NFS(5) File Formats Manual NFS(5)
NAME
nfs - fstab format and options for the nfs file systems
SYNOPSIS
/etc/fstab
DESCRIPTION
NFS is an Internet Standard protocol created by Sun Microsystems in
1984. NFS was 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 ac-
companied by an interface identifier. See ipv6(7) for details on spec-
ifying raw IPv6 addresses.
The fstype field contains "nfs". Use of the "nfs4" fstype in
/etc/fstab is deprecated.
MOUNT OPTIONS
Refer to mount(8) for a description of generic mount options available
for all file 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 suit-
able 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 sys-
tems
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 re-
trans 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 op-
tion 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 re-
quest.
For NFS over TCP the default timeo value is 600 (60 sec-
onds). 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 re-
transmission, 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 be-
fore 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" 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 ne-
gotiate the largest rsize value that they can both sup-
port.
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 ne-
gotiate the largest wsize value that they can both sup-
port.
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 at-
tributes. 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 be-
come 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 ex-
tracts a significant performance penalty. As such, ju-
dicious 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 at-
tribute 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 at-
tribute 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 at-
tempt 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).
rdirplus / nordirplus
Selects whether to use NFS v3 or v4 READDIRPLUS re-
quests. 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 directo-
ries.
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 CONSIDER-
ATIONS 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 op-
tion 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.
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 impor-
tant 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 at-
tributes expire.
If pos or positive is specified, the client assumes pos-
itive entries are valid until their parent directory's
cached attributes expire, but always revalidates nega-
tive 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 ap-
plication and server performance.
The DATA AND METADATA COHERENCE section contains a de-
tailed discussion of these trade-offs.
fsc / nofsc Enable/Disables the cache of (read-only) data pages to
the local disk using the FS-Cache facility. See
cachefilesd(8) and <kernel_soruce>/Documenta-
tion/filesystems/caching for detail on how to configure
the FS-Cache 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 com-
municate 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 METH-
ODS 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 METH-
ODS 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 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 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, differ-
ent 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 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) re-
ports the proper maximum component length to applica-
tions 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 se-
mantics. 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.
local_lock=mechanism
Specifies whether to use local locking for any or both
of the flock and the POSIX locking mechanisms. mecha-
nism can be one of all, flock, posix, or none. This op-
tion 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 speci-
fied, 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 com-
municate 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 ver-
sion 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 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.
cto / nocto Selects whether to use close-to-open cache coherence se-
mantics 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 op-
tion. 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 au-
tomatically. 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.
migration / nomigration
Selects whether the client uses an identification string
that is compatible with NFSv4 Transparent State Migra-
tion (TSM). If the mounted server supports NFSv4 migra-
tion with TSM, specify the migration option.
Some server features misbehave in the face of a migra-
tion-compatible identification string. The nomigration
option retains the use of a traditional client indenti-
fication 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 mi-
grated transparently when it identifies itself via a
traditional identification string.
This mount option has no effect with NFSv4 minor ver-
sions newer than zero, which always use TSM-compatible
client identification strings.
nfs4 FILE SYSTEM TYPE
The nfs4 file system type is an old syntax for specifying NFSv4 usage.
It can still be used with all NFSv4-specific and common options, ex-
cepted the nfsvers mount option.
MOUNT CONFIGURATION FILE
If the mount command is configured to do so, all of the mount options
described in the previous section can also be configured in the
/etc/nfsmount.conf file. See nfsmount.conf(5) for details.
EXAMPLES
To mount an export using NFS version 2, use the nfs file system type
and specify the nfsvers=2 mount option. To mount using NFS version 3,
use the nfs file system type and specify the nfsvers=3 mount option.
To mount using NFS version 4, use either the nfs file system type, with
the nfsvers=4 mount option, or the nfs4 file system type.
The following example from an /etc/fstab file causes the mount command
to negotiate 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 Ker-
beros 5 mutual authentication.
server:/export /mnt nfs4 sec=krb5 0 0
This example shows how to mount using NFS version 4 over TCP with Ker-
beros 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-lo-
cal address.
[fe80::215:c5ff:fb3e:e2b1%eth0]:/export /mnt nfs defaults 0 0
TRANSPORT METHODS
NFS clients send requests to NFS servers via Remote Procedure Calls, or
RPCs. The RPC client discovers remote service endpoints automatically,
handles per-request 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 au-
tomatically 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 in-
significant packet loss rate results in the loss of whole NFS requests;
as such, retransmit timeouts are usually in the subsecond range to al-
low clients to recover quickly from dropped requests, but this can re-
sult 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 net-
work environments that enable jumbo Ethernet frames). In such environ-
ments, 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 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 op-
tion), it assumes a network partition has occurred, and attempts to re-
connect to the server on a fresh socket. Since TCP itself makes network
data transfer reliable, rsize and wsize can safely be allowed to de-
fault to the largest values supported by both client and server, inde-
pendent 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 Man-
ager (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 (NF-
SACL) 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 ser-
vices.
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 trans-
ports 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 right-
most instance of each of these options takes effect.
Using NFS over UDP on high-speed links
Using NFS over UDP on high-speed links such as Gigabit can cause silent
data corruption.
