29.4 Port Mapper

29.4 Port Mapper

The RPC server programs containing the remote procedures use ephemeral ports, not well-known ports. This requires a "registrar" of some form that keeps track of which RPC programs are using which ephemeral ports. In Sun RPC this registrar is called the port mapper.

The term "port" in this name originates from the TCP and UDP port numbers , features of the Internet protocol suite. Since TI-RPC works over any transport layer, and not just TCP and UDP, the name of the port mapper in systems using TI-RPC (SVR4 and Solaris 2.2, for example) has become rpcbind. We'll continue to use the more familiar name of port mapper.

Naturally, the port mapper itself must have a well-known port: UDP port 111 and TCP port 111. The port mapper is also just an RPC server program. It has a program number (100000), a version number (2), a TCP port of 111, and a UDP port of 111. Servers register themselves with the port mapper using RPC calls, and clients query the port mapper using RPC calls. The port mapper provides four server procedures:

1. PMAPPROC_SET. Called by an RPC server on startup to register a program number, version number, and protocol with a port number.

2. PMAPPROC_UNSET. Called by server to remove a previously registered mapping.

3. PMAPPROC_GETPORT. Called by an RPC client on startup to obtain the port number for a given program number, version number, and protocol.

4. PMAPPROC_DUMP. Returns all entries (program number, version number, protocol, and port number) in the port mapper database.

When an RPC server program starts, and is later called by an RPC client program, the following steps take place.

1. The port mapper must be started first, normally when the system is bootstrapped. It creates a TCP end point and does a passive open on TCP port 111. It also creates a UDP end point and waits for a UDP datagram to arrive for UDP port 111.

2. When the RPC server program starts, it creates a TCP end point and a UDP end point for each version of the program that it supports. (A given RPC program can support multiple versions. The client specifies which version it wants when it calls a server procedure.) An ephemeral port number is bound to both end points. (It doesn't matter whether the TCP port number is the same or different from the UDP port number.) The server registers each program, version, protocol, and port number by making a remote procedure call to the port mapper's PMAPPROC_SET procedure.

3. When the RPC client program starts, it calls the port mapper's PMAPPROC_GETPORT procedure to obtain the ephemeral port number for a given program, version, and protocol.

4. The client sends an RPC call message to the port number returned in step 3. If UDP is being used, the client just sends a UDP datagram containing an RPC call message (Figure 29.1) to the server's UDP port number. The server responds by sending a UDP datagram containing an RPC reply message (Figure 29.2) back to the client.

   If TCP is being used, the client does an active open to the server's TCP port number, and then sends an RPC call message across the connection. The server responds with an RPC reply message across the connection.

The program rpcinfo (8) prints out the port mapper's current mappings. (It calls the port mapper's PMAPPROC_DUMP procedure.) Here is some typical output:

 sun %  /usr/etc/rpcinfo -p  program vers proto  port         100005    1   tcp   702  mountd  mount daemon for NFS  100005    1   udp   699  mountd         100005    2   tcp   702  mountd         100005    2   udp   699  mountd         100003    2   udp  2049  nfs  NFS itself  100021    1   tcp   709  nlockmgr  NFS lock manager  100021    1   udp  1036  nlockmgr         100021    2   tcp   721  nlockmgr         100021    2   udp  1039  nlockmgr         100021    3   tcp   713  nlockmgr         100021    3   udp  1037  nlockmgr 

We see that some programs do support multiple versions, and each combination of a program number, version number, and protocol has its own port number mapping maintained by the port mapper.

Both versions of the mount daemon are accessed through the same TCP port number (702) and the same UDP port number (699), but each version of the lock manager has its own port number.