NFS(5)NFS(5)NAMEnfs - fstab format and options for the nfs and nfs4 file systems
SYNOPSIS
/etc/fstab
DESCRIPTION
NFS is an Internet Standard protocol created by Sun Microsystems in
1984. NFS was developed to allow file sharing between systems residing
on a local area network. 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, or a dotted quad
IPv4 address. The fstype field contains either "nfs" (for version 2 or
version 3 NFS mounts) or "nfs4" (for NFS version 4 mounts). The nfs
and nfs4 file system types share similar mount options, which are
described below.
MOUNT OPTIONS
Refer to mount(8) for a description of generic mount options available
for all file systems. If you do not need to specify any mount options,
use the generic option defaults in /etc/fstab.
Valid options for either the nfs or nfs4 file system type
These options are valid to use when mounting either nfs or nfs4 file
system types. They imply the same behavior and have the same default
for both file system types.
soft / hard Determines the recovery behavior of the NFS client after
an NFS request times out. If neither option is speci‐
fied (or if the hard option is specified), NFS requests
are retried indefinitely. If the soft option is speci‐
fied, 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.
timeo=n The time (in tenths of a second) the NFS client waits
for a response before it retries an NFS request. If this
option is not specified, requests are retried every 60
seconds for NFS over TCP. The NFS client does not per‐
form any kind of timeout backoff for NFS over TCP.
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 sec‐
onds.
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" mes‐
sage after retrans retries, then attempts further recov‐
ery (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) com‐
mand 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 inte‐
gral 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) com‐
mand 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 inter‐
vals 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.
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.
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.
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.
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 con‐
tinues to attempt to mount the export. The parent imme‐
diately 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. Alter‐
natively these issues can be addressed using an auto‐
mounter (refer to automount(8) for details).
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).
sec=mode The RPCGSS security flavor to use for accessing files on
this mount point. If the sec option is not specified,
or if sec=sys is specified, the NFS client uses the
AUTH_SYS security flavor for all NFS requests on this
mount point. Valid security flavors are none, sys,
krb5, krb5i, krb5p, lkey, lkeyi, lkeyp, spkm, spkmi, and
spkmp. 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 noshare‐
cache 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.
Valid options for the nfs file system type
Use these options, along with the options in the above subsection, for
mounting the nfs file system type.
proto=netid The transport protocol used by the NFS client to trans‐
mit requests to the NFS server for this mount point.
netid can be either udp or tcp. 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 proto=tcp
forces all traffic from the mount(8) command and the NFS
client to use TCP. Specifying proto=udp forces all
traffic types to use UDP.
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.
tcp The tcp option is an alternative to specifying
proto=tcp. It is included for compatibility with other
operating systems.
port=n The numeric value of the server's NFS service port. If
the server's NFS service is not available on the speci‐
fied 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 ser‐
vice port number advertised by the server's rpcbind ser‐
vice. 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 speci‐
fied 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 ser‐
vice, 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 protocol used by the NFS client to trans‐
mit requests to the NFS server's mountd service when
performing this mount request, and when later unmounting
this mount point. netid can be either udp or tcp.
This option can be used when mounting an NFS server
through a firewall that blocks a particular transport
protocol. When used in combination with the proto
option, different transports for mountd requests and NFS
requests can be specified. If the server's mountd ser‐
vice is not available via the specified transport, the
mount request fails.
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 ser‐
vices 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 negotia‐
tion. Using this option ensures that pathconf(3)
reports the proper maximum component length to applica‐
tions in such cases.
nfsvers=n The NFS protocol version number used to contact the
server's NFS service. The Linux client supports version
2 and version 3 of the NFS protocol when using the file
system type nfs. If the server does not support the
requested version, the mount request fails. If this
option is not specified, the client attempts to use ver‐
sion 3, but negotiates the NFS version with the server
if version 3 support is not available.
vers=n This option is an alternative to the nfsvers option. It
is included for compatibility with other operating sys‐
tems.
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.
intr / nointr Selects whether to allow signals to interrupt file oper‐
ations on this mount point. If neither option is speci‐
fied (or if nointr is specified), signals do not inter‐
rupt NFS file operations. If intr is specified, system
calls return EINTR if an in-progress NFS operation is
interrupted by a signal.
Using the intr option is preferred to using the soft
option because it is significantly less likely to result
in data corruption.
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 coher‐
ence 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 sup‐
ports 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.
rdirplus / nordirplus
Selects whether to use NFS version 3 READDIRPLUS
requests. If this option is not specified, the NFS
client uses READDIRPLUS requests on NFS version 3 mounts
to read small directories. Some applications perform
better if the client uses only READDIR requests for all
directories.
Valid options for the nfs4 file system type
Use these options, along with the options in the first subsection
above, for mounting the nfs4 file system type.
proto=netid The transport protocol used by the NFS client to trans‐
mit requests to the NFS server for this mount point.
netid can be either udp or tcp. All NFS version 4
servers are required to support TCP, so if this mount
option is not specified, the NFS version 4 client uses
the TCP transport protocol. 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 speci‐
fied 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.
intr / nointr Selects whether to allow signals to interrupt file oper‐
ations on this mount point. If neither option is speci‐
fied (or if intr is specified), system calls return
EINTR if an in-progress NFS operation is interrupted by
a signal. If nointr is specified, signals do not inter‐
rupt NFS operations.
Using the intr option is preferred to using the soft
option because it is significantly less likely to result
in data corruption.
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 dis‐
cusses the behavior of this option in more detail.
clientaddr=n.n.n.n
Specifies a single IPv4 address (in dotted-quad
form) that the NFS client advertises to allow servers to
perform NFS version 4 callback requests against files on
this mount point. If the server is unable to establish
callback connections to clients, performance 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 atypi‐
cal network topologies, the exact address to use for
callbacks may be nontrivial to determine.
