EXPORTS(5)EXPORTS(5)NAMEexports - NFS file systems being exported
SYNOPSIS
/etc/exports
DESCRIPTION
The file /etc/exports serves as the access control list for file sys‐
tems 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 sev‐
eral 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 parame‐
ters 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 qual‐
ified 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. How‐
ever, 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 direc‐
tory name as the public root directory (see the section on Web‐
NFS 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 sepa‐
rate 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.
rw Allow the client to modify files and directories. The default is
to restrict the client to read-only request, which can be made
explicit by using the ro option.
noaccess
This makes everything below the directory inaccessible for the
named client. This is useful when you want to export a direc‐
tory 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 opera‐
tion.
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 anony‐
mous 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) map‐
ping 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 ser‐
vice 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 sen‐
sitive, such as user bin.
no_root_squash
Turn off root squashing. This option is mainly useful for disk‐
less clients.
squash_uids and squash_gids
This option specifies a list of uids or gids that should be sub‐
ject 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 set‐
ting.
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 exam‐
ple section below, which maps all requests to uid 150 (which is
supposedly that of user joe).
EXAMPLE
# sample /etc/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 pri‐
vate 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 /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.
FILES
/etc/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.
CYGWIN
The Cygwin port of 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, then nfsd tries to obtain the anonymous uid and gid by
looking up user Guest in the password file. If neither of these pass‐
word entries are found, a uid and gid of -2 (i.e. 65534) is used. As
always, these values can also be overridden by the anonuid and anongid
options.
SEE ALSOmountd(8), nfsd(8)4.2 Berkeley Distribution 11 August 1997 EXPORTS(5)
