nfs(5) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | MOUNT OPTIONS | nfs4 FILE SYSTEM TYPE | MOUNT CONFIGURATION FILE | EXAMPLES | TRANSPORT METHODS | DATA AND METADATA COHERENCE | SECURITY CONSIDERATIONS | THE REMOUNT OPTION | FILES | NOTES | SEE ALSO | COLOPHON

NFS(5)                     File Formats Manual                    NFS(5)

NAME         top

       nfs - fstab format and options for the nfs file systems

SYNOPSIS         top

       /etc/fstab

DESCRIPTION         top

       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 2,
       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         top

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

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

       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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       acdirmin=n
              The minimum time (in seconds) that the NFS client caches
              attributes of a directory before it requests fresh
              attribute information from a server.  If this option is
              not specified, the NFS client uses a 30-second minimum.
              See the DATA AND METADATA COHERENCE section for a full
              discussion of attribute caching.

       acdirmax=n
              The maximum time (in seconds) that the NFS client caches
              attributes of a directory before it requests fresh
              attribute information from a server.  If this option is
              not specified, the NFS client uses a 60-second maximum.
              See the DATA AND METADATA COHERENCE section for a full
              discussion of attribute caching.

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

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

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

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

       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
              READDIRPLUS requests on NFS v3 or v4 mounts to read small
              directories.  Some applications perform better if the
              client uses only READDIR requests for all directories.

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

              Note that this only affects how many retries are made and
              doesn't affect the delay caused by each retry.  For UDP
              each retry takes the time determined by the timeo and
              retrans options, which by default will be about 7 seconds.
              For TCP the default is 3 minutes, but system TCP
              connection timeouts will sometimes limit the timeout of
              each retransmission to around 2 minutes.

       sec=flavors
              A colon-separated list of one or more security flavors to
              use for accessing files on the mounted export. If the
              server does not support any of these flavors, the mount
              operation fails.  If sec= is not specified, the client
              attempts to find a security flavor that both the client
              and the server supports.  Valid flavors are none, sys,
              krb5, krb5i, and krb5p.  Refer to the SECURITY
              CONSIDERATIONS section for details.

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

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

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

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

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

              Refer to the SECURITY CONSIDERATIONS section for important
              details.

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

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

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

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

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

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

       fsc / nofsc
              Enable/Disables the cache of (read-only) data pages to the
              local disk using the FS-Cache facility. See cachefilesd(8)
              and <kernel_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.

   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 indentification
              string which is compatible with legacy NFS servers.  This
              is also the behavior if neither option is specified.  A
              client's open and lock state cannot be migrated
              transparently when it identifies itself via a traditional
              identification string.

              This mount option has no effect with NFSv4 minor versions
              newer than zero, which always use TSM-compatible client
              identification strings.

nfs4 FILE SYSTEM TYPE         top

       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         top

       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         top

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

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

               server:/export  /mnt  nfs   defaults                      0 0

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

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

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

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

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

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

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

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

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

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

TRANSPORT METHODS         top

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Using NFS over UDP on high-speed links
       Using NFS over UDP on high-speed links such as Gigabit can cause
       silent data corruption.

       The problem can be triggered at high loads, and is caused by
       problems in IP fragment reassembly. NFS read and writes typically
       transmit UDP packets of 4 Kilobytes or more, which have to be
       broken up into several fragments in order to be sent over the
       Ethernet link, which limits packets to 1500 bytes by default.
       This process happens at the IP network layer and is called
       fragmentation.

       In order to identify fragments that belong together, IP assigns a
       16bit IP ID value to each packet; fragments generated from the
       same UDP packet will have the same IP ID. The receiving system
       will collect these fragments and combine them to form the
       original UDP packet. This process is called reassembly. The
       default timeout for packet reassembly is 30 seconds; if the
       network stack does not receive all fragments of a given packet
       within this interval, it assumes the missing fragment(s) got lost
       and discards those it already received.

       The problem this creates over high-speed links is that it is
       possible to send more than 65536 packets within 30 seconds. In
       fact, with heavy NFS traffic one can observe that the IP IDs
       repeat after about 5 seconds.

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

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

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

       It is therefore strongly recommended to use NFS over TCP where
       possible, since TCP does not perform fragmentation.

       If you absolutely have to use NFS over UDP over Gigabit Ethernet,
       some steps can be taken to mitigate the problem and reduce the
       probability of corruption:

       Jumbo frames:  Many Gigabit network cards are capable of
                      transmitting frames bigger than the 1500 byte
                      limit of traditional Ethernet, typically 9000
                      bytes. Using jumbo frames of 9000 bytes will allow
                      you to run NFS over UDP at a page size of 8K
                      without fragmentation. Of course, this is only
                      feasible if all involved stations support jumbo
                      frames.

                      To enable a machine to send jumbo frames on cards
                      that support it, it is sufficient to configure the
                      interface for a MTU value of 9000.

       Lower reassembly timeout:
                      By lowering this timeout below the time it takes
                      the IP ID counter to wrap around, incorrect
                      reassembly of fragments can be prevented as well.
                      To do so, simply write the new timeout value (in
                      seconds) to the file
                      /proc/sys/net/ipv4/ipfrag_time.

                      A value of 2 seconds will greatly reduce the
                      probability of IPID clashes on a single Gigabit
                      link, while still allowing for a reasonable
                      timeout when receiving fragmented traffic from
                      distant peers.

DATA AND METADATA COHERENCE         top

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

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

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

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

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

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

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

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

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

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

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

SECURITY CONSIDERATIONS         top

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

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

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

THE REMOUNT OPTION         top

       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         top

       /etc/fstab
              file system table

       /etc/nfsmount.conf
              Configuration file for NFS mounts

NOTES         top

       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.

SEE ALSO         top

       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 7530 for the NFS version 4.0 specification.
       RFC 5661 for the NFS version 4.1 specification.
       RFC 7862 for the NFS version 4.2 specification.

COLOPHON         top

       This page is part of the nfs-utils (NFS utilities) project.
       Information about the project can be found at 
       ⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩.  If you have a
       bug report for this manual page, see
       ⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩.  This page was
       obtained from the project's upstream Git repository
       ⟨http://git.linux-nfs.org/?p=steved/nfs-utils.git;a=summary⟩ on
       2021-08-27.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-08-21.)  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to man-pages@man7.org

                             9 October 2012                       NFS(5)

Pages that refer to this page: pmdanfsclient(1)flock(2)filesystems(5)nfsmount.conf(5)systemd.mount(5)blkmapd(8)mount(8)mountd(8)mount.nfs(8)nfsstat(8)rpcdebug(8)rpc.rquotad(8)sm-notify(8)statd(8)umount.nfs(8)