nfs(4) Kernel Interfaces Manual nfs(4)
NAME
nfs, nfs_intro - Network File System (NFS) information
DESCRIPTION
NFS is a facility for sharing files in a heterogeneous environment of processors, operating systems, and networks. Sharing is accomplished
by mounting a remote file system or directory on a local system and then reading or writing the files as though they were local.
Sharing file systems has the following advantages: Removes the need to copy files across the network from one system to another Provides
easier access to remote files Reduces the risk of having out-of-date copies of the same file on different systems
The NFS Environment
NFS is based on the client-server model, in which client systems request resources from other systems called servers. A server is any host
system or process that provides a network service. A client is any host system or process that uses services from a server.
A single host, or server, can provide more than one service. Servers are passive; they do not call clients, they wait for clients to call
them.
A one-to-one correspondence between servers, clients, and systems does not always exist. A system that acts as a server can also act as a
client. A server that exports file systems and directories can also mount remote file systems and directories exported by other systems,
thus becoming a client. To export a file system or directory is to make it available for NFS clients to mount remotely on their local sys-
tems.
The /etc/exports file defines the NFS file systems and directories that can be exported. The following table summarizes the distinctions
between client and server systems for NFS mounts.
--------------------------------------------------------------
Client Server
--------------------------------------------------------------
Requests a remote mount Responds to the mount request
Reads the /etc/fstab file Reads the /etc/exports file
Checks if the server is known Checks if the client is known
--------------------------------------------------------------
The client always initiates the remote mount. The server completes the bindings, subject to access-control rules specific to NFS. Because
most network administration problems occur at bind time, you should know how a client binds to a server and what access control policy each
server uses. For more information on access control, see the NFS Service section and the Network Administration manual.
You mount a remote file system by using either the mount command or an automatic mounting daemon, such as Automount or AutoFS. In addi-
tion, you can use the WebNFS protocol, which bypasses mount and gives direct access to the WebNFS served file system.
An NFS client selects a specific server from which to mount a file system or directory. A client can mount file systems and directories
from several different servers.
Network Transport Protocols
NFS can use either the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) protocol. The TCP protocol is best for con-
gested wide area networks because of its superior retransmit features and congestion control. The UDP protocol is best in networks where
congestion and lost messages are not a problem because its lower overhead might yield higher throughput. The client selects the transport
protocol in the mount command.
The NFS Service
Once a remote file system or directory is mounted, it can be used as a local file system by client programs. Typically, a client mounts
one or more remote file systems and directories at start up time, if lines similar to the following are in the /etc/fstab file. The mount
program reads these lines when the system starts up: titan:/usr2 /usr2 nfs rw,bg 0 0 venus:/usr/man /usr/man nfs rw,bg 0 0 The first line
in the example shows that the /usr2 directory at system titan is mounted at local mount point /usr2 when you boot the local system. The
rest of this line describes the mount. See fstab(4) for a description of the file format.
NFS servers control access to their resources by limiting named file systems and directories to a specific set of clients with an entry in
the /etc/exports file. The /etc/exports file allows you to limit access to NFS clients but not to individual remote users. By default in
Tru64 UNIX, mounts can be limited to client superusers only.
The following four programs implement the NFS service: portmap mountd nfsiod nfsd
A client's mount request is transmitted to the remote server's mountd daemon after obtaining its address from portmap. A port mapper is a
Remote Procedure Call (RPC) daemon that maps RPC program numbers of network services to their UDP or TCP protocol port numbers.
The mountd daemon checks the access permission of the client and returns a pointer to the file system or directory. After the mount is
completed, access to files and directories at and below that mount point go through the pointer to the server's NFS daemon (nfsd) using
remote procedure calls. Some file access requests (write-behind and read-ahead) are handled by the block I/O daemon (nfsiod) on the
client.
