.\"@(#)nfs.5"
-.TH NFS 5 "2 November 2007"
+.TH NFS 5 "9 October 2012"
.SH NAME
nfs \- fstab format and options for the
.B nfs
-and
-.B nfs4
file systems
.SH SYNOPSIS
.I /etc/fstab
file describes how
.BR mount (8)
should assemble a system's file name hierarchy
-from various independent file systems
+from various independent file systems
(including file systems exported by NFS servers).
Each line in the
.I /etc/fstab
The fifth and sixth fields on each line are not used
by NFS, thus conventionally each contain the digit zero. For example:
.P
-.SP
-.NF
-.TA 2.5i +0.75i +0.75i +1.0i
+.nf
+.ta 8n +14n +14n +9n +20n
server:path /mountpoint fstype option,option,... 0 0
-.FI
+.fi
.P
The server's hostname and export pathname
are separated by a colon, while
-the mount options are separated by commas. The remaining fields
+the mount options are separated by commas. The remaining fields
are separated by blanks or tabs.
+.P
The server's hostname can be an unqualified hostname,
a fully qualified domain name,
-or a dotted quad IPv4 address.
+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
+.BR ipv6 (7)
+for details on specifying raw IPv6 addresses.
+.P
The
.I fstype
-field contains either "nfs" (for version 2 or version 3 NFS mounts)
-or "nfs4" (for NFS version 4 mounts).
-The
-.B nfs
-and
-.B nfs4
-file system types share similar mount options,
-which are described below.
+field contains "nfs". Use of the "nfs4" fstype in
+.I /etc/fstab
+is deprecated.
.SH "MOUNT OPTIONS"
-Refer to
+Refer to
.BR 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
+available for all file systems. If you do not need to
+specify any mount options, use the generic option
.B defaults
in
.IR /etc/fstab .
-.
.DT
-.SS "Valid options for either the nfs or nfs4 file system type"
-These options are valid to use when mounting either
-.B nfs
-or
-.B nfs4
-file system types.
-They imply the same behavior
-and have the same default for both file system types.
+.SS "Options supported by all versions"
+These options are valid to use with any NFS version.
.TP 1.5i
.BR soft " / " hard
Determines the recovery behavior of the NFS client
If the
.B soft
option is specified, then the NFS client fails an NFS request
-after
+after
.B retrans
retransmissions have been sent,
causing the NFS client to return an error
option.
.TP 1.5i
.BI timeo= n
-The time (in tenths of a second) the NFS client waits for a
-response before it retries an NFS request. If this
-option is not specified, requests are retried after
-60 seconds for NFS over TCP, and are retried after 7/10 of a second for
-NFS over UDP.
+The time in deciseconds (tenths of a second) the NFS client waits for a
+response before it retries an NFS request.
.IP
-For NFS over TCP, the client uses a fixed timeout, as specified by the
+For NFS over TCP the default
.B timeo
-option. However, for NFS over UDP, the client uses an adaptive
+value is 600 (60 seconds).
+The NFS client performs linear backoff: After each retransmission the
+timeout is increased by
+.BR timeo
+up to the maximum of 600 seconds.
+.IP
+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
+request types (such as READ and WRITE requests), but uses the
.B timeo
setting for infrequently used request types (such as FSINFO requests).
-After each retransmission, the NFS client doubles the timeout for that
-request, up to a maximum timeout length of 60 seconds.
+If the
+.B 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.
.TP 1.5i
.BI retrans= n
The number of times the NFS client retries a request before
-it attempts further recovery action. If the
+it attempts further recovery action. If the
.B retrans
-option is not specified, the NFS client retries each request
+option is not specified, the NFS client tries each request
three times.
.IP
The NFS client generates a "server not responding" message
-after
+after
.B retrans
retries, then attempts further recovery (depending on whether the
.B hard
The
.B rsize
value is a positive integral multiple of 1024.
-Specified
+Specified
.B 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
+range but not a multiple of 1024, it is rounded down to the nearest
multiple of 1024.
.IP
If an
.B rsize
-value is not specified, or if the specified
-.B rsize
+value is not specified, or if the specified
+.B rsize
value is larger than the maximum that either client or server can support,
the client and server negotiate the largest
.B rsize
-value that they can both support.
+value that they can both support.
