Dynamic Host Configuration Protocol (DHCP)

The Dynamic Host Configuration Protocol (DHCP) is a network service that enables host computers to be automatically assigned settings from a server as opposed to manually configuring each network host. Computers configured to be DHCP clients have no control over the settings they receive from the DHCP server, and the configuration is transparent to the computer's user.

The most common settings provided by a DHCP server to DHCP clients include:

  • IP-Address and Netmask

  • DNS

  • WINS

However, a DHCP server can also supply configuration properties such as:

  • Host Name

  • Domain Name

  • Default Gateway

  • Time Server

  • Print Server

The advantage of using DHCP is that changes to the network, for example a change in the address of the DNS server, need only be changed at the DHCP server, and all network hosts will be reconfigured the next time their DHCP clients poll the DHCP server. As an added advantage, it is also easier to integrate new computers into the network, as there is no need to check for the availability of an IP address. Conflicts in IP address allocation are also reduced.

A DHCP server can provide configuration settings using two methods:

MAC Address

This method entails using DHCP to identify the unique hardware address of each network card connected to the network and then continually supplying a constant configuration each time the DHCP client makes a request to the DHCP server using that network device.

Address Pool

This method entails defining a pool (sometimes also called a range or scope) of IP addresses from which DHCP clients are supplied their configuration properties dynamically and on a "first come, first served" basis. When a DHCP client is no longer on the network for a specified period, the configuration is expired and released back to the address pool for use by other DHCP Clients.

Ubuntu is shipped with both DHCP server and client. The server is dhcpd (dynamic host configuration protocol daemon). The client provided with Ubuntu is dhclient and should be installed on all computers required to be automatically configured. Both programs are easy to install and configure and will be automatically started at system boot.

Instal·lació

At a terminal prompt, enter the following command to install dhcpd:

sudo apt-get install dhcp3-server

You will probably need to change the default configuration by editing /etc/dhcp3/dhcpd.conf to suit your needs and particular configuration.

You also need to edit /etc/default/dhcp3-server to specify the interfaces dhcpd should listen to. By default it listens to eth0.

NOTE: dhcpd's messages are being sent to syslog. Look there for diagnostics messages.

Configuration

The error message the installation ends with might be a little confusing, but the following steps will help you configure the service:

Most commonly, what you want to do is assign an IP address randomly. This can be done with settings as follows:

# Sample /etc/dhcpd.conf
# (add your comments here) 
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 "mydomain.example";

subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.10 192.168.1.100;
range 192.168.1.150 192.168.1.200;
} 

This will result in the DHCP server giving a client an IP address from the range 192.168.1.10-192.168.1.100 or 192.168.1.150-192.168.1.200. It will lease an IP address for 600 seconds if the client doesn't ask for a specific time frame. Otherwise the maximum (allowed) lease will be 7200 seconds. The server will also "advise" the client that it should use 255.255.255.0 as its subnet mask, 192.168.1.255 as its broadcast address, 192.168.1.254 as the router/gateway and 192.168.1.1 and 192.168.1.2 as its DNS servers.

If you need to specify a WINS server for your Windows clients, you will need to include the netbios-name-servers option, e.g.

option netbios-name-servers 192.168.1.1; 

Dhcpd configuration settings are taken from the DHCP mini-HOWTO, which can be found here.

References