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 accompanied by an
interface identifier. See
ipv6(7) for details on specifying raw IPv6
addresses.
The
fstype field contains "nfs". Use of the "nfs4"
fstype in
/etc/fstab is deprecated.
MOUNT OPTIONS¶
Refer to
mount(8) for a description of generic mount options available
for all file 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.
- soft / hard
- Determines the recovery behavior of the NFS client after an NFS request
times out. If neither option is specified (or if the hard option is
specified), NFS requests are retried indefinitely. If the soft
option is specified, then the NFS client fails an NFS request after
retrans retransmissions have been sent, causing the NFS client to
return an error to the calling application.
- NB: A so-called "soft" timeout can cause silent data
corruption in certain cases. As such, use the soft option only when
client responsiveness is more important than data integrity. Using NFS
over TCP or increasing the value of the retrans option may mitigate
some of the risks of using the soft option.
- timeo=n
- The time in deciseconds (tenths of a second) the NFS client waits for a
response before it retries an NFS request.
- For NFS over TCP the default timeo value is 600 (60 seconds). The
NFS client performs linear backoff: After each retransmission the timeout
is increased by timeo up to the maximum of 600 seconds.
- However, for NFS over UDP, the client uses an adaptive algorithm to
estimate an appropriate timeout value for frequently used request types
(such as READ and WRITE requests), but uses the timeo setting for
infrequently used request types (such as FSINFO requests). If the
timeo option is not specified, infrequently used request types are
retried after 1.1 seconds. After each retransmission, the NFS client
doubles the timeout for that request, up to a maximum timeout length of 60
seconds.
- retrans=n
- The number of times the NFS client retries a request before it attempts
further recovery action. If the retrans option is not specified,
the NFS client tries each 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 multiple of 1024.
- If an rsize value is not specified, or if the specified
rsize value is larger than the maximum that either client or server
can support, the client and server negotiate the largest rsize
value that they can both support.
- The rsize mount option as specified on the mount(8) command
line appears in the /etc/mtab file. However, the effective
rsize value negotiated by the client and server is reported in the
/proc/mounts file.
- wsize=n
- The maximum number of bytes per network WRITE request that the NFS client
can send when writing data to a file on an NFS server. The actual data
payload size of each NFS WRITE request is equal to or smaller than the
wsize setting. The largest write payload supported by the Linux NFS
client is 1,048,576 bytes (one megabyte).
- Similar to rsize , the wsize value is a positive integral
multiple of 1024. Specified wsize values lower than 1024 are
replaced with 4096; values larger than 1048576 are replaced with 1048576.
If a specified value is within the supported range but not a multiple of
1024, it is rounded down to the nearest multiple of 1024.
- If a wsize value is not specified, or if the specified wsize
value is larger than the maximum that either client or server can support,
the client and server negotiate the largest wsize value that they
can both support.
- The wsize mount option as specified on the mount(8) command
line appears in the /etc/mtab file. However, the effective
wsize value negotiated by the client and server is reported in the
/proc/mounts file.
- ac / noac
- Selects whether the client may cache file attributes. If neither option is
specified (or if ac is specified), the client caches file
attributes.
- To improve performance, NFS clients cache file attributes. Every few
seconds, an NFS client checks the server's version of each file's
attributes for updates. Changes that occur on the server in those small
intervals remain undetected until the client checks the server again. The
noac option prevents clients from caching file attributes so that
applications can more quickly detect file changes on the server.
- In addition to preventing the client from caching file attributes, the
noac option forces application writes to become synchronous so that
local changes to a file become visible on the server immediately. That
way, other clients can quickly detect recent writes when they check the
file's attributes.
- Using the noac option provides greater cache coherence among NFS
clients accessing the same files, but it extracts a significant
performance penalty. As such, judicious use of file locking is encouraged
instead. The DATA AND METADATA COHERENCE section contains a detailed
discussion of these trade-offs.
- acregmin=n
- The minimum time (in seconds) that the NFS client caches attributes of a
regular file before it requests fresh attribute information from a server.
If this option is not specified, the NFS client uses a 3-second
minimum.
- 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).
- rdirplus / nordirplus
- Selects whether to use NFS v3 or v4 READDIRPLUS requests. If this option
is not specified, the NFS client uses READDIRPLUS requests on NFS v3 or v4
mounts to read small directories. Some applications perform better if the
client uses only READDIR requests for all directories.
