NFS(5) Linux Programmer's Manual NFS(5)NAMEnfs - nfs and nfs4 fstab format and options
SYNOPSIS
/etc/fstab
DESCRIPTION
The fstab file contains information about which filesystems to mount
where and with what options. For NFS mounts, it contains the server
name and exported server directory to mount from, the local directory
that is the mount point, and the NFS specific options that control the
way the filesystem is mounted.
Three different versions of the NFS protocol are supported by the Linux
NFS client: NFS version 2, NFS version 3, and NFS version 4. To mount
via NFS version 2, use the nfs file system type and specify nfsvers=2.
To mount via NFS version 3, use the nfs file system type and specify
nfsvers=3. Version 3 is the default protocol version for the nfs file
system type when nfsvers= is not specified on the mount command. To
mount via NFS version 4, use the nfs4 file system type. The nfsvers=
keyword is not supported for the nfs4 file system type.
These file system types share similar mount options; the differences
are listed below.
Here is an example from an /etc/fstab file for an NFSv2 mount over UDP.
server:/usr/local/pub /pub nfs rsize=32768,wsize=32768,timeo=14,intr
Here is an example for an NFSv4 mount over TCP using Kerberos 5 mutual
authentication.
server:/usr/local/pub /pub nfs4 proto=tcp,sec=krb5,hard,intr
Options for the nfs file system type
rsize=n The number of bytes NFS uses when reading files from an
NFS server. The rsize is negotiated between the server
and client to determine the largest block size that both
can support. The value specified by this option is the
maximum size that could be used; however, the actual
size used may be smaller. Note: Setting this size to a
value less than the largest supported block size will
adversely affect performance.
wsize=n The number of bytes NFS uses when writing files to an
NFS server. The wsize is negotiated between the server
and client to determine the largest block size that both
can support. The value specified by this option is the
maximum size that could be used; however, the actual
size used may be smaller. Note: Setting this size to a
value less than the largest supported block size will
adversely affect performance.
timeo=n The value in tenths of a second before sending the first
retransmission after an RPC timeout. The default value
is 7 tenths of a second. After the first timeout, the
timeout is doubled after each successive timeout until a
maximum timeout of 60 seconds is reached or the enough
retransmissions have occured to cause a major timeout.
Then, if the filesystem is hard mounted, each new time‐
out cascade restarts at twice the initial value of the
previous cascade, again doubling at each retransmission.
The maximum timeout is always 60 seconds. Better over‐
all performance may be achieved by increasing the time‐
out when mounting on a busy network, to a slow server,
or through several routers or gateways.
retrans=n The number of minor timeouts and retransmissions that
must occur before a major timeout occurs. The default
is 3 timeouts. When a major timeout occurs, the file
operation is either aborted or a "server not responding"
message is printed on the console.
acregmin=n The minimum time in seconds that attributes of a regular
file should be cached before requesting fresh informa‐
tion from a server. The default is 3 seconds.
acregmax=n The maximum time in seconds that attributes of a regular
file can be cached before requesting fresh information
from a server. The default is 60 seconds.
acdirmin=n The minimum time in seconds that attributes of a direc‐
tory should be cached before requesting fresh informa‐
tion from a server. The default is 30 seconds.
acdirmax=n The maximum time in seconds that attributes of a direc‐
tory can be cached before requesting fresh information
from a server. The default is 60 seconds.
actimeo=n Using actimeo sets all of acregmin, acregmax, acdirmin,
and acdirmax to the same value. There is no default
value.
retry=n The number of minutes to retry an NFS mount operation in
the foreground or background before giving up. The
default value for forground mounts is 2 minutes. The
default value for background mounts is 10000 minutes,
which is roughly one week.
namlen=n When an NFS server does not support version two of the
RPC mount protocol, this option can be used to specify
the maximum length of a filename that is supported on
the remote filesystem. This is used to support the
POSIX pathconf functions. The default is 255 charac‐
ters.
port=n The numeric value of the port to connect to the NFS
server on. If the port number is 0 (the default) then
query the remote host's portmapper for the port number
to use. If the remote host's NFS daemon is not regis‐
tered with its portmapper, the standard NFS port number
2049 is used instead.
proto=n Mount the NFS filesystem using a specific network
protocol instead of the default TCP protocol.
Valid protocol types are udp and tcp.
mountport=n The numeric value of the mountd port. moun‐
thost=name The name of the host running mountd .
mountprog=n Use an alternate RPC program number to contact
the mount daemon on the remote host. This option
is useful for hosts that can run multiple NFS
servers. The default value is 100005 which is
the standard RPC mount daemon program number.
mountvers=n Use an alternate RPC version number to contact
the mount daemon on the remote host. This option
is useful for hosts that can run multiple NFS
servers. The default value depends on which ker‐
nel you are using.
nfsprog=n Use an alternate RPC program number to contact
the NFS daemon on the remote host. This option
is useful for hosts that can run multiple NFS
servers. The default value is 100003 which is
the standard RPC NFS daemon program number.
nfsvers=n Use an alternate RPC version number to contact
the NFS daemon on the remote host. This option
is useful for hosts that can run multiple NFS
servers. The default value depends on which ker‐
nel you are using.
