Dynamic Host Configuration Protocol (Dhcp); Why Use Dhcp; Configuring A Dhcp Server; Configuration File - Red Hat ENTERPRISE LINUX 4 System Administration Manual

Chapter 23.
Dynamic Host Configuration Protocol
(DHCP)
Dynamic Host Configuration Protocol (DHCP) is a network protocol for automatically assigning TCP/
IP information to client machines. Each DHCP client connects to the centrally-located DHCP server
which returns that client's network configuration, including the IP address, gateway, and DNS servers.

23.1. Why Use DHCP?

DHCP is useful for automatic configuration of client network interfaces. When configuring the client
system, the administrator can choose DHCP and instead of entering an IP address, netmask,
gateway, or DNS servers. The client retrieves this information from the DHCP server. DHCP is also
useful if an administrator wants to change the IP addresses of a large number of systems. Instead of
reconfiguring all the systems, he can just edit one DHCP configuration file on the server for the new
set of IP addresses. If the DNS servers for an organization changes, the changes are made on the
DHCP server, not on the DHCP clients. Once the network is restarted on the clients (or the clients are
rebooted), the changes take effect.
Furthermore, if a laptop or any type of mobile computer is configured for DHCP, it can be moved from
office to office without being reconfigured as long as each office has a DHCP server that allows it to
connect to the network.

23.2. Configuring a DHCP Server

To configure a DHCP server, the /etc/dhcpd.conf configuration file must be created. A sample file
can be found at /usr/share/doc/dhcp-<version>/dhcpd.conf.sample.
DHCP also uses the file /var/lib/dhcp/dhcpd.leases to store the client lease database. Refer
Section 23.2.2, "Lease Database"
to for more information.

23.2.1. Configuration File

The first step in configuring a DHCP server is to create the configuration file that stores the network
information for the clients. Global options can be declared for all clients, while other options can be
declared for individual client systems.
The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are case-
insensitive and lines beginning with a hash mark (#) are considered comments.
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. The DHCP server must be configured to use one of the two current 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;
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