29.6. Automatic Network Configuration (DHCP)

Written by Greg Sutter.

29.6.1. 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.

29.6.2. 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.

29.6.3. 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).

29.6.4. 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 required to have the system use DHCP upon startup:

  • Make sure that the bpf device is compiled into the kernel. To do this, add device bpf to the kernel configuration file, and rebuild the kernel. For more information about building kernels, see Chapter 9, Configuring the FreeBSD Kernel.

    The bpf device is already part of the GENERIC kernel that is supplied with FreeBSD, thus there is no need to build a custom kernel for DHCP. In the case of a custom kernel configuration file, this device must be present for DHCP to function properly.


    For those who are particularly security conscious, take note 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; however, the security sensitive types should probably not add bpf to the kernel in the expectation that at some point in the future the system 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:


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



    Replace the fxp0 shown in these examples with the name of the interface to be dynamically configured, as described in Section 12.8, “Setting Up Network Interface Cards”.

    When using a different file system location for dhclient, or if additional flags must be passed to dhclient, 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.

29.6.5. 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.

29.6.6. Further Reading

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

29.6.7. Installing and Configuring a DHCP Server What This Section Covers

This section provides information on how to configure a FreeBSD system to act as a DHCP server using the ISC (Internet Systems Consortium) implementation of the DHCP server.

The server is not provided as part of FreeBSD, and so the net/isc-dhcp42-server port must be installed to provide this service. See Chapter 5, Installing Applications: Packages and Ports for more information on using the Ports Collection. DHCP Server Installation

In order to configure the FreeBSD system as a DHCP server, first ensure that the bpf(4) device is compiled into the kernel. To do this, add device bpf to the kernel configuration file, and rebuild the kernel. For more information about building kernels, see Chapter 9, Configuring the FreeBSD Kernel.

The bpf device is already part of the GENERIC kernel that is supplied with FreeBSD, so there is no need to create a custom kernel in order to get DHCP working.


Those who are particularly security conscious should note that bpf is also the device that allows packet sniffers to function correctly (although such programs still need privileged access). The bpf device is required to use DHCP, but if the sensitivity of the system's security is high, this device should not be included in the kernel purely because the use of DHCP may, at some point in the future, be desired.

An example configuration file is installed by the net/isc-dhcp42-server port. Copy the example /usr/local/etc/dhcpd.conf.example to the actual configuration file, /usr/local/etc/dhcpd.conf. Edits will be made to this new file. Configuring the DHCP Server

dhcpd.conf is comprised of declarations regarding subnets and hosts, and is perhaps most easily explained using an example :

option domain-name "example.com";1 option domain-name-servers;2 option subnet-mask;3 default-lease-time 3600;4 max-lease-time 86400;5 ddns-update-style none;6 subnet netmask { range;7 option routers;8 } host mailhost { hardware ethernet 02:03:04:05:06:07;9 fixed-address mailhost.example.com;10 }


This option specifies the domain that will be provided to clients as the default search domain. See resolv.conf(5) for more information on what this means.


This option specifies a comma separated list of DNS servers that the client should use.


The netmask that will be provided to clients.


A client may request a specific length of time that a lease will be valid. Otherwise the server will assign a lease with this expiry value (in seconds).


This is the maximum length of time that the server will lease for. Should a client request a longer lease, a lease will be issued, although it will only be valid for max-lease-time seconds.


This option specifies whether the DHCP server should attempt to update DNS when a lease is accepted or released. In the ISC implementation, this option is required.


This denotes which IP addresses should be used in the pool reserved for allocating to clients. IP addresses between, and including, the ones stated are handed out to clients.


Declares the default gateway that will be provided to clients.


The hardware MAC address of a host (so that the DHCP server can recognize a host when it makes a request).


Specifies that the host should always be given the same IP address. Note that using a hostname is correct here, since the DHCP server will resolve the hostname itself before returning the lease information.

Once the configuration of dhcpd.conf has been completed, enable the DHCP server in /etc/rc.conf, i.e., by adding:

dhcpd_enable="YES" dhcpd_ifaces="dc0"

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

Proceed to start the server by issuing the following command:

# service isc-dhcpd start

Any future changes to the configuration of the server will require the sending of a SIGTERM signal to dhcpd rather than a SIGHUP. It is definitely more simple to use service(8) to completely restart the service. Files

  • /usr/local/sbin/dhcpd

    dhcpd is statically linked and resides in /usr/local/sbin. The dhcpd(8) manual page installed with the port gives more information about dhcpd.

  • /usr/local/etc/dhcpd.conf

    dhcpd requires a configuration file, /usr/local/etc/dhcpd.conf before it will start providing service to clients. This file needs to contain all the information that should be provided to clients that are being serviced, along with information regarding the operation of the server. This configuration file is described by the dhcpd.conf(5) manual page installed by the port.

  • /var/db/dhcpd.leases

    The DHCP server keeps a database of leases it has issued in this file, which is written as a log. The manual page dhcpd.leases(5), installed by the port gives a slightly longer description.

  • /usr/local/sbin/dhcrelay

    dhcrelay is used in advanced environments where one DHCP server forwards a request from a client to another DHCP server on a separate network. If this functionality is required, then install the net/isc-dhcp42-relay port. The dhcrelay(8) manual page provided with the port contains more detail.

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

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