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NFS(5) NFS(5)
NAME
nfs - 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 devel-
oped 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 ver-
sion 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 ver-
sion 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 sys-
tems. 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 specified (or if the hard option is speci-
fied), NFS requests are retried indefinitely. If the soft option is speci-
fied, then the NFS client fails an NFS request after retrans retransmis-
sions have been sent, causing the NFS client to return an error to the
calling application.
NB: A so-called "soft" timeout can cause silent data corruption in certain
cases. As such, use the soft option only when client responsiveness is more
important than data integrity. Using NFS over TCP or increasing the value
of the retrans option may mitigate some of the risks of using the soft
option.
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 perform
any kind of timeout backoff for NFS over TCP.
However, for NFS over UDP, the client uses an adaptive algorithm to esti-
mate an appropriate timeout value for frequently used request types (such
as READ and WRITE requests), but uses the timeo setting for infrequently
used request types (such as FSINFO requests). If the timeo option is not
specified, infrequently used request types are retried after 1.1 seconds.
After each retransmission, the NFS client doubles the timeout for that
request, up to a maximum timeout length of 60 seconds.
retrans=n The number of times the NFS client retries a request before it attempts
further recovery action. If the retrans option is not specified, the NFS
client tries each request three times.
The NFS client generates a "server not responding" message after retrans
retries, then attempts further recovery (depending on whether the hard
mount option is in effect).
rsize=n The maximum number of bytes in each network READ request that the NFS
client can receive when reading data from a file on an NFS server. The
actual data payload size of each NFS READ request is equal to or smaller
than the rsize setting. The largest read payload supported by the Linux NFS
client is 1,048,576 bytes (one megabyte).
The rsize value is a positive integral multiple of 1024. Specified rsize
values lower than 1024 are replaced with 4096; values larger than 1048576
are replaced with 1048576. If a specified value is within the supported
range but not a multiple of 1024, it is rounded down to the nearest multi-
ple of 1024.
If an rsize value is not specified, or if the specified rsize value is
larger than the maximum that either client or server can support, the
client and server negotiate the largest rsize value that they can both sup-
port.
The rsize mount option as specified on the mount(8) command line appears in
the /etc/mtab file. However, the effective rsize value negotiated by the
client and server is reported in the /proc/mounts file.
wsize=n The maximum number of bytes per network WRITE request that the NFS client
can send when writing data to a file on an NFS server. The actual data pay-
load size of each NFS WRITE request is equal to or smaller than the wsize
setting. The largest write payload supported by the Linux NFS client is
1,048,576 bytes (one megabyte).
Similar to rsize , the wsize value is a positive integral multiple of 1024.
Specified wsize values lower than 1024 are replaced with 4096; values
larger than 1048576 are replaced with 1048576. If a specified value is
within the supported range but not a multiple of 1024, it is rounded down
to the nearest multiple of 1024.
If a wsize value is not specified, or if the specified wsize value is
larger than the maximum that either client or server can support, the
client and server negotiate the largest wsize value that they can both sup-
port.
The wsize mount option as specified on the mount(8) command line appears in
the /etc/mtab file. However, the effective wsize value negotiated by the
client and server is reported in the /proc/mounts file.
ac / noac Selects whether the client may cache file attributes. If neither option is
specified (or if ac is specified), the client caches file attributes.
To improve performance, NFS clients cache file attributes. Every few sec-
onds, an NFS client checks the server's version of each file's attributes
for updates. Changes that occur on the server in those small intervals
remain undetected until the client checks the server again. The noac option
prevents clients from caching file attributes so that applications can more
quickly detect file changes on the server.
In addition to preventing the client from caching file attributes, the noac
option forces application writes to become synchronous so that local
changes to a file become visible on the server immediately. That way,
other clients can quickly detect recent writes when they check the file's
attributes.
Using the noac option provides greater cache coherence among NFS clients
accessing the same files, but it extracts a significant performance
penalty. As such, judicious use of file locking is encouraged instead.
The DATA AND METADATA COHERENCE section contains a detailed discussion of
these trade-offs.
acregmin=n The minimum time (in seconds) that the NFS client caches attributes of a
regular file before it requests fresh attribute information from a server.
If this option is not specified, the NFS client uses a 3-second minimum.
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 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).
retry=n The number of minutes that the mount(8) command retries an NFS mount opera-
tion in the foreground or background before giving up. If this option is
not specified, the default value for foreground mounts is 2 minutes, and
the default value for background mounts is 10000 minutes (80 minutes shy of
one week).
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 con-
tents to applications when the same remote file is accessed via different
mount points.
