nfs.7 (2010 09)

n
nfs(7) nfs(7)
NAME
nfs, NFS - network file system
DESCRIPTION
The Network File System (NFS) allows a client node to perform transparent file access over the network.
By using NFS, a client node operates on files residing on a variety of servers and server architectures,
and across a variety of operating systems. File access calls on the client (such as read requests) are con-
verted to NFS protocol requests and sent to the server system over the network. The server receives the
request, performs the actual file system operation, and sends a response back to the client.
NFS operates in a stateless manner using remote procedure calls (RPC) built on top of an external data
representation (XDR) protocol. The RPC protocol enables version and authentication parameters to be
exchanged for security over the network.
A server grants access to a specific file system to clients by adding an entry for that file system to the
server’s
/etc/dfs/dfstab file.
A client gains access to that file system using the
mount command to request a file handle for the file sys-
tem (see mount (1M)). (A file handle is the means by which NFS identifies remote files.) Once a client
mounts the file system, the server issues a file handle to the client for each file (or directory) the client
accesses. If the file is removed on the server side, the file handle becomes stale (dissociated with a known
file), and the server returns an error with
errno set to [ESTALE].
A server can also be a client with respect to file systems it has mounted over the network; however, its
clients cannot directly access those file systems. If a client attempts to mount a file system for which the
server is an NFS client, the server returns with
errno set to [EREMOTE]. The client must mount the
file system directly from the server on which the file system resides.
The user ID and group ID mappings must be the same between client and server. However, the server
maps UID 0 (the superuser) to UID −2 before performing access checks for a client. This process prevents
gaining superuser privileges on remote file systems.
RETURN VALUE
Generally, physical disk I/O errors detected at the server are returned to the client for action. If the
server is down or inaccessible, the client receives the message:
NFS: file server xxx not responding: still trying.
where xxx is the hostname of the NFS server. The client continues resending the request until it
receives an acknowledgement from the server. Therefore, the server can crash or power down, and come
back up without any special action required by the client. The client process requesting the I/O will
block, but remains sensitive to signals (unless mounted with the nointr option) until the server recov-
ers. However, if mounted with the soft option, the client process returns an error instead of waiting
indefinitely.
AUTHOR
nfs was developed by Sun Microsystems, Inc.
SEE ALSO
exportfs(1M), share(1M), mount(1M), mount_nfs(1M), nfsd(1M), mount(2), fstab(4), dfstab(4).
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1