.\"@(#)nfs.5"
.TH NFS 5 "9 October 2012"
.SH NAME
nfs \- fstab format and options for the
.B nfs
file systems
.SH SYNOPSIS
.I /etc/fstab
.SH DESCRIPTION
NFS is an Internet Standard protocol
created by Sun Microsystems in 1984. NFS was developed
to allow file sharing between systems residing
on a local area network.
The Linux NFS client supports three versions
of the NFS protocol:
NFS version 2 [RFC1094],
NFS version 3 [RFC1813],
and NFS version 4 [RFC3530].
.P
The
.BR mount (8)
command attaches a file system to the system's
name space hierarchy at a given mount point.
The
.I /etc/fstab
file describes how
.BR mount (8)
should assemble a system's file name hierarchy
from various independent file systems
(including file systems exported by NFS servers).
Each line in the
.I /etc/fstab
file describes a single file system, its mount point,
and a set of default mount options for that mount point.
.P
For NFS file system mounts, a line in the
.I /etc/fstab
file specifies the server name,
the path name of the exported server directory to mount,
the local directory that is the mount point,
the type of file system that is being mounted,
and a list of mount options that control
the way the filesystem is mounted and
how the NFS client behaves when accessing
files on this mount point.
The fifth and sixth fields on each line are not used
by NFS, thus conventionally each contain the digit zero. For example:
.P
.nf
.ta 8n +14n +14n +9n +20n
	server:path	/mountpoint	fstype	option,option,...	0 0
.fi
.P
The server's hostname and export pathname
are separated by a colon, while
the mount options are separated by commas. The remaining fields
are separated by blanks or tabs.
.P
The server's hostname can be an unqualified hostname,
a fully qualified domain name,
a dotted quad IPv4 address, or
an IPv6 address enclosed in square brackets.
Link-local and site-local IPv6 addresses must be accompanied by an
interface identifier.
See
.BR ipv6 (7)
for details on specifying raw IPv6 addresses.
.P
The
.I fstype
field contains "nfs".  Use of the "nfs4" fstype in
.I /etc/fstab
is deprecated.
.SH "MOUNT OPTIONS"
Refer to
.BR mount (8)
for a description of generic mount options
available for all file systems. If you do not need to
specify any mount options, use the generic option
.B defaults
in
.IR /etc/fstab .
.DT
.SS "Options supported by all versions"
These options are valid to use with any NFS version.
.TP 1.5i
.BI nfsvers= n
The NFS protocol version number used to contact the server's NFS service.
If the server does not support the requested version, the mount request 
fails.
If this option is not specified, the client negotiates a suitable version 
with
the server, trying version 4 first, version 3 second, and version 2 last.
.TP 1.5i
.BI vers= n
This option is an alternative to the
.B nfsvers
option.
It is included for compatibility with other operating systems
.TP 1.5i
.BR soft " / " hard
Determines the recovery behavior of the NFS client
after an NFS request times out.
If neither option is specified (or if the
.B hard
option is specified), NFS requests are retried indefinitely.
If the
.B soft
option is specified, then the NFS client fails an NFS request
after
.B retrans
retransmissions have been sent,
causing the NFS client to return an error
to the calling application.
.IP
.I NB:
A so-called "soft" timeout can cause
silent data corruption in certain cases. As such, use the
.B soft
option only when client responsiveness
is more important than data integrity.
Using NFS over TCP or increasing the value of the
.B retrans
option may mitigate some of the risks of using the
.B soft
option.
.TP 1.5i
.BR intr " / " nointr
This option is provided for backward compatibility.
It is ignored after kernel 2.6.25.
.TP 1.5i
.BI timeo= n
The time in deciseconds (tenths of a second) the NFS client waits for a
response before it retries an NFS request.
.IP
For NFS over TCP the default
.B timeo
value is 600 (60 seconds).
The NFS client performs linear backoff: After each retransmission the 
timeout is increased by
.BR timeo 
up to the maximum of 600 seconds.
.IP
However, for NFS over UDP, the client uses an adaptive
algorithm to estimate an appropriate timeout value for frequently used
request types (such as READ and WRITE requests), but uses the
.B timeo
setting for infrequently used request types (such as FSINFO requests).
If the
.B timeo
option is not specified,
infrequently used request types are retried after 1.1 seconds.
After each retransmission, the NFS client doubles the timeout for
that request,
up to a maximum timeout length of 60 seconds.
.TP 1.5i
.BI retrans= n
The number of times the NFS client retries a request before
it attempts further recovery action. If the
.B retrans
option is not specified, the NFS client tries each request
three times.
.IP
The NFS client generates a "server not responding" message
after
.B retrans
retries, then attempts further recovery (depending on whether the
.B hard
mount option is in effect).
.TP 1.5i
.BI rsize= n
The maximum number of bytes in each network READ request
that the NFS client can receive when reading data from a file
on an NFS server.
The actual data payload size of each NFS READ request is equal to
or smaller than the
.B rsize
setting. The largest read payload supported by the Linux NFS client
is 1,048,576 bytes (one megabyte).
.IP
The
.B rsize
value is a positive integral multiple of 1024.
Specified
.B rsize
values lower than 1024 are replaced with 4096; values larger than
1048576 are replaced with 1048576. If a specified value is within the supported
range but not a multiple of 1024, it is rounded down to the nearest
multiple of 1024.
.IP
If an
.B rsize
value is not specified, or if the specified
.B rsize
value is larger than the maximum that either client or server can support,
the client and server negotiate the largest
.B rsize
value that they can both support.
.IP
The
.B rsize
mount option as specified on the
.BR mount (8)
command line appears in the
.I /etc/mtab
file. However, the effective
.B rsize
value negotiated by the client and server is reported in the
.I /proc/mounts
file.
.TP 1.5i
.BI wsize= n
The maximum number of bytes per network WRITE request
that the NFS client can send when writing data to a file
on an NFS server. The actual data payload size of each
NFS WRITE request is equal to
or smaller than the
.B wsize
setting. The largest write payload supported by the Linux NFS client
is 1,048,576 bytes (one megabyte).
.IP
Similar to
.B rsize
, the
.B wsize
value is a positive integral multiple of 1024.
Specified
.B wsize
values lower than 1024 are replaced with 4096; values larger than
1048576 are replaced with 1048576. If a specified value is within the supported
range but not a multiple of 1024, it is rounded down to the nearest
multiple of 1024.
.IP
If a
.B wsize
value is not specified, or if the specified
.B wsize
value is larger than the maximum that either client or server can support,
the client and server negotiate the largest
.B wsize
value that they can both support.
