NFS(5) File Formats Manual NFS(5)
nfs - fstab format and options for the nfs file systems
/etc/fstab
NFS is an Internet Standard protocol created by Sun Microsystems in
1984. NFS was developed to allow file sharing between systems
residing on a local area network. The Linux NFS client supports
three versions of the NFS protocol: NFS version 2 [RFC1094], NFS
version 3 [RFC1813], and NFS version 4 [RFC3530].
The mount(8) command attaches a file system to the system's name
space hierarchy at a given mount point. The /etc/fstab file
describes how mount(8) should assemble a system's file name hierarchy
from various independent file systems (including file systems
exported by NFS servers). Each line in the /etc/fstab file describes
a single file system, its mount point, and a set of default mount
options for that mount point.
For NFS file system mounts, a line in the /etc/fstab file specifies
the server name, the path name of the exported server directory to
mount, the local directory that is the mount point, the type of file
system that is being mounted, and a list of mount options that
control the way the filesystem is mounted and how the NFS client
behaves when accessing files on this mount point. The fifth and
sixth fields on each line are not used by NFS, thus conventionally
each contain the digit zero. For example:
server:path /mountpoint fstype option,option,... 0 0
The server's hostname and export pathname are separated by a colon,
while the mount options are separated by commas. The remaining fields
are separated by blanks or tabs.
The server's hostname can be an unqualified hostname, a fully
qualified domain name, a dotted quad IPv4 address, or an IPv6 address
enclosed in square brackets. Link-local and site-local IPv6
addresses must be accompanied by an interface identifier. See
ipv6(7) for details on specifying raw IPv6 addresses.
The fstype field contains "nfs". Use of the "nfs4" fstype in
/etc/fstab is deprecated.
Refer to mount(8) for a description of generic mount options
available for all file systems. If you do not need to specify any
mount options, use the generic option defaults in /etc/fstab.
Options supported by all versions
These options are valid to use with any NFS version.
nfsvers=n The NFS protocol version number used to contact the
server's NFS service. If the server does not support
the requested version, the mount request fails. If
this option is not specified, the client negotiates a
suitable version with the server, trying version 4
first, version 3 second, and version 2 last.
vers=n This option is an alternative to the nfsvers option.
It is included for compatibility with other operating
systems
soft / hard Determines the recovery behavior of the NFS client
after an NFS request times out. If neither option is
specified (or if the hard option is specified), NFS
requests are retried indefinitely. If the soft option
is specified, then the NFS client fails an NFS request
after retrans retransmissions have been sent, causing
the NFS client to return an error to the calling
application.
NB: A so-called "soft" timeout can cause silent data
corruption in certain cases. As such, use the soft
option only when client responsiveness is more
important than data integrity. Using NFS over TCP or
increasing the value of the retrans option may
mitigate some of the risks of using the soft option.
intr / nointr This option is provided for backward compatibility.
It is ignored after kernel 2.6.25.
timeo=n The time in deciseconds (tenths of a second) the NFS
client waits for a response before it retries an NFS
request.
For NFS over TCP the default timeo value is 600 (60
seconds). The NFS client performs linear backoff:
After each retransmission the timeout is increased by
timeo up to the maximum of 600 seconds.
However, for NFS over UDP, the client uses an adaptive
algorithm to estimate an appropriate timeout value for
frequently used request types (such as READ and WRITE
requests), but uses the timeo setting for infrequently
used request types (such as FSINFO requests). If the
timeo option is not specified, infrequently used
request types are retried after 1.1 seconds. After
each retransmission, the NFS client doubles the
timeout for that request, up to a maximum timeout
length of 60 seconds.
retrans=n The number of times the NFS client retries a request
before it attempts further recovery action. If the
retrans option is not specified, the NFS client tries
each UDP request three times and each TCP request
twice.
The NFS client generates a "server not responding"
message after retrans retries, then attempts further
recovery (depending on whether the hard mount option
is in effect).
rsize=n The maximum number of bytes in each network READ
request that the NFS client can receive when reading
data from a file on an NFS server. The actual data
payload size of each NFS READ request is equal to or
smaller than the rsize setting. The largest read
payload supported by the Linux NFS client is 1,048,576
bytes (one megabyte).
The rsize value is a positive integral multiple of
1024. Specified rsize values lower than 1024 are
replaced with 4096; values larger than 1048576 are
replaced with 1048576. If a specified value is within
the supported range but not a multiple of 1024, it is
rounded down to the nearest multiple of 1024.
If an rsize value is not specified, or if the
specified rsize value is larger than the maximum that
either client or server can support, the client and
server negotiate the largest rsize value that they can
both support.
The rsize mount option as specified on the mount(8)
command line appears in the /etc/mtab file. However,
the effective rsize value negotiated by the client and
server is reported in the /proc/mounts file.
wsize=n The maximum number of bytes per network WRITE request
that the NFS client can send when writing data to a
file on an NFS server. The actual data payload size of
each NFS WRITE request is equal to or smaller than the
wsize setting. The largest write payload supported by
the Linux NFS client is 1,048,576 bytes (one
megabyte).
Similar to rsize , the wsize value is a positive
integral multiple of 1024. Specified wsize values
lower than 1024 are replaced with 4096; values larger
than 1048576 are replaced with 1048576. If a specified
value is within the supported range but not a multiple
of 1024, it is rounded down to the nearest multiple of
1024.
If a wsize value is not specified, or if the specified
wsize value is larger than the maximum that either
client or server can support, the client and server
negotiate the largest wsize value that they can both
support.
The wsize mount option as specified on the mount(8)
command line appears in the /etc/mtab file. However,
the effective wsize value negotiated by the client and
server is reported in the /proc/mounts file.
ac / noac Selects whether the client may cache file attributes.
If neither option is specified (or if ac is
specified), the client caches file attributes.
