23.5 Automatic Network Configuration (DHCP)

23.5.1 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.

23.5.2 What This Section Covers

This section describes both the client-side and server-side components of the ISC DHCP system. The client-side program, dhclient, comes integrated within FreeBSD, and the server-side portion is available from the net/isc-dhcp3-server port. The dhclient(8), dhcp-options(5), and dhclient.conf(5) manual pages, in addition to the references below, are useful resources.

23.5.3 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).

23.5.4 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 second question asked is: “Do you want to try DHCP configuration of the 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:

• Make sure that the bpf device is compiled into your kernel. To do this, add device bpf (pseudo-device bpf under FreeBSD 4.X) to your kernel configuration file, and rebuild the kernel. For more information about building kernels, see Chapter 8.

  The bpf device is already part of the GENERIC kernel that is supplied with FreeBSD, so if you do not have a custom kernel, you should not need to create one in order to get DHCP working.

  Note: For those who are particularly security conscious, you should be warned that bpf is also the device that allows packet sniffers to work correctly (although they still have to be run as root). bpf is required to use DHCP, but if you are very sensitive about security, you probably should not add bpf to your kernel in the expectation that at some point in the future you will be using DHCP.

• Edit your /etc/rc.conf to include the following:

  ifconfig_fxp0="DHCP"
  

  Note: Be sure to replace fxp0 with the designation for the interface that you wish to dynamically configure, as described in Section 11.8.

  If you are using a different location for dhclient, or if you wish to pass additional flags to dhclient, also include the following (editing as necessary):

  dhcp_program="/sbin/dhclient"
  dhcp_flags=""
  

The DHCP server, dhcpd, is included as part of the net/isc-dhcp3-server port in the ports collection. This port contains the ISC DHCP server and documentation.

23.5.5 Files

• /etc/dhclient.conf

  dhclient requires a configuration file, /etc/dhclient.conf. Typically the file contains only comments, the defaults being reasonably sane. This configuration file is described by the dhclient.conf(5) manual page.

• /sbin/dhclient

  dhclient is statically linked and resides in /sbin. The dhclient(8) manual page gives more information about dhclient.

• /sbin/dhclient-script

  dhclient-script is the FreeBSD-specific DHCP client configuration script. It is described in dhclient-script(8), but should not need any user modification to function properly.

• /var/db/dhclient.leases

  The DHCP client keeps a database of valid leases in this file, which is written as a log. dhclient.leases(5) gives a slightly longer description.

23.5.6 Further Reading

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

23.5.7 Installing and Configuring a DHCP Server

option domain-name "example.com";
option domain-name-servers 192.168.4.100;
option subnet-mask 255.255.255.0;

default-lease-time 3600;
max-lease-time 86400;
ddns-update-style none;

subnet 192.168.4.0 netmask 255.255.255.0 {
  range 192.168.4.129 192.168.4.254;
  option routers 192.168.4.1;
}

host mailhost {
  hardware ethernet 02:03:04:05:06:07;
  fixed-address mailhost.example.com;
}

# /usr/local/etc/rc.d/isc-dhcpd.sh start