.IP
The
.B wsize
mount option as specified on the
.BR mount (8)
command line appears in the
.I /etc/mtab
file. However, the effective
.B wsize
value negotiated by the client and server is reported in the
.I /proc/mounts
file.
.TP 1.5i
.BR ac " / " noac
Selects whether the client may cache file attributes. If neither
option is specified (or if
.B ac
is specified), the client caches file
attributes.
.IP
To improve performance, NFS clients cache file
attributes. Every few seconds, an NFS client checks the server's version of each
file's attributes for updates.  Changes that occur on the server in
those small intervals remain undetected until the client checks the
server again. The
.B noac
option prevents clients from caching file
attributes so that applications can more quickly detect file changes
on the server.
.IP
In addition to preventing the client from caching file attributes,
the
.B noac
option forces application writes to become synchronous so
that local changes to a file become visible on the server
immediately.  That way, other clients can quickly detect recent
writes when they check the file's attributes.
.IP
Using the
.B noac
option provides greater cache coherence among NFS clients
accessing the same files,
but it extracts a significant performance penalty.
As such, judicious use of file locking is encouraged instead.
The DATA AND METADATA COHERENCE section contains a detailed discussion
of these trade-offs.
.TP 1.5i
.BI acregmin= n
The minimum time (in seconds) that the NFS client caches
attributes of a regular file before it requests
fresh attribute information from a server.
If this option is not specified, the NFS client uses
a 3-second minimum.
See the DATA AND METADATA COHERENCE section
for a full discussion of attribute caching.
.TP 1.5i
.BI acregmax= n
The maximum time (in seconds) that the NFS client caches
attributes of a regular file before it requests
fresh attribute information from a server.
If this option is not specified, the NFS client uses
a 60-second maximum.
See the DATA AND METADATA COHERENCE section
for a full discussion of attribute caching.
.TP 1.5i
.BI acdirmin= n
The minimum time (in seconds) that the NFS client caches
attributes of a directory before it requests
fresh attribute information from a server.
If this option is not specified, the NFS client uses
a 30-second minimum.
See the DATA AND METADATA COHERENCE section
for a full discussion of attribute caching.
.TP 1.5i
.BI acdirmax= n
The maximum time (in seconds) that the NFS client caches
attributes of a directory before it requests
fresh attribute information from a server.
If this option is not specified, the NFS client uses
a 60-second maximum.
See the DATA AND METADATA COHERENCE section
for a full discussion of attribute caching.
.TP 1.5i
.BI actimeo= n
Using
.B actimeo
sets all of
.BR acregmin ,
.BR acregmax ,
.BR acdirmin ,
and
.B acdirmax
to the same value.
If this option is not specified, the NFS client uses
the defaults for each of these options listed above.
.TP 1.5i
.BR bg " / " fg
Determines how the
.BR mount (8)
command behaves if an attempt to mount an export fails.
The
.B fg
option causes
.BR mount (8)
to exit with an error status if any part of the mount request
times out or fails outright.
This is called a "foreground" mount,
and is the default behavior if neither the
.B fg
nor
.B bg
mount option is specified.
.IP
If the
.B bg
option is specified, a timeout or failure causes the
.BR mount (8)
command to fork a child which continues to attempt
to mount the export.
The parent immediately returns with a zero exit code.
This is known as a "background" mount.
.IP
If the local mount point directory is missing, the
.BR mount (8)
command acts as if the mount request timed out.
This permits nested NFS mounts specified in
.I /etc/fstab
to proceed in any order during system initialization,
even if some NFS servers are not yet available.
Alternatively these issues can be addressed
using an automounter (refer to
.BR automount (8)
for details).
.TP 1.5i
.BR rdirplus " / " nordirplus
Selects whether to use NFS v3 or v4 READDIRPLUS requests.
If this option is not specified, the NFS client uses READDIRPLUS requests
on NFS v3 or v4 mounts to read small directories.
Some applications perform better if the client uses only READDIR requests
for all directories.
.TP 1.5i
.BI retry= n
The number of minutes that the
.BR mount (8)
command retries an NFS mount operation
in the foreground or background before giving up.
If this option is not specified, the default value for foreground mounts
is 2 minutes, and the default value for background mounts is 10000 minutes
(80 minutes shy of one week).
If a value of zero is specified, the
.BR mount (8)
command exits immediately after the first failure.
.TP 1.5i
.BI sec= flavors
A colon-separated list of one or more security flavors to use for accessing
files on the mounted export. If the server does not support any of these
flavors, the mount operation fails.
If
.B sec=
is not specified, the client attempts to find
a security flavor that both the client and the server supports.
Valid
.I flavors
are
.BR none ,
.BR sys ,
.BR krb5 ,
.BR krb5i ,
and
.BR krb5p .
Refer to the SECURITY CONSIDERATIONS section for details.
.TP 1.5i
.BR sharecache " / " nosharecache
Determines how the client's data cache and attribute cache are shared
when mounting the same export more than once concurrently.  Using the
same cache reduces memory requirements on the client and presents
identical file contents to applications when the same remote file is
accessed via different mount points.
.IP
If neither option is specified, or if the
.B sharecache
option is
specified, then a single cache is used for all mount points that
access the same export.  If the
.B nosharecache
option is specified,
then that mount point gets a unique cache.  Note that when data and
attribute caches are shared, the mount options from the first mount
point take effect for subsequent concurrent mounts of the same export.
.IP
As of kernel 2.6.18, the behavior specified by
.B nosharecache
is legacy caching behavior. This
is considered a data risk since multiple cached copies
of the same file on the same client can become out of sync
following a local update of one of the copies.
.TP 1.5i
.BR resvport " / " noresvport
Specifies whether the NFS client should use a privileged source port
when communicating with an NFS server for this mount point.
If this option is not specified, or the
.B resvport
option is specified, the NFS client uses a privileged source port.
If the
.B noresvport
option is specified, the NFS client uses a non-privileged source port.
This option is supported in kernels 2.6.28 and later.
.IP
Using non-privileged source ports helps increase the maximum number of
NFS mount points allowed on a client, but NFS servers must be configured
to allow clients to connect via non-privileged source ports.
.IP
Refer to the SECURITY CONSIDERATIONS section for important details.
.TP 1.5i
.BI lookupcache= mode
Specifies how the kernel manages its cache of directory entries
for a given mount point.
.I mode
can be one of
.BR all ,
.BR none ,
.BR pos ,
or
.BR positive .
This option is supported in kernels 2.6.28 and later.
.IP
The Linux NFS client caches the result of all NFS LOOKUP requests.
If the requested directory entry exists on the server,
the result is referred to as
.IR positive .
If the requested directory entry does not exist on the server,
the result is referred to as
.IR negative .
