# nfs

## NAME



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


## SYNOPSIS



           _/etc/fstab_
    


## DESCRIPTION



           NFS is an Internet Standard protocol created by Sun Microsystems
           in 1984. NFS was developed to allow file sharing between systems
           residing on a local area network.  Depending on kernel
           configuration, the Linux NFS client may support NFS versions 3,
           4.0, 4.1, or 4.2.
    
           The mount(8) command attaches a file system to the system's name
           space hierarchy at a given mount point.  The _/etc/fstab_ file
           describes how mount(8) should assemble a system's file name
           hierarchy from various independent file systems (including file
           systems exported by NFS servers).  Each line in the _/etc/fstab_
           file describes a single file system, its mount point, and a set of
           default mount options for that mount point.
    
           For NFS file system mounts, a line in the _/etc/fstab_ file
           specifies the server name, the path name of the exported server
           directory to mount, the local directory that is the mount point,
           the type of file system that is being mounted, and a list of mount
           options that control the way the filesystem is mounted and how the
           NFS client behaves when accessing files on this mount point.  The
           fifth and sixth fields on each line are not used by NFS, thus
           conventionally each contain the digit zero. For example:
    
                   server:path   /mountpoint   fstype   option,option,...   0 0
    
           The server's hostname and export pathname are separated by a
           colon, while the mount options are separated by commas. The
           remaining fields are separated by blanks or tabs.
    
           The server's hostname can be an unqualified hostname, a fully
           qualified domain name, a dotted quad IPv4 address, or an IPv6
           address enclosed in square brackets.  Link-local and site-local
           IPv6 addresses must be accompanied by an interface identifier.
           See ipv6(7) for details on specifying raw IPv6 addresses.
    
           The _fstype_ field contains "nfs".  Use of the "nfs4" fstype in
           _/etc/fstab_ is deprecated.
    


## MOUNT OPTIONS



           Refer to mount(8) for a description of generic mount options
           available for all file systems. If you do not need to specify any
           mount options, use the generic option **defaults** in _/etc/fstab_.
    
       **Options supported by all versions**
           These options are valid to use with any NFS version.
    
           **nfsvers=**_n_
                  The NFS protocol version number used to contact the
                  server's NFS service.  If the server does not support the
                  requested version, the mount request fails.  If this option
                  is not specified, the client tries version 4.2 first, then
                  negotiates down until it finds a version supported by the
                  server.
    
           **vers=**_n_ This option is an alternative to the **nfsvers** option.  It is
                  included for compatibility with other operating systems
    
           **soft** / **softerr** / **hard**
                  Determines the recovery behavior of the NFS client after an
                  NFS request times out.  If no option is specified (or if
                  the **hard** option is specified), NFS requests are retried
                  indefinitely.  If either the **soft** or **softerr** option is
                  specified, then the NFS client fails an NFS request after
                  **retrans** retransmissions have been sent, causing the NFS
                  client to return either the error **EIO**(for the **soft** option)
                  or **ETIMEDOUT**(for the **softerr** option) to the calling
                  application.
    
                  _NB:_ A so-called "soft" timeout can cause silent data
                  corruption in certain cases. As such, use the **soft** or
                  **softerr** 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** or **softerr** option.
    
           **softreval** / **nosoftreval**
                  In cases where the NFS server is down, it may be useful to
                  allow the NFS client to continue to serve up paths and
                  attributes from cache after **retrans** attempts to revalidate
                  that cache have timed out.  This may, for instance, be
                  helpful when trying to unmount a filesystem tree from a
                  server that is permanently down.
    
                  It is possible to combine **softreval** with the **soft** mount
                  option, in which case operations that cannot be served up
                  from cache will time out and return an error after **retrans**
                  attempts. The combination with the default **hard** mount
                  option implies those uncached operations will continue to
                  retry until a response is received from the server.
    
                  Note: the default mount option is **nosoftreval** which
                  disallows fallback to cache when revalidation fails, and
                  instead follows the behavior dictated by the **hard** or **soft**
                  mount option.
    
           **intr** / **nointr**
                  This option is provided for backward compatibility.  It is
                  ignored after kernel 2.6.25.
    
           **timeo=**_n_
                  The time in deciseconds (tenths of a second) the NFS client
                  waits for a response before it retries an NFS request.
    
                  For NFS over TCP the default **timeo** value is 600 (60
                  seconds).  The NFS client performs linear backoff: After
                  each retransmission the timeout is increased by **timeo** up to
                  the maximum of 600 seconds.
    
                  However, for NFS over UDP, the client uses an adaptive
                  algorithm to estimate an appropriate timeout value for
                  frequently used request types (such as READ and WRITE
                  requests), but uses the **timeo** setting for infrequently used
                  request types (such as FSINFO requests).  If the **timeo**
                  option is not specified, infrequently used request types
                  are retried after 1.1 seconds.  After each retransmission,
                  the NFS client doubles the timeout for that request, up to
                  a maximum timeout length of 60 seconds.
    
                  Any timeo value greater than default value will be set to
                  the default value.  For TCP and RDMA, default value is 600
                  (60 seconds).  For UDP, default value is 60 (6 seconds).
    
           **retrans=**_n_
                  The number of times the NFS client retries a request before
                  it attempts further recovery action. If the **retrans** option
                  is not specified, the NFS client tries each UDP request
                  three times and each TCP request twice.
    
                  The NFS client generates a "server not responding" message
                  after **retrans** retries, then attempts further recovery
                  (depending on whether the **hard** mount option is in effect).
    
           **rsize=**_n_
                  The maximum number of bytes in each network READ request
                  that the NFS client can receive when reading data from a
                  file on an NFS server.  The actual data payload size of
                  each NFS READ request is equal to or smaller than the **rsize**
                  setting. The largest read payload supported by the Linux
                  NFS client is 1,048,576 bytes (one megabyte).
    
                  The allowed **rsize** value is a positive integral multiple of
                  system's page size or a power of two if it is less than
                  system's page size.  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 such an allowed value, it is
                  rounded down to the nearest allowed value.
    
                  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 allowed **wsize** value is a positive
                  integral multiple of system's page size or a power of two
                  if it is less than system's page size.  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 such
                  an allowed value, it is rounded down to the nearest allowed
                  value.
    
                  If a **wsize** value is not specified, or if the specified
                  **wsize** value is larger than the maximum that either client
                  or server can support, the client and server negotiate the
                  largest **wsize** value that they can both support.
    
                  The **wsize** mount option as specified on the mount(8) command
                  line appears in the _/etc/mtab_ file. However, the effective
                  **wsize** value negotiated by the client and server is reported
                  in the _/proc/mounts_ file.
    
           **ac** / **noac**
                  Selects whether the client may cache file attributes. If
                  neither option is specified (or if **ac** is specified), the
                  client caches file attributes.
    
                  To improve performance, NFS clients cache file attributes.
                  Every few seconds, an NFS client checks the server's
                  version of each file's attributes for updates.  Changes
                  that occur on the server in those small intervals remain
                  undetected until the client checks the server again. The
                  **noac** option prevents clients from caching file attributes
                  so that applications can more quickly detect file changes
                  on the server.
    
