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 Also, each line may have one or more specifications for default options
26 after the path name, in the form of a dash ("\-") followed by an option
27 list. The option list is used for all subsequent exports on that line
30 Blank lines are ignored. A pound sign ("#") introduces a comment to the
31 end of the line. Entries may be continued across newlines using a
32 backslash. If an export name contains spaces it should be quoted using
33 double quotes. You can also specify spaces or other unusual character in
34 the export name using a backslash followed by the character code as three
37 .SS Machine Name Formats
38 NFS clients may be specified in a number of ways:
40 This is the most common format. You may specify a host either by an
41 abbreviated name recognized be the resolver, the fully qualified domain
42 name, or an IP address.
44 NIS netgroups may be given as
46 Only the host part of each
47 netgroup members is consider in checking for membership. Empty host
48 parts or those containing a single dash (\-) are ignored.
50 Machine names may contain the wildcard characters \fI*\fR and \fI?\fR.
51 This can be used to make the \fIexports\fR file more compact; for instance,
52 \fI*.cs.foo.edu\fR matches all hosts in the domain
53 \fIcs.foo.edu\fR. As these characters also match the dots in a domain
54 name, the given pattern will also match all hosts within any subdomain
57 You can also export directories to all hosts on an IP (sub-) network
58 simultaneously. This is done by specifying an IP address and netmask pair
61 where the netmask can be specified in dotted-decimal format, or as a
62 contiguous mask length (for example, either `/255.255.252.0' or `/22' appended
63 to the network base address result in identical subnetworks with 10 bits of
64 host). Wildcard characters generally do not work on IP addresses, though they
65 may work by accident when reverse DNS lookups fail.
68 '''This is a special ``hostname'' that identifies the given directory name
69 '''as the public root directory (see the section on WebNFS in
71 '''for a discussion of WebNFS and the public root handle). When using this
74 '''must be the only entry on this line, and must have no export options
75 '''associated with it. Note that this does
77 '''actually export the named directory; you still have to set the exports
78 '''options in a separate entry.
80 '''The public root path can also be specified by invoking
83 '''.B \-\-public\-root
84 '''option. Multiple specifications of a public root will be ignored.
86 .SS RPCSEC_GSS security
87 To restrict access to an export using rpcsec_gss security, use the special
88 string "gss/krb5" as the client. It is not possible to simultaneously require
89 rpcsec_gss and to make requirements on the IP address of the client.
93 understands the following export options:
96 This option requires that requests originate on an Internet port less
97 than IPPORT_RESERVED (1024). This option is on by default. To turn it
102 Allow both read and write requests on this NFS volume. The
103 default is to disallow any request which changes the filesystem.
104 This can also be made explicit by using
109 This option allows the NFS server to violate the NFS protocol and
110 reply to requests before any changes made by that request have been
111 committed to stable storage (e.g. disc drive).
113 Using this option usually improves performance, but at the cost that
114 an unclean server restart (i.e. a crash) can cause data to be lost or
119 Reply to requests only after the changes have been committed to stable
124 In releases of nfs-utils up to and including 1.0.0, this option was the
125 default. In all subsequence releases,
129 must be explicitly requested if needed.
130 To help make system administrators aware of this change, 'exportfs'
131 will issue a warning if neither
138 This option has no effect if
140 is also set. The NFS server will normally delay committing a write request
141 to disc slightly if it suspects that another related write request may be in
142 progress or may arrive soon. This allows multiple write requests to
143 be committed to disc with the one operation which can improve
144 performance. If an NFS server received mainly small unrelated
145 requests, this behaviour could actually reduce performance, so
147 is available to turn it off.
148 The default can be explicitly requested with the
152 This option is based on the option of the same name provided in IRIX
153 NFS. Normally, if a server exports two filesystems one of which is
154 mounted on the other, then the client will have to mount both
155 filesystems explicitly to get access to them. If it just mounts the
156 parent, it will see an empty directory at the place where the other
157 filesystem is mounted. That filesystem is "hidden".
