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 23-1.
Example 23-1 The Configuration File /etc/dhcpd.conf
default-lease-time 600; # 10 minutes max-lease-time 7200; # 2 hours option domain-name "example.com"; option domain-name-servers 192.168.1.116; option broadcast-address 192.168.2.255; option routers 192.168.2.1; option subnet-mask 255.255.255.0; subnet 192.168.2.0 netmask 255.255.255.0 { range 192.168.2.10 192.168.2.20; range 192.168.2.100 192.168.2.200; }
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 is not started.
The 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 line option domain-name defines the default domain of your network.
With the entry option domain-name-servers, specify up to three values for the DNS servers used to resolve IP addresses into hostnames and vice versa. Ideally, configure a name server on your machine or somewhere else in your network before setting up DHCP. That name server should also define a hostname for each dynamic address and vice versa. To learn how to configure your own name server, read Section 22.0, The Domain Name System.
The line option broadcast-address defines the broadcast address the requesting client should use.
With option routers, set where the server should send data packets that cannot be delivered to a host on the local network (according to the source and target host address and the subnet mask provided). In most cases, especially in smaller networks, this router is identical to the Internet gateway.
With option subnet-mask, specify the netmask assigned to clients.
The last section of the file defines 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 Example 23-1, clients may be given any address between 192.168.2.10 and 192.168.2.20 as well as 192.168.2.100 and 192.168.2.200.
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. 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 openSUSE 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.
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 23-2, are added to the configuration file of Example 23-1, the DHCP daemon always assigns the same set of data to the corresponding client.
Example 23-2 Additions to the Configuration File
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 hostname 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 openSUSE 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, 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-ng daemon, there is an additional entry SYSLOGD_ADDITIONAL_SOCKET_DHCP in the file /etc/sysconfig/syslog.