses this way:

Subnet 1 DHCP Server 10.10.1.100-10.10.1.175 10.10.20.176-10.10.20.200

Subnet 2 DHCP Server 10.10.20.100-10.10.20.175 10.10.1.176-10.10.1.200

Each server is assigned part of the address range for each subnet. When a client requests an IP address, the DHCP server always assigns one from the appropriate subnet (if none is available, the request fails). Splitting the addresses this way provides for some fault tolerance. If either server goes down, a DHCP relay (see below) can be set up temporarily on its subnet pointing to the other server, and IP address assignment will still ...