                  In addition to preventing the client from caching file
                  attributes, the **noac** option forces application writes to
                  become synchronous so that local changes to a file become
                  visible on the server immediately.  That way, other clients
                  can quickly detect recent writes when they check the file's
                  attributes.
    
                  Using the **noac** option provides greater cache coherence
                  among NFS clients accessing the same files, but it extracts
                  a significant performance penalty.  As such, judicious use
                  of file locking is encouraged instead.  The DATA AND
                  METADATA COHERENCE section contains a detailed discussion
                  of these trade-offs.
    
           **acregmin=**_n_
                  The minimum time (in seconds) that the NFS client caches
                  attributes of a regular file before it requests fresh
                  attribute information from a server.  If this option is not
                  specified, the NFS client uses a 3-second minimum.  See the
                  DATA AND METADATA COHERENCE section for a full discussion
                  of attribute caching.
    
           **acregmax=**_n_
                  The maximum time (in seconds) that the NFS client caches
                  attributes of a regular file before it requests fresh
                  attribute information from a server.  If this option is not
                  specified, the NFS client uses a 60-second maximum.  See
                  the DATA AND METADATA COHERENCE section for a full
                  discussion of attribute caching.
    
           **acdirmin=**_n_
                  The minimum time (in seconds) that the NFS client caches
                  attributes of a directory before it requests fresh
                  attribute information from a server.  If this option is not
                  specified, the NFS client uses a 30-second minimum.  See
                  the DATA AND METADATA COHERENCE section for a full
                  discussion of attribute caching.
    
           **acdirmax=**_n_
                  The maximum time (in seconds) that the NFS client caches
                  attributes of a directory before it requests fresh
                  attribute information from a server.  If this option is not
                  specified, the NFS client uses a 60-second maximum.  See
                  the DATA AND METADATA COHERENCE section for a full
                  discussion of attribute caching.
    
           **actimeo=**_n_
                  Using **actimeo** sets all of **acregmin** , **acregmax** , **acdirmin** , and
                  **acdirmax** to the same value.  If this option is not
                  specified, the NFS client uses the defaults for each of
                  these options listed above.
    
           **bg** / **fg**
                  Determines how the mount(8) command behaves if an attempt
                  to mount an export fails.  The **fg** option causes mount(8) to
                  exit with an error status if any part of the mount request
                  times out or fails outright.  This is called a "foreground"
                  mount, and is the default behavior if neither the **fg** nor **bg**
                  mount option is specified.
    
                  If the **bg** option is specified, a timeout or failure causes
                  the mount(8) command to fork a child which continues to
                  attempt to mount the export.  The parent immediately
                  returns with a zero exit code.  This is known as a
                  "background" mount.
    
                  If the local mount point directory is missing, the mount(8)
                  command acts as if the mount request timed out.  This
                  permits nested NFS mounts specified in _/etc/fstab_ to
                  proceed in any order during system initialization, even if
                  some NFS servers are not yet available.  Alternatively
                  these issues can be addressed using an automounter (refer
                  to automount(8) for details).
    
           **nconnect=** n
                  When using a connection oriented protocol such as TCP, it
                  may sometimes be advantageous to set up multiple
                  connections between the client and server. For instance, if
                  your clients and/or servers are equipped with multiple
                  network interface cards (NICs), using multiple connections
                  to spread the load may improve overall performance.  In
                  such cases, the **nconnect** option allows the user to specify
                  the number of connections that should be established
                  between the client and server up to a limit of 16.
    
                  Note that the **nconnect** option may also be used by some pNFS
                  drivers to decide how many connections to set up to the
                  data servers.
    
           **rdirplus** / **nordirplus**
                  Selects whether to use NFS v3 or v4 READDIRPLUS requests.
                  If this option is not specified, the NFS client uses a
                  heuristic to optimize performance by choosing READDIR vs
                  READDIRPLUS based on how often the calling process uses the
                  additional attributes returned from READDIRPLUS.  Some
                  applications perform better if the client uses only READDIR
                  requests for all directories.
    
           **rdirplus={none|force}**
                  If set to "force", the NFS client always attempts to use
                  READDIRPLUS requests.  If set to "none", the behavior is
                  the same as **nordirplus.**
    
           **retry=**_n_
                  The number of minutes that the mount(8) command retries an
                  NFS mount operation in the foreground or background before
                  giving up.  If this option is not specified, the default
                  value for foreground mounts is 2 minutes, and the default
                  value for background mounts is 10000 minutes (80 minutes
                  shy of one week).  If a value of zero is specified, the
                  mount(8) command exits immediately after the first failure.
    
                  Note that this only affects how many retries are made and
                  doesn't affect the delay caused by each retry.  For UDP
                  each retry takes the time determined by the **timeo** and
                  **retrans** options, which by default will be about 7 seconds.
                  For TCP the default is 3 minutes, but system TCP connection
                  timeouts will sometimes limit the timeout of each
                  retransmission to around 2 minutes.
    
           **sec=**_flavors_
                  A colon-separated list of one or more security flavors to
                  use for accessing files on the mounted export. If the
                  server does not support any of these flavors, the mount
                  operation fails.  If **sec=** is not specified, the client
                  attempts to find a security flavor that both the client and
                  the server supports.  Valid _flavors_ are **none** , **sys** , **krb5** ,
                  **krb5i** , and **krb5p**.  Refer to the SECURITY CONSIDERATIONS
                  section for details.
    
           **sharecache** / **nosharecache**
                  Determines how the client's data cache and attribute cache
                  are shared when mounting the same export more than once
                  concurrently.  Using the same cache reduces memory
                  requirements on the client and presents identical file
                  contents to applications when the same remote file is
                  accessed via different mount points.
    
                  If neither option is specified, or if the **sharecache** option
                  is specified, then a single cache is used for all mount
                  points that access the same export.  If the **nosharecache**
                  option is specified, then that mount point gets a unique
                  cache.  Note that when data and attribute caches are
                  shared, the mount options from the first mount point take
                  effect for subsequent concurrent mounts of the same export.
    
                  As of kernel 2.6.18, the behavior specified by **nosharecache**
                  is legacy caching behavior. This is considered a data risk
                  since multiple cached copies of the same file on the same
                  client can become out of sync following a local update of
                  one of the copies.
    
           **resvport** / **noresvport**
                  Specifies whether the NFS client should use a privileged
                  source port when communicating with an NFS server for this
                  mount point.  If this option is not specified, or the
                  **resvport** option is specified, the NFS client uses a
                  privileged source port.  If the **noresvport** option is
                  specified, the NFS client uses a non-privileged source
                  port.  This option is supported in kernels 2.6.28 and
                  later.
    
                  Using non-privileged source ports helps increase the
                  maximum number of NFS mount points allowed on a client, but
                  NFS servers must be configured to allow clients to connect
                  via non-privileged source ports.
    
                  Refer to the SECURITY CONSIDERATIONS section for important
                  details.
    
           **lookupcache=**_mode_
                  Specifies how the kernel manages its cache of directory
                  entries for a given mount point.  _mode_ can be one of **all** ,
                  **none** , **pos** , or **positive**.  This option is supported in
                  kernels 2.6.28 and later.
    
