DHCP Server

The DHCP server manages the IP address space of networks directly connected to that server. To extend this environment into other networks, DHCP servers or BOOTP relay agents must be set up on those networks.

A DHCP server can act as a primary or a secondary server. To be a primary server, it must have a range of IP addresses for which it is responsible.

The term primary is used differently for the client and the server.

When a DHCP server is added to a network that already has a primary DHCP server, the new server can be configured to provide primary and secondary service, or secondary service only. If the server is configured for both services, both servers can perform the duties of a primary server, (they can give out IP addresses) as long as each is primarily responsible for a different IP range. Each server can act as a secondary server for the other, by confirming existing configurations supplied by a primary server when the primary server is unable to respond to requests for confirmation. Every primary server automatically acts as a secondary server.

A DHCP server's range of IP addresses is specified during the installation and configuration of the software on the server. As a primary DHCP server, the server can give out an IP address to a client requesting a new configuration from the range of IP addresses for which it is responsible. When a client asks for confirmation of its existing configuration, the server responsible for that client's IP address confirms the configuration. Acting as a secondary server, it can confirm configurations that were supplied by another DHCP server on the network.

To provide secondary service, the DHCP server confirms configurations that were supplied by another DHCP server on the network. It does this when the primary server responsible for those IP addresses cannot respond. After a waiting period, the secondary server responds in its place.

DHCP servers can be configured as secondary only. If you decide you want to configure a DHCP server as a secondary server only, you can do this through the dhcpconfig program, by choosing to configure the server without a range of IP addresses that it can give out to clients requesting a new configuration. In this configuration, the DHCP servers should be using NIS+ for their data storage.

DHCP service is enabled and configured on the machine on which it is run with the dhcpconfig utility. This utility allows you to set startup options, configure the DHCP service database type and location, and initialize the dhcptab and dhcp_network tables for any locally attached or remote networks.

When dhcpconfig is invoked, it presents a menu offering the option of configuring the DHCP service, configuring a BOOTP relay agent, removing the DHCP configuration or relay service configuration, or exiting. When the administrator selects one of the menu options, he or she is presented with a series of questions to collect the required information. Then dhcpconfig performs the appropriate steps to turn on the selected functions.

Multiple DHCP servers on the same network operate much more efficiently if they share DHCP databases through NIS+ or NFS. Sharing allows DHCP servers to communicate through a common datastore, increasing redundancy and balancing load among cooperating servers.

When a new DHCP client is added to the network, the client broadcasts a message intended to locate all available DHCP and/or BOOTP servers within reach. Any DHCP server that receives the message first checks to see if any IP addresses are available for assignment. If they are, the server verifies that a potential IP address is not already in use. If it is not, the server offers the IP address and other configuration information to the client. If the IP address is in use, the server marks this IP address as unusable, notifies the network administrator of its status, and selects another IP address.

The client selects an IP address offered to it based on its own criteria, and broadcasts a message that identifies its selection.

Server Databases

The DHCP/BOOTP server uses two types of databases: the dhcptab database and the dhcp_network databases (see dhcp_network(4).

The dhcptab database contains macros defined using a termcap-like syntax. This syntax permits network administrators to define groups of DHCP configuration parameters to be returned to clients. There are currently 77 predefined parameters.

A DHCP/BOOTP server returns hostname, network broadcast address, network subnet mask, or IP maximum transfer unit (MTU), if this information is requested by a client attached to the same network as the server. This information does not have to be explicitly configured in the dhcptab. The dhtadm command manages the dhcptab service configuration table.

If there are two servers sharing a distributed dhcptab table, the administrator can configure the DHCP parameters in the table so the servers back each other up, provided that they are in the same NIS+ domain. However, each should be primarily responsible for a different range of IP addresses. Each network might require a BOOTP relay agent as well, so its clients can reach the server on the other network.

The dhcp_network databases contain client identifier-to-IP address mappings. These databases are named after the network they support. There is one dhcp_network database for each network that offers DHCP/BOOTP services. The dhcp_network databases are located dynamically by the server and consulted during runtime. A client request received from a network for which no dhcp_network database exists is ignored.

The dhcp_network database maps a DHCP client's client identifier to an IP address and the configuration parameter associated with that IP address. This database is located by the DHCP server at runtime by generating a dhcp_network database name by using the IP network address and subnet mask for the network where the DHCP request originated. For example, a dhcp_network database that supports the 10.0.0.0 network would be called 10_0_0_0. The dhcp_network databases can exist as NIS+ tables or ASCII files. Use the pntadm command to manage the dhcp_network databases.

The in.dhcpd daemon has two run modes, DHCP server (with optional BOOTP compatibility mode) and BOOTP relay agent mode (see in.dhcpd(1M).