To improve performance, NFS clients cache file
attributes. Every few seconds, an NFS client checks
the server's version of each file's attributes for
updates. Changes that occur on the server in those
small intervals remain undetected until the client
checks the server again. The noac option prevents
clients from caching file attributes so that
applications can more quickly detect file changes on
the server.
In addition to preventing the client from caching file
attributes, the noac option forces application writes
to become synchronous so that local changes to a file
become visible on the server immediately. That way,
other clients can quickly detect recent writes when
they check the file's attributes.
Using the noac option provides greater cache coherence
among NFS clients accessing the same files, but it
extracts a significant performance penalty. As such,
judicious use of file locking is encouraged instead.
The DATA AND METADATA COHERENCE section contains a
detailed discussion of these trade-offs.
acregmin=n The minimum time (in seconds) that the NFS client
caches attributes of a regular file before it requests
fresh attribute information from a server. If this
option is not specified, the NFS client uses a
3-second minimum. See the DATA AND METADATA COHERENCE
section for a full discussion of attribute caching.
acregmax=n The maximum time (in seconds) that the NFS client
caches attributes of a regular file before it requests
fresh attribute information from a server. If this
option is not specified, the NFS client uses a
60-second maximum. See the DATA AND METADATA
COHERENCE section for a full discussion of attribute
caching.
acdirmin=n The minimum time (in seconds) that the NFS client
caches attributes of a directory before it requests
fresh attribute information from a server. If this
option is not specified, the NFS client uses a
30-second minimum. See the DATA AND METADATA
COHERENCE section for a full discussion of attribute
caching.
acdirmax=n The maximum time (in seconds) that the NFS client
caches attributes of a directory before it requests
fresh attribute information from a server. If this
option is not specified, the NFS client uses a
60-second maximum. See the DATA AND METADATA
COHERENCE section for a full discussion of attribute
caching.
actimeo=n Using actimeo sets all of acregmin, acregmax,
acdirmin, and acdirmax to the same value. If this
option is not specified, the NFS client uses the
defaults for each of these options listed above.
bg / fg Determines how the mount(8) command behaves if an
attempt to mount an export fails. The fg option
causes mount(8) to exit with an error status if any
part of the mount request times out or fails outright.
This is called a "foreground" mount, and is the
default behavior if neither the fg nor bg mount option
is specified.
If the bg option is specified, a timeout or failure
causes the mount(8) command to fork a child which
continues to attempt to mount the export. The parent
immediately returns with a zero exit code. This is
known as a "background" mount.
If the local mount point directory is missing, the
mount(8) command acts as if the mount request timed
out. This permits nested NFS mounts specified in
/etc/fstab to proceed in any order during system
initialization, even if some NFS servers are not yet
available. Alternatively these issues can be
addressed using an automounter (refer to automount(8)
for details).
rdirplus / nordirplus
Selects whether to use NFS v3 or v4 READDIRPLUS
requests. If this option is not specified, the NFS
client uses READDIRPLUS requests on NFS v3 or v4
mounts to read small directories. Some applications
perform better if the client uses only READDIR
requests for all directories.
retry=n The number of minutes that the mount(8) command
retries an NFS mount operation in the foreground or
background before giving up. If this option is not
specified, the default value for foreground mounts is
2 minutes, and the default value for background mounts
is 10000 minutes (80 minutes shy of one week). If a
value of zero is specified, the mount(8) command exits
immediately after the first failure.
Note that this only affects how many retries are made
and doesn't affect the delay caused by each retry.
For UDP each retry takes the time determined by the
timeo and retrans options, which by default will be
about 7 seconds. For TCP the default is 3 minutes,
but system TCP connection timeouts will sometimes
limit the timeout of each retransmission to around 2
minutes.
sec=flavors A colon-separated list of one or more security flavors
to use for accessing files on the mounted export. If
the server does not support any of these flavors, the
mount operation fails. If sec= is not specified, the
client attempts to find a security flavor that both
the client and the server supports. Valid flavors are
none, sys, krb5, krb5i, and krb5p. Refer to the
SECURITY CONSIDERATIONS section for details.
sharecache / nosharecache
Determines how the client's data cache and attribute
cache are shared when mounting the same export more
than once concurrently. Using the same cache reduces
memory requirements on the client and presents
identical file contents to applications when the same
remote file is accessed via different mount points.
If neither option is specified, or if the sharecache
option is specified, then a single cache is used for
all mount points that access the same export. If the
nosharecache option is specified, then that mount
point gets a unique cache. Note that when data and
attribute caches are shared, the mount options from
the first mount point take effect for subsequent
concurrent mounts of the same export.
As of kernel 2.6.18, the behavior specified by
nosharecache is legacy caching behavior. This is
considered a data risk since multiple cached copies of
the same file on the same client can become out of
sync following a local update of one of the copies.
resvport / noresvport
Specifies whether the NFS client should use a
privileged source port when communicating with an NFS
server for this mount point. If this option is not
specified, or the resvport option is specified, the
NFS client uses a privileged source port. If the
noresvport option is specified, the NFS client uses a
non-privileged source port. This option is supported
in kernels 2.6.28 and later.
Using non-privileged source ports helps increase the
maximum number of NFS mount points allowed on a
client, but NFS servers must be configured to allow
clients to connect via non-privileged source ports.
Refer to the SECURITY CONSIDERATIONS section for
important details.
lookupcache=mode
Specifies how the kernel manages its cache of
directory entries for a given mount point. mode can
be one of all, none, pos, or positive. This option is
supported in kernels 2.6.28 and later.
The Linux NFS client caches the result of all NFS
LOOKUP requests. If the requested directory entry
exists on the server, the result is referred to as
positive. If the requested directory entry does not
exist on the server, the result is referred to as
negative.
If this option is not specified, or if all is
specified, the client assumes both types of directory
cache entries are valid until their parent directory's
cached attributes expire.