                  The Linux NFS client caches the result of all NFS LOOKUP
                  requests.  If the requested directory entry exists on the
                  server, the result is referred to as _positive_.  If the
                  requested directory entry does not exist on the server, the
                  result is referred to as _negative_.
    
                  If this option is not specified, or if **all** is specified,
                  the client assumes both types of directory cache entries
                  are valid until their parent directory's cached attributes
                  expire.
    
                  If **pos** or **positive** is specified, the client assumes
                  positive entries are valid until their parent directory's
                  cached attributes expire, but always revalidates negative
                  entires before an application can use them.
    
                  If **none** is specified, the client revalidates both types of
                  directory cache entries before an application can use them.
                  This permits quick detection of files that were created or
                  removed by other clients, but can impact application and
                  server performance.
    
                  The DATA AND METADATA COHERENCE section contains a detailed
                  discussion of these trade-offs.
    
           **fsc** / **nofsc**
                  Enable/Disables the cache of (read-only) data pages to the
                  local disk using the FS-Cache facility. See cachefilesd(8)
                  and <kernel_source>/Documentation/filesystems/caching for
                  detail on how to configure the FS-Cache facility.  Default
                  value is nofsc.
    
           **sloppy** The **sloppy** option is an alternative to specifying **mount.nfs**
                  -s **option.**
    
           **xprtsec=**_policy_
                  Specifies the use of transport layer security to protect
                  NFS network traffic on behalf of this mount point.  _policy_
                  can be one of **none** , **tls** , or **mtls**.
    
                  If **none** is specified, transport layer security is forced
                  off, even if the NFS server supports transport layer
                  security.
    
                  If **tls** is specified, the client uses RPC-with-TLS to
                  provide in-transit confidentiality.
    
                  If **mtls** is specified, the client uses RPC-with-TLS to
                  authenticate itself and to provide in-transit
                  confidentiality.
    
                  If either **tls** or **mtls** is specified and the server does not
                  support RPC-with-TLS or peer authentication fails, the
                  mount attempt fails.
    
                  If the **xprtsec=** option is not specified, the default
                  behavior depends on the kernel version, but is usually
                  equivalent to **xprtsec=none**.
    
           **noalignwrite**
                  This option disables the default behavior of extending
                  buffered write operations to full page boundaries.
    
                  Normally, the NFS client rounds non-aligned buffered writes
                  up to the system page size, which can lead to "lost writes"
                  when multiple clients write concurrently to distinct non-
                  overlapping regions. Use this option when your applications
                  perform non-aligned buffered writes and you can guarantee
                  that file regions do not overlap, thus avoiding the need
                  for file locking.
    
       **Options for NFS versions 2 and 3 only**
           Use these options, along with the options in the above subsection,
           for NFS versions 2 and 3 only.
    
           **proto=**_netid_
                  The _netid_ determines the transport that is used to
                  communicate with the NFS server.  Available options are
                  **udp** , **udp6** , **tcp** , **tcp6** , **rdma** , and **rdma6**.  Those which end in
                  **6** use IPv6 addresses and are only available if support for
                  TI-RPC is built in. Others use IPv4 addresses.
    
                  Each transport protocol uses different default **retrans** and
                  **timeo** settings.  Refer to the description of these two
                  mount options for details.
    
                  In addition to controlling how the NFS client transmits
                  requests to the server, this mount option also controls how
                  the mount(8) command communicates with the server's rpcbind
                  and mountd services.  Specifying a netid that uses TCP
                  forces all traffic from the mount(8) command and the NFS
                  client to use TCP.  Specifying a netid that uses UDP forces
                  all traffic types to use UDP.
    
                  **Before using NFS over UDP, refer to the TRANSPORT METHODS**
                  **section.**
    
                  If the **proto** mount option is not specified, the mount(8)
                  command discovers which protocols the server supports and
                  chooses an appropriate transport for each service.  Refer
                  to the TRANSPORT METHODS section for more details.
    
           **udp** The **udp** option is an alternative to specifying **proto=udp.**
                  It is included for compatibility with other operating
                  systems.
    
                  **Before using NFS over UDP, refer to the TRANSPORT METHODS**
                  **section.**
    
           **tcp** The **tcp** option is an alternative to specifying **proto=tcp.**
                  It is included for compatibility with other operating
                  systems.
    
           **rdma** The **rdma** option is an alternative to specifying **proto=rdma.**
    
           **port=**_n_ The numeric value of the server's NFS service port.  If the
                  server's NFS service is not available on the specified
                  port, the mount request fails.
    
                  If this option is not specified, or if the specified port
                  value is 0, then the NFS client uses the NFS service port
                  number advertised by the server's rpcbind service.  The
                  mount request fails if the server's rpcbind service is not
                  available, the server's NFS service is not registered with
                  its rpcbind service, or the server's NFS service is not
                  available on the advertised port.
    
           **mountport=**_n_
                  The numeric value of the server's mountd port.  If the
                  server's mountd service is not available on the specified
                  port, the mount request fails.
    
                  If this option is not specified, or if the specified port
                  value is 0, then the mount(8) command uses the mountd
                  service port number advertised by the server's rpcbind
                  service.  The mount request fails if the server's rpcbind
                  service is not available, the server's mountd service is
                  not registered with its rpcbind service, or the server's
                  mountd service is not available on the advertised port.
    
                  This option can be used when mounting an NFS server through
                  a firewall that blocks the rpcbind protocol.
    
           **mountproto=**_netid_
                  The transport the NFS client uses to transmit requests to
                  the NFS server's mountd service when performing this mount
                  request, and when later unmounting this mount point.
    
                  _netid_ may be one of **udp** , and **tcp** which use IPv4 address or,
                  if TI-RPC is built into the **mount.nfs** command, **udp6** , and
                  **tcp6** which use IPv6 addresses.
    
                  This option can be used when mounting an NFS server through
                  a firewall that blocks a particular transport.  When used
                  in combination with the **proto** option, different transports
                  for mountd requests and NFS requests can be specified.  If
                  the server's mountd service is not available via the
                  specified transport, the mount request fails.
    
                  Refer to the TRANSPORT METHODS section for more on how the
                  **mountproto** mount option interacts with the **proto** mount
                  option.
    
           **mounthost=**_name_
                  The hostname of the host running mountd.  If this option is
                  not specified, the mount(8) command assumes that the mountd
                  service runs on the same host as the NFS service.
    
           **mountvers=**_n_
                  The RPC version number used to contact the server's mountd.
                  If this option is not specified, the client uses a version
                  number appropriate to the requested NFS version.  This
                  option is useful when multiple NFS services are running on
                  the same remote server host.
    
           **namlen=**_n_
                  The maximum length of a pathname component on this mount.
                  If this option is not specified, the maximum length is
                  negotiated with the server. In most cases, this maximum
                  length is 255 characters.
    
                  Some early versions of NFS did not support this
                  negotiation.  Using this option ensures that pathconf(3)
                  reports the proper maximum component length to applications
                  in such cases.
    
