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.  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
       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.

       soft / hard    Determines the recovery behavior of the NFS client after
                      an NFS request times out.  If neither option  is  speci-
                      fied  (or if the hard option is specified), NFS requests
                      are retried indefinitely.  If the soft option is  speci-
                      fied,  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.

       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 sec-
                      onds).   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 sec-
                      onds.

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

                      The  NFS client generates a "server not responding" mes-
                      sage after retrans retries, then attempts further recov-
                      ery  (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) com-
                      mand  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  inte-
                      gral  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) com-
                      mand  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  inter-
                      vals  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.

       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.

       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.

       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.

       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 con-
                      tinues to attempt to mount the export.  The parent imme-
                      diately 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.   Alter-
                      natively  these  issues  can be addressed using an auto-
                      mounter (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.

       sec=flavor     The security flavor to use for accessing files  on  this
                      mount  point.   If the server does not support this fla-
                      vor, the mount operation fails.  If sec= is  not  speci-
                      fied, 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  noshare-
                      cache  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  impor-
                      tant 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 pos-
                      itive entries are valid until their  parent  directory's
                      cached  attributes  expire, but always revalidates nega-
                      tive 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>/Documenta-
                      tion/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  com-
                      municate  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  METH-
                      ODS 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  METH-
                      ODS 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 speci-
                      fied 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 ser-
                      vice port number advertised by the server's rpcbind ser-
                      vice.   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 speci-
                      fied 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 ser-
                      vice, 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, differ-
                      ent 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  ser-
                      vices 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 negotia-
                      tion.    Using  this  option  ensures  that  pathconf(3)
                      reports the proper maximum component length to  applica-
                      tions in such cases.

       nfsvers=n      The  NFS  protocol  version  number  used to contact the
                      server's NFS service.  If the server  does  not  support
                      the requested version, the mount request fails.  If this
                      option is not specified, the client negotiates  a  suit-
                      able  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 sys-
                      tems.

       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.

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

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

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

       cto / nocto    Selects  whether  to  use  close-to-open cache coherence
                      semantics.  If neither option is specified (or if cto is
                      specified),  the  client uses close-to-open cache coher-
                      ence 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  sup-
                      ports 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.  mecha-
                      nism 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 speci-
                      fied, 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  com-
                      municate  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 ver-
                      sion 4 client uses  the  TCP  protocol.   Refer  to  the
                      TRANSPORT METHODS section for more details.

       port=n         The  numeric value of the server's NFS service port.  If
                      the server's NFS service is not available on the  speci-
                      fied 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.

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

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

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

       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 directo-
                      ries.

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

       clientaddr=n.n.n.n

       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  atypi-
                      cal  network  topologies,  the  exact address to use for
                      callbacks may be nontrivial to determine.

nfs4 FILE SYSTEM TYPE
       The nfs4 file system type is an old syntax for specifying NFSv4  usage.
       It  can  still  be  used  with  all  NFSv4-specific and common options,
       excepted the nfsvers mount option.

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

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

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

               server:/export  /mnt  nfs   defaults                      0 0

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

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

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

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

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

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

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

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

TRANSPORT METHODS
       NFS clients send requests to NFS servers via Remote Procedure Calls, or
       RPCs.  The RPC client discovers remote service endpoints automatically,
       handles per-request authentication, adjusts request parameters for dif-
       ferent  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  set-
       tings  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 net-
       work environments that enable jumbo Ethernet frames).  In such environ-
       ments, 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 implemen-
       tations.  It performs well in almost every conceivable network environ-
       ment 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  set-
       tings  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  net-
       work  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 Man-
       ager (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  ser-
       vices.

       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 trans-
       ports 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 right-
       most 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 net-
       work 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 sec-
       onds.

       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 ran-
       dom (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  possi-
       ble, 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 frag-
                      ments  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 frag-
                      mented traffic from distant peers.

DATA AND METADATA COHERENCE
       Some modern cluster file systems provide perfect cache coherence  among
       their  clients.  Perfect cache coherence among disparate NFS clients is
       expensive to achieve, especially on wide area networks.  As  such,  NFS
       settles  for  weaker cache coherence that satisfies the requirements of
       most file sharing types. Normally, file sharing is  completely  sequen-
       tial:  first client A opens a file, writes something to it, then closes
       it; then client B opens the same file, and reads the changes.