.IP
The
.B rsize
.BI 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
+on an NFS server. The actual data payload size of each
NFS WRITE request is equal to
or smaller than the
.B wsize
Similar to
.B rsize
, the
-.B wsize
+.B wsize
value is a positive integral multiple of 1024.
-Specified
+Specified
.B 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
+range but not a multiple of 1024, it is rounded down to the nearest
multiple of 1024.
.IP
If a
.B wsize
-value is not specified, or if the specified
-.B wsize
+value is not specified, or if the specified
+.B wsize
value is larger than the maximum that either client or server can support,
the client and server negotiate the largest
.B wsize
.B wsize
value negotiated by the client and server is reported in the
.I /proc/mounts
-file.
+file.
.TP 1.5i
.BR ac " / " noac
-Selects whether the client may cache file attributes. If neither
-option is specified (or if
+Selects whether the client may cache file attributes. If neither
+option is specified (or if
.B ac
-is specified), the client caches file
-attributes.
+is specified), the client caches file
+attributes.
.IP
-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
+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
.B noac
-option prevents clients from caching file
-attributes so that applications can more quickly detect file changes
+option prevents clients from caching file
+attributes so that applications can more quickly detect file changes
on the server.
.IP
-In addition to preventing the client from caching file attributes,
-the
+In addition to preventing the client from caching file attributes,
+the
.B 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
+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.
.IP
Using the
.BR automount (8)
for details).
.TP 1.5i
+.BR 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.
+.TP 1.5i
.BI retry= n
The number of minutes that the
.BR 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).
+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
+.BR mount (8)
+command exits immediately after the first failure.
.TP 1.5i
-.BI sec= mode
-The RPCGSS security flavor to use for accessing files on this mount point.
-If the
-.B sec
-option is not specified, or if
-.B sec=sys
-is specified, the NFS client uses the AUTH_SYS security flavor
-for all NFS requests on this mount point.
-Valid security flavors are
+.BI sec= flavor
+The security flavor to use for accessing files on this mount point.
+If the server does not support this flavor, the mount operation fails.
+If
+.B sec=
+is not specified, the client attempts to find
+a security flavor that both the client and the server supports.
+Valid
+.I flavors
+are
.BR none ,
.BR sys ,
.BR krb5 ,
.BR krb5i ,
-.BR krb5p ,
-.BR lkey ,
-.BR lkeyi ,
-.BR lkeyp ,
-.BR spkm ,
-.BR spkmi ,
and
-.BR spkmp .
+.BR krb5p .
Refer to the SECURITY CONSIDERATIONS section for details.
.TP 1.5i
.BR 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
+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.
.IP
-If neither option is specified, or if the
+If neither option is specified, or if the
.B sharecache
-option is
-specified, then a single cache is used for all mount points that
-access the same export. If the
+option is
+specified, then a single cache is used for all mount points that
+access the same export. If the
.B 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
+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.
.IP
As of kernel 2.6.18, the behavior specified by
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.
-.SS "Valid options for the nfs file system type"
+.TP 1.5i
+.BR 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
+.B resvport
+option is specified, the NFS client uses a privileged source port.
+If the
+.B noresvport
+option is specified, the NFS client uses a non-privileged source port.
+This option is supported in kernels 2.6.28 and later.
+.IP
+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.
+.IP
+Refer to the SECURITY CONSIDERATIONS section for important details.
+.TP 1.5i
+.BI lookupcache= mode
+Specifies how the kernel manages its cache of directory entries
+for a given mount point.
+.I mode
+can be one of
+.BR all ,
+.BR none ,
+.BR pos ,
+or
+.BR positive .
+This option is supported in kernels 2.6.28 and later.
+.IP
+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
+.IR positive .
+If the requested directory entry does not exist on the server,
+the result is referred to as
+.IR negative .
+.IP
+If this option is not specified, or if
+.B all
+is specified, the client assumes both types of directory cache entries
+are valid until their parent directory's cached attributes expire.
+.IP
+If
+.BR 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.
+.IP
+If
+.B 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.
+.IP
+The DATA AND METADATA COHERENCE section contains a
+detailed discussion of these trade-offs.