           **lock** / **nolock**
                  Selects whether to use the NLM sideband protocol to lock
                  files on the server.  If neither option is specified (or if
                  **lock** is specified), NLM locking is used for this mount
                  point.  When using the **nolock** option, applications can lock
                  files, but such locks provide exclusion only against other
                  applications running on the same client.  Remote
                  applications are not affected by these locks.
    
                  NLM locking must be disabled with the **nolock** option when
                  using NFS to mount _/var_ because _/var_ contains files used by
                  the NLM implementation on Linux.  Using the **nolock** option
                  is also required when mounting exports on NFS servers that
                  do not support the NLM protocol.
    
           **cto** / **nocto**
                  Selects whether to use close-to-open cache coherence
                  semantics.  If neither option is specified (or if **cto** is
                  specified), the client uses close-to-open cache coherence
                  semantics. If the **nocto** option is specified, the client
                  uses a non-standard heuristic to determine when files on
                  the server have changed.
    
                  Using the **nocto** option may improve performance for read-
                  only mounts, but should be used only if the data on the
                  server changes only occasionally.  The DATA AND METADATA
                  COHERENCE section discusses the behavior of this option in
                  more detail.
    
           **acl** / **noacl**
                  Selects whether to use the NFSACL sideband protocol on this
                  mount point.  The NFSACL sideband protocol is a proprietary
                  protocol implemented in Solaris that manages Access Control
                  Lists. NFSACL was never made a standard part of the NFS
                  protocol specification.
    
                  If neither **acl** nor **noacl** option is specified, the NFS
                  client negotiates with the server to see if the NFSACL
                  protocol is supported, and uses it if the server supports
                  it.  Disabling the NFSACL sideband protocol may be
                  necessary if the negotiation causes problems on the client
                  or server.  Refer to the SECURITY CONSIDERATIONS section
                  for more details.
    
           **local_lock=** mechanism
                  Specifies whether to use local locking for any or both of
                  the flock and the POSIX locking mechanisms.  _mechanism_ can
                  be one of **all** , **flock** , **posix** , or **none**.  This option is
                  supported in kernels 2.6.37 and later.
    
                  The Linux NFS client provides a way to make locks local.
                  This means, the applications can lock files, but such locks
                  provide exclusion only against other applications running
                  on the same client. Remote applications are not affected by
                  these locks.
    
                  If this option is not specified, or if **none** is specified,
                  the client assumes that the locks are not local.
    
                  If **all** is specified, the client assumes that both flock and
                  POSIX locks are local.
    
                  If **flock** is specified, the client assumes that only flock
                  locks are local and uses NLM sideband protocol to lock
                  files when POSIX locks are used.
    
                  If **posix** is specified, the client assumes that POSIX locks
                  are local and uses NLM sideband protocol to lock files when
                  flock locks are used.
    
                  To support legacy flock behavior similar to that of NFS
                  clients < 2.6.12, use 'local_lock=flock'. This option is
                  required when exporting NFS mounts via Samba as Samba maps
                  Windows share mode locks as flock. Since NFS clients >
                  2.6.12 implement flock by emulating POSIX locks, this will
                  result in conflicting locks.
    
                  NOTE: When used together, the 'local_lock' mount option
                  will be overridden by 'nolock'/'lock' mount option.
    
       **Options for NFS version 4 only**
           Use these options, along with the options in the first subsection
           above, for NFS version 4.0 and newer.
    
           **proto=**_netid_
                  The _netid_ determines the transport that is used to
                  communicate with the NFS server.  Supported options are
                  **tcp** , **tcp6** , **rdma** , and **rdma6**.  **tcp6** use IPv6 addresses and is
                  only available if support for TI-RPC is built in. Both
                  others use IPv4 addresses.
    
                  All NFS version 4 servers are required to support TCP, so
                  if this mount option is not specified, the NFS version 4
                  client uses the TCP protocol.  Refer to the TRANSPORT
                  METHODS section for more details.
    
           **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.  Before kernel 2.6.38, the minor
                  version is always zero, and this option is not recognized.
                  After this kernel, specifying "minorversion=1" enables a
                  number of advanced features, such as NFSv4 sessions.
    
                  Recent kernels allow the minor version to be specified
                  using the **vers=** option.  For example, specifying **vers=4.1**
                  is the same as specifying **vers=4,minorversion=1**.
    
           **port=**_n_ The numeric value of the server's NFS service port.  If the
                  server's NFS service is not available on the specified
                  port, the mount request fails.
    
                  If this mount option is not specified, the NFS client uses
                  the standard NFS port number of 2049 without first checking
                  the server's rpcbind service.  This allows an NFS version 4
                  client to contact an NFS version 4 server through a
                  firewall that may block rpcbind requests.
    
                  If the specified port value is 0, then the NFS client uses
                  the NFS service port number advertised by the server's
                  rpcbind service.  The mount request fails if the server's
                  rpcbind service is not available, the server's NFS service
                  is not registered with its rpcbind service, or the server's
                  NFS service is not available on the advertised port.
    
           **cto** / **nocto**
                  Selects whether to use close-to-open cache coherence
                  semantics for NFS directories on this mount point.  If
                  neither **cto** nor **nocto** is specified, the default is to use
                  close-to-open cache coherence semantics for directories.
    
                  File data caching behavior is not affected by this option.
                  The DATA AND METADATA COHERENCE section discusses the
                  behavior of this option in more detail.
    
           **clientaddr=**_n.n.n.n_
    
           **clientaddr=**_n:n:_**...**_:n_
                  Specifies a single IPv4 address (in dotted-quad form), or a
                  non-link-local IPv6 address, that the NFS client advertises
                  to allow servers to perform NFS version 4.0 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.  Can specify a value of IPv4_ANY
                  (0.0.0.0) or equivalent IPv6 any address which will signal
                  to the NFS server that this NFS client does not want
                  delegations.
    
                  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.
    
                  NFS protocol versions 4.1 and 4.2 use the client-
                  established TCP connection for callback requests, so do not
                  require the server to connect to the client.  This option
                  is therefore only affect NFS version 4.0 mounts.
    
           **migration** / **nomigration**
                  Selects whether the client uses an identification string
                  that is compatible with NFSv4 Transparent State Migration
                  (TSM).  If the mounted server supports NFSv4 migration with
                  TSM, specify the **migration** option.
    
                  Some server features misbehave in the face of a migration-
                  compatible identification string.  The **nomigration** option
                  retains the use of a traditional client identification
                  string which is compatible with legacy NFS servers.  This
                  is also the behavior if neither option is specified.  A
                  client's open and lock state cannot be migrated
                  transparently when it identifies itself via a traditional
                  identification string.
    
                  This mount option has no effect with NFSv4 minor versions
                  newer than zero, which always use TSM-compatible client
                  identification strings.
    
           **max_connect=** n
                  While **nconnect** option sets a limit on the number of
                  connections that can be established to a given server IP,
                  **max_connect** option allows the user to specify maximum
                  number of connections to different server IPs that belong
                  to the same NFSv4.1+ server (session trunkable connections)
                  up to a limit of 16. When client discovers that it
                  established a client ID to an already existing server,
                  instead of dropping the newly created network transport,
                  the client will add this new connection to the list of
                  available transports for that RPC client.
    
