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DHCP
Configuration Files
Most Linux distributions ship with the DHCP
server written by the Internet Software Consortium (
of early 2002, many Linux distributions still ship with the older 2.0 version. Most
of the configuration features described here apply to both 2.0 and 3.0 versions
of the DHCP server, but 3.0 adds support for some advanced features, including
the DNS integration described in the section "
The DHCP server relies on a configuration
file called dhcpd.conf . This file usually resides in the /etc or /etc/dhcp directory. Like other Linux configuration files, dhcpd.conf is a plain
text file, so you can edit it with your favorite text editor. In addition to
this file, dhcpd creates its own status file, dhcp.leases , which
usually resides in /var/lib/dhcp . This file contains information on leases
issued by dhcpd . DHCP operates on the principle that the DHCP server orchestrates
the allocation of IP addresses. It does this by telling a DHCP client that it
may use a particular IP address for a particular period of timethat is, the
DHCP client obtains a lease, which it must periodically renew. The dhcp.leases file also
contains information on the Ethernet addresses of the clients. The dhcp.leases file isn't a configuration file per se, though;
you won't edit it, although there are a few circumstances in which you might
want to examine it, such as when you are troubleshooting or if you need to
locate a client's media access control (MAC) address.
The main dhcpd.conf file, like
many configuration files, may contain comments, which begin with the pound sign
( # ). The file's noncomment lines are known as statements, and they fall into two broad categories:
Parameters Parameters tell the DHCP server whether to do something (such as
providing addresses to unknown clients), how to do something (such as how long
a lease to offer), or what information to provide to clients (such as the
network's gateway address).
Declarations Declarations describe the topology of the network (what addresses
are associated with particular interfaces, for instance), specify IP addresses
that can be assigned to clients, or associate a set of parameters with a set of
declarations.
Some declarations rely on information
described in parameters. When this is the case, the parameter must precede the
declaration in the file. As described shortly, most configuration files begin
with a series of global parameters, then move on to declarations.
Some declarations span multiple lines, and
such declarations may include parameters within the grouping. Such multi-line
declarations are indicated by the use of curly braces ( {} )
surrounding the enclosed lines. For instance, the following lines define an
individual computer and the IP address it's to be given, as discussed in the
upcoming section "
host teela { hardware Ethernet 00:05:02:a7:76:da;
fixed-address 192.168.1.2;
}
The details of what this declaration does are
discussed later. For now, you should note that the host keyword begins the
declaration, it's followed by an additional option (the name of the computer, teela ), and
the curly braces enclose additional configuration lines that define key
features needed for this declaration. Such multi-line declarations may be
nested within each other in certain circumstances. It's common practice to
indent the nested declarations a few spaces in such cases, to help you identify
the nested components at a glance. Such indentation isn't required, though; dhcpd ignores
extra spaces and lines except when they're enclosed in quotes.
