NFS(5) Linux Programmer's Manual NFS(5)
NAME
nfs - 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. Version 2 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 3, use the nfs file system type and specify nfsvers=3. 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 sup-
ported 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 sup-
ported 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 retransmis-
sions 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 maxi-
mum timeout is always 60 seconds. Better overall performance may be
achieved by increasing the timeout 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 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 sec-
onds.
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 sec-
onds.
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 sec-
onds.
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 characters.
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.
mountport=n The numeric value of the mountd port.
mounthost=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 dae-
mon 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 kernel 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 kernel you are using.
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 back-
ground. 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 fore-
ground. This is the complement 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
interrupted.
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
support 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 signifi-
cant 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.
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. 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.
udp Mount the NFS filesystem using the UDP protocol instead of the default TCP
protocol.
nordirplus Disables NFSv3 READDIRPLUS RPCs. Use this option when mounting servers that
don't support or have broken READDIRPLUS implementations.
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 sup-
ported 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 max-
imum 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 maxi-
mum 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 previ-
ous 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 time-
outs for proto=tcp. When a major timeout occurs, the file operation is
either aborted or a "server not responding" message is printed on the con-
sole.
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 sec-
onds.
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 sec-
onds.
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 sec-
onds.
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.
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 UDP protocol. Many NFS version 4 servers only support TCP. Valid
protocol types are udp and tcp.
clientaddr=n On a multi-homed client, this causes the client to use a specific callback
address when communicating 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 fore-
ground. This is the complement 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
interrupted.
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.
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 ALSO
fstab(5), mount(8), umount(8), exports(5)
AUTHOR
"Rick Sladkey" <>
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 kernel even if noac is used.
Linux 0.99 20 November 1993 NFS(5)
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