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NFS(5) File Formats Manual NFS(5)
NAME
nfs - fstab format and options for the nfs file systems
SYNOPSIS
/etc/fstab
DESCRIPTION
NFS is an Internet Standard protocol created by Sun Microsystems in 1984. NFS was devel-
oped 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
Refer to mount(8) for a description of generic mount options available for all file sys-
tems. 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 4.1
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 speci-
fied), NFS requests are retried indefinitely. If the soft option is speci-
fied, then the NFS client fails an NFS request after retrans retransmis-
sions 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.
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 esti-
mate an appropriate timeout value for frequently used request types (such
as READ and WRITE requests), but uses the timeo setting for infrequently
used request types (such as FSINFO requests). If the timeo option is not
specified, infrequently used request types are retried after 1.1 seconds.
After each retransmission, the NFS client doubles the timeout for that
request, up to a maximum timeout length of 60 seconds.
retrans=n The number of times the NFS client retries a request before it attempts
further recovery action. If the retrans option is not specified, the NFS
client tries each request three times.
The NFS client generates a "server not responding" message after retrans
retries, then attempts further recovery (depending on whether the hard
mount option is in effect).
rsize=n The maximum number of bytes in each network READ request that the NFS
client can receive when reading data from a file on an NFS server. The
actual data payload size of each NFS READ request is equal to or smaller
than the rsize setting. The largest read payload supported by the Linux NFS
client is 1,048,576 bytes (one megabyte).
The rsize value is a positive integral multiple of 1024. Specified rsize
values lower than 1024 are replaced with 4096; values larger than 1048576
are replaced with 1048576. If a specified value is within the supported
range but not a multiple of 1024, it is rounded down to the nearest multi-
ple 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 sup-
port.
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 pay-
load 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 sup-
port.
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 sec-
onds, 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).
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 opera-
tion 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.
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 sup-
ports. 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 con-
tents 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 ker-
nels 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 direc-
tory'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_soruce>/Documenta-
tion/filesystems/caching for detail on how to configure the FS-Cache facil-
ity. Default value is nofsc.
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, and rdma. 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 communi-
cates 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 adver-
tised port.
mountport=n The numeric value of the server's mountd port. If the server's mountd ser-
vice 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 unmount-
ing 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 speci-
fied 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, appli-
cations 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 nec-
essary 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, and rdma. 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 ver-
sioning," 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 directo-
ries.
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, perfor-
mance 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 net-
work 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 sup-
ports NFSv4 migration with TSM, specify the migration option.
Some server features misbehave in the face of a migration-compatible iden-
tification 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
The nfs4 file system type is an old syntax for specifying NFSv4 usage. It can still be
used with all NFSv4-specific and common options, excepted the nfsvers mount option.
MOUNT CONFIGURATION FILE
If the mount command is configured to do so, all of the mount options described in the
previous section can also be configured in the /etc/nfsmount.conf file. See nfs-
mount.conf(5) for details.
EXAMPLES
To mount an export using NFS version 2, use the nfs file system type and specify the
nfsvers=2 mount option. To mount using NFS version 3, use the nfs file system type and
specify the nfsvers=3 mount option. To mount using NFS version 4, use either the nfs file
system type, with the nfsvers=4 mount option, or the nfs4 file system type.
The following example from an /etc/fstab file causes the mount command to negotiate rea-
sonable 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
NFS clients send requests to NFS servers via Remote Procedure Calls, or RPCs. The RPC
client discovers remote service endpoints automatically, handles per-request authentica-
tion, 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 negoti-
ate 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 pre-
vent 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 net-
work 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 per-
forms well in almost every conceivable network environment and provides excellent guaran-
tees 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 trans-
port 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 speci-
fied, 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 trans-
port 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 net-
work layer and is called fragmentation.
In order to identify fragments that belong together, IP assigns a 16bit IP ID value to
each packet; fragments generated from the same UDP packet will have the same IP ID. The
receiving system will collect these fragments and combine them to form the original UDP
packet. This process is called reassembly. The default timeout for packet reassembly is 30
seconds; if the network stack does not receive all fragments of a given packet within this
interval, it assumes the missing fragment(s) got lost and discards those it already
received.
The problem this creates over high-speed links is that it is possible to send more than
65536 packets within 30 seconds. In fact, with heavy NFS traffic one can observe that the
IP IDs repeat after about 5 seconds.
