rpc.mountd - NFS mount daemon
daemon implements the server side of the NFS MOUNT
protocol, an NFS side protocol used by NFS version 2 [RFC1094] and NFS version
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.
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)
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
the pathname and the sender's IP address against its export table. If the
sender is permitted to access the requested export, rpc.mountd
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.
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
, 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)
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
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
- -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
- 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
- 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
- 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
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
section include manage-gids
which each have the same effect
as the option with the same name.
The values recognized in the [nfsd]
section include TCP
, and vers4
which each have same
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
. Use the daemon name mountd
even if the
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)
TI-RPC is a pre-requisite for supporting NFS on IPv6. If TI-RPC support is built
, 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
- input file for exportfs, listing exports, export
options, and access control lists
- table of clients accessing server's exports
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.