Provided by: nfs-common_1.2.2-4ubuntu5_i386 bug

NAME

       nfs - fstab format and options for the nfs and nfs4 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 either "nfs" (for version 2 or version 3  NFS
       mounts)  or  "nfs4"  (for NFS version 4 mounts).  The nfs and nfs4 file
       system types share similar mount options, which are described below.

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.

   Valid options for either the nfs or nfs4 file system type
       These options are valid to use when mounting either nfs  or  nfs4  file
       system  types.   They imply the same behavior and have the same default
       for both file system types.

       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.

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

                      However, for NFS over UDP, the client uses  an  adaptive
                      algorithm  to  estimate an appropriate timeout value for
                      frequently used request types (such as  READ  and  WRITE
                      requests),  but  uses the timeo setting for 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 request three times.

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

       rsize=n        The maximum number of bytes in each network READ request
                      that the NFS client can receive when reading data from a
                      file  on an NFS server.  The actual data payload size of
                      each NFS READ request is equal to or  smaller  than  the
                      rsize setting. The largest read payload supported by the
                      Linux NFS client is 1,048,576 bytes (one megabyte).

                      The rsize value is a positive integral multiple of 1024.
                      Specified rsize values lower than 1024 are replaced with
                      4096; values  larger  than  1048576  are  replaced  with
                      1048576.  If  a  specified value is within the supported
                      range but not a multiple of 1024, it is rounded down  to
                      the nearest multiple of 1024.

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

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

       wsize=n        The maximum number of bytes per  network  WRITE  request
                      that the NFS client can send when writing data to a file
                      on an NFS server. The actual data payload size  of  each
                      NFS  WRITE request is equal to or smaller than the wsize
                      setting. The largest  write  payload  supported  by  the
                      Linux NFS client is 1,048,576 bytes (one megabyte).

                      Similar  to  rsize  ,  the  wsize  value  is  a positive
                      integral multiple of 1024.  Specified wsize values lower
                      than  1024  are  replaced  with 4096; values larger than
                      1048576 are replaced with 1048576. If a specified  value
                      is  within  the  supported  range  but not a multiple of
                      1024, it is rounded down  to  the  nearest  multiple  of
                      1024.

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

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

       ac / noac      Selects whether the client may cache file attributes. If
                      neither option is specified (or if ac is 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.

       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
                      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).

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

       sharecache / nosharecache
                      Determines  how  the  client's  data cache and attribute
                      cache are shared when mounting the same export more than
                      once  concurrently.  Using the same cache reduces memory
                      requirements on the client and presents  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.

   Valid options for the nfs file system type
       Use  these options, along with the options in the above subsection, for
       mounting the nfs file system type.

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

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

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

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

                      If the proto mount option is not specified, the mount(8)
                      command  discovers  which  protocols the server 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.

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

       port=n         The  numeric value of the server's NFS service port.  If
                      the  server's  NFS  service  is  not  available  on  the
                      specified port, the mount request fails.

                      If  this  option  is  not specified, or if the specified
                      port value is 0,  then  the  NFS  client  uses  the  NFS
                      service  port  number advertised by the server's rpcbind
                      service.   The  mount  request  fails  if  the  server's
                      rpcbind  service  is  not  available,  the  server's NFS
                      service is not registered with its rpcbind  service,  or
                      the  server's  NFS  service  is  not  available  on  the
                      advertised port.

       mountport=n    The numeric value of the server's mountd port.   If  the
                      server's   mountd   service  is  not  available  on  the
                      specified port, the mount request fails.

                      If this option is not specified,  or  if  the  specified
                      port  value  is  0,  then  the mount(8) command uses the
                      mountd service port number advertised  by  the  server's
                      rpcbind   service.   The  mount  request  fails  if  the
                      server's rpcbind service is not available, the  server's
                      mountd  service  is  not  registered  with  its  rpcbind
                      service, or the server's mountd service is not available
                      on the advertised port.

