The purpose of the Dynamic Host Configuration Protocol is to assign network settings centrally from a server rather than configuring them locally on each and every workstation. A client configured to use DHCP does not have control over its own static address. It is enabled to configure itself completely and automatically according to directions from the server.

One way to use DHCP is to identify each client using the hardware address of its network card (which is fixed in most cases) then supply that client with identical settings each time it connects to the server. DHCP can also be configured so the server assigns addresses to each interested host dynamically from an address pool set up for that purpose. In the latter case, the DHCP server will try to assign the same address to the client each time it receives a request from it (even over longer periods). This, of course, will not work if there are more client hosts in the network than network addresses available.

With these possibilities, DHCP can make life easier for system administrators in two ways. Any changes (even bigger ones) related to addresses and the network configuration in general can be implemented centrally by editing the server's configuration file. This is much more convenient than reconfiguring lots of client machines. Also it is much easier to integrate machines, particularly new machines, into the network, as they can be given an IP address from the pool. Retrieving the appropriate network settings from a DHCP server can be especially useful in the case of laptops regularly used in different networks.

A DHCP server not only supplies the IP address and the netmask, but also the host name, domain name, gateway, and name server addresses to be used by the client. In addition to that, DHCP allows for a number of other parameters to be configured in a centralized way, for example, a time server from which clients may poll the current time or even a print server.

The following section gives an overview of DHCP without describing the service in every detail. In particular, we want to show how to use the DHCP server dhcpd in your own network to easily manage its entire setup from one central point.

Both a DHCP server and DHCP clients are available for SUSE LINUX. The DHCP server available is dhcpd (published by the Internet Software Consortium). On the client side, choose between two different DHCP client programs: dhclient (also from ISC) and the DHCP client daemon in the dhcpcd package.

SUSE LINUX installs dhcpcd by default. The program is very easy to handle and will be launched automatically on each system boot to watch for a DHCP server. It does not need a configuration file to do its job and should work out of the box in most standard setups. For more complex situations, use the ISC dhclient, which is controlled by means of the configuration file /etc/dhclient.conf.

The core of any DHCP system is the dynamic host configuration protocol daemon. This server leases addresses and watches how they are used, according to the settings as defined in the configuration file /etc/dhcpd.conf. By changing the parameters and values in this file, a system administrator can influence the program's behavior in numerous ways. Look at a basic sample /etc/dhcpd.conf file:

default-lease-time 600;         # 10 minutes
max-lease-time 7200;            # 2  hours

option domain-name "kosmos.all";
option domain-name-servers 192.168.1.1, 192.168.1.2;
option broadcast-address 192.168.1.255;
option routers 192.168.1.254;
option subnet-mask 255.255.255.0;

subnet 192.168.1.0 netmask 255.255.255.0
 {
  range 192.168.1.10 192.168.1.20;
  range 192.168.1.100 192.168.1.200;
 }

This simple configuration file should be sufficient to get the DHCP server to assign IP addresses in the network. Make sure a semicolon is inserted at the end of each line, because otherwise dhcpd will not be started.

The above sample file can be divided into three sections. The first one defines how many seconds an IP address is leased to a requesting host by default (default-lease-time) before it should apply for renewal. The section also includes a statement of the maximum period for which a machine may keep an IP address assigned by the DHCP server without applying for renewal (max-lease-time).

In the second part, some basic network parameters are defined on a global level:

The last section of the file is there to define a network, including a subnet mask. To finish, specify the address range that the DHCP daemon should use to assign IP addresses to interested clients. In our example, clients may be given any address between 192.168.1.10 and 192.168.1.20 as well as 192.168.1.100 and 192.168.1.200.

After editing these few lines, you should be able to activate the DHCP daemon with the command rcdhcpd start. It will be ready for use immediately. You can also use the command rcdhcpd check-syntax to perform a brief syntax check. If you encounter any unexpected problems with your configuration — the server aborts with an error or does not return “done” on start — you should be able to find out what has gone wrong by looking for information either in the main system log /var/log/messages or on console 10 (Ctrl + Alt + F10).

