Computer Science
EXPORTS(5) EXPORTS(5)
NAME
exports - NFS file systems being exported
SYNOPSIS
/etc/exports
DESCRIPTION
The file /etc/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 back-
slash.
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 discus-
sion 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 actu-
ally 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 specifica-
tions 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, spec-
ify 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 inacces-
sible 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 question-
able, 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 squash-
ing') 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 run-
ning ugidd, everybody can query the client host for a list
of valid user names. You can protect yourself by restrict-
ing 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 use-
ful 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 associ-
ated 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 argu-
ment to the map_nis options, e.g.
map_nis=foo.com
Note that it may not be sufficient to simply spec-
ify the NIS domain here; you may have to take addi-
tional actions before nfsd is actually able to con-
tact 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 con-
figured 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 /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 net-
groups (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 /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.
11 August 1997 1
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