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

       rpc.mountd - NFS mount daemon

       /usr/sbin/rpc.mountd [options]

       The  rpc.mountd daemon implements the server side of the NFS MOUNT pro-
       tocol, 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 cur-
       rently 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

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

       -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)

              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 statd  state  information.
              If  this  option is not specified the default of /var/lib/nfs is

       -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 situa-

       -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 cryp-
              tographic  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.

       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 sup-
       port is built into rpc.mountd, it attempts to start listeners  on  net-
       work  transports  marked  'visible'  in  /etc/netconfig.  As long as at
       least one network transport listener  starts  successfully,  rpc.mountd
       will operate.

       /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

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

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

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

                                  31 Dec 2009                    rpc.mountd(8)
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