Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual page 320

Chapter 21. Dynamic Host Configuration Protocol (DHCP)
Two DNS update schemes are currently implemented — the ad-hoc DNS update mode and the
interim DHCP-DNS interaction draft update mode. If and when these two are accepted as part of the
Internet Engineering Task Force (IETF) standards process, there will be a third mode — the standard
DNS update method. You must configure the DNS server for compatibility with these schemes.
Version 3.0b2pl11 and previous versions used the ad-hoc mode; however, it has been deprecated. To
keep the same behavior, add the following line to the top of the configuration file:
ddns-update-style ad-hoc;
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.
The parameters that start with the keyword option are reffered to as options. These options control
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 21.1, "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.
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Declaration", the routers, subnet-mask, domain-name, domain-