rpcbind

rpcbind is a daemon that provides a mapping between RPC program numbers and network ports on a host. It is the modern name for what historically was called portmapper in Sun RPC implementations. The service listens on TCP and UDP port 111 and acts as a directory for RPC services on the local machine. RPC services register themselves with rpcbind by providing their program number, version, protocol, and port number; clients query rpcbind to learn which port to contact for a given RPC program.

In operation, when an RPC service starts, it registers its details with rpcbind. A client that wishes to use a particular RPC service first asks rpcbind for the port associated with the service’s program number and version, then connects to that port to issue requests. If the service stops, it unregisters from rpcbind. This mechanism allows clients to discover and communicate with distributed RPC services without needing to know fixed port numbers in advance.

Implementation and usage vary by system. On many Unix-like systems, including most Linux distributions, rpcbind runs as a background daemon and is managed by systemd or another init system. It is commonly required for services that rely on Sun RPC, such as NFS and certain directory services, though as RPC-based services shrink in some environments, rpcbind may be disabled or removed if not needed. Some distributions retain the older term portmapper in reference to the same concept.

Security considerations include the fact that rpcbind exposes UDP and TCP port 111, which can be probed by remote actors to discover available RPC services. To reduce risk, administrators often restrict access with firewalls, bind rpcbind to trusted networks, or disable RPC services and rpcbind when they are not necessary on a given host.