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 the nfs4 file system type. The
nfsvers mount option is not supported for 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
Try this example to mount using NFS version 4 over TCP with Kerberos 5
mutual authentication.
server:/export /mnt nfs4 sec=krb5 0 0
This example can be used to mount /usr over NFS.
server:/export /usr nfs ro,nolock,nocto,actimeo=3600 0 0
TRANSPORT METHODS
NFS clients send requests to NFS servers via Remote Procedure Calls, or
RPCs. The RPC client discovers remote service endpoints automatically,
handles per-request authentication, adjusts request parameters for dif‐
ferent 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 set‐
tings 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
network’s MTU is large relative to NFS’s data transfer size (such as
network environments that enable jumbo Ethernet frames). In such envi‐
ronments, 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 sin‐
gle 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 implemen‐
tations. It performs well in almost every conceivable network environ‐
ment 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 set‐
tings 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 net‐
work 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.
DATA AND METADATA COHERENCE
Some modern cluster file systems provide perfect cache coherence among
their clients. Perfect cache coherence among disparate NFS clients is
expensive to achieve, especially on wide area networks. As such, NFS
settles for weaker cache coherence that satisfies the requirements of
most file sharing types. Normally, file sharing is completely sequen‐
tial: first client A opens a file, writes something to it, then closes
it; then client B opens the same file, and reads the changes.
Close-to-open cache consistency
When an application opens a file stored on an NFS server, the NFS
client checks that it still exists on the server and is permitted to
the opener by sending a GETATTR or ACCESS request. When the applica‐
tion 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 any server write errors to the
application via the return code from close(2). The behavior of check‐
ing at open time and flushing at close time is referred to as close-to-
open cache consistency.
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 consis‐
tency" (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.
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 appli‐
cation 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 applica‐
tion 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 sig‐
nificant 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 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 con‐
tains files used by the NLM implementation on Linux.
Specifying the nolock option may also be advised to improve the perfor‐
mance 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 sim‐
ilar to that of earlier versions. However, NFS version 4 adds two fea‐
tures that improve cache behavior: change attributes and file delega‐
tion.
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 modifica‐
tion and change time stamps as a way for clients to validate the con‐
tent 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 delega‐
tion means that the server notifies the client about any other clients
that want to write to the file. A write delegation means that the
client gets notified about either read or write accessors.
Servers grant file delegations when a file is opened, and can recall
delegations at any time when another client wants access to the file
that conflicts with any delegations already granted. Delegations on
directories are not supported.
In order to support delegation callback, the server checks the network
return path to the client during the client's initial contact with the
server. If contact with the client cannot be established, the server
simply does not grant any delegations to that client.
SECURITY CONSIDERATIONS
NFS servers control access to file data, but they depend on their RPC
implementation to provide authentication of NFS requests. Traditional
NFS access control mimics the standard mode bit access control provided
in local file systems. Traditional RPC authentication uses a number to
represent each user (usually the user's own uid), a number to represent
the user's group (the user's gid), and a set of up to 16 auxiliary
group numbers to represent other groups of which the user may be a mem‐
ber.
Typically, file data and user ID values appear unencrypted (i.e. "in
the clear") on the network. Moreover, NFS versions 2 and 3 use sepa‐
rate sideband protocols for mounting, locking and unlocking files, and
reporting system status of clients and servers. These auxiliary proto‐
cols use no authentication.
In addition to combining these sideband protocols with the main NFS
protocol, NFS version 4 introduces more advanced forms of access con‐
trol, authentication, and in-transit data protection. The NFS version
4 specification mandates NFSv4 ACLs, RPCGSS authentication, and RPCGSS
security flavors that provide per-RPC integrity checking and encryp‐
tion. Because NFS version 4 combines the function of the sideband pro‐
tocols into the main NFS protocol, the new security features apply to
all NFS version 4 operations including mounting, file locking, and so
on. RPCGSS authentication can also be used with NFS versions 2 and 3,
but does not protect their sideband protocols.
The sec mount option specifies the RPCGSS security mode that is in
effect on a given NFS mount point. Specifying sec=krb5 provides cryp‐
tographic proof of a user's identity in each RPC request. This pro‐
vides 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 (such as lipkey and SPKM3) is
also available.
The NFS version 4 protocol allows clients and servers to negotiate
among multiple security flavors during mount processing. However,
Linux does not yet implement such negotiation. The Linux client speci‐
fies a single security flavor at mount time which remains in effect for
the lifetime of the mount. If the server does not support this flavor,
the initial mount request is rejected by the server.
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 mecha‐
nisms 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 num‐
ber (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 spec‐
ify 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 Con‐
trol 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 stan‐
dard part of the NFS version 3 specification, however.
The NFS version 4 specification mandates a new version of Access Con‐
trol Lists that are semantically richer than POSIX ACLs. NFS version 4
ACLs are not fully compatible with POSIX ACLs; as such, some transla‐
tion between the two is required in an environment that mixes POSIX
ACLs and NFS version 4.
FILES
/etc/fstab file system table
BUGS
The generic remount option is not fully supported. Generic options,
such as rw and ro can be modified using the remount option, but NFS-
specific options are not all supported. 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 mixture of a generic
option, sync, and an NFS-specific option actimeo=0.
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 refer‐
rals, and named attributes.
SEE ALSOfstab(5), mount(8), umount(8), mount.nfs(5), umount.nfs(5), exports(5),
nfsd(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.
2 November 2007 NFS(5)
