mountd(8) — Linux manual page


rpc.mountd(8)              System Manager's Manual             rpc.mountd(8)

NAME         top

       rpc.mountd - NFS mount daemon

SYNOPSIS         top

       /usr/sbin/rpc.mountd [options]

DESCRIPTION         top

       The rpc.mountd daemon implements the server side of the NFS MOUNT
       protocol, an NFS side protocol used by NFS version 2 [RFC1094] and
       NFS version 3 [RFC1813].

       An NFS server maintains a table of local physical file systems that
       are accessible to NFS clients.  Each file system in this table is
       referred to as an exported file system, or export, for short.

       Each file system in the export table has an access control list.
       rpc.mountd uses these access control lists to determine whether an
       NFS client is permitted to access a given file system.  For details
       on how to manage your NFS server's export table, see the exports(5)
       and exportfs(8) man pages.

   Mounting exported NFS File Systems
       The NFS MOUNT protocol has several procedures.  The most important of
       these are MNT (mount an export) and UMNT (unmount an export).

       A MNT request has two arguments: an explicit argument that contains
       the pathname of the root directory of the export to be mounted, and
       an implicit argument that is the sender's IP address.

       When receiving a MNT request from an NFS client, rpc.mountd checks
       both the pathname and the sender's IP address against its export
       table.  If the sender is permitted to access the requested export,
       rpc.mountd returns an NFS file handle for the export's root directory
       to the client.  The client can then use the root file handle and NFS
       LOOKUP requests to navigate the directory structure of the export.

   The rmtab File
       The rpc.mountd daemon registers every successful MNT request by
       adding an entry to the /var/lib/nfs/rmtab file.  When receivng a UMNT
       request from an NFS client, rpc.mountd simply removes the matching
       entry from /var/lib/nfs/rmtab, as long as the access control list for
       that export allows that sender to access the export.

       Clients can discover the list of file systems an NFS server is
       currently exporting, or the list of other clients that have mounted
       its exports, by using the showmount(8) command.  showmount(8) uses
       other procedures in the NFS MOUNT protocol to report information
       about the server's exported file systems.

       Note, however, that there is little to guarantee that the contents of
       /var/lib/nfs/rmtab are accurate.  A client may continue accessing an
       export even after invoking UMNT.  If the client reboots without
       sending a UMNT request, stale entries remain for that client in

OPTIONS         top

       -d kind  or  --debug kind
              Turn on debugging. Valid kinds are: all, auth, call, general
              and parse.

       -F  or  --foreground
              Run in foreground (do not daemonize)

       -h  or  --help
              Display usage message.

       -o num  or  --descriptors num
              Set the limit of the number of open file descriptors to num.
              The default is to leave the limit unchanged.

       -N mountd-version  or  --no-nfs-version mountd-version
              This option can be used to request that rpc.mountd do not
              offer certain versions of NFS. The current version of
              rpc.mountd can support both NFS version 2, 3 and 4. If the
              either one of these version should not be offered, rpc.mountd
              must be invoked with the option --no-nfs-version <vers> .

       -n  or  --no-tcp
              Don't advertise TCP for mount.

       -p num  or  -P num  or  --port num
              Specifies the port number used for RPC listener sockets.  If
              this option is not specified, rpc.mountd will try to consult
              /etc/services, if gets port succeed, set the same port for all
              listener socket, otherwise chooses a random ephemeral port for
              each listener socket.

              This option can be used to fix the port value of rpc.mountd's
              listeners when NFS MOUNT requests must traverse a firewall
              between clients and servers.

       -H  prog or  --ha-callout prog
              Specify a high availability callout program.  This program
              receives callouts for all MOUNT and UNMOUNT requests.  This
              allows rpc.mountd to be used in a High Availability NFS (HA-
              NFS) environment.

              The callout program is run with 4 arguments.  The first is
              mount or unmount depending on the reason for the callout.  The
              second will be the name of the client performing the mount.
              The third will be the path that the client is mounting.  The
              last is the number of concurrent mounts that we believe the
              client has of that path.

              This callout is not needed with 2.6 and later kernels.
              Instead, mount the nfsd filesystem on /proc/fs/nfsd.

