MOUNT_NFS(8) System Manager's Manual MOUNT_NFS(8)
mount_nfs – mount NFS file systems
mount_nfs [-o options] server:/path directory
The mount_nfs command calls the mount(2) system call to prepare and graft a
remote NFS file system ( server:/path ) on to the file system tree at the
This command is expected to be executed by the mount(8) command. Direct
use of mount_nfs to mount NFS file systems is strongly discouraged because
there is little practical benefit of using it instead of mount(8).
For NFS versions that use a separate mount protocol, mount_nfs implements
the mount protocol as described in RFC 1094, Appendix A and NFS: Network
File System Version 3 Protocol Specification, RFC 1813, Appendix I.
By default, mount_nfs will attempt the mount twice before exiting with an
error. If the -o 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 -o retrycnt=⟨num⟩ can be used if
a different retry behavior is desired for a mount.
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 -o intr and
-o soft mount options.
Mount options are specified with a -o flag followed by a comma separated
string of options. See the mount(8) man page for possible options and
their meanings. The following NFS-specific options are also available:
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.
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. Setting retrycnt to 0, in addition to only trying to
establish connection once, will case nfs client code to use quick
time out value (default 8s) instead of regular mount time out
(default 30s) value while establishing initial connection.
udp Use UDP transport protocol.
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
inet Use only IPv4 addresses.
inet6 Use only IPv6 addresses.
Use the transport protocol and address family as specified by the
given ONC RPC Netid (RFC 5665). Valid netid values are: tcp (TCP
over IPv4), udp (UDP over IPv4), tcp6 (TCP over IPv6), and udp6
(UDP over IPv6). Note that this option differs from the separate
tcp and udp options described above in that each netid value
specifies both a transport protocol and address family (IP
mntudp Force the mount protocol to use UDP transport, even for TCP NFS
mounts. (Necessary for some old BSD servers.)
Connect to the NFS server's mount daemon using the given port
Connect to an NFS server at the given port number.
noconn Do not connect UDP sockets. For UDP mount points, do not do a
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.
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.)
intr Make the mount interruptible, which implies that file system calls
that are delayed due to an unresponsive server will fail with EINTR
when a termination signal is posted for the process.
soft Make the mount soft, which means that file system calls will fail
after retrans round trip timeout intervals. Note: mounts which are
both soft and read-only will also have the locallocks mount option
enabled by default - unless explicitly overridden with a lock
option (for example, nolocks or nolocallocks ).
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. A range of versions can be specified
by including a dash and another version with no spaces between
versions and the dash. In that case the highest version is tried
first and if not successful fall back to each version down to the
lowest version specified until the mount succeeds or the lowest
version fails. Note minor versions may be specified for versions
greater than or equal to four by appending a dot and then the minor
version number. Currently NFSv4 is the highest supported version
with a minor version of zero. If no minor version is specified,
zero is assumed. Specifying a non supported version or minor
version will print a warning and ignore the vers or nfsvers option.
Versions 2 or 3 do not support minor versioning so minor versions
greater than zero are treated as above.
nfsv4 Deprecated. Use -o vers=⟨num⟩ to specify NFS protocol version.
Force a specific security mechanism to be used for the mount, where
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.
Use the specified encryption type for the mount, where
encryption-type is one of: des3, des3-cbc-sha1, des3-cbc-sha1-kd,
aes128, aes128-cts-hmac-sha1, aes128-cts-hmac-sha1-96, aes256-cts-
hmac-sha1, or aes256-cts-hmac-sha1-96.
Use the default credential for realm or security domain. For
Kerberos realms are usually uppercase. If the realm specified does
not begin with an “@”, an “@” 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.
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.
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
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 “fragments dropped due to timeout” value is getting
large while actively using a mount point. (Use netstat(1) with the
-s option to see what the “fragments dropped due to timeout” value
Set the write data size to the specified value. Ditto the comments
w.r.t. the rsize option, but using the “fragments dropped due to
timeout” value on the server instead of the client. Note that both
the rsize and wsize options should only be used as a last ditch
effort at improving performance when mounting servers that do not
support TCP mounts.
Set both the read data size and write data size to the specified
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
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.
rdirplus / nordirplus
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. “ls -F” and “find . -type f” ). Note that
the long directory listing format case (i.e. “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.
Default value is rdirplus.
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.
Set all attribute cache timeouts to the same value.
noac Disable attribute caching. Equivalent to setting actimeo to 0.
Disable negative name caching.
For NFSv2/v3 mounts, 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
locallocks mount option enabled by default - unless explicitly
overridden with a lock option (for example, nolocks or nolocallocks
nonlm For NFSv2/v3 mounts, do not support NFS file locking operations.
Any attempt to perform file locking operations on this mount will
return the error ENOTSUP regardless of whether or not the NFS
server supports NFS file locking.
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 ENOTSUP
regardless of whether or not the NFS server supports the RQUOTA
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.