+.TP 1.5i
+.BR fsc " / " nofsc
+Enable/Disables the cache of (read-only) data pages to the local disk
+using the FS-Cache facility. See cachefilesd(8)
+and <kernel_soruce>/Documentation/filesystems/caching
+for detail on how to configure the FS-Cache facility.
+Default value is nofsc.
+.SS "Options for NFS versions 2 and 3 only"
Use these options, along with the options in the above subsection,
-for mounting the
-.B nfs
-file system type.
+for NFS versions 2 and 3 only.
.TP 1.5i
.BI proto= netid
-The transport protocol used by the NFS client
-to transmit requests to the NFS server for this mount point.
+The
.I netid
-can be either
-.B udp
-or
-.BR tcp .
+determines the transport that is used to communicate with the NFS
+server. Available options are
+.BR udp ", " udp6 ", "tcp ", " tcp6 ", and " rdma .
+Those which end in
+.B 6
+use IPv6 addresses and are only available if support for TI-RPC is
+built in. Others use IPv4 addresses.
+.IP
Each transport protocol uses different default
.B retrans
and
.B timeo
-settings; refer to the description of these two mount options for details.
+settings.
+Refer to the description of these two mount options for details.
.IP
In addition to controlling how the NFS client transmits requests to
the server, this mount option also controls how the
.BR mount (8)
command communicates with the server's rpcbind and mountd services.
-Specifying
-.B proto=tcp
-forces all traffic from the
+Specifying a netid that uses TCP forces all traffic from the
.BR mount (8)
command and the NFS client to use TCP.
-Specifying
-.B proto=udp
-forces all traffic types to use UDP.
+Specifying a netid that uses UDP forces all traffic types to use UDP.
+.IP
+.B Before using NFS over UDP, refer to the TRANSPORT METHODS section.
.IP
If the
.B proto
option is an alternative to specifying
.BR proto=udp.
It is included for compatibility with other operating systems.
+.IP
+.B Before using NFS over UDP, refer to the TRANSPORT METHODS section.
.TP 1.5i
.B tcp
The
.BR proto=tcp.
It is included for compatibility with other operating systems.
.TP 1.5i
+.B rdma
+The
+.B rdma
+option is an alternative to specifying
+.BR proto=rdma.
+.TP 1.5i
.BI 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,
This option can be used when mounting an NFS server
through a firewall that blocks the rpcbind protocol.
.TP 1.5i
+.BI 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.
+.IP
+.I netid
+may be one of
+.BR udp ", and " tcp
+which use IPv4 address or, if TI-RPC is built into the
+.B mount.nfs
+command,
+.BR udp6 ", and " tcp6
+which use IPv6 addresses.
+.IP
+This option can be used when mounting an NFS server
+through a firewall that blocks a particular transport.
+When used in combination with the
+.B 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.
+.IP
+Refer to the TRANSPORT METHODS section for more on how the
+.B mountproto
+mount option interacts with the
+.B proto
+mount option.
+.TP 1.5i
.BI mounthost= name
The hostname of the host running mountd.
If this option is not specified, the
.TP 1.5i
.BI nfsvers= n
The NFS protocol version number used to contact the server's NFS service.
-The Linux client supports version 2 and version 3 of the NFS protocol
-when using the file system type
-.BR nfs .
-If the server does not support the requested version,
-the mount request fails.
-If this option is not specified, the client attempts to use version 3,
-but negotiates the NFS version with the server if version 3 support
-is not available.
+If the server does not support the requested version, the mount request fails.
+If this option is not specified, the client negotiates a suitable version with
+the server, trying version 4 first, version 3 second, and version 2 last.
.TP 1.5i
.BI vers= n
This option is an alternative to the
.TP 1.5i
.BR lock " / " nolock
Selects whether to use the NLM sideband protocol to lock files on the server.
-If neither option is specified (or if
-.B lock
-is specified), NLM locking is used for this mount point.
+If neither option is specified (or if
+.B lock
+is specified), NLM locking is used for this mount point.
When using the
.B nolock
option, applications can lock files,
.TP 1.5i
.BR intr " / " nointr
Selects whether to allow signals to interrupt file operations
-on this mount point. If neither option
-is specified (or if
+on this mount point. If neither option
+is specified (or if
.B nointr
is specified),
signals do not interrupt NFS file operations. If
-.B intr
-is specified, system calls return EINTR if an in-progress NFS operation is interrupted by
+.B intr
+is specified, system calls return EINTR if an in-progress NFS operation is interrupted by
a signal.
