rpc.mountd
Section: Maintenance Commands (8)
Updated: 31 Dec 2009
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NAME
rpc.mountd - NFS mount daemon
SYNOPSIS
/usr/sbin/rpc.mountd [options]
DESCRIPTION
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
/var/lib/nfs/rmtab.
OPTIONS
- -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)
- -f or --exports-file
-
This option specifies the exports file, listing the clients that this
server is prepared to serve and parameters to apply to each
such mount (see
exports(5)).
By default, export information is read from
/etc/exports.
- -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) 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 statd state information.
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
-
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.
- -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 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.
TCP_WRAPPERS SUPPORT
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
- /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
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"
AUTHOR
Olaf Kirch, H. J. Lu, G. Allan Morris III, and a host of others.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Mounting exported NFS File Systems
-
- The rmtab File
-
- OPTIONS
-
- TCP_WRAPPERS SUPPORT
-
- IPv6 and TI-RPC support
-
- FILES
-
- SEE ALSO
-
- AUTHOR
-
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