Red Hat Enterprise Linux 3: System Administration Guide
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# 25.2. Configuring a DHCP Server

To configure a DHCP server, modify the configuration file `/etc/dhcpd.conf`.

DHCP also uses the file `/var/lib/dhcp/dhcpd.leases` to store the client lease database. Refer to Section 25.2.2 Lease Database for more information.

## 25.2.1. 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, and 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.

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 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 version 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.

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`.

In Example 25-1, the `routers`, `subnet-mask`, `domain-name`, `domain-name-servers`, and `time-offset` options are used for any `host` statements declared below it.

As shown in Example 25-1, 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                  192.168.1.254;
            option subnet-mask              255.255.255.0;

            option domain-name              "example.com";
            option domain-name-servers       192.168.1.1;

            option time-offset              -18000;     # Eastern Standard Time

    	range 192.168.1.10 192.168.1.100;
    }

Example 25-1. Subnet Declaration

All subnets that share the same physical network should be declared within a `shared-network` declaration as shown in Example 25-2. 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.

    shared-network name {
        option domain-name              "test.redhat.com";
        option domain-name-servers      ns1.redhat.com, ns2.redhat.com;
        option routers                  192.168.1.254;
        more parameters for EXAMPLE shared-network
        subnet 192.168.1.0 netmask 255.255.255.0 {
            parameters for subnet
            range 192.168.1.1 192.168.1.31;
        }
        subnet 192.168.1.32 netmask 255.255.255.0 {
            parameters for subnet
            range 192.168.1.33 192.168.1.63;
        }
    }

Example 25-2. Shared-network Declaration

As demonstrated in Example 25-3, the `group` declaration can be used to apply global parameters to a group of declarations. For example, shared networks, subnets, hosts, or other groups can be grouped.

    group {
       option routers                  192.168.1.254;
       option subnet-mask              255.255.255.0;

       option domain-name              "example.com";
       option domain-name-servers       192.168.1.1;

       option time-offset              -18000;     # Eastern Standard Time

       host apex {
          option host-name "apex.example.com";
          hardware ethernet 00:A0:78:8E:9E:AA;
          fixed-address 192.168.1.4;
       }

       host raleigh {
          option host-name "raleigh.example.com";
          hardware ethernet 00:A1:DD:74:C3:F2;
          fixed-address 192.168.1.6;
       }
    }

Example 25-3. Group Declaration

To configure a DHCP server that leases a dynamic IP address to a system within a subnet, modify Example 25-4 with your values. It declares a default lease time, maximum lease time, and network configuration values for the clients. This example assigns IP addresses in the `range` 192.168.1.10 and 192.168.1.100 to client systems.

    default-lease-time 600;
    max-lease-time 7200;
    option subnet-mask 255.255.255.0;
    option broadcast-address 192.168.1.255;
    option routers 192.168.1.254;
    option domain-name-servers 192.168.1.1, 192.168.1.2;
    option domain-name "example.com";

    subnet 192.168.1.0 netmask 255.255.255.0 {
       range 192.168.1.10 192.168.1.100;
    }

Example 25-4. Range Parameter

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 Example 25-5, the `host apex` declaration specifies that the network interface card with the MAC address 00:A0:78:8E:9E:AA always receives the IP address 192.168.1.4.

Notice that the optional parameter `host-name` can be used to assign a host name to the client.

    host apex {
       option host-name "apex.example.com";
       hardware ethernet 00:A0:78:8E:9E:AA;
       fixed-address 192.168.1.4;
    }

Example 25-5. Static IP Address using DHCP

| Tip

The sample configuration file provided can be used as a starting point. Custom configuration options can be added to it. To copy it to the proper location, use the following command:

    cp /usr/share/doc/dhcp-<version-number>/dhcpd.conf.sample /etc/dhcpd.conf

(where `<version-number>` is the DHCP version number).

For a complete list of option statements and what they do, refer to the `dhcp-options` man page.

## 25.2.2. Lease Database

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 retrieve the lease.

All times in the lease database are in Greenwich Mean Time (GMT), not local time.

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, the `dhcpd.leases` file does not exist, but it is required to start the service. Do not create a new lease file. 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.

## 25.2.3. Starting and Stopping the Server

| Important
| When the DHCP server is started for the first time, it will fail unless the `dhcpd.leases` file exists. Use the command `touch /var/lib/dhcp/dhcpd.leases` to create the file if it does not exist.

To start the DHCP service, use the command `/sbin/service dhcpd start`. To stop the DHCP server, use the command `/sbin/service dhcpd stop`. To configure the daemon to start automatically at boot time, refer to Chapter 21 Controlling Access to Services for information on how to manage services.

If more than one network interface is attached to the system, but the DHCP server should only be started on one of the interface, configure the DHCP server to start only on that device. In `/etc/sysconfig/dhcpd`, add the name of the interface to the list of `DHCPDARGS`:

    # Command line options here
    DHCPDARGS=eth0

This is useful for a firewall machine with two network cards. One network card can be configured as a DHCP client to retrieve an IP address to the Internet. The other network card can be used as a DHCP server for the internal network behind the firewall. Specifying only the network card connected to the internal network makes the system more secure because users can not connect to the daemon via the Internet.

Other command line options that can be specified in `/etc/sysconfig/dhcpd` include:

- `-p `<portnum>`` — Specify the udp port number on which `dhcpd` should listen. The default is port 67. The DHCP server transmits responses to the DHCP clients at a port number one greater than the udp port specified. For example, if the default port 67 is used, the server listens on port 67 for requests and responses to the client on port 68. If a port is specified here and the DHCP relay agent is used, the same port on which the DHCP relay agent should listen must be specified. Refer to Section 25.2.4 DHCP Relay Agent for details.

- `-f` — Run the daemon as a foreground process. This is mostly used for debugging.

- `-d` — Log the DHCP server daemon to the standard error descriptor. This is mostly used for debugging. If this is not specified, the log is written to `/var/log/messages`.

- `-cf `<filename>`` — Specify the location of the configuration file. The default location is `/etc/dhcpd.conf`.

- `-lf `<filename>`` — Specify the location of the lease database file. If a lease database file already exists, it is very important that the same file be used every time the DHCP server is started. It is strongly recommended that this option only be used for debugging purposes on non-production machines. The default location is `/var/lib/dhcp/dhcpd.leases`.

- `-q` — Do not print the entire copyright message when starting the daemon.

## 25.2.4. DHCP Relay Agent

The DHCP Relay Agent (`dhcrelay`) allows for the relay of DHCP and BOOTP requests from a subnet with no DHCP server on it to 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.

The DHCP Relay Agent listens for DHCP requests on all interfaces unless the interfaces are specified in `/etc/sysconfig/dhcrelay` with the `INTERFACES` directive.

To start the DHCP Relay Agent, use the command `service dhcrelay start`.

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