nfs(5) - man - phpMan

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NFS(5) File Formats Manual 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. Depending on kernel
configuration, the Linux NFS client may support NFS versions 3, 4.0, 4.1, or 4.2.

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 (including 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 convention‐
ally 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 specify 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.

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 tries version 4.2 first, then ne‐
gotiates down until it finds a version supported by the server.

vers=n This option is an alternative to the nfsvers option. It is included for com‐
patibility with other operating systems

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,
causing 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.

softreval / nosoftreval
In cases where the NFS server is down, it may be useful to allow the NFS
client to continue to serve up paths and attributes from cache after retrans
attempts to revalidate that cache have timed out. This may, for instance, be
helpful when trying to unmount a filesystem tree from a server that is perma‐
nently down.

It is possible to combine softreval with the soft mount option, in which case
operations that cannot be served up from cache will time out and return an er‐
ror after retrans attempts. The combination with the default hard mount option
implies those uncached operations will continue to retry until a response is
received from the server.

Note: the default mount option is nosoftreval which disallows fallback to
cache when revalidation fails, and instead follows the behavior dictated by
the hard or soft mount option.

intr / nointr This option is provided for backward compatibility. It is ignored after ker‐
nel 2.6.25.

timeo=n The time in deciseconds (tenths of a second) the NFS client waits for a re‐
sponse before it retries an NFS request.

For NFS over TCP the default timeo value is 600 (60 seconds). The NFS client
performs linear backoff: After each retransmission the timeout is increased by
timeo up to the maximum of 600 seconds.

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 infrequently used request
types (such as FSINFO requests). If the timeo option is not specified, infre‐
quently used request types are retried after 1.1 seconds. After each retrans‐
mission, 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 fur‐
ther recovery action. If the retrans option is not specified, the NFS client
tries each UDP request three times and each TCP request twice.

The NFS client generates a "server not responding" message after retrans re‐
tries, then attempts further recovery (depending on whether the hard mount op‐
tion 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 val‐
ues lower than 1024 are replaced with 4096; values larger than 1048576 are re‐
placed 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 specified), 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 up‐
dates. Changes that occur on the server in those small intervals remain unde‐
tected until the client checks the server again. The noac option prevents
clients from caching file attributes so that applications can more quickly de‐
tect 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 ac‐
cessing the same files, but it extracts a significant performance penalty. As
such, judicious use of file locking is encouraged instead. The DATA AND META‐
DATA 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 regu‐
lar 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.

acregmax=n The maximum time (in seconds) that the NFS client caches attributes of a regu‐
lar 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.

acdirmin=n The minimum time (in seconds) that the NFS client caches attributes of a di‐
rectory 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.

acdirmax=n The maximum time (in seconds) that the NFS client caches attributes of a di‐
rectory 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.

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 "fore‐
ground" mount, and is the default behavior if neither the fg nor bg mount op‐
tion is specified.

If the bg option is specified, a timeout or failure causes the mount(8) com‐
mand to fork a child which continues to attempt to mount the export. The par‐
ent immediately returns with a zero exit code. This is known as a "back‐
ground" 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 initialization, even if some
NFS servers are not yet available. Alternatively these issues can be ad‐
dressed using an automounter (refer to automount(8) for details).

nconnect=n When using a connection oriented protocol such as TCP, it may sometimes be ad‐
vantageous to set up multiple connections between the client and server. For
instance, if your clients and/or servers are equipped with multiple network
interface cards (NICs), using multiple connections to spread the load may im‐
prove overall performance. In such cases, the nconnect option allows the user
to specify the number of connections that should be established between the
client and server up to a limit of 16.

Note that the nconnect option may also be used by some pNFS drivers to decide
how many connections to set up to the data servers.

max_connect=n While nconnect option sets a limit on the number of connections that can be
established to a given server IP, max_connect option allows the user to spec‐
ify maximum number of connections to different server IPs that belong to the
same NFSv4.1+ server (session trunkable connections) up to a limit of 16. When
client discovers that it established a client ID to an already existing
server, instead of dropping the newly created network transport, the client
will add this new connection to the list of available transports for that RPC
client.

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.

retry=n The number of minutes that the mount(8) command retries an NFS mount operation
in the foreground or background before giving up. If this option is not spec‐
ified, 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.