- retry=n
- The number of minutes that the mount(8) command retries an NFS
mount operation in the foreground or background before giving up. If this
option is not specified, the default value for foreground mounts is 2
minutes, and the default value for background mounts is 10000 minutes (80
minutes shy of one week). If a value of zero is specified, the
mount(8) command exits immediately after the first failure.
- sec=flavor
- The security flavor to use for accessing files on this mount point. If the
server does not support this flavor, the mount operation fails. If
sec= is not specified, the client attempts to find a security
flavor that both the client and the server supports. Valid flavors
are none, sys, krb5, krb5i, and krb5p.
Refer to the SECURITY CONSIDERATIONS section for details.
- sharecache / nosharecache
- Determines how the client's data cache and attribute cache are shared when
mounting the same export more than once concurrently. Using the same cache
reduces memory requirements on the client and presents identical file
contents to applications when the same remote file is accessed via
different mount points.
- If neither option is specified, or if the sharecache option is
specified, then a single cache is used for all mount points that access
the same export. If the nosharecache option is specified, then that
mount point gets a unique cache. Note that when data and attribute caches
are shared, the mount options from the first mount point take effect for
subsequent concurrent mounts of the same export.
- As of kernel 2.6.18, the behavior specified by nosharecache is
legacy caching behavior. This is considered a data risk since multiple
cached copies of the same file on the same client can become out of sync
following a local update of one of the copies.
- resvport / noresvport
- Specifies whether the NFS client should use a privileged source port when
communicating with an NFS server for this mount point. If this option is
not specified, or the resvport option is specified, the NFS client
uses a privileged source port. If the noresvport option is
specified, the NFS client uses a non-privileged source port. This option
is supported in kernels 2.6.28 and later.
- Using non-privileged source ports helps increase the maximum number of NFS
mount points allowed on a client, but NFS servers must be configured to
allow clients to connect via non-privileged source ports.
- Refer to the SECURITY CONSIDERATIONS section for important details.
- lookupcache=mode
- Specifies how the kernel manages its cache of directory entries for a
given mount point. mode can be one of all, none,
pos, or positive. This option is supported in kernels 2.6.28
and later.
- The Linux NFS client caches the result of all NFS LOOKUP requests. If the
requested directory entry exists on the server, the result is referred to
as positive. If the requested directory entry does not exist on the
server, the result is referred to as negative.
- If this option is not specified, or if all is specified, the client
assumes both types of directory cache entries are valid until their parent
directory's cached attributes expire.
- If pos or positive is specified, the client assumes positive
entries are valid until their parent directory's cached attributes expire,
but always revalidates negative entires before an application can use
them.
- If none is specified, the client revalidates both types of
directory cache entries before an application can use them. This permits
quick detection of files that were created or removed by other clients,
but can impact application and server performance.
- The DATA AND METADATA COHERENCE section contains a detailed discussion of
these trade-offs.
- fsc / nofsc
- Enable/Disables the cache of (read-only) data pages to the local disk
using the FS-Cache facility. See cachefilesd(8) and
<kernel_soruce>/Documentation/filesystems/caching for detail on how
to configure the FS-Cache facility. Default value is nofsc.
Options for NFS versions 2 and 3 only¶
Use these options, along with the options in the above subsection, for NFS
versions 2 and 3 only.
- proto=netid
- The netid determines the transport that is used to communicate with
the NFS server. Available options are udp, udp6, tcp,
tcp6, and rdma. Those which end in 6 use IPv6
addresses and are only available if support for TI-RPC is built in. Others
use IPv4 addresses.
- Each transport protocol uses different default retrans and
timeo settings. Refer to the description of these two mount options
for details.
- In addition to controlling how the NFS client transmits requests to the
server, this mount option also controls how the mount(8) command
communicates with the server's rpcbind and mountd services. Specifying a
netid that uses TCP forces all traffic from the mount(8) command
and the NFS client to use TCP. Specifying a netid that uses UDP forces all
traffic types to use UDP.
- Before using NFS over UDP, refer to the TRANSPORT METHODS
section.
- If the proto mount option is not specified, the mount(8)
command discovers which protocols the server supports and chooses an
appropriate transport for each service. Refer to the TRANSPORT METHODS
section for more details.
- udp
- The udp option is an alternative to specifying proto=udp. It
is included for compatibility with other operating systems.
- Before using NFS over UDP, refer to the TRANSPORT METHODS
section.
- tcp
- The tcp option is an alternative to specifying proto=tcp. It
is included for compatibility with other operating systems.
