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NFS(5) NFS(5)

NAME
nfs - fstab format and options for the nfs file systems

SYNOPSIS
/etc/fstab

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].

The mount(8) command attaches a file system to the system's name space hierarchy at a given mount point. The /etc/fstab
file describes how mount(8) should assemble a system's file name hierarchy from various independent file systems (includ-
ing file systems exported by NFS servers). Each line in the /etc/fstab file describes a single file system, its mount
point, and a set of default mount options for that mount point.

For NFS file system mounts, a line in the /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:

server:path /mountpoint fstype option,option,... 0 0

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.

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 ipv6(7) for details on specifying raw IPv6 addresses.

The fstype field contains "nfs". Use of the "nfs4" fstype in /etc/fstab is deprecated.

MOUNT OPTIONS
Refer to mount(8) for a description of generic mount options available for all file systems. If you do not need to spec-
ify any mount options, use the generic option defaults in /etc/fstab.

Options supported by all versions
These options are valid to use with any NFS version.

soft / hard Determines the recovery behavior of the NFS client after an NFS request times out. If neither option is
specified (or if the hard option is specified), NFS requests are retried indefinitely. If the soft option
is specified, then the NFS client fails an NFS request after retrans retransmissions have been sent, caus-
ing the NFS client to return an error to the calling application.

NB: A so-called "soft" timeout can cause silent data corruption in certain cases. As such, use the soft
option only when client responsiveness is more important than data integrity. Using NFS over TCP or
increasing the value of the retrans option may mitigate some of the risks of using the soft option.

timeo=n The time (in tenths of a second) the NFS client waits for a response before it retries an NFS request. If
this option is not specified, requests are retried every 60 seconds for NFS over TCP. The NFS client does
not perform any kind of timeout backoff for NFS over TCP.

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 timeo setting for infre-
quently used request types (such as FSINFO requests). If the 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.

retrans=n The number of times the NFS client retries a request before it attempts further recovery action. If the
retrans option is not specified, the NFS client tries each request three times.

The NFS client generates a "server not responding" message after retrans retries, then attempts further
recovery (depending on whether the hard mount option is in effect).

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 rsize setting. The largest read payload supported by the Linux NFS client is 1,048,576
bytes (one megabyte).

The rsize value is a positive integral multiple of 1024. Specified 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.

If an rsize value is not specified, or if the specified rsize value is larger than the maximum that either
client or server can support, the client and server negotiate the largest rsize value that they can both
support.

The rsize mount option as specified on the mount(8) command line appears in the /etc/mtab file. However,
the effective rsize value negotiated by the client and server is reported in the /proc/mounts file.

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 wsize setting. The largest write payload supported by the Linux NFS client is 1,048,576 bytes (one
megabyte).

Similar to rsize , the wsize value is a positive integral multiple of 1024. Specified 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.

If a wsize value is not specified, or if the specified wsize value is larger than the maximum that either
client or server can support, the client and server negotiate the largest wsize value that they can both
support.

The wsize mount option as specified on the mount(8) command line appears in the /etc/mtab file. However,
the effective wsize value negotiated by the client and server is reported in the /proc/mounts file.

ac / noac Selects whether the client may cache file attributes. If neither option is specified (or if ac is speci-
fied), the client caches file attributes.

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 noac option prevents clients
from caching file attributes so that applications can more quickly detect file changes on the server.

In addition to preventing the client from caching file attributes, the 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.

Using the 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.

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-sec-
ond minimum.

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-sec-
ond maximum.

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-sec-
ond minimum.

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-sec-
ond maximum.

actimeo=n Using actimeo sets all of acregmin, acregmax, acdirmin, and acdirmax to the same value. If this option is
not specified, the NFS client uses the defaults for each of these options listed above.

bg / fg Determines how the mount(8) command behaves if an attempt to mount an export fails. The fg option causes
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 fg nor bg mount option is spec-
ified.

If the bg option is specified, a timeout or failure causes the mount(8) command to fork a child which con-
tinues to attempt to mount the export. The parent immediately returns with a zero exit code. This is
known as a "background" mount.

