32.2. The ISC DHCP Server

The most common DHCP server for UNIX system is the ISC server, of which several versions have been released. The latest is version 3, but version 2 is still in common use. Release 1 uses a very different configuration file format to later versions and is not seen much anymore. The ISC DHCP server supports a wide range of options and can be configured to behave differently for different clients, networks, and address ranges.

The ISC server can be used to assign fixed addresses to hosts or addresses from certain ranges. Every host is identified by its MAC addresses, which on an Ethernet LAN is the address on the host's Ethernet card. A static IP address and other options can be associated with a particular hardware address, which allows you to fix the address that certain systems receive while using dynamic allocation for others.

The server's configuration file contains four different types of entries, which contain options that affect different clients:

Subnet A subnet is an entire IP network, such as 192.168.1.0. Entries of this type are used to dynamically allocate addresses within certain ranges to clients within the network.

Shared network A shared network is a group of subnets that share the same physical network.

Host This is a single client host identified by its MAC address and assigned a fixed IP address.

Group This is a group of hosts for which the same options can be set.

Entries in the server configuration are arranged in a hierarchy that determines what client options and other settings apply to a particular client. Options in higher-level entries are overridden by those lower in the hierarchy, which allows an administrator to avoid repeating configuration information while still being able to set individual options for specific hosts. Figure 32.1 below shows which kinds of entries can be defined under which other types.

The ISC DHCP server's primary configuration file is called dhcpd.conf and can usually be found in the /etc directory. Other configuration files can be included by the primary file, but on most systems only dhcpd.conf is used. The only other file used by the server is dhcpd.leases, which contains all granted leases and is always kept up-to-date. Whenever the server is started, it rereads this file to find out which leases are currently active. This means that there is no danger of lease information being lost if the server is stopped and restarted, which is necessary for it to reread the primary configuration file.

Webmin's DHCP Server module directly updates the configuration and lease files when you manage subnets, hosts, groups, and leases. To activate the current configuration, it kills the server process and reruns it, as there is no way to signal the server to reread its configuration file.