161 option on a filesystem causes it not to be hidden, and an
162 appropriately authorised client will be able to move from the parent to
163 that filesystem without noticing the change.
165 However, some NFS clients do not cope well with this situation as, for
166 instance, it is then possible for two files in the one apparent
167 filesystem to have the same inode number.
171 option is currently only effective on
173 exports. It does not work reliably with netgroup, subnet, or wildcard
176 This option can be very useful in some situations, but it should be
177 used with due care, and only after confirming that the client system
178 copes with the situation effectively.
180 The option can be explicitly disabled with
184 This option is similar to
186 but it makes it possible for clients to move from the filesystem marked
187 with crossmnt to exported filesystems mounted on it. Thus when a child
188 filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has
189 the same effect as setting "nohide" on B.
192 This option disables subtree checking, which has mild security
193 implications, but can improve reliability in some circumstances.
195 If a subdirectory of a filesystem is exported, but the whole
196 filesystem isn't then whenever a NFS request arrives, the server must
197 check not only that the accessed file is in the appropriate filesystem
198 (which is easy) but also that it is in the exported tree (which is
199 harder). This check is called the
202 In order to perform this check, the server must include some
203 information about the location of the file in the "filehandle" that is
204 given to the client. This can cause problems with accessing files that
205 are renamed while a client has them open (though in many simple cases
208 subtree checking is also used to make sure that files inside
209 directories to which only root has access can only be accessed if the
210 filesystem is exported with
212 (see below), even if the file itself allows more general access.
214 As a general guide, a home directory filesystem, which is normally
215 exported at the root and may see lots of file renames, should be
216 exported with subtree checking disabled. A filesystem which is mostly
217 readonly, and at least doesn't see many file renames (e.g. /usr or
218 /var) and for which subdirectories may be exported, should probably be
219 exported with subtree checks enabled.
221 The default of having subtree checks enabled, can be explicitly
225 From release 1.1.0 of nfs-utils onwards, the default will be
227 as subtree_checking tends to cause more problems than it is worth.
228 If you genuinely require subtree checking, you should explicitly put
231 file. If you put neither option,
233 will warn you that the change is pending.
239 This option (the two names are synonymous) tells the NFS server not to require authentication of
240 locking requests (i.e. requests which use the NLM protocol). Normally
241 the NFS server will require a lock request to hold a credential for a
242 user who has read access to the file. With this flag no access checks
245 Early NFS client implementations did not send credentials with lock
246 requests, and many current NFS clients still exist which are based on
247 the old implementations. Use this flag if you find that you can only
248 lock files which are world readable.
250 The default behaviour of requiring authentication for NLM requests can
251 be explicitly requested with either of the synonymous
257 On some specially patched kernels, and when exporting filesystems that
258 support ACLs, this option tells nfsd not to reveal ACLs to clients, so
259 they will see only a subset of actual permissions on the given file
260 system. This option is safe for filesystems used by NFSv2 clients and
261 old NFSv3 clients that perform access decisions locally. Current
262 NFSv3 clients use the ACCESS RPC to perform all access decisions on
263 the server. Note that the
265 option only has effect on kernels specially patched to support it, and
266 when exporting filesystems with ACL support. The default is to export
267 with ACL support (i.e. by default,
273 '''This makes everything below the directory inaccessible for the named
274 '''client. This is useful when you want to export a directory hierarchy to
275 '''a client, but exclude certain subdirectories. The client's view of a
276 '''directory flagged with noaccess is very limited; it is allowed to read
277 '''its attributes, and lookup `.' and `..'. These are also the only entries
278 '''returned by a readdir.
281 '''Convert absolute symbolic links (where the link contents start with a
282 '''slash) into relative links by prepending the necessary number of ../'s
283 '''to get from the directory containing the link to the root on the
284 '''server. This has subtle, perhaps questionable, semantics when the file
285 '''hierarchy is not mounted at its root.
