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 @(#)mount_nfs.8 8.3 (Berkeley) 3/29/95

.Dd February 28, 2010
.Dt MOUNT_NFS 8
.Os 
.Sh NAME
.Nm mount_nfs
.Nd mount NFS file systems
.Sh SYNOPSIS
.Nm mount_nfs
.Op Fl o Ar options
.Ar server: Ns Ar /path directory
.Sh DESCRIPTION
The
.Nm mount_nfs
command
calls the
.Xr mount 2
system call to prepare and graft a remote NFS file system (
.Ar server:/path
) on to the file system tree at the point
.Ar directory .

p
This command is expected to be executed by the
.Xr mount 8
command. Direct use of
.Nm
to mount NFS file systems is strongly discouraged because there is
little practical benefit of using it instead of
.Xr mount 8 .

p
For NFS versions that use a separate mount protocol,
.Nm
implements the mount protocol as described in RFC 1094, Appendix A and
.%T "NFS: Network File System Version 3 Protocol Specification" ,
RFC 1813, Appendix I.

p
By default,
.Nm
will attempt the mount twice before exiting with an error. If the
.Fl o Cm bg
option is given, it will attempt the mount once and then background
itself to continue trying another 10,000 times (pausing for one minute
between attempts). The option
.Fl o Cm retrycnt Ns = Ns Aq Ar num
can be used if a different retry behavior is desired for a mount.

p
If the server becomes unresponsive while an NFS file system is
mounted, any new or outstanding file operations on that file system
will hang uninterruptibly until the server comes back (or that NFS
file system is forcibly unmounted).
To modify this default behaviour, see the
.Fl o Cm intr
and
.Fl o Cm soft
mount options.

p
Mount options are specified with a
.Fl o
flag followed by a comma separated string of options.
See the
.Xr mount 8
man page for possible options and their meanings.
The following NFS-specific options are also available:

p
l -tag -width indent -compact t Cm bg Retry mount in background.
If an initial attempt to contact the server fails, fork off a child to keep
trying the mount in the background. Useful for startup scripts
where the file system mount is not critical to multiuser operation.

p
t Cm retrycnt Ns = Ns Aq Ar num Set the retry count for doing the mount to the specified value. The
default is 1 for foreground mounts and 10,000 for background mounts.

p
t Cm udp Use UDP transport protocol.

p
t Cm tcp Use the TCP transport protocol instead of UDP. The default is to try TCP first,
then fall back to UDP if the server doesn't support TCP.

p
t Cm inet Use only IPv4 addresses.

p
t Cm inet6 Use only IPv6 addresses.

p
t Cm proto Ns = Ns Aq Ar netid Use the transport protocol and address family as specified by the given ONC RPC Netid (RFC 5665).
Valid netid values are:
.Cm tcp
(TCP over IPv4),
.Cm udp
(UDP over IPv4),
.Cm tcp6
(TCP over IPv6),
and
.Cm udp6
(UDP over IPv6).
Note that this option differs from the separate
.Cm tcp
and
.Cm udp
options described above in that each netid value specifies both a
transport protocol and address family (IP version).

p
t Cm mntudp Force the mount protocol to use UDP transport, even for TCP NFS mounts.
(Necessary for some old BSD servers.)

p
t Cm mountport Ns = Ns Aq Ar port-number Connect to the NFS server's mount daemon using the given port number.

p
t Cm port Ns = Ns Aq Ar port-number Connect to an NFS server at the given port number.

p
t Cm noconn Do not connect UDP sockets.
For UDP mount points, do not do a
.Xr connect 2 .
This must be used for servers that do not reply to requests from the
standard NFS port number 2049. It may also be required for servers
with more than one IP address if replies come from an address other
than the one specified in the requests.

p
t Cm resvport Use a reserved socket port number. This is useful for mounting servers
that require clients to use a reserved port number on the mistaken belief
that this makes NFS more secure. (For the rare case where the client has
a trusted root account but untrustworthy users and the network cables
are in secure areas this does help, but for normal desktop clients this
does not apply.)