If the local mount point directory is missing, the mount(8) command acts as if the mount request timed
out. This permits nested NFS mounts specified in /etc/fstab to proceed in any order during system ini-
tialization, even if some NFS servers are not yet available. Alternatively these issues can be addressed
using an automounter (refer to automount(8) for details).

retry=n The number of minutes that the mount(8) command retries an NFS mount operation in the foreground or back-
ground 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 mount(8) command exits immediately after the first failure.

sec=mode The RPCGSS security flavor to use for accessing files on this mount point. If the sec option is not spec-
ified, or if sec=sys is specified, the NFS client uses the AUTH_SYS security flavor for all NFS requests
on this mount point. Valid security flavors are none, sys, krb5, krb5i, krb5p, lkey, lkeyi, lkeyp, spkm,
spkmi, and spkmp. Refer to the SECURITY CONSIDERATIONS section for details.

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 iden-
tical file contents to applications when the same remote file is accessed via different mount points.

If neither option is specified, or if the sharecache option is specified, then a single cache is used for
all mount points that access the same export. If the 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.

As of kernel 2.6.18, the behavior specified by 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 fol-
lowing a local update of one of the copies.

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 resvport option is specified, the NFS
client uses a privileged source port. If the noresvport option is specified, the NFS client uses a non-
privileged source port. This option is supported in kernels 2.6.28 and later.

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.

Refer to the SECURITY CONSIDERATIONS section for important details.

lookupcache=mode
Specifies how the kernel manages its cache of directory entries for a given mount point. mode can be one
of all, none, pos, or positive. This option is supported in kernels 2.6.28 and later.

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 positive. If the requested directory entry does not
exist on the server, the result is referred to as negative.

If this option is not specified, or if all is specified, the client assumes both types of directory cache
entries are valid until their parent directory's cached attributes expire.

If pos or positive is specified, the client assumes positive entries are valid until their parent direc-
tory's cached attributes expire, but always revalidates negative entires before an application can use
them.

If 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.

The DATA AND METADATA COHERENCE section contains a detailed discussion of these trade-offs.

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.

proto=netid The transport protocol name and protocol family the NFS client uses to transmit requests to the NFS server
for this mount point. If an NFS server has both an IPv4 and an IPv6 address, using a specific netid will
force the use of IPv4 or IPv6 networking to communicate with that server.

If support for TI-RPC is built into the mount.nfs command, netid is a valid netid listed in /etc/netcon-
fig. The value "rdma" may also be specified. If the mount.nfs command does not have TI-RPC support, then
netid is one of "tcp," "udp," or "rdma," and only IPv4 may be used.

Each transport protocol uses different default retrans and timeo settings. Refer to the description of
these two mount options for details.

In addition to controlling how the NFS client transmits requests to the server, this mount option also
controls how the mount(8) command communicates with the server's rpcbind and mountd services. Specifying
a netid that uses TCP forces all traffic from the mount(8) command and the NFS client to use TCP. Speci-
fying a netid that uses UDP forces all traffic types to use UDP.

If the proto mount option is not specified, the mount(8) command discovers which protocols the server sup-
ports and chooses an appropriate transport for each service. Refer to the TRANSPORT METHODS section for
more details.

udp The udp option is an alternative to specifying proto=udp. It is included for compatibility with other
operating systems.

tcp The tcp option is an alternative to specifying proto=tcp. It is included for compatibility with other
operating systems.

rdma The rdma option is an alternative to specifying proto=rdma.

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.

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.

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.

If this option is not specified, or if the specified port value is 0, then the 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.

This option can be used when mounting an NFS server through a firewall that blocks the rpcbind protocol.

mountproto=netid
The transport protocol name and protocol family 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.

If support for TI-RPC is built into the mount.nfs command, netid is a valid netid listed in /etc/netcon-
fig. Otherwise, netid is one of "tcp" or "udp," and only IPv4 may be used.

This option can be used when mounting an NFS server through a firewall that blocks a particular transport.
When used in combination with the 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.

Refer to the TRANSPORT METHODS section for more on how the mountproto mount option interacts with the
proto mount option.

mounthost=name The hostname of the host running mountd. If this option is not specified, the mount(8) command assumes
that the mountd service runs on the same host as the NFS service.

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.

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.