This has serious effects on reassembly: if one fragment gets lost, another fragment from a
different packet but with the same IP ID will arrive within the 30 second timeout, and the
network stack will combine these fragments to form a new packet. Most of the time, network
layers above IP will detect this mismatched reassembly - in the case of UDP, the UDP
checksum, which is a 16 bit checksum over the entire packet payload, will usually not
match, and UDP will discard the bad packet.
However, the UDP checksum is 16 bit only, so there is a chance of 1 in 65536 that it will
match even if the packet payload is completely random (which very often isn't the case).
If that is the case, silent data corruption will occur.
This potential should be taken seriously, at least on Gigabit Ethernet. Network speeds of
100Mbit/s should be considered less problematic, because with most traffic patterns IP ID
wrap around will take much longer than 30 seconds.
It is therefore strongly recommended to use NFS over TCP where possible, since TCP does
not perform fragmentation.
If you absolutely have to use NFS over UDP over Gigabit Ethernet, some steps can be taken
to mitigate the problem and reduce the probability of corruption:
Jumbo frames: Many Gigabit network cards are capable of transmitting frames bigger than
the 1500 byte limit of traditional Ethernet, typically 9000 bytes. Using
jumbo frames of 9000 bytes will allow you to run NFS over UDP at a page
size of 8K without fragmentation. Of course, this is only feasible if all
involved stations support jumbo frames.
To enable a machine to send jumbo frames on cards that support it, it is
sufficient to configure the interface for a MTU value of 9000.
Lower reassembly timeout:
By lowering this timeout below the time it takes the IP ID counter to wrap
around, incorrect reassembly of fragments can be prevented as well. To do
so, simply write the new timeout value (in seconds) to the file
/proc/sys/net/ipv4/ipfrag_time.
A value of 2 seconds will greatly reduce the probability of IPID clashes on
a single Gigabit link, while still allowing for a reasonable timeout when
receiving fragmented traffic from distant peers.
DATA AND METADATA COHERENCE
Some modern cluster file systems provide perfect cache coherence among their clients.
Perfect cache coherence among disparate NFS clients is expensive to achieve, especially on
wide area networks. As such, NFS settles for weaker cache coherence that satisfies the
requirements of most file sharing types.
Close-to-open cache consistency
Typically file sharing is completely sequential. First client A opens a file, writes
something to it, then closes it. Then client B opens the same file, and reads the
changes.
When an application opens a file stored on an NFS version 3 server, the NFS client checks
that the file exists on the server and is permitted to the opener by sending a GETATTR or
ACCESS request. The NFS client sends these requests regardless of the freshness of the
file's cached attributes.
When the application closes the file, the NFS client writes back any pending changes to
the file so that the next opener can view the changes. This also gives the NFS client an
opportunity to report write errors to the application via the return code from close(2).
The behavior of checking at open time and flushing at close time is referred to as close-
to-open cache consistency, or CTO. It can be disabled for an entire mount point using the
nocto mount option.
Weak cache consistency
There are still opportunities for a client's data cache to contain stale data. The NFS
version 3 protocol introduced "weak cache consistency" (also known as WCC) which provides
a way of efficiently checking a file's attributes before and after a single request. This
allows a client to help identify changes that could have been made by other clients.
When a client is using many concurrent operations that update the same file at the same
time (for example, during asynchronous write behind), it is still difficult to tell
whether it was that client's updates or some other client's updates that altered the file.
Attribute caching
Use the noac mount option to achieve attribute cache coherence among multiple clients.
Almost every file system operation checks file attribute information. The client keeps
this information cached for a period of time to reduce network and server load. When noac
is in effect, a client's file attribute cache is disabled, so each operation that needs to
check a file's attributes is forced to go back to the server. This permits a client to
see changes to a file very quickly, at the cost of many extra network operations.
Be careful not to confuse the noac option with "no data caching." The noac mount option
prevents the client from caching file metadata, but there are still races that may result
in data cache incoherence between client and server.
The NFS protocol is not designed to support true cluster file system cache coherence with-
out 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 maintainence
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 stricta-
time/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 direc-
tory'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 inter-
fere 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 applica-
tion 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 cre-
ated 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 immedi-
ately 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 pro-
tocol, 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 advi-
sory 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 propri-
etary 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 ear-
lier versions. However, NFS version 4 adds two features that improve cache behavior:
change attributes and file delegation.