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
Set the initial retransmit timeout to the specified value (in
tenths of a second). 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
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 dumbtimer option
should be specified when using this option to manually tune the
Set the retransmit timeout count for soft mounts to the specified
value. The default value is 10.
If the mount is still unresponsive 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 deadtimeout mount option set
to 60 seconds. This can be explicitly overridden by setting
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.
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
nfs.client.allow_async option in nfs.conf(5) is also enabled.
sync Perform I/O requests (specifically, write requests) synchronously.
The operation will not return until a response is received from the
server. (The default, nosync, behavior is to return once the I/O
has been queued up.)
For NFSv4 mounts, don't support callback requests from the server.
This should effectively disable features that require callback
requests such as delegations.
For NFSv4 mounts, don't support named attributes even if the server
does. This is the default.
For NFSv4 mounts, 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).
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 acl option.
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 noacl
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.
nfs.conf(5) can be used to configure some NFS client options. In
particular, 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 nfs.conf(5)
correspond to kernel configuration values which will get set by mount_nfs
when performing a mount. To update these values without performing a
mount, use the command: mount_nfs configupdate.
The following mount_nfs command line flags have equivalent -o option forms
(shown in parentheses) and their use is strongly discouraged. These
command line flags are deprecated and the -o option forms should be used
-2 (vers=2), -3 (vers=3), -4 (vers=4), -L (nolocks), -P (resvport), -T
(tcp), -U (mntudp), -b (bg), -c (noconn), -d (dumbtimer), -i (intr), -l
(rdirplus), -m (nordirplus), -s (soft), -I readdirsize (dsize=#), -R
retrycnt (retrycnt=#), -a maxreadahead (readahead=#), -g maxgroups
(maxgroups=#), -r readsize (rsize=#), -t timeout (timeo=#), -w writesize
(wsize=#), -x retrans (retrans=#).
The simplest way to invoke mount_nfs is with a command like:
mount remotehost:/filesystem /localmountpoint
mount -t nfs remotehost:/filesystem /localmountpoint
As can be derived from the comments accompanying the options, performance
tuning of NFS can be a non-trivial task. Here are some common points to
• Use of the sync option will probably have a detrimental affect on
performance. Its use is discouraged as it provides little benefit.
• Use of the 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
• Increasing the read and write size with the rsize and wsize options
respectively will increase throughput if the network interface can
handle the larger packet sizes.
The default read and write sizes are 8K when using UDP, and 32K
when using TCP. Values over 16K are only supported for TCP, where
2M is the maximum.
Any value over 32K is unlikely to get you more performance, unless
you have a very fast network.
• 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 NFS activity.
This can especially happen with lossy network connections (e.g.
wireless networks) which can lead to a lot of dropped packets.
In this case, decreasing the read and write size, using TCP, or a
combination of both will usually lead to better throughput.
• For connections that are not on the same LAN, and/or may experience
packet loss, using TCP is strongly recommended.
Some common problems with mount_nfs can be difficult for first time users
mount_nfs: can't access /foo: Permission denied
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
exports(5) file is exporting the proper directories. The program
showmount(8) can be used to see a server's exports list. The command
“showmount -e remotehostname” will display what file systems the remote
host is exporting.
A common mistake is that mountd(8) will not export a file system with the
-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 -alldirs option.
The following error:
NFS Portmap: RPC: Program not registered
means that the remote host is not running nfsd(8). or mountd(8). The
program rpcinfo(8) can be used to determine if the remote host is running
nfsd and mountd by issuing the command:
rpcinfo -p remotehostname
If the remote host is running nfsd, mountd, rpc.statd, and rpc.lockd it
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100005 1 udp 950 mountd
100005 3 udp 950 mountd
100005 1 tcp 884 mountd
100005 3 tcp 884 mountd
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
100024 1 udp 644 status
100024 1 tcp 918 status
100021 0 udp 630 nlockmgr
100021 1 udp 630 nlockmgr
100021 3 udp 630 nlockmgr
100021 4 udp 630 nlockmgr
100021 0 tcp 917 nlockmgr
100021 1 tcp 917 nlockmgr
100021 3 tcp 917 nlockmgr
100021 4 tcp 917 nlockmgr
The following error:
mount_nfs: can't resolve host
indicates that mount_nfs could not resolve the name of the remote host.
The following error:
mount_nfs: can't mount <dst-path> from <dst-ip> onto <local-path>:
RPC prog. not avail
In case the remote host is not running rpc.statd and the client is mounting
the NFS directory with remote locks (enabled by default), mount_nfs should
fail with EPROGUNAVAIL. rpc.statd status can be verified using rpcinfo (see
mount(2), unmount(2), mount(8), umount(8), nfsstat(1), netstat(1),
rpcinfo(8), showmount(8), automount(8), nfs.conf(5), nfs(5)
An NFS server shouldn't loopback-mount its own exported file systems
because it's fundamentally prone to deadlock.
macOS 12.1 February 28, 2010 macOS 12.1