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
309 .IR fsid= num|root|uuid
310 NFS needs to be able to identify each filesystem that it exports.
311 Normally it will use a UUID for the filesystem (if the filesystem has
312 such a thing) or the device number of the device holding the
313 filesystem (if the filesystem is stored on the device).
315 As not all filesystems are stored on devices, and not all filesystems
316 have UUIDs, it is sometimes necessary to explicitly tell NFS how to
317 identify a filesystem. This is done with the
321 For NFSv4, there is a distinguished filesystem which is the root of
322 all exported filesystem. This is specified with
326 both of which mean exactly the same thing.
328 Other filesystems can be identified with a small integer, or a UUID
329 which should contain 32 hex digits and arbitrary punctuation.
331 Linux kernels version 2.6.20 and earlier do not understand the UUID
332 setting so a small integer must be used if an fsid option needs to be
333 set for such kernels. Setting both a small number and a UUID is
334 supported so the same configuration can be made to work on old and new
340 bases its access control to files on the server machine on the uid and
341 gid provided in each NFS RPC request. The normal behavior a user would
342 expect is that she can access her files on the server just as she would
343 on a normal file system. This requires that the same uids and gids are
344 used on the client and the server machine. This is not always true, nor
345 is it always desirable.
347 Very often, it is not desirable that the root user on a client machine
348 is also treated as root when accessing files on the NFS server. To this
349 end, uid 0 is normally mapped to a different id: the so-called
352 uid. This mode of operation (called `root squashing') is the default,
353 and can be turned off with
358 '''tries to obtain the anonymous uid and gid by looking up user
360 '''in the password file at startup time. If it isn't found, a uid and gid
362 chooses a uid and gid
363 of 65534 for squashed access. These values can also be overridden by
365 .IR anonuid " and " anongid
368 '''In addition to this,
370 '''lets you specify arbitrary uids and gids that should be mapped to user
372 Finally, you can map all user requests to the
373 anonymous uid by specifying the
374 .IR all_squash " option.
376 '''For the benefit of installations where uids differ between different
379 '''provides several mechanism to dynamically map server uids to client
380 '''uids and vice versa: static mapping files, NIS-based mapping, and
385 '''mapping is enabled with the
387 '''option, and uses the UGID RPC protocol. For this to work, you have to run
390 '''mapping daemon on the client host. It is the least secure of the three methods,
391 '''because by running
393 '''everybody can query the client host for a list of valid user names. You
394 '''can protect yourself by restricting access to
396 '''to valid hosts only. This can be done by entering the list of valid
401 '''file. The service name is
403 '''For a description of the file's syntax, please read
404 '''.IR hosts_access (5).
406 '''Static mapping is enabled by using the
408 '''option, which takes a file name as an argument that describes the mapping.
409 '''NIS-based mapping queries the client's NIS server to obtain a mapping from
410 '''user and group names on the server host to user and group names on the
413 Here's the complete list of mapping options:
416 Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
417 not apply to any other uids or gids that might be equally sensitive, such as
424 Turn off root squashing. This option is mainly useful for diskless clients.
426 '''.IR squash_uids " and " squash_gids
427 '''This option specifies a list of uids or gids that should be subject to
428 '''anonymous mapping. A valid list of ids looks like this:
430 '''.IR squash_uids=0-15,20,25-50
432 '''Usually, your squash lists will look a lot simpler.
435 Map all uids and gids to the anonymous user. Useful for NFS-exported
436 public FTP directories, news spool directories, etc. The opposite option
439 which is the default setting.
442 '''This option turns on dynamic uid/gid mapping. Each uid in an NFS request
443 '''will be translated to the equivalent server uid, and each uid in an
444 '''NFS reply will be mapped the other way round. This option requires that
446 '''runs on the client host. The default setting is
447 '''.IR map_identity ,
448 '''which leaves all uids untouched. The normal squash options apply regardless
449 '''of whether dynamic mapping is requested or not.
452 '''This option enables static mapping. It specifies the name of the file
453 '''that describes the uid/gid mapping, e.g.
455 '''.IR map_static=/etc/nfs/foobar.map
457 '''The file's format looks like this
461 '''# Mapping for client foobar:
463 '''uid 0-99 - # squash these
464 '''uid 100-500 1000 # map 100-500 to 1000-1400
465 '''gid 0-49 - # squash these
466 '''gid 50-100 700 # map 50-100 to 700-750
470 '''This option enables NIS-based uid/gid mapping. For instance, when
471 '''the server encounters the uid 123 on the server, it will obtain the
472 '''login name associated with it, and contact the NFS client's NIS server
473 '''to obtain the uid the client associates with the name.
475 '''In order to do this, the NFS server must know the client's NIS domain.
476 '''This is specified as an argument to the
480 '''.I map_nis=foo.com
482 '''Note that it may not be sufficient to simply specify the NIS domain
483 '''here; you may have to take additional actions before
485 '''is actually able to contact the server. If your distribution uses
486 '''the NYS library, you can specify one or more NIS servers for the
487 '''client's domain in
488 '''.IR /etc/yp.conf .
489 '''If you are using a different NIS library, you may have to obtain a
492 '''daemon that can be configured via
495 .IR anonuid " and " anongid
496 These options explicitly set the uid and gid of the anonymous account.
497 This option is primarily useful for PC/NFS clients, where you might want
498 all requests appear to be from one user. As an example, consider the
501 in the example section below, which maps all requests to uid 150 (which
502 is supposedly that of user joe).
508 # sample /etc/exports file
509 / master(rw) trusty(rw,no_root_squash)
510 /projects proj*.local.domain(rw)
511 /usr *.local.domain(ro) @trusted(rw)
512 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
513 /pub (ro,insecure,all_squash)
514 /srv/www \-sync,rw server @trusted @external(ro)
515 '''/pub/private (noaccess)
518 The first line exports the entire filesystem to machines master and trusty.
519 In addition to write access, all uid squashing is turned off for host
520 trusty. The second and third entry show examples for wildcard hostnames
521 and netgroups (this is the entry `@trusted'). The fourth line shows the
522 entry for the PC/NFS client discussed above. Line 5 exports the
523 public FTP directory to every host in the world, executing all requests
524 under the nobody account. The
526 option in this entry also allows clients with NFS implementations that
527 don't use a reserved port for NFS.
528 The sixth line exports a directory read-write to the machine 'server'
529 as well as the `@trusted' netgroup, and read-only to netgroup `@external',
530 all three mounts with the `sync' option enabled.
531 ''' The last line denies all NFS clients
532 '''access to the private directory.
534 '''Unlike other NFS server implementations, this
536 '''allows you to export both a directory and a subdirectory thereof to
537 '''the same host, for instance
538 '''.IR /usr " and " /usr/X11R6 .
539 '''In this case, the mount options of the most specific entry apply. For
540 '''instance, when a user on the client host accesses a file in
542 '''the mount options given in the
544 '''entry apply. This is also true when the latter is a wildcard or netgroup
555 '''An error parsing the file is reported using syslogd(8) as level NOTICE from
556 '''a DAEMON whenever nfsd(8) or mountd(8) is started up. Any unknown
557 '''host is reported at that time, but often not all hosts are not yet known
558 '''to named(8) at boot time, thus as hosts are found they are reported
559 '''with the same syslogd(8) parameters.
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