17.8 DHCP

Written by Greg Sutter.

17.8.1 What Is DHCP?

DHCP, the Dynamic Host Configuration Protocol, describes the means by which a system can connect to a network and obtain the necessary information for communication upon that network. FreeBSD uses the ISC (Internet Software Consortium) DHCP implementation, so all implementation-specific information here is for use with the ISC distribution.

17.8.2 What this Section Covers

This section attempts to describe only the parts of the DHCP system that are integrated with FreeBSD; consequently, the server portions are not described. The DHCP manual pages, in addition to the references below, are useful resources.

17.8.3 How It Works

When dhclient, the DHCP client, is executed on the client machine, it begins broadcasting requests for configuration information. By default, these requests are on UDP port 68. The server replies on UDP 67, giving the client an IP address and other relevant network information such as netmask, router, and DNS servers. All of this information comes in the form of a DHCP "lease" and is only valid for a certain time (configured by the DHCP server maintainer). In this manner, stale IP addresses for clients no longer connected to the network can be automatically reclaimed.

DHCP clients can obtain a great deal of information from the server. An exhaustive list may be found in dhcp-options(5).

17.8.4 FreeBSD Integration

FreeBSD fully integrates the ISC DHCP client, dhclient. DHCP client support is provided within both the installer and the base system, obviating the need for detailed knowledge of network configurations on any network that runs a DHCP server. dhclient has been included in all FreeBSD distributions since 3.2.

DHCP is supported by sysinstall. When configuring a network interface within sysinstall, the first question asked is, "Do you want to try DHCP configuration of this interface?" Answering affirmatively will execute dhclient, and if successful, will fill in the network configuration information automatically.

There are two things you must do to have your system use DHCP upon startup:

The DHCP server, dhcpd, is included as part of the isc-dhcp2 port in the ports collection. This port contains the full ISC DHCP distribution, consisting of client, server, relay agent and documentation.

17.8.5 Files

17.8.6 Further Reading

The DHCP protocol is fully described in RFC 2131. An informational resource has also been set up at dhcp.org.

This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.

For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.