       -s, --state-directory-path directory
              Specify a directory in which to place state information (etab
              and rmtab).  If this option is not specified the default of
              /var/lib/nfs is used.

       -r, --reverse-lookup
              rpc.mountd tracks IP addresses in the rmtab file.  When a DUMP
              request is made (by someone running showmount -a, for
              instance), it returns IP addresses instead of hostnames by
              default. This option causes rpc.mountd to perform a reverse
              lookup on each IP address and return that hostname instead.
              Enabling this can have a substantial negative effect on
              performance in some situations.

       -t N or --num-threads=N or --num-threads N
              This option specifies the number of worker threads that
              rpc.mountd spawns.  The default is 1 thread, which is probably
              enough.  More threads are usually only needed for NFS servers
              which need to handle mount storms of hundreds of NFS mounts in
              a few seconds, or when your DNS server is slow or unreliable.

       -u  or  --no-udp
              Don't advertise UDP for mounting

       -V version  or  --nfs-version version
              This option can be used to request that rpc.mountd offer
              certain versions of NFS. The current version of rpc.mountd can
              support both NFS version 2 and the newer version 3.

       -v  or  --version
              Print the version of rpc.mountd and exit.

       -g  or  --manage-gids
              Accept requests from the kernel to map user id numbers into
              lists of group id numbers for use in access control.  An NFS
              request will normally (except when using Kerberos or other
              cryptographic authentication) contains a user-id and a list of
              group-ids.  Due to a limitation in the NFS protocol, at most
              16 groups ids can be listed.  If you use the -g flag, then the
              list of group ids received from the client will be replaced by
              a list of group ids determined by an appropriate lookup on the
              server. Note that the 'primary' group id is not affected so a
              newgroup command on the client will still be effective.  This
              function requires a Linux Kernel with version at least 2.6.21.


       Many of the options that can be set on the command line can also be
       controlled through values set in the [mountd] or, in some cases, the
       [nfsd] sections of the /etc/nfs.conf configuration file.  Values
       recognized in the [mountd] section include manage-gids, descriptors,
       port, threads, reverse-lookup, and state-directory-path, ha-callout
       which each have the same effect as the option with the same name.

       The values recognized in the [nfsd] section include TCP, UDP, vers2,
       vers3, and vers4 which each have same same meaning as given by


       You can protect your rpc.mountd listeners using the tcp_wrapper
       library or iptables(8).

       Note that the tcp_wrapper library supports only IPv4 networking.

       Add the hostnames of NFS peers that are allowed to access rpc.mountd
       to /etc/hosts.allow.  Use the daemon name mountd even if the
       rpc.mountd binary has a different name.

       Hostnames used in either access file will be ignored when they can
       not be resolved into IP addresses.  For further information see the
       tcpd(8) and hosts_access(5) man pages.

   IPv6 and TI-RPC support
       TI-RPC is a pre-requisite for supporting NFS on IPv6.  If TI-RPC
       support is built into rpc.mountd, it attempts to start listeners on
       network transports marked 'visible' in /etc/netconfig.  As long as at
       least one network transport listener starts successfully, rpc.mountd
       will operate.

FILES         top

       /etc/exports             input file for exportfs, listing exports,
                                export options, and access control lists

       /var/lib/nfs/rmtab       table of clients accessing server's exports

SEE ALSO         top

       exportfs(8), exports(5), showmount(8), rpc.nfsd(8), rpc.rquotad(8),
       nfs(5), nfs.conf(5), tcpd(8), hosts_access(5), iptables(8),

       RFC 1094 - "NFS: Network File System Protocol Specification"
       RFC 1813 - "NFS Version 3 Protocol Specification"

AUTHOR         top

       Olaf Kirch, H. J. Lu, G. Allan Morris III, and a host of others.

COLOPHON         top

       This page is part of the nfs-utils (NFS utilities) project.
       Information about the project can be found at 
       ⟨⟩.  If you have a bug
       report for this manual page, see
       ⟨⟩.  This page was
       obtained from the project's upstream Git repository
       ⟨;a=summary⟩ on
       2020-09-18.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-09-18.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

                                 31 Dec 2009                   rpc.mountd(8)

Pages that refer to this page: exports(5)nfsd(7)mount(8)