If neither option is specified, or if the sharecache option is specified,
then a single cache is used for all mount points that access the same
export. If the nosharecache option is specified, then that mount point
gets a unique cache. Note that when data and attribute caches are shared,
the mount options from the first mount point take effect for subsequent
concurrent mounts of the same export.
As of kernel 2.6.18, the behavior specified by nosharecache is legacy
caching behavior. This is considered a data risk since multiple cached
copies of the same file on the same client can become out of sync following
a local update of one of the copies.
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 transmit 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 communi-
cates 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 specified port, the mount request fails.
If this option is not specified, or if the specified port value is 0, then
the NFS client uses the NFS service port number advertised by the server's
rpcbind service. The mount request fails if the server's rpcbind service
is not available, the server's NFS service is not registered with its
rpcbind service, or the server's NFS service is not available on the adver-
tised port.
mountport=n The numeric value of the server's mountd port. If the server's mountd ser-
vice is not available on the specified port, the mount request fails.
If this option is not specified, or if the specified port value is 0, then
the mount(8) command uses the mountd service port number advertised by the
server's rpcbind service. The mount request fails if the server's rpcbind
service is not available, the server's mountd service is not registered
with its rpcbind service, or the server's mountd service is not available
on the advertised port.
This option can be used when mounting an NFS server through a firewall that
blocks the rpcbind protocol.
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.
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 speci-
fied, the client attempts to use version 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 systems.
lock / nolock Selects whether to use the NLM sideband protocol to lock files on the
server. If neither option is specified (or if lock is specified), NLM
locking is used for this mount point. When using the nolock option, appli-
cations can lock files, but such locks provide exclusion only against other
applications running on the same client. Remote applications are not
affected by these locks.
NLM locking must be disabled with the nolock option when using NFS to mount
/var because /var contains files used by the NLM implementation on Linux.
Using the nolock option is also required when mounting exports on NFS
servers that do not support the NLM protocol.
intr / nointr Selects whether to allow signals to interrupt file operations on this mount
point. If neither option is specified (or if nointr is specified), signals
do not interrupt 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 coherence semantics. If the nocto option is specified, the client
uses a non-standard heuristic to determine when files on the server have
changed.
Using the nocto option may improve performance for read-only mounts, but
should be used only if the data on the server changes only occasionally.
The DATA AND METADATA COHERENCE section discusses the behavior of this
option in more detail.
acl / noacl Selects whether to use the NFSACL sideband protocol on this mount point.
The NFSACL sideband protocol is a proprietary protocol implemented in
Solaris that manages Access Control Lists. NFSACL was never made a standard
part of the NFS protocol specification.
If neither acl nor noacl option is specified, the NFS client negotiates
with the server to see if the NFSACL protocol is supported, and uses it if
the server supports it. Disabling the NFSACL sideband protocol may be nec-
essary if the negotiation causes problems on the client or server. Refer
to the SECURITY CONSIDERATIONS section for more details.
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 transmit 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 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.
intr / nointr Selects whether to allow signals to interrupt file operations on this mount
point. If neither option is specified (or if intr is specified), system
calls return EINTR if an in-progress NFS operation is interrupted by a sig-
nal. If nointr is specified, signals do not interrupt 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 discusses 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 net-
work interfaces, special routing policies, or atypical 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 sys-
tem 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 rea-
sonable defaults for NFS behavior.
server:/export /mnt nfs defaults 0 0
Here is an example from an /etc/fstab file for an NFS version 2 mount over UDP.
server:/export /mnt nfs nfsvers=2,proto=udp 0 0
Try this example to mount using NFS version 4 over TCP with Kerberos 5 mutual authentica-
tion.
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 authentica-
tion, adjusts request parameters for different byte endianness on client and server, and
retransmits requests that may have been lost by the network or server. RPC requests and
replies flow over a network transport.
In most cases, the mount(8) command, NFS client, and NFS server can automatically negoti-
ate proper transport and data transfer size settings for a mount point. In some cases,
however, it pays to specify these settings explicitly using mount options.
Traditionally, NFS clients used the UDP transport exclusively for transmitting requests to
servers. Though its implementation is simple, NFS over UDP has many limitations that pre-
vent smooth operation and good performance in some common deployment environments. Even
an insignificant packet loss rate results in the loss of whole NFS requests; as such,
retransmit timeouts are usually in the subsecond range to allow clients to recover quickly
from dropped requests, but this can result in extraneous network traffic and server load.