.IP
If this option is not specified, or if
.B all
is specified, the client assumes both types of directory cache entries
are valid until their parent directory's cached attributes expire.
.IP
If
.BR pos " or " positive
is specified, the client assumes positive entries are valid
until their parent directory's cached attributes expire, but
always revalidates negative entires before an application
can use them.
.IP
If
.B none
is specified,
the client revalidates both types of directory cache entries
before an application can use them.
This permits quick detection of files that were created or removed
by other clients, but can impact application and server performance.
.IP
The DATA AND METADATA COHERENCE section contains a
detailed discussion of these trade-offs.
.TP 1.5i
.BR fsc " / " nofsc
Enable/Disables the cache of (read-only) data pages to the local disk 
using the FS-Cache facility. See cachefilesd(8) 
and <kernel_soruce>/Documentation/filesystems/caching
for detail on how to configure the FS-Cache facility.
Default value is nofsc.
.SS "Options for NFS versions 2 and 3 only"
Use these options, along with the options in the above subsection,
for NFS versions 2 and 3 only.
.TP 1.5i
.BI proto= netid
The
.I netid
determines the transport that is used to communicate with the NFS
server.  Available options are
.BR udp ", " udp6 ", "tcp ", " tcp6 ", and " rdma .
Those which end in
.B 6
use IPv6 addresses and are only available if support for TI-RPC is
built in. Others use IPv4 addresses.
.IP
Each transport protocol uses different default
.B retrans
and
.B timeo
settings.
Refer to the description of these two mount options for details.
.IP
In addition to controlling how the NFS client transmits requests to
the server, this mount option also controls how the
.BR mount (8)
command communicates with the server's rpcbind and mountd services.
Specifying a netid that uses TCP forces all traffic from the
.BR mount (8)
command and the NFS client to use TCP.
Specifying a netid that uses UDP forces all traffic types to use UDP.
.IP
.B Before using NFS over UDP, refer to the TRANSPORT METHODS section.
.IP
If the
.B proto
mount option is not specified, the
.BR mount (8)
command discovers which protocols the server supports
and chooses an appropriate transport for each service.
Refer to the TRANSPORT METHODS section for more details.
.TP 1.5i
.B udp
The
.B udp
option is an alternative to specifying
.BR proto=udp.
It is included for compatibility with other operating systems.
.IP
.B Before using NFS over UDP, refer to the TRANSPORT METHODS section.
.TP 1.5i
.B tcp
The
.B tcp
option is an alternative to specifying
.BR proto=tcp.
It is included for compatibility with other operating systems.
.TP 1.5i
.B rdma
The
.B rdma
option is an alternative to specifying
.BR proto=rdma.
.TP 1.5i
.BI port= n
The numeric value of the server's NFS service port.
If the server's NFS service is not available on the specified port,
the mount request fails.
.IP
If this option is not specified, or if the specified port value is 0,
then the NFS client uses the NFS service port number
advertised by the server's rpcbind service.
The mount request fails if the server's rpcbind service is not available,
the server's NFS service is not registered with its rpcbind service,
or the server's NFS service is not available on the advertised port.
.TP 1.5i
.BI mountport= n
The numeric value of the server's mountd port.
If the server's mountd service is not available on the specified port,
the mount request fails.
.IP
If this option is not specified,
or if the specified port value is 0, then the
.BR mount (8)
command uses the mountd service port number
advertised by the server's rpcbind service.
The mount request fails if the server's rpcbind service is not available,
the server's mountd service is not registered with its rpcbind service,
or the server's mountd service is not available on the advertised port.
.IP
This option can be used when mounting an NFS server
through a firewall that blocks the rpcbind protocol.
.TP 1.5i
.BI mountproto= netid
The transport the NFS client uses
to transmit requests to the NFS server's mountd service when performing
this mount request, and when later unmounting this mount point.
.IP
.I netid
may be one of
.BR udp ", and " tcp
which use IPv4 address or, if TI-RPC is built into the
.B mount.nfs
command,
.BR udp6 ", and " tcp6
which use IPv6 addresses.
.IP
This option can be used when mounting an NFS server
through a firewall that blocks a particular transport.
When used in combination with the
.B proto
option, different transports for mountd requests and NFS requests
can be specified.
If the server's mountd service is not available via the specified
transport, the mount request fails.
.IP
Refer to the TRANSPORT METHODS section for more on how the
.B mountproto
mount option interacts with the
.B proto
mount option.
.TP 1.5i
.BI mounthost= name
The hostname of the host running mountd.
If this option is not specified, the
.BR mount (8)
command assumes that the mountd service runs
on the same host as the NFS service.
.TP 1.5i
.BI mountvers= n
The RPC version number used to contact the server's mountd.
If this option is not specified, the client uses a version number
appropriate to the requested NFS version.
This option is useful when multiple NFS services
are running on the same remote server host.
.TP 1.5i
.BI namlen= n
The maximum length of a pathname component on this mount.
If this option is not specified, the maximum length is negotiated
with the server. In most cases, this maximum length is 255 characters.
.IP
Some early versions of NFS did not support this negotiation.
Using this option ensures that
.BR pathconf (3)
reports the proper maximum component length to applications
in such cases.
.TP 1.5i
.BR lock " / " nolock
Selects whether to use the NLM sideband protocol to lock files on the server.
If neither option is specified (or if
.B lock
is specified), NLM locking is used for this mount point.
When using the
.B nolock
option, applications can lock files,
but such locks provide exclusion only against other applications
running on the same client.
Remote applications are not affected by these locks.
.IP
NLM locking must be disabled with the
.B nolock
option when using NFS to mount
.I /var
because
.I /var
contains files used by the NLM implementation on Linux.
Using the
.B nolock
option is also required when mounting exports on NFS servers
that do not support the NLM protocol.
.TP 1.5i
.BR cto " / " nocto
Selects whether to use close-to-open cache coherence semantics.
If neither option is specified (or if
.B cto
is specified), the client uses close-to-open
cache coherence semantics. If the
.B nocto
option is specified, the client uses a non-standard heuristic to determine when
files on the server have changed.
.IP
Using the
.B nocto
option may improve performance for read-only mounts,
but should be used only if the data on the server changes only occasionally.
The DATA AND METADATA COHERENCE section discusses the behavior
of this option in more detail.
.TP 1.5i
.BR acl " / " noacl
Selects whether to use the NFSACL sideband protocol on this mount point.
The NFSACL sideband protocol is a proprietary protocol
implemented in Solaris that manages Access Control Lists. NFSACL was never
made a standard part of the NFS protocol specification.
