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 To apply changes to this file, run exportfs \-ra or /etc/init.d/nfs-kernel-server
33 reload (the latter being a distribution-specific extension).
35 .SS Machine Name Formats
36 NFS clients may be specified in a number of ways:
38 This is the most common format. You may specify a host either by an
39 abbreviated name recognized be the resolver, the fully qualified domain
40 name, or an IP address.
42 NIS netgroups may be given as
44 Only the host part of each
45 netgroup members is consider in checking for membership. Empty host
46 parts or those containing a single dash (\-) are ignored.
48 Machine names may contain the wildcard characters \fI*\fR and \fI?\fR.
49 This can be used to make the \fIexports\fR file more compact; for instance,
50 \fI*.cs.foo.edu\fR matches all hosts in the domain
51 \fIcs.foo.edu\fR. As these characters also match the dots in a domain
52 name, the given pattern will also match all hosts within any subdomain
55 You can also export directories to all hosts on an IP (sub-) network
56 simultaneously. This is done by specifying an IP address and netmask pair
59 where the netmask can be specified in dotted-decimal format, or as a
60 contiguous mask length (for example, either `/255.255.252.0' or `/22' appended
61 to the network base address result in identical subnetworks with 10 bits of
62 host). Wildcard characters generally do not work on IP addresses, though they
63 may work by accident when reverse DNS lookups fail.
66 '''This is a special ``hostname'' that identifies the given directory name
67 '''as the public root directory (see the section on WebNFS in
69 '''for a discussion of WebNFS and the public root handle). When using this
72 '''must be the only entry on this line, and must have no export options
73 '''associated with it. Note that this does
75 '''actually export the named directory; you still have to set the exports
76 '''options in a separate entry.
78 '''The public root path can also be specified by invoking
81 '''.B \-\-public\-root
82 '''option. Multiple specifications of a public root will be ignored.
84 .SS RPCSEC_GSS security
85 To restrict access to an export using rpcsec_gss security, use the special
86 string "gss/krb5" as the client. It is not possible to simultaneously require
87 rpcsec_gss and to make requirements on the IP address of the client.
91 understands the following export options:
94 This option requires that requests originate on an internet port less
95 than IPPORT_RESERVED (1024). This option is on by default. To turn it
100 Allow both read and write requests on this NFS volume. The
101 default is to disallow any request which changes the filesystem.
102 This can also be made explicit by using
107 This option allows the NFS server to violate the NFS protocol and
108 reply to requests before any changes made by that request have been
109 committed to stable storage (e.g. disc drive).
111 Using this option usually improves performance, but at the cost that
112 an unclean server restart (i.e. a crash) can cause data to be lost or
117 Reply to requests only after the changes have been committed to stable
122 In releases of nfs-utils upto and including 1.0.0, this option was the
123 default. In all subsequence releases,
127 must be explicitly requested if needed.
128 To help make system adminstrators aware of this change, 'exportfs'
129 will issue a warning if neither
136 This option has no effect if
138 is also set. The NFS server will normally delay committing a write request
139 to disc slightly if it suspects that another related write request may be in
140 progress or may arrive soon. This allows multiple write requests to
141 be committed to disc with the one operation which can improve
142 performance. If an NFS server received mainly small unrelated
143 requests, this behaviour could actually reduce performance, so
145 is available to turn it off.
146 The default can be explicitly requested with the
150 This option is based on the option of the same name provided in IRIX
151 NFS. Normally, if a server exports two filesystems one of which is
152 mounted on the other, then the client will have to mount both
153 filesystems explicitly to get access to them. If it just mounts the
154 parent, it will see an empty directory at the place where the other
155 filesystem is mounted. That filesystem is "hidden".
159 option on a filesystem causes it not to be hidden, and an
160 appropriately authorised client will be able to move from the parent to
161 that filesystem without noticing the change.
163 However, some NFS clients do not cope well with this situation as, for
164 instance, it is then possible for two files in the one apparent
165 filesystem to have the same inode number.
169 option is currently only effective on
171 exports. It does not work reliably with netgroup, subnet, or wildcard
174 This option can be very useful in some situations, but it should be
175 used with due care, and only after confirming that the client system
176 copes with the situation effectively.
178 The option can be explicitly disabled with
182 This option is similar to
184 but it makes it possible for clients to move from the filesystem marked
185 with crossmnt to exported filesystems mounted on it. Thus when a child
186 filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has
187 the same effect as setting "nohide" on B.
190 This option disables subtree checking, which has mild security
191 implications, but can improve reliability in some circumstances.
193 If a subdirectory of a filesystem is exported, but the whole
194 filesystem isn't then whenever a NFS request arrives, the server must
195 check not only that the accessed file is in the appropriate filesystem
196 (which is easy) but also that it is in the exported tree (which is
197 harder). This check is called the
200 In order to perform this check, the server must include some
201 information about the location of the file in the "filehandle" that is
202 given to the client. This can cause problems with accessing files that
203 are renamed while a client has them open (though in many simple cases
206 subtree checking is also used to make sure that files inside
207 directories to which only root has access can only be accessed if the
208 filesystem is exported with
210 (see below), even if the file itself allows more general access.
212 As a general guide, a home directory filesystem, which is normally
213 exported at the root and may see lots of file renames, should be
214 exported with subtree checking disabled. A filesystem which is mostly
215 readonly, and at least doesn't see many file renames (e.g. /usr or
216 /var) and for which subdirectories may be exported, should probably be
217 exported with subtree checks enabled.
219 The default of having subtree checks enabled, can be explicitly
223 From release 1.1.0 of nfs-utils onwards, the default will be
225 as subtree_checking tends to cause more problems than it is worth.