However, UDP can be quite effective in specialized settings where the networks MTU is
large relative to NFSs data transfer size (such as network environments that enable jumbo
Ethernet frames). In such environments, trimming the rsize and wsize settings so that
each NFS read or write request fits in just a few network frames (or even in a single
frame) is advised. This reduces the probability that the loss of a single MTU-sized net-
work frame results in the loss of an entire large read or write request.
TCP is the default transport protocol used for all modern NFS implementations. It per-
forms well in almost every conceivable network environment and provides excellent guaran-
tees against data corruption caused by network unreliability. TCP is often a requirement
for mounting a server through a network firewall.
Under normal circumstances, networks drop packets much more frequently than NFS servers
drop requests. As such, an aggressive retransmit timeout setting for NFS over TCP is
unnecessary. Typical timeout settings for NFS over TCP are between one and ten minutes.
After the client exhausts its retransmits (the value of the retrans mount option), it
assumes a network partition has occurred, and attempts to reconnect to the server on a
fresh socket. Since TCP itself makes network data transfer reliable, rsize and wsize can
safely be allowed to default to the largest values supported by both client and server,
independent of the network's MTU size.
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 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.
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 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 any server 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.
Weak cache consistency
There are still opportunities for a client's data cache to contain stale data. The NFS
version 3 protocol introduced "weak cache consistency" (also known as WCC) which provides
a way of efficiently checking a file's attributes before and after a single request. This
allows a client to help identify changes that could have been made by other clients.
When a client is using many concurrent operations that update the same file at the same
time (for example, during asynchronous write behind), it is still difficult to tell
whether it was that client's updates or some other client's updates that altered the file.
Attribute caching
Use the noac mount option to achieve attribute cache coherence among multiple clients.
Almost every file system operation checks file attribute information. The client keeps
this information cached for a period of time to reduce network and server load. When noac
is in effect, a client's file attribute cache is disabled, so each operation that needs to
check a file's attributes is forced to go back to the server. This permits a client to
see changes to a file very quickly, at the cost of many extra network operations.
Be careful not to confuse the noac option with "no data caching." The noac mount option
prevents the client from caching file metadata, but there are still races that may result
in data cache incoherence between client and server.
The NFS protocol is not designed to support true cluster file system cache coherence with-
out some type of application serialization. If absolute cache coherence among clients is
required, applications should use file locking. Alternatively, applications can also open
their files with the O_DIRECT flag to disable data caching entirely.
The sync mount option
The NFS client treats the sync mount option differently than some other file systems
(refer to mount(8) for a description of the generic sync and async mount options). If
neither sync nor async is specified (or if the async option is specified), the NFS client
delays sending application writes to the server until any of these events occur:
Memory pressure forces reclamation of system memory resources.
An application flushes file data explicitly with sync(2), msync(2), or fsync(3).
An application closes a file with close(2).
The file is locked/unlocked via fcntl(2).
In other words, under normal circumstances, data written by an application may not immedi-
ately appear on the server that hosts the file.
If the sync option is specified on a mount point, any system call that writes data to
files on that mount point causes that data to be flushed to the server before the system
call returns control to user space. This provides greater data cache coherence among
clients, but at a significant performance cost.
Applications can use the O_SYNC open flag to force application writes to individual files
to go to the server immediately without the use of the sync mount option.
Using file locks with NFS
The Network Lock Manager protocol is a separate sideband protocol used to manage file
locks in NFS version 2 and version 3. To support lock recovery after a client or server
reboot, a second sideband protocol -- known as the Network Status Manager protocol -- is
also required. In NFS version 4, file locking is supported directly in the main NFS pro-
tocol, and the NLM and NSM sideband protocols are not used.
In most cases, NLM and NSM services are started automatically, and no extra configuration
is required. Configure all NFS clients with fully-qualified domain names to ensure that
NFS servers can find clients to notify them of server reboots.
NLM supports advisory file locks only. To lock NFS files, use fcntl(2) with the F_GETLK
and F_SETLK commands. The NFS client converts file locks obtained via flock(2) to advi-
sory locks.
When mounting servers that do not support the NLM protocol, or when mounting an NFS server
through a firewall that blocks the NLM service port, specify the nolock mount option. NLM
locking must be disabled with the nolock option when using NFS to mount /var because /var
contains files used by the NLM implementation on Linux.
Specifying the nolock option may also be advised to improve the performance of a propri-
etary application which runs on a single client and uses file locks extensively.
NFS version 4 caching features
The data and metadata caching behavior of NFS version 4 clients is similar to that of ear-
lier versions. However, NFS version 4 adds two features that improve cache behavior:
change attributes and file delegation.