If pos or positive is specified, the client assumes
positive entries are valid until their parent
directory's cached attributes expire, but always
revalidates negative entires before an application can
use them.
If none is specified, the client revalidates both
types of directory cache entries before an application
can use them. This permits quick detection of files
that were created or removed by other clients, but can
impact application and server performance.
The DATA AND METADATA COHERENCE section contains a
detailed discussion of these trade-offs.
fsc / nofsc Enable/Disables the cache of (read-only) data pages to
the local disk using the FS-Cache facility. See
cachefilesd(8) and
<kernel_soruce>/Documentation/filesystems/caching for
detail on how to configure the FS-Cache facility.
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 communicates
with the server's rpcbind and mountd services.
Specifying a netid that uses TCP forces all traffic
from the mount(8) command and the NFS client to use
TCP. Specifying a netid that uses UDP forces all
traffic types to use UDP.
Before using NFS over UDP, refer to the TRANSPORT
METHODS section.
If the proto mount option is not specified, the
mount(8) command discovers which protocols the server
supports and chooses an appropriate transport for each
service. Refer to the TRANSPORT METHODS section for
more details.
udp The udp option is an alternative to specifying
proto=udp. It is included for compatibility with
other operating systems.
Before using NFS over UDP, refer to the TRANSPORT
METHODS section.
tcp The tcp option is an alternative to specifying
proto=tcp. It is included for compatibility with
other operating systems.
rdma The rdma option is an alternative to specifying
proto=rdma.
port=n The numeric value of the server's NFS service port.
If the server's NFS service is not available on the
specified port, the mount request fails.
If this option is not specified, or if the specified
port value is 0, then the NFS client uses the NFS
service port number advertised by the server's rpcbind
service. The mount request fails if the server's
rpcbind service is not available, the server's NFS
service is not registered with its rpcbind service, or
the server's NFS service is not available on the
advertised port.
mountport=n The numeric value of the server's mountd port. If the
server's mountd service is not available on the
specified port, the mount request fails.
If this option is not specified, or if the specified
port value is 0, then the mount(8) command uses the
mountd service port number advertised by the server's
rpcbind service. The mount request fails if the
server's rpcbind service is not available, the
server's mountd service is not registered with its
rpcbind service, or the server's mountd service is not
available on the advertised port.
This option can be used when mounting an NFS server
through a firewall that blocks the rpcbind protocol.
mountproto=netid
The transport the NFS client uses to transmit requests
to the NFS server's mountd service when performing
this mount request, and when later unmounting this
mount point.
netid may be one of udp, and tcp which use IPv4
address or, if TI-RPC is built into the mount.nfs
command, udp6, and tcp6 which use IPv6 addresses.
This option can be used when mounting an NFS server
through a firewall that blocks a particular transport.
When used in combination with the proto option,
different transports for mountd requests and NFS
requests can be specified. If the server's mountd
service is not available via the specified transport,
the mount request fails.
Refer to the TRANSPORT METHODS section for more on how
the mountproto mount option interacts with the proto
mount option.
mounthost=name The hostname of the host running mountd. If this
option is not specified, the mount(8) command assumes
that the mountd service runs on the same host as the
NFS service.
mountvers=n The RPC version number used to contact the server's
mountd. If this option is not specified, the client
uses a version number appropriate to the requested NFS
version. This option is useful when multiple NFS
services are running on the same remote server host.
namlen=n The maximum length of a pathname component on this
mount. If this option is not specified, the maximum
length is negotiated with the server. In most cases,
this maximum length is 255 characters.
Some early versions of NFS did not support this
negotiation. Using this option ensures that
pathconf(3) reports the proper maximum component
length to applications in such cases.
lock / nolock Selects whether to use the NLM sideband protocol to
lock files on the server. If neither option is
specified (or if lock is specified), NLM locking is
used for this mount point. When using the nolock
option, applications can lock files, but such locks
provide exclusion only against other applications
running on the same client. Remote applications are
not affected by these locks.
NLM locking must be disabled with the nolock option
when using NFS to mount /var because /var contains
files used by the NLM implementation on Linux. Using
the nolock option is also required when mounting
exports on NFS servers that do not support the NLM
protocol.
cto / nocto Selects whether to use close-to-open cache coherence
semantics. If neither option is specified (or if cto
is specified), the client uses close-to-open cache
coherence semantics. If the nocto option is specified,
the client uses a non-standard heuristic to determine
when files on the server have changed.
Using the nocto option may improve performance for
read-only mounts, but should be used only if the data
on the server changes only occasionally. The DATA AND
METADATA COHERENCE section discusses the behavior of
this option in more detail.
acl / noacl Selects whether to use the NFSACL sideband protocol on
this mount point. The NFSACL sideband protocol is a
proprietary protocol implemented in Solaris that
manages Access Control Lists. NFSACL was never made a
standard part of the NFS protocol specification.
If neither acl nor noacl option is specified, the NFS
client negotiates with the server to see if the NFSACL
protocol is supported, and uses it if the server
supports it. Disabling the NFSACL sideband protocol
may be necessary if the negotiation causes problems on
the client or server. Refer to the SECURITY
CONSIDERATIONS section for more details.
local_lock=mechanism
Specifies whether to use local locking for any or both
of the flock and the POSIX locking mechanisms.
mechanism can be one of all, flock, posix, or none.
This option is supported in kernels 2.6.37 and later.
The Linux NFS client provides a way to make locks
local. This means, the applications can lock files,
but such locks provide exclusion only against other
applications running on the same client. Remote
applications are not affected by these locks.
If this option is not specified, or if none is
specified, the client assumes that the locks are not
local.
If all is specified, the client assumes that both
flock and POSIX locks are local.
If flock is specified, the client assumes that only
flock locks are local and uses NLM sideband protocol
to lock files when POSIX locks are used.