- rdma
- The rdma option is an alternative to specifying
proto=rdma.
- port=n
- The numeric value of the server's NFS service port. If the server's NFS
service is not available on the specified port, the mount request
fails.
- If this option is not specified, or if the specified port value is 0, then
the NFS client uses the NFS service port number advertised by the server's
rpcbind service. The mount request fails if the server's rpcbind service
is not available, the server's NFS service is not registered with its
rpcbind service, or the server's NFS service is not available on the
advertised port.
- mountport=n
- The numeric value of the server's mountd port. If the server's mountd
service is not available on the specified port, the mount request
fails.
- If this option is not specified, or if the specified port value is 0, then
the mount(8) command uses the mountd service port number advertised
by the server's rpcbind service. The mount request fails if the server's
rpcbind service is not available, the server's mountd service is not
registered with its rpcbind service, or the server's mountd service is not
available on the advertised port.
- This option can be used when mounting an NFS server through a firewall
that blocks the rpcbind protocol.
- mountproto=netid
- The transport the NFS client uses to transmit requests to the NFS server's
mountd service when performing this mount request, and when later
unmounting this mount point.
- netid may be one of udp, and tcp which use IPv4
address or, if TI-RPC is built into the mount.nfs command,
udp6, and tcp6 which use IPv6 addresses.
- This option can be used when mounting an NFS server through a firewall
that blocks a particular transport. When used in combination with the
proto option, different transports for mountd requests and NFS
requests can be specified. If the server's mountd service is not available
via the specified transport, the mount request fails.
- Refer to the TRANSPORT METHODS section for more on how the
mountproto mount option interacts with the proto mount
option.
- mounthost=name
- The hostname of the host running mountd. If this option is not specified,
the mount(8) command assumes that the mountd service runs on the
same host as the NFS service.
- mountvers=n
- The RPC version number used to contact the server's mountd. If this option
is not specified, the client uses a version number appropriate to the
requested NFS version. This option is useful when multiple NFS services
are running on the same remote server host.
- namlen=n
- The maximum length of a pathname component on this mount. If this option
is not specified, the maximum length is negotiated with the server. In
most cases, this maximum length is 255 characters.
- Some early versions of NFS did not support this negotiation. Using this
option ensures that pathconf(3) reports the proper maximum
component length to applications in such cases.
- nfsvers=n
- The NFS protocol version number used to contact the server's NFS service.
If the server does not support the requested version, the mount request
fails. If this option is not specified, the client negotiates a suitable
version with the server, trying version 4 first, version 3 second, and
version 2 last.
- vers=n
- This option is an alternative to the nfsvers option. It is included
for compatibility with other operating systems.
- lock / nolock
- Selects whether to use the NLM sideband protocol to lock files on the
server. If neither option is specified (or if lock is specified),
NLM locking is used for this mount point. When using the nolock
option, applications can lock files, but such locks provide exclusion only
against other applications running on the same client. Remote applications
are not affected by these locks.
- NLM locking must be disabled with the nolock option when using NFS
to mount /var because /var contains files used by the NLM
implementation on Linux. Using the nolock option is also required
when mounting exports on NFS servers that do not support the NLM
protocol.
- intr / nointr
- Selects whether to allow signals to interrupt file 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.
- The intr / nointr mount option is deprecated after kernel
2.6.25. Only SIGKILL can interrupt a pending NFS operation on these
kernels, and if specified, this mount option is ignored to provide
backwards compatibility with older kernels.
- cto / nocto
- Selects whether to use close-to-open cache coherence semantics. If neither
option is specified (or if cto is specified), the client uses
close-to-open cache coherence semantics. If the nocto option is
specified, the client uses a non-standard heuristic to determine when
files on the server have changed.
- Using the nocto option may improve performance for read-only
mounts, but should be used only if the data on the server changes only
occasionally. The DATA AND METADATA COHERENCE section discusses the
behavior of this option in more detail.
- acl / noacl
- Selects whether to use the NFSACL sideband protocol on this mount point.
The NFSACL sideband protocol is a proprietary protocol implemented in
Solaris that manages Access Control Lists. NFSACL was never made a
standard part of the NFS protocol specification.
- If neither acl nor noacl option is specified, the NFS client
negotiates with the server to see if the NFSACL protocol is supported, and
uses it if the server supports it. Disabling the NFSACL sideband protocol
may be necessary if the negotiation causes problems on the client or
server. Refer to the SECURITY CONSIDERATIONS section for more
details.
