NAME | SYNOPSIS | DESCRIPTION | MOUNT OPTIONS | nfs4 FILE SYSTEM TYPE | MOUNT CONFIGURATION FILE | EXAMPLES | TRANSPORT METHODS | DATA AND METADATA COHERENCE | SECURITY CONSIDERATIONS | THE REMOUNT OPTION | FILES | BUGS | SEE ALSO | COLOPHON

NFS(5)                       File Formats Manual                      NFS(5)

NAME         top

       nfs - fstab format and options for the nfs file systems

SYNOPSIS         top

       /etc/fstab

DESCRIPTION         top

       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 (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 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         top

       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 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 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.

       intr / nointr  This option is provided for backward compatibility.
                      It is ignored after kernel 2.6.25.

       timeo=n        The time in deciseconds (tenths of a second) the NFS
                      client waits for a response 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, 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 UDP request three times and each TCP request
                      twice.

                      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
                      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
                      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-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 regular file before it requests
                      fresh attribute information from a server.  If this
                      option is not specified, the NFS client uses a
                      60-second maximum.  See the DATA AND METADATA
                      COHERENCE section for a full discussion of attribute
                      caching.

       acdirmin=n     The minimum time (in seconds) that the NFS client
                      caches attributes of a directory before it requests
                      fresh attribute information from a server.  If this
                      option is not specified, the NFS client uses a
                      30-second minimum.  See the DATA AND METADATA
                      COHERENCE section for a full discussion of attribute
                      caching.

       acdirmax=n     The maximum time (in seconds) that the NFS client
                      caches attributes of a directory before it requests
                      fresh attribute information from a server.  If this
                      option is not specified, the NFS client uses a
                      60-second maximum.  See the DATA AND METADATA
                      COHERENCE section for a full discussion of attribute
                      caching.

       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 specified.

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

                      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
                      addressed using an automounter (refer to automount(8)
                      for details).

       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
                      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.

                      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 sometimes
                      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 flavors, the
                      mount operation fails.  If sec= is not specified, the
                      client attempts 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 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
                      directory'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.

       fsc / nofsc    Enable/Disables the cache of (read-only) data pages to
                      the local disk using the FS-Cache facility. See
                      cachefilesd(8) and
                      <kernel_soruce>/Documentation/filesystems/caching for
                      detail on how to configure the FS-Cache facility.
                      Default value is nofsc.

   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, and rdma.  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 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 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 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.

       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
                      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.

       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 SECURITY
                      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 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.

                      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 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.

       proto=netid    The netid determines the transport that is used to
                      communicate with the NFS server.  Supported options
                      are tcp, tcp6, and rdma.  tcp6 use IPv6 addresses 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 versioning," where NFS protocol
                      enhancements can be introduced without bumping the NFS
                      protocol version number.  Before kernel 2.6.38, the
                      minor version is always zero, and this option is not
                      recognized.  After this kernel, specifying
                      "minorversion=1" enables a number of advanced
                      features, such as NFSv4 sessions.

                      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,minorversion=1.

       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
                      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 service 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 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
                      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
                      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.

       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 identification 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 traditional 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         top

       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         top

       If the mount command is configured to do so, all of the mount options
       described in the previous section can also be configured in the
       /etc/nfsmount.conf file. See nfsmount.conf(5) for details.

EXAMPLES         top

       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

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

               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         top

       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 implementation is
       simple, NFS over UDP has many limitations that prevent smooth
       operation and good performance in some common deployment
       environments.  Even an insignificant packet loss rate results in the
       loss of whole NFS requests; as such, retransmit timeouts are usually
       in the subsecond range to allow clients to recover quickly from
       dropped requests, but this can result in extraneous network traffic
       and server load.

       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 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 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 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 transport is used for
       the initial mountd request, but the mount command attempts to
       discover what the server supports for the NFS protocol, preferring
       TCP if both transports are supported.

       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.

   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 reassembly. NFS read and writes typically transmit UDP
       packets of 4 Kilobytes or more, which have to be broken up into
       several fragments in order to be sent over the Ethernet link, which
       limits packets to 1500 bytes by default. This process happens at the
       IP network layer and is called fragmentation.

       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
       assumes 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 checksum, 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 stations support
                      jumbo frames.

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

       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
                      receiving fragmented traffic from distant peers.

