NFS(4P)NFS(4P)NAME
nfs, NFS - network file system
SYNOPSIS
options NFS
DESCRIPTION
The Network File System, or NFS, allows a client workstation to perform
transparent file access over the network. Using it, a client
workstation can operate on files that reside on a variety of servers,
server architectures and across a variety of operating systems. Client
file access calls are converted to NFS protocol requests, and are 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.
The Nework File System operates in a stateless fashion using remote
procedure (RPC) calls built on top of external data representation
(XDR) protocol. The RPC protocol provides for version and
authentication parameters to be exchanged to ensure security over the
network.
A server can grant access to a specific filesystem to certain clients
by adding an entry for that filesystem to the server's /etc/exports
file.
A client gains access to that filesystem with the mount(2) system call,
which requests a file handle for the filesystem itself. Once the
filesystem is mounted by the client, the server issues a file handle to
the client for each file (or directory) the client accesses. If the
file is somehow removed on the server side, the file handle becomes
stale (dissociated with a known file).
A server may also be a client with respect to filesystems it has
mounted over the network, but its clients cannot gain access to those
filesystems. Instead, the client must mount a filesystem directly from
the server on which it resides.
The user ID and group ID mappings must be the same between client and
server. However, the server maps uid 0 (the super-user) to uid -2
before performing access checks for a client. This inhibits super-user
privileges on remote filesystems.
ERRORS
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 will see the console message:
NFS: file server not responding: still trying.
The client continues (forever) to resend the request until it receives
an acknowledgement from the server. This means the server can crash or
power down, and come back up, without any special action required by
the client. It also means the client process requesting the I/O will
block and remain insensitive to signals, sleeping inside the kernel at
PRIBIO.
SEE ALSOexports(5), fstab(5), mntent(5), mount(2), mount(8), nfsd(8)
9 July 1986 NFS(4P)