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

       mountproto=netid
                      The transport protocol name and protocol family the  NFS
                      client  uses  to  transmit  requests to the NFS server's
                      mountd service when performing this mount  request,  and
                      when later unmounting this mount point.

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

                      This option can be used  when  mounting  an  NFS  server
                      through  a  firewall that blocks a particular transport.
                      When  used  in  combination  with  the   proto   option,
                      different   transports   for  mountd  requests  and  NFS
                      requests can  be  specified.   If  the  server's  mountd
                      service  is  not  available via the specified transport,
                      the mount request fails.

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

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

       mountvers=n    The RPC version number  used  to  contact  the  server's
                      mountd.   If  this  option  is not specified, the client
                      uses a version number appropriate to the  requested  NFS
                      version.   This  option  is  useful  when  multiple  NFS
                      services are running on the same remote server host.

       namlen=n       The maximum length  of  a  pathname  component  on  this
                      mount.   If  this  option  is not specified, the maximum
                      length is negotiated with the  server.  In  most  cases,
                      this maximum length is 255 characters.

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

       nfsvers=n      The NFS protocol version  number  used  to  contact  the
                      server's NFS service.  The Linux client supports version
                      2 and version 3 of the NFS protocol when using the  file
                      system  type  nfs.   If  the server does not support the
                      requested version, the mount  request  fails.   If  this
                      option  is  not  specified,  the  client attempts to use
                      version 3, but  negotiates  the  NFS  version  with  the
                      server if version 3 support is not available.

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

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

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

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

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

   Valid options for the nfs4 file system type
       Use these options, along with  the  options  in  the  first  subsection
       above, for mounting the nfs4 file system type.

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

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

                      All  NFS  version 4 servers are required to support TCP,
                      so if this  mount  option  is  not  specified,  the  NFS
                      version  4  client  uses the TCP protocol.  Refer to the
                      TRANSPORT METHODS section for more details.

       port=n         The numeric value of the server's NFS service port.   If
                      the  server's  NFS  service  is  not  available  on  the
                      specified port, the mount request fails.

                      If this mount option is not specified,  the  NFS  client
                      uses  the standard NFS port number of 2049 without first
                      checking the server's rpcbind service.  This  allows  an
                      NFS  version 4 client to contact an NFS 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
                      operations  on  this  mount  point. If neither option is
                      specified (or if intr is specified), system calls return
                      EINTR  if an in-progress NFS operation is interrupted by
                      a signal.   If  nointr  is  specified,  signals  do  not
                      interrupt NFS operations.

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

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

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  the  nfs4  file  system  type.   The
       nfsvers mount option is not supported for the nfs4 file system type.

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

            server:/export /mnt nfs  defaults  0 0

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

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

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

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

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

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

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

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

TRANSPORT METHODS

       NFS clients send requests to NFS servers via Remote Procedure Calls, or
       RPCs.  The RPC client discovers remote service endpoints automatically,
       handles  per-request  authentication,  adjusts  request  parameters for
       different  byte  endianness  on  client  and  server,  and  retransmits
       requests  that  may  have  been  lost  by  the  network or server.  RPC
       requests and replies flow over a network transport.

       In most cases, the mount(8) command, NFS client,  and  NFS  server  can
       automatically   negotiate  proper  transport  and  data  transfer  size
       settings for a mount point.  In some cases, however, it pays to specify
       these settings explicitly using mount options.

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

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
       sequential: first client A opens a file, writes something to  it,  then
       closes it; then client B opens the same file, and reads the changes.

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

   Weak cache consistency
       There  are  still  opportunities  for  a client's data cache to contain
       stale  data.   The  NFS  version  3  protocol  introduced  "weak  cache
       consistency"  (also  known  as WCC) which provides a way of efficiently
       checking a file's attributes before and after a single  request.   This
       allows  a  client to help identify changes that could have been made by
       other clients.