           **trunkdiscovery** / **notrunkdiscovery**
                  When the client discovers a new filesystem on a NFSv4.1+
                  server, the **trunkdiscovery** mount option will cause it to
                  send a GETATTR for the fs_locations attribute.  If is
                  receives a non-zero length reply, it will iterate through
                  the response, and for each server location it will
                  establish a connection, send an EXCHANGE_ID, and test for
                  session trunking.  If the trunking test succeeds, the
                  connection will be added to the existing set of transports
                  for the server, subject to the limit specified by the
                  **max_connect** option.  The default is **notrunkdiscovery**.
    


## nfs4 FILE SYSTEM TYPE



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


## MOUNT CONFIGURATION FILE



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


## EXAMPLES



           To mount 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
    
           This example shows how to mount using NFS version 4 over TCP with
           Kerberos 5 mutual authentication.
    
                   server:/export  /mnt  nfs4  sec=krb5                      0 0
    
           This example shows how to mount using NFS version 4 over TCP with
           Kerberos 5 privacy or data integrity mode.
    
                   server:/export  /mnt  nfs4  sec=krb5p:krb5i               0 0
    
           This example can be used to mount /usr over NFS.
    
                   server:/export  /usr  nfs   ro,nolock,nocto,actimeo=3600  0 0
    
           This example shows how to mount an NFS server using a raw IPv6
           link-local address.
    
                   [fe80::215:c5ff:fb3e:e2b1%eth0]:/export /mnt nfs defaults 0 0
    


## TRANSPORT METHODS



           NFS clients send requests to NFS servers via Remote Procedure
           Calls, or _RPCs_.  The RPC client discovers remote service endpoints
           automatically, handles per-request authentication, adjusts request
           parameters for different byte endianness on client and server, and
           retransmits requests that may have been lost by the network or
           server.  RPC requests and replies flow over a network transport.
    
           In most cases, the mount(8) command, NFS client, and NFS server
           can automatically negotiate proper transport and data transfer
           size settings for a mount point.  In some cases, however, it pays
           to specify these settings explicitly using mount options.
    
           Traditionally, NFS clients used the UDP transport exclusively for
           transmitting requests to servers.  Though its implementation is
           simple, NFS over UDP has many limitations that prevent smooth
           operation and good performance in some common deployment
           environments.  Even an insignificant packet loss rate results in
           the loss of whole NFS requests; as such, retransmit timeouts are
           usually in the subsecond range to allow clients to recover quickly
           from dropped requests, but this can result in extraneous network
           traffic and server load.
    
           However, UDP can be quite effective in specialized settings where
           the networks MTU is large relative to NFSs data transfer size
           (such as network environments that enable jumbo Ethernet frames).
           In such environments, trimming the **rsize** and **wsize** settings so
           that each NFS read or write request fits in just a few network
           frames (or even in  a single  frame) is advised.  This reduces the
           probability that the loss of a single MTU-sized network frame
           results in the loss of an entire large read or write request.
    
           TCP is the default transport protocol used for all modern NFS
           implementations.  It performs well in almost every conceivable
           network environment and provides excellent guarantees against data
           corruption caused by network unreliability.  TCP is often a
           requirement for mounting a server through a network firewall.
    
           Under normal circumstances, networks drop packets much more
           frequently than NFS servers drop requests.  As such, an aggressive
           retransmit timeout  setting for NFS over TCP is unnecessary.
           Typical timeout settings for NFS over TCP are between one and ten
           minutes.  After  the client exhausts its retransmits (the value of
           the **retrans** mount option), it assumes a network partition has
           occurred, and attempts to reconnect to the server on a fresh
           socket. Since TCP itself makes network data transfer reliable,
           **rsize** and **wsize** can safely be allowed to default to the largest
           values supported by both client and server, independent of the
           network's MTU size.
    
       **Using the mountproto mount option**
           This section applies only to NFS version 3 mounts since NFS
           version 4 does not use a separate protocol for mount requests.
    
           The Linux NFS client can use a different transport for contacting
           an NFS server's rpcbind service, its mountd service, its Network
           Lock Manager (NLM) service, and its NFS service.  The exact
           transports employed by the Linux NFS client for each mount point
           depends on the settings of the transport mount options, which
           include **proto** , **mountproto** , **udp** , and **tcp**.
    
           The client sends Network Status Manager (NSM) notifications via
           UDP no matter what transport options are specified, but listens
           for server NSM notifications on both UDP and TCP.  The NFS Access
           Control List (NFSACL) protocol shares the same transport as the
           main NFS service.
    
           If no transport options are specified, the Linux NFS client uses
           UDP to contact the server's mountd service, and TCP to contact its
           NLM and NFS services by default.
    
           If the server does not support these transports for these
           services, the mount(8) command attempts to discover what the
           server supports, and then retries the mount request once using the
           discovered transports.  If the server does not advertise any
           transport supported by the client or is misconfigured, the mount
           request fails.  If the **bg** option is in effect, the mount command
           backgrounds itself and continues to attempt the specified mount
           request.
    
           When the **proto** option, the **udp** option, or the **tcp** option is
           specified but the **mountproto** option is not, the specified
           transport is used to contact both the server's mountd service and
           for the NLM and NFS services.
    
           If the **mountproto** option is specified but none of the **proto** , **udp**
           or **tcp** options are specified, then the specified transport is used
           for the initial mountd request, but the mount command attempts to
           discover what the server supports for the NFS protocol, preferring
           TCP if both transports are supported.
    
           If both the **mountproto** and **proto**(or **udp** or **tcp**) options are
           specified, then the transport specified by the **mountproto** option
           is used for the initial mountd request, and the transport
           specified by the **proto** option (or the **udp** or **tcp** options) is used
           for NFS, no matter what order these options appear.  No automatic
           service discovery is performed if these options are specified.
    
           If any of the **proto** , **udp** , **tcp** , or **mountproto** options are specified
           more than once on the same mount command line, then the value of
           the rightmost instance of each of these options takes effect.
    
       **Using NFS over UDP on high-speed links**
           Using NFS over UDP on high-speed links such as Gigabit **can cause**
           **silent data corruption**.
    
           The problem can be triggered at high loads, and is caused by
           problems in IP fragment reassembly. NFS read and writes typically
           transmit UDP packets of 4 Kilobytes or more, which have to be
           broken up into several fragments in order to be sent over the
           Ethernet link, which limits packets to 1500 bytes by default. This
           process happens at the IP network layer and is called
           fragmentation.
    
           In order to identify fragments that belong together, IP assigns a
           16bit _IP ID_ value to each packet; fragments generated from the
           same UDP packet will have the same IP ID. The receiving system
           will collect these fragments and combine them to form the original
           UDP packet. This process is called reassembly. The default timeout
           for packet reassembly is 30 seconds; if the network stack does not
           receive all fragments of a given packet within this interval, it
           assumes the missing fragment(s) got lost and discards those it
           already received.
    
           The problem this creates over high-speed links is that it is
           possible to send more than 65536 packets within 30 seconds. In
           fact, with heavy NFS traffic one can observe that the IP IDs
           repeat after about 5 seconds.
    
