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NFS(5)									NFS(5)

NAME
       nfs - fstab format and options for the nfs file systems

SYNOPSIS
       /etc/fstab

DESCRIPTION
       NFS  is	an  Internet  Standard protocol created by Sun Microsystems in
       1984. NFS was developed to allow file sharing between systems  residing
       on  a local area network.  The Linux NFS client supports three versions
       of the NFS protocol: NFS version 2 [RFC1094], NFS version 3  [RFC1813],
       and NFS version 4 [RFC3530].

       The  mount(8) command attaches a file system to the system's name space
       hierarchy at a given mount point.  The /etc/fstab  file	describes  how
       mount(8)	 should	 assemble  a system's file name hierarchy from various
       independent file	 systems  (including  file  systems  exported  by  NFS
       servers).   Each	 line  in  the /etc/fstab file describes a single file
       system, its mount point, and a set of default mount  options  for  that
       mount point.

       For NFS file system mounts, a line in the /etc/fstab file specifies the
       server name, the path name of the exported server directory  to	mount,
       the  local  directory  that is the mount point, the type of file system
       that is being mounted, and a list of mount options that control the way
       the filesystem is mounted and how the NFS client behaves when accessing
       files on this mount point.  The fifth and sixth fields on each line are
       not  used  by NFS, thus conventionally each contain the digit zero. For
       example:

	       server:path   /mountpoint   fstype   option,option,...	0 0

       The server's hostname and export pathname are  separated	 by  a	colon,
       while  the  mount options are separated by commas. The remaining fields
       are separated by blanks or tabs.

       The server's hostname can be an unqualified hostname, a fully qualified
       domain name, a dotted quad IPv4 address, or an IPv6 address enclosed in
       square brackets.	 Link-local and	 site-local  IPv6  addresses  must  be
       accompanied  by	an  interface  identifier.  See ipv6(7) for details on
       specifying raw IPv6 addresses.

       The  fstype  field  contains  "nfs".   Use  of  the  "nfs4"  fstype  in
       /etc/fstab is deprecated.

MOUNT OPTIONS
       Refer  to mount(8) for a description of generic mount options available
       for all file systems. If you do not need to specify any mount  options,
       use the generic option defaults in /etc/fstab.

   Options supported by all versions
       These options are valid to use with any NFS version.

       nfsvers=n      The  NFS	protocol  version  number  used to contact the
		      server's NFS service.  If the server  does  not  support
		      the requested version, the mount request fails.  If this
		      option is not specified, the client negotiates  a	 suit‐
		      able  version  with  the server, trying version 4 first,
		      version 3 second, and version 2 last.

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

       soft / hard    Determines the recovery behavior of the NFS client after
		      an  NFS  request times out.  If neither option is speci‐
		      fied (or if the hard option is specified), NFS  requests
		      are  retried indefinitely.  If the soft option is speci‐
		      fied, then the NFS client fails  an  NFS	request	 after
		      retrans  retransmissions have been sent, causing the NFS
		      client to return an error to the calling application.

		      NB: A so-called "soft" timeout  can  cause  silent  data
		      corruption  in  certain  cases.  As  such,  use the soft
		      option only when client responsiveness is more important
		      than  data  integrity.  Using NFS over TCP or increasing
		      the value of the retrans option may mitigate some of the
		      risks of using the soft option.

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

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

		      However, for NFS over UDP, the client uses  an  adaptive
		      algorithm	 to  estimate an appropriate timeout value for
		      frequently used request types (such as  READ  and	 WRITE
		      requests),  but  uses the timeo setting for infrequently
		      used request types (such as FSINFO  requests).   If  the
		      timeo option is not specified, infrequently used request
		      types  are  retried  after  1.1  seconds.	  After	  each
		      retransmission,  the  NFS client doubles the timeout for
		      that request, up to a maximum timeout length of 60  sec‐
		      onds.

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

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

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

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

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

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

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

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

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

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

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

		      To  improve  performance,	  NFS	clients	  cache	  file
		      attributes.  Every few seconds, an NFS client checks the
		      server's version of each file's attributes for  updates.
		      Changes  that  occur on the server in those small inter‐
		      vals remain  undetected  until  the  client  checks  the
		      server  again.  The  noac	 option	 prevents clients from
		      caching file attributes so that  applications  can  more
		      quickly detect file changes on the server.

		      In  addition  to preventing the client from caching file
		      attributes, the noac option forces application writes to
		      become  synchronous  so  that  local  changes  to a file
		      become visible on the  server  immediately.   That  way,
		      other clients can quickly detect recent writes when they
		      check the file's attributes.

		      Using the noac option provides greater  cache  coherence
		      among  NFS  clients  accessing  the  same	 files, but it
		      extracts a significant performance  penalty.   As	 such,
		      judicious	 use  of  file	locking is encouraged instead.
		      The DATA	AND  METADATA  COHERENCE  section  contains  a
		      detailed discussion of these trade-offs.

       acregmin=n     The minimum time (in seconds) that the NFS client caches
		      attributes of a regular file before  it  requests	 fresh
		      attribute	 information from a server.  If this option is
		      not specified, the NFS client uses a 3-second minimum.

       acregmax=n     The maximum time (in seconds) that the NFS client caches
		      attributes  of  a	 regular file before it requests fresh
		      attribute information from a server.  If this option  is
		      not specified, the NFS client uses a 60-second maximum.

       acdirmin=n     The minimum time (in seconds) that the NFS client caches
		      attributes of  a	directory  before  it  requests	 fresh
		      attribute	 information from a server.  If this option is
		      not specified, the NFS client uses a 30-second minimum.

       acdirmax=n     The maximum time (in seconds) that the NFS client caches
		      attributes  of  a	 directory  before  it	requests fresh
		      attribute information from a server.  If this option  is
		      not specified, the NFS client uses a 60-second maximum.

       actimeo=n      Using  actimeo sets all of acregmin, acregmax, acdirmin,
		      and acdirmax to the same value.  If this option  is  not
		      specified,  the NFS client uses the defaults for each of
		      these options listed above.

       bg / fg	      Determines  how  the  mount(8)  command  behaves	if  an
		      attempt  to mount an export fails.  The fg option causes
		      mount(8) to exit with an error status if any part of the
		      mount  request  times  out  or  fails outright.  This is
		      called a "foreground" mount, and is the default behavior
		      if neither the fg nor bg mount option is specified.

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

		      If  the  local  mount  point  directory  is missing, the
		      mount(8) command acts as if the mount request timed out.
		      This  permits  nested NFS mounts specified in /etc/fstab
		      to proceed in any order  during  system  initialization,
		      even  if some NFS servers are not yet available.	Alter‐
		      natively these issues can be addressed  using  an	 auto‐
		      mounter (refer to automount(8) for details).

       rdirplus / nordirplus
		      Selects	whether	 to  use  NFS  v3  or  v4  READDIRPLUS
		      requests.	 If this option	 is  not  specified,  the  NFS
		      client  uses READDIRPLUS requests on NFS v3 or v4 mounts
		      to read small directories.   Some	 applications  perform
		      better  if the client uses only READDIR requests for all
		      directories.

       retry=n	      The number of minutes that the mount(8) command  retries
		      an  NFS  mount operation in the foreground or background
		      before giving up.	 If this option is not specified,  the
		      default  value  for  foreground mounts is 2 minutes, and
		      the default value for background mounts is 10000 minutes
		      (80  minutes  shy	 of  one week).	 If a value of zero is
		      specified, the mount(8) command exits immediately	 after
		      the first failure.

       sec=flavors    A	 colon-separated  list of one or more security flavors
		      to use for accessing files on the mounted export. If the
		      server  does not support any of these flavors, the mount
		      operation fails.	If sec= is not specified,  the	client
		      attempts	to find a security flavor that both the client
		      and the server supports.	Valid flavors are  none,  sys,
		      krb5, krb5i, and krb5p.  Refer to the SECURITY CONSIDER‐
		      ATIONS section for details.

       sharecache / nosharecache
		      Determines how the client's  data	 cache	and  attribute
		      cache are shared when mounting the same export more than
		      once concurrently.  Using the same cache reduces	memory
		      requirements  on	the client and presents identical file
		      contents to applications when the same  remote  file  is
		      accessed via different mount points.

		      If  neither  option  is  specified, or if the sharecache
		      option is specified, then a single cache is used for all
		      mount  points  that  access  the	same  export.	If the
		      nosharecache option is specified, then that mount	 point
		      gets  a unique cache.  Note that when data and attribute
		      caches are shared, the  mount  options  from  the	 first
		      mount point take effect for subsequent concurrent mounts
		      of the same export.

		      As of kernel 2.6.18, the behavior specified by  noshare‐
		      cache  is	 legacy caching behavior. This is considered a
		      data risk since multiple cached copies of the same  file
		      on  the  same  client can become out of sync following a
		      local update of one of the copies.

       resvport / noresvport
		      Specifies whether the NFS client should use a privileged
		      source  port  when  communicating with an NFS server for
		      this mount point.	 If this option is not	specified,  or
		      the  resvport option is specified, the NFS client uses a
		      privileged source port.  If  the	noresvport  option  is
		      specified,  the  NFS client uses a non-privileged source
		      port.  This option is supported in  kernels  2.6.28  and
		      later.

		      Using  non-privileged  source  ports  helps increase the
		      maximum number of NFS mount points allowed on a  client,
		      but  NFS	servers must be configured to allow clients to
		      connect via non-privileged source ports.

		      Refer to the SECURITY CONSIDERATIONS section for	impor‐
		      tant details.

       lookupcache=mode
		      Specifies	 how the kernel manages its cache of directory
		      entries for a given mount point.	mode  can  be  one  of
		      all,  none,  pos, or positive.  This option is supported
		      in kernels 2.6.28 and later.

		      The Linux NFS client caches the result of all NFS LOOKUP
		      requests.	  If  the  requested directory entry exists on
		      the server, the result is referred to as	positive.   If
		      the  requested  directory	 entry	does  not exist on the
		      server, the result is referred to as negative.

		      If this option is not specified, or if all is specified,
		      the client assumes both types of directory cache entries
		      are  valid  until	 their	 parent	  directory's	cached
		      attributes expire.

		      If pos or positive is specified, the client assumes pos‐
		      itive entries are valid until their  parent  directory's
		      cached  attributes  expire, but always revalidates nega‐
		      tive entires before an application can use them.

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

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

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

   Options for NFS versions 2 and 3 only
       Use  these options, along with the options in the above subsection, for
       NFS versions 2 and 3 only.

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

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

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

		      Before using NFS over UDP, refer to the TRANSPORT	 METH‐
		      ODS section.

		      If the proto mount option is not specified, the mount(8)
		      command discovers which protocols	 the  server  supports
		      and  chooses  an appropriate transport for each service.
		      Refer to the TRANSPORT METHODS section for more details.

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

		      Before using NFS over UDP, refer to the TRANSPORT	 METH‐
		      ODS section.

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

       rdma	      The   rdma   option  is  an  alternative	to  specifying
		      proto=rdma.

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

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

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

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

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

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

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

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

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

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

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

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

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

       lock / nolock  Selects whether to use the NLM sideband protocol to lock
		      files on the server.  If neither option is specified (or
		      if  lock	is  specified),	 NLM  locking is used for this
		      mount point.  When using the nolock option, applications
		      can  lock	 files,	 but such locks provide exclusion only
		      against other applications running on the	 same  client.
		      Remote applications are not affected by these locks.

		      NLM locking must be disabled with the nolock option when
		      using NFS to mount /var because /var contains files used
		      by  the  NLM  implementation on Linux.  Using the nolock
		      option is also required when  mounting  exports  on  NFS
		      servers that do not support the NLM protocol.

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

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

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

       cto / nocto    Selects whether to  use  close-to-open  cache  coherence
		      semantics.  If neither option is specified (or if cto is
		      specified), the client uses close-to-open	 cache	coher‐
		      ence  semantics.	If  the nocto option is specified, the
		      client uses a non-standard heuristic to  determine  when
		      files on the server have changed.

