Red Hat ENTERPRISE LINUX 4 System Administration Manual page 244

Chapter 23. Dynamic Host Configuration Protocol (DHCP)
To use the recommended mode, add the following line to the top of the configuration file:
ddns-update-style interim;
Refer to the dhcpd.conf man page for details about the different modes.
There are two types of statements in the configuration file:
• Parameters — State how to perform a task, whether to perform a task, or what network
configuration options to send to the client.
• Declarations — Describe the topology of the network, describe the clients, provide addresses for the
clients, or apply a group of parameters to a group of declarations.
Some parameters must start with the option keyword and are referred to as options. Options
configure DHCP options; whereas, parameters configure values that are not optional or control how
the DHCP server behaves.
Parameters (including options) declared before a section enclosed in curly brackets ({ }) are
considered global parameters. Global parameters apply to all the sections below it.
Important
If the configuration file is changed, the changes do not take effect until the DHCP daemon
is restarted with the command service dhcpd restart.
Tip
Instead of changing a DHCP configuration file and restarting the service each time, using
the omshell command provides an interactive way to connect to, query, and change the
configuration of a DHCP server. By using omshell, all changes can be made while the
server is running. For more information on omshell, refer to the omshell man page.
Example 23.1, "Subnet
In
name-servers, and time-offset options are used for any host statements declared below it.
Additionally, a subnet can be declared, a subnet declaration must be included for every subnet in
the network. If it is not, the DHCP server fails to start.
In this example, there are global options for every DHCP client in the subnet and a range declared.
Clients are assigned an IP address within the range.
subnet 192.168.1.0 netmask 255.255.255.0 {
option routers
option subnet-mask
option domain-name
option domain-name-servers
option time-offset
226
Declaration", the routers, subnet-mask, domain-name, domain-
192.168.1.254;
255.255.255.0;
"example.com";
192.168.1.1;
-18000; # Eastern Standard Time