Note that this only affects how many retries are made and doesn't affect the
delay caused by each retry. For UDP each retry takes the time determined by
the timeo and retrans options, which by default will be about 7 seconds. For
TCP the default is 3 minutes, but system TCP connection timeouts will some‐
times limit the timeout of each retransmission to around 2 minutes.

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 fla‐
vors, the mount operation fails. If sec= is not specified, the client at‐
tempts to find a security flavor that both the client and the server supports.
Valid flavors are none, sys, krb5, krb5i, and krb5p. 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 identical 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 following a local update
of one of the copies.

resvport / noresvport
Specifies whether the NFS client should use a privileged source port when com‐
municating 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 privi‐
leged 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 re‐
quested 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 directory's cached attributes expire, but always revali‐
dates negative entires before an application can use them.

If none is specified, the client revalidates both types of directory cache en‐
tries before an application can use them. This permits quick detection of
files that were created or removed by other clients, but can impact applica‐
tion and server performance.

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

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_source>/Documenta‐
tion/filesystems/caching for detail on how to configure the FS-Cache facility.
Default value is nofsc.

sloppy The sloppy option is an alternative to specifying mount.nfs -s option.

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 netid determines the transport that is used to communicate with the NFS
server. Available options are udp, udp6, tcp, tcp6, rdma, and rdma6. Those
which end in 6 use IPv6 addresses and are only available if support for TI-RPC
is built in. Others use IPv4 addresses.

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. Specifying a netid that uses UDP forces all traffic types to use UDP.

Before using NFS over UDP, refer to the TRANSPORT METHODS section.

If the proto mount option is not specified, the 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.

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

Before using NFS over UDP, refer to the TRANSPORT METHODS section.

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 ser‐
vice 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 ser‐
vice, 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 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.

netid may be one of udp, and tcp which use IPv4 address or, if TI-RPC is built
into the mount.nfs command, udp6, and tcp6 which use IPv6 addresses.

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 op‐
tion, different transports for mountd requests and NFS requests can be speci‐
fied. 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 op‐
tion ensures that pathconf(3) reports the proper maximum component length to
applications in such cases.

lock / nolock Selects whether to use the NLM sideband protocol to lock files on the server.
If neither option is specified (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 run‐
ning 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. Us‐
ing the nolock option is also required when mounting exports on NFS servers
that do not support the NLM protocol.

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 protocol may be necessary
if the negotiation causes problems on the client or server. Refer to the SE‐
CURITY CONSIDERATIONS section for more details.

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 ap‐
plications 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.

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
'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 conflict‐
ing 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 4.0
and newer.

proto=netid The netid determines the transport that is used to communicate with the NFS
server. Supported options are tcp, tcp6, rdma, and rdma6. tcp6 use IPv6 ad‐
dresses and is only available if support for TI-RPC is built in. Both others
use IPv4 addresses.

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.

minorversion=n Specifies the protocol minor version number. NFSv4 introduces "minor version‐
ing," where NFS protocol enhancements can be introduced without bumping the
NFS protocol version number. Before kernel 2.6.38, the minor version is al‐
ways zero, and this option is not recognized. After this kernel, specifying
"minorversion=1" enables a number of advanced features, such as NFSv4 ses‐
sions.

Recent kernels allow the minor version to be specified using the vers= option.
For example, specifying vers=4.1 is the same as specifying vers=4,minorver‐‐
sion=1.

port=n The numeric value of the server's NFS service port. If the server's NFS ser‐
vice 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 version 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 ser‐
vice is not registered with its rpcbind service, or the server's NFS service
is not available on the advertised port.

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

File data caching behavior is not affected by this option. The DATA AND META‐
DATA COHERENCE section discusses the behavior of this option in more detail.

clientaddr=n.n.n.n

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 4.0 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. Can specify a value of
IPv4_ANY (0.0.0.0) or equivalent IPv6 any address which will signal to the NFS
server that this NFS client does not want delegations.

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 inter‐
faces, special routing policies, or atypical network topologies, the exact ad‐
dress to use for callbacks may be nontrivial to determine.

NFS protocol versions 4.1 and 4.2 use the client-established TCP connection
for callback requests, so do not require the server to connect to the client.
This option is therefore only affect NFS version 4.0 mounts.