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

       NFS servers control access to file data, but they depend on  their  RPC
       implementation  to provide authentication of NFS requests.  Traditional
       NFS access control mimics the standard mode bit access control provided
       in local file systems.  Traditional RPC authentication uses a number to
       represent each user (usually the user's own uid), a number to represent
       the  user's  group  (the  user's  gid), and a set of up to 16 auxiliary
       group numbers to represent other groups of which  the  user  may  be  a
       member.

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

       In  addition  to  combining  these sideband protocols with the main NFS
       protocol, NFS version  4  introduces  more  advanced  forms  of  access
       control,  authentication,  and  in-transit  data  protection.   The NFS
       version 4 specification mandates NFSv4 ACLs, RPCGSS authentication, and
       RPCGSS  security  flavors  that  provide per-RPC integrity checking and
       encryption.  Because  NFS  version  4  combines  the  function  of  the
       sideband  protocols  into  the  main  NFS  protocol,  the  new security
       features apply to all NFS version 4 operations including mounting, file
       locking,  and  so  on.  RPCGSS authentication can also be used with NFS
       versions 2 and 3, but does not protect their sideband protocols.

       The sec mount option specifies the RPCGSS  security  mode  that  is  in
       effect  on  a  given  NFS  mount  point.   Specifying sec=krb5 provides
       cryptographic proof of a user's identity in  each  RPC  request.   This
       provides strong verification of the identity of users accessing data on
       the server.  Note that additional  configuration  besides  adding  this
       mount  option  is required in order to enable Kerberos security.  Refer
       to the rpc.gssd(8) man page for details.

       Two additional flavors of Kerberos security are  supported:  krb5i  and
       krb5p.   The  krb5i security flavor provides a 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 (such as lipkey and SPKM3) is
       also available.

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

   Using non-privileged source ports
       NFS clients usually communicate with NFS servers via  network  sockets.
       Each end of a socket is assigned a port value, which is simply a number
       between 1 and 65535 that distinguishes socket endpoints at the same  IP
       address.   A  socket  is  uniquely defined by a tuple that includes the
       transport protocol (TCP or UDP) and the port values and IP addresses of
       both endpoints.

       The  NFS  client  can choose any source port value for its sockets, but
       usually chooses a privileged port.  A privileged port is a  port  value
       less  than  1024.   Only  a  process  with root privileges may create a
       socket with a privileged source port.

       The exact range of privileged source ports that can be chosen is set by
       a pair of sysctls to avoid choosing a well-known port, such as the port
       used by ssh.  This means the number of source ports available  for  the
       NFS  client, and therefore the number of socket connections that can be
       used at the same time, is practically limited to only a few hundred.

       As described above, the traditional default NFS authentication  scheme,
       known  as  AUTH_SYS,  relies  on  sending  local UID and GID numbers to
       identify users making NFS requests.  An NFS server assumes  that  if  a
       connection comes from a privileged port, the UID and GID numbers in the
       NFS requests on this connection have  been  verified  by  the  client's
       kernel or some other local authority.  This is an easy system to spoof,
       but on a trusted physical network between trusted hosts, it is entirely
       adequate.

       Roughly  speaking,  one  socket is used for each NFS mount point.  If a
       client could use non-privileged source ports as  well,  the  number  of
       sockets  allowed,  and  thus  the  maximum  number  of concurrent mount
       points, would be much larger.

       Using  non-privileged  source  ports  may  compromise  server  security
       somewhat, since any user on AUTH_SYS mount points can now pretend to be
       any other when making NFS requests.  Thus NFS servers  do  not  support
       this  by  default.   They  explicitly  allow  it  usually via an export
       option.

       To retain  good  security  while  allowing  as  many  mount  points  as
       possible, it is best to allow non-privileged client 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.

FILES

       /etc/fstab     file system table

BUGS

       The generic remount option is not fully  supported.   Generic  options,
       such  as  rw  and ro can be modified using the remount option, but NFS-
       specific options are not all supported.  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 mixture of a generic
       option, sync, and an NFS-specific option actimeo=0.

       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

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

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

                                2 November 2007                         NFS(5)