.IP
Using the
option is preferred to using the
.B soft
option because it is significantly less likely to result in data corruption.
+.IP
+The
+.BR intr " / " nointr
+mount option is deprecated after kernel 2.6.25.
+Only SIGKILL can interrupt a pending NFS operation on these kernels,
+and if specified, this mount option is ignored to provide backwards
+compatibility with older kernels.
.TP 1.5i
.BR cto " / " nocto
Selects whether to use close-to-open cache coherence semantics.
-If neither option is specified (or if
+If neither option is specified (or if
.B cto
is specified), the client uses close-to-open
-cache coherence semantics. If the
-.B nocto
+cache coherence semantics. If the
+.B nocto
option is specified, the client uses a non-standard heuristic to determine when
-files on the server have changed.
+files on the server have changed.
.IP
Using the
.B nocto
.BR 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
+implemented in Solaris that manages Access Control Lists. NFSACL was never
made a standard part of the NFS protocol specification.
.IP
-If neither
+If neither
.B acl
-nor
-.B noacl
+nor
+.B noacl
option is specified,
the NFS client negotiates with the server
to see if the NFSACL protocol is supported,
if the negotiation causes problems on the client or server.
Refer to the SECURITY CONSIDERATIONS section for more details.
.TP 1.5i
-.BR rdirplus " / " nordirplus
-Selects whether to use NFS version 3 READDIRPLUS requests.
-If this option is not specified, the NFS client uses READDIRPLUS requests
-on NFS version 3 mounts to read small directories.
-Some applications perform better if the client uses only READDIR requests
-for all directories.
-.SS "Valid options for the nfs4 file system type"
+.BR local_lock= mechanism
+Specifies whether to use local locking for any or both of the flock and the
+POSIX locking mechanisms.
+.I mechanism
+can be one of
+.BR all ,
+.BR flock ,
+.BR posix ,
+or
+.BR none .
+This option is supported in kernels 2.6.37 and later.
+.IP
+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.
+.IP
+If this option is not specified, or if
+.B none
+is specified, the client assumes that the locks are not local.
+.IP
+If
+.BR all
+is specified, the client assumes that both flock and POSIX locks are local.
+.IP
+If
+.BR 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.
+.IP
+If
+.BR posix
+is specified, the client assumes that POSIX locks are local and uses NLM
+sideband protocol to lock files when flock locks are used.
+.IP
+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.
+.IP
+NOTE: When used together, the 'local_lock' mount option will be overridden
+by 'nolock'/'lock' mount option.
+.SS "Options for NFS version 4 only"
Use these options, along with the options in the first subsection above,
-for mounting the
-.B nfs4
-file system type.
+for NFS version 4 and newer.
.TP 1.5i
.BI proto= netid
-The transport protocol used by the NFS client
-to transmit requests to the NFS server for this mount point.
+The
.I netid
-can be either
-.B udp
-or
-.BR tcp .
+determines the transport that is used to communicate with the NFS
+server. Supported options are
+.BR tcp ", " tcp6 ", and " rdma .
+.B tcp6
+use IPv6 addresses and is only available if support for TI-RPC is
+built in. Both others use IPv4 addresses.
+.IP
All NFS version 4 servers are required to support TCP,
-so if this mount option is not specified, the NFS version 4 client
-uses the TCP transport protocol.
+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.
.TP 1.5i
.BI port= n
.TP 1.5i
.BR intr " / " nointr
Selects whether to allow signals to interrupt file operations
-on this mount point. If neither option is specified (or if
-.B intr
-is specified), system calls return EINTR if an in-progress NFS operation
-is interrupted by a signal. If
+on this mount point. If neither option is specified (or if
+.B intr
+is specified), system calls return EINTR if an in-progress NFS operation
+is interrupted by a signal. If
.B nointr
-is specified, signals do not
+is specified, signals do not
interrupt NFS operations.
.IP
Using the
option is preferred to using the
.B soft
option because it is significantly less likely to result in data corruption.
+.IP
+The
+.BR intr " / " nointr
+mount option is deprecated after kernel 2.6.25.
+Only SIGKILL can interrupt a pending NFS operation on these kernels,
+and if specified, this mount option is ignored to provide backwards
+compatibility with older kernels.