           This has serious effects on reassembly: if one fragment gets lost,
           another fragment _from a different packet_ but with the _same IP ID_
           will arrive within the 30 second timeout, and the network stack
           will combine these fragments to form a new packet. Most of the
           time, network layers above IP will detect this mismatched
           reassembly - in the case of UDP, the UDP checksum, which is a 16
           bit checksum over the entire packet payload, will usually not
           match, and UDP will discard the bad packet.
    
           However, the UDP checksum is 16 bit only, so there is a chance of
           1 in 65536 that it will match even if the packet payload is
           completely random (which very often isn't the case). If that is
           the case, silent data corruption will occur.
    
           This potential should be taken seriously, at least on Gigabit
           Ethernet.  Network speeds of 100Mbit/s should be considered less
           problematic, because with most traffic patterns IP ID wrap around
           will take much longer than 30 seconds.
    
           It is therefore strongly recommended to use **NFS over TCP where**
           **possible** , since TCP does not perform fragmentation.
    
           If you absolutely have to use NFS over UDP over Gigabit Ethernet,
           some steps can be taken to mitigate the problem and reduce the
           probability of corruption:
    
           _Jumbo frames:_  Many Gigabit network cards are capable of
                          transmitting frames bigger than the 1500 byte limit
                          of traditional Ethernet, typically 9000 bytes.
                          Using jumbo frames of 9000 bytes will allow you to
                          run NFS over UDP at a page size of 8K without
                          fragmentation. Of course, this is only feasible if
                          all involved stations support jumbo frames.
    
                          To enable a machine to send jumbo frames on cards
                          that support it, it is sufficient to configure the
                          interface for a MTU value of 9000.
    
           _Lower reassembly timeout:_
                          By lowering this timeout below the time it takes
                          the IP ID counter to wrap around, incorrect
                          reassembly of fragments can be prevented as well.
                          To do so, simply write the new timeout value (in
                          seconds) to the file
                          **/proc/sys/net/ipv4/ipfrag_time**.
    
                          A value of 2 seconds will greatly reduce the
                          probability of IPID clashes on a single Gigabit
                          link, while still allowing for a reasonable timeout
                          when receiving fragmented traffic from distant
                          peers.
    


## DATA AND METADATA COHERENCE



           Some modern cluster file systems provide perfect cache coherence
           among their clients.  Perfect cache coherence among disparate NFS
           clients is expensive to achieve, especially on wide area networks.
           As such, NFS settles for weaker cache coherence that satisfies the
           requirements of most file sharing types.
    
       **Close-to-open cache consistency**
           Typically file sharing is completely sequential.  First client A
           opens a file, writes something to it, then closes it.  Then client
           B opens the same file, and reads the changes.
    
           When an application opens a file stored on an NFS version 3
           server, the NFS client checks that the file exists on the server
           and is permitted to the opener by sending a GETATTR or ACCESS
           request.  The NFS client sends these requests regardless of the
           freshness of the file's cached attributes.
    
           When the application closes the file, the NFS client writes back
           any pending changes to the file so that the next opener can view
           the changes.  This also gives the NFS client an opportunity to
           report write errors to the application via the return code from
           close(2).
    
           The behavior of checking at open time and flushing at close time
           is referred to as _close-to-open cache consistency_ , or _CTO_.  It can
           be disabled for an entire mount point using the **nocto** mount
           option.
    
       **Weak cache consistency**
           There are still opportunities for a client's data cache to contain
           stale data.  The NFS version 3 protocol introduced "weak cache
           consistency" (also known as WCC) which provides a way of
           efficiently checking a file's attributes before and after a single
           request.  This allows a client to help identify changes that could
           have been made by other clients.
    
           When a client is using many concurrent operations that update the
           same file at the same time (for example, during asynchronous write
           behind), it is still difficult to tell whether it was that
           client's updates or some other client's updates that altered the
           file.
    
       **Attribute caching**
           Use the **noac** mount option to achieve attribute cache coherence
           among multiple clients.  Almost every file system operation checks
           file attribute information.  The client keeps this information
           cached for a period of time to reduce network and server load.
           When **noac** is in effect, a client's file attribute cache is
           disabled, so each operation that needs to check a file's
           attributes is forced to go back to the server.  This permits a
           client to see changes to a file very quickly, at the cost of many
           extra network operations.
    
           Be careful not to confuse the **noac** option with "no data caching."
           The **noac** mount option prevents the client from caching file
           metadata, but there are still races that may result in data cache
           incoherence between client and server.
    
           The NFS protocol is not designed to support true cluster file
           system cache coherence without some type of application
           serialization.  If absolute cache coherence among clients is
           required, applications should use file locking. Alternatively,
           applications can also open their files with the O_DIRECT flag to
           disable data caching entirely.
    
       **File timestamp maintenance**
           NFS servers are responsible for managing file and directory
           timestamps (**atime** , **ctime** , and **mtime**).  When a file is accessed or
           updated on an NFS server, the file's timestamps are updated just
           like they would be on a filesystem local to an application.
    
           NFS clients cache file attributes, including timestamps.  A file's
           timestamps are updated on NFS clients when its attributes are
           retrieved from the NFS server.  Thus there may be some delay
           before timestamp updates on an NFS server appear to applications
           on NFS clients.
    
           To comply with the POSIX filesystem standard, the Linux NFS client
           relies on NFS servers to keep a file's **mtime** and **ctime** timestamps
           properly up to date.  It does this by flushing local data changes
           to the server before reporting **mtime** to applications via system
           calls such as stat(2).
    
           The Linux client handles **atime** updates more loosely, however.  NFS
           clients maintain good performance by caching data, but that means
           that application reads, which normally update **atime** , are not
           reflected to the server where a file's **atime** is actually
           maintained.
    
           Because of this caching behavior, the Linux NFS client does not
           support generic atime-related mount options.  See mount(8) for
           details on these options.
    
           In particular, the **atime** /**noatime** , **diratime** /**nodiratime** ,
           **relatime** /**norelatime** , and **strictatime** /**nostrictatime** mount options
           have no effect on NFS mounts.
    
           _/proc/mounts_ may report that the **relatime** mount option is set on
           NFS mounts, but in fact the **atime** semantics are always as
           described here, and are not like **relatime** semantics.
    
       **Directory entry caching**
           The Linux NFS client caches the result of all NFS LOOKUP requests.
           If the requested directory entry exists on the server, the result
           is referred to as a _positive_ lookup result.  If the requested
           directory entry does not exist on the server (that is, the server
           returned ENOENT), the result is referred to as _negative_ lookup
           result.
    
           To detect when directory entries have been added or removed on the
           server, the Linux NFS client watches a directory's mtime.  If the
           client detects a change in a directory's mtime, the client drops
           all cached LOOKUP results for that directory.  Since the
           directory's mtime is a cached attribute, it may take some time
           before a client notices it has changed.  See the descriptions of
           the **acdirmin** , **acdirmax** , and **noac** mount options for more
           information about how long a directory's mtime is cached.
    
           Caching directory entries improves the performance of applications
           that do not share files with applications on other clients.  Using
           cached information about directories can interfere with
           applications that run concurrently on multiple clients and need to
           detect the creation or removal of files quickly, however.  The
           **lookupcache** mount option allows some tuning of directory entry
           caching behavior.
    
