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NFS(5)									NFS(5)

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 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  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  per‐
		      form 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 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).

       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  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.

   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 com‐
		      mand, 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	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 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  com‐
		      mand,  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, 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.  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 ver‐
		      sion  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 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.

       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 com‐
		      mand, 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 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
		      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.

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 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.

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 NFSv4 ACLs, RPCGSS authentication, and	RPCGSS
       security	 flavors  that	provide per-RPC integrity checking and encryp‐
       tion.  Because NFS version 4 combines the function of the sideband pro‐
       tocols  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 cryp‐
       tographic proof of a user's identity in each RPC	 request.   This  pro‐
       vides  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 speci‐
       fies 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 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.

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 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.

				2 November 2007				NFS(5)
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