Provided by: nfs-common_1.2.5-3ubuntu3_amd64 bug

NAME

       nfs - fstab format and options for the nfs file systems

SYNOPSIS

       /etc/fstab

DESCRIPTION

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

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

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

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

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

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

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

MOUNT OPTIONS

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

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

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

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

       timeo=n        The time in 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 seconds).  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 seconds.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                      Refer to the SECURITY CONSIDERATIONS section for important details.

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

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

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

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

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

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

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

                      If support for TI-RPC is built into the mount.nfs command, netid is a valid
                      netid  listed  in  /etc/netconfig.  The value "rdma" may also be specified.
                      If the mount.nfs command does not have TI-RPC support, then netid is one of
                      "tcp," "udp," or "rdma," and only IPv4 may be used.

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

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

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

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

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

       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 specified port, the mount request fails.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       local_lock=mechanism
                      Specifies whether to use local locking for any or both of the flock and the
                      POSIX locking mechanisms.  mechanism 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  specified,  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 transport protocol name and protocol family  the  NFS  client  uses  to
                      transmit requests to the NFS server for this mount point.  If an NFS server
                      has both an IPv4 and an IPv6 address, using a specific netid will force the
                      use of IPv4 or IPv6 networking to communicate with that server.

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

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

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

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

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

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

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

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

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

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

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

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

nfs4 FILE SYSTEM TYPE

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

MOUNT CONFIGURATION FILE

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

EXAMPLES

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

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

               server:/export  /mnt  nfs   defaults                      0 0

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

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

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

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

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

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

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

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

TRANSPORT METHODS

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

DATA AND METADATA COHERENCE

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

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

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

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

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

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

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

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

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

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

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

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

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

              Memory pressure forces reclamation of system memory resources.

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

              An application closes a file with close(2).

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

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

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

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

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

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

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

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

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

   NFS version 4 caching features
       The data and metadata caching behavior of NFS version 4 clients  is  similar  to  that  of
       earlier  versions.   However, NFS version 4 adds two features that improve cache behavior:
       change attributes and file delegation.

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

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

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

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

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

SECURITY CONSIDERATIONS

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

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

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

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

       Two  additional  flavors  of  Kerberos security are supported: krb5i and krb5p.  The krb5i
       security flavor provides a cryptographically strong guarantee that the data  in  each  RPC
       request  has not been tampered with.  The krb5p security flavor encrypts every RPC request
       to prevent data exposure during network transit; however, expect some  performance  impact
       when  using  integrity  checking  or  encryption.   Similar  support  for  other  forms of
       cryptographic security (such as lipkey and SPKM3) is also available.

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

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

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

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

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

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

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

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

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

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

              What network port each RPC-based service is using

              What transport protocols each RPC-based service supports

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

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

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

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

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 subcommand depends on
       knowing the original set of mount options  used  to  perform  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 referrals, and named attributes.

SEE ALSO

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

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

                                         2 November 2007                                   NFS(5)