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MOUNT_NFS(8)                System Manager's Manual               MOUNT_NFS(8)

     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
             performance, 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 consistency issues.

     -a maxreadahead
             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
             deprecated and connectionless UDP mounts are the default.

     -D deadthresh
             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 a

     -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

     -g maxgroups
             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.

     -I readdirsize
             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.

     -L leaseterm
             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.

     -o options
             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      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
             provided 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.

     -R retrycnt
             Set the retry count for doing the mount to the specified value.
             The default is 10000.

     -r readsize
             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.

     -t timeout
             Set the initial retransmit timeout to the specified value in 0.1
             seconds.  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 -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.

     -w writesize
             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
             introduces some overhead.

     -x retrans
             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
               interfaces.  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 packets.

               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

               options NFS_RSIZE=value
               options NFS_WSIZE=value

               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
               experience packet loss, using TCP is strongly recommended.

     Some common problems with mount_nfs can be difficult for first time users
     to understand.

           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

     The error:

           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 10.99                   January 24, 2021                   NetBSD 10.99