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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | CONFIGURATION FILE | TCP_WRAPPERS SUPPORT | FILES | SEE ALSO | AUTHOR | COLOPHON |
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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
protocol, an NFS side protocol used by NFS version 2 [RFC1094] and
NFS version 3 [RFC1813]. It also responds to requests from the
Linux kernel to authenticate clients and provides details of
access permissions.
The NFS server (nfsd) maintains a cache of authentication and
authorization information which is used to identify the source of
each request, and then what access permissions that source has to
any local filesystem. When required information is not found in
the cache, the server sends a request to mountd to fill in the
missing information. Mountd uses a table of information stored in
/var/lib/nfs/etab and maintained by exportfs(8), possibly based on
the contents of exports(5), to respond to each request.
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 /var/lib/nfs/rmtab.
Mounting File Systems with NFSv4
Version 4 (and later) of NFS does not use a separate NFS MOUNT
protocol. Instead mounting is performed using regular NFS
requests handled by the NFS server in the Linux kernel (nfsd).
Consequently /var/lib/nfs/rmtab is not updated to reflect any
NFSv4 activity.
-d kind or --debug kind
Turn on debugging. Valid kinds are: all, auth, call,
general and parse.
-l or --log-auth
Enable logging of responses to authentication and access
requests from nfsd. Each response is then cached by the
kernel for 30 minutes (or as set by --ttl below), and will
be refreshed after 15 minutes (half the ttl time) if the
relevant client remains active. Note that -l is equivalent
to -d auth and so can be enabled in /etc/nfs.conf with
"debug = auth" in the [mountd] section.
rpc.mountd will always log authentication responses to
MOUNT requests when NFSv3 is used, but to get similar logs
for NFSv4, this option is required.
-i or --cache-use-ipaddr
Normally each client IP address is matched against each
host identifier (name, wildcard, netgroup etc) found in
/etc/exports and a combined identity is formed from all
matching identifiers. Often many clients will map to the
same combined identity so performing this mapping reduces
the number of distinct access details that the kernel needs
to store. Specifying the -i option suppresses this mapping
so that access to each filesystem is requested and cached
separately for each client IP address. Doing this can
increase the burden of updating the cache slightly, but can
make the log messages produced by the -l option easier to
read.
-T or --ttl
Provide a time-to-live (TTL) for cached information given
to the kernel. The kernel will normally request an update
if the information is needed after half of this time has
expired. Increasing the provided number, which is in
seconds, reduces the rate of cache update requests, and
this is particularly noticeable when these requests are
logged with -l. However increasing also means that changes
to hostname to address mappings can take longer to be
noticed. The default TTL is 1800 (30 minutes).
-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, cache-use-ipaddr, descriptors, port, threads, ttl, 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,
vers3, and vers4 which each have the same meaning as given by
rpc.nfsd(8).
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.
/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), nfs.conf(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"
RFC 7530 - "Network File System (NFS) Version 4 Protocol"
RFC 8881 - "Network File System (NFS) Version 4 Minor Version 1
Protocol"
Olaf Kirch, H. J. Lu, G. Allan Morris III, and a host of others.
This page is part of the nfs-utils (NFS utilities) project.
Information about the project can be found at
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. If you have a bug
report for this manual page, see
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. This page was
obtained from the project's upstream Git repository
⟨git://git.linux-nfs.org/projects/steved/nfs-utils.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-06-27.) 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 [email protected]
31 Dec 2009 rpc.mountd(8)
Pages that refer to this page: exports(5), nfs.conf(5), nfsd(7), exportfs(8), mount(8), nfsd(8), showmount(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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