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MOUNT_NFS(8) NetBSD System Manager's Manual MOUNT_NFS(8)
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mount_nfs -- mount NFS file systems
mount_nfs [-23AbCcdilPpqsTUuX] [-a maxreadahead] [-D deadthresh]
[-g maxgroups] [-I readdirsize] [-L leaseterm] [-o options]
[-R retrycnt] [-r readsize] [-t timeout] [-w writesize]
[-x retrans] rhost:path node
The mount_nfs command calls the mount(2) system call to prepare and graft
a remote NFS file system (rhost:path) on to the file system tree at the
mount point node. The directory specified by node is converted to an
absolute path before use. This command is normally executed by mount(8).
It implements the mount protocol as described in RFC 1094, Appendix A and
NFS: Network File System Version 3 Protocol Specification, Appendix I.
The options are:
-2 Use the NFS Version 2 protocol.
-3 Use the NFS Version 3 protocol. The default is to try version 3
first, and fall back to version 2 if the mount fails.
-A Disable the attribute cache in the client, making it consult the
server for the current version of a given file's attributes every
time the file is accessed.
By default the attribute cache is enabled, making the client only
consult the server every handful of seconds. This improves per-
formance, however at the cost of the risk of a mid-air collision
when changes are made simultaneously by different NFS clients
and/or locally on the NFS server that affect the same file. In
that case changes made elsewhere may be missed, leading to con-
Set the read-ahead count to the specified value. This may be in
the range of 0 - 4, and determines how many blocks will be read
ahead when a large file is being read sequentially. Trying a
value greater than 1 for this is suggested for mounts with a
large bandwidth * delay product.
-b If an initial attempt to contact the server fails, fork off a
child to keep trying the mount in the background. Useful for
fstab(5), where the filesystem mount is not critical to multiuser
-C For UDP mount points, do a connect(2). Although this flag
increases the efficiency of UDP mounts it cannot be used for
servers that do not reply to requests from the standard NFS port
number 2049, or for servers with multiple network interfaces. In
these cases if the socket is connected and the server replies
from a different port number or a different network interface the
client will get ICMP port unreachable and the mount will hang.
-c For UDP mount points, do not do a connect(2). This flag is dep-
recated and connectionless UDP mounts are the default.
Set the ``dead server threshold'' to the specified number of
round trip timeout intervals. After a ``dead server threshold''
of retransmit timeouts, ``not responding'' message is printed to
-d 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 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 response
from the mount point.
Set the readdir read size to the specified value. The value
should normally be a multiple of DIRBLKSIZ that is <= the read
size for the mount.
-i 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.
Ignored. It used to be NQNFS lease term.
-l Used with NFS Version 3 to specify that the ReaddirPlus() RPC
should be used. This option reduces RPC traffic for cases such
as ls -l, but tends to flood the attribute and name caches with
prefetched entries. 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.
Options are specified with a -o flag followed by a comma sepa-
rated string of options. See the mount(8) man page for possible
options and their meanings.
The following NFS specific options are also available:
bg Same as -b.
conn Same as -C.
Same as -D deadthresh.
Same as -d.
intr Same as -i.
Same as -L leaseterm.
Same as -g maxgroups.
mntudp Same as -U.
nfsv2 Same as -2.
nfsv3 Same as -3.
noac Same as -A.
Same as -p.
nqnfs Same as -q.
Use the specified port number for NFS requests. The
default is to query the portmapper for the NFS port.
Same as -l.
Same as -a maxreadahead.
Same as -r readsize.
soft Same as -s.
tcp Same as -T.
udp Same as -u.
Same as -t timeout.
Same as -w writesize.
-P Use a reserved socket port number. This is the default, and
available for backwards compatibility purposes only.
-p Do not use a reserved port number for RPCs. This option is pro-
vided only to be able to mimic the old default behavior of not
using a reserved port, and should rarely be useful.
-q A synonym of -3. It used to specify NQNFS.
Set the retry count for doing the mount to the specified value.
The default is 10000.
Set the read data size to the specified value in bytes. It
should normally be a power of 2 greater than or equal to 1024.
