1 .TH EXPORTS 5 "4 March 2005" "Linux" "Linux File Formats Manual"
3 exports \- NFS file systems being exported (for Kernel based NFS)
9 serves as the access control list for file systems which may be
10 exported to NFS clients. It is used by
12 to give information to
14 and to the kernel based NFS file server daemon
17 The file format is similar to the SunOS
19 file. Each line contains an export point and a whitespace-separated list
20 of clients allowed to mount the file system at that point. Each listed
21 client may be immediately followed by a parenthesized, comma-separated
22 list of export options for that client. No whitespace is permitted
23 between a client and its option list.
25 Blank lines are ignored. A pound sign ("#") introduces a comment to the
26 end of the line. Entries may be continued across newlines using a
27 backslash. If an export name contains spaces it should be quoted using
28 double quotes. You can also specify spaces or other unusual character in
29 the export name using a backslash followed by the character code as three
32 .SS Machine Name Formats
33 NFS clients may be specified in a number of ways:
35 This is the most common format. You may specify a host either by an
36 abbreviated name recognized be the resolver, the fully qualified domain
37 name, or an IP address.
39 NIS netgroups may be given as
41 Only the host part of each
42 netgroup members is consider in checking for membership. Empty host
43 parts or those containing a single dash (\-) are ignored.
45 Machine names may contain the wildcard characters \fI*\fR and \fI?\fR.
46 This can be used to make the \fIexports\fR file more compact; for instance,
47 \fI*.cs.foo.edu\fR matches all hosts in the domain
48 \fIcs.foo.edu\fR. As these characters also match the dots in a domain
49 name, the given pattern will also match all hosts within any subdomain
52 You can also export directories to all hosts on an IP (sub-) network
53 simultaneously. This is done by specifying an IP address and netmask pair
56 where the netmask can be specified in dotted-decimal format, or as a
57 contiguous mask length (for example, either `/255.255.252.0' or `/22' appended
58 to the network base address result in identical subnetworks with 10 bits of
59 host). Wildcard characters generally do not work on IP addresses, though they
60 may work by accident when reverse DNS lookups fail.
63 '''This is a special ``hostname'' that identifies the given directory name
64 '''as the public root directory (see the section on WebNFS in
66 '''for a discussion of WebNFS and the public root handle). When using this
69 '''must be the only entry on this line, and must have no export options
70 '''associated with it. Note that this does
72 '''actually export the named directory; you still have to set the exports
73 '''options in a separate entry.
75 '''The public root path can also be specified by invoking
78 '''.B \-\-public\-root
79 '''option. Multiple specifications of a public root will be ignored.
81 .SS RPCSEC_GSS security
82 To restrict access to an export using rpcsec_gss security, use the special
83 string "gss/krb5" as the client. It is not possible to simultaneously require
84 rpcsec_gss and to make requirements on the IP address of the client.
88 understands the following export options:
91 This option requires that requests originate on an internet port less
92 than IPPORT_RESERVED (1024). This option is on by default. To turn it
97 Allow both read and write requests on this NFS volume. The
98 default is to disallow any request which changes the filesystem.
99 This can also be made explicit by using
104 This option allows the NFS server to violate the NFS protocol and
105 reply to requests before any changes made by that request have been
106 committed to stable storage (e.g. disc drive).
108 Using this option usually improves performance, but at the cost that
109 an unclean server restart (i.e. a crash) can cause data to be lost or
112 In releases of nfs-utils upto and including 1.0.0, this option was the
113 default. In this and future releases,
117 must be explicit requested if needed.
118 To help make system adminstrators aware of this change, 'exportfs'
119 will issue a warning if neither
126 This option has no effect if
128 is also set. The NFS server will normally delay committing a write request
129 to disc slightly if it suspects that another related write request may be in
130 progress or may arrive soon. This allows multiple write requests to
131 be committed to disc with the one operation which can improve
132 performance. If an NFS server received mainly small unrelated
133 requests, this behaviour could actually reduce performance, so
135 is available to turn it off.
136 The default can be explicitly requested with the
140 This option is based on the option of the same name provided in IRIX
141 NFS. Normally, if a server exports two filesystems one of which is
142 mounted on the other, then the client will have to mount both
143 filesystems explicitly to get access to them. If it just mounts the
144 parent, it will see an empty directory at the place where the other
145 filesystem is mounted. That filesystem is "hidden".
149 option on a filesystem causes it not to be hidden, and an
150 appropriately authorised client will be able to move from the parent to
151 that filesystem without noticing the change.
153 However, some NFS clients do not cope well with this situation as, for
154 instance, it is then possible for two files in the one apparent
155 filesystem to have the same inode number.
159 option is currently only effective on
161 exports. It does not work reliably with netgroup, subnet, or wildcard
164 This option can be very useful in some situations, but it should be
165 used with due care, and only after confirming that the client system
166 copes with the situation effectively.
168 The option can be explicitly disabled with
172 This option disables subtree checking, which has mild security
173 implications, but can improve reliability in some circumstances.