p
t Cm intr Make the mount interruptible, which implies that file system calls that
are delayed due to an unresponsive server will fail with
.Er EINTR
when a termination signal is posted for the process.

p
t Cm soft Make the mount soft, which means that file system calls will fail after
.Cm retrans
round trip timeout intervals. Note: mounts which are both soft and
read-only will also have the
.Cm locallocks
mount option enabled by default - unless explicitly overridden with a
lock option (for example,
.Cm nolocks
or
.Cm nolocallocks
).

p
t Cm vers Ns = Ns Aq Ar num t Cm nfsvers Ns = Ns Aq Ar num Set the NFS protocol version number - 2 for NFSv2, 3 for NFSv3 and 4 for
NFSv4. The default is to try version 3 first, and fall back to version 2
if the mount fails.

p
t Cm nfsv2 t Cm nfsv3 t Cm nfsv4 Deprecated. Use
.Fl o Cm vers Ns = Ns Aq Ar num
to specify NFS protocol version.

p
t Cm sec Ns = Ns Aq Ar mechanism Force a specific security mechanism to be used for the mount, where
.Ar mechanism
is one of: krb5p, krb5i, krb5, or sys. When this option is not given
the security mechanism will be negotiated transparently with the remote
server.

p
t Cm realm Ns = Ns Aq Ar realm Use the default credential for realm or security domain. For
Kerberos realms are usually uppercase. If the realm specified
does not begin with an 
.Dq "@" ,
an 
.Dq "@"
sign will be prepended to it.
Note specifying the realm is typically used for automounter maps when clients may have multiple credential caches, and tells the client
what cache to use on the mount.

p
t Cm principal Ns = Ns Aq Ar principal Use the specified principal for acquiring credentials for the mount.
That principal will be used for all accesses by the mounting
credential on the mounted file system. Note specifying a principal is useful for user initiated command line mounts, where the user knows the particular credential to use.

p
t t Cm sprincipal Ns = Ns Aq Ar server-principal Use the specified server-principal for establishing credentials
for the mount. That server principal will be use for all mount
access. If no server principal is specified, then the
GSS_C_NT_HOSTBASED_SERVICE nfs@server is used, where server is taken
from the mount argument server:/path. Note its rare to use this
option.

p
t Cm rsize Ns = Ns Aq Ar readsize Set the read data size to the specified value.
The default is 8192 for UDP mounts and 32768 for TCP mounts.
It should normally be a power of 2 greater than or equal to 1024.
Values greater than 4096 should be multiples of 4096.
It may need to be lowered for UDP mounts when the
.Dq "fragments dropped due to timeout"
value is getting large while actively using a mount point.
(Use
.Xr netstat 1
with the
.Fl s
option to see what the
.Dq "fragments dropped due to timeout"
value is.)

p
t Cm wsize Ns = Ns Aq Ar writesize Set the write data size to the specified value.
Ditto the comments w.r.t. the
.Cm rsize
option, but using the
.Dq "fragments dropped due to timeout"
value on the server instead of the client.
Note that both the
.Cm rsize
and
.Cm wsize
options should only be used as a last ditch effort at improving performance
when mounting servers that do not support TCP mounts.

p
t Cm rwsize Ns = Ns Aq Ar size Set both the read data size and write data size to the specified value.

p
t Cm dsize Ns = Ns Aq Ar readdirsize Set the directory read size to the specified value. The value should
normally be a multiple of DIRBLKSIZ that is <= the read size for the
mount. The default is 8192 for UDP mounts and 32768 for TCP mounts.

p
t Cm readahead Ns = Ns Aq Ar maxreadahead Set the maximum read-ahead count to the specified value. The default is 16. This
may be in the range of 0 - 128, and determines how many blocks will be
read ahead when a large file is being read sequentially. Trying larger
values for this is suggested for mounts with a large bandwidth * delay
product.