.IP
If neither
.B acl
nor
.B noacl
option is specified,
the NFS client negotiates with the server
to see if the NFSACL protocol is supported,
and uses it if the server supports it.
Disabling the NFSACL sideband protocol may be necessary
if the negotiation causes problems on the client or server.
Refer to the SECURITY CONSIDERATIONS section for more details.
.TP 1.5i
.BR local_lock= mechanism
Specifies whether to use local locking for any or both of the flock and the
POSIX locking mechanisms.
.I mechanism
can be one of
.BR all ,
.BR flock ,
.BR posix ,
or
.BR none .
This option is supported in kernels 2.6.37 and later.
.IP
The Linux NFS client provides a way to make locks local. This means, the
applications can lock files, but such locks provide exclusion only against
other applications running on the same client. Remote applications are not
affected by these locks.
.IP
If this option is not specified, or if
.B none
is specified, the client assumes that the locks are not local.
.IP
If
.BR all
is specified, the client assumes that both flock and POSIX locks are local.
.IP
If
.BR flock
is specified, the client assumes that only flock locks are local and uses
NLM sideband protocol to lock files when POSIX locks are used.
.IP
If
.BR posix
is specified, the client assumes that POSIX locks are local and uses NLM
sideband protocol to lock files when flock locks are used.
.IP
To support legacy flock behavior similar to that of NFS clients < 2.6.12, 
use 'local_lock=flock'. This option is required when exporting NFS mounts via
Samba as Samba maps Windows share mode locks as flock. Since NFS clients >
2.6.12 implement flock by emulating POSIX locks, this will result in
conflicting locks.
.IP
NOTE: When used together, the 'local_lock' mount option will be overridden
by 'nolock'/'lock' mount option.
.SS "Options for NFS version 4 only"
Use these options, along with the options in the first subsection above,
for NFS version 4 and newer.
.TP 1.5i
.BI proto= netid
The
.I netid
determines the transport that is used to communicate with the NFS
server.  Supported options are
.BR tcp ", " tcp6 ", and " rdma .
.B tcp6
use IPv6 addresses and is only available if support for TI-RPC is
built in. Both others use IPv4 addresses.
.IP
All NFS version 4 servers are required to support TCP,
so if this mount option is not specified, the NFS version 4 client
uses the TCP protocol.
Refer to the TRANSPORT METHODS section for more details.
.TP 1.5i
.BI port= n
The numeric value of the server's NFS service port.
If the server's NFS service is not available on the specified port,
the mount request fails.
.IP
If this mount option is not specified,
the NFS client uses the standard NFS port number of 2049
without first checking the server's rpcbind service.
This allows an NFS version 4 client to contact an NFS version 4
server through a firewall that may block rpcbind requests.
.IP
If the specified port value is 0,
then the NFS client uses the NFS service port number
advertised by the server's rpcbind service.
The mount request fails if the server's rpcbind service is not available,
the server's NFS service is not registered with its rpcbind service,
or the server's NFS service is not available on the advertised port.
.TP 1.5i
.BR cto " / " nocto
Selects whether to use close-to-open cache coherence semantics
for NFS directories on this mount point.
If neither
.B cto
nor
.B nocto
is specified,
the default is to use close-to-open cache coherence
semantics for directories.
.IP
File data caching behavior is not affected by this option.
The DATA AND METADATA COHERENCE section discusses
the behavior of this option in more detail.
.TP 1.5i
.BI clientaddr= n.n.n.n
.TP 1.5i
.BI clientaddr= n:n: ... :n
Specifies a single IPv4 address (in dotted-quad form),
or a non-link-local IPv6 address,
that the NFS client advertises to allow servers
to perform NFS version 4 callback requests against
files on this mount point. If  the  server is unable to
establish callback connections to clients, performance
may degrade, or accesses to files may temporarily hang.
.IP
If this option is not specified, the
.BR mount (8)
command attempts to discover an appropriate callback address automatically.
The automatic discovery process is not perfect, however.
In the presence of multiple client network interfaces,
special routing policies,
or atypical network topologies,
the exact address to use for callbacks may be nontrivial to determine.
.TP 1.5i
.BR migration " / " nomigration
Selects whether the client uses an identification string that is compatible
with NFSv4 Transparent State Migration (TSM).
If the mounted server supports NFSv4 migration with TSM, specify the
.B migration
option.
.IP
Some server features misbehave in the face of a migration-compatible
identification string.
The
.B nomigration
option retains the use of a traditional client indentification string
which is compatible with legacy NFS servers.
This is also the behavior if neither option is specified.
A client's open and lock state cannot be migrated transparently
when it identifies itself via a traditional identification string.
.IP
This mount option has no effect with NFSv4 minor versions newer than zero,
which always use TSM-compatible client identification strings.
.SH nfs4 FILE SYSTEM TYPE
The
.BR nfs4
file system type is an old syntax for specifying NFSv4 usage. It can still 
be used with all NFSv4-specific and common options, excepted the
.B nfsvers
mount option.
.SH MOUNT CONFIGURATION FILE
If the mount command is configured to do so, all of the mount options 
described in the previous section can also be configured in the 
.I /etc/nfsmount.conf 
file. See 
.BR nfsmount.conf(5)
for details.
.SH EXAMPLES
To mount an export using NFS version 2,
use the
.B nfs
file system type and specify the
.B nfsvers=2
mount option.
To mount using NFS version 3,
use the
.B nfs
file system type and specify the
.B nfsvers=3
mount option.
To mount using NFS version 4,
use either the
.B nfs
file system type, with the
.B nfsvers=4
mount option, or the 
.B nfs4
file system type.
.P
The following example from an
.I /etc/fstab
file causes the mount command to negotiate
reasonable defaults for NFS behavior.
.P
.nf
.ta 8n +16n +6n +6n +30n
	server:/export	/mnt	nfs	defaults	0 0
.fi
.P
Here is an example from an /etc/fstab file for an NFS version 2 mount over UDP.
.P
.nf
.ta 8n +16n +6n +6n +30n
	server:/export	/mnt	nfs	nfsvers=2,proto=udp	0 0
.fi
.P
This example shows how to mount using NFS version 4 over TCP
with Kerberos 5 mutual authentication.
.P
.nf
.ta 8n +16n +6n +6n +30n
	server:/export	/mnt	nfs4	sec=krb5	0 0
.fi
.P
This example shows how to mount using NFS version 4 over TCP
with Kerberos 5 privacy or data integrity mode.
.P
.nf
.ta 8n +16n +6n +6n +30n
	server:/export	/mnt	nfs4	sec=krb5p:krb5i	0 0
.fi
.P
This example can be used to mount /usr over NFS.