If posix is specified, the client assumes that POSIX
locks are local and uses NLM sideband protocol to lock
files when flock locks are used.
To support legacy flock behavior similar to that of
NFS clients < 2.6.12, use 'local_lock=flock'. This
option is required when exporting NFS mounts via Samba
as Samba maps Windows share mode locks as flock. Since
NFS clients > 2.6.12 implement flock by emulating
POSIX locks, this will result in conflicting locks.
NOTE: When used together, the 'local_lock' mount
option will be overridden by 'nolock'/'lock' mount
option.
Options for NFS version 4 only
Use these options, along with the options in the first subsection
above, for NFS version 4 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.
port=n The numeric value of the server's NFS service port.
If the server's NFS service is not available on the
specified port, the mount request fails.
If this mount option is not specified, the NFS client
uses the standard NFS port number of 2049 without
first checking the server's rpcbind service. This
allows an NFS version 4 client to contact an NFS
version 4 server through a firewall that may block
rpcbind requests.
If the specified port value is 0, then the NFS client
uses the NFS service port number advertised by the
server's rpcbind service. The mount request fails if
the server's rpcbind service is not available, the
server's NFS service is not registered with its
rpcbind service, or the server's NFS service is not
available on the advertised port.
cto / nocto Selects whether to use close-to-open cache coherence
semantics for NFS directories on this mount point. If
neither cto nor nocto is specified, the default is to
use close-to-open cache coherence semantics for
directories.
File data caching behavior is not affected by this
option. The DATA AND METADATA COHERENCE section
discusses the behavior of this option in more detail.
clientaddr=n.n.n.n
clientaddr=n:n:...:n
Specifies a single IPv4 address (in dotted-quad form),
or a non-link-local IPv6 address, that the NFS client
advertises to allow servers to perform NFS version 4
callback requests against files on this mount point.
If the server is unable to establish callback
connections to clients, performance may degrade, or
accesses to files may temporarily hang.
If this option is not specified, the mount(8) command
attempts to discover an appropriate callback address
automatically. The automatic discovery process is not
perfect, however. In the presence of multiple client
network interfaces, special routing policies, or
atypical network topologies, the exact address to use
for callbacks may be nontrivial to determine.
migration / nomigration
Selects whether the client uses an identification
string that is compatible with NFSv4 Transparent State
Migration (TSM). If the mounted server supports NFSv4
migration with TSM, specify the migration option.
Some server features misbehave in the face of a
migration-compatible identification string. The
nomigration option retains the use of a traditional
client indentification string which is compatible with
legacy NFS servers. This is also the behavior if
neither option is specified. A client's open and lock
state cannot be migrated transparently when it
identifies itself via a traditional identification
string.
This mount option has no effect with NFSv4 minor
versions newer than zero, which always use TSM-
compatible client identification strings.
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.
If the mount command is configured to do so, all of the mount options
described in the previous section can also be configured in the
/etc/nfsmount.conf file. See nfsmount.conf(5) for details.
To mount an export using NFS version 2, use the nfs file system type
and specify the nfsvers=2 mount option. To mount using NFS version
3, use the nfs file system type and specify the nfsvers=3 mount
option. To mount using NFS version 4, use either the nfs file system
type, with the nfsvers=4 mount option, or the nfs4 file system type.
The following example from an /etc/fstab file causes the mount
command to negotiate reasonable defaults for NFS behavior.
server:/export /mnt nfs defaults 0 0
Here is an example from an /etc/fstab file for an NFS version 2 mount
over UDP.
server:/export /mnt nfs nfsvers=2,proto=udp 0 0
This example shows how to mount using NFS version 4 over TCP with
Kerberos 5 mutual authentication.
server:/export /mnt nfs4 sec=krb5 0 0
This example shows how to mount using NFS version 4 over TCP with
Kerberos 5 privacy or data integrity mode.
server:/export /mnt nfs4 sec=krb5p:krb5i 0 0
This example can be used to mount /usr over NFS.
server:/export /usr nfs ro,nolock,nocto,actimeo=3600 0 0
This example shows how to mount an NFS server using a raw IPv6 link-
local address.
[fe80::215:c5ff:fb3e:e2b1%eth0]:/export /mnt nfs defaults 0 0
NFS clients send requests to NFS servers via Remote Procedure Calls,
or RPCs. The RPC client discovers remote service endpoints
automatically, handles per-request authentication, adjusts request
parameters for different byte endianness on client and server, and
retransmits requests that may have been lost by the network or
server. RPC requests and replies flow over a network transport.
In most cases, the mount(8) command, NFS client, and NFS server can
automatically negotiate proper transport and data transfer size
settings for a mount point. In some cases, however, it pays to
specify these settings explicitly using mount options.
Traditionally, NFS clients used the UDP transport exclusively for
transmitting requests to servers. Though its implementation is
simple, NFS over UDP has many limitations that prevent smooth
operation and good performance in some common deployment
environments. Even an insignificant packet loss rate results in the
loss of whole NFS requests; as such, retransmit timeouts are usually
in the subsecond range to allow clients to recover quickly from
dropped requests, but this can result in extraneous network traffic
and server load.
However, UDP can be quite effective in specialized settings where the
networks MTU is large relative to NFSs data transfer size (such as
network environments that enable jumbo Ethernet frames). In such
environments, trimming the rsize and wsize settings so that each NFS
read or write request fits in just a few network frames (or even in
a single frame) is advised. This reduces the probability that the
loss of a single MTU-sized network frame results in the loss of an
entire large read or write request.
TCP is the default transport protocol used for all modern NFS
implementations. It performs well in almost every conceivable
network environment and provides excellent guarantees against data
corruption caused by network unreliability. TCP is often a
requirement for mounting a server through a network firewall.