175 If a subdirectory of a filesystem is exported, but the whole
176 filesystem isn't then whenever a NFS request arrives, the server must
177 check not only that the accessed file is in the appropriate filesystem
178 (which is easy) but also that it is in the exported tree (which is
179 harder). This check is called the
182 In order to perform this check, the server must include some
183 information about the location of the file in the "filehandle" that is
184 given to the client. This can cause problems with accessing files that
185 are renamed while a client has them open (though in many simple cases
188 subtree checking is also used to make sure that files inside
189 directories to which only root has access can only be accessed if the
190 filesystem is exported with
192 (see below), even if the file itself allows more general access.
194 As a general guide, a home directory filesystem, which is normally
195 exported at the root and may see lots of file renames, should be
196 exported with subtree checking disabled. A filesystem which is mostly
197 readonly, and at least doesn't see many file renames (e.g. /usr or
198 /var) and for which subdirectories may be exported, should probably be
199 exported with subtree checks enabled.
201 The default of having subtree checks enabled, can be explicitly
209 This option (the two names are synonymous) tells the NFS server not to require authentication of
210 locking requests (i.e. requests which use the NLM protocol). Normally
211 the NFS server will require a lock request to hold a credential for a
212 user who has read access to the file. With this flag no access checks
215 Early NFS client implementations did not send credentials with lock
216 requests, and many current NFS clients still exist which are based on
217 the old implementations. Use this flag if you find that you can only
218 lock files which are world readable.
220 The default behaviour of requiring authentication for NLM requests can
221 be explicitly requested with either of the synonymous
227 On some specially patched kernels, and when exporting filesystems that
228 support ACLs, this option tells nfsd not to reveal ACLs to clients, so
229 they will see only a subset of actual permissions on the given file
230 system. This option is safe for filesystems used by NFSv2 clients and
231 old NFSv3 clients that perform access decisions locally. Current
232 NFSv3 clients use the ACCESS RPC to perform all access decisions on
233 the server. Note that the
235 option only has effect on kernels specially patched to support it, and
236 when exporting filesystems with ACL support. The default is to export
237 with ACL support (i.e. by default,
243 '''This makes everything below the directory inaccessible for the named
244 '''client. This is useful when you want to export a directory hierarchy to
245 '''a client, but exclude certain subdirectories. The client's view of a
246 '''directory flagged with noaccess is very limited; it is allowed to read
247 '''its attributes, and lookup `.' and `..'. These are also the only entries
248 '''returned by a readdir.
251 '''Convert absolute symbolic links (where the link contents start with a
252 '''slash) into relative links by prepending the necessary number of ../'s
253 '''to get from the directory containing the link to the root on the
254 '''server. This has subtle, perhaps questionable, semantics when the file
255 '''hierarchy is not mounted at its root.
258 '''Leave all symbolic link as they are. This is the default operation.
264 This option makes it possible to only export a directory if it has
265 successfully been mounted.
266 If no path is given (e.g.
267 .IR mountpoint " or " mp )
268 then the export point must also be a mount point. If it isn't then
269 the export point is not exported. This allows you to be sure that the
270 directory underneath a mountpoint will never be exported by accident
271 if, for example, the filesystem failed to mount due to a disc error.
273 If a path is given (e.g.
274 .IR mountpoint= "/path or " mp= /path)
275 then the nominted path must be a mountpoint for the exportpoint to be
280 This option forces the filesystem identification portion of the file
281 handle and file attributes used on the wire to be
283 instead of a number derived from the major and minor number of the
284 block device on which the filesystem is mounted. Any 32 bit number
285 can be used, but it must be unique amongst all the exported filesystems.
287 This can be useful for NFS failover, to ensure that both servers of
288 the failover pair use the same NFS file handles for the shared filesystem
289 thus avoiding stale file handles after failover.
291 Some Linux filesystems are not mounted on a block device; exporting
292 these via NFS requires the use of the
294 option (although that may still not be enough).
296 The value 0 has a special meaning when use with NFSv4. NFSv4 has a
297 concept of a root of the overall exported filesystem. The export point
298 exported with fsid=0 will be used as this root.
303 bases its access control to files on the server machine on the uid and
304 gid provided in each NFS RPC request. The normal behavior a user would
305 expect is that she can access her files on the server just as she would
306 on a normal file system. This requires that the same uids and gids are
307 used on the client and the server machine. This is not always true, nor
308 is it always desirable.
310 Very often, it is not desirable that the root user on a client machine
311 is also treated as root when accessing files on the NFS server. To this
312 end, uid 0 is normally mapped to a different id: the so-called
315 uid. This mode of operation (called `root squashing') is the default,
316 and can be turned off with
321 '''tries to obtain the anonymous uid and gid by looking up user
323 '''in the password file at startup time. If it isn't found, a uid and gid
325 chooses a uid and gid
326 of -2 (i.e. 65534) for squashed access. These values can also be overridden by
328 .IR anonuid " and " anongid
331 '''In addition to this,
333 '''lets you specify arbitrary uids and gids that should be mapped to user
335 Finally, you can map all user requests to the
336 anonymous uid by specifying the
337 .IR all_squash " option.