.P
.nf
.ta 8n +16n +6n +6n +30n
	server:/export	/usr	nfs	ro,nolock,nocto,actimeo=3600	0 0
.fi
.P
This example shows how to mount an NFS server
using a raw IPv6 link-local address.
.P
.nf
.ta 8n +40n +5n +4n +9n
	[fe80::215:c5ff:fb3e:e2b1%eth0]:/export	/mnt	nfs	defaults	0 0
.fi
.SH "TRANSPORT METHODS"
NFS clients send requests to NFS servers via
Remote Procedure Calls, or
.IR RPCs .
The RPC client discovers remote service endpoints automatically,
handles per-request authentication,
adjusts request parameters for different byte endianness on client and server,
and retransmits requests that may have been lost by the network or server.
RPC requests and replies flow over a network transport.
.P
In most cases, the
.BR mount (8)
command, NFS client, and NFS server
can automatically negotiate proper transport
and data transfer size settings for a mount point.
In some cases, however, it pays to specify
these settings explicitly using mount options.
.P
Traditionally, NFS clients used the UDP transport exclusively for
transmitting requests to servers.  Though its implementation is
simple, NFS over UDP has many limitations that prevent smooth
operation and good performance in some common deployment
environments.  Even an insignificant packet loss rate results in the
loss of whole NFS requests; as such, retransmit timeouts are usually
in the subsecond range to allow clients to recover quickly from
dropped requests, but this can result in extraneous network traffic
and server load.
.P
However, UDP can be quite effective in specialized settings where
the networks MTU is large relative to NFSs data transfer size (such
as network environments that enable jumbo Ethernet frames).  In such
environments, trimming the
.B rsize
and
.B wsize
settings so that each
NFS read or write request fits in just a few network frames (or even
in  a single  frame) is advised.  This reduces the probability that
the loss of a single MTU-sized network frame results in the loss of
an entire large read or write request.
.P
TCP is the default transport protocol used for all modern NFS
implementations.  It performs well in almost every conceivable
network environment and provides excellent guarantees against data
corruption caused by network unreliability.  TCP is often a
requirement for mounting a server through a network firewall.
.P
Under normal circumstances, networks drop packets much more
frequently than NFS servers drop requests.  As such, an aggressive
retransmit timeout  setting for NFS over TCP is unnecessary. Typical
timeout settings for NFS over TCP are between one and ten minutes.
After  the client exhausts its retransmits (the value of the
.B retrans
mount option), it assumes a network partition has occurred,
and attempts to reconnect to the server on a fresh socket. Since
TCP itself makes network data transfer reliable,
.B rsize
and
.B wsize
can safely be allowed to default to the largest values supported by
both client and server, independent of the network's MTU size.
.SS "Using the mountproto mount option"
This section applies only to NFS version 2 and version 3 mounts
since NFS version 4 does not use a separate protocol for mount
requests.
.P
The Linux NFS client can use a different transport for
contacting an NFS server's rpcbind service, its mountd service,
its Network Lock Manager (NLM) service, and its NFS service.
The exact transports employed by the Linux NFS client for
each mount point depends on the settings of the transport
mount options, which include
.BR proto ,
.BR mountproto ,
.BR udp ", and " tcp .
.P
The client sends Network Status Manager (NSM) notifications
via UDP no matter what transport options are specified, but
listens for server NSM notifications on both UDP and TCP.
The NFS Access Control List (NFSACL) protocol shares the same
transport as the main NFS service.
.P
If no transport options are specified, the Linux NFS client
uses UDP to contact the server's mountd service, and TCP to
contact its NLM and NFS services by default.
.P
If the server does not support these transports for these services, the
.BR mount (8)
command attempts to discover what the server supports, and then retries
the mount request once using the discovered transports.
If the server does not advertise any transport supported by the client
or is misconfigured, the mount request fails.
If the
.B bg
option is in effect, the mount command backgrounds itself and continues
to attempt the specified mount request.
.P
When the
.B proto
option, the
.B udp
option, or the
.B tcp
option is specified but the
.B mountproto
option is not, the specified transport is used to contact
both the server's mountd service and for the NLM and NFS services.
.P
If the
.B mountproto
option is specified but none of the
.BR proto ", " udp " or " tcp
options are specified, then the specified transport is used for the
initial mountd request, but the mount command attempts to discover
what the server supports for the NFS protocol, preferring TCP if
both transports are supported.
.P
If both the
.BR mountproto " and " proto
(or
.BR udp " or " tcp )
options are specified, then the transport specified by the
.B mountproto
option is used for the initial mountd request, and the transport
specified by the
.B proto
option (or the
.BR udp " or " tcp " options)"
is used for NFS, no matter what order these options appear.
No automatic service discovery is performed if these options are
specified.
.P
If any of the
.BR proto ", " udp ", " tcp ", "
or
.B mountproto
options are specified more than once on the same mount command line,
then the value of the rightmost instance of each of these options
takes effect.
.SS "Using NFS over UDP on high-speed links"
Using NFS over UDP on high-speed links such as Gigabit
.BR "can cause silent data corruption" .
.P
The problem can be triggered at high loads, and is caused by problems in
IP fragment reassembly. NFS read and writes typically transmit UDP packets
of 4 Kilobytes or more, which have to be broken up into several fragments
in order to be sent over the Ethernet link, which limits packets to 1500
bytes by default. This process happens at the IP network layer and is
called fragmentation.
.P
In order to identify fragments that belong together, IP assigns a 16bit
.I IP ID
value to each packet; fragments generated from the same UDP packet
will have the same IP ID. The receiving system will collect these
fragments and combine them to form the original UDP packet. This process
is called reassembly. The default timeout for packet reassembly is
30 seconds; if the network stack does not receive all fragments of
a given packet within this interval, it assumes the missing fragment(s)
got lost and discards those it already received.
.P
The problem this creates over high-speed links is that it is possible
to send more than 65536 packets within 30 seconds. In fact, with
heavy NFS traffic one can observe that the IP IDs repeat after about
5 seconds.
.P
This has serious effects on reassembly: if one fragment gets lost,
another fragment
.I from a different packet
but with the
.I same IP ID
will arrive within the 30 second timeout, and the network stack will
combine these fragments to form a new packet. Most of the time, network
layers above IP will detect this mismatched reassembly - in the case
of UDP, the UDP checksum, which is a 16 bit checksum over the entire
packet payload, will usually not match, and UDP will discard the
bad packet.
.P
However, the UDP checksum is 16 bit only, so there is a chance of 1 in
65536 that it will match even if the packet payload is completely
random (which very often isn't the case). If that is the case,
silent data corruption will occur.
