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, or options
can be declared for each client system.
The configuration file can contain any extra tabs or blank lines for
easier formatting. The keywords are case-insensitive, and lines
beginning with a hash mark (#) are considered comments.
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 sent 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. Options configure DHCP options; whereas, parameters configure
values that are not optional or control how the DHCP server behaves.
 | Important |
|---|
| | If you change the configuration file, the changes will not take effect
until you restart the DHCP daemon with the command service
dhcpd restart.
|
Parameters (including options) declared before a section enclosed in
curly brackets ({ }) are considered global parameters. Global
parameters apply to all the sections below it.
In Figure 12-1, the routers,
subnet-mask, domain-name,
domain-name-servers, and
time-offset options are used for both the apex
and raleigh host statements.
As shown in Figure 12-1, you can declare a
subnet. You must include a
subnet declaration for every subnet in your
network. If you do not, the DHCP server will fail 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.
All subnets that share the same physical network should be declared
within a shared-network declaration as shown in
Figure 12-2. Parameters within the
shared-network but outside the enclosed
subnet declarations are considered global
parameters. The name of the shared-network
should be a descriptive title for the network such as test-lab to
describe all the subnets in a test lab environment.
As demonstrated in Figure 12-3, the
group declaration can be used to apply global
parameters to a group of declarations. You can group shared networks,
subnets, hosts, or other groups.
To configure a DHCP server that leases dynamic IP address to system
within a subnet, modify Figure 12-4 with your
values. It declares a default lease time, maximum lease time, and
network configuration values for the clients. This example assigns IP
address in the range 192.168.1.10 and
192.168.1.100 to client systems.
To assign an IP address to a client based on the MAC address of the
network interface card, use the hardware ethernet
parameter within a host declaration. As
demonstrated in Figure 12-5, the
host apex declaration specifies that the network
interface card with the MAC address 00:A0:78:8E:9E:AA should always
been leased the IP address 192.168.1.4.
Notice that you can also use the optional parameter
host-name to assign a host name to the client.
 | Tip |
|---|
| | You can use the sample configuration file in Red Hat Linux 7.2 as a
starting point and then add your own custom configuration options to
it. Copy it to its proper location with the command
cp /usr/share/doc/dhcp-<version-number>/dhcpd.conf.sample /etc/dhcpd.conf |
(where <version-number> is the DHCP
version you are using).
|
For a complete list of option statements and what they do, refer to
the dhcp-options man page.
On the DHCP server, the file
/var/lib/dhcp/dhcpd.leases stores the DHCP client
lease database. This file should not be modified by hand. DHCP lease
information for each recently assigned IP address is automatically
stored in the lease database. The information includes the length of
the lease, to whom the IP address has been assigned, the start and end
dates for the lease, and the MAC address of the network interface card
that was used to retreive the lease.
All times in the lease database are in Greenwich Mean Time (GMT), not
local time.
The first time you start the DHCP service, it will fail unless the
lease database exists. Use the command touch
/var/lib/dhcpd.leases to create the file before starting the
server for the first time. Once the file exists and the server has
been started, do not try to create a new lease database file.
The lease database is recreated from time to time so that it is not
too large. First, all known leases are saved in a temporary lease
database. The dhcpd.leases file is renamed
dhcpd.leases~, and the temporary lease database
is written to dhcpd.leases.
The DHCP daemon could be killed or the system could crash after the
lease database has been renamed to the backup file but before the new
file has been written. If this happens, there is no
dhcpd.leases file that is required to start the
service. Do not create a new lease file if this occurs. If you do, all
the old leases will be lost and cause many problems. The correct
solution is to rename the dhcpd.leases~ backup
file to dhcpd.leases and then start the daemon.
| Important |
|---|
| | Before you start the DHCP server for the first time, it will fail
unless there is an existing dhcpd.leases
file. Use the command touch
/var/lib/dhcp/dhcpd.leases to create the file before
starting the service for the first time (and the first time
only). Once the file exists, you do not have to perform this step
again.
|
To start the DHCP service, use the command /sbin/service
dhcpd start. To stop the DHCP server, use the command
/sbin/service dhcpd stop. If you want the daemon to
start automatically at boot time, see Chapter 8
for information on how to manage services.
If you have more than more network interface attached to the system,
but you only want the DHCP server to start on one of the interface,
you can modify the init script to start only on that device. In the
init script located at /etc/rc.d/init.d/dhcpd,
there is a section that declares what happens when you try to start
the service:
start() {
# Start daemons.
echo -n "Starting dhcpd: "
daemon /usr/sbin/dhcpd
RETVAL=$?
echo
[ $RETVAL -eq 0 ] && touch /var/lock/subsys/dhcpd
return $RETVAL
} |
Modify the line that starts the daemon to include the ethernet device
on which you want the DHCP server to run:
daemon /usr/sbin/dhcpd eth0 |
This is useful if you have a firewall machine with two network
cards. One network card can be configured as a DHCP client to retreive
an IP address to the Internet. The other network card can be used as a
DHCP server for the internal network behind the firewall.
The DHCP Relay Agent (dhcrelay) allows you to relay
DHCP and BOOTP requests from a subnet with no DHCP server on it to a
one or more DHCP servers on other subnets.
When a DHCP client requests information, the DHCP Relay Agent forwards
the request to the list of DHCP servers specified when the DHCP Relay
Agent is started. When a DHCP server returns a reply, the reply is
broadcast or unicast on the network that sent the original request.
To start the DHCP Relay Agent, use the dhcrelay
command. It can be started with the following options:
Table 12-1. Graphical Update Agent Options
| Argument | Description |
|---|
| -i | Names of the network interfaces to configure. If no interface
is specified, all network interfaces will be configured,
eliminating non-broadcast interfaces if it can.
|
| -p | Port on which dhcrelay should listen. The
DHCP Relay Agent transmits requests to the servers on this
port and transmits responses to the clients on the
port one greater than this port.
|
| -d | Force dhcrelay to run in the foreground
always.
|
| -q | Disable printing the network configuration of
dhcrelay on startup.
|