226 If you genuinely require subtree checking, you should explicitly put
229 file. If you put neither option,
231 will warn you that the change is pending.
237 This option (the two names are synonymous) tells the NFS server not to require authentication of
238 locking requests (i.e. requests which use the NLM protocol). Normally
239 the NFS server will require a lock request to hold a credential for a
240 user who has read access to the file. With this flag no access checks
243 Early NFS client implementations did not send credentials with lock
244 requests, and many current NFS clients still exist which are based on
245 the old implementations. Use this flag if you find that you can only
246 lock files which are world readable.
248 The default behaviour of requiring authentication for NLM requests can
249 be explicitly requested with either of the synonymous
255 On some specially patched kernels, and when exporting filesystems that
256 support ACLs, this option tells nfsd not to reveal ACLs to clients, so
257 they will see only a subset of actual permissions on the given file
258 system. This option is safe for filesystems used by NFSv2 clients and
259 old NFSv3 clients that perform access decisions locally. Current
260 NFSv3 clients use the ACCESS RPC to perform all access decisions on
261 the server. Note that the
263 option only has effect on kernels specially patched to support it, and
264 when exporting filesystems with ACL support. The default is to export
265 with ACL support (i.e. by default,
271 '''This makes everything below the directory inaccessible for the named
272 '''client. This is useful when you want to export a directory hierarchy to
273 '''a client, but exclude certain subdirectories. The client's view of a
274 '''directory flagged with noaccess is very limited; it is allowed to read
275 '''its attributes, and lookup `.' and `..'. These are also the only entries
276 '''returned by a readdir.
279 '''Convert absolute symbolic links (where the link contents start with a
280 '''slash) into relative links by prepending the necessary number of ../'s
281 '''to get from the directory containing the link to the root on the
282 '''server. This has subtle, perhaps questionable, semantics when the file
283 '''hierarchy is not mounted at its root.
286 '''Leave all symbolic link as they are. This is the default operation.
292 This option makes it possible to only export a directory if it has
293 successfully been mounted.
294 If no path is given (e.g.
295 .IR mountpoint " or " mp )
296 then the export point must also be a mount point. If it isn't then
297 the export point is not exported. This allows you to be sure that the
298 directory underneath a mountpoint will never be exported by accident
299 if, for example, the filesystem failed to mount due to a disc error.
301 If a path is given (e.g.
302 .IR mountpoint= "/path or " mp= /path)
303 then the nominted path must be a mountpoint for the exportpoint to be
308 This option forces the filesystem identification portion of the file
309 handle and file attributes used on the wire to be
311 instead of a number derived from the major and minor number of the
312 block device on which the filesystem is mounted. Any 32 bit number
313 can be used, but it must be unique amongst all the exported filesystems.
315 This can be useful for NFS failover, to ensure that both servers of
316 the failover pair use the same NFS file handles for the shared filesystem
317 thus avoiding stale file handles after failover.
319 Some Linux filesystems are not mounted on a block device; exporting
320 these via NFS requires the use of the
322 option (although that may still not be enough).
324 The value 0 has a special meaning when use with NFSv4. NFSv4 has a
325 concept of a root of the overall exported filesystem. The export point
326 exported with fsid=0 will be used as this root.
331 bases its access control to files on the server machine on the uid and
332 gid provided in each NFS RPC request. The normal behavior a user would
333 expect is that she can access her files on the server just as she would
334 on a normal file system. This requires that the same uids and gids are
335 used on the client and the server machine. This is not always true, nor
336 is it always desirable.
338 Very often, it is not desirable that the root user on a client machine
339 is also treated as root when accessing files on the NFS server. To this
340 end, uid 0 is normally mapped to a different id: the so-called
343 uid. This mode of operation (called `root squashing') is the default,
344 and can be turned off with
349 '''tries to obtain the anonymous uid and gid by looking up user
351 '''in the password file at startup time. If it isn't found, a uid and gid
353 chooses a uid and gid
354 of 65534 for squashed access. These values can also be overridden by
356 .IR anonuid " and " anongid
359 '''In addition to this,
361 '''lets you specify arbitrary uids and gids that should be mapped to user
363 Finally, you can map all user requests to the
364 anonymous uid by specifying the
365 .IR all_squash " option.
367 '''For the benefit of installations where uids differ between different
370 '''provides several mechanism to dynamically map server uids to client
371 '''uids and vice versa: static mapping files, NIS-based mapping, and
376 '''mapping is enabled with the
378 '''option, and uses the UGID RPC protocol. For this to work, you have to run
381 '''mapping daemon on the client host. It is the least secure of the three methods,
382 '''because by running
384 '''everybody can query the client host for a list of valid user names. You
385 '''can protect yourself by restricting access to
387 '''to valid hosts only. This can be done by entering the list of valid
392 '''file. The service name is
394 '''For a description of the file's syntax, please read
395 '''.IR hosts_access (5).
397 '''Static mapping is enabled by using the
399 '''option, which takes a file name as an argument that describes the mapping.
400 '''NIS-based mapping queries the client's NIS server to obtain a mapping from
401 '''user and group names on the server host to user and group names on the
404 Here's the complete list of mapping options:
407 Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
408 not apply to any other uids or gids that might be equally sensitive, such as
415 Turn off root squashing. This option is mainly useful for diskless clients.
417 '''.IR squash_uids " and " squash_gids
418 '''This option specifies a list of uids or gids that should be subject to
419 '''anonymous mapping. A valid list of ids looks like this:
421 '''.IR squash_uids=0-15,20,25-50
423 '''Usually, your squash lists will look a lot simpler.
426 Map all uids and gids to the anonymous user. Useful for NFS-exported
427 public FTