nfs

NFS(5)                        File Formats Manual                       NFS(5)

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

SYNOPSIS
       /etc/fstab

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

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

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

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

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

       The server's hostname can be an unqualified hostname, a fully qualified
       domain name, a dotted quad IPv4 address, or an IPv6 address enclosed in
       square brackets.  Link-local and site-local IPv6 addresses must be  ac-
       companied by an interface identifier.  See ipv6(7) for details on spec-
       ifying 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 re-
                      trans  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 op-
                      tion only when client responsiveness is  more  important
                      than  data  integrity.  Using NFS over TCP or increasing
                      the value of the retrans option may mitigate some of the
                      risks of using the soft option.

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

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

                      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 re-
                      transmission, 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 be-
                      fore it attempts further recovery action. If the retrans
                      option is not specified, the NFS client tries  each  UDP
                      request three times and each TCP request twice.

                      The  NFS client generates a "server not responding" 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  ne-
                      gotiate  the largest rsize value that they can both sup-
                      port.

                      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  ne-
                      gotiate  the largest wsize value that they can both sup-
                      port.

                      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 at-
                      tributes. 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  be-
                      come 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  ex-
                      tracts  a significant performance penalty.  As such, ju-
                      dicious use of file locking is encouraged instead.   The
                      DATA  AND METADATA COHERENCE section contains a detailed
                      discussion of these trade-offs.

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

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

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

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

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

       bg / fg        Determines  how  the  mount(8) command behaves if an at-
                      tempt 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  re-
                      quests.  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  directo-
                      ries.

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

                      Note that this only affects how many  retries  are  made
                      and  doesn't affect the delay caused by each retry.  For
                      UDP each retry takes the time determined  by  the  timeo
                      and  retrans  options,  which by default will be about 7
                      seconds.  For TCP the default is 3 minutes,  but  system
                      TCP connection timeouts will sometimes limit the timeout
                      of each retransmission to around 2 minutes.

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

                      The  DATA  AND METADATA COHERENCE section contains a de-
                      tailed 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.

       sloppy         The  sloppy  option  is  an  alternative  to  specifying
                      mount.nfs -s option.

   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) re-
                      ports 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.

       cto / nocto    Selects whether to use close-to-open cache coherence se-
                      mantics.  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 op-
                      tion 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.

       cto / nocto    Selects whether to use close-to-open cache coherence se-
                      mantics 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 op-
                      tion.  The DATA AND METADATA COHERENCE section discusses
                      the behavior of this option in more detail.

       clientaddr=n.n.n.n

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

                      If  this  option  is not specified, the mount(8) command
                      attempts to discover an appropriate callback address au-
                      tomatically.   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.

       migration / nomigration
                      Selects whether the client uses an identification string
                      that is compatible with NFSv4 Transparent  State  Migra-
                      tion (TSM).  If the mounted server supports NFSv4 migra-
                      tion with TSM, specify the migration option.

                      Some server features misbehave in the face of  a  migra-
                      tion-compatible  identification string.  The nomigration
                      option retains the use of a traditional client  indenti-
                      fication  string  which  is  compatible  with legacy NFS
                      servers.  This is also the behavior if neither option is
                      specified.  A client's open and lock state cannot be mi-
                      grated transparently when it  identifies  itself  via  a
                      traditional identification string.

                      This  mount  option  has no effect with NFSv4 minor ver-
                      sions newer than zero, which always  use  TSM-compatible
                      client identification strings.

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,  ex-
       cepted 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-lo-
       cal 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 au-
       tomatically negotiate proper transport and data transfer size  settings
       for  a  mount  point.  In some cases, however, it pays to specify these
       settings explicitly using mount options.

       Traditionally, NFS clients  used  the  UDP  transport  exclusively  for
       transmitting requests to servers.  Though its implementation is simple,
       NFS over UDP has many limitations that  prevent  smooth  operation  and
       good  performance  in some common deployment environments.  Even an in-
       significant packet loss rate results in the loss of whole NFS requests;
       as  such, retransmit timeouts are usually in the subsecond range to al-
       low clients to recover quickly from dropped requests, but this can  re-
       sult 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  op-
       tion), it assumes a network partition has occurred, and attempts to re-
       connect to the server on a fresh socket. Since TCP itself makes network
       data  transfer  reliable,  rsize and wsize can safely be allowed to de-
       fault to the largest values supported by both client and server,  inde-
       pendent 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 (NF-
       SACL) 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,  an-
       other fragment from a different packet but with the same IP ID will ar-
       rive 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,  be-
       cause  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.

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

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

       When the application closes the file, the NFS client  writes  back  any
       pending  changes  to  the  file  so  that  the next opener can view the
       changes.  This also gives the NFS client an opportunity to report write
       errors to the application via the return code from close(2).

       The behavior of checking at open time and flushing at close time is re-
       ferred to as close-to-open cache consistency, or CTO.  It can  be  dis-
       abled for an entire mount point using the nocto mount option.

   Weak cache consistency
       There  are  still  opportunities  for  a client's data cache to contain
       stale data.  The NFS version 3 protocol introduced "weak cache  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  at-
       tribute  information.   The  client keeps this information cached for a
       period of time to reduce network and server load.  When noac is in  ef-
       fect,  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 ab-
       solute  cache  coherence among clients is required, applications should
       use file locking. Alternatively, applications can also open their files
       with the O_DIRECT flag to disable data caching entirely.

   File timestamp maintainence
       NFS  servers are responsible for managing file and directory timestamps
       (atime, ctime, and mtime).  When a file is accessed or  updated  on  an
       NFS  server,  the file's timestamps are updated just like they would be
       on a filesystem local to an application.

       NFS clients cache file  attributes,  including  timestamps.   A  file's
       timestamps are updated on NFS clients when its attributes are retrieved
       from the NFS server.  Thus there may be some delay before timestamp up-
       dates on an NFS server appear to applications on NFS clients.

       To  comply with the POSIX filesystem standard, the Linux NFS client re-
       lies on NFS servers to keep a file's mtime and ctime  timestamps  prop-
       erly  up  to  date.  It does this by flushing local data changes to the
       server before reporting mtime to applications via system calls such  as
       stat(2).

       The  Linux  client  handles  atime  updates more loosely, however.  NFS
       clients maintain good performance by caching data, but that means  that
       application  reads,  which  normally update atime, are not reflected to
       the server where a file's atime is actually maintained.

       Because of this caching behavior, the Linux NFS client does not support
       generic atime-related mount options.  See mount(8) for details on these
       options.

       In particular, the atime/noatime, diratime/nodiratime, relatime/norela-
       time, and strictatime/nostrictatime mount options have no effect on NFS
       mounts.

       /proc/mounts may report that the relatime mount option is  set  on  NFS
       mounts,  but  in fact the atime semantics are always as described here,
       and are not like relatime semantics.

   Directory entry caching
       The Linux NFS client caches the result of all NFS LOOKUP requests.   If
       the  requested  directory entry exists on the server, the result is re-
       ferred to as a positive lookup result.  If the requested directory  en-
       try 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 re-
       moval  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 ex-
       act 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.  Be-
       cause NFS version 4 combines the function  of  the  sideband  protocols
       into  the main NFS protocol, the new security features apply to all NFS
       version 4 operations including  mounting,  file  locking,  and  so  on.
       RPCGSS  authentication  can also be used with NFS versions 2 and 3, but
       it does not protect their sideband protocols.

       The sec mount option specifies the security flavor used for  operations
       on  behalf  of users on that NFS mount point.  Specifying sec=krb5 pro-
       vides 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  im-
       pact  when using integrity checking or encryption.  Similar support for
       other forms of cryptographic security is also available.

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

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

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

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

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

       The Linux NFS client establishes one lease for each server.  Lease man-
       agement  operations, such as lease renewal, are not done on behalf of a
       particular file, lock, user, or mount point, but on behalf of the whole
       client  that owns that lease.  These operations must use the same secu-
       rity flavor and principal that was used when the lease was established,
       even across client reboots.

       When  Kerberos  is  configured  on a Linux NFS client (i.e., there is a
       /etc/krb5.keytab on that client), the client attempts to use a Kerberos
       security  flavor  for  its  lease management operations.  This provides
       strong authentication of the client to each server it contacts.  By de-
       fault,  the  client  uses  the  host/  or nfs/ service principal in its
       /etc/krb5.keytab for this purpose.

       If the client has Kerberos configured, but the server does not,  or  if
       the  client does not have a keytab or the requisite service principals,
       the client uses AUTH_SYS and UID 0 for lease management.

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

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

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

       As described above, the traditional default NFS authentication  scheme,
       known as AUTH_SYS, relies on sending local UID and GID numbers to 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 de-
       tails 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 ac-
       cess to specific NFS service ports.  Client administrators then specify
       the  port  number  for  the  mountd  service via the mount(8) command's
       mountport option.  It may also be necessary to enforce the use  of  TCP
       or UDP if the firewall blocks one of those transports.

   NFS Access Control Lists
       Solaris allows NFS version 3 clients direct access to POSIX Access 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  So-
       laris  NFS implementation.  The NFSACL protocol never became a standard
       part of the NFS version 3 specification, however.

       The NFS version 4 specification mandates a new version of  Access  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

       /etc/nfsmount.conf
                      Configuration file for NFS mounts

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),
       nfsmount.conf(5),   netconfig(5),   ipv6(7),   nfsd(8),   sm-notify(8),
       rpc.statd(8), rpc.idmapd(8), rpc.gssd(8), rpc.svcgssd(8), kerberos(1)

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

                                9 October 2012                          NFS(5)
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