Chapter 35: DHCP Server

Overview

Many networks now provide the DHCP service that automatically provides network configuration for all connected hosts. They use a protocol called DHCP (Dynamic Host Configuration Protocol). With the DHCP protocol an administrator can use a pool of IP addresses from which it can assign an IP address to a host as needed. The protocol can also be used to provide all necessary network connection information such as the gateway address for the network or the netmask. Instead of having to configure each host separately, network configuration can be handled by a central DHCP server. The length of time that an address can be used can be controlled by means of leases, making effective use of available addresses. If your network is configuring your system with DHCP, you will not have to configure it manually. All necessary information will be automatically entered into your network configuration files.

DHCP provides configuration information to systems connected to a TCP/IP network, whether the Internet or an intranet. The machines on the network operate as DHCP clients, obtaining their network configuration information from a DHCP server on their network. A machine on the network runs a DHCP client daemon that automatically downloads its network configuration information from its network's DHCP server. The information includes its IP address, along with the network's name server, gateway, and proxy addresses, including the netmask. Nothing has to be configured manually on the local system. This has the added advantage of centralizing control over network configuration for the different systems on the network. A network administrator can manage the network configurations for all the systems on the network from the DHCP server.

Note

DHCP is based on the BOOTP protocol developed for diskless workstations. Check DHCP documentation for options specific to machines designed to work with BOOTP.

A DHCP server also supports several methods for IP address allocation: automatic, dynamic, and manual. Automatic allocation assigns a permanent IP address for a host. Manual allocation assigns an IP address designated by the network administrator. With dynamic allocation, a DHCP server can allocate an IP address to a host on the network only when the host actually needs to use it. Dynamic allocation takes addresses from a pool of IP addresses that hosts can use when needed and release when they are finished.

The current version of DHCP now supports the DHCP failover protocol in which two DHCP servers support the same address pool. Should one fail, the other can continue to provide DHCP services for a network. Primary and secondary servers in this scheme are designated with the primary and secondary statements.

A variety of DHCP servers and clients are available for different operating systems. For Linux, you can obtain DHCP software from the Internet Software Consortium (ISC) at www.isc.org. The software package includes a DHCP server, a client, and a relay agent. Linux includes a DHCP server and client. The DHCP client is called dhcpcd, and the server is called dhcpd. The network information a DHCP client downloads is kept in its own network configuration files in the /etc/dhcpc directory. For example, here you can find a resolv.conf file for your network's name servers.