288 '''Leave all symbolic link as they are. This is the default operation.
294 This option makes it possible to only export a directory if it has
295 successfully been mounted.
296 If no path is given (e.g.
297 .IR mountpoint " or " mp )
298 then the export point must also be a mount point. If it isn't then
299 the export point is not exported. This allows you to be sure that the
300 directory underneath a mountpoint will never be exported by accident
301 if, for example, the filesystem failed to mount due to a disc error.
303 If a path is given (e.g.
304 .IR mountpoint= "/path or " mp= /path)
305 then the nominated path must be a mountpoint for the exportpoint to be
310 This option forces the filesystem identification portion of the file
311 handle and file attributes used on the wire to be
313 instead of a number derived from the major and minor number of the
314 block device on which the filesystem is mounted. Any 32 bit number
315 can be used, but it must be unique amongst all the exported filesystems.
317 This can be useful for NFS failover, to ensure that both servers of
318 the failover pair use the same NFS file handles for the shared filesystem
319 thus avoiding stale file handles after failover.
321 Some Linux filesystems are not mounted on a block device; exporting
322 these via NFS requires the use of the
324 option (although that may still not be enough).
326 The value 0 has a special meaning when use with NFSv4. NFSv4 has a
327 concept of a root of the overall exported filesystem. The export point
328 exported with fsid=0 will be used as this root.
333 bases its access control to files on the server machine on the uid and
334 gid provided in each NFS RPC request. The normal behavior a user would
335 expect is that she can access her files on the server just as she would
336 on a normal file system. This requires that the same uids and gids are
337 used on the client and the server machine. This is not always true, nor
338 is it always desirable.
340 Very often, it is not desirable that the root user on a client machine
341 is also treated as root when accessing files on the NFS server. To this
342 end, uid 0 is normally mapped to a different id: the so-called
345 uid. This mode of operation (called `root squashing') is the default,
346 and can be turned off with
351 '''tries to obtain the anonymous uid and gid by looking up user
353 '''in the password file at startup time. If it isn't found, a uid and gid
355 chooses a uid and gid
356 of 65534 for squashed access. These values can also be overridden by
358 .IR anonuid " and " anongid
361 '''In addition to this,
363 '''lets you specify arbitrary uids and gids that should be mapped to user
365 Finally, you can map all user requests to the
366 anonymous uid by specifying the
367 .IR all_squash " option.
369 '''For the benefit of installations where uids differ between different
372 '''provides several mechanism to dynamically map server uids to client
373 '''uids and vice versa: static mapping files, NIS-based mapping, and
378 '''mapping is enabled with the
380 '''option, and uses the UGID RPC protocol. For this to work, you have to run
383 '''mapping daemon on the client host. It is the least secure of the three methods,
384 '''because by running
386 '''everybody can query the client host for a list of valid user names. You
387 '''can protect yourself by restricting access to
389 '''to valid hosts only. This can be done by entering the list of valid
394 '''file. The service name is
396 '''For a description of the file's syntax, please read
397 '''.IR hosts_access (5).
399 '''Static mapping is enabled by using the
401 '''option, which takes a file name as an argument that describes the mapping.
402 '''NIS-based mapping queries the client's NIS server to obtain a mapping from
403 '''user and group names on the server host to user and group names on the
406 Here's the complete list of mapping options:
409 Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
410 not apply to any other uids or gids that might be equally sensitive, such as
417 Turn off root squashing. This option is mainly useful for diskless clients.
419 '''.IR squash_uids " and " squash_gids
420 '''This option specifies a list of uids or gids that should be subject to
421 '''anonymous mapping. A valid list of ids looks like this:
423 '''.IR squash_uids=0-15,20,25-50
425 '''Usually, your squash lists will look a lot simpler.
428 Map all uids and gids to the anonymous user. Useful for NFS-exported
429 public FTP directories, news spool directories, etc. The opposite option
432 which is the default setting.