Under normal circumstances, networks drop packets much more
frequently than NFS servers drop requests. As such, an aggressive
retransmit timeout setting for NFS over TCP is unnecessary. Typical
timeout settings for NFS over TCP are between one and ten minutes.
After the client exhausts its retransmits (the value of the retrans
mount option), it assumes a network partition has occurred, and
attempts to reconnect to the server on a fresh socket. Since TCP
itself makes network data transfer reliable, rsize and wsize can
safely be allowed to default to the largest values supported by both
client and server, independent of the network's MTU size.
Using the mountproto mount option
This section applies only to NFS version 2 and version 3 mounts since
NFS version 4 does not use a separate protocol for mount requests.
The Linux NFS client can use a different transport for contacting an
NFS server's rpcbind service, its mountd service, its Network Lock
Manager (NLM) service, and its NFS service. The exact transports
employed by the Linux NFS client for each mount point depends on the
settings of the transport mount options, which include proto,
mountproto, udp, and tcp.
The client sends Network Status Manager (NSM) notifications via UDP
no matter what transport options are specified, but listens for
server NSM notifications on both UDP and TCP. The NFS Access Control
List (NFSACL) protocol shares the same transport as the main NFS
service.
If no transport options are specified, the Linux NFS client uses UDP
to contact the server's mountd service, and TCP to contact its NLM
and NFS services by default.
If the server does not support these transports for these services,
the mount(8) command attempts to discover what the server supports,
and then retries the mount request once using the discovered
transports. If the server does not advertise any transport supported
by the client or is misconfigured, the mount request fails. If the
bg option is in effect, the mount command backgrounds itself and
continues to attempt the specified mount request.
When the proto option, the udp option, or the tcp option is specified
but the mountproto option is not, the specified transport is used to
contact both the server's mountd service and for the NLM and NFS
services.
If the mountproto option is specified but none of the proto, udp or
tcp options are specified, then the specified transport is used for
the initial mountd request, but the mount command attempts to
discover what the server supports for the NFS protocol, preferring
TCP if both transports are supported.
If both the mountproto and proto (or udp or tcp) options are
specified, then the transport specified by the mountproto option is
used for the initial mountd request, and the transport specified by
the proto option (or the udp or tcp options) is used for NFS, no
matter what order these options appear. No automatic service
discovery is performed if these options are specified.
If any of the proto, udp, tcp, or mountproto options are specified
more than once on the same mount command line, then the value of the
rightmost instance of each of these options takes effect.
Using NFS over UDP on high-speed links
Using NFS over UDP on high-speed links such as Gigabit can cause
silent data corruption.
The problem can be triggered at high loads, and is caused by problems
in IP fragment reassembly. NFS read and writes typically transmit UDP
packets of 4 Kilobytes or more, which have to be broken up into
several fragments in order to be sent over the Ethernet link, which
limits packets to 1500 bytes by default. This process happens at the
IP network layer and is called fragmentation.
In order to identify fragments that belong together, IP assigns a
16bit IP ID value to each packet; fragments generated from the same
UDP packet will have the same IP ID. The receiving system will
collect these fragments and combine them to form the original UDP
packet. This process is called reassembly. The default timeout for
packet reassembly is 30 seconds; if the network stack does not
receive all fragments of a given packet within this interval, it
assumes the missing fragment(s) got lost and discards those it
already received.
The problem this creates over high-speed links is that it is possible
to send more than 65536 packets within 30 seconds. In fact, with
heavy NFS traffic one can observe that the IP IDs repeat after about
5 seconds.
This has serious effects on reassembly: if one fragment gets lost,
another fragment from a different packet but with the same IP ID will
arrive within the 30 second timeout, and the network stack will
combine these fragments to form a new packet. Most of the time,
network layers above IP will detect this mismatched reassembly - in
the case of UDP, the UDP checksum, which is a 16 bit checksum over
the entire packet payload, will usually not match, and UDP will
discard the bad packet.
However, the UDP checksum is 16 bit only, so there is a chance of 1
in 65536 that it will match even if the packet payload is completely
random (which very often isn't the case). If that is the case, silent
data corruption will occur.
This potential should be taken seriously, at least on Gigabit
Ethernet. Network speeds of 100Mbit/s should be considered less
problematic, because with most traffic patterns IP ID wrap around
will take much longer than 30 seconds.
It is therefore strongly recommended to use NFS over TCP where
possible, since TCP does not perform fragmentation.
If you absolutely have to use NFS over UDP over Gigabit Ethernet,
some steps can be taken to mitigate the problem and reduce the
probability of corruption:
Jumbo frames: Many Gigabit network cards are capable of transmitting
frames bigger than the 1500 byte limit of traditional
Ethernet, typically 9000 bytes. Using jumbo frames of
9000 bytes will allow you to run NFS over UDP at a
page size of 8K without fragmentation. Of course, this
is only feasible if all involved stations support
jumbo frames.
To enable a machine to send jumbo frames on cards that
support it, it is sufficient to configure the
interface for a MTU value of 9000.
Lower reassembly timeout:
By lowering this timeout below the time it takes the
IP ID counter to wrap around, incorrect reassembly of
fragments can be prevented as well. To do so, simply
write the new timeout value (in seconds) to the file
/proc/sys/net/ipv4/ipfrag_time.
A value of 2 seconds will greatly reduce the
probability of IPID clashes on a single Gigabit link,
while still allowing for a reasonable timeout when
receiving fragmented traffic from distant peers.
Some modern cluster file systems provide perfect cache coherence
among their clients. Perfect cache coherence among disparate NFS
clients is expensive to achieve, especially on wide area networks.
As such, NFS settles for weaker cache coherence that satisfies the
requirements of most file sharing types.
Close-to-open cache consistency
Typically file sharing is completely sequential. First client A
opens a file, writes something to it, then closes it. Then client B
opens the same file, and reads the changes.
