The DHCP server

Where to find a DHCP server?

The Internet Software Consortium develops a free DHCP server for Unix-like system. This is the most spread server and the one that respects at best RFCs. The latest version is 3.0 but it is still a beta version. previous versions are working very well, even if the ISC releases regular patches. One of the most interesting innovation of the version 3 is the possibility to dynamically update a DNS server so that IP addresses and names perfectly match. Actually the first draft for this feature was released in march 1996 and it is not properly implemented yet... To know more about dynamic DNS update.

Obviously, Microsoft has its own DHCP server for Windows, but only the version for Windows 2000 Server can dynamically update DNS. Microsoft already had such a feature but for its proprietary name system using a WINS server.

What follows in this page deals with the installation and configuration of the ISC DHCP server under a Unix-like system.

Compiling the server

This is the very first thing to do once you have downloaded the sources.

Visit and download a DHCP server from the ISC web site, or simply this version that implements in a tricky way the dynamic DNS update feature. Copy this file in a directory.

Uncompress the archive: tar xzf dhcp-2.0pl5.tar.gz

A new directory is created. Go into it (with cd) and type: ./configure

This prepares the Makefile files corresponding to your system. Now, type: make to compile the server. Finally, type: make install to definitely install the DHCP server at the right place on your system.

Before typing ./configure, it is highly advised to read the README file that explains in a deeper way how to install the server. For instance with the version provided on this web site, you could learn how to compile the server so that it will update a DNS server: type ./configure --with-nsupdate to build a server with dynamic DNS update support. make install will copy some perl scripts in the /usr/local/DHCP-DNS-0.52mdn.

dhcpd.conf

This file is used to configure the server. By default, it is located in /etc but it is possible to put it somewhere else. It is made up of several sections, some of them delimited with braces ({ and }):

• global parameters,
• shared-network { ... },
• subnet { ... },
• host { ... },
• group { ... }.

Every section may contain parameters and options. A group section may contain several host sections. At the very beginning of the file, one can write global parameters, such as the lease duration, that will be used for the rest of the configuration.

Every line of this file must end with a ;, unless there is a brace.

Comments are allowed with a # at the beginning of the line.

Global parameters

Nearly everything can be used as a global parameters, provided that they have a real meaning for the configuration in a whole. For instance, it is possible to define the duration for leases (max-lease-time and default-lease-time), avoid the server to reply to messages from undeclared hosts (deny unknown-clients;), give the domain name, DNS servers addresses... See an example.

shared-network

This statement is used to gather several subnet sections when they are sharing the same physical network. Parameters provided at the beginning of the statement will be used for booting hosts, unless you use host sections to prevent the server to reply for these hosts. It is recommended to use it every time several subnet sections concern the same physical network.

Syntax:

shared-network FOO-BAR {
	filename "boot";

	subnet 192.168.2.0 netmask 255.255.255.224 {
		range 192.168.2.10 192.168.2.30;
	}

	subnet 192.168.2.32 netmask 255.255.255.224 {
		range 192.168.2.40 192.168.2.50;
	}
}

subnet

This section is used to define subnetworks on which the DHCP server must intervene. It is maybe the most important part of the configuration file: without them, the server will never work.

The syntax is the following:

subnet subnet_address netmask netmask {
	[ global parameters... ]
	[ declarations... ]
}

subnet_addess and netmask are formatted in the IP address way (255.255.248.0 for instance). There is an example above.

It is obviously possible to use global parameters that should be valid only for computers from this subnet. For instance, we can give the domain name for this subnet (option domain-name). We can use host statements.

The most important global parameter is range: range [ dynamic-bootp ] lower_address [ upper_address ]; that defines the range of IP addresses the server can distribute for this subnet. We can use several range parameters if needed. If we do not give any upper-range address, the DHCP server will use only lower one. dynamic-bootp must be used to ask the server to reply to BOOTP request with an address from this range.

host

This statement is used to define host typical parameters. This section is not mandatory but it can be useful if there is a deny unknown-clients; at the beginning of the file to give a sense to the DHCP server (it will never work, otherwise).

host is used like this:

host name {
	parameters...
}

A host may be recognized by 2 ways: whether by its name (the name that comes just after host) or by its hardware address (ethernet or token-ring). In this latter case, one must add a line in the host statement: hardware ethernet|token-ring adresse-hardware;. It is highly recommended to identify hosts using their hardware address rather than using their name, all the more so they are not supposed to have a name and this name may change. However, the name provided after host can be internally used in the configuration file as an indentifier.