This should be used for UDP mounts when the ``fragments dropped
after timeout'' value is getting large while actively using a
mount point. Use netstat(1) with the -s option to see what the
``fragments dropped after timeout'' value is. See the mount_nfs
-w option also.
-s A soft mount, which implies that file system calls will fail
after retrans round trip timeout intervals.
-T Use TCP transport instead of UDP. This is the default; the flag
is maintained for backwards compatibility.
Set the initial retransmit timeout to the specified value in 0.1
seconds. May be useful for fine tuning UDP mounts over internet-
works 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. Nor-
mally, the -d option should be specified when using this option
to manually tune the timeout interval. The default is 3 seconds.
-U Force the mount protocol to use UDP transport, even for TCP NFS
mounts. This is necessary for some old BSD servers.
-u Use UDP transport instead of TCP. This may be necessary for some
very old servers.
Set the write data size to the specified value in bytes.
The same logic applies for use of this option as with the
mount_nfs -r option, but using the ``fragments dropped after
timeout'' value on the NFS server instead of the client. Note
that both the -r and -w options should only be used as a last
ditch effort at improving performance when mounting servers that
do not support TCP mounts.
-X Perform 32 <-> 64 bit directory cookie translation for version 3
mounts. This may be needed in the case of a server using the
upper 32 bits of version 3 directory cookies, and when you are
running emulated binaries that access such a filesystem. Native
NetBSD binaries will never need this option. This option intro-
duces some overhead.
Set the retransmit timeout count for soft mounts to the specified
value. The default is 10.
The simplest way to invoke mount_nfs is with a command like:
mount -t nfs remotehost:/filesystem /localmountpoint
It is also possible to automatically mount filesystems at boot from your
/etc/fstab by using a line like:
remotehost:/home /home nfs rw 0 0
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
· Increasing the read and write size with the -r and -w options
respectively will increase throughput if the network interface
can handle the larger packet sizes.
The default size for NFS version 2 is 8K when using UDP, 64K
when using TCP.
The default size for NFS version 3 is platform dependent: on
NetBSD/aarch64, NetBSD/amd64, and NetBSD/i386, the default is
32K, for other platforms it is 8K. Values over 32K are only
supported for TCP, where 64K 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 older Ethernet network inter-
faces. What happens is that either the receive buffer on the
network interface on the client side is overflowing, or that
similar events occur on the server, leading to a lot of dropped
In this case, decreasing the read and write size, using TCP, or
a combination of both will usually lead to better throughput.
Should you need to decrease the read and write size for all
your NFS mounts because of a slow Ethernet network interface
(e.g. a USB 1.1 to 10/100 Ethernet network interface), you can
in your kernel config(1) file to avoid having do specify the
sizes for all mounts.
· For connections that are not on the same LAN, and/or may expe-
rience 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
filesystem you requested, or is not exporting it to your host. If you
believe the remote host is indeed exporting a filesystem to you, make
sure the exports(5) file is exporting the proper directories.
A common mistake is that mountd(8) will not export a filesystem 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 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, and mountd, it would display:
100005 3 udp 719 mountd
100005 1 tcp 720 mountd
100005 3 tcp 720 mountd
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
mount_nfs: can't get net id for host
indicates that mount_nfs cannot resolve the name of the remote host.
nfsstat(1), mount(2), unmount(2), options(4), exports(5), fstab(5),
mount(8), mountd(8), rpcinfo(8)
NFS: Network File System Protocol specification, RFC 1094, March 1989.
NFS Version 2 and Version 3 Security Issues and the NFS Protocol's Use of
RPCSEC_GCC and Kerberos V5, RFC 2623, June 1999.
NFS Version 4 Design Considerations, RFC 2624, June 1999.
Authentication Mechanisms for ONC RPC, RFC 2695, September 1999.
A version of the mount_nfs utility appeared in 4.4BSD.
An NFS server should not mount its own exported file systems (loopback
fashion) because doing so is fundamentally prone to deadlock.
NetBSD 9.99 January 24, 2021 NetBSD 9.99