EXPORTS
NAME
exports - NFS file systems being exported
SYNOPSIS
/.et/exports
DESCRIPTION
The file
/.et/exports
serves as the access control list for file systems which may be
exported to NFS clients. It it 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:
- single host
-
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 networks
-
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.
General Options
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
-
Allow only read-only requests on this NFS volume. The default is to
allow write requests as well, which can also be made explicit by using
the
rw option.
- 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.
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 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=/.et/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
/.et/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).
-
EXAMPLE
# sample /.et/exports file
/ master(rw) trusty(rw,no_root_squash)
/projects proj*.local.domain(rw)
/usr *.local.domain(ro) @trusted(rw)
/home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
/pub (ro,insecure,all_squash)
/pub/private (noaccess)
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 private directory.
CAVEATS
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 /.us/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
/.us/X11R6,
the mount options given in the
/.us/X11R6
entry apply. This is also true when the latter is a wildcard or netgroup
entry.
FILES
/.et/exports
DIAGNOSTICS
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.
SEE ALSO
mountd(8), nfsd(8)