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rpc.mountd(8) 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 directory to the client. The client can then use the root file handle and NFS LOOKUP requests to navi-
gate 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 proto-
col 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 con-
tinue 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 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 call-
out. 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 hun-
dreds 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 infor-
mation 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), net-
config(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)
