Provided by: nfs-user-server_2.2beta47-22_i386
exports - NFS file systems being exported
The file /etc/exports serves as the access control list for file
systems which may be exported to NFS clients. It is used by both the
NFS mount daemon, mountd(8) and the NFS file server daemon nfsd(8).
The file format is similar to the SunOS exports file, except that
several additional options are permitted. Each line contains a mount
point and a list of machine or netgroup names allowed to mount the file
system at that point. An optional parenthesized list of mount
parameters may follow each machine name. Blank lines are ignored, and
a # introduces a comment to the end of the line. Entries may be
continued across newlines using a backslash.
Machine Name Formats
NFS clients may be specified in a number of ways:
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.
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
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
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.
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
The public root path can also be specified by invoking nfsd with the
--public-root option. Multiple specifications of a public root will be
mountd and nfsd understand the following export options:
secure 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
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.
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.
Leave all symbolic link as they are. This is the default
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.
User ID Mapping
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
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:
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.
Turn off root squashing. This option is mainly useful for
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
Usually, your squash lists will look a lot simpler.
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
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.
This option enables static mapping. It specifies the name of the
file that describes the uid/gid mapping, e.g.
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
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
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
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).
# sample /etc/exports file
/ master(rw) trusty(rw,no_root_squash)
/usr *.local.domain(ro) @trusted(rw)
The first line exports the entire filesystem to machines master and
trusty. In addition to write access, all uid squashing is turned off
for host trusty. The second and third entry show examples for wildcard
hostnames and netgroups (this is the entry ‘@trusted’). The fourth line
shows the entry for the PC/NFS client discussed above. Line 5 exports
the public FTP directory to every host in the world, executing all
requests under the nobody account. The insecure option in this entry
also allows clients with NFS implementations that don’t use a reserved
port for NFS. The last line denies all NFS clients access to the
Unlike other NFS server implementations, this nfsd allows you to export
both a directory and a subdirectory thereof to the same host, for
instance /usr and /usr/X11R6. In this case, the mount options of the
most specific entry apply. For instance, when a user on the client host
accesses a file in /usr/X11R6, the mount options given in the
/usr/X11R6 entry apply. This is also true when the latter is a wildcard
or netgroup entry.
An error parsing the file is reported using syslogd(8) as level NOTICE
from a DAEMON whenever nfsd(8) or mountd(8) is started up. Any unknown
host is reported at that time, but often not all hosts are not yet
known to named(8) at boot time, thus as hosts are found they are
reported with the same syslogd(8) parameters.