On a default SUSE LINUX system, the DHCP daemon is started in a chroot environment for security reasons. The configuration files must be copied to the chroot environment so the daemon can find them. Normally, there is no need to worry about this because the command rcdhcpd start automatically copies the files.

As mentioned above, DHCP can also be used to assign a predefined, static address to a specific host for each request. As might be expected, addresses assigned explicitly always take priority over addresses from the pool of dynamic addresses. Furthermore, a static address never expires in the way a dynamic address would, for example, if there were not enough addresses available so the server needed to redistribute them among hosts.

To identify a host configured with a static address, dhcpd uses the hardware address, which is a globally unique, fixed numerical code consisting of six octet pairs for the identification of all network devices (for example 00:00:45:12:EE:F4). If the respective lines, like the ones in File 14.29. “Additions to the Configuration File”, are added to the configuration file of File 14.28. “The Configuration File /etc/dhcpd.conf”, the DHCP daemon will assign the same set of data to the corresponding host under all circumstances.

host earth {
hardware ethernet 00:00:45:12:EE:F4;
fixed-address 192.168.1.21;
}

The structure of this entry is almost self-explanatory: The name of the respective host (host <host name>) is entered in the first line and the MAC address in the second line. On Linux hosts, this address can be determined with the command ifstatus followed by the network device (for example eth0). If necessary, activate the network card first: ifup eth0. The output should contain something like

link/ether 00:00:45:12:EE:F4

In the above example, a host with a network card having the MAC address 00:00:45:12:EE:F4 is assigned the IP address 192.168.1.21 and the host name earth automatically. The type of hardware to enter is ethernet in nearly all cases, although token-ring, which is often found on IBM systems, is also supported.

To improve security, the SUSE version of the ISC's DHCP server comes with the non-root/chroot patch by Ari Edelkind applied. This enables dhcpd to:

To make this possible, the configuration file /etc/dhcpd.conf needs to be located in /var/lib/dhcp/etc. The corresponding init script automatically copies the file to this directory upon starting.

The server's behavior with regard to this feature can be controlled through the configuration file /etc/sysconfig/dhcpd. To continue running dhcpd without the chroot environment, set the variable DHCPD_RUN_CHROOTED in /etc/sysconfig/dhcpd to “no”.

To enable dhcpd to resolve host names even from within the chroot environment, some other configuration files need to be copied as well, namely:

These files are copied to /var/lib/dhcp/etc/ when starting the init script. These copies must be taken into account for any changes that they require, if they are dynamically modified by scripts like /etc/ppp/ip-up. However, there should be no need to worry about this if the configuration file only specifies IP addresses (instead of host names).

If your configuration includes additional files that should be copied into the chroot environment, specify these under the variable DHCPD_CONF_INCLUDE_FILES in the file etc/sysconfig/dhcpd. To make sure the DHCP logging facility keeps working even after a restart of the syslog daemon, it is necessary to add the option "-a /var/lib/dhcp/dev/log" under SYSLOGD_PARAMS in the file /etc/sysconfig/syslog.

For more information, the page of the Internet Software Consortium on the subject (http://www.isc.org/products/DHCP/) is a good source to read about the details of DHCP, including about version 3 of the protocol, currently in beta testing. Apart from that, you can always rely on the man pages for further help. Try man dhcpd, man dhcpd.conf, man dhcpd.leases, and man dhcp-options. Also, several books about DHCP have been published over the years that take an in-depth look at the topic.

With dhcpd, it is even possible to offer a file to a requesting host, as defined with the parameter filename, and for this file to contain a bootable Linux kernel. This allows you to build client hosts that do not need a hard disk — they are enabled to load both their operating system and their network data over the network (diskless clients), which could be an interesting option for both cost and security reasons.