DHCP client support is included in the FreeBSD installer, making it easy to configure a newly installed system to automatically receive its networking addressing information from an existing DHCP server. Refer to Section 2.8, “Post-Installation” for examples of network configuration.

When dhclient is executed on the client machine, it begins broadcasting requests for configuration information. By default, these requests use UDP port 68. The server replies on UDP port 67, giving the client an IP address and other relevant network information such as a subnet mask, default gateway, and DNS server addresses. This information is in the form of a DHCP “lease” and is valid for a configurable time. This allows stale IP addresses for clients no longer connected to the network to automatically be reused. DHCP clients can obtain a great deal of information from the server. An exhaustive list may be found in dhcp-options(5).

By default, when a FreeBSD system boots, its DHCP client runs in the background, or asynchronously. Other startup scripts continue to run while the DHCP process completes, which speeds up system startup.

Background DHCP works well when the DHCP server responds quickly to the client's requests. However, DHCP may take a long time to complete on some systems. If network services attempt to run before DHCP has assigned the network addressing information, they will fail. Using DHCP in synchronous mode prevents this problem as it pauses startup until the DHCP configuration has completed.

This line in /etc/rc.conf is used to configure background or asynchronous mode:

ifconfig_fxp0="DHCP"

This line may already exist if the system was configured to use DHCP during installation. Replace the fxp0 shown in these examples with the name of the interface to be dynamically configured, as described in Section 12.5, “Setting Up Network Interface Cards”.

To instead configure the system to use synchronous mode, and to pause during startup while DHCP completes, use “SYNCDHCP”:

ifconfig_fxp0="SYNCDHCP"

Additional client options are available. Search for dhclient in rc.conf(5) for details.

The DHCP client uses the following files:

This section demonstrates how to configure a FreeBSD system to act as a DHCP server using the Internet Systems Consortium (ISC) implementation of the DHCP server. This implementation and its documentation can be installed using the net/isc-dhcp42-server package or port.

The installation of net/isc-dhcp42-server installs a sample configuration file. Copy /usr/local/etc/dhcpd.conf.example to /usr/local/etc/dhcpd.conf and make any edits to this new file.

The configuration file is comprised of declarations for subnets and hosts which define the information that is provided to DHCP clients. For example, these lines configure the following:

option domain-name "example.org";
option domain-name-servers ns1.example.org;
option subnet-mask 255.255.255.0;

default-lease-time 600;
max-lease-time 72400;
ddns-update-style none;

subnet 10.254.239.0 netmask 255.255.255.224 {
  range 10.254.239.10 10.254.239.20;
  option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
}

host fantasia {
  hardware ethernet 08:00:07:26:c0:a5;
  fixed-address fantasia.fugue.com;
}

This configuration file supports many more options. Refer to dhcpd.conf(5), installed with the server, for details and examples.

Once the configuration of dhcpd.conf is complete, enable the DHCP server in /etc/rc.conf:

dhcpd_enable="YES"
dhcpd_ifaces="dc0"

Replace the dc0 with the interface (or interfaces, separated by whitespace) that the DHCP server should listen on for DHCP client requests.

Start the server by issuing the following command:

# service isc-dhcpd start

Any future changes to the configuration of the server will require the dhcpd service to be stopped and then started using service(8).

The DHCP server uses the following files. Note that the manual pages are installed with the server software.