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NFS(5)                                                                  NFS(5)



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

SYNOPSIS
       /etc/fstab

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                      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  near-
                      est 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 appli-
                      cation 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  lock-
                      ing  is encouraged instead.  The DATA AND METADATA COHERENCE section contains a detailed discus-
                      sion 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  dur-
                      ing  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 fore-
                      ground 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 immedi-
                      ately after the first failure.

       sec=mode       The RPCGSS security flavor to use for accessing files on this mount point.  If the sec option is
                      not  specified, or if sec=sys is specified, the NFS client uses the AUTH_SYS security flavor for
                      all NFS requests on this mount point.  Valid security flavors are none, sys,  krb5,  krb5i,  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 direc-
                      tory 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 par-
                      ent directory's cached attributes expire, but always  revalidates  negative  entires  before  an
                      application can use them.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       mountvers=n    The  RPC  version  number used to contact the server's mountd.  If this option is not specified,
                      the client uses a version number appropriate to the requested NFS version.  This option is  use-
                      ful 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  charac-
                      ters.

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

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

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

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

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

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

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

                      The intr / nointr mount option is deprecated after kernel 2.6.25.  Only SIGKILL can interrupt  a
                      pending  NFS  operation on these kernels, and if specified, this mount option is ignored to pro-
                      vide 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  dis-
                      cusses the behavior of this option in more detail.

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

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

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

       local_lock=mechanism
                      Specifies whether to use local locking for any or both of the flock and the POSIX locking mecha-
                      nisms.  mechanism can be one of all, flock, posix, or none.

                      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 pro-
                      tocol to lock files when flock locks are used.

                      To support legacy flock behavior similar to that of NFS clients < 2.6.12,  use  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.

       minorversion=n Specifies  the  protocol  minor  version number.  NFSv4 introduces "minor versioning," where NFS
                      protocol enhancements can be introduced without bumping the NFS protocol version number.

                      The minor version can also be be specified using the  vers=  option.   For  example,  specifying
                      vers=4.1 is the same as specifying vers=4,minorversion=1.

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

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

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

       port=n         The numeric value of the server's NFS service port.  If the server's NFS service is  not  avail-
                      able 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 adver-
                      tised 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 pro-
                      vide 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 coher-
                      ence semantics for directories.

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

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

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

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

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

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

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

            server:/export /mnt nfs  defaults  0 0

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

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

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

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

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

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

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

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

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

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

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

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

       TCP is the default transport protocol used for all modern NFS implementations.   It  performs  well  in  almost
       every  conceivable network environment and provides excellent guarantees against data corruption caused by net-
       work 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 speci-
       fied, but listens for server NSM notifications on both UDP and TCP.  The NFS Access Control List (NFSACL)  pro-
       tocol 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 speci-
       fied 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 dis-
       covery 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, dur-
       ing 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 sys-
       tem operation checks file attribute information.  The client keeps this information cached for a period of time
       to  reduce  network  and  server load.  When noac is in effect, a client's file attribute cache is disabled, so
       each operation that needs to check a file's attributes is forced to go back to  the  server.   This  permits  a
       client to see changes to a file very quickly, at the cost of many extra network operations.

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

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

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

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

       Caching directory entries improves the performance of applications that do not share files with applications on
       other clients.  Using cached information about directories can interfere with  applications  that  run  concur-
       rently  on  multiple clients and need to detect the creation or removal of files quickly, however.  The lookup-
       cache 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 appli-
       cations to detect new directory entries created by other clients quickly while still providing some of the per-
       formance 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 perfor-
       mance 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  pro-
       vides 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.  Config-
       ure 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.  How-
       ever, 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,  how-
       ever.

       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  call-
       back  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 directo-
       ries 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 sys-
       tem 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 specifica-
       tion 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 side-
       band protocols.

       The sec mount option specifies the RPCGSS security mode that is in effect on a given NFS mount point.  Specify-
       ing sec=krb5 provides cryptographic proof of a user's identity in each RPC request.  This provides strong veri-
       fication 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 cryp-
       tographic security 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 secu-
       rity flavor at mount time which remains in effect for the lifetime of the mount.  If the server does  not  sup-
       port 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  lim-
       ited 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 physi-
       cal 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  sys-
       tems.   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  trans-
       port 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  direc-
       tory, 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.



                                2 November 2007                         NFS(5)