Provided by: nfs-common_1.2.0-2ubuntu8_i386 bug

NAME

       nfs - fstab format and options for the nfs and nfs4 file systems

SYNOPSIS

       /etc/fstab

DESCRIPTION

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

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

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

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

       The server’s hostname and export pathname are  separated  by  a  colon,
       while  the  mount options are separated by commas. The remaining fields
       are separated by blanks or tabs.   The  server’s  hostname  can  be  an
       unqualified  hostname,  a fully qualified domain name, or a dotted quad
       IPv4 address.  The fstype field contains either "nfs" (for version 2 or
       version  3  NFS  mounts) or "nfs4" (for NFS version 4 mounts).  The nfs
       and nfs4 file system types  share  similar  mount  options,  which  are
       described below.

MOUNT OPTIONS

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

   Valid options for either the nfs or nfs4 file system type
       These  options  are  valid to use when mounting either nfs or nfs4 file
       system types.  They imply the same behavior and have the  same  default
       for both file system types.

       soft / hard    Determines the recovery behavior of the NFS client after
                      an  NFS  request  times  out.   If  neither  option   is
                      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  tenths of a second) the NFS client waits
                      for a response before it retries an NFS request. If this
                      option  is  not specified, requests are retried every 60
                      seconds for NFS over  TCP.   The  NFS  client  does  not
                      perform any kind of timeout backoff for NFS over TCP.

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

       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.

   Valid options for the nfs file system type
       Use these options, along with the options in the above subsection,  for
       mounting the nfs file system type.

       proto=transport
                      The  transport  the NFS client uses to transmit requests
                      to the NFS server for this mount point.   transport  can
                      be  either  udp  or  tcp.  Each transport 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   proto=tcp
                      forces all traffic from the mount(8) command and the NFS
                      client to use  TCP.   Specifying  proto=udp  forces  all
                      traffic types to use UDP.

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

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

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

       port=n         The  numeric value of the server’s NFS service port.  If
                      the  server’s  NFS  service  is  not  available  on  the
                      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=transport
                      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.  transport can be either udp or tcp.

                      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.  The Linux client supports version
                      2 and version 3 of the NFS protocol when using the  file
                      system  type  nfs.   If  the server does not support the
                      requested version, the mount  request  fails.   If  this
                      option  is  not  specified,  the  client attempts to use
                      version 3, but  negotiates  the  NFS  version  with  the
                      server if version 3 support is not available.

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

   Valid options for the nfs4 file system type
       Use these options, along with  the  options  in  the  first  subsection
       above, for mounting the nfs4 file system type.

       proto=transport
                      The  transport  the NFS client uses to transmit requests
                      to the NFS server for this mount point.   transport  can
                      be  either  udp  or  tcp.  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
                      transport 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) 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.

EXAMPLES

       To  mount  an  export using NFS version 2, use the nfs file system type
       and specify the nfsvers=2 mount option.  To mount using NFS version  3,
       use  the  nfs  file system type and specify the nfsvers=3 mount option.
       To mount using NFS version 4, use  the  nfs4  file  system  type.   The
       nfsvers mount option is not supported for the nfs4 file system type.

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

            server:/export /mnt nfs  defaults  0 0

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

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

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

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

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

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

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
       network’s MTU is large relative to NFS’s 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.

   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.

FILES

       /etc/fstab     file system table

BUGS

       The  generic  remount  option is not fully supported.  Generic options,
       such as rw and ro can be modified using the remount  option,  but  NFS-
       specific  options  are  not all supported.  The underlying transport or
       NFS version cannot be changed by a remount, for example.  Performing  a
       remount  on  an  NFS  file system mounted with the noac option may have
       unintended consequences.  The noac option is a  mixture  of  a  generic
       option, sync, and an NFS-specific option actimeo=0.

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

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

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

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

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

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

SEE ALSO

       fstab(5), mount(8), umount(8), mount.nfs(5), umount.nfs(5), exports(5),
       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)