Registering Ports

Every port mapper on every host is associated with port number 111. The port mapper is the only network service that must have a dedicated port.

Other network services can be assigned port numbers either statically or dynamically, as long as the services register their ports with their host's port mapper. Typically, a server program based on an RPC library gets a port number at run time by calling an RPC library procedure.

The delegation of port-to-remote program mapping to a port mapper also automates port number administration. Statically mapping ports and remote programs in a file duplicated on each client requires updating all mapping files whenever a new remote program is introduced to a network. The alternative solution, placing the port-to-program mappings in a shared Network File System (NFS) file, would be too centralized. If the file server were to go down in this case, the entire network would also.

The port-to-program mappings, which are maintained by the port mapper server, are called a portmap. The port mapper is started automatically whenever a machine is booted. Both the server programs and the client programs call port mapper procedures. As part of its initialization, a server program calls its host's port mapper to create a portmap entry. Whereas server programs call port mapper programs to update portmap entries, clients call port mapper programs to query portmap entries. To find a remote program's port, a client sends an RPC call message to a server's port mapper. If the remote program is supported on the server, the port mapper returns the relevant port number in an RPC reply message. The client program can then send RPC call messages to the remote program's port. A client program can minimize port mapper calls by caching the port numbers of recently called remote programs.