Some early versions of NFS did not support this negotiation. Using this option ensures that pathconf(3)
reports the proper maximum component length to applications in such cases.

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.

vers=n This option is an alternative to the nfsvers option. It is included for compatibility with other operat-
ing systems.

lock / nolock Selects whether to use the NLM sideband protocol to lock files on the server. If neither option is speci-
fied (or if lock is specified), NLM locking is used for this mount point. When using the 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.

NLM locking must be disabled with the nolock option when using NFS to mount /var because /var contains
files used by the NLM implementation on Linux. Using the nolock option is also required when mounting
exports on NFS servers that do not support the NLM protocol.

intr / nointr Selects whether to allow signals to interrupt file operations on this mount point. If neither option is
specified (or if nointr is specified), signals do not interrupt NFS file operations. If intr is specified,
system calls return EINTR if an in-progress NFS operation is interrupted by a signal.

Using the intr option is preferred to using the soft option because it is significantly less likely to
result in data corruption.

The intr / nointr mount option is deprecated after kernel 2.6.25. Only SIGKILL can interrupt a pending
NFS operation on these kernels, and if specified, this mount option is ignored to provide backwards com-
patibility with older kernels.

cto / nocto Selects whether to use close-to-open cache coherence semantics. If neither option is specified (or if cto
is specified), the client uses close-to-open cache coherence semantics. If the nocto option is specified,
the client uses a non-standard heuristic to determine when files on the server have changed.

Using the 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.

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.

If neither acl nor 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 proto-
col may be necessary if the negotiation causes problems on the client or server. Refer to the SECURITY
CONSIDERATIONS section for more details.

rdirplus / nordirplus
Selects whether to use NFS version 3 READDIRPLUS requests. If this option is not specified, the NFS
client uses READDIRPLUS requests on NFS version 3 mounts to read small directories. Some applications
perform better if the client uses only READDIR requests for all directories.

local_lock=mechanism
Specifies whether to use local locking for any or both of the flock and the POSIX locking mechanisms.
mechanism can be one of all, flock, posix, or none. This option is supported in kernels 2.6.37 and later.

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 applica-
tions are not affected by these locks.

If this option is not specified, or if none is specified, the client assumes that the locks are not local.

If all is specified, the client assumes that both flock and POSIX locks are local.

If 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.

If posix is specified, the client assumes that POSIX locks are local and uses NLM sideband protocol to
lock files when flock locks are used.

To support legacy flock behavior similar to that of NFS clients < 2.6.12, use 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.

NOTE: When used together, the 'local_lock' mount option will be overridden by 'nolock'/'lock' mount
option.

Options for NFS version 4 only
Use these options, along with the options in the first subsection above, for NFS version 4 and newer.

If support for TI-RPC is built into the mount.nfs command, netid is a valid netid listed in /etc/netcon-
fig. Otherwise, netid is one of "tcp" or "udp," and only IPv4 may be used.

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.

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.

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 ver-
sion 4 server through a firewall that may block rpcbind requests.

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 avail-
able on the advertised port.

intr / nointr Selects whether to allow signals to interrupt file operations on this mount point. If neither option is
specified (or if intr is specified), system calls return EINTR if an in-progress NFS operation is inter-
rupted by a signal. If nointr is specified, signals do not interrupt NFS operations.

Using the intr option is preferred to using the soft option because it is significantly less likely to
result in data corruption.

cto / nocto Selects whether to use close-to-open cache coherence semantics for NFS directories on this mount point.
If neither cto nor nocto is specified, the default is to use close-to-open cache coherence semantics for
directories.

File data caching behavior is not affected by this option. The DATA AND METADATA COHERENCE section dis-
cusses the behavior of this option in more detail.

clientaddr=n.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.

If this option is not specified, the 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.

nfs4 FILE SYSTEM TYPE
The 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 nfsvers mount option.

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 con-
figured in the /etc/nfsmount.conf file. See nfsmount.conf(5) for details.

EXAMPLES
To mount an export using NFS version 2, use the nfs file system type and specify the nfsvers=2 mount option. To mount
using NFS version 3, use the nfs file system type and specify the nfsvers=3 mount option. To mount using NFS version 4,
use either the nfs file system type, with the nfsvers=4 mount option, or the nfs4 file system type.

