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19.10 DHCP

Written by Greg Sutter.

19.10.1 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.

19.10.2 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 port. The dhclient(8), dhcp-options(5), and dhclient.conf(5) manual pages, in addition to the references below, are useful resources.

19.10.3 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).

19.10.4 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:

Make sure that the bpf device is compiled into your kernel. To do this, add pseudo-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.
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 6.8.

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 port in the ports collection. This port contains the full ISC DHCP distribution, consisting of client, server, relay agent and documentation.

19.10.5 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.

19.10.6 Further Reading

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

19.10.7 Installing and Configuring a DHCP Server

19.10.7.1 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 Software Consortium) implementation of the DHCP suite.

The server portion of the suite is not provided as part of FreeBSD, and so you will need to install the net/isc-dhcp3 port to provide this service. See Chapter 4 for more information on using the ports collection.

19.10.7.2 DHCP Server Installation

In order to configure your FreeBSD system as a DHCP server, you will need to ensure that the bpf(4) device is compiled into your kernel. To do this, add pseudo-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 you do not need to create a custom kernel in order to get DHCP working.

Note: Those who are particularly security conscious should note that bpf is also the device that allows packet sniffers to work correctly (although such programs still need privileged access). bpf is required to use DHCP, but if you are very sensitive about security, you probably should not include bpf in your kernel purely because you expect to use DHCP at some point in the future.

The next thing that you will need to do is edit the sample dhcpd.conf which was installed by the net/isc-dhcp3 port. By default, this will be /usr/local/etc/dhcpd.conf.sample, and you should copy this to /usr/local/etc/dhcpd.conf before proceeding to make changes.

19.10.7.3 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";
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;
}

: 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 a hostname is OK here, since the DHCP server will resolve the hostname itself before returning the lease information.

Once you have finished writing your dhcpd.conf, you can proceed to start the server by issuing the following command:

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

Should you need to make changes to the configuration of your server in the future, it is important to note that sending a SIGHUP signal to dhcpd does not result in the configuration being reloaded, as it does with most daemons. You will need to send a SIGTERM signal to stop the process, and then restart it using the command above.

19.10.7.4 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. The dhcrelay(8) manual page provided with the port contains more detail.

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