		      Using the nocto option may improve performance for read-
		      only mounts, but should be used only if the data on  the
		      server changes only occasionally.	 The DATA AND METADATA
		      COHERENCE section discusses the behavior of this	option
		      in more detail.

       acl / noacl    Selects  whether	to use the NFSACL sideband protocol on
		      this mount point.	 The NFSACL  sideband  protocol	 is  a
		      proprietary protocol implemented in Solaris that manages
		      Access Control Lists. NFSACL was never made  a  standard
		      part of the NFS protocol specification.

		      If  neither  acl	nor noacl option is specified, the NFS
		      client negotiates with the server to see if  the	NFSACL
		      protocol	is  supported,	and uses it if the server sup‐
		      ports it.	 Disabling the NFSACL sideband protocol may be
		      necessary	 if  the  negotiation  causes  problems on the
		      client or server.	 Refer to the SECURITY	CONSIDERATIONS
		      section for more details.

       local_lock=mechanism
		      Specifies	 whether  to use local locking for any or both
		      of the flock and the POSIX locking  mechanisms.	mecha‐
		      nism  can	 be  one  of all, flock, posix, or none.  This
		      option is supported in kernels 2.6.37 and later.

		      The Linux NFS client provides a way to make locks local.
		      This  means,  the	 applications can lock files, but such
		      locks provide exclusion only against other  applications
		      running  on the same client. Remote applications are not
		      affected by these locks.

		      If this option is not specified, or if  none  is	speci‐
		      fied, the client assumes that the locks are not local.

		      If  all is specified, the client assumes that both flock
		      and POSIX locks are local.

		      If flock is specified,  the  client  assumes  that  only
		      flock  locks are local and uses NLM sideband protocol to
		      lock files when POSIX locks are used.

		      If posix is specified, the  client  assumes  that	 POSIX
		      locks  are  local and uses NLM sideband protocol to lock
		      files when flock locks are used.

		      To support legacy flock behavior similar to that of  NFS
		      clients < 2.6.12, use 'local_lock=flock'. This option is
		      required when exporting NFS mounts via  Samba  as	 Samba
		      maps  Windows  share  mode  locks	 as  flock.  Since NFS
		      clients > 2.6.12	implement  flock  by  emulating	 POSIX
		      locks, this will result in conflicting locks.

		      NOTE:  When used together, the 'local_lock' mount option
		      will be overridden by 'nolock'/'lock' mount option.

   Options for NFS version 4 only
       Use these options, along with  the  options  in	the  first  subsection
       above, for NFS version 4 and newer.

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

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

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

		      If this mount option is not specified,  the  NFS	client
		      uses  the standard NFS port number of 2049 without first
		      checking the server's rpcbind service.  This  allows  an
		      NFS  version 4 client to contact an NFS version 4 server
		      through a firewall that may block rpcbind requests.

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

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

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

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

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

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

       clientaddr=n.n.n.n

       clientaddr=n:n:...:n
		      Specifies	 a  single IPv4 address (in dotted-quad form),
		      or a non-link-local IPv6 address, that  the  NFS	client
		      advertises  to  allow  servers  to perform NFS version 4
		      callback requests against files on this mount point.  If
		      the   server is unable to establish callback connections
		      to clients, performance  may  degrade,  or  accesses  to
		      files may temporarily hang.

		      If  this	option	is not specified, the mount(8) command
		      attempts to discover  an	appropriate  callback  address
		      automatically.   The  automatic discovery process is not
		      perfect, however.	 In the presence  of  multiple	client
		      network  interfaces, special routing policies, or atypi‐
		      cal network topologies, the exact	 address  to  use  for
		      callbacks may be nontrivial to determine.

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

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

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

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

	       server:/export  /mnt  nfs   defaults			 0 0

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

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

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

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

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

	       server:/export  /mnt  nfs4  sec=krb5p:krb5i		 0 0

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

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

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

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

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

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

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

       However,	 UDP  can be quite effective in specialized settings where the
       networks MTU is large relative to NFSs data transfer size (such as net‐
       work environments that enable jumbo Ethernet frames).  In such environ‐
       ments, trimming the rsize and wsize settings so that each NFS  read  or
       write  request  fits in just a few network frames (or even in  a single
       frame) is advised.  This reduces the probability that  the  loss	 of  a
       single  MTU-sized  network frame results in the loss of an entire large
       read or write request.