The following example from an /etc/fstab file causes the mount command to negotiate reasonable defaults for NFS behavior.

server:/export /mnt nfs defaults 0 0

Here is an example from an /etc/fstab file for an NFS version 2 mount over UDP.

server:/export /mnt nfs nfsvers=2,proto=udp 0 0

Try this example to mount using NFS version 4 over TCP with Kerberos 5 mutual authentication.

server:/export /mnt nfs4 sec=krb5 0 0

This example can be used to mount /usr over NFS.

server:/export /usr nfs ro,nolock,nocto,actimeo=3600 0 0

This example shows how to mount an NFS server using a raw IPv6 link-local address.

[fe80::215:c5ff:fb3e:e2b1%eth0]:/export /mnt nfs defaults 0 0

TRANSPORT METHODS
NFS clients send requests to NFS servers via Remote Procedure Calls, or 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.

In most cases, the 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.

Traditionally, NFS clients used the UDP transport exclusively for transmitting requests to servers. Though its implemen-
tation 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.

However, UDP can be quite effective in specialized settings where the networks MTU is large relative to NFSs data trans-
fer size (such as network environments that enable jumbo Ethernet frames). In such environments, trimming the rsize and
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.

TCP is the default transport protocol used for all modern NFS implementations. It performs well in almost every conceiv-
able 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.

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 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, rsize and wsize can safely be allowed to default to the largest values supported by
both client and server, independent of the network's MTU size.

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.

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 proto, mountproto, udp, and tcp.

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.

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.

If the server does not support these transports for these services, the 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 adver-
tise any transport supported by the client or is misconfigured, the mount request fails. If the bg option is in effect,
the mount command backgrounds itself and continues to attempt the specified mount request.

When the proto option, the udp option, or the tcp option is specified but the mountproto option is not, the specified
transport is used to contact both the server's mountd service and for the NLM and NFS services.

If the mountproto option is specified but none of the proto, udp or tcp options are specified, then the specified trans-
port 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.

If both the mountproto and proto (or udp or tcp) options are specified, then the transport specified by the mountproto
option is used for the initial mountd request, and the transport specified by the proto option (or the 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.

If any of the proto, udp, tcp, or 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.

DATA AND METADATA COHERENCE
Some modern cluster file systems provide perfect cache coherence among their clients. Perfect cache coherence among dis-
parate 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. Normally, 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.

Close-to-open cache consistency
When an application opens a file stored on an NFS server, the NFS client checks that it still exists on the server and is
permitted to the opener by sending a GETATTR or ACCESS request. 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 any server write errors to the application via the return code from close(2). The behavior of
checking at open time and flushing at close time is referred to as close-to-open cache consistency.

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.

When a client is using many concurrent operations that update the same file at the same time (for example, during asyn-
chronous 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.

Attribute caching
Use the noac mount option to achieve attribute cache coherence among multiple clients. Almost every file system opera-
tion checks file attribute information. The client keeps this information cached for a period of time to reduce network
and server load. When 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.

Be careful not to confuse the noac option with "no data caching." The 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.

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. Alterna-
tively, applications can also open their files with the O_DIRECT flag to disable data caching entirely.

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 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 negative lookup result.

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 direc-
tory. 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 acdirmin, acdirmax, and noac mount options for more information about how long a directory's
mtime is cached.

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 lookupcache mount option allows some
tuning of directory entry caching behavior.

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 lookup-
cache=positive.

If the client ignores its cache and validates every application lookup request with the server, that client can immedi-
ately detect when a new directory entry has been either created or removed by another client. You can specify this
behavior using 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 noac, and has
no effect on how the NFS client caches the attributes of files.

The sync mount option
The NFS client treats the sync mount option differently than some other file systems (refer to mount(8) for a description
of the generic sync and async mount options). If neither sync nor async is specified (or if the async option is speci-
fied), the NFS client delays sending application writes to the server until any of these events occur:

Memory pressure forces reclamation of system memory resources.

An application flushes file data explicitly with sync(2), msync(2), or fsync(3).

An application closes a file with close(2).

The file is locked/unlocked via fcntl(2).

In other words, under normal circumstances, data written by an application may not immediately appear on the server that
hosts the file.