p
t Cm rdirplus Used with NFS v3/v4 to specify that directory read operations should
retrieve additional information about each entry (e.g. use the NFSv3
ReaddirPlus RPC). This option typically reduces RPC traffic for
cases such as directory listings that use or display basic attributes
(e.g.
.Dq "ls -F"
and
.Dq "find . -type f"
).
Note that the long directory listing format case (i.e.
.Dq "ls -l"
) may not be helped much when the file system does not natively support
extended attributes.
Older implementations tended to flood the vnode and name caches with
prefetched entries which may not be referenced. The current implementation
avoids creating those entries until they are referenced. Try this
option and see whether performance improves or degrades. Probably most
useful for client to server network interconnects with a large bandwidth
times delay product.

p
t Cm acregmin Ns = Ns Aq Ar seconds t Cm acregmax Ns = Ns Aq Ar seconds t Cm acdirmin Ns = Ns Aq Ar seconds t Cm acdirmax Ns = Ns Aq Ar seconds These options set the minimum and maximum attribute cache timeouts for
directories and "regular" (non-directory) files. The default minimum
is 5 seconds and the default maximum is 60 seconds. Setting both the
minimum and maximum to zero will disable attribute caching. The algorithm
to calculate the timeout is based on the age of the file or directory.
The older it is, the longer the attribute cache is considered valid,
subject to the limits above. Note that the effectiveness of this
algorithm depends on how well the clocks on the client and server are
synchronized.

p
t Cm actimeo Ns = Ns Aq Ar seconds Set all attribute cache timeouts to the same value.

p
t Cm noac Disable attribute caching. Equivalent to setting
.Cm actimeo
to 0.

p
t Cm nonegnamecache Disable negative name caching.

p
t Cm locallocks Perform all file locking operations locally on the NFS client (in the VFS
layer) instead of on the NFS server. This option can provide file locking
support on an NFS file system for which the server does not support file locking.
However, because the file locking is only performed on the client, the
NFS server and other NFS clients will have no knowledge of the locks.
Note: mounts which are both soft and read-only will also have the
.Cm locallocks
mount option enabled by default - unless explicitly overridden with a
lock option (for example,
.Cm nolocks
or
.Cm nolocallocks
).

p
t Cm nolocks t Cm nolockd t Cm nolock t Cm nonlm Do not support NFS file locking operations. Any attempt to perform file
locking operations on this mount will return the error
.Er ENOTSUP
regardless of whether or not the NFS server supports NFS file locking.

p
t Cm noquota Do not support file system quota operations that would normally be
serviced by using the RQUOTA protocol. Any attempt to perform quota
operations on this mount will return the error
.Er ENOTSUP
regardless of whether or not the NFS server supports the RQUOTA service.

p
t Cm maxgroups Ns = Ns Aq Ar num Set the maximum size of the group list for the credentials to the specified
value. This should be used for mounts on old servers that cannot handle
a group list size of 16, as specified in RFC 1057. Try 8, if users in
a lot of groups cannot get a response from the mount point.

p
t Cm dumbtimer Turn off the dynamic retransmit timeout estimator.
This may be useful for UDP mounts that exhibit high retry rates, since it
is possible that the dynamically estimated timeout interval is too short.

p
t Cm timeo Ns = Ns Aq Ar timeout Set the initial retransmit timeout to the specified value.
The default is 1 second.
May be useful for fine tuning UDP mounts over internetworks
with high packet loss rates or an overloaded server.
Try increasing the interval if
.Xr nfsstat 1
shows high retransmit rates while the file system is active or reducing the
value if there is a low retransmit rate but long response delay observed.
(Normally, the
.Cm dumbtimer
option should be specified when using this option to manually
tune the timeout interval.)

p
t Cm retrans Ns = Ns Aq Ar count Set the retransmit timeout count for soft mounts to the specified value.
The default value is 10.

p
t Cm deadtimeout Ns = Ns Aq Ar timeout If the mount is still unresponsive
.Ar timeout
seconds after it is initially reported unresponsive, then mark the
mount as dead so that it will be forcibly unmounted.
Note: mounts which are both soft and read-only will also have the
.Cm deadtimeout
mount option set to 60 seconds. This can be explicitly overridden by setting 
.Cm deadtimeout Ns = Ns 0 .