vers=n vers is an alternative to nfsvers and is compati‐
ble with many other operating systems.
nolock Disable NFS locking. Do not start lockd. This
has to be used with some old NFS servers that
don't support locking.
bg If the first NFS mount attempt times out, retry
the mount in the background. After a mount oper‐
ation is backgrounded, all subsequent mounts on
the same NFS server will be backgrounded immedi‐
ately, without first attempting the mount. A
missing mount point is treated as a timeout, to
allow for nested NFS mounts.
fg If the first NFS mount attempt times out, retry
the mount in the foreground. This is the comple‐
ment of the bg option, and also the default
behavior.
soft If an NFS file operation has a major timeout then
report an I/O error to the calling program. The
default is to continue retrying NFS file opera‐
tions indefinitely.
hard If an NFS file operation has a major timeout then
report "server not responding" on the console and
continue retrying indefinitely. This is the
default.
intr If an NFS file operation has a major timeout and
it is hard mounted, then allow signals to
interupt the file operation and cause it to
return EINTR to the calling program. The default
is to not allow file operations to be inter‐
rupted.
posix Mount the NFS filesystem using POSIX semantics.
This allows an NFS filesystem to properly support
the POSIX pathconf command by querying the mount
server for the maximum length of a filename. To
do this, the remote host must support version two
of the RPC mount protocol. Many NFS servers sup‐
port only version one.
nocto Suppress the retrieval of new attributes when
creating a file.
noac Disable all forms of attribute caching entirely.
This extracts a significant performance penalty
but it allows two different NFS clients to get
reasonable results when both clients are actively
writing to a common export on the server.
noacl Disables Access Control List (ACL) processing.
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 spec‐
ified, the NFS client uses a privileged source
port. If the noresvport option is specified, the
NFS client uses a non-privileged source port.
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.
sec=mode Set the security flavor for this mount to "mode".
The default setting is sec=sys, which uses local
unix uids and gids to authenticate NFS operations
(AUTH_SYS). Other currently supported settings
are: sec=krb5, which uses Kerberos V5 instead of
local unix uids and gids to authenticate users;
sec=krb5i, which uses Kerberos V5 for user
authentication and performs integrity checking of
NFS operations using secure checksums to prevent
data tampering; and sec=krb5p, which uses Ker‐
beros V5 for user authentication and integrity
checking, and encrypts NFS traffic to prevent
traffic sniffing (this is the most secure set‐
ting). Note that there is a performance penalty
when using integrity or privacy.
tcp Mount the NFS filesystem using the TCP protocol.
This is the default protocol.
udp Mount the NFS filesystem using the UDP protocol
instead of the default TCP protocol.
nordirplus Disables NFSv3 READDIRPLUS RPCs. Use this options
when mounting servers that don't support or have
broken READDIRPLUS implementations.
nosharecache As of kernel 2.6.18, it is no longer possible to
mount the same same filesystem with different
mount options to a new mountpoint. It was deemed
unsafe to do so, since cached data cannot be
shared between the two mountpoints. In conse‐
quence, files or directories that were common to
both mountpoint subtrees could often be seen to
be out of sync following an update.
This option allows administrators to select the
pre-2.6.18 behaviour, permitting the same
filesystem to be mounted with different mount
options.
Beware: Use of this option is not recommended
unless you are certain that there are no hard
links or subtrees of this mountpoint that are
mounted elsewhere.
lookupcache=type
This option dictates how directories and files
should be cached when they are accessed -- i.e.
"looked up" -- on the server. A lookup can be
either positive (directory/file was found) or
negative (directory/file was not found); both
types of lookups can be cached.
By default, both positive and negative lookups
are cached ( lookupcache=all ). lookupcache=pos
prevents negative lookups from being cached,
while lookupcache=none prevents all lookups from
being cached.
Note: lookupcache=none can adversely affect per‐
formance, but may be necessary if shared files
created or deleted on the server need to be imme‐
diately visible to any applications running on
NFS clients.
All of the non-value options have corresponding nooption forms.
For example, nointr means don't allow file operations to be
interrupted.
Options for the nfs4 file system type
rsize=n The number of bytes nfs4 uses when reading files
from the server. The rsize is negotiated between
the server and client to determine the largest
block size that both can support. The value
specified by this option is the maximum size that
could be used; however, the actual size used may
be smaller. Note: Setting this size to a value
less than the largest supported block size will
adversely affect performance.
wsize=n The number of bytes nfs4 uses when writing files
to the server. The wsize is negotiated between
the server and client to determine the largest
block size that both can support. The value
specified by this option is the maximum size that
could be used; however, the actual size used may
be smaller. Note: Setting this size to a value
less than the largest supported block size will
adversely affect performance.
timeo=n The value in tenths of a second before sending
the first retransmission after an RPC timeout.
