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rpc.mountd(8) System Manager's Manual rpc.mountd(8)
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 direc-
tory 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 sim-
ply 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 export-file or --exports-file export-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 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 Avail-
ability NFS (HA-NFS) environment.
The callout program is run with 4 arguments. The first is mount or unmount depend-
ing on the reason for the callout. The second will be the name of the client per-
forming 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 sub-
stantial 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 ver-
sion 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 num-
bers 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 com-
mand on the client will still be effective. This function requires a Linux Kernel
with version at least 2.6.21.
CONFIGURATION FILE
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,
descriptors, port, threads, 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, vers2, vers3, and vers4
which each have same same meaning as given by rpc.nfsd(8).
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), 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"
AUTHOR
Olaf Kirch, H. J. Lu, G. Allan Morris III, and a host of others.
31 Dec 2009 rpc.mountd(8)
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