If the 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.

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 sync mount option.

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.

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.

NLM supports advisory file locks only. To lock NFS files, use fcntl(2) with the F_GETLK and F_SETLK commands. The NFS
client converts file locks obtained via flock(2) to advisory locks.

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 nolock mount option. NLM locking must be disabled with the nolock option when using NFS
to mount /var because /var contains files used by the NLM implementation on Linux.

Specifying the 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.

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: change attributes and file delegation.

The 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.

A file delegation is a contract between an NFS version 4 client and server that allows the client to treat a file tempo-
rarily 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.

File delegations come in two flavors: read and write. A read delegation means that the server notifies the client about
any other clients that want to write to the file. A write delegation means that the client gets notified about either
read or write accessors.

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.

In order to support delegation callback, the server checks the network return path to the client during the client's ini-
tial contact with the server. If contact with the client cannot be established, the server simply does not grant any
delegations to that client.

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.

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.

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 NFSv4
ACLs, RPCGSS authentication, and RPCGSS 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 does not protect their sideband protocols.

The sec mount option specifies the RPCGSS security mode that is in effect on a given NFS mount point. Specifying
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 rpc.gssd(8) man page for details.

Two additional flavors of Kerberos security are supported: krb5i and krb5p. The krb5i security flavor provides a crypto-
graphically strong guarantee that the data in each RPC request has not been tampered with. The 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 (such as lipkey and
SPKM3) is also available.

The NFS version 4 protocol allows clients and servers to negotiate among multiple security flavors during mount process-
ing. However, Linux does not yet implement such negotiation. The Linux client specifies a single security flavor at
mount time which remains in effect for the lifetime of the mount. If the server does not support this flavor, the ini-
tial mount request is rejected by the server.

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.

The NFS client can choose any source port value for its sockets, but usually chooses a 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.

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.

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.

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.

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.

To retain good security while allowing as many mount points as possible, it is best to allow non-privileged client con-
nections only if the server and client both require strong authentication, such as Kerberos.

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 mount(8) command's
automatic service endpoint discovery mechanisms may not work; this requires you to provide specific endpoint details via
NFS mount options.

NFS servers normally run a portmapper or rpcbind daemon to advertise their service endpoints to clients. Clients use the
rpcbind daemon to determine:

What network port each RPC-based service is using

What transport protocols each RPC-based service supports

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.

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 mount(8) command's mountport
option. It may also be necessary to enforce the use of TCP or UDP if the firewall blocks one of those transports.

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.

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.

THE REMOUNT OPTION
Generic mount options such as rw and sync can be modified on NFS mount points using the remount option. See mount(8) for
more information on generic mount options.

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.

Performing a remount on an NFS file system mounted with the noac option may have unintended consequences. The noac
option is a combination of the generic option sync, and the NFS-specific option actimeo=0.

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.

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,

mount -o remount,ro /mnt

merges the mount option ro with the mount options already saved on disk for the NFS server mounted at /mnt.

FILES
/etc/fstab file system table

BUGS
Before 2.4.7, the Linux NFS client did not support NFS over TCP.

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.

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.

Before 2.6.0, the Linux NFS client did not support NFS version 4.

Before 2.6.8, the Linux NFS client used only synchronous reads and writes when the rsize and wsize settings were smaller
than the system's page size.

The Linux NFS client does not yet support certain optional features of the NFS version 4 protocol, such as security nego-
tiation, server referrals, and named attributes.

SEE ALSO
fstab(5), mount(8), umount(8), mount.nfs(5), umount.nfs(5), exports(5), netconfig(5), ipv6(7), nfsd(8), sm-notify(8),
rpc.statd(8), rpc.idmapd(8), rpc.gssd(8), rpc.svcgssd(8), kerberos(1)

RFC 768 for the UDP specification.
RFC 793 for the TCP specification.
RFC 1094 for the NFS version 2 specification.
RFC 1813 for the NFS version 3 specification.
RFC 1832 for the XDR specification.
RFC 1833 for the RPC bind specification.
RFC 2203 for the RPCSEC GSS API protocol specification.
RFC 3530 for the NFS version 4 specification.

2 November 2007 NFS(5)