Setting up reserved (privileged) ports

The z/OS NFS client uses a reserved (privileged) port to prevent the NFS server from rejecting a client request. The z/OS client attempts to use reserved port 1023 and if that port is not available, the z/OS client will subtract one from 1023 until a reserve port is available. If no reserve ports are available, the z/OS client will issue error message GFSC724E.

The amount of reserved ports the z/OS client can use is based on the client attribute biod. The amount of reserved ports can be calculated from the following formula:

   reserved ports = 8 + ( #biod * 4 )

The privileged ports should be reserved in the tcpip.profile file using the PORTRANGE statement. The default biod(6) and 8 additional ports correspond to 32 privileged ports that can be used by the z/OS client. For biod(6), the tcpip.profile file should include the following PORTRANGE statement:

  PORTRANGE 991 32 UDP MVSNFSC
  PORTRANGE 991 32 TCP MVSNFSC

This allows ports 991 through 1023 to be used by the z/OS client. Note that MVSNFSC is the default z/OS NFS client start-up procedure. Please specify the correct z/OS NFS client start-up procedure if it is different than the default.

When specifying secure(udp) or proto(udp), the z/OS client uses the privileged UDP ports to communicate with the NFS servers. When specifying proto(tcp) the z/OS client uses the privileged TCP ports to communicate the MOUNT RPC or UNMOUNT RPC with the NFS server. However, the z/OS client uses the ephemeral TCP ports to communicate NFS RPC with the NFS server. As a result, the z/OS client does not work with NFS servers that require all source TCP ports to be privileged.