- local_lock=mechanism
- Specifies whether to use local locking for any or both of the flock and
the POSIX locking mechanisms. mechanism can be one of all,
flock, posix, or none. This option is supported in
kernels 2.6.37 and later.
- The Linux NFS client provides a way to make locks local. This means, the
applications can lock files, but such locks provide exclusion only against
other applications running on the same client. Remote applications are not
affected by these locks.
- If this option is not specified, or if none is specified, the
client assumes that the locks are not local.
- If all is specified, the client assumes that both flock and POSIX
locks are local.
- If flock is specified, the client assumes that only flock locks are
local and uses NLM sideband protocol to lock files when POSIX locks are
used.
- If posix is specified, the client assumes that POSIX locks are
local and uses NLM sideband protocol to lock files when flock locks are
used.
- To support legacy flock behavior similar to that of NFS clients <
2.6.12, use 'local_lock=flock'. This option is required when exporting NFS
mounts via Samba as Samba maps Windows share mode locks as flock. Since
NFS clients > 2.6.12 implement flock by emulating POSIX locks, this
will result in conflicting locks.
- NOTE: When used together, the 'local_lock' mount option will be overridden
by 'nolock'/'lock' mount option.
Options for NFS version 4 only¶
Use these options, along with the options in the first subsection above, for NFS
version 4 and newer.
- proto=netid
- The netid determines the transport that is used to communicate with
the NFS server. Supported options are tcp, tcp6, and
rdma. tcp6 use IPv6 addresses and is only available if
support for TI-RPC is built in. Both others use IPv4 addresses.
- All NFS version 4 servers are required to support TCP, so if this mount
option is not specified, the NFS version 4 client uses the TCP protocol.
Refer to the TRANSPORT METHODS section for more details.
- port=n
- The numeric value of the server's NFS service port. If the server's NFS
service is not available on the specified port, the mount request
fails.
- If this mount option is not specified, the NFS client uses the standard
NFS port number of 2049 without first checking the server's rpcbind
service. This allows an NFS version 4 client to contact an NFS version 4
server through a firewall that may block rpcbind requests.
- If the specified port value is 0, then the NFS client uses the NFS service
port number advertised by the server's rpcbind service. The mount request
fails if the server's rpcbind service is not available, the server's NFS
service is not registered with its rpcbind service, or the server's NFS
service is not available on the advertised port.
- 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 signal. 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.
- The intr / nointr mount option is deprecated after kernel
2.6.25. Only SIGKILL can interrupt a pending NFS operation on these
kernels, and if specified, this mount option is ignored to provide
backwards compatibility with older kernels.
- cto / nocto
- Selects whether to use close-to-open cache coherence semantics for NFS
directories on this mount point. If neither cto nor nocto is
specified, the default is to use close-to-open cache coherence semantics
for directories.
- File data caching behavior is not affected by this option. The DATA AND
METADATA COHERENCE section discusses the behavior of this option in more
detail.
- clientaddr=n.n.n.n
- clientaddr=n:n:...:n
- Specifies a single IPv4 address (in dotted-quad form), or a non-link-local
IPv6 address, that the NFS client advertises to allow servers to perform
NFS version 4 callback requests against files on this mount point. If the
server is unable to establish callback connections to clients, performance
may degrade, or accesses to files may temporarily hang.
- If this option is not specified, the mount(8) command attempts to
discover an appropriate callback address automatically. The automatic
discovery process is not perfect, however. In the presence of multiple
client network interfaces, special routing policies, or atypical network
topologies, the exact address to use for callbacks may be nontrivial to
determine.
nfs4 FILE SYSTEM TYPE¶
The
nfs4 file system type is an old syntax for specifying NFSv4 usage. It
can still be used with all NFSv4-specific and common options, excepted the
nfsvers mount option.
MOUNT CONFIGURATION FILE¶
If the mount command is configured to do so, all of the mount options described
in the previous section can also be configured in the
/etc/nfsmount.conf file. See
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
Try this example to mount using NFS version 4 over TCP with Kerberos 5 mutual
authentication.
server:/export /mnt nfs4 sec=krb5 0 0
This example can be used to mount /usr over NFS.
server:/export /usr nfs ro,nolock,nocto,actimeo=3600 0 0
This example shows how to mount an NFS server using a raw IPv6 link-local
address.