The change attribute is a new part of NFS file and directory metadata which tracks data
changes. It replaces the use of a file's modification and change time stamps as a way for
clients to validate the content of their caches. Change attributes are independent of the
time stamp resolution on either the server or client, however.
A file delegation is a contract between an NFS version 4 client and server that allows the
client to treat a file temporarily as if no other client is accessing it. The server
promises to notify the client (via a callback request) if another client attempts to
access that file. Once a file has been delegated to a client, the client can cache that
file's data and metadata aggressively without contacting the server.
File delegations come in two flavors: read and write. A read delegation means that the
server notifies the client about any other clients that want to write to the file. A
write delegation means that the client gets notified about either read or write accessors.
Servers grant file delegations when a file is opened, and can recall delegations at any
time when another client wants access to the file that conflicts with any delegations
already granted. Delegations on directories are not supported.
In order to support delegation callback, the server checks the network return path to the
client during the client's initial contact with the server. If contact with the client
cannot be established, the server simply does not grant any delegations to that client.
SECURITY CONSIDERATIONS
NFS servers control access to file data, but they depend on their RPC implementation to
provide authentication of NFS requests. Traditional NFS access control mimics the stan-
dard mode bit access control provided in local file systems. Traditional RPC authentica-
tion uses a number to represent each user (usually the user's own uid), a number to repre-
sent the user's group (the user's gid), and a set of up to 16 auxiliary group numbers to
represent other groups of which the user may be a member.
Typically, file data and user ID values appear unencrypted (i.e. "in the clear") on the
network. Moreover, NFS versions 2 and 3 use separate sideband protocols for mounting,
locking and unlocking files, and reporting system status of clients and servers. These
auxiliary protocols use no authentication.
In addition to combining these sideband protocols with the main NFS protocol, NFS version
4 introduces more advanced forms of access control, authentication, and in-transit data
protection. The NFS version 4 specification mandates NFSv4 ACLs, RPCGSS authentication,
and RPCGSS 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 mount-
ing, 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 cryptographic proof of a user's identity in
each RPC request. This provides strong verification of the identity of users accessing
data on the server. Note that additional configuration besides adding this mount option
is required in order to enable Kerberos security. Refer to the rpc.gssd(8) man page for
details.
Two additional flavors of Kerberos security are supported: krb5i and krb5p. The krb5i
security flavor provides a cryptographically strong guarantee that the data in each RPC
request has not been tampered with. The krb5p security flavor encrypts every RPC request
to prevent data exposure during network transit; however, expect some performance impact
when using integrity checking or encryption. Similar support for other forms of crypto-
graphic security (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 specifies a single security flavor at mount time which remains in effect
for the lifetime of the mount. If the server does not support this flavor, the initial
mount request is rejected by the server.
Mounting through a firewall
A firewall may reside between an NFS client and server, or the client or server may block
some of its own ports via IP filter rules. It is still possible to mount an NFS server
through a firewall, though some of the mount(8) command's automatic service endpoint dis-
covery mechanisms may not work; this requires you to provide specific endpoint details via
NFS mount options.
NFS servers normally run a portmapper or rpcbind daemon to advertise their service end-
points to clients. Clients use the rpcbind daemon to determine:
What network port each RPC-based service is using
What transport protocols each RPC-based service supports
The rpcbind daemon uses a well-known port number (111) to help clients find a service end-
point. Although NFS often uses a standard port number (2049), auxiliary services such as
the NLM service can choose any unused port number at random.
Common firewall configurations block the well-known rpcbind port. In the absense of an
rpcbind service, the server administrator fixes the port number of NFS-related services so
that the firewall can allow access to specific NFS service ports. Client administrators
then specify the port number for the mountd service via the mount(8) command's mountport
option. It may also be necessary to enforce the use of TCP or UDP if the firewall blocks
one of those transports.
NFS Access Control Lists
Solaris allows NFS version 3 clients direct access to POSIX Access Control Lists stored in
its local file systems. This proprietary sideband protocol, known as NFSACL, provides
richer access control than mode bits. Linux implements this protocol for compatibility
with the Solaris NFS implementation. The NFSACL protocol never became a standard part of
the NFS version 3 specification, however.
The NFS version 4 specification mandates a new version of Access Control Lists that are
semantically richer than POSIX ACLs. NFS version 4 ACLs are not fully compatible with
POSIX ACLs; as such, some translation between the two is required in an environment that
mixes POSIX ACLs and NFS version 4.
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. Perform-
ing a remount on an NFS file system mounted with the noac option may have unintended con-
sequences. 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 referrals, and named attributes.
SEE ALSO
fstab(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)
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