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 migration option.

Some server features misbehave in the face of a migration-compatible identifi‐
cation string. The 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 tradi‐
tional identification string.

This mount option has no effect with NFSv4 minor versions newer than zero,
which always use TSM-compatible client identification strings.

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 previ‐
ous section can also be configured in the /etc/nfsmount.conf file. See nfsmount.conf(5) for
details.

EXAMPLES
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 reason‐
able defaults for NFS behavior.

server:/export /mnt nfs defaults 0 0

This example shows how to mount using NFS version 4 over TCP with Kerberos 5 mutual authenti‐
cation.

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

This example shows how to mount using NFS version 4 over TCP with Kerberos 5 privacy or data
integrity mode.

server:/export /mnt nfs4 sec=krb5p:krb5i 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 re‐
quests 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 implementation is simple, NFS over UDP has many limitations that prevent
smooth operation and good performance in some common deployment environments. Even an in‐
significant 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 transfer 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 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.

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 unneces‐
sary. 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 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 at‐
tempts 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 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 op‐
tion 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 speci‐
fied, then the specified transport is used for the initial mountd request, but the mount com‐
mand 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 or‐
der 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.

Using NFS over UDP on high-speed links
Using NFS over UDP on high-speed links such as Gigabit can cause silent data corruption.

The problem can be triggered at high loads, and is caused by problems in IP fragment reassem‐
bly. 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.

In order to identify fragments that belong together, IP assigns a 16bit 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 as‐
sumes the missing fragment(s) got lost and discards those it already received.

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.

This has serious effects on reassembly: if one fragment gets lost, another fragment from a
different packet but with the 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 check‐
sum, which is a 16 bit checksum over the entire packet payload, will usually not match, and
UDP will discard the bad packet.

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.

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.

It is therefore strongly recommended to use NFS over TCP where possible, since TCP does not
perform fragmentation.

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:

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 sta‐
tions support jumbo frames.

To enable a machine to send jumbo frames on cards that support it, it is suf‐
ficient to configure the interface for a MTU value of 9000.

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
/proc/sys/net/ipv4/ipfrag_time.

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 re‐
ceiving fragmented traffic from distant peers.

DATA AND METADATA COHERENCE
Some modern cluster file systems provide perfect cache coherence among their clients. Per‐
fect 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 require‐
ments of most file sharing types.

Close-to-open cache consistency
Typically file sharing is completely sequential. First client A opens a file, writes some‐
thing to it, then closes it. Then client B opens the same file, and reads the changes.

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 AC‐
CESS request. The NFS client sends these requests regardless of the freshness of the file's
cached attributes.

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 oppor‐
tunity to report 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, or CTO. It can be disabled for an entire mount point using the nocto
mount option.

Weak cache consistency
There are still opportunities for a client's data cache to contain stale data. The NFS ver‐
sion 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 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.

Attribute caching
Use the noac mount option to achieve attribute cache coherence among multiple clients. Al‐
most 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 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 pre‐
vents 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 re‐
quired, applications should use file locking. Alternatively, applications can also open their
files with the O_DIRECT flag to disable data caching entirely.

File timestamp maintenance
NFS servers are responsible for managing file and directory timestamps (atime, ctime, and
mtime). When a file is accessed or updated on an NFS server, the file's timestamps are up‐
dated just like they would be on a filesystem local to an application.

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.

To comply with the POSIX filesystem standard, the Linux NFS client relies on NFS servers to
keep a file's mtime and ctime timestamps properly up to date. It does this by flushing local
data changes to the server before reporting mtime to applications via system calls such as
stat(2).

The Linux client handles atime updates more loosely, however. NFS clients maintain good per‐
formance by caching data, but that means that application reads, which normally update atime,
are not reflected to the server where a file's atime is actually maintained.

Because of this caching behavior, the Linux NFS client does not support generic atime-related
mount options. See mount(8) for details on these options.

In particular, the atime/noatime, diratime/nodiratime, relatime/norelatime, and stricta‐‐
time/nostrictatime mount options have no effect on NFS mounts.

/proc/mounts may report that the relatime mount option is set on NFS mounts, but in fact the
atime semantics are always as described here, and are not like relatime semantics.

