Registration and the Portmapper

Servers on a host need to let clients know their logical addresses and which services they offer.

In ONC RPC, servers generally do this by registering with a utility service called the Portmapper. This maintains a list of mappings from program/version numbers (also qualified by protocol used) to TCP/IP port numbers on a host.

The Portmapper itself can always be located by clients because it is always on well-known port 111 on a given host. If using low-level calls, the client first asks the Portmapper for the port number for the particular remote procedure, and then calls that port directly. The high-level call, callrpc, performs the same function transparently to the user.