.TP 1.5i
.BR cto " / " nocto
Selects whether to use close-to-open cache coherence semantics
If neither
.B cto
nor
-.B nocto
+.B nocto
is specified,
the default is to use close-to-open cache coherence
semantics for directories.
the behavior of this option in more detail.
.TP 1.5i
.BI clientaddr= n.n.n.n
-Specifies a single IPv4 address (in dotted-quad form)
-that the NFS client advertises to allow servers
-to perform NFS version 4 callback requests against
-files on this mount point. If the server is unable to
-establish callback connections to clients, performance
+.TP 1.5i
+.BI clientaddr= n:n: ... :n
+Specifies a single IPv4 address (in dotted-quad form),
+or a non-link-local IPv6 address,
+that the NFS client advertises to allow servers
+to perform NFS version 4 callback requests against
+files on this mount point. If the server is unable to
+establish callback connections to clients, performance
may degrade, or accesses to files may temporarily hang.
.IP
If this option is not specified, the
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.
+the exact address to use for callbacks may be nontrivial to determine.
+.SH nfs4 FILE SYSTEM TYPE
+The
+.BR 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
+.B nfsvers
+mount option.
+.SH 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
+.I /etc/nfsmount.conf
+file. See
+.BR nfsmount.conf(5)
+for details.
.SH EXAMPLES
To mount an export using NFS version 2,
use the
.B nfsvers=3
mount option.
To mount using NFS version 4,
-use the
-.B nfs4
-file system type.
-The
-.B nfsvers
-mount option is not supported for the
+use either the
+.B nfs
+file system type, with the
+.B nfsvers=4
+mount option, or the
.B nfs4
file system type.
.P
file causes the mount command to negotiate
reasonable defaults for NFS behavior.
.P
-.NF
-.TA 2.5i +0.7i +0.7i +.7i
+.nf
+.ta 8n +16n +6n +6n +30n
server:/export /mnt nfs defaults 0 0
-.FI
+.fi
.P
Here is an example from an /etc/fstab file for an NFS version 2 mount over UDP.
.P
-.NF
-.TA 2.5i +0.7i +0.7i +.7i
+.nf
+.ta 8n +16n +6n +6n +30n
server:/export /mnt nfs nfsvers=2,proto=udp 0 0
-.FI
+.fi
.P
Try this example to mount using NFS version 4 over TCP
with Kerberos 5 mutual authentication.
.P
-.NF
-.TA 2.5i +0.7i +0.7i +.7i
+.nf
+.ta 8n +16n +6n +6n +30n
server:/export /mnt nfs4 sec=krb5 0 0
-.FI
+.fi
.P
This example can be used to mount /usr over NFS.
.P
-.NF
-.TA 2.5i +0.7i +0.7i +.7i
+.nf
+.ta 8n +16n +6n +6n +30n
server:/export /usr nfs ro,nolock,nocto,actimeo=3600 0 0
-.FI
+.fi
+.P
+This example shows how to mount an NFS server
+using a raw IPv6 link-local address.
+.P
+.nf
+.ta 8n +40n +5n +4n +9n
+ [fe80::215:c5ff:fb3e:e2b1%eth0]:/export /mnt nfs defaults 0 0
+.fi
.SH "TRANSPORT METHODS"
NFS clients send requests to NFS servers via
Remote Procedure Calls, or
In some cases, however, it pays to specify
these settings explicitly using mount options.
.P
-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
+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.
.P
-However, UDP can be quite effective in specialized settings where
-the network’s MTU is large relative to NFS’s data transfer size (such
-as network environments that enable jumbo Ethernet frames). In such
-environments, trimming the
-.B rsize
-and
-.B 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
+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
+.B rsize
+and
+.B 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.
.P
-TCP is the default transport protocol used for all modern NFS
+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
+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.
.P
Under normal circumstances, networks drop packets much more
.B 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,
+TCP itself makes network data transfer reliable,
.B rsize
-and
+and
.B wsize
-can safely be allowed to default to the largest values supported by
+can safely be allowed to default to the largest values supported by
both client and server, independent of the network's MTU size.
+.SS "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.
+.P
+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
+.BR proto ,
+.BR mountproto ,
+.BR udp ", and " tcp .