Directory entry caching
The Linux NFS client caches the result of all NFS LOOKUP requests. If the requested direc‐
tory 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 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 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 di‐
rectory 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 lookupcache=pos‐‐
itive.

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 lookupcache=none. The ex‐
tra NFS requests needed if the client does not cache directory entries can exact a perfor‐
mance 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 specified), 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 immedi‐
ately 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 re‐
turns 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 3. To support lock recovery after a client or server reboot, a second side‐
band 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 at‐
tributes 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 temporarily as if no other client is accessing it. The server prom‐
ises 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 initial contact with the server. If contact with the client can‐
not 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 pro‐
vide 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 net‐
work. 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 protec‐
tion. The NFS version 4 specification mandates support for strong authentication and secu‐
rity 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.

The sec mount option specifies the security flavor used for operations on behalf of users on
that 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 de‐
tails.

Two additional flavors of Kerberos security are supported: krb5i and krb5p. The krb5i secu‐
rity flavor provides a cryptographically 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 in‐
tegrity checking or encryption. Similar support for other forms of cryptographic security is
also available.

NFS version 4 filesystem crossing
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 ac‐
cesses of the new filesystem.

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 set‐
tings than the pseudo-fs.

NFS version 4 Leases
In NFS version 4, a lease is a period during which a server irrevocably grants a client file
locks. Once the lease expires, the server may revoke those locks. Clients periodically re‐
new their leases to prevent lock revocation.

After an NFS version 4 server reboots, each client tells the server about existing file open
and lock state under its lease before operation can continue. If a client reboots, the
server frees all open and lock state associated with that client's lease.

When establishing a lease, therefore, a client must identify itself to a server. Each client
presents an arbitrary string to distinguish itself from other clients. The client adminis‐
trator can supplement the default identity string using the nfs4.nfs4_unique_id module param‐
eter to avoid collisions with other client identity strings.

A client also uses a unique security flavor and principal when it establishes its lease. If
two clients present the same identity string, a server can use client principals to distin‐
guish between them, thus securely preventing one client from interfering with the other's
lease.

The Linux NFS client establishes one lease on each NFS version 4 server. Lease management
operations, such as lease renewal, are not done on behalf of a particular file, lock, user,
or mount point, but on behalf of the client that owns that lease. A client uses a consistent
identity string, security flavor, and principal across client reboots to ensure that the
server can promptly reap expired lease state.

When Kerberos is configured on a Linux NFS client (i.e., there is a /etc/krb5.keytab on that
client), the client attempts to use a Kerberos security flavor for its lease management oper‐
ations. Kerberos provides secure authentication of each client. By default, the client uses
the host/ or nfs/ service principal in its /etc/krb5.keytab for this purpose, as described in
rpc.gssd(8).

If the client has Kerberos configured, but the server does not, or if the client does not
have a keytab or the requisite service principals, the client uses AUTH_SYS and UID 0 for
lease management.

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 in‐
cludes the transport protocol (TCP or UDP) and the port values and IP addresses of both end‐
points.

The NFS client can choose any source port value for its sockets, but usually chooses a privi‐
leged 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, re‐
lies 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 op‐
tion.

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 authenti‐
cation, 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 discov‐
ery 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 end‐
point. 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 So‐
laris NFS implementation. The NFSACL protocol never became a standard part of the NFS ver‐
sion 3 specification, however.

The NFS version 4 specification mandates a new version of Access Control Lists that are se‐
mantically 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 re‐‐
mount 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-spe‐
cific 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 re‐
mount. 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

/etc/nfsmount.conf
Configuration file for NFS mounts

NOTES
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 de‐
termine 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 client's support for protocol versions depend on whether the kernel was built with
options CONFIG_NFS_V2, CONFIG_NFS_V3, CONFIG_NFS_V4, CONFIG_NFS_V4_1, and CONFIG_NFS_V4_2.

SEE ALSO
fstab(5), mount(8), umount(8), mount.nfs(5), umount.nfs(5), exports(5), nfsmount.conf(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 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 7530 for the NFS version 4.0 specification.
RFC 5661 for the NFS version 4.1 specification.
RFC 7862 for the NFS version 4.2 specification.

9 October 2012 NFS(5)