Date: Mon, 26 Nov 2001 11:00:03 -0800 (PST) From: setantae <setantae@submonkey.net> To: freebsd-doc@freebsd.org Subject: Re: docs/32229: Omission from Handbook Chapter 17.8 (DHCP) Message-ID: <200111261900.fAQJ03809531@freefall.freebsd.org>
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The following reply was made to PR docs/32229; it has been noted by GNATS.
From: setantae <setantae@submonkey.net>
To: FreeBSD-gnats-submit@freebsd.org
Cc:
Subject: Re: docs/32229: Omission from Handbook Chapter 17.8 (DHCP)
Date: Mon, 26 Nov 2001 18:50:05 +0000
OK, how about this ?
All feedback more than welcome.
Ceri
<sect1 id="dhcp-server">
<sect1info>
<authorgroup>
<author>
<firstname>Ceri</firstname>
<surname>Davies</surname>
<contrib>Written by </contrib>
</author>
</authorgroup>
</sect1info>
<title>DHCP</title>
<sect2>
<title>What Is DHCP?</title>
<indexterm>
<primary>Dynamic Host Configuration Protocol</primary>
<see>DHCP</see>
</indexterm>
<indexterm>
<primary>Internet Software Consortium (ISC)</primary>
</indexterm>
<para>DHCP, the Dynamic Host Configuration Protocol, is used to
provide information to a system on how to connect to a network and
for a system to request that information.</para>
</sect2>
<sect2>
<title>What this Section Covers</title>
<para>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.</para>
<para>The server portion of the suite is not provided as part of
FreeBSD, and so you will need to install the <port>net/isc-dhcp2</port>
port to provide this service. See <xref linkend="ports"></xref> for more
information on using the ports collection.</para>
</sect2>
<sect2>
<title>How It Works</title>
<indexterm><primary>UDP</primary></indexterm>
<para>When the server daemon <command>dhcpd</command> starts up,
it sleeps and listens for broadcast requests for network
configuration information. By default, it will listen on UDP port 67.
When such a request is received, then the server will reply to
the client machine on UDP port 68, providing details required to
connect to the network such as IP address, subnet mask, default
gateway and DNS servers.</para>
<para>Also included with this reply is a length of time for which
this information can be used by that particular client. This is
known as a DHCP "lease" and a new lease must be acquired by the
client when it expires. The length of time for which a lease is
valid is decided by the administrator of the DHCP server.</para>
<para>DHCP clients can obtain a great deal of information from
the server. An exhaustive list may be found in
&man.dhcp-options.5;.</para>
</sect2>
<sect2>
<title>DHCP Server Installation</title>
<indexterm>
<primary>DHCP</primary>
<secondary>installation</secondary>
</indexterm>
<para>In order to configure your FreeBSD system as a DHCP server,
you will need to ensure that the <devicename>bpf</devicename>
device is compiled into your kernel. To do this, add
<literal>pseudo-device bpf</literal> to your kernel
configuration file, and rebuild the kernel. For more
information about building kernels, see <xref
linkend="kernelconfig"></xref>.</para>
<para>The <devicename>bpf</devicename> device is already
part of the <filename>GENERIC</filename> 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.</para>
<note>
<para>For those who are particularly security conscious,
you should be warned that <devicename>bpf</devicename>
is also the device that allows packet sniffers to work
correctly (although they still have to be run as
root). <devicename>bpf</devicename>
<emphasis>is</emphasis> required to use DHCP, but if
you are very sensitive about security, you probably
should not add <devicename>bpf</devicename> to your
kernel in the expectation that at some point in the
future you will be using DHCP.</para>
</note>
<para>The next thing that you will need to do is edit the file
<filename>dhcpd.conf</filename> that was installed by the
<port>net/isc-dhcp2</port> port. By default, this will be
<filename>/usr/local/etc/dhcpd.conf</filename>.</para>
</sect2>
<sect2>
<title>Configuring the DHCP Server</title>
<indexterm>
<primary>DHCP</primary>
<secondary>configuration</secondary>
</indexterm>
<para>The file <filename>/usr/local/etc/dhcpd.conf</filename> is
comprised of declarations regarding subnets and hosts, and is
perhaps most easily explained using an example :</para>