+.P
+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.
+.P
+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.
+.P
+If the server does not support these transports for these services, the
+.BR 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
+.B bg
+option is in effect, the mount command backgrounds itself and continues
+to attempt the specified mount request.
+.P
+When the
+.B proto
+option, the
+.B udp
+option, or the
+.B tcp
+option is specified but the
+.B mountproto
+option is not, the specified transport is used to contact
+both the server's mountd service and for the NLM and NFS services.
+.P
+If the
+.B mountproto
+option is specified but none of the
+.BR 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.
+.P
+If both the
+.BR mountproto " and " proto
+(or
+.BR udp " or " tcp )
+options are specified, then the transport specified by the
+.B mountproto
+option is used for the initial mountd request, and the transport
+specified by the
+.B proto
+option (or the
+.BR 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.
+.P
+If any of the
+.BR proto ", " udp ", " tcp ", "
+or
+.B 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.
+.SS "Using NFS over UDP on high-speed links"
+Using NFS over UDP on high-speed links such as Gigabit
+.BR "can cause silent data corruption" .
+.P
+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.
+.P
+In order to identify fragments that belong together, IP assigns a 16bit
+.I 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.
+.P
+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.
+.P
+This has serious effects on reassembly: if one fragment gets lost,
+another fragment
+.I from a different packet
+but with the
+.I 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.
+.P
+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.
+.P
+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.
+.P
+It is therefore strongly recommended to use
+.BR "NFS over TCP where possible" ,
+since TCP does not perform fragmentation.
+.P
+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:
+.TP +1.5i
+.I 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.
+.IP
+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.
+.TP +1.5i
+.I 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
+.BR /proc/sys/net/ipv4/ipfrag_time .
+.IP
+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.
.SH "DATA AND METADATA COHERENCE"
Some modern cluster file systems provide
perfect cache coherence among their clients.
Perfect cache coherence among disparate NFS clients
is expensive to achieve, especially on wide area networks.
-As such, NFS settles for weaker cache coherence that
-satisfies the requirements of most file sharing types. Normally,
+As such, NFS settles for weaker cache coherence that
+satisfies the requirements of most file sharing types. Normally,
file sharing is completely sequential:
first client A opens a file, writes something to it, then closes it;
then client B opens the same file, and reads the changes.
that client's updates or some other client's updates
that altered the file.
.SS "Attribute caching"
-Use the
+Use the
.B noac
mount option to achieve attribute cache coherence
among multiple clients.
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
+applications should use file locking. Alternatively, applications
can also open their files with the O_DIRECT flag
to disable data caching entirely.
+.SS "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
+.IR 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
+.IR negative " lookup result.
+.P
+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
+.BR acdirmin ", " acdirmax ", and " noac
+mount options for more information about
+how long a directory's mtime is cached.
+.P
+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
+.B lookupcache
+mount option allows some tuning of directory entry caching behavior.
+.P
+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
+.BR lookupcache=positive .
+.P
+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
+.BR 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
+.BR noac ,
+and has no effect on how the NFS client caches the attributes of files.
+.P
.SS "The sync mount option"
The NFS client treats the
.B sync
mount option differently than some other file systems
-(refer to
+(refer to
.BR mount (8)
for a description of the generic
.B sync
.B sync
nor
.B async
-is specified (or if the
-.B async
+is specified (or if the
+.B async
option is specified),
the NFS client delays sending application
writes to the server
-until any of these events occur:
+until any of these events occur:
.IP
Memory pressure forces reclamation of system memory resources.
.IP
-An application flushes file data explicitly with
+An application flushes file data explicitly with
.BR sync (2),
.BR msync (2),
or
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
-.B sync
+.B sync
mount option.
.SS "Using file locks with NFS"
The Network Lock Manager protocol is a separate sideband protocol
.B nolock
option when using NFS to mount
.I /var
-because
-.I /var
+because
+.I /var
contains files used by the NLM implementation on Linux.
.P
Specifying the
.B 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.
+and uses file locks extensively.
.SS "NFS version 4 caching features"
The data and metadata caching behavior of NFS version 4
clients is similar to that of earlier versions.
In addition to combining these sideband protocols with the main NFS protocol,
NFS version 4 introduces more advanced forms of access control,
authentication, and in-transit data protection.
