Provided by: nfs-common_1.3.4-2.2ubuntu3_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.

       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

       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.

       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 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 UDP request three times and each TCP request twice.

                      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.
                      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 attribute 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 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.

       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).

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

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

                      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 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.

       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>/Documentation/filesystems/caching  for  detail  on  how  to
                      configure the FS-Cache facility.  Default value is nofsc.

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

       proto=netid    The netid determines the transport that is used to communicate 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 METHODS 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 METHODS 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 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 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,  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.

       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 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.

       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 netid determines the transport that is used to communicate 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 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.

       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

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

                      If this option is not specified, the mount(8) command attempts to  discover
                      an  appropriate  callback  address  automatically.  The automatic discovery
                      process is not perfect,  however.   In  the  presence  of  multiple  client
                      network   interfaces,   special   routing  policies,  or  atypical  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  Migration  (TSM).   If  the  mounted  server
                      supports NFSv4 migration with TSM, specify the migration option.

                      Some  server  features  misbehave  in  the  face  of a migration-compatible
                      identification string.   The  nomigration  option  retains  the  use  of  a
                      traditional  client  indentification 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 migrated transparently when it
                      identifies itself via a traditional identification string.

                      This mount option has no effect with NFSv4 minor versions 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, excepted the nfsvers mount option.

MOUNT CONFIGURATION FILE

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

EXAMPLES

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

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

               server:/export  /mnt  nfs   defaults                      0 0

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

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

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

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

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

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

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

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

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

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

TRANSPORT METHODS

       NFS clients send requests to NFS servers via Remote Procedure Calls,  or  RPCs.   The  RPC
       client   discovers   remote   service   endpoints   automatically,   handles   per-request
       authentication, adjusts request parameters for 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.

   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
       network 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 seconds.

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

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

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

       It  is  therefore  strongly recommended to use NFS over TCP where possible, 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 fragments 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 fragmented 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 referred to as close-
       to-open cache consistency, or CTO.  It can be disabled 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 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.

   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 updates on  an  NFS  server  appear  to  applications  on  NFS
       clients.

       To  comply  with the POSIX filesystem standard, the Linux NFS client relies on NFS servers
       to keep a file's mtime and ctime timestamps properly 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/norelatime,  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  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 support  for  strong  authentication
       and  security flavors that provide per-RPC integrity checking and encryption.  Because NFS
       version 4 combines the function of the sideband protocols into the main NFS protocol,  the
       new  security  features  apply  to  all  NFS version 4 operations including mounting, file
       locking, and so on.  RPCGSS authentication can also be used with NFS versions 2 and 3, but
       it does not protect their sideband protocols.

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

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

   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 filesystem.

       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 irrevocably 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 itself 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 performing 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 interfering with the other's lease.

       The Linux NFS client establishes one lease for each server.  Lease management  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 security 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  default,  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 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

       /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 referrals, 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)