Configuring a Dynamic Host Configuration Protocol (DHCP) server

How the SCO DHCP server works

``Steps in serving configuration parameters to a DHCP client'' shows the steps that the SCO DHCP server normally undertakes to service a DHCP client with its configuration parameters.

Steps in serving configuration parameters to a DHCP client

The numbered steps in the figure show the sequence of events:

When the SCO DHCP server receives a ``discover'' message from a client requesting configuration parameters, it constructs the set of parameters corresponding to the client from the DHCP configuration file. This set of parameters can be built from any combination of global options, subnet-specific options, class-specific options, and client-specific options. See ``DHCP options''.
If there is no entry for the client in the configuration file, or if the client's entry does not contain an IP address, the DHCP server leases an IP address to the client from the Address Allocation Server (AAS).
If the client's entry in the configuration file already contains an IP address, the DHCP server assigns that address to the client. See ``Manually assigning IP addresses''.
If requested, the AAS server allocates a dynamically assigned IP address for the DHCP server to assign (see ``Dynamically assigning IP addresses''). The AAS server takes the address from a pool of addresses that are consistent with the client's subnet.
The DHCP server constructs an ``offer'' message containing the client's configuration parameters (including an IP address) and sends it to the client.
The server waits for a ``request'' from the client accepting the parameters and address.
After the client accepts the offered parameters, the server sends an ``ack'' (acknowledgment) message completing the process.

Steps 5 and 6 are repeated whenever a DHCP client needs to renew the lease on its IP address. If the server sends a ``nak'' (negative acknowledgment) message instead of an ``ack'' message in step 6, the client must start again from step 1. This may happen if the requested IP address is no longer available.

For more information on DHCP message types and message exchange, see RFC 2131.

More than one SCO DHCP server can be configured on a network, if necessary, but these servers cannot share the same pool(s) of addresses. The DHCP server can configure clients on different subnets by using BOOTP gateways. See ``Setting up a BOOTP gateway'' for more information.