-The NFS version 4 specification mandates NFSv4 ACLs,
-RPCGSS authentication, and RPCGSS security flavors
+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
+Because NFS version 4 combines the
function of the sideband protocols into the main NFS protocol,
the new security features apply to all NFS version 4 operations
including mounting, file locking, and so on.
RPCGSS authentication can also be used with NFS versions 2 and 3,
-but does not protect their sideband protocols.
+but it does not protect their sideband protocols.
.P
The
.B sec
-mount option specifies the RPCGSS security mode
+mount option specifies the security flavor
that is in effect on a given NFS mount point.
Specifying
.B sec=krb5
provides cryptographic proof of a user's identity in each RPC request.
-This provides strong verification of the identity of users
+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
+Refer to the
.BR rpc.gssd (8)
man page for details.
.P
to prevent data exposure during network transit; however,
expect some performance impact
when using integrity checking or encryption.
-Similar support for other forms of cryptographic security (such as lipkey and SPKM3)
+Similar support for other forms of cryptographic security
is also available.
.P
The NFS version 4 protocol allows
-clients and servers to negotiate among multiple security flavors
-during mount processing.
-However, Linux does not yet implement such negotiation.
-The Linux client specifies a single security flavor at mount time
-which remains in effect for the lifetime of the mount.
-If the server does not support this flavor,
-the initial mount request is rejected by the server.
+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.
+.P
+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.
+.SS "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.
+.P
+The NFS client can choose any source port value for its sockets,
+but usually chooses a
+.I 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.
+.P
+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.
+.P
+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.
+.P
+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.
+.P
+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.
+.P
+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.
.SS "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
It is still possible to mount an NFS server through a firewall,
though some of the
.BR mount (8)
-command's automatic service endpoint discovery mechanisms may not work; this
+command's automatic service endpoint discovery mechanisms may not work; this
requires you to provide specific endpoint details via NFS mount options.
.P
NFS servers normally run a portmapper or rpcbind daemon to advertise
-their service endpoints to clients. Clients use the rpcbind daemon to determine:
+their service endpoints to clients. Clients use the rpcbind daemon to determine:
.IP
What network port each RPC-based service is using
.IP
-What transport protocols each RPC-based service supports
+What transport protocols each RPC-based service supports
.P
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),
.P
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,
+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.
-.SH FILES
-.TP 1.5i
-.I /etc/fstab
-file system table
-.SH BUGS
-The generic
-.B remount
-option is not fully supported.
-Generic options, such as
-.BR rw " and " ro
-can be modified using the
-.B remount
-option,
-but NFS-specific options are not all supported.
+in an environment that mixes POSIX ACLs and NFS version 4.
+.SH "THE REMOUNT OPTION"
+Generic mount options such as
+.BR rw " and " sync
+can be modified on NFS mount points using the
+.BR remount
+option.
+See
+.BR mount (8)
+for more information on generic mount options.
+.P
+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.
+.P
Performing a remount on an NFS file system mounted with the
.B noac
option may have unintended consequences.
The
.B noac
-option is a mixture of a generic option,
+option is a combination of the generic option
.BR sync ,
-and an NFS-specific option
+and the NFS-specific option
.BR actimeo=0 .
+.SS "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.
+.P
+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,
+.P
+.nf
+.ta 8n
+ mount -o remount,ro /mnt
+.fi
.P
+merges the mount option
+.B ro
+with the mount options already saved on disk for the NFS server mounted at /mnt.
+.SH FILES
+.TP 1.5i
+.I /etc/fstab
+file system table
+.SH BUGS
Before 2.4.7, the Linux NFS client did not support NFS over TCP.
.P
Before 2.4.20, the Linux NFS client used a heuristic
.BR mount.nfs (5),
.BR umount.nfs (5),
.BR exports (5),
+.BR netconfig (5),
+.BR ipv6 (7),
.BR nfsd (8),
+.BR sm-notify (8),
+.BR rpc.statd (8),
.BR rpc.idmapd (8),
.BR rpc.gssd (8),
.BR rpc.svcgssd (8),
.br
RFC 2203 for the RPCSEC GSS API protocol specification.
.br
-RFC 3530 for the NFS version 4 specification.
+RFC 3530 for the NFS version 4 specification.
projects / nfs-utils.git / blobdiff