NFS clients may be specified in a number of ways:

"single

This is the most common format. You may specify a host either by an abbreviated name recognizued be the resolver, the fully qualified domain name, or an IP address.

"netgroups

NIS netgroups may be given as @group. Only the host part of all netgroup members is extracted and added to the access list. Empty host parts or those containing a single dash (-) are ignored.

"wildcards

Machine names may contain the wildcard characters * and ?. This can be used to make the exports file more compact; for instance, *.cs.foo.edu matches all hosts in the domain cs.foo.edu. However, these wildcard characters do not match the dots in a domain name, so the above pattern does not include hosts such as a.b.cs.foo.edu.

"IP

You can also export directories to all hosts on an IP (sub-) network simultaneously. This is done by specifying an IP address and netmask pair as address/netmask.

=public

This is a special ``hostname'' that identifies the given directory name as the public root directory (see the section on WebNFS in nfsd(8) for a discussion of WebNFS and the public root handle). When using this convention, =public must be the only entry on this line, and must have no export options associated with it. Note that this does not actually export the named directory; you still have to set the exports options in a separate entry.

The public root path can also be specified by invoking nfsd with the --public-root option. Multiple specifications of a public root will be ignored.

mountd and nfsd understand the following export options:

secure"d

This option requires that requests originate on an internet port less than IPPORT_RESERVED (1024). This option is on by default. To turn it off, specify insecure.

ro

Disallow the client to modify files and directories. The client is only allowed to issue read-only requests.

rw

Allow the client to modify files and directories. This is the default.

noaccess

This makes everything below the directory inaccessible for the named client. This is useful when you want to export a directory hierarchy to a client, but exclude certain subdirectories. The client's view of a directory flagged with noaccess is very limited; it is allowed to read its attributes, and lookup `.' and `..'. These are also the only entries returned by a readdir.

link_relative

Convert absolute symbolic links (where the link contents start with a slash) into relative links by prepending the necessary number of ../'s to get from the directory containing the link to the root on the server. This has subtle, perhaps questionable, semantics when the file hierarchy is not mounted at its root.

link_absolute

Leave all symbolic link as they are. This is the default operation.

Entries where hosts are not specified are known as anonymous entries. They have different default settings compared to normal entries. The differences include all_squash, no_secure, and ro.

nfsd bases its access control to files on the server machine on the uid and gid provided in each NFS RPC request. The normal behavior a user would expect is that she can access her files on the server just as she would on a normal file system. This requires that the same uids and gids are used on the client and the server machine. This is not always true, nor is it always desirable.

Very often, it is not desirable that the root user on a client machine is also treated as root when accessing files on the NFS server. To this end, uid 0 is normally mapped to a different id: the so-called anonymous or nobody uid. This mode of operation (called `root squashing') is the default, and can be turned off with no_root_squash.

By default, nfsd tries to obtain the anonymous uid and gid by looking up user nobody in the password file at startup time. If it isn't found, a uid and gid of -2 (i.e. 65534) is used. These values can also be overridden by the anonuid and anongid options.

In addition to this, nfsd lets you specify arbitrary uids and gids that should be mapped to user nobody as well. Finally, you can map all user requests to the anonymous uid by specifying the all_squash"option.

For the benefit of installations where uids differ between different machines, nfsd provides several mechanism to dynamically map server uids to client uids and vice versa: static mapping files, NIS-based mapping, and ugidd-based mapping.

ugidd-based mapping is enabled with the map_daemon option, and uses the UGID RPC protocol. For this to work, you have to run the ugidd(8) mapping daemon on the client host. It is the least secure of the three methods, because by running ugidd, everybody can query the client host for a list of valid user names. You can protect yourself by restricting access to ugidd to valid hosts only. This can be done by entering the list of valid hosts into the hosts.allow or hosts.deny file. The service name is ugidd. For a description of the file's syntax, please read hosts_access(5).

Static mapping is enabled by using the map_static option, which takes a file name as an argument that describes the mapping. NIS-based mapping queries the client's NIS server to obtain a mapping from user and group names on the server host to user and group names on the client.

Here's the complete list of mapping options:

root_squash

Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does not apply to any other uids that might be equally sensitive, such as user bin.

no_root_squash

Turn off root squashing. This option is mainly useful for diskless clients.

squash_uids and squash_gids

This option specifies a list of uids or gids that should be subject to anonymous mapping. A valid list of ids looks like this:

squash_uids=0-15,20,25-50

Usually, your squash lists will look a lot simpler.

all_squash

Map all uids and gids to the anonymous user. Useful for NFS-exported public FTP directories, news spool directories, etc. The opposite option is no_all_squash, which is the default setting.

map_daemon

This option turns on dynamic uid/gid mapping. Each uid in an NFS request will be translated to the equivalent server uid, and each uid in an NFS reply will be mapped the other way round. This option requires that rpc.ugidd(8) runs on the client host. The default setting is map_identity, which leaves all uids untouched. The normal squash options apply regardless of whether dynamic mapping is requested or not.

map_static

This option enables static mapping. It specifies the name of the file that describes the uid/gid mapping, e.g.

map_static=/etc/nfs/foobar.map

The file's format looks like this

# Mapping for client foobar:
#    remote     local
uid  0-99       -       # squash these
uid  100-500    1000    # map 100-500 to 1000-1500
gid  0-49       -       # squash these
gid  50-100     700     # map 50-100 to 700-750

map_nis

This option enables NIS-based uid/gid mapping. For instance, when the server encounters the uid 123 on the server, it will obtain the login name associated with it, and contact the NFS client's NIS server to obtain the uid the client associates with the name.

In order to do this, the NFS server must know the client's NIS domain. This is specified as an argument to the map_nis options, e.g.

map_nis=foo.com

Note that it may not be sufficient to simply specify the NIS domain here; you may have to take additional actions before nfsd is actually able to contact the server. If your distribution uses the NYS library, you can specify one or more NIS servers for the client's domain in /etc/yp.conf. If you are using a different NIS library, you may have to obtain a special ypbind(8) daemon that can be configured via yp.conf.

anonuid and anongid

These options explicitly set the uid and gid of the anonymous account. This option is primarily useful for PC/NFS clients, where you might want all requests appear to be from one user. As an example, consider the export entry for /home/joe in the example section below, which maps all requests to uid 150 (which is supposedly that of user joe).