       TCP is the default transport protocol used for all modern NFS implemen‐
       tations.	 It performs well in almost every conceivable network environ‐
       ment and provides excellent guarantees against data  corruption	caused
       by  network  unreliability.   TCP is often a requirement for mounting a
       server through a network firewall.

       Under normal circumstances, networks drop packets much more  frequently
       than  NFS  servers  drop	 requests.   As such, an aggressive retransmit
       timeout	setting for NFS over TCP is unnecessary. Typical timeout  set‐
       tings  for  NFS	over  TCP are between one and ten minutes.  After  the
       client exhausts	its  retransmits  (the	value  of  the	retrans	 mount
       option),	 it  assumes a network partition has occurred, and attempts to
       reconnect to the server on a fresh socket. Since TCP itself makes  net‐
       work  data  transfer reliable, rsize and wsize can safely be allowed to
       default to the largest values supported	by  both  client  and  server,
       independent of the network's MTU size.

   Using the mountproto mount option
       This  section  applies only to NFS version 2 and version 3 mounts since
       NFS version 4 does not use a separate protocol for mount requests.

       The Linux NFS client can use a different transport  for	contacting  an
       NFS server's rpcbind service, its mountd service, its Network Lock Man‐
       ager (NLM) service, and its NFS service.	 The exact transports employed
       by the Linux NFS client for each mount point depends on the settings of
       the transport mount options, which include proto, mountproto, udp,  and
       tcp.

       The  client sends Network Status Manager (NSM) notifications via UDP no
       matter what transport options are specified, but listens for server NSM
       notifications  on  both	UDP  and  TCP.	 The  NFS  Access Control List
       (NFSACL) protocol shares the same transport as the main NFS service.

       If no transport options are specified, the Linux NFS client uses UDP to
       contact the server's mountd service, and TCP to contact its NLM and NFS
       services by default.

       If the server does not support these transports for these services, the
       mount(8)	 command  attempts  to	discover what the server supports, and
       then retries the mount request once using  the  discovered  transports.
       If  the server does not advertise any transport supported by the client
       or is misconfigured, the mount request fails.  If the bg option	is  in
       effect,	the  mount command backgrounds itself and continues to attempt
       the specified mount request.

       When the proto option, the udp option, or the tcp option	 is  specified
       but  the	 mountproto  option is not, the specified transport is used to
       contact both the server's mountd service and for the NLM and  NFS  ser‐
       vices.

       If the mountproto option is specified but none of the proto, udp or tcp
       options are specified, then the specified transport  is	used  for  the
       initial mountd request, but the mount command attempts to discover what
       the server supports for the NFS protocol, preferring TCP if both trans‐
       ports are supported.

       If both the mountproto and proto (or udp or tcp) options are specified,
       then the transport specified by the mountproto option is used  for  the
       initial mountd request, and the transport specified by the proto option
       (or the udp or tcp options) is used for NFS, no matter what order these
       options	appear.	  No automatic service discovery is performed if these
       options are specified.

       If any of the proto, udp, tcp, or mountproto options are specified more
       than  once on the same mount command line, then the value of the right‐
       most instance of each of these options takes effect.

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

       The  problem  can be triggered at high loads, and is caused by problems
       in IP fragment reassembly. NFS read and writes typically	 transmit  UDP
       packets of 4 Kilobytes or more, which have to be broken up into several
       fragments in order to be sent over  the	Ethernet  link,	 which	limits
       packets	to  1500 bytes by default. This process happens at the IP net‐
       work layer and is called fragmentation.

       In order to identify fragments that belong together, IP assigns a 16bit
       IP  ID  value  to  each	packet;	 fragments generated from the same UDP
       packet will have the same IP ID.	 The  receiving	 system	 will  collect
       these  fragments and combine them to form the original UDP packet. This
       process is called reassembly. The default timeout for packet reassembly
       is 30 seconds; if the network stack does not receive all fragments of a
       given packet within this interval, it assumes the  missing  fragment(s)
       got lost and discards those it already received.