DATA AND METADATA COHERENCE         top

       Some modern cluster file systems provide perfect cache coherence
       among their clients.  Perfect cache coherence among disparate NFS
       clients is expensive to achieve, especially on wide area networks.
       As such, NFS settles for weaker cache coherence that satisfies the
       requirements of most file sharing types.

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

       When an application opens a file stored on an NFS version 3 server,
       the NFS client checks that the file exists on the server and is
       permitted to the opener by sending a GETATTR or ACCESS request.  The
       NFS client sends these requests regardless of the freshness of the
       file's cached attributes.

       When the application closes the file, the NFS client writes back any
       pending changes to the file so that the next opener can view the
       changes.  This also gives the NFS client an opportunity to report
       write errors to the application via the return code from 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 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 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.  Almost every file system operation checks file
       attribute information.  The client keeps this information cached for
       a period of time to reduce network and server load.  When 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. Alternatively, applications can also open
       their files with the O_DIRECT flag to disable data caching entirely.

   File timestamp maintainence
       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 updated 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 performance 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 strictatime/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 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 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 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 lookupcache=positive.

       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 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 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 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 temporarily as if no
       other client is accessing it.  The server promises to notify the
       client (via a callback request) if another client attempts to access
       that file.  Once a file has been delegated to a client, the client
       can cache that file's data and metadata aggressively without
       contacting the server.

       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 cannot be established,
       the server simply does not grant any delegations to that client.

SECURITY CONSIDERATIONS         top

       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 support for strong authentication
       and security flavors that provide per-RPC integrity checking and
       encryption.  Because NFS version 4 combines the function of the
       sideband protocols into the main NFS protocol, the new security
       features apply to all NFS version 4 operations including mounting,
       file locking, and so on.  RPCGSS authentication can also be used with
       NFS versions 2 and 3, but it does not protect their sideband
       protocols.

       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
       details.

       Two additional flavors of Kerberos security are supported: krb5i and
       krb5p.  The krb5i security 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 integrity 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 accesses 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 settings than
       the pseudo-fs.

   NFS version 4 Leases
       In NFS version 4, a lease is a period of time during which a server
       irrevocably grants a file lock to a client.  If the lease expires,
       the server is allowed to revoke that lock.  Clients periodically
       renew their leases to prevent lock revocation.

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

       As part of establishing a lease, therefore, a client must identify
       itself to a server.  A fixed string is used to distinguish that
       client from others, and a changeable verifier is used to indicate
       when the client has rebooted.

       A client uses a particular security flavor and principal when
       performing the operations to establish a lease.  If two clients
       happen to present the same identity string, a server can use their
       principals to detect that they are different clients, and prevent one
       client from interfering with the other's lease.

       The Linux NFS client establishes one lease for each 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 whole client that owns that lease.  These operations must use the
       same security flavor and principal that was used when the lease was
       established, even across client reboots.

       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 operations.  This
       provides strong authentication of the client to each server it
       contacts.  By default, the client uses the host/ or nfs/ service
       principal in its /etc/krb5.keytab for this purpose.

       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
       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 connections 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         top

       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         top

       /etc/fstab     file system table

       /etc/nfsmount.conf
                      Configuration file for NFS mounts

BUGS         top

       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 negotiation, server
       referrals, and named attributes.

SEE ALSO         top

       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 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.

COLOPHON         top

       This page is part of the nfs-utils (NFS utilities) project.
       Information about the project can be found at 
       ⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩.  If you have a bug
       report for this manual page, see 
       ⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩.  This page was
       obtained from the project's upstream Git repository 
       ⟨http://git.linux-nfs.org/?p=steved/nfs-utils.git;a=summary⟩ on
       2017-09-15.  If you discover any rendering problems in this HTML ver‐
       sion of the page, or you believe there is a better or more up-to-date
       source for the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original man‐
       ual page), send a mail to man-pages@man7.org

                               9 October 2012                         NFS(5)

Pages that refer to this page: pmdanfsclient(1)flock(2)filesystems(5)nfsmount.conf(5)systemd.mount(5)blkmapd(8)mount(8)mountd(8)mount.nfs(8)nfsstat(8)rpcdebug(8)rpc.rquotad(8)sm-notify(8)statd(8)umount.nfs(8)