30.5 Automatic Network Configuration (DHCP)

30.5.1 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 OpenBSD dhclient taken from OpenBSD 3.7. All information here regarding dhclient is for use with either of the ISC or OpenBSD DHCP clients. The DHCP server is the one included in the ISC distribution.

30.5.2 What This Section Covers

This section describes both the client-side components of the ISC and OpenBSD DHCP client 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-dhcp42-server port. The dhclient(8), dhcp-options(5), and dhclient.conf(5) manual pages, in addition to the references below, are useful resources.

30.5.3 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).

30.5.4 FreeBSD Integration

FreeBSD fully integrates the OpenBSD 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.

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 to your kernel configuration file, and rebuild the kernel. For more information about building kernels, see Chapter 9.

  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.

• By default, DHCP configuration on FreeBSD runs in the background, or asynchronously. Other startup scripts continue to run while DHCP completes, speeding up system startup.

  Background DHCP works well when the DHCP server responds quickly to requests and the DHCP configuration process goes quickly. However, DHCP may take a long time to complete on some systems. If network services attempt to run before DHCP has completed, they will fail. Using DHCP in synchronous mode prevents the problem, pausing startup until DHCP configuration has completed.

  To connect to a DHCP server in the background while other startup continues (asynchronous mode), use the “DHCP” value in /etc/rc.conf:

  ifconfig_fxp0="DHCP"
  

  To pause startup while DHCP completes, use synchronous mode with the “SYNCDHCP” value:

  ifconfig_fxp0="SYNCDHCP"
  

  Note: Replace the fxp0 shown in these examples with the name of the interface to be dynamically configured, as described in Section 12.8.

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

  dhclient_program="/sbin/dhclient"
  dhclient_flags=""
  

The DHCP server, dhcpd, is included as part of the net/isc-dhcp42-server port in the ports collection. This port contains the ISC DHCP server and documentation.

30.5.5 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.interface

  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.

30.5.6 Further Reading

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

30.5.7 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;
}

dhcpd_enable="YES"
dhcpd_ifaces="dc0"

# service isc-dhcpd start