[fe80::215:c5ff:fb3e:e2b1%eth0]:/export /mnt nfs defaults 0 0
TRANSPORT METHODS¶
NFS clients send requests to NFS servers via Remote Procedure Calls, or
RPCs. The RPC client discovers remote service endpoints automatically,
handles per-request authentication, adjusts request parameters for different
byte endianness on client and server, and retransmits requests that may have
been lost by the network or server. RPC requests and replies flow over a
network transport.
In most cases, the
mount(8) command, NFS client, and NFS server can
automatically negotiate proper transport and data transfer size settings for a
mount point. In some cases, however, it pays to specify these settings
explicitly using mount options.
Traditionally, NFS clients used the UDP transport exclusively for transmitting
requests to servers. Though its implementation is simple, NFS over UDP has
many limitations that prevent smooth operation and good performance in some
common deployment environments. Even an insignificant packet loss rate results
in the loss of whole NFS requests; as such, retransmit timeouts are usually in
the subsecond range to allow clients to recover quickly from dropped requests,
but this can result in extraneous network traffic and server load.
However, UDP can be quite effective in specialized settings where the networks
MTU is large relative to NFSs data transfer size (such as network environments
that enable jumbo Ethernet frames). In such environments, trimming the
rsize and
wsize settings so that each NFS read or write request
fits in just a few network frames (or even in a single frame) is advised. This
reduces the probability that the loss of a single MTU-sized network frame
results in the loss of an entire large read or write request.
TCP is the default transport protocol used for all modern NFS implementations.
It performs well in almost every conceivable network environment and provides
excellent guarantees against data corruption caused by network unreliability.
TCP is often a requirement for mounting a server through a network firewall.
Under normal circumstances, networks drop packets much more frequently than NFS
servers drop requests. As such, an aggressive retransmit timeout setting for
NFS over TCP is unnecessary. Typical timeout settings for NFS over TCP are
between one and ten minutes. After the client exhausts its retransmits (the
value of the
retrans mount option), it assumes a network partition has
occurred, and attempts to reconnect to the server on a fresh socket. Since TCP
itself makes network data transfer reliable,
rsize and
wsize can
safely be allowed to default to the largest values supported by both client
and server, independent of the network's MTU size.
Using the mountproto mount option¶
This section applies only to NFS version 2 and version 3 mounts since NFS
version 4 does not use a separate protocol for mount requests.
The Linux NFS client can use a different transport for contacting an NFS
server's rpcbind service, its mountd service, its Network Lock Manager (NLM)
service, and its NFS service. The exact transports employed by the Linux NFS
client for each mount point depends on the settings of the transport mount
options, which include
proto,
mountproto,
udp, and
tcp.
The client sends Network Status Manager (NSM) notifications via UDP no matter
what transport options are specified, but listens for server NSM notifications
on both UDP and TCP. The NFS Access Control List (NFSACL) protocol shares the
same transport as the main NFS service.
If no transport options are specified, the Linux NFS client uses UDP to contact
the server's mountd service, and TCP to contact its NLM and NFS services by
default.
If the server does not support these transports for these services, the
mount(8) command attempts to discover what the server supports, and
then retries the mount request once using the discovered transports. If the
server does not advertise any transport supported by the client or is
misconfigured, the mount request fails. If the
bg option is in effect,
the mount command backgrounds itself and continues to attempt the specified
mount request.
When the
proto option, the
udp option, or the
tcp option is
specified but the
mountproto option is not, the specified transport is
used to contact both the server's mountd service and for the NLM and NFS
services.
If the
mountproto option is specified but none of the
proto,
udp or
tcp options are specified, then the specified transport
is used for the initial mountd request, but the mount command attempts to
discover what the server supports for the NFS protocol, preferring TCP if both
transports are supported.
If both the
mountproto and
proto (or
udp or
tcp)
options are specified, then the transport specified by the
mountproto
option is used for the initial mountd request, and the transport specified by
the
proto option (or the
udp or
tcp options) is used for
NFS, no matter what order these options appear. No automatic service discovery
is performed if these options are specified.
If any of the
proto,
udp,
tcp, or
mountproto options
are specified more than once on the same mount command line, then the value of
the rightmost instance of each of these options takes effect.
Using NFS over UDP on high-speed links¶
Using NFS over UDP on high-speed links such as Gigabit
can cause silent data
corruption.
The problem can be triggered at high loads, and is caused by problems in IP
fragment reassembly. NFS read and writes typically transmit UDP packets of 4
Kilobytes or more, which have to be broken up into several fragments in order
to be sent over the Ethernet link, which limits packets to 1500 bytes by
default. This process happens at the IP network layer and is called
fragmentation.