p
t Cm mutejukebox When NFS requests repeatedly get jukebox errors (NFS3ERR_JUKEBOX,
NFS4ERR_DELAY) from the server the NFS file system is reported as being
unresponsive. Use of this option will prevent the file system from
being included in the list of unresponsive file systems that would be
included in a dialog presented to the user. This option may be useful
when a file system is expected to get such errors during normal
operation. For example, when it's backed by a hierarchical storage
management system.

p
t Cm async Assume that unstable write requests have actually been committed
to stable storage on the server, and thus will not require resending
in the event that the server crashes. Use of this option may improve
performance but only at the risk of data loss if the server crashes.
Note: this mount option will only be honored if the
.Cm nfs.client.allow_async
option in
.Xr nfs.conf 5
is also enabled.

p
t Cm sync Perform I/O requests (specifically, write requests) synchronously.
The operation will not return until a response is received from the
server. (The default,
.Cm nosync ,
behavior is to return once the I/O has been queued up.)

p
t Cm nocallback For NFSv4 mounts, don't support callback requests from the server.
This should effectively disable features that require callback requests
such as delegations.

p
t Cm nonamedattr For NFSv4 mounts, don't support named attributes even if the server does.
By default, if the server appears to support named attributes, they
will be used to store extended attributes and named streams (e.g. FinderInfo
and resource forks).

p
t Cm noacl For NFSv4 mounts, don't support ACLs even if the server does.
ACLs are currently disabled by default to avoid issues with the way
ACLs and modes are handled differently on other operating systems.
This may be overriden by specifying the
.Cm acl
option.

p
t Cm aclonly For NFSv4 mounts, only support ACLs; do not support the mode attribute.
(Any mode attribute values returned will have all permission bits set -
regardless of the value of any ACL or access mode stored in the file
system.) This option overrides the
.Cm noacl
option.

p
t Cm nfc Convert name strings to Unicode Normalization Form C (NFC) when sending
them to the NFS server. This option may be used to improve interoperability
with NFS clients and servers that typically use names in the NFC form.
.El

p
.Xr nfs.conf 5
can be used to configure some NFS client options. In particular,
.Cm nfs.client.mount.options
can be used to specify default mount options. This can be useful in
situations where it is not easy to configure the command-line options.
Some NFS client options in
.Xr nfs.conf 5
correspond to kernel configuration values which will get set by
.Nm
when performing a mount. To update these values without performing a
mount, use the command:
.Nm
.Cm configupdate .
.Sh COMPATIBILITY
The following
.Nm
command line flags have equivalent
.Fl o Ar option
forms (shown in parentheses) and their use is strongly discouraged.
These command line flags are deprecated and the
.Fl o Ar option
forms should be used instead.

p
.Fl 2
(vers=2),
.Fl 3
(vers=3),
.Fl 4
(vers=4),
.Fl L
(nolocks),
.Fl P
(resvport),
.Fl T
(tcp),
.Fl U
(mntudp),
.Fl b
(bg),
.Fl c
(noconn),
.Fl d
(dumbtimer),
.Fl i
(intr),
.Fl l
(rdirplus),
.Fl s
(soft),
.Fl I Ar readdirsize
(dsize=#),
.Fl R Ar retrycnt
(retrycnt=#),
.Fl a Ar maxreadahead
(readahead=#),
.Fl g Ar maxgroups
(maxgroups=#),
.Fl r Ar readsize
(rsize=#),
.Fl t Ar timeout
(timeo=#),
.Fl w Ar writesize
(wsize=#),
.Fl x Ar retrans
(retrans=#).
.Sh EXAMPLES
The simplest way to invoke
.Nm
is with a command like:

p
.D1 Ic mount remotehost:/filesystem /localmountpoint

p
or:

p
.D1 Ic mount -t nfs remotehost:/filesystem /localmountpoint
.Sh PERFORMANCE
As can be derived from the comments accompanying the options, performance
tuning of
.Tn NFS
can be a non-trivial task.
Here are some common points
to watch:
l -bullet -width indent t Use of the
.Cm sync
option will probably have a detrimental affect on performance.
Its use is discouraged as it provides little benefit.
t Use of the
.Cm async
option may improve performance, but only at the risk of losing data
if the server crashes because the client will not be making sure that
all data is committed to stable storage on the server.
t Increasing the read and write size with the
.Cm rsize
and
.Cm wsize
options respectively will increase throughput if the network
interface can handle the larger packet sizes.