The change attribute is a new part of NFS file and directory metadata which tracks data
changes. It replaces the use of a file's modification and change time stamps as a way for
clients to validate the content of their caches. Change attributes are independent of the
time stamp resolution on either the server or client, however.
A file delegation is a contract between an NFS version 4 client and server that allows the
client to treat a file temporarily as if no other client is accessing it. The server
promises to notify the client (via a callback request) if another client attempts to
access that file. Once a file has been delegated to a client, the client can cache that
file's data and metadata aggressively without contacting the server.
File delegations come in two flavors: read and write. A read delegation means that the
server notifies the client about any other clients that want to write to the file. A
write delegation means that the client gets notified about either read or write accessors.
Servers grant file delegations when a file is opened, and can recall delegations at any
time when another client wants access to the file that conflicts with any delegations
already granted. Delegations on directories are not supported.
In order to support delegation callback, the server checks the network return path to the
client during the client's initial contact with the server. If contact with the client
cannot be established, the server simply does not grant any delegations to that client.
SECURITY CONSIDERATIONS
NFS servers control access to file data, but they depend on their RPC implementation to
provide authentication of NFS requests. Traditional NFS access control mimics the stan-
dard mode bit access control provided in local file systems. Traditional RPC authentica-
tion uses a number to represent each user (usually the user's own uid), a number to repre-
sent 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 lock-
ing, 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 that is in effect on a given NFS mount
point. Specifying sec=krb5 provides cryptographic proof of a user's identity in each RPC
request. This provides strong verification of the identity of users accessing data on the
server. Note that additional configuration besides adding this mount option is required
in order to enable Kerberos security. Refer to the rpc.gssd(8) man page for details.
Two additional flavors of Kerberos security are supported: krb5i and krb5p. The krb5i
security flavor provides a cryptographically strong guarantee that the data in each RPC
request has not been tampered with. The krb5p security flavor encrypts every RPC request
to prevent data exposure during network transit; however, expect some performance impact
when using integrity checking or encryption. Similar support for other forms of crypto-
graphic security is also available.
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.
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 distin-
guishes 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 connec-
tions 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 num-
ber 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 dis-
covery 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 end-
points 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 end-
point. 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
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 unin-
tended 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 know-
ing the original set of mount options used to perform the MNT operation. These options
are stored on disk by the NFS mount subcommand, and can be erased by a remount.
To ensure that the saved mount options are not erased during a remount, specify either the
local mount directory, or the server hostname and export pathname, but not both, during a
remount. For example,
mount -o remount,ro /mnt
merges the mount option ro with the mount options already saved on disk for the NFS server
mounted at /mnt.
FILES
/etc/fstab file system table
/etc/nfsmount.conf
Configuration file for NFS mounts
NOTES
Before 2.4.7, the Linux NFS client did not support NFS over TCP.
Before 2.4.20, the Linux NFS client used a heuristic to determine whether cached file data
was still valid rather than using the standard close-to-open cache coherency method
described above.
Starting with 2.4.22, the Linux NFS client employs a Van Jacobsen-based RTT estimator to
determine retransmit timeout values when using NFS over UDP.
Before 2.6.0, the Linux NFS client did not support NFS version 4.
Before 2.6.8, the Linux NFS client used only synchronous reads and writes when the rsize
and wsize settings were smaller than the system's page size.
The Linux client's support for protocol versions depend on whether the kernel was built
with options CONFIG_NFS_V2, CONFIG_NFS_V3, CONFIG_NFS_V4, CONFIG_NFS_V4_1, and CON-
FIG_NFS_V4_2.
SEE ALSO
fstab(5), mount(8), umount(8), mount.nfs(5), umount.nfs(5), exports(5), nfsmount.conf(5),
netconfig(5), ipv6(7), nfsd(8), sm-notify(8), rpc.statd(8), rpc.idmapd(8), rpc.gssd(8),
rpc.svcgssd(8), kerberos(1)
RFC 768 for the UDP specification.
RFC 793 for the TCP specification.
RFC 1094 for the NFS version 2 specification.
RFC 1813 for the NFS version 3 specification.
RFC 1832 for the XDR specification.
RFC 1833 for the RPC bind specification.
RFC 2203 for the RPCSEC GSS API protocol specification.
RFC 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.
9 October 2012 NFS(5)
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