In order to identify fragments that belong together, IP assigns a 16bit
IP
ID value to each packet; fragments generated from the same UDP packet will
have the same IP ID. The receiving system will collect these fragments and
combine them to form the original UDP packet. This process is called
reassembly. The default timeout for packet reassembly is 30 seconds; if the
network stack does not receive all fragments of a given packet within this
interval, it assumes the missing fragment(s) got lost and discards those it
already received.
The problem this creates over high-speed links is that it is possible to send
more than 65536 packets within 30 seconds. In fact, with heavy NFS traffic one
can observe that the IP IDs repeat after about 5 seconds.
This has serious effects on reassembly: if one fragment gets lost, another
fragment
from a different packet but with the
same IP ID will
arrive within the 30 second timeout, and the network stack will combine these
fragments to form a new packet. Most of the time, network layers above IP will
detect this mismatched reassembly - in the case of UDP, the UDP checksum,
which is a 16 bit checksum over the entire packet payload, will usually not
match, and UDP will discard the bad packet.
However, the UDP checksum is 16 bit only, so there is a chance of 1 in 65536
that it will match even if the packet payload is completely random (which very
often isn't the case). If that is the case, silent data corruption will occur.
This potential should be taken seriously, at least on Gigabit Ethernet. Network
speeds of 100Mbit/s should be considered less problematic, because with most
traffic patterns IP ID wrap around will take much longer than 30 seconds.
It is therefore strongly recommended to use
NFS over TCP where possible,
since TCP does not perform fragmentation.
If you absolutely have to use NFS over UDP over Gigabit Ethernet, some steps can
be taken to mitigate the problem and reduce the probability of corruption:
- Jumbo frames:
- Many Gigabit network cards are capable of transmitting frames bigger than
the 1500 byte limit of traditional Ethernet, typically 9000 bytes. Using
jumbo frames of 9000 bytes will allow you to run NFS over UDP at a page
size of 8K without fragmentation. Of course, this is only feasible if all
involved stations support jumbo frames.
- To enable a machine to send jumbo frames on cards that support it, it is
sufficient to configure the interface for a MTU value of 9000.
- Lower reassembly timeout:
- By lowering this timeout below the time it takes the IP ID counter to wrap
around, incorrect reassembly of fragments can be prevented as well. To do
so, simply write the new timeout value (in seconds) to the file
/proc/sys/net/ipv4/ipfrag_time.
- A value of 2 seconds will greatly reduce the probability of IPID clashes
on a single Gigabit link, while still allowing for a reasonable timeout
when receiving fragmented traffic from distant peers.
Some modern cluster file systems provide perfect cache coherence among their
clients. Perfect cache coherence among disparate NFS clients is expensive to
achieve, especially on wide area networks. As such, NFS settles for weaker
cache coherence that satisfies the requirements of most file sharing types.
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 without some type of application serialization. If absolute cache
coherence among clients is required, applications should use file locking.
Alternatively, applications can also open their files with the O_DIRECT flag
to disable data caching entirely.
Directory entry caching¶
The Linux NFS client caches the result of all NFS LOOKUP requests. If the
requested directory entry exists on the server, the result is referred to as a
positive lookup result. If the requested directory entry does not exist
on the server (that is, the server returned ENOENT), the result is referred to
as
negative lookup result.
To detect when directory entries have been added or removed on the server, the
Linux NFS client watches a directory's mtime. If the client detects a change
in a directory's mtime, the client drops all cached LOOKUP results for that
directory. Since the directory's mtime is a cached attribute, it may take some
time before a client notices it has changed. See the descriptions of the
acdirmin,
acdirmax, and
noac mount options for more
information about how long a directory's mtime is cached.
Caching directory entries improves the performance of applications that do not
share files with applications on other clients. Using cached information about
directories can interfere with applications that run concurrently on multiple
clients and need to detect the creation or removal of files quickly, however.
The
lookupcache mount option allows some tuning of directory entry
caching behavior.
Before kernel release 2.6.28, the Linux NFS client tracked only positive lookup
results. This permitted applications to detect new directory entries created
by other clients quickly while still providing some of the performance
benefits of caching. If an application depends on the previous lookup caching
behavior of the Linux NFS client, you can use
lookupcache=positive.