The default value depends on whether proto=udp or
proto=tcp is in effect (see below). The default
value for UDP is 7 tenths of a second. The
default value for TCP is 60 seconds. After the
first timeout, the timeout is doubled after each
successive timeout until a maximum timeout of 60
seconds is reached or the enough retransmissions
have occured to cause a major timeout. Then, if
the filesystem is hard mounted, each new timeout
cascade restarts at twice the initial value of
the previous cascade, again doubling at each
retransmission. The maximum timeout is always 60
seconds.
retrans=n The number of minor timeouts and retransmissions
that must occur before a major timeout occurs.
The default is 5 timeouts for proto=udp and 2
timeouts for proto=tcp. When a major timeout
occurs, the file operation is either aborted or a
"server not responding" message is printed on the
console.
acregmin=n The minimum time in seconds that attributes of a
regular file should be cached before requesting
fresh information from a server. The default is
3 seconds.
acregmax=n The maximum time in seconds that attributes of a
regular file can be cached before requesting
fresh information from a server. The default is
60 seconds.
acdirmin=n The minimum time in seconds that attributes of a
directory should be cached before requesting
fresh information from a server. The default is
30 seconds.
acdirmax=n The maximum time in seconds that attributes of a
directory can be cached before requesting fresh
information from a server. The default is 60
seconds.
actimeo=n Using actimeo sets all of acregmin, acregmax,
acdirmin, and acdirmax to the same value. There
is no default value.
retry=n The number of minutes to retry an NFS mount oper‐
ation in the foreground or background before giv‐
ing up. The default value for forground mounts
is 2 minutes. The default value for background
mounts is 10000 minutes, which is roughly one
week.
port=n The numeric value of the port to connect to the
NFS server on. If the port number is 0 (the
default) then query the remote host's portmapper
for the port number to use. If the remote host's
NFS daemon is not registered with its portmapper,
the standard NFS port number 2049 is used
instead.
proto=n Mount the NFS filesystem using a specific network
protocol instead of the default TCP protocol.
Valid protocol types are udp and tcp. Many NFS
version 4 servers only support the TCP protocol.
clientaddr=n On a multi-homed client, this causes the client
to use a specific callback address when communi‐
cating with an NFS version 4 server. This option
is currently ignored.
sec=mode Same as sec=mode for the nfs filesystem type (see
above).
bg If an NFS mount attempt times out, retry the
mount in the background. After a mount operation
is backgrounded, all subsequent mounts on the
same NFS server will be backgrounded immediately,
without first attempting the mount. A missing
mount point is treated as a timeout, to allow for
nested NFS mounts.
fg If the first NFS mount attempt times out, retry
the mount in the foreground. This is the comple‐
ment of the bg option, and also the default
behavior.
soft If an NFS file operation has a major timeout then
report an I/O error to the calling program. The
default is to continue retrying NFS file opera‐
tions indefinitely.
hard If an NFS file operation has a major timeout then
report "server not responding" on the console and
continue retrying indefinitely. This is the
default.
intr If an NFS file operation has a major timeout and
it is hard mounted, then allow signals to
interupt the file operation and cause it to
return EINTR to the calling program. The default
is to not allow file operations to be inter‐
rupted.
nocto Suppress the retrieval of new attributes when
creating a file.
noac Disable attribute caching, and force synchronous
writes. This extracts a server performance
penalty but it allows two different NFS clients
to get reasonable good results when both clients
are actively writing to common filesystem on the
server.
nosharecache As of kernel 2.6.18, it is no longer possible to
mount the same same filesystem with different
mount options to a new mountpoint. It was deemed
unsafe to do so, since cached data cannot be
shared between the two mountpoints. In conse‐
quence, files or directories that were common to
both mountpoint subtrees could often be seen to
be out of sync following an update.
This option allows administrators to select the
pre-2.6.18 behaviour, permitting the same
filesystem to be mounted with different mount
options.
Beware: Use of this option is not recommended
unless you are certain that there are no hard
links or subtrees of this mountpoint that are
mounted elsewhere.
lookupcache=type
This option dictates how directories and files
should be cached when they are accessed -- i.e.
"looked up" -- on the server. A lookup can be
either positive (directory/file was found) or
negative (directory/file was not found); both
types of lookups can be cached.
By default, both positive and negative lookups
are cached ( lookupcache=all ). lookupcache=pos
prevents negative lookups from being cached,
while lookupcache=none prevents all lookups from
being cached.
Note: lookupcache=none can adversely affect per‐
formance, but may be necessary if shared files
created or deleted on the server need to be imme‐
diately visible to any applications running on
NFS clients.
All of the non-value options have corresponding nooption forms.
For example, nointr means don't allow file operations to be
interrupted.
All of the non-value options have corresponding nooption forms.
For example, nointr means don't allow file operations to be
interrupted.
FILES
/etc/fstab
SEE ALSOfstab(5), mount(8), umount(8), exports(5)AUTHOR
"Rick Sladkey" <jrs@world.std.com>
BUGS
The posix option is implemented but is currently ignored by the
Linux kernel.
Checking files on NFS filesystem referenced by file descriptors
(i.e. the fcntl and ioctl families of functions) may lead to
inconsistent result due to the lack of consistency check in ker‐
nel even if noac is used.
Linux 0.99 20 November 1993 NFS(5)
