Chapter 23. DHCP


23.1. Configuring a DHCP Server with YaST
23.2. DHCP Software Packages
23.3. The DHCP Server dhcpd
23.4. For More Information


The purpose of the dynamic host configuration protocol (DHCP) is to assign network settings centrally from a server rather than configuring them locally on each and every workstation. A host configured to use DHCP does not have control over its own static address. It is enabled to configure itself completely and automatically according to directions from the server. If you use the NetworkManager on the client side, you do not need to configure the client at all. This is useful if you have changing environments and only one interface active at a time. Never use NetworkManager on a machine that runs a DHCP server.

One way to configure a DHCP server is to identify each client using the hardware address of its network card (which is fixed in most cases), then supply that client with identical settings each time it connects to the server. DHCP can also be configured to assign addresses to each interested client dynamically from an address pool set up for that purpose. In the latter case, the DHCP server tries to assign the same address to the client each time it receives a request, even over longer periods. This works only if the network does not have more clients than addresses.

DHCP makes life easier for system administrators. Any changes, even bigger ones, related to addresses and the network configuration in general can be implemented centrally by editing the server's configuration file. This is much more convenient than reconfiguring numerous workstations. Also it is much easier to integrate machines, particularly new machines, into the network, because they can be given an IP address from the pool. Retrieving the appropriate network settings from a DHCP server is especially useful in the case of laptops regularly used in different networks.

A DHCP server supplies not only the IP address and the netmask, but also the hostname, domain name, gateway, and name server addresses for the client to use. In addition to that, DHCP allows a number of other parameters to be configured in a centralized way, for example, a time server from which clients may poll the current time or even a print server.

23.1. Configuring a DHCP Server with YaST

When the module is started for the first time, a wizard starts, prompting you to make a few basic decision concerning server administration. Completing this initial setup produces a very basic server configuration that should function in essential aspects. The expert mode can be used to deal with more advanced configuration tasks.

Card Selection

In the first step, YaST looks for the network interfaces available on your system then displays them in a list. From the list, select the interface on which the DHCP server should listen and click Add. After this, select Open Firewall for Selected Interfaces to open the firewall for this interface. See Figure 23.1, “DHCP Server: Card Selection”.

Figure 23.1. DHCP Server: Card Selection

DHCP Server: Card Selection
Global Settings

In the entry fields, provide the network specifics for all clients the DHCP server should manage. These specifics are the domain name, address of a time server, addresses of the primary and secondary name server, addresses of a print and a WINS server (for a mixed network with both Windows and Linux clients), gateway address, and lease time. See Figure 23.2, “DHCP Server: Global Settings”.

Figure 23.2. DHCP Server: Global Settings

DHCP Server: Global Settings
Dynamic DHCP

In this step, configure how dynamic IP addresses should be assigned to clients. To do so, specify an IP range from which the server can assign addresses to DHCP clients. All these addresses must be covered by the same netmask. Also specify the lease time during which a client may keep its IP address without needing to request an extension of the lease. Optionally, specify the maximum lease time—the period during which the server reserves an IP address for a particular client. See Figure 23.3, “DHCP Server: Dynamic DHCP”.

Figure 23.3. DHCP Server: Dynamic DHCP

DHCP Server: Dynamic DHCP
Finishing the Configuration and Setting the Start Mode

After the third part of the configuration wizard, a last dialog is shown in which you can define how the DHCP server should be started. Here, specify whether to start the DHCP server automatically when the system is booted or manually when needed (for example, for test purposes). Click Finish to complete the configuration of the server. See Figure 23.4, “DHCP Server: Start-Up”.

Figure 23.4. DHCP Server: Start-Up

DHCP Server: Start-Up