           Before kernel release 2.6.28, the Linux NFS client tracked only
           positive lookup results.  This permitted applications to detect
           new directory entries created by other clients quickly while still
           providing some of the performance benefits of caching.  If an
           application depends on the previous lookup caching behavior of the
           Linux NFS client, you can use **lookupcache=positive**.
    
           If the client ignores its cache and validates every application
           lookup request with the server, that client can immediately detect
           when a new directory entry has been either created or removed by
           another client.  You can specify this behavior using
           **lookupcache=none**.  The extra NFS requests needed if the client
           does not cache directory entries can exact a performance penalty.
           Disabling lookup caching should result in less of a performance
           penalty than using **noac** , and has no effect on how the NFS client
           caches the attributes of files.
    
       **The sync mount option**
           The NFS client treats the **sync** mount option differently than some
           other file systems (refer to mount(8) for a description of the
           generic **sync** and **async** mount options).  If neither **sync** nor **async**
           is specified (or if the **async** option is specified), the NFS client
           delays sending application writes to the server until any of these
           events occur:
    
                  Memory pressure forces reclamation of system memory
                  resources.
    
                  An application flushes file data explicitly with sync(2),
                  msync(2), or **fsync**(3).
    
                  An application closes a file with close(2).
    
                  The file is locked/unlocked via fcntl(2).
    
           In other words, under normal circumstances, data written by an
           application may not immediately appear on the server that hosts
           the file.
    
           If the **sync** option is specified on a mount point, any system call
           that writes data to files on that mount point causes that data to
           be flushed to the server before the system call returns control to
           user space.  This provides greater data cache coherence among
           clients, but at a significant performance cost.
    
           Applications can use the O_SYNC open flag to force application
           writes to individual files to go to the server immediately without
           the use of the **sync** mount option.
    
       **Using file locks with NFS**
           The Network Lock Manager protocol is a separate sideband protocol
           used to manage file locks in NFS version 3.  To support lock
           recovery after a client or server reboot, a second sideband
           protocol -- known as the Network Status Manager protocol -- is
           also required.  In NFS version 4, file locking is supported
           directly in the main NFS protocol, and the NLM and NSM sideband
           protocols are not used.
    
           In most cases, NLM and NSM services are started automatically, and
           no extra configuration is required.  Configure all NFS clients
           with fully-qualified domain names to ensure that NFS servers can
           find clients to notify them of server reboots.
    
           NLM supports advisory file locks only.  To lock NFS files, use
           fcntl(2) with the F_GETLK and F_SETLK commands.  The NFS client
           converts file locks obtained via flock(2) to advisory locks.
    
           When mounting servers that do not support the NLM protocol, or
           when mounting an NFS server through a firewall that blocks the NLM
           service port, specify the **nolock** mount option. NLM locking must be
           disabled with the **nolock** option when using NFS to mount _/var_
           because _/var_ contains files used by the NLM implementation on
           Linux.
    
           Specifying the **nolock** option may also be advised to improve the
           performance of a proprietary application which runs on a single
           client and uses file locks extensively.
    
       **NFS version 4 caching features**
           The data and metadata caching behavior of NFS version 4 clients is
           similar to that of earlier versions.  However, NFS version 4 adds
           two features that improve cache behavior: _change attributes_ and
           _file delegation_.
    
           The _change attribute_ is a new part of NFS file and directory
           metadata which tracks data changes.  It replaces the use of a
           file's modification and change time stamps as a way for clients to
           validate the content of their caches.  Change attributes are
           independent of the time stamp resolution on either the server or
           client, however.
    
           A _file delegation_ is a contract between an NFS version 4 client
           and server that allows the client to treat a file temporarily as
           if no other client is accessing it.  The server promises to notify
           the client (via a callback request) if another client attempts to
           access that file.  Once a file has been delegated to a client, the
           client can cache that file's data and metadata aggressively
           without contacting the server.
    
           File delegations come in two flavors: _read_ and _write_.  A _read_
           delegation means that the server notifies the client about any
           other clients that want to write to the file.  A _write_ delegation
           means that the client gets notified about either read or write
           accessors.
    
           Servers grant file delegations when a file is opened, and can
           recall delegations at any time when another client wants access to
           the file that conflicts with any delegations already granted.
           Delegations on directories are not supported.
    
           In order to support delegation callback, the server checks the
           network return path to the client during the client's initial
           contact with the server.  If contact with the client cannot be
           established, the server simply does not grant any delegations to
           that client.
    


## SECURITY CONSIDERATIONS



           NFS servers control access to file data, but they depend on their
           RPC implementation to provide authentication of NFS requests.
           Traditional NFS access control mimics the standard mode bit access
           control provided in local file systems.  Traditional RPC
           authentication uses a number to represent each user (usually the
           user's own uid), a number to represent the user's group (the
           user's gid), and a set of up to 16 auxiliary group numbers to
           represent other groups of which the user may be a member.
    
           Typically, file data and user ID values appear unencrypted (i.e.
           "in the clear") on the network.  Moreover, NFS versions 2 and 3
           use separate sideband protocols for mounting, locking and
           unlocking files, and reporting system status of clients and
           servers.  These auxiliary protocols use no authentication.
    
           In addition to combining these sideband protocols with the main
           NFS protocol, NFS version 4 introduces more advanced forms of
           access control, authentication, and in-transit data protection.
           The NFS version 4 specification mandates support for strong
           authentication and security flavors that provide per-RPC integrity
           checking and encryption.  Because NFS version 4 combines the
           function of the sideband protocols into the main NFS protocol, the
           new security features apply to all NFS version 4 operations
           including mounting, file locking, and so on.  RPCGSS
           authentication can also be used with NFS versions 2 and 3, but it
           does not protect their sideband protocols.
    
           The **sec** mount option specifies the security flavor used for
           operations on behalf of users on that NFS mount point.  Specifying
           **sec=krb5** provides cryptographic proof of a user's identity in each
           RPC request.  This provides strong verification of the identity of
           users accessing data on the server.  Note that additional
           configuration besides adding this mount option is required in
           order to enable Kerberos security.  Refer to the rpc.gssd(8) man
           page for details.
    
           Two additional flavors of Kerberos security are supported: **krb5i**
           and **krb5p**.  The **krb5i** security flavor provides a cryptographically
           strong guarantee that the data in each RPC request has not been
           tampered with.  The **krb5p** security flavor encrypts every RPC
           request to prevent data exposure during network transit; however,
           expect some performance impact when using integrity checking or
           encryption.  Similar support for other forms of cryptographic
           security is also available.
    
       **NFS version 4 filesystem crossing**
           The NFS version 4 protocol allows a client to renegotiate the
           security flavor when the client crosses into a new filesystem on
           the server.  The newly negotiated flavor effects only accesses of
           the new filesystem.
    
           Such negotiation typically occurs when a client crosses from a
           server's pseudo-fs into one of the server's exported physical
           filesystems, which often have more restrictive security settings
           than the pseudo-fs.
    
