nfs, NFS - network file system
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 converted 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/exports file.
A client gains access to that file system using the mount command to
request a file handle for the file system (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.
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 not responding: still trying.
The client continues resending the request until it receives an
acknowledgement from the server. Therefore, the server can crash or
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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 recovers. However, if mounted with the soft option,
the client process returns an error instead of waiting indefinitely.
nfs was developed by Sun Microsystems, Inc.
exportfs(1M), mount(1M), mount_nfs(1M), nfsd(1M), mount(2), fstab(4),
Hewlett-Packard Company - 2 - HP-UX Release 11i: November 2000