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, vers2, vers3, and vers4 which each have same same meaning as given by rpc.nfsd(8).
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.
input file for exportfs, listing exports, export options, and access control lists
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), 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.
exportfs(8), exports(5), fixmount(8), fsinfo(8), mount(8), nfs.conf(5), nfsd(7), nfsd(8), showmount(8).
The man page rpc.mountd(8) is an alias of mountd(8).