If the client ignores its cache and validates every application lookup request
with the server, that client can immediately detect when a new directory entry
has been either created or removed by another client. You can specify this
behavior using
lookupcache=none. The extra NFS requests needed if the
client does not cache directory entries can exact a performance penalty.
Disabling lookup caching should result in less of a performance penalty than
using
noac, and has no effect on how the NFS client caches the
attributes of files.
The sync mount option¶
The NFS client treats the
sync mount option differently than some other
file systems (refer to
mount(8) for a description of the generic
sync and
async mount options). If neither
sync nor
async is specified (or if the
async option is specified), the
NFS client delays sending application writes to the server until any of these
events occur:
- Memory pressure forces reclamation of system memory resources.
- An application flushes file data explicitly with sync(2),
msync(2), or fsync(3).
- An application closes a file with close(2).
- The file is locked/unlocked via fcntl(2).
In other words, under normal circumstances, data written by an application may
not immediately appear on the server that hosts the file.
If the
sync option is specified on a mount point, any system call that
writes data to files on that mount point causes that data to be flushed to the
server before the system call returns control to user space. This provides
greater data cache coherence among clients, but at a significant performance
cost.
Applications can use the O_SYNC open flag to force application writes to
individual files to go to the server immediately without the use of the
sync mount option.
Using file locks with NFS¶
The Network Lock Manager protocol is a separate sideband protocol used to manage
file locks in NFS version 2 and version 3. To support lock recovery after a
client or server reboot, a second sideband protocol -- known as the Network
Status Manager protocol -- is also required. In NFS version 4, file locking is
supported directly in the main NFS protocol, and the NLM and NSM sideband
protocols are not used.
In most cases, NLM and NSM services are started automatically, and no extra
configuration is required. Configure all NFS clients with fully-qualified
domain names to ensure that NFS servers can find clients to notify them of
server reboots.
NLM supports advisory file locks only. To lock NFS files, use
fcntl(2)
with the F_GETLK and F_SETLK commands. The NFS client converts file locks
obtained via
flock(2) to advisory locks.
When mounting servers that do not support the NLM protocol, or when mounting an
NFS server through a firewall that blocks the NLM service port, specify the
nolock mount option. NLM locking must be disabled with the
nolock option when using NFS to mount
/var because
/var
contains files used by the NLM implementation on Linux.
Specifying the
nolock option may also be advised to improve the
performance of a proprietary application which runs on a single client and
uses file locks extensively.
NFS version 4 caching features¶
The data and metadata caching behavior of NFS version 4 clients is similar to
that of earlier versions. However, NFS version 4 adds two features that
improve cache behavior:
change attributes and
file delegation.
The
change attribute is a new part of NFS file and directory metadata
which tracks data changes. It replaces the use of a file's modification and
change time stamps as a way for clients to validate the content of their
caches. Change attributes are independent of the time stamp resolution on
either the server or client, however.
A
file delegation is a contract between an NFS version 4 client and
server that allows the client to treat a file temporarily as if no other
client is accessing it. The server promises to notify the client (via a
callback request) if another client attempts to access that file. Once a file
has been delegated to a client, the client can cache that file's data and
metadata aggressively without contacting the server.
File delegations come in two flavors:
read and
write. A
read delegation means that the server notifies the client about any
other clients that want to write to the file. A
write delegation means
that the client gets notified about either read or write accessors.
Servers grant file delegations when a file is opened, and can recall delegations
at any time when another client wants access to the file that conflicts with
any delegations already granted. Delegations on directories are not supported.
In order to support delegation callback, the server checks the network return
path to the client during the client's initial contact with the server. If
contact with the client cannot be established, the server simply does not
grant any delegations to that client.
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 member.
Typically, file data and user ID values appear unencrypted (i.e. "in the
clear") on the network. Moreover, NFS versions 2 and 3 use separate
sideband protocols for mounting, locking and unlocking files, and reporting
system status of clients and servers. These auxiliary protocols use no
authentication.
In addition to combining these sideband protocols with the main NFS protocol,
NFS version 4 introduces more advanced forms of access control,
authentication, and in-transit data protection. The NFS version 4
specification mandates support for strong authentication and security flavors
that provide per-RPC integrity checking and encryption. Because NFS version 4
combines the function of the sideband protocols into the main NFS protocol,
the new security features apply to all NFS version 4 operations including
mounting, file locking, and so on. RPCGSS authentication can also be used with
NFS versions 2 and 3, but it does not protect their sideband protocols.
The
sec mount option specifies the security flavor 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
cryptographic security is also available.