p
The default read and write sizes are 8K when using
.Tn UDP ,
and 32K when using
.Tn TCP .
Values over 16K are only supported for
.Tn TCP ,
where 64K is the maximum.

p
Any value over 32K is unlikely to get you more performance, unless
you have a very fast network.
t If the network interface cannot handle larger packet sizes or a
long train of back to back packets, you may see low performance
figures or even temporary hangups during
.Tn NFS
activity.

p
This can especially happen with lossy network connections
(e.g. wireless networks) which can lead to a lot of dropped packets.

p
In this case, decreasing the read and write size, using
.Tn TCP ,
or a combination of both will usually lead to better throughput.
t For connections that are not on the same
.Tn LAN ,
and/or may experience packet loss, using
.Tn TCP
is strongly recommended.
.El
.Sh ERRORS
Some common problems with
.Nm
can be difficult for first time users to understand.

p
.Dl "mount_nfs: can't access /foo: Permission denied

p
This message means that the remote host is either not exporting
the file system you requested or is not exporting it to your host.
If you believe the remote host is indeed exporting a file system to you,
make sure the
.Xr exports 5
file is exporting the proper directories.
The program
.Xr showmount 8
can be used to see a server's exports list.
The command
.Dq "showmount -e remotehostname"
will display what file systems the remote host is exporting.

p
A common mistake is that
.Xr mountd 8
will not export a file system with the
.Fl alldirs
option, unless it
is a mount point on the exporting host.
It is not possible to remotely
mount a subdirectory of an exported mount, unless it is exported with the
.Fl alldirs
option.

p
The following error:

p
.Dl "NFS Portmap: RPC: Program not registered

p
means that the remote host is not running
.Xr nfsd 8 .
or
.Xr mountd 8 .
The program
.Xr rpcinfo 8
can be used to determine if the remote host is running nfsd and mountd by issuing
the command:

p
.Dl rpcinfo -p remotehostname

p
If the remote host is running nfsd, mountd, rpc.statd,
and rpc.lockd it would display:

p
.Dl "program vers proto port
.Dl " 100000 2 tcp 111 portmapper
.Dl " 100000 2 udp 111 portmapper
.Dl " 100005 1 udp 950 mountd
.Dl " 100005 3 udp 950 mountd
.Dl " 100005 1 tcp 884 mountd
.Dl " 100005 3 tcp 884 mountd
.Dl " 100003 2 udp 2049 nfs
.Dl " 100003 3 udp 2049 nfs
.Dl " 100003 2 tcp 2049 nfs
.Dl " 100003 3 tcp 2049 nfs
.Dl " 100024 1 udp 644 status
.Dl " 100024 1 tcp 918 status
.Dl " 100021 0 udp 630 nlockmgr
.Dl " 100021 1 udp 630 nlockmgr
.Dl " 100021 3 udp 630 nlockmgr
.Dl " 100021 4 udp 630 nlockmgr
.Dl " 100021 0 tcp 917 nlockmgr
.Dl " 100021 1 tcp 917 nlockmgr
.Dl " 100021 3 tcp 917 nlockmgr
.Dl " 100021 4 tcp 917 nlockmgr

p
The error:

p
.Dl "mount_nfs: can't resolve host

p
indicates that
.Nm
could not resolve the name of the remote host.
.Sh SEE ALSO
.Xr mount 2 ,
.Xr unmount 2 ,
.Xr mount 8 ,
.Xr umount 8 ,
.Xr nfsstat 1 ,
.Xr netstat 1 ,
.Xr rpcinfo 8 ,
.Xr showmount 8 ,
.Xr automount 8 ,
.Xr nfs.conf 5
.Sh CAVEATS
An NFS server shouldn't loopback-mount its own exported file systems because
it's fundamentally prone to deadlock.