Important point: we must use a host statement if we want the DHCP server to allocate a fixed IP address to a given host: fixed-address 192.168.2.4;. Obviously, such IP addresses must be removed in the range... parameters.

group

It is just used to gather host statement to apply a common set of global parameters:

group {
	option domain-name "bar.org";
	option routers 192.168.1.254;

	host foo1 {
		...
	}

	host foo2 {
		...
	}
}

Options

Parameter lines beginning with "option" are options defined in RFC2132. There are only about 60 options defined in this RFC, even if it is possible to have up to 254 options (options 0 and 255 are reserved). To find the complete list of options and their names, you can check the file common/tables.c located in the sources of the server. Be careful, names can vary from one version to another.

The most common options are the following ones:

• subnet-mask (option 1) is used to give the subnet-makc value to the client.
• routers (option 3) that points out routers and gateways.
• domain-name-servers (option 6) that points out the DNS servers. It is possible to give the names of the DNS servers as well as their IP addresses.
• host-name (option 12) is used to give its name to a host.
• domain-name (option 15) that points out the domain name.
• broadcast-address (option 28) is used to tell the broadcast address available on the local network.
• dhcp-lease-time (option 51) is used to tell how long the lease is valid.

Other options (60 in particular) can be used to personalize a DHCP message.

Complete example for dhcpd.conf

max-lease-time 240;
default-lease-time 240;
deny unknown-clients;
option domain-name "bar.com";
option domain-name-servers foo1.bar.com, foo2.bar.com;

subnet 192.168.1.0 netmask 255.255.255.0 {
	range 192.168.1.2 192.168.1.100;
	range 192.168.1.110 192.168.1.254;
	option broadcast-address 192.168.1.255;
}

group {
	option routers 192.168.2.101;

	host foo3 {
		hardware ethernet 00:c0:c3:11:90:23;
		option host-name pc3;
	}

	host foo4 {
		hardware ethernet 00:c0:c3:cc:0a:8f;
		fixed-address 192.168.1.105;
	}
}

host foo5 {
	hardware ethernet 00:c0:c3:2a:34:f5;
	server-name "bootp.bar.com";
	filename "boot";
}

Explanation:

The first 5 lines define global parameters. The first 2 ones deal with leases. The next line asks the server not to reply to DHCP messages from unknown hosts (i.e. not declared in dhcpd.conf). Finally, we define global parameters for the network (domain name and DNS servers).

Then, we define the subnetwork with which the DHCP server should work; it is the line starting with "subnet...". In this subnet, we would like the server to distribute IP addresses in 2 specific ranges ("range..."). The last line of the subnet statement defines the broadcast address that should be used by DHCP clients for this subnet.

Then we create a group whose unique role is to give a gateway address to some specific machines (recognized thanks to their MAC addresses). Let's notice that foo4.bar.com should get a fixed address.

Finally, foo5 is a machine that will boot remotely, connecting to the server called bootp.bar.com and asking for the file "boot".

Starting the dhcpd daemon

To start the server, you must be root on the system. Then, you have to use this command line:

dhcpd -lf lease_database -cf config_file adpater1 adapter2...

The DHCP server is requested to work on the network adapters adapter1, adapter2... The server will also find its configuration in the "config_file" file, and will use the file "lease_database" to store the leases it will grant. If no arguments are provided, the DHCP server will look for its default files, i.e. those specified at the compilation process in the includes/dhcpd.h file, and will use eth0 as a unique network adapter to work on. Of course, it is possible to change all these parameters.

How to start automatically the server when the server boots?

To make the DHCP server start when your server machine boots, you need to add a launching script in /etc/rc.d/init.d/. This script will be actually used to start and stop the daemon. This file is actually not provided in the ISC package, so you have to write it by yourself by taking example on those that are already in this directory. Nevertheless, you can simply use this one. Check it to give the correct parameter. Do a chmod 755 dhcpd to set the rights.

Now, you have to ask Linux to use this script at its boot process. This is performed by creating symbol links in /etc/rc.d/rcx.d/ where x is an enteger that represents the level at which the server should be started or stopped. However, with a correctly written script, you will just have to execute: chkconfig --add dhcpd and all the right symbol link will be created.

You can now start you machine and the DHCP server will start automatically.

WARNING! linuxconf may take control of your server. If you do not want linuxconf to do that, you will have to tell it not to do so (start its configuration tool and look in the menu).

Documentation

make install should have copied standard manual pages onto your system. To access them type:

• man dhcpd to learn how dhcpd works,
• man dhcpd.conf to learn a bit more about writting a perfect dhcpd.conf file,
• man dhcpd.leases to know more about leases and the lease database format.

This documentation is unfortunately not complete or simple; for instance, DHCP options are not detailed. The best thing to learn a lot, but it is tedious, is to read the RFCs which, for once, are quite clear and simple.

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