The problem can be triggered at high loads, and is caused by problems
in IP fragment reassembly. NFS read and writes typically transmit UDP
packets of 4 Kilobytes or more, which have to be broken up into several
fragments in order to be sent over the Ethernet link, which limits
packets to 1500 bytes by default. This process happens at the IP net-
work layer and is called fragmentation.
In order to identify fragments that belong together, IP assigns a 16bit
IP ID value to each packet; fragments generated from the same UDP
packet will have the same IP ID. The receiving system will collect
these fragments and combine them to form the original UDP packet. This
process is called reassembly. The default timeout for packet reassembly
is 30 seconds; if the network stack does not receive all fragments of a
given packet within this interval, it assumes the missing fragment(s)
got lost and discards those it already received.
The problem this creates over high-speed links is that it is possible
to send more than 65536 packets within 30 seconds. In fact, with heavy
NFS traffic one can observe that the IP IDs repeat after about 5 sec-
onds.
This has serious effects on reassembly: if one fragment gets lost, an-
other fragment from a different packet but with the same IP ID will ar-
rive 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 ran-
dom (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, be-
cause 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 possi-
ble, 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 frag-
ments 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 frag-
mented traffic from distant peers.
DATA AND METADATA COHERENCE
Some modern cluster file systems provide perfect cache coherence among
their clients. Perfect cache coherence among disparate NFS clients is
expensive to achieve, especially on wide area networks. As such, NFS
settles for weaker cache coherence that satisfies the requirements of
most file sharing types.
Close-to-open cache consistency
Typically file sharing is completely sequential. First client A opens
a file, writes something to it, then closes it. Then client B opens
the same file, and reads the changes.
When an application opens a file stored on an NFS version 3 server, the
NFS client checks that the file exists on the server and is permitted
to the opener by sending a GETATTR or ACCESS request. The NFS client
sends these requests regardless of the freshness of the file's cached
attributes.
When the application closes the file, the NFS client writes back any
pending changes to the file so that the next opener can view the
changes. This also gives the NFS client an opportunity to report write
errors to the application via the return code from close(2).
The behavior of checking at open time and flushing at close time is re-
ferred to as close-to-open cache consistency, or CTO. It can be dis-
abled 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 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 at-
tribute information. The client keeps this information cached for a
period of time to reduce network and server load. When noac is in ef-
fect, 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 ab-
solute 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 up-
dates on an NFS server appear to applications on NFS clients.
To comply with the POSIX filesystem standard, the Linux NFS client re-
lies on NFS servers to keep a file's mtime and ctime timestamps prop-
erly 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/norela-
time, 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 re-
ferred to as a positive lookup result. If the requested directory en-
try 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 re-
moval 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 posi-
tive lookup results. This permitted applications to detect new direc-
tory 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 ex-
act 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 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 support for strong authentication and security
flavors that provide per-RPC integrity checking and encryption. Be-
cause 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 pro-
vides 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 im-
pact 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 filesys-
tem.
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 ir-
revocably 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 it-
self 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 perform-
ing 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 in-
terfering with the other's lease.
The Linux NFS client establishes one lease for each server. Lease man-
agement 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 secu-
rity 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 de-
fault, 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 iden-
tify users making NFS requests. An NFS server assumes that if a con-
nection comes from a privileged port, the UID and GID numbers in the
NFS requests on this connection have been verified by the client's ker-
nel 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 some-
what, 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 possi-
ble, 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 mecha-
nisms may not work; this requires you to provide specific endpoint de-
tails 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 ac-
cess 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 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 So-
laris 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 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.
THE REMOUNT OPTION
Generic mount options such as rw and sync can be modified on NFS mount
points using the remount option. See mount(8) for more information on
generic mount options.
With few exceptions, NFS-specific options are not able to be modified
during a remount. The underlying transport or NFS version cannot be
changed by a remount, for example.
Performing a remount on an NFS file system mounted with the noac option
may have 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 subcom-
mand depends on knowing the original set of mount options used to per-
form the MNT operation. These options are stored on disk by the NFS
mount subcommand, and can be erased by a remount.
To ensure that the saved mount options are not erased during a remount,
specify either the local mount directory, or the server hostname and
export pathname, but not both, during a remount. For example,
mount -o remount,ro /mnt
merges the mount option ro with the mount options already saved on disk
for the NFS server mounted at /mnt.
FILES
/etc/fstab file system table
/etc/nfsmount.conf
Configuration file for NFS mounts
BUGS
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 ALSO
fstab(5), mount(8), umount(8), mount.nfs(5), umount.nfs(5), exports(5),
nfsmount.conf(5), netconfig(5), ipv6(7), nfsd(8), sm-notify(8),
rpc.statd(8), rpc.idmapd(8), rpc.gssd(8), rpc.svcgssd(8), kerberos(1)
RFC 768 for the UDP specification.
RFC 793 for the TCP specification.
RFC 1094 for the NFS version 2 specification.
RFC 1813 for the NFS version 3 specification.
RFC 1832 for the XDR specification.
RFC 1833 for the RPC bind specification.
RFC 2203 for the RPCSEC GSS API protocol specification.
RFC 3530 for the NFS version 4 specification.
9 October 2012 NFS(5)