When an application opens a file stored on an NFS version 3 server,
the NFS client checks that the file exists on the server and is
permitted to the opener by sending a GETATTR or ACCESS request. The
NFS client sends these requests regardless of the freshness of the
file's cached attributes.
When the application closes the file, the NFS client writes back any
pending changes to the file so that the next opener can view the
changes. This also gives the NFS client an opportunity to report
write errors to the application via the return code from close(2).
The behavior of checking at open time and flushing at close time is
referred to as close-to-open cache consistency, or CTO. It can be
disabled for an entire mount point using the nocto mount option.
Weak cache consistency
There are still opportunities for a client's data cache to contain
stale data. The NFS version 3 protocol introduced "weak cache
consistency" (also known as WCC) which provides a way of efficiently
checking a file's attributes before and after a single request. This
allows a client to help identify changes that could have been made by
other clients.
When a client is using many concurrent operations that update the
same file at the same time (for example, during asynchronous write
behind), it is still difficult to tell whether it was that client's
updates or some other client's updates that altered the file.
Attribute caching
Use the noac mount option to achieve attribute cache coherence among
multiple clients. Almost every file system operation checks file
attribute information. The client keeps this information cached for
a period of time to reduce network and server load. When noac is in
effect, a client's file attribute cache is disabled, so each
operation that needs to check a file's attributes is forced to go
back to the server. This permits a client to see changes to a file
very quickly, at the cost of many extra network operations.
Be careful not to confuse the noac option with "no data caching."
The noac mount option prevents the client from caching file metadata,
but there are still races that may result in data cache incoherence
between client and server.
The NFS protocol is not designed to support true cluster file system
cache coherence without some type of application serialization. If
absolute cache coherence among clients is required, applications
should use file locking. Alternatively, applications can also open
their files with the O_DIRECT flag to disable data caching entirely.
File timestamp 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 strictatime/nostrictatime mount options have
no effect on NFS mounts.
/proc/mounts may report that the relatime mount option is set on NFS
mounts, but in fact the atime semantics are always as described here,
and are not like relatime semantics.
Directory entry caching
The Linux NFS client caches the result of all NFS LOOKUP requests.
If the requested directory entry exists on the server, the result is
referred to as a positive lookup result. If the requested directory
entry does not exist on the server (that is, the server returned
ENOENT), the result is referred to as negative lookup result.
To detect when directory entries have been added or removed on the
server, the Linux NFS client watches a directory's mtime. If the
client detects a change in a directory's mtime, the client drops all
cached LOOKUP results for that directory. Since the directory's
mtime is a cached attribute, it may take some time before a client
notices it has changed. See the descriptions of the acdirmin,
acdirmax, and noac mount options for more information about how long
a directory's mtime is cached.
Caching directory entries improves the performance of applications
that do not share files with applications on other clients. Using
cached information about directories can interfere with applications
that run concurrently on multiple clients and need to detect the
creation or removal of files quickly, however. The lookupcache mount
option allows some tuning of directory entry caching behavior.
Before kernel release 2.6.28, the Linux NFS client tracked only
positive lookup results. This permitted applications to detect new
directory entries created by other clients quickly while still
providing some of the performance benefits of caching. If an
application depends on the previous lookup caching behavior of the
Linux NFS client, you can use lookupcache=positive.
If the client ignores its cache and validates every application
lookup request with the server, that client can immediately detect
when a new directory entry has been either created or removed by
another client. You can specify this behavior using
lookupcache=none. The extra NFS requests needed if the client does
not cache directory entries can exact a performance penalty.
Disabling lookup caching should result in less of a performance
penalty than using noac, and has no effect on how the NFS client
caches the attributes of files.
The sync mount option
The NFS client treats the sync mount option differently than some
other file systems (refer to mount(8) for a description of the
generic sync and async mount options). If neither sync nor async is
specified (or if the async option is specified), the NFS client
delays sending application writes to the server until any of these
events occur:
Memory pressure forces reclamation of system memory resources.
An application flushes file data explicitly with sync(2),
msync(2), or fsync(3).
An application closes a file with close(2).
The file is locked/unlocked via fcntl(2).
In other words, under normal circumstances, data written by an
application may not immediately appear on the server that hosts the
file.
If the sync option is specified on a mount point, any system call
that writes data to files on that mount point causes that data to be
flushed to the server before the system call returns control to user
space. This provides greater data cache coherence among clients, but
at a significant performance cost.
Applications can use the O_SYNC open flag to force application writes
to individual files to go to the server immediately without the use
of the sync mount option.
Using file locks with NFS
The Network Lock Manager protocol is a separate sideband protocol
used to manage file locks in NFS version 2 and version 3. To support
lock recovery after a client or server reboot, a second sideband
protocol -- known as the Network Status Manager protocol -- is also
required. In NFS version 4, file locking is supported directly in
the main NFS protocol, and the NLM and NSM sideband protocols are not
used.
In most cases, NLM and NSM services are started automatically, and no
extra configuration is required. Configure all NFS clients with
fully-qualified domain names to ensure that NFS servers can find
clients to notify them of server reboots.
NLM supports advisory file locks only. To lock NFS files, use
fcntl(2) with the F_GETLK and F_SETLK commands. The NFS client
converts file locks obtained via flock(2) to advisory locks.
When mounting servers that do not support the NLM protocol, or when
mounting an NFS server through a firewall that blocks the NLM service
port, specify the nolock mount option. NLM locking must be disabled
with the nolock option when using NFS to mount /var because /var
contains files used by the NLM implementation on Linux.
Specifying the nolock option may also be advised to improve the
performance of a proprietary application which runs on a single
client and uses file locks extensively.
NFS version 4 caching features
The data and metadata caching behavior of NFS version 4 clients is
similar to that of earlier versions. However, NFS version 4 adds two
features that improve cache behavior: change attributes and file
delegation.