435 '''This option turns on dynamic uid/gid mapping. Each uid in an NFS request
436 '''will be translated to the equivalent server uid, and each uid in an
437 '''NFS reply will be mapped the other way round. This option requires that
439 '''runs on the client host. The default setting is
440 '''.IR map_identity ,
441 '''which leaves all uids untouched. The normal squash options apply regardless
442 '''of whether dynamic mapping is requested or not.
445 '''This option enables static mapping. It specifies the name of the file
446 '''that describes the uid/gid mapping, e.g.
448 '''.IR map_static=/etc/nfs/foobar.map
450 '''The file's format looks like this
454 '''# Mapping for client foobar:
456 '''uid 0-99 - # squash these
457 '''uid 100-500 1000 # map 100-500 to 1000-1400
458 '''gid 0-49 - # squash these
459 '''gid 50-100 700 # map 50-100 to 700-750
463 '''This option enables NIS-based uid/gid mapping. For instance, when
464 '''the server encounters the uid 123 on the server, it will obtain the
465 '''login name associated with it, and contact the NFS client's NIS server
466 '''to obtain the uid the client associates with the name.
468 '''In order to do this, the NFS server must know the client's NIS domain.
469 '''This is specified as an argument to the
473 '''.I map_nis=foo.com
475 '''Note that it may not be sufficient to simply specify the NIS domain
476 '''here; you may have to take additional actions before
478 '''is actually able to contact the server. If your distribution uses
479 '''the NYS library, you can specify one or more NIS servers for the
480 '''client's domain in
481 '''.IR /etc/yp.conf .
482 '''If you are using a different NIS library, you may have to obtain a
485 '''daemon that can be configured via
488 .IR anonuid " and " anongid
489 These options explicitly set the uid and gid of the anonymous account.
490 This option is primarily useful for PC/NFS clients, where you might want
491 all requests appear to be from one user. As an example, consider the
494 in the example section below, which maps all requests to uid 150 (which
495 is supposedly that of user joe).
501 # sample /etc/exports file
502 / master(rw) trusty(rw,no_root_squash)
503 /projects proj*.local.domain(rw)
504 /usr *.local.domain(ro) @trusted(rw)
505 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
506 /pub (ro,insecure,all_squash)
507 /srv/www \-sync,rw server @trusted @external(ro)
508 '''/pub/private (noaccess)
511 The first line exports the entire filesystem to machines master and trusty.
512 In addition to write access, all uid squashing is turned off for host
513 trusty. The second and third entry show examples for wildcard hostnames
514 and netgroups (this is the entry `@trusted'). The fourth line shows the
515 entry for the PC/NFS client discussed above. Line 5 exports the
516 public FTP directory to every host in the world, executing all requests
517 under the nobody account. The
519 option in this entry also allows clients with NFS implementations that
520 don't use a reserved port for NFS.
521 The sixth line exports a directory read-write to the machine 'server'
522 as well as the `@trusted' netgroup, and read-only to netgroup `@external',
523 all three mounts with the `sync' option enabled.
524 ''' The last line denies all NFS clients
525 '''access to the private directory.
527 '''Unlike other NFS server implementations, this
529 '''allows you to export both a directory and a subdirectory thereof to
530 '''the same host, for instance
531 '''.IR /usr " and " /usr/X11R6 .
532 '''In this case, the mount options of the most specific entry apply. For
533 '''instance, when a user on the client host accesses a file in
535 '''the mount options given in the
537 '''entry apply. This is also true when the latter is a wildcard or netgroup
548 '''An error parsing the file is reported using syslogd(8) as level NOTICE from
549 '''a DAEMON whenever nfsd(8) or mountd(8) is started up. Any unknown
550 '''host is reported at that time, but often not all hosts are not yet known
551 '''to named(8) at boot time, thus as hosts are found they are reported
552 '''with the same syslogd(8) parameters.
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