339 '''For the benefit of installations where uids differ between different
342 '''provides several mechanism to dynamically map server uids to client
343 '''uids and vice versa: static mapping files, NIS-based mapping, and
348 '''mapping is enabled with the
350 '''option, and uses the UGID RPC protocol. For this to work, you have to run
353 '''mapping daemon on the client host. It is the least secure of the three methods,
354 '''because by running
356 '''everybody can query the client host for a list of valid user names. You
357 '''can protect yourself by restricting access to
359 '''to valid hosts only. This can be done by entering the list of valid
364 '''file. The service name is
366 '''For a description of the file's syntax, please read
367 '''.IR hosts_access (5).
369 '''Static mapping is enabled by using the
371 '''option, which takes a file name as an argument that describes the mapping.
372 '''NIS-based mapping queries the client's NIS server to obtain a mapping from
373 '''user and group names on the server host to user and group names on the
376 Here's the complete list of mapping options:
379 Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
380 not apply to any other uids that might be equally sensitive, such as user
384 Turn off root squashing. This option is mainly useful for diskless clients.
386 '''.IR squash_uids " and " squash_gids
387 '''This option specifies a list of uids or gids that should be subject to
388 '''anonymous mapping. A valid list of ids looks like this:
390 '''.IR squash_uids=0-15,20,25-50
392 '''Usually, your squash lists will look a lot simpler.
395 Map all uids and gids to the anonymous user. Useful for NFS-exported
396 public FTP directories, news spool directories, etc. The opposite option
399 which is the default setting.
402 '''This option turns on dynamic uid/gid mapping. Each uid in an NFS request
403 '''will be translated to the equivalent server uid, and each uid in an
404 '''NFS reply will be mapped the other way round. This option requires that
406 '''runs on the client host. The default setting is
407 '''.IR map_identity ,
408 '''which leaves all uids untouched. The normal squash options apply regardless
409 '''of whether dynamic mapping is requested or not.
412 '''This option enables static mapping. It specifies the name of the file
413 '''that describes the uid/gid mapping, e.g.
415 '''.IR map_static=/etc/nfs/foobar.map
417 '''The file's format looks like this
421 '''# Mapping for client foobar:
423 '''uid 0-99 - # squash these
424 '''uid 100-500 1000 # map 100-500 to 1000-1400
425 '''gid 0-49 - # squash these
426 '''gid 50-100 700 # map 50-100 to 700-750
430 '''This option enables NIS-based uid/gid mapping. For instance, when
431 '''the server encounters the uid 123 on the server, it will obtain the
432 '''login name associated with it, and contact the NFS client's NIS server
433 '''to obtain the uid the client associates with the name.
435 '''In order to do this, the NFS server must know the client's NIS domain.
436 '''This is specified as an argument to the
440 '''.I map_nis=foo.com
442 '''Note that it may not be sufficient to simply specify the NIS domain
443 '''here; you may have to take additional actions before
445 '''is actually able to contact the server. If your distribution uses
446 '''the NYS library, you can specify one or more NIS servers for the
447 '''client's domain in
448 '''.IR /etc/yp.conf .
449 '''If you are using a different NIS library, you may have to obtain a
452 '''daemon that can be configured via
455 .IR anonuid " and " anongid
456 These options explicitly set the uid and gid of the anonymous account.
457 This option is primarily useful for PC/NFS clients, where you might want
458 all requests appear to be from one user. As an example, consider the
461 in the example section below, which maps all requests to uid 150 (which
462 is supposedly that of user joe).
468 # sample /etc/exports file
469 / master(rw) trusty(rw,no_root_squash)
470 /projects proj*.local.domain(rw)
471 /usr *.local.domain(ro) @trusted(rw)
472 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
473 /pub (ro,insecure,all_squash)
474 '''/pub/private (noaccess)
477 The first line exports the entire filesystem to machines master and trusty.
478 In addition to write access, all uid squashing is turned off for host
479 trusty. The second and third entry show examples for wildcard hostnames
480 and netgroups (this is the entry `@trusted'). The fourth line shows the
481 entry for the PC/NFS client discussed above. Line 5 exports the
482 public FTP directory to every host in the world, executing all requests
483 under the nobody account. The
485 option in this entry also allows clients with NFS implementations that
486 don't use a reserved port for NFS.
487 ''' The last line denies all NFS clients
488 '''access to the private directory.
490 '''Unlike other NFS server implementations, this
492 '''allows you to export both a directory and a subdirectory thereof to
493 '''the same host, for instance
494 '''.IR /usr " and " /usr/X11R6 .
495 '''In this case, the mount options of the most specific entry apply. For
496 '''instance, when a user on the client host accesses a file in
498 '''the mount options given in the
500 '''entry apply. This is also true when the latter is a wildcard or netgroup
511 '''An error parsing the file is reported using syslogd(8) as level NOTICE from
512 '''a DAEMON whenever nfsd(8) or mountd(8) is started up. Any unknown
513 '''host is reported at that time, but often not all hosts are not yet known
514 '''to named(8) at boot time, thus as hosts are found they are reported
515 '''with the same syslogd(8) parameters.
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