The change attribute is a new part of NFS file and directory metadata
which tracks data changes. It replaces the use of a file's
modification and change time stamps as a way for clients to validate
the content of their caches. Change attributes are independent of
the time stamp resolution on either the server or client, however.
A file delegation is a contract between an NFS version 4 client and
server that allows the client to treat a file temporarily as if no
other client is accessing it. The server promises to notify the
client (via a callback request) if another client attempts to access
that file. Once a file has been delegated to a client, the client
can cache that file's data and metadata aggressively without
contacting the server.
File delegations come in two flavors: read and write. A read
delegation means that the server notifies the client about any other
clients that want to write to the file. A write delegation means
that the client gets notified about either read or write accessors.
Servers grant file delegations when a file is opened, and can recall
delegations at any time when another client wants access to the file
that conflicts with any delegations already granted. Delegations on
directories are not supported.
In order to support delegation callback, the server checks the
network return path to the client during the client's initial contact
with the server. If contact with the client cannot be established,
the server simply does not grant any delegations to that client.
NFS servers control access to file data, but they depend on their RPC
implementation to provide authentication of NFS requests.
Traditional NFS access control mimics the standard mode bit access
control provided in local file systems. Traditional RPC
authentication uses a number to represent each user (usually the
user's own uid), a number to represent the user's group (the user's
gid), and a set of up to 16 auxiliary group numbers to represent
other groups of which the user may be a member.
Typically, file data and user ID values appear unencrypted (i.e. "in
the clear") on the network. Moreover, NFS versions 2 and 3 use
separate sideband protocols for mounting, locking and unlocking
files, and reporting system status of clients and servers. These
auxiliary protocols use no authentication.
In addition to combining these sideband protocols with the main NFS
protocol, NFS version 4 introduces more advanced forms of access
control, authentication, and in-transit data protection. The NFS
version 4 specification mandates support for strong authentication
and security flavors that provide per-RPC integrity checking and
encryption. Because NFS version 4 combines the function of the
sideband protocols into the main NFS protocol, the new security
features apply to all NFS version 4 operations including mounting,
file locking, and so on. RPCGSS authentication can also be used with
NFS versions 2 and 3, but it does not protect their sideband
protocols.
The sec mount option specifies the security flavor used for
operations on behalf of users on that NFS mount point. Specifying
sec=krb5 provides cryptographic proof of a user's identity in each
RPC request. This provides strong verification of the identity of
users accessing data on the server. Note that additional
configuration besides adding this mount option is required in order
to enable Kerberos security. Refer to the rpc.gssd(8) man page for
details.
Two additional flavors of Kerberos security are supported: krb5i and
krb5p. The krb5i security flavor provides a cryptographically strong
guarantee that the data in each RPC request has not been tampered
with. The krb5p security flavor encrypts every RPC request to
prevent data exposure during network transit; however, expect some
performance impact when using integrity checking or encryption.
Similar support for other forms of cryptographic security is also
available.
NFS version 4 filesystem crossing
The NFS version 4 protocol allows a client to renegotiate the
security flavor when the client crosses into a new filesystem on the
server. The newly negotiated flavor effects only accesses of the new
filesystem.
Such negotiation typically occurs when a client crosses from a
server's pseudo-fs into one of the server's exported physical
filesystems, which often have more restrictive security settings than
the pseudo-fs.
NFS version 4 Leases
In NFS version 4, a lease is a period of time during which a server
irrevocably grants a file lock to a client. If the lease expires,
the server is allowed to revoke that lock. Clients periodically
renew their leases to prevent lock revocation.
After an NFS version 4 server reboots, each client tells the server
about all file open and lock state under its lease before operation
can continue. If the client reboots, the server frees all open and
lock state associated with that client's lease.
As part of establishing a lease, therefore, a client must identify
itself to a server. A fixed string is used to distinguish that
client from others, and a changeable verifier is used to indicate
when the client has rebooted.
A client uses a particular security flavor and principal when
performing the operations to establish a lease. If two clients
happen to present the same identity string, a server can use their
principals to detect that they are different clients, and prevent one
client from interfering with the other's lease.
The Linux NFS client establishes one lease for each server. Lease
management operations, such as lease renewal, are not done on behalf
of a particular file, lock, user, or mount point, but on behalf of
the whole client that owns that lease. These operations must use the
same security flavor and principal that was used when the lease was
established, even across client reboots.
When Kerberos is configured on a Linux NFS client (i.e., there is a
/etc/krb5.keytab on that client), the client attempts to use a
Kerberos security flavor for its lease management operations. This
provides strong authentication of the client to each server it
contacts. By default, the client uses the host/ or nfs/ service
principal in its /etc/krb5.keytab for this purpose.
If the client has Kerberos configured, but the server does not, or if
the client does not have a keytab or the requisite service
principals, the client uses AUTH_SYS and UID 0 for lease management.
Using non-privileged source ports
NFS clients usually communicate with NFS servers via network sockets.
Each end of a socket is assigned a port value, which is simply a
number between 1 and 65535 that distinguishes socket endpoints at the
same IP address. A socket is uniquely defined by a tuple that
includes the transport protocol (TCP or UDP) and the port values and
IP addresses of both endpoints.
The NFS client can choose any source port value for its sockets, but
usually chooses a privileged port. A privileged port is a port value
less than 1024. Only a process with root privileges may create a
socket with a privileged source port.
The exact range of privileged source ports that can be chosen is set
by a pair of sysctls to avoid choosing a well-known port, such as the
port used by ssh. This means the number of source ports available
for the NFS client, and therefore the number of socket connections
that can be used at the same time, is practically limited to only a
few hundred.