PC-NFS
The PC-NFS facility provides the benefits of NFS to personal computers on your network. PC-NFS enables personal computers to share
resources and exchange files, and like NFS, is based on the client-server model. The client software runs on the personal computer; the
server software resides in the /usr/sbin directory.
For information on the PC-NFS client, see your PC-NFS client documentation.
For instructions on making the PC-NFS server available to clients, refer to Network Administration.
The NFS Locking Service
The NFS Locking Service (rpc.lockd) allows you to create advisory locks on files and file regions on local and remotely mounted file sys-
tems. Advisory locks are not enforced.
To make use of the NFS Locking Service, a programmer needs to understand how to use the fcntl system call or the lockf subroutine. See
fcntl(2) and lockf(3) for programming information.
File locking is a way to manage access to shared files. A process takes the following steps when locking a file or region of a file:
Determines if the file or region within the file is locked Applies a lock if the file or region is not locked Makes the changes to the file
or region Unlocks the file or region
The NFS Locking Service coordinates the dispersal of locks to remote file systems. It communicates with the kernel and status monitor
(rpc.statd) of the local system, as well as with the other lock daemons on the network.
The NFS Locking Service uses a stateless approach to failure recovery. The fundamental element of this approach is that the status monitor
detects both client and server recoveries. This approach is passive. When the client status monitor detects that a failed server has
reinitialized (recovered), it notifies the local locking daemon of the failure. The lock daemon then activates the appropriate recovery
mechanism.
If the NFS server fails and the NFS Locking Service is enabled, all the locks managed by the server's local processes are lost. However,
when the server recovers, the lock manager daemons on the client systems send reclaim requests for the NFS locks. The server lock manager
reestablishes the previously acquired locks associated with the reclaim requests, provided the requests are received within the grace
period built into the NFS Locking Service.
During the grace period, the server lock manager honors only reclaim requests. Once the grace period expires, reclaim requests are no
longer valid, and the server lock manager accepts only new requests. At this time, the server lock manager cannot reestablish old locks.
A user process whose lock could not be reclaimed is sent a SIGLOST signal. The client lock manager can create new locks by using the
interface primitives in the fcntl system call or the lockf subroutine.
If a client fails while it is using the NFS Locking Service, then when the client recovers, the status monitor daemon notifies the appro-
priate servers. The server lock manager then releases the locks. The client applications can then issue new lock requests as part of
their recovery procedure.
Automatic Mounting Daemons
NFS clients can optionally run an automatic mounting daemon, such as Automount or AutoFS. These daemons are client interfaces that perform
remote mounts automatically, and only when they are needed. If you need to access a remotely mounted file or directory, the selected dae-
mon mounts it, keeping it mounted as long as you need it.
The automount and autofsd daemons are told which remote file systems to mount from a database file called a map. A map lists the remote
file systems that the selected daemon monitors, their locations, and any mount options. The maps can be shared through NIS.
There are three types of maps: Master - Although not required, the master map helps to organize mounts. If a master map exists, it is read
first when the automount or autofsmount commands are invoked, acting as a pointer to other maps.
Both commands look for an NIS map called auto.master when invoked. They can also be instructed to use a local master map. Direct -
Specifies a key that is the pathname of the mount point, and a location, which is the location of the file system or directory in
which it resides, specified in this format: server:pathname. For direct maps, a local mount point is specified as an absolute path-
name, such as /doclib. Indirect - Like a direct map, specifies the pathname of the mount point and the location of the file system
on which it resides. For indirect maps, a local mount point is specified as a simple pathname, such as docsrc because the whole map
is associated with a directory.
For more information these daemons, see automount(8), autofsd(8), and autofsmount(8). Or, refer to the Network Administration manual.
RELATED INFORMATION
Commands: autofsmount(8), automount(8), nfsstat(8)
Daemons: autofsd(8), automount(8), mountd(8), nfsd(8), nfsiod(8), rpc.lockd(8), rpc.statd(8)
Files: advfs(4), cdfs(4), fstab(4)
Technical Overview, Network Administration delim off
nfs(4)