.P
This potential should be taken seriously, at least on Gigabit
Ethernet.
Network speeds of 100Mbit/s should be considered less
problematic, because with most traffic patterns IP ID wrap around
will take much longer than 30 seconds.
.P
It is therefore strongly recommended to use
.BR "NFS over TCP where possible" ,
since TCP does not perform fragmentation.
.P
If you absolutely have to use NFS over UDP over Gigabit Ethernet,
some steps can be taken to mitigate the problem and reduce the
probability of corruption:
.TP +1.5i
.I Jumbo frames:
Many Gigabit network cards are capable of transmitting
frames bigger than the 1500 byte limit of traditional Ethernet, typically
9000 bytes. Using jumbo frames of 9000 bytes will allow you to run NFS over
UDP at a page size of 8K without fragmentation. Of course, this is
only feasible if all involved stations support jumbo frames.
.IP
To enable a machine to send jumbo frames on cards that support it,
it is sufficient to configure the interface for a MTU value of 9000.
.TP +1.5i
.I Lower reassembly timeout:
By lowering this timeout below the time it takes the IP ID counter
to wrap around, incorrect reassembly of fragments can be prevented
as well. To do so, simply write the new timeout value (in seconds)
to the file
.BR /proc/sys/net/ipv4/ipfrag_time .
.IP
A value of 2 seconds will greatly reduce the probability of IPID clashes on
a single Gigabit link, while still allowing for a reasonable timeout
when receiving fragmented traffic from distant peers.
.SH "DATA AND METADATA COHERENCE"
Some modern cluster file systems provide
perfect cache coherence among their clients.
Perfect cache coherence among disparate NFS clients
is expensive to achieve, especially on wide area networks.
As such, NFS settles for weaker cache coherence that
satisfies the requirements of most file sharing types.
.SS "Close-to-open cache consistency"
Typically file sharing is completely sequential.
First client A opens a file, writes something to it, then closes it.
Then client B opens the same file, and reads the changes.
.P
When an application opens a file stored on an NFS version 3 server,
the NFS client checks that the file exists on the server
and is permitted to the opener by sending a GETATTR or ACCESS request.
The NFS client sends these requests
regardless of the freshness of the file's cached attributes.
.P
When the application closes the file,
the NFS client writes back any pending changes
to the file so that the next opener can view the changes.
This also gives the NFS client an opportunity to report
write errors to the application via the return code from
.BR close (2).
.P
The behavior of checking at open time and flushing at close time
is referred to as
.IR "close-to-open cache consistency" ,
or
.IR CTO .
It can be disabled for an entire mount point using the
.B nocto
mount option.
.SS "Weak cache consistency"
There are still opportunities for a client's data cache
to contain stale data.
The NFS version 3 protocol introduced "weak cache consistency"
(also known as WCC) which provides a way of efficiently checking
a file's attributes before and after a single request.
This allows a client to help identify changes
that could have been made by other clients.
.P
When a client is using many concurrent operations
that update the same file at the same time
(for example, during asynchronous write behind),
it is still difficult to tell whether it was
that client's updates or some other client's updates
that altered the file.
.SS "Attribute caching"
Use the
.B noac
mount option to achieve attribute cache coherence
among multiple clients.
Almost every file system operation checks
file attribute information.
The client keeps this information cached
for a period of time to reduce network and server load.
When
.B noac
is in effect, a client's file attribute cache is disabled,
so each operation that needs to check a file's attributes
is forced to go back to the server.
This permits a client to see changes to a file very quickly,
at the cost of many extra network operations.
.P
Be careful not to confuse the
.B noac
option with "no data caching."
The
.B noac
mount option prevents the client from caching file metadata,
but there are still races that may result in data cache incoherence
between client and server.
.P
The NFS protocol is not designed to support
true cluster file system cache coherence
without some type of application serialization.
If absolute cache coherence among clients is required,
applications should use file locking. Alternatively, applications
can also open their files with the O_DIRECT flag
to disable data caching entirely.
.SS "File timestamp maintainence"
NFS servers are responsible for managing file and directory timestamps
.RB ( atime ,
.BR ctime ", and"
.BR mtime ).
When a file is accessed or updated on an NFS server,
the file's timestamps are updated just like they would be on a filesystem
local to an application.
.P
NFS clients cache file attributes, including timestamps.
A file's timestamps are updated on NFS clients when its attributes
are retrieved from the NFS server.
Thus there may be some delay before timestamp updates
on an NFS server appear to applications on NFS clients.
.P
To comply with the POSIX filesystem standard, the Linux NFS client
relies on NFS servers to keep a file's
.B mtime
and
.B ctime
timestamps properly up to date.
It does this by flushing local data changes to the server
before reporting
.B mtime
to applications via system calls such as
.BR stat (2).
.P
The Linux client handles
.B atime
updates more loosely, however.
NFS clients maintain good performance by caching data,
but that means that application reads, which normally update
.BR atime ,
are not reflected to the server where a file's
.B atime
is actually maintained.
.P
Because of this caching behavior,
the Linux NFS client does not support generic atime-related mount options.
See
.BR mount (8)
for details on these options.
.P
In particular, the
.BR atime / noatime ,
.BR diratime / nodiratime ,
.BR relatime / norelatime ,
and
.BR strictatime / nostrictatime
mount options have no effect on NFS mounts.
.P
.I /proc/mounts
may report that the
.B relatime
mount option is set on NFS mounts, but in fact the
.B atime
semantics are always as described here, and are not like
.B relatime
semantics.
.SS "Directory entry caching"
The Linux NFS client caches the result of all NFS LOOKUP requests.
If the requested directory entry exists on the server,
the result is referred to as a
.IR positive " lookup result.
If the requested directory entry does not exist on the server
(that is, the server returned ENOENT),
the result is referred to as
.IR negative " lookup result.
.P
To detect when directory entries have been added or removed
on the server,
the Linux NFS client watches a directory's mtime.
If the client detects a change in a directory's mtime,
the client drops all cached LOOKUP results for that directory.
Since the directory's mtime is a cached attribute, it may
take some time before a client notices it has changed.
See the descriptions of the
.BR acdirmin ", " acdirmax ", and " noac
mount options for more information about
how long a directory's mtime is cached.
.P
Caching directory entries improves the performance of applications that
do not share files with applications on other clients.
Using cached information about directories can interfere
with applications that run concurrently on multiple clients and
need to detect the creation or removal of files quickly, however.
The
.B lookupcache
mount option allows some tuning of directory entry caching behavior.
.P
Before kernel release 2.6.28,
the Linux NFS client tracked only positive lookup results.