       **NFS version 4 Leases**
           In NFS version 4, a lease is a period during which a server
           irrevocably grants a client file locks.  Once the lease expires,
           the server may revoke those locks.  Clients periodically renew
           their leases to prevent lock revocation.
    
           After an NFS version 4 server reboots, each client tells the
           server about existing file open and lock state under its lease
           before operation can continue.  If a client reboots, the server
           frees all open and lock state associated with that client's lease.
    
           When establishing a lease, therefore, a client must identify
           itself to a server.  Each client presents an arbitrary string to
           distinguish itself from other clients.  The client administrator
           can supplement the default identity string using the
           _nfs4.nfs4_unique_id_ module parameter to avoid collisions with
           other client identity strings.
    
           A client also uses a unique security flavor and principal when it
           establishes its lease.  If two clients present the same identity
           string, a server can use client principals to distinguish between
           them, thus securely preventing one client from interfering with
           the other's lease.
    
           The Linux NFS client establishes one lease on each NFS version 4
           server.  Lease management operations, such as lease renewal, are
           not done on behalf of a particular file, lock, user, or mount
           point, but on behalf of the client that owns that lease.  A client
           uses a consistent identity string, security flavor, and principal
           across client reboots to ensure that the server can promptly reap
           expired lease state.
    
           When Kerberos is configured on a Linux NFS client (i.e., there is
           a _/etc/krb5.keytab_ on that client), the client attempts to use a
           Kerberos security flavor for its lease management operations.
           Kerberos provides secure authentication of each client.  By
           default, the client uses the _host/_ or _nfs/_ service principal in
           its _/etc/krb5.keytab_ for this purpose, as described in
           rpc.gssd(8).
    
           If the client has Kerberos configured, but the server does not, or
           if the client does not have a keytab or the requisite service
           principals, the client uses _AUTH_SYS_ and UID 0 for lease
           management.
    
       **Using non-privileged source ports**
           NFS clients usually communicate with NFS servers via network
           sockets.  Each end of a socket is assigned a port value, which is
           simply a number between 1 and 65535 that distinguishes socket
           endpoints at the same IP address.  A socket is uniquely defined by
           a tuple that includes the transport protocol (TCP or UDP) and the
           port values and IP addresses of both endpoints.
    
           The NFS client can choose any source port value for its sockets,
           but usually chooses a _privileged_ port.  A privileged port is a
           port value less than 1024.  Only a process with root privileges
           may create a socket with a privileged source port.
    
           The exact range of privileged source ports that can be chosen is
           set by a pair of sysctls to avoid choosing a well-known port, such
           as the port used by ssh.  This means the number of source ports
           available for the NFS client, and therefore the number of socket
           connections that can be used at the same time, is practically
           limited to only a few hundred.
    
           As described above, the traditional default NFS authentication
           scheme, known as AUTH_SYS, relies on sending local UID and GID
           numbers to identify users making NFS requests.  An NFS server
           assumes that if a connection comes from a privileged port, the UID
           and GID numbers in the NFS requests on this connection have been
           verified by the client's kernel or some other local authority.
           This is an easy system to spoof, but on a trusted physical network
           between trusted hosts, it is entirely adequate.
    
           Roughly speaking, one socket is used for each NFS mount point.  If
           a client could use non-privileged source ports as well, the number
           of sockets allowed, and thus the maximum number of concurrent
           mount points, would be much larger.
    
           Using non-privileged source ports may compromise server security
           somewhat, since any user on AUTH_SYS mount points can now pretend
           to be any other when making NFS requests.  Thus NFS servers do not
           support this by default.  They explicitly allow it usually via an
           export option.
    
           To retain good security while allowing as many mount points as
           possible, it is best to allow non-privileged client connections
           only if the server and client both require strong authentication,
           such as Kerberos.
    
       **Mounting through a firewall**
           A firewall may reside between an NFS client and server, or the
           client or server may block some of its own ports via IP filter
           rules.  It is still possible to mount an NFS server through a
           firewall, though some of the mount(8) command's automatic service
           endpoint discovery mechanisms may not work; this requires you to
           provide specific endpoint details via NFS mount options.
    
           NFS servers normally run a portmapper or rpcbind daemon to
           advertise their service endpoints to clients. Clients use the
           rpcbind daemon to determine:
    
                  What network port each RPC-based service is using
    
                  What transport protocols each RPC-based service supports
    
           The rpcbind daemon uses a well-known port number (111) to help
           clients find a service endpoint.  Although NFS often uses a
           standard port number (2049), auxiliary services such as the NLM
           service can choose any unused port number at random.
    
           Common firewall configurations block the well-known rpcbind port.
           In the absence of an rpcbind service, the server administrator
           fixes the port number of NFS-related services so that the firewall
           can allow access to specific NFS service ports.  Client
           administrators then specify the port number for the mountd service
           via the mount(8) command's **mountport** option.  It may also be
           necessary to enforce the use of TCP or UDP if the firewall blocks
           one of those transports.
    
       **NFS Access Control Lists**
           Solaris allows NFS version 3 clients direct access to POSIX Access
           Control Lists stored in its local file systems.  This proprietary
           sideband protocol, known as NFSACL, provides richer access control
           than mode bits.  Linux implements this protocol for compatibility
           with the Solaris NFS implementation.  The NFSACL protocol never
           became a standard part of the NFS version 3 specification,
           however.
    
           The NFS version 4 specification mandates a new version of Access
           Control Lists that are semantically richer than POSIX ACLs.  NFS
           version 4 ACLs are not fully compatible with POSIX ACLs; as such,
           some translation between the two is required in an environment
           that mixes POSIX ACLs and NFS version 4.
    


## THE REMOUNT OPTION



           Generic mount options such as **rw** and **sync** can be modified on NFS
           mount points using the **remount** option.  See mount(8) for more
           information on generic mount options.
    
           With few exceptions, NFS-specific options are not able to be
           modified during a remount.  The underlying transport or NFS
           version cannot be changed by a remount, for example.
    
           Performing a remount on an NFS file system mounted with the **noac**
           option may have unintended consequences.  The **noac** option is a
           combination of the generic option **sync** , and the NFS-specific
           option **actimeo=0**.
    
       **Unmounting after a remount**
           For mount points that use NFS versions 2 or 3, the NFS umount
           subcommand depends on knowing the original set of mount options
           used to perform the MNT operation.  These options are stored on
           disk by the NFS mount subcommand, and can be erased by a remount.
    
           To ensure that the saved mount options are not erased during a
           remount, specify either the local mount directory, or the server
           hostname and export pathname, but not both, during a remount.  For
           example,
    
                   mount -o remount,ro /mnt
    
           merges the mount option **ro** with the mount options already saved on
           disk for the NFS server mounted at /mnt.
    


## FILES



           _/etc/fstab_
                  file system table
    
           _/etc/nfsmount.conf_
                  Configuration file for NFS mounts
    


## NOTES



           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 client's support for protocol versions depend on whether
           the kernel was built with options CONFIG_NFS_V2, CONFIG_NFS_V3,
           CONFIG_NFS_V4, CONFIG_NFS_V4_1, and CONFIG_NFS_V4_2.
    


* * *