portmap daemon

The portmap daemon helps clients map program number and version number pairs to the port number of a server.

Each RPC application has associated with it a program number and a version number. These numbers are used to communicate with a server application on a system. When making a request from a server, the client needs to know what port number the server is accepting requests on. This port number is associated with the User Datagram Protocol (UDP) or Transmission Control Protocol (TCP) that is being used by the service. The client knows the program number, the version number, and the system name or host name where the service resides. The client needs a way to map the program number and version number pair to the port number of the server application. This is done with the help of the portmap daemon.

The portmap daemon runs on the same system as the NFS application. When the server starts running, it registers with the portmap daemon. As a function of this registration, the server supplies its program number, version number, and UDP or TCP port number. The portmap daemon keeps a table of server applications. When the client tries to make a request of the server, it first contacts the portmap daemon to find out which port the server is using. The portmap daemon responds to the client with the port of the server that the client is requesting. Upon receipt of the port number, the client is able to make all of its future requests directly to the server application.