       The  problem  this creates over high-speed links is that it is possible
       to send more than 65536 packets within 30 seconds. In fact, with	 heavy
       NFS  traffic  one can observe that the IP IDs repeat after about 5 sec‐
       onds.

       This has serious effects on reassembly:	if  one	 fragment  gets	 lost,
       another	fragment  from a different packet but with the same IP ID will
       arrive within the 30 second timeout, and the network stack will combine
       these  fragments to form a new packet. Most of the time, network layers
       above IP will detect this mismatched reassembly - in the case  of  UDP,
       the  UDP	 checksum,  which  is a 16 bit checksum over the entire packet
       payload, will usually not match, and UDP will discard the bad packet.

       However, the UDP checksum is 16 bit only, so there is a chance of 1  in
       65536  that it will match even if the packet payload is completely ran‐
       dom (which very often isn't the case). If that is the case, silent data
       corruption will occur.

       This potential should be taken seriously, at least on Gigabit Ethernet.
       Network speeds of 100Mbit/s  should  be	considered  less  problematic,
       because	with  most  traffic  patterns IP ID wrap around will take much
       longer than 30 seconds.

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

       If  you absolutely have to use NFS over UDP over Gigabit Ethernet, some
       steps can be taken to mitigate the problem and reduce  the  probability
       of corruption:

       Jumbo frames:  Many  Gigabit  network cards are capable of transmitting
		      frames bigger than the 1500 byte	limit  of  traditional
		      Ethernet,	 typically  9000  bytes. Using jumbo frames of
		      9000 bytes will allow you to run NFS over UDP at a  page
		      size  of	8K  without  fragmentation. Of course, this is
		      only feasible if all  involved  stations	support	 jumbo
		      frames.

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

       Lower reassembly timeout:
		      By  lowering this timeout below the time it takes the IP
		      ID counter to wrap around, incorrect reassembly of frag‐
		      ments  can  be prevented as well. To do so, simply write
		      the  new	timeout	 value	(in  seconds)  to   the	  file
		      /proc/sys/net/ipv4/ipfrag_time.

		      A value of 2 seconds will greatly reduce the probability
		      of IPID clashes on a single Gigabit  link,  while	 still
		      allowing	for  a reasonable timeout when receiving frag‐
		      mented traffic from distant peers.

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

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

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

       When  a client is using many concurrent operations that update the same
       file at the same time (for example, during asynchronous write  behind),
       it  is  still difficult to tell whether it was that client's updates or
       some other client's updates that altered the file.

   Attribute caching
       Use the noac mount option to achieve attribute  cache  coherence	 among
       multiple	 clients.   Almost  every  file	 system	 operation checks file
       attribute information.  The client keeps this information cached for  a
       period  of  time	 to  reduce  network and server load.  When noac is in
       effect, a client's file attribute cache is disabled, so each  operation
       that  needs  to	check  a file's attributes is forced to go back to the
       server.	This permits a client to see changes to a file	very  quickly,
       at the cost of many extra network operations.

       Be  careful not to confuse the noac option with "no data caching."  The
       noac mount option prevents the client from caching file	metadata,  but
       there are still races that may result in data cache incoherence between
       client and server.

       The NFS protocol is not designed to support true	 cluster  file	system
       cache  coherence	 without  some	type of application serialization.  If
       absolute cache coherence among clients is required, applications should
       use file locking. Alternatively, applications can also open their files
       with the O_DIRECT flag to disable data caching entirely.

   Directory entry caching
       The Linux NFS client caches the result of all NFS LOOKUP requests.   If
       the  requested  directory  entry	 exists	 on  the server, the result is
       referred to as a positive lookup result.	 If  the  requested  directory
       entry  does  not	 exist	on  the	 server	 (that is, the server returned
       ENOENT), the result is referred to as negative lookup result.

