27.3. The DHCP Server dhcpd

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 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 the basic sample /etc/dhcpd.conf file in Example 27.1, “The Configuration File /etc/dhcpd.conf”.

Example 27.1. The Configuration File /etc/dhcpd.conf

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

option domain-name "cosmos.all";
option domain-name-servers,;
option broadcast-address;
option routers;
option subnet-mask;

subnet netmask

This simple configuration file should be sufficient to get the DHCP server to assign IP addresses in the network. Make sure that 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 client 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 this example, clients may be given any address between and as well as and

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. 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.

27.3.1. Clients with Fixed IP Addresses

As mentioned above, DHCP can also be used to assign a predefined, static address to a specific client for each request. Addresses assigned explicitly always take priority over dynamic addresses from the pool. 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 clients.

To identify a client 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 Example 27.2, “Additions to the Configuration File”, are added to the configuration file of Example 27.1, “The Configuration File /etc/dhcpd.conf”, the DHCP daemon always assigns the same set of data to the corresponding client under all circumstances.

Example 27.2. Additions to the Configuration File

host earth {
hardware ethernet 00:00:45:12:EE:F4;

The name of the respective client (host hostname, here earth) 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 with ifup eth0. The output should contain something like

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

In the above example, a client with a network card having the MAC address 00:00:45:12:EE:F4 is assigned the IP address and the hostname 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.

27.3.2. The SUSE LINUX Version

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 run with the user ID nobody and run in a chroot environment (/var/lib/dhcp). To make this possible, the configuration file dhcpd.conf must be located in /var/lib/dhcp/etc. The init script automatically copies the file to this directory when starting.

Control the server's behavior regarding this feature by means of entries in the file /etc/sysconfig/dhcpd. To run dhcpd without the chroot environment, set the variable DHCPD_RUN_CHROOTED in /etc/sysconfig/dhcpd to “no”.

To enable dhcpd to resolve hostnames even from within the chroot environment, some other configuration files must be copied as well:

  • /etc/localtime

  • /etc/host.conf

  • /etc/hosts

  • /etc/resolv.conf

These files are copied to /var/lib/dhcp/etc/ when starting the init script. Take these copies 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 hostnames).

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.

SUSE LINUX Administration Guide 9.3