<programlisting>
option domain-name "example.com";<co id="domain-name"></co>
option domain-name-servers 192.168.4.100;<co id="domain-name-servers"></co>
option subnet-mask 255.255.255.0;<co id="subnet-mask"></co>
default-lease-time 3600;<co id="default-lease-time"></co>
max-lease-time 86400;<co id="max-lease-time"></co>
subnet 192.168.4.0 netmask 255.255.255.0 {
range 192.168.4.129 192.168.4.254;<co id="range"></co>
option routers 192.168.4.1;<co id="routers"></co>
}
host mailhost {
hardware ethernet 02:03:04:05:06:07;<co id="hardware"></co>
fixed-address mailhost.example.com;<co id="fixed-address"></co>
}
</programlisting>
<calloutlist>
<callout arearefs="domain-name">
<para>This option specifies the domain that will be provided
to clients as the default search domain. See &man.resolv.conf.5;
for more information on what this means.</para>
</callout>
<callout arearefs="domain-name-servers">
<para>This option specifies a comma seperated list of DNS
servers that the client should use.</para>
</callout>
<callout arearefs="subnet-mask">
<para>The netmask that will be provided to clients.</para>
</callout>
<callout arearefs="default-lease-time">
<para>A client may request a specific length of time that a
lease will be valid. If it doesn't, then the server will assign
a lease with this expiry value (in seconds).</para>
</callout>
<callout arearefs="max-lease-time">
<para>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
<literal>max-lease-time</literal> seconds.</para>
</callout>
<callout arearefs="range">
<para>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.</para>
</callout>
<callout arearefs="routers">
<para>Declares the default gateway that will be provided to
clients.</para>
</callout>
<callout arearefs="hardware">
<para>The hardware MAC address of a host (so that the DHCP server
can recognise a host when it makes a request).</para>
</callout>
<callout arearefs="fixed-address">
<para>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.</para>
</callout
</calloutlist>
<para>Once you have finished writing your <filename>dhcpd.conf</filename>,
you can proceed to start the server by issuing the command:</para>
<screen>
&prompt.root; <userinput>/usr/local/etc/rc.d/isc-dhcpd.sh start</userinput>
</screen>
<para>Should you need to make changes to the configuration of your
server in the future, it's important to note that sending a
<literal>SIGHUP</literal> signal to <application>dhcpd</application>
does not result in the configuration being reloaded, as it does with
most daemons. You will need to send a <literal>SIGTERM</literal>
signal to stop the process, and then restart it using the command above.</para>
</sect2>
<sect2>
<title>Files</title>
<indexterm>
<primary>DHCP</primary>
<secondary>configuration files</secondary>
</indexterm>
<itemizedlist>
<listitem><para><filename>/usr/local/sbin/dhcpd</filename></para>
<para><command>dhcpd</command> is statically linked and
resides in <filename>/usr/local/sbin</filename>. The &man.dhcpd.8;
manual page gives more information about
<command>dhcpd</command>.</para>
</listitem>
<listitem><para><filename>/usr/local/etc/dhcpd.conf</filename></para>
<para><command>dhcpd</command> requires a configuration file,
<filename>/usr/local/etc/dhcpd.conf</filename> 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
&man.dhcpd.conf.5; manual page.</para>
</listitem>
<listitem><para><filename>/var/db/dhcpd.leases</filename></para>
<para>The DHCP server keeps a database of leases it has issued
in this file, which is written as a log. &man.dhcpd.leases.5;
gives a slightly longer description.</para>
</listitem>
<listitem><para><filename>/usr/local/sbin/dhcrelay</filename></para>
<para><command>dhcrelay</command> is used in advanced environments
where one DHCP server forwards a request from a client to
another DHCP server on a separate network. The &man.dhcrelay.8;
manual page provides more information.</para>
</listitem>
</itemizedlist>
</sect2>
<sect2>
<title>Further Reading</title>
<para>The DHCP protocol is fully described in
<ulink url="http://www.freesoft.org/CIE/RFC/2131/">RFC 2131</ulink>.
An informational resource has also been set up at
<ulink url="http://www.dhcp.org/">dhcp.org</ulink>.</para>;
</sect2>
</sect1>
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