       To detect when directory entries have been  added  or  removed  on  the
       server,	the  Linux  NFS	 client	 watches  a directory's mtime.	If the
       client detects a change in a directory's mtime, the  client  drops  all
       cached  LOOKUP results for that directory.  Since the directory's mtime
       is a cached attribute, it may take some time before a client notices it
       has  changed.  See the descriptions of the acdirmin, acdirmax, and noac
       mount options for more information about how long a  directory's	 mtime
       is cached.

       Caching directory entries improves the performance of applications that
       do not share files with applications on other  clients.	 Using	cached
       information  about directories can interfere with applications that run
       concurrently on multiple clients and need to  detect  the  creation  or
       removal of files quickly, however.  The lookupcache mount option allows
       some tuning of directory entry caching behavior.

       Before kernel release 2.6.28, the Linux NFS client tracked  only	 posi‐
       tive  lookup results.  This permitted applications to detect new direc‐
       tory entries created by other clients  quickly  while  still  providing
       some of the performance benefits of caching.  If an application depends
       on the previous lookup caching behavior of the Linux  NFS  client,  you
       can use lookupcache=positive.

       If  the client ignores its cache and validates every application lookup
       request with the server, that client can immediately detect when a  new
       directory  entry	 has been either created or removed by another client.
       You can specify this behavior using lookupcache=none.   The  extra  NFS
       requests	 needed	 if  the  client  does not cache directory entries can
       exact a performance penalty.  Disabling lookup caching should result in
       less of a performance penalty than using noac, and has no effect on how
       the NFS client caches the attributes of files.

   The sync mount option
       The NFS client treats the sync mount option differently than some other
       file  systems  (refer to mount(8) for a description of the generic sync
       and async mount options).  If neither sync nor async is	specified  (or
       if the async option is specified), the NFS client delays sending appli‐
       cation writes to the server until any of these events occur:

	      Memory pressure forces reclamation of system memory resources.

	      An  application  flushes	file  data  explicitly	with  sync(2),
	      msync(2), or fsync(3).

	      An application closes a file with close(2).

	      The file is locked/unlocked via fcntl(2).

       In other words, under normal circumstances, data written by an applica‐
       tion may not immediately appear on the server that hosts the file.

       If the sync option is specified on a mount point, any system call  that
       writes data to files on that mount point causes that data to be flushed
       to the server before the system call returns  control  to  user	space.
       This provides greater data cache coherence among clients, but at a sig‐
       nificant performance cost.

       Applications can use the O_SYNC open flag to force  application	writes
       to  individual files to go to the server immediately without the use of
       the sync mount option.

   Using file locks with NFS
       The Network Lock Manager protocol is a separate sideband protocol  used
       to  manage  file locks in NFS version 2 and version 3.  To support lock
       recovery after a client or server reboot, a second sideband protocol --
       known  as  the Network Status Manager protocol -- is also required.  In
       NFS version 4, file locking is supported directly in the main NFS  pro‐
       tocol, and the NLM and NSM sideband protocols are not used.

       In  most	 cases, NLM and NSM services are started automatically, and no
       extra configuration is required.	 Configure all NFS clients with fully-
       qualified  domain  names to ensure that NFS servers can find clients to
       notify them of server reboots.

       NLM supports advisory file locks only.  To lock NFS files, use fcntl(2)
       with  the  F_GETLK  and F_SETLK commands.  The NFS client converts file
       locks obtained via flock(2) to advisory locks.

       When mounting servers that do not support the  NLM  protocol,  or  when
       mounting	 an  NFS server through a firewall that blocks the NLM service
       port, specify the nolock mount option. NLM  locking  must  be  disabled
       with  the  nolock option when using NFS to mount /var because /var con‐
       tains files used by the NLM implementation on Linux.

       Specifying the nolock option may also be advised to improve the perfor‐
       mance  of  a  proprietary application which runs on a single client and
       uses file locks extensively.

   NFS version 4 caching features
       The data and metadata caching behavior of NFS version 4 clients is sim‐
       ilar to that of earlier versions.  However, NFS version 4 adds two fea‐
       tures that improve cache behavior: change attributes and	 file  delega‐
       tion.