The NFS version 4 protocol allows a client to renegotiate the security flavor
when the client crosses into a new filesystem on the server. The newly
negotiated flavor effects only accesses of the new filesystem.
Such negotiation typically occurs when a client crosses from a server's
pseudo-fs into one of the server's exported physical filesystems, which often
have more restrictive security settings than the pseudo-fs.
Using non-privileged source ports¶
NFS clients usually communicate with NFS servers via network sockets. Each end
of a socket is assigned a port value, which is simply a number between 1 and
65535 that distinguishes socket endpoints at the same IP address. A socket is
uniquely defined by a tuple that includes the transport protocol (TCP or UDP)
and the port values and IP addresses of both endpoints.
The NFS client can choose any source port value for its sockets, but usually
chooses a
privileged port. A privileged port is a port value less than
1024. Only a process with root privileges may create a socket with a
privileged source port.
The exact range of privileged source ports that can be chosen is set by a pair
of sysctls to avoid choosing a well-known port, such as the port used by ssh.
This means the number of source ports available for the NFS client, and
therefore the number of socket connections that can be used at the same time,
is practically limited to only a few hundred.
As described above, the traditional default NFS authentication scheme, known as
AUTH_SYS, relies on sending local UID and GID numbers to identify users making
NFS requests. An NFS server assumes that if a connection comes from a
privileged port, the UID and GID numbers in the NFS requests on this
connection have been verified by the client's kernel or some other local
authority. This is an easy system to spoof, but on a trusted physical network
between trusted hosts, it is entirely adequate.
Roughly speaking, one socket is used for each NFS mount point. If a client could
use non-privileged source ports as well, the number of sockets allowed, and
thus the maximum number of concurrent mount points, would be much larger.
Using non-privileged source ports may compromise server security somewhat, since
any user on AUTH_SYS mount points can now pretend to be any other when making
NFS requests. Thus NFS servers do not support this by default. They explicitly
allow it usually via an export option.
To retain good security while allowing as many mount points as possible, it is
best to allow non-privileged client connections only if the server and client
both require strong authentication, such as Kerberos.
Mounting through a firewall¶
A firewall may reside between an NFS client and server, or the client or server
may block some of its own ports via IP filter rules. It is still possible to
mount an NFS server through a firewall, though some of the
mount(8)
command's automatic service endpoint discovery mechanisms may not work; this
requires you to provide specific endpoint details via NFS mount options.
NFS servers normally run a portmapper or rpcbind daemon to advertise their
service endpoints to clients. Clients use the rpcbind daemon to determine:
- What network port each RPC-based service is using
- What transport protocols each RPC-based service supports
The rpcbind daemon uses a well-known port number (111) to help clients find a
service endpoint. Although NFS often uses a standard port number (2049),
auxiliary services such as the NLM service can choose any unused port number
at random.
Common firewall configurations block the well-known rpcbind port. In the absense
of an rpcbind service, the server administrator fixes the port number of
NFS-related services so that the firewall can allow access to specific NFS
service ports. Client administrators then specify the port number for the
mountd service via the
mount(8) command's
mountport option. It
may also be necessary to enforce the use of TCP or UDP if the firewall blocks
one of those transports.
NFS Access Control Lists¶
Solaris allows NFS version 3 clients direct access to POSIX Access Control Lists
stored in its local file systems. This proprietary sideband protocol, known as
NFSACL, provides richer access control than mode bits. Linux implements this
protocol for compatibility with the Solaris NFS implementation. The NFSACL
protocol never became a standard part of the NFS version 3 specification,
however.
The NFS version 4 specification mandates a new version of Access Control Lists
that are semantically richer than POSIX ACLs. NFS version 4 ACLs are not fully
compatible with POSIX ACLs; as such, some translation between the two is
required in an environment that mixes POSIX ACLs and NFS version 4.
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 subcommand depends
on knowing the original set of mount options used to perform the MNT
operation. These options are stored on disk by the NFS mount subcommand, and
can be erased by a remount.
To ensure that the saved mount options are not erased during a remount, specify
either the local mount directory, or the server hostname and export pathname,
but not both, during a remount. For example,
mount -o remount,ro /mnt
merges the mount option
ro with the mount options already saved on disk
for the NFS server mounted at /mnt.
FILES¶
- /etc/fstab
- file system table
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 referrals, and named
attributes.
SEE ALSO¶
fstab(5),
mount(8),
umount(8),
mount.nfs(5),
umount.nfs(5),
exports(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.