This permitted applications to detect new directory entries
created by other clients quickly while still providing some of the
performance benefits of caching.
If an application depends on the previous lookup caching behavior
of the Linux NFS client, you can use
.BR lookupcache=positive .
.P
If the client ignores its cache and validates every application
lookup request with the server,
that client can immediately detect when a new directory
entry has been either created or removed by another client.
You can specify this behavior using
.BR lookupcache=none .
The extra NFS requests needed if the client does not
cache directory entries can exact a performance penalty.
Disabling lookup caching
should result in less of a performance penalty than using
.BR noac ,
and has no effect on how the NFS client caches the attributes of files.
.P
.SS "The sync mount option"
The NFS client treats the
.B sync
mount option differently than some other file systems
(refer to
.BR mount (8)
for a description of the generic
.B sync
and
.B async
mount options).
If neither
.B sync
nor
.B async
is specified (or if the
.B async
option is specified),
the NFS client delays sending application
writes to the server
until any of these events occur:
.IP
Memory pressure forces reclamation of system memory resources.
.IP
An application flushes file data explicitly with
.BR sync (2),
.BR msync (2),
or
.BR fsync (3).
.IP
An application closes a file with
.BR close (2).
.IP
The file is locked/unlocked via
.BR fcntl (2).
.P
In other words, under normal circumstances,
data written by an application may not immediately appear
on the server that hosts the file.
.P
If the
.B sync
option is specified on a mount point,
any system call that writes data to files on that mount point
causes that data to be flushed to the server
before the system call returns control to user space.
This provides greater data cache coherence among clients,
but at a significant performance cost.
.P
Applications can use the O_SYNC open flag to force application
writes to individual files to go to the server immediately without
the use of the
.B sync
mount option.
.SS "Using file locks with NFS"
The Network Lock Manager protocol is a separate sideband protocol
used to manage file locks in NFS version 2 and version 3.
To support lock recovery after a client or server reboot,
a second sideband protocol --
known as the Network Status Manager protocol --
is also required.
In NFS version 4,
file locking is supported directly in the main NFS protocol,
and the NLM and NSM sideband protocols are not used.
.P
In most cases, NLM and NSM services are started automatically,
and no extra configuration is required.
Configure all NFS clients with fully-qualified domain names
to ensure that NFS servers can find clients to notify them of server reboots.
.P
NLM supports advisory file locks only.
To lock NFS files, use
.BR fcntl (2)
with the F_GETLK and F_SETLK commands.
The NFS client converts file locks obtained via
.BR flock (2)
to advisory locks.
.P
When mounting servers that do not support the NLM protocol,
or when mounting an NFS server through a firewall
that blocks the NLM service port,
specify the
.B nolock
mount option. NLM locking must be disabled with the
.B nolock
option when using NFS to mount
.I /var
because
.I /var
contains files used by the NLM implementation on Linux.
.P
Specifying the
.B nolock
option may also be advised to improve the performance
of a proprietary application which runs on a single client
and uses file locks extensively.
.SS "NFS version 4 caching features"
The data and metadata caching behavior of NFS version 4
clients is similar to that of earlier versions.
However, NFS version 4 adds two features that improve
cache behavior:
.I change attributes
and
.IR "file delegation" .
.P
The
.I change attribute
is a new part of NFS file and directory metadata
which tracks data changes.
It replaces the use of a file's modification
and change time stamps
as a way for clients to validate the content
of their caches.
Change attributes are independent of the time stamp
resolution on either the server or client, however.
.P
A
.I file delegation
is a contract between an NFS version 4 client
and server that allows the client to treat a file temporarily
as if no other client is accessing it.
The server promises to notify the client (via a callback request) if another client
attempts to access that file.
Once a file has been delegated to a client, the client can
cache that file's data and metadata aggressively without
contacting the server.
.P
File delegations come in two flavors:
.I read
and
.IR write .
A
.I read
delegation means that the server notifies the client
about any other clients that want to write to the file.
A
.I write
delegation means that the client gets notified about
either read or write accessors.
.P
Servers grant file delegations when a file is opened,
and can recall delegations at any time when another
client wants access to the file that conflicts with
any delegations already granted.
Delegations on directories are not supported.
.P
In order to support delegation callback, the server
checks the network return path to the client during
the client's initial contact with the server.
If contact with the client cannot be established,
the server simply does not grant any delegations to
that client.
.SH "SECURITY CONSIDERATIONS"
NFS servers control access to file data,
but they depend on their RPC implementation
to provide authentication of NFS requests.
Traditional NFS access control mimics
the standard mode bit access control provided in local file systems.
Traditional RPC authentication uses a number
to represent each user
(usually the user's own uid),
a number to represent the user's group (the user's gid),
and a set of up to 16 auxiliary group numbers
to represent other groups of which the user may be a member.
.P
Typically, file data and user ID values appear unencrypted
(i.e. "in the clear") on the network.
Moreover, NFS versions 2 and 3 use
separate sideband protocols for mounting,
locking and unlocking files,
and reporting system status of clients and servers.
These auxiliary protocols use no authentication.
.P
In addition to combining these sideband protocols with the main NFS protocol,
NFS version 4 introduces more advanced forms of access control,
authentication, and in-transit data protection.
The NFS version 4 specification mandates support for
strong authentication and security flavors
that provide per-RPC integrity checking and encryption.
Because NFS version 4 combines the
function of the sideband protocols into the main NFS protocol,
the new security features apply to all NFS version 4 operations
including mounting, file locking, and so on.
RPCGSS authentication can also be used with NFS versions 2 and 3,
but it does not protect their sideband protocols.
.P
The
.B sec
mount option specifies the security flavor
that is in effect on a given NFS mount point.
Specifying
.B sec=krb5
provides cryptographic proof of a user's identity in each RPC request.
This provides strong verification of the identity of users
accessing data on the server.
Note that additional configuration besides adding this mount option
is required in order to enable Kerberos security.
Refer to the
.BR rpc.gssd (8)
man page for details.
.P
Two additional flavors of Kerberos security are supported:
.B krb5i
and
.BR krb5p .
The
.B krb5i
security flavor provides a cryptographically strong guarantee
that the data in each RPC request has not been tampered with.
The
.B krb5p
security flavor encrypts every RPC request
to prevent data exposure during network transit; however,
expect some performance impact
when using integrity checking or encryption.
Similar support for other forms of cryptographic security
is also available.
.P
The NFS version 4 protocol allows
a client to renegotiate the security flavor
when the client crosses into a new filesystem on the server.
The newly negotiated flavor effects only accesses of the new filesystem.
.P
Such negotiation typically occurs when a client crosses
from a server's pseudo-fs
into one of the server's exported physical filesystems,
which often have more restrictive security settings than the pseudo-fs.
