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 versions prior to 6.0 use the ISC (Internet Software Consortium) DHCP
client (dhclient(8))
implementation. Later versions use 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.
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-dhcp3-server port. The dhclient(8), dhcp-options(5), and
dhclient.conf(5)
manual pages, in addition to the references below, are useful resources.
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).
FreeBSD fully integrates the ISC or OpenBSD DHCP client, dhclient (according to the FreeBSD version you run). 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 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).
bpfis
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.
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.
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
server.
The server is not provided as part of FreeBSD, and so you will need to install the net/isc-dhcp3-server port to provide this service. See Chapter 4 for more information on using the Ports Collection.
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 device bpf 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 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). bpfis
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-server 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.
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 you have finished writing your dhcpd.conf, you should
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 your DHCP server should listen on for DHCP
client requests.
Then, 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.
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
you require this functionality, then install the net/isc-dhcp3-relay port. The dhcrelay(8)
manual page provided with the port contains more detail.