40 DHCP #

When the module is started for the first time, a wizard starts, prompting you to make a few basic decisions concerning server administration. Completing this initial setup produces a very basic server configuration that should function in its essential aspects. The expert mode can be used to deal with more advanced configuration tasks. Proceed as follows:

In addition to the configuration method discussed earlier, there is also an expert configuration mode that allows you to change the DHCP server setup in every detail. Start the expert configuration by clicking DHCP Server Expert Configuration in the Start-Up dialog (see Figure 40.4, “DHCP server: start-up”).

After completing all configuration steps, close the dialog with OK. The server is now started with its new configuration.

DHCP can also be used to assign a predefined, static address to a specific client. Addresses assigned explicitly always take priority over dynamic addresses from the pool. A static address never expires in the way a dynamic address would, for example, if there were not enough addresses available and 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, 00:30:6E:08:EC:80). If the respective lines, like the ones in Example 40.2, “Additions to the configuration file”, are added to the configuration file of Example 40.1, “The configuration file /etc/dhcpd.conf”, the DHCP daemon always assigns the same set of data to the corresponding client.

host jupiter {
hardware ethernet 00:30:6E:08:EC:80;
fixed-address 192.168.2.100;
}

The name of the respective client (host HOSTNAME, here jupiter) is entered in the first line and the MAC address in the second line. On Linux hosts, find the MAC address with the command ip link show followed by the network device (for example, eth0). The output should contain something like

link/ether 00:30:6E:08:EC:80

In the preceding example, a client with a network card having the MAC address 00:30:6E:08:EC:80 is assigned the IP address 192.168.2.100 and the host name jupiter 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 Linux Enterprise Server 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 host names even from within the chroot environment, some other configuration files must be copied as well:

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 host names).

If your configuration includes additional files that should be copied into the chroot environment, set these under the variable DHCPD_CONF_INCLUDE_FILES in the file /etc/sysconfig/dhcpd. To ensure that the DHCP logging facility keeps working even after a restart of the syslog daemon, there is an additional entry SYSLOGD_ADDITIONAL_SOCKET_DHCP in the file /etc/sysconfig/syslog.