Chapter 15: 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 IP address, gateway, and DNS servers.

DHCP is useful for fast delivery of client network configuration. When configuring the client system, the administrator can choose DHCP and not have to enter 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, the administrator can edit just one DHCP configuration file on the server for the new set of IP address.

If the DNS servers for an organization change, 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.

Configuring a DHCP Server

You can configure a DHCP server using the configuration file /etc/dhcpd.conf. DHCP also uses the file /var/lib/dhcp/dhcpd.leases to store the client lease database. Refer to the “Lease Database” section of this chapter for more information.

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, 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.

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 versions used the ad hoc mode; however, it has been deprecated. If you want 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;

Read 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 braces ({ }) are considered global parameters. Global parameters apply to all the sections that follow them.

Note

If you change the configuration file, the changes will not take effect until you restart the DHCP daemon with the command service dhcpd restart.

In Listing 15-1, the routers, subnet-mask, domain-name, domain-name-servers, and time-offset options are used for any host statements declared below them. As shown in Listing 15-1, 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.

Listing 15-1: Subnet declaration

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; }

All subnets that share the same physical network should be declared within a shared-network declaration as shown in Listing 15-2. 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.

Listing 15-2: shared-network declaration

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;           } }

As demonstrated in Listing 15-3, 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.

Listing 15-3: group declaration

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;      } }

To configure a DHCP server that leases a dynamic IP address to a system within a subnet, modify Listing 15-4 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.

Listing 15-4: Range parameter

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; }

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 Listing 15-5, 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 you can also use the optional parameter host-name to assign a hostname to the client.

Listing 15-5: Static IP address using DHCP

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

Tip

You can use the sample configuration file in Red Hat Linux 8.0 as a starting point and 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.

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 manually. 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 re-created from time to time so that it is not too large. 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, which 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 this will cause many problems. The correct solution is to rename the dhcpd.leases~ backup file to dhcpd.leases and then start the daemon.

Tip

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 if it does not exist.

Starting and Stopping the Server

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 21 for information on how to manage services.

If you have more than one network interface attached to the system, but you want the DHCP server to start on only one of the interface, you can 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 if you have 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 cannot connect to the daemon via the Internet.

Other command-line options that can be specified in /etc/sysconfig/dhcpd are:

• -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 you accept the default of port 67, the server listens on port 67 for requests and on port 68 for responses to the client. If you specify a port here and use the DHCP relay agent, you must specify the same port on which the DHCP relay agent should listen. See the “DHCP Relay Agent” section of this chapter for details.

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

• -d — Log the DHCP server daemon to the standard error descriptor. This is used mostly 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 be used only 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.

DHCP Relay Agent

The DHCP Relay Agent (dhcrelay) allows you to relay 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.