As described above, the traditional default NFS authentication
scheme, known as AUTH_SYS, relies on sending local UID and GID
numbers to identify users making NFS requests. An NFS server assumes
that if a connection comes from a privileged port, the UID and GID
numbers in the NFS requests on this connection have been verified by
the client's kernel or some other local authority. This is an easy
system to spoof, but on a trusted physical network between trusted
hosts, it is entirely adequate.
Roughly speaking, one socket is used for each NFS mount point. If a
client could use non-privileged source ports as well, the number of
sockets allowed, and thus the maximum number of concurrent mount
points, would be much larger.
Using non-privileged source ports may compromise server security
somewhat, since any user on AUTH_SYS mount points can now pretend to
be any other when making NFS requests. Thus NFS servers do not
support this by default. They explicitly allow it usually via an
export option.
To retain good security while allowing as many mount points as
possible, it is best to allow non-privileged client connections only
if the server and client both require strong authentication, such as
Kerberos.
Mounting through a firewall
A firewall may reside between an NFS client and server, or the client
or server may block some of its own ports via IP filter rules. It is
still possible to mount an NFS server through a firewall, though some
of the mount(8) command's automatic service endpoint discovery
mechanisms may not work; this requires you to provide specific
endpoint details via NFS mount options.
NFS servers normally run a portmapper or rpcbind daemon to advertise
their service endpoints to clients. Clients use the rpcbind daemon to
determine:
What network port each RPC-based service is using
What transport protocols each RPC-based service supports
The rpcbind daemon uses a well-known port number (111) to help
clients find a service endpoint. Although NFS often uses a standard
port number (2049), auxiliary services such as the NLM service can
choose any unused port number at random.
Common firewall configurations block the well-known rpcbind port. In
the absense of an rpcbind service, the server administrator fixes the
port number of NFS-related services so that the firewall can allow
access to specific NFS service ports. Client administrators then
specify the port number for the mountd service via the mount(8)
command's mountport option. It may also be necessary to enforce the
use of TCP or UDP if the firewall blocks one of those transports.
NFS Access Control Lists
Solaris allows NFS version 3 clients direct access to POSIX Access
Control Lists stored in its local file systems. This proprietary
sideband protocol, known as NFSACL, provides richer access control
than mode bits. Linux implements this protocol for compatibility
with the Solaris NFS implementation. The NFSACL protocol never
became a standard part of the NFS version 3 specification, however.
The NFS version 4 specification mandates a new version of Access
Control Lists that are semantically richer than POSIX ACLs. NFS
version 4 ACLs are not fully compatible with POSIX ACLs; as such,
some translation between the two is required in an environment that
mixes POSIX ACLs and NFS version 4.
Generic mount options such as rw and sync can be modified on NFS
mount points using the remount option. See mount(8) for more
information on generic mount options.
With few exceptions, NFS-specific options are not able to be modified
during a remount. The underlying transport or NFS version cannot be
changed by a remount, for example.
Performing a remount on an NFS file system mounted with the noac
option may have unintended consequences. The noac option is a
combination of the generic option sync, and the NFS-specific option
actimeo=0.
Unmounting after a remount
For mount points that use NFS versions 2 or 3, the NFS umount
subcommand depends on knowing the original set of mount options used
to perform the MNT operation. These options are stored on disk by
the NFS mount subcommand, and can be erased by a remount.
To ensure that the saved mount options are not erased during a
remount, specify either the local mount directory, or the server
hostname and export pathname, but not both, during a remount. For
example,
mount -o remount,ro /mnt
merges the mount option ro with the mount options already saved on
disk for the NFS server mounted at /mnt.
/etc/fstab file system table
/etc/nfsmount.conf
Configuration file for NFS mounts
Before 2.4.7, the Linux NFS client did not support NFS over TCP.
Before 2.4.20, the Linux NFS client used a heuristic to determine
whether cached file data was still valid rather than using the
standard close-to-open cache coherency method described above.
Starting with 2.4.22, the Linux NFS client employs a Van Jacobsen-
based RTT estimator to determine retransmit timeout values when using
NFS over UDP.
Before 2.6.0, the Linux NFS client did not support NFS version 4.
Before 2.6.8, the Linux NFS client used only synchronous reads and
writes when the rsize and wsize settings were smaller than the
system's page size.
The Linux NFS client does not yet support certain optional features
of the NFS version 4 protocol, such as security negotiation, server
referrals, and named attributes.
fstab(5), mount(8), umount(8), mount.nfs(5), umount.nfs(5),
exports(5), nfsmount.conf(5), netconfig(5), ipv6(7), nfsd(8),
sm-notify(8), rpc.statd(8), rpc.idmapd(8), rpc.gssd(8),
rpc.svcgssd(8), kerberos(1)
RFC 768 for the UDP specification.
RFC 793 for the TCP specification.
RFC 1094 for the NFS version 2 specification.
RFC 1813 for the NFS version 3 specification.
RFC 1832 for the XDR specification.
RFC 1833 for the RPC bind specification.
RFC 2203 for the RPCSEC GSS API protocol specification.
RFC 3530 for the NFS version 4 specification.
This page is part of the nfs-utils (NFS utilities) project.
Information about the project can be found at
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. If you have a bug
report for this manual page, see
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. This page was
obtained from the project's upstream Git repository
⟨http://git.linux-nfs.org/?p=steved/nfs-utils.git;a=summary⟩ on
2017-07-05. If you discover any rendering problems in this HTML ver‐
sion of the page, or you believe there is a better or more up-to-date
source for the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original man‐
ual page), send a mail to man-pages@man7.org
9 October 2012 NFS(5)
Pages that refer to this page: pmdanfsclient(1), flock(2), filesystems(5), nfsmount.conf(5), systemd.mount(5), blkmapd(8), mount(8), mountd(8), mount.nfs(8), nfsstat(8), rpcdebug(8), rpc.rquotad(8), sm-notify(8), statd(8), umount.nfs(8)