How File Systems Are Mounted in NFS Version 3

The information in this section applies to NFS Version 3 mounts. The NFS Version 4 mount process does not include the portmap service or the MOUNT protocol.

When a client attempts to mount a file system from a server, the client must obtain a file handle from the server. The file handle must correspond to the file system. This process requires that several transactions occur between the client and the server. In this example, the client is attempting to mount /home/user from the server. A snoop trace for this transaction follows:

client -> server PORTMAP C GETPORT prog=100005 (MOUNT) vers=3 proto=UDP
server -> client PORTMAP R GETPORT port=33482
client -> server MOUNT3 C Null
server -> client MOUNT3 R Null 
client -> server MOUNT3 C Mount /export/home9/user
server -> client MOUNT3 R Mount OK FH=9000 Auth=unix
client -> server PORTMAP C GETPORT prog=100003 (NFS) vers=3 proto=TCP
server -> client PORTMAP R GETPORT port=2049
client -> server NFS C NULL3
server -> client NFS R NULL3 
client -> server NFS C FSINFO3 FH=9000
server -> client NFS R FSINFO3 OK
client -> server NFS C GETATTR3 FH=9000
server -> client NFS R GETATTR3 OK

In this trace, the client first requests the mount port number from the portmap service on the NFS server. After the client receives the mount port number (33492), that number is used to test the availability of the service on the server. After the client has determined that a service is running on that port number, the client then makes a mount request. When the server responds to this request, the server includes the file handle for the file system (9000) being mounted. The client then sends a request for the NFS port number. When the client receives the number from the server, the client tests the availability of the NFS service (nfsd). Also, the client requests NFS information about the file system that uses the file handle.

In the following trace, the client is mounting the file system with the –public option:

client -> server NFS C LOOKUP3 FH=0000 /export/home9/user
server -> client NFS R LOOKUP3 OK FH=9000
client -> server NFS C FSINFO3 FH=9000
server -> client NFS R FSINFO3 OK
client -> server NFS C GETATTR3 FH=9000
server -> client NFS R GETATTR3 OK

By using the default public file handle (which is 0000), all the transactions to obtain information from the portmap service and to determine the NFS port number are skipped.