.SS "Using non-privileged source ports"
NFS clients usually communicate with NFS servers via network sockets.
Each end of a socket is assigned a port value, which is simply a number
between 1 and 65535 that distinguishes socket endpoints at the same
IP address.
A socket is uniquely defined by a tuple that includes the transport
protocol (TCP or UDP) and the port values and IP addresses of both
endpoints.
.P
The NFS client can choose any source port value for its sockets,
but usually chooses a
.I privileged
port.
A privileged port is a port value less than 1024.
Only a process with root privileges may create a socket
with a privileged source port.
.P
The exact range of privileged source ports that can be chosen is
set by a pair of sysctls to avoid choosing a well-known port, such as
the port used by ssh.
This means the number of source ports available for the NFS client,
and therefore the number of socket connections that can be used
at the same time,
is practically limited to only a few hundred.
.P
As described above, the traditional default NFS authentication scheme,
known as AUTH_SYS, relies on sending local UID and GID numbers to identify
users making NFS requests.
An NFS server assumes that if a connection comes from a privileged port,
the UID and GID numbers in the NFS requests on this connection have been
verified by the client's kernel or some other local authority.
This is an easy system to spoof, but on a trusted physical network between
trusted hosts, it is entirely adequate.
.P
Roughly speaking, one socket is used for each NFS mount point.
If a client could use non-privileged source ports as well,
the number of sockets allowed,
and thus the maximum number of concurrent mount points,
would be much larger.
.P
Using non-privileged source ports may compromise server security somewhat,
since any user on AUTH_SYS mount points can now pretend to be any other
when making NFS requests.
Thus NFS servers do not support this by default.
They explicitly allow it usually via an export option.
.P
To retain good security while allowing as many mount points as possible,
it is best to allow non-privileged client connections only if the server
and client both require strong authentication, such as Kerberos.
.SS "Mounting through a firewall"
A firewall may reside between an NFS client and server,
or the client or server may block some of its own ports via IP
filter rules.
It is still possible to mount an NFS server through a firewall,
though some of the
.BR mount (8)
command's automatic service endpoint discovery mechanisms may not work; this
requires you to provide specific endpoint details via NFS mount options.
.P
NFS servers normally run a portmapper or rpcbind daemon to advertise
their service endpoints to clients. Clients use the rpcbind daemon to determine:
.IP
What network port each RPC-based service is using
.IP
What transport protocols each RPC-based service supports
.P
The rpcbind daemon uses a well-known port number (111) to help clients find a service endpoint.
Although NFS often uses a standard port number (2049),
auxiliary services such as the NLM service can choose
any unused port number at random.
.P
Common firewall configurations block the well-known rpcbind port.
In the absense of an rpcbind service,
the server administrator fixes the port number
of NFS-related services so that the firewall
can allow access to specific NFS service ports.
Client administrators then specify the port number
for the mountd service via the
.BR mount (8)
command's
.B mountport
option.
It may also be necessary to enforce the use of TCP or UDP
if the firewall blocks one of those transports.
.SS "NFS Access Control Lists"
Solaris allows NFS version 3 clients direct access
to POSIX Access Control Lists stored in its local file systems.
This proprietary sideband protocol, known as NFSACL,
provides richer access control than mode bits.
Linux implements this protocol
for compatibility with the Solaris NFS implementation.
The NFSACL protocol never became a standard part
of the NFS version 3 specification, however.
.P
The NFS version 4 specification mandates a new version
of Access Control Lists that are semantically richer than POSIX ACLs.
NFS version 4 ACLs are not fully compatible with POSIX ACLs; as such,
some translation between the two is required
in an environment that mixes POSIX ACLs and NFS version 4.
.SH "THE REMOUNT OPTION"
Generic mount options such as
.BR rw " and " sync
can be modified on NFS mount points using the
.BR remount
option.
See
.BR mount (8)
for more information on generic mount options.
.P
With few exceptions, NFS-specific options
are not able to be modified during a remount.
The underlying transport or NFS version
cannot be changed by a remount, for example.
.P
Performing a remount on an NFS file system mounted with the
.B noac
option may have unintended consequences.
The
.B noac
option is a combination of the generic option
.BR sync ,
and the NFS-specific option
.BR actimeo=0 .
.SS "Unmounting after a remount"
For mount points that use NFS versions 2 or 3, the NFS umount subcommand
depends on knowing the original set of mount options used to perform the
MNT operation.
These options are stored on disk by the NFS mount subcommand,
and can be erased by a remount.
.P
To ensure that the saved mount options are not erased during a remount,
specify either the local mount directory, or the server hostname and
export pathname, but not both, during a remount.  For example,
.P
.nf
.ta 8n
	mount -o remount,ro /mnt
.fi
.P
merges the mount option
.B ro
with the mount options already saved on disk for the NFS server mounted at /mnt.
.SH FILES
.TP 1.5i
.I /etc/fstab
file system table
.SH BUGS
Before 2.4.7, the Linux NFS client did not support NFS over TCP.
.P
Before 2.4.20, the Linux NFS client used a heuristic
to determine whether cached file data was still valid
rather than using the standard close-to-open cache coherency method
described above.
.P
Starting with 2.4.22, the Linux NFS client employs
a Van Jacobsen-based RTT estimator to determine retransmit
timeout values when using NFS over UDP.
.P
Before 2.6.0, the Linux NFS client did not support NFS version 4.
.P
Before 2.6.8, the Linux NFS client used only synchronous reads and writes
when the
.BR rsize " and " wsize
settings were smaller than the system's page size.
.P
The Linux NFS client does not yet support
certain optional features of the NFS version 4 protocol,
such as security negotiation, server referrals, and named attributes.
.SH "SEE ALSO"
.BR fstab (5),
.BR mount (8),
.BR umount (8),
.BR mount.nfs (5),
.BR umount.nfs (5),
.BR exports (5),
.BR netconfig (5),
.BR ipv6 (7),
.BR nfsd (8),
.BR sm-notify (8),
.BR rpc.statd (8),
.BR rpc.idmapd (8),
.BR rpc.gssd (8),
.BR rpc.svcgssd (8),
.BR kerberos (1)
.sp
RFC 768 for the UDP specification.
.br
RFC 793 for the TCP specification.
.br
RFC 1094 for the NFS version 2 specification.
.br
RFC 1813 for the NFS version 3 specification.
.br
RFC 1832 for the XDR specification.
.br
RFC 1833 for the RPC bind specification.
.br
RFC 2203 for the RPCSEC GSS API protocol specification.
.br
RFC 3530 for the NFS version 4 specification.