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
-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 or --no-nfs-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 Ignored (compatibility with unfsd??).
-p 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 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
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
rpc.mountd tracks IP addresses in the rmtab file. When a DUMP
few seconds, or when your DNS server is slow or unreliable.
-V or --nfs-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
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
/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)
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