       The  change  attribute is a new part of NFS file and directory metadata
       which tracks data changes.  It replaces the use of a  file's  modifica‐
       tion  and  change time stamps as a way for clients to validate the con‐
       tent of their caches.  Change attributes are independent	 of  the  time
       stamp resolution on either the server or client, however.

       A  file	delegation  is	a contract between an NFS version 4 client and
       server that allows the client to treat a	 file  temporarily  as	if  no
       other client is accessing it.  The server promises to notify the client
       (via a callback request) if another  client  attempts  to  access  that
       file.  Once a file has been delegated to a client, the client can cache
       that file's data	 and  metadata	aggressively  without  contacting  the
       server.

       File  delegations  come in two flavors: read and write.	A read delega‐
       tion means that the server notifies the client about any other  clients
       that  want  to  write  to  the file.  A write delegation means that the
       client gets notified about either read or write accessors.

       Servers grant file delegations when a file is opened,  and  can	recall
       delegations  at	any  time when another client wants access to the file
       that conflicts with any delegations already  granted.   Delegations  on
       directories are not supported.

       In  order to support delegation callback, the server checks the network
       return path to the client during the client's initial contact with  the
       server.	 If  contact with the client cannot be established, the server
       simply does not grant any delegations to that client.

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

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

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

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

       Two additional flavors of Kerberos security are	supported:  krb5i  and
       krb5p.	The  krb5i security flavor provides a cryptographically strong
       guarantee that the data in each RPC request has not been tampered with.
       The  krb5p  security  flavor encrypts every RPC request to prevent data
       exposure during	network	 transit;  however,  expect  some  performance
       impact  when  using  integrity checking or encryption.  Similar support
       for other forms of cryptographic security is also available.

       The NFS version 4 protocol allows a client to renegotiate the  security
       flavor  when  the  client  crosses into a new filesystem on the server.
       The newly negotiated flavor effects only accesses of the	 new  filesys‐
       tem.

       Such negotiation typically occurs when a client crosses from a server's
       pseudo-fs into one of the server's exported physical filesystems, which
       often have more restrictive security settings than the pseudo-fs.

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

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

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

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

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

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

       To retain good security while allowing as many mount points  as	possi‐
       ble,  it is best to allow non-privileged client connections only if the
       server and client both require strong authentication, such as Kerberos.

   Mounting through a firewall
       A firewall may reside between an NFS client and server, or  the	client
       or  server  may block some of its own ports via IP filter rules.	 It is
       still possible to mount an NFS server through a firewall,  though  some
       of  the	mount(8) command's automatic service endpoint discovery mecha‐
       nisms may not work; this requires  you  to  provide  specific  endpoint
       details via NFS mount options.

       NFS  servers  normally  run a portmapper or rpcbind daemon to advertise
       their service endpoints to clients. Clients use the rpcbind  daemon  to
       determine:

	      What network port each RPC-based service is using

	      What transport protocols each RPC-based service supports

       The  rpcbind daemon uses a well-known port number (111) to help clients
       find a service endpoint.	 Although NFS often uses a standard port  num‐
       ber  (2049),  auxiliary services such as the NLM service can choose any
       unused port number at random.

       Common firewall configurations block the well-known rpcbind  port.   In
       the  absense  of an rpcbind service, the server administrator fixes the
       port number of NFS-related services so  that  the  firewall  can	 allow
       access to specific NFS service ports.  Client administrators then spec‐
       ify the port number for the mountd service via the  mount(8)  command's
       mountport  option.   It may also be necessary to enforce the use of TCP
       or UDP if the firewall blocks one of those transports.

   NFS Access Control Lists
       Solaris allows NFS version 3 clients direct access to POSIX Access Con‐
       trol Lists stored in its local file systems.  This proprietary sideband
       protocol, known as NFSACL, provides richer  access  control  than  mode
       bits.   Linux  implements  this	protocol  for  compatibility  with the
       Solaris NFS implementation.  The NFSACL protocol never became  a	 stan‐
       dard part of the NFS version 3 specification, however.

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

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

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

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

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

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

	       mount -o remount,ro /mnt

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

FILES
       /etc/fstab     file system table

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

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

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

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

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

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

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

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

				9 October 2012				NFS(5)
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