   Close-to-open cache consistency
       When an application opens a file stored  on  an  NFS  server,  the  NFS
       client  checks  that  it still exists on the server and is permitted to
       the opener by sending a GETATTR or ACCESS request.  When  the  applica-
       tion closes the file, the NFS client writes back any pending changes to
       the file so that the next opener can view the changes.  This also gives
       the  NFS client an opportunity to report any server write errors to the
       application via the return code from close(2).  The behavior of  check-
       ing at open time and flushing at close time is referred to as close-to-
       open cache consistency.

   Weak cache consistency
       There are still opportunities for a  client's  data  cache  to  contain
       stale  data.  The NFS version 3 protocol introduced "weak cache consis-
       tency" (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.

   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  posi-
       tive  lookup results.  This permitted applications to detect new direc-
       tory 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 appli-
       cation 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 applica-
       tion 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 sig-
       nificant 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  pro-
       tocol, 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 con-
       tains files used by the NLM implementation on Linux.

       Specifying the nolock option may also be advised to improve the perfor-
       mance  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 sim-
       ilar to that of earlier versions.  However, NFS version 4 adds two fea-
       tures that improve cache behavior: change attributes and  file  delega-
       tion.

       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  modifica-
       tion  and  change time stamps as a way for clients to validate the con-
       tent 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 delega-
       tion 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
       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 mem-
       ber.

       Typically,  file  data  and user ID values appear unencrypted (i.e. "in
       the clear") on the network.  Moreover, NFS versions 2 and 3  use  sepa-
       rate  sideband protocols for mounting, locking and unlocking files, and
       reporting system status of clients and servers.  These auxiliary proto-
       cols use no authentication.

       In  addition  to  combining  these sideband protocols with the main NFS
       protocol, NFS version 4 introduces more advanced forms of  access  con-
       trol,  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 that is in effect on
       a  given  NFS  mount point.  Specifying sec=krb5 provides cryptographic
       proof of a user's identity in each RPC request.  This  provides  strong
       verification  of  the  identity  of users accessing data on the server.
       Note that additional configuration besides adding this mount option  is
       required   in   order  to  enable  Kerberos  security.   Refer  to  the
       rpc.gssd(8) man page for details.

       Two additional flavors of Kerberos security are  supported:  krb5i  and
       krb5p.   The  krb5i security flavor provides a 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.

       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  filesys-
       tem.

       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.

   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 iden-
       tify  users  making NFS requests.  An NFS server assumes that if a con-
       nection comes from a privileged port, the UID and GID  numbers  in  the
       NFS requests on this connection have been verified by the client's ker-
       nel 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 some-
       what, 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  possi-
       ble,  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 mecha-
       nisms 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  num-
       ber  (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 spec-
       ify 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 Con-
       trol 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  stan-
       dard part of the NFS version 3 specification, however.

       The  NFS  version 4 specification mandates a new version of Access Con-
       trol Lists that are semantically richer than POSIX ACLs.  NFS version 4
       ACLs  are  not fully compatible with POSIX ACLs; as such, some transla-
       tion between the two is required in an  environment  that  mixes  POSIX
       ACLs and NFS version 4.

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

       With  few  exceptions, NFS-specific options are not able to be modified
       during a remount.  The underlying transport or NFS  version  cannot  be
       changed by a remount, for example.

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

   Unmounting after a remount
       For  mount  points that use NFS versions 2 or 3, the NFS umount subcom-
       mand depends on knowing the original set of mount options used to  per-
       form  the  MNT  operation.  These options are stored on disk by the NFS
       mount subcommand, and can be erased by a remount.

       To ensure that the saved mount options are not erased during a remount,
       specify  either  the  local mount directory, or the server hostname and
       export pathname, but not both, during a remount.  For example,

               mount -o remount,ro /mnt

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

FILES
       /etc/fstab     file system table

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

       Before  2.4.20,  the  Linux  NFS  client  used a heuristic to determine
       whether cached file data was still valid rather than using the standard
       close-to-open cache coherency method described above.

       Starting with 2.4.22, the Linux NFS client employs a Van Jacobsen-based
       RTT estimator to determine retransmit timeout  values  when  using  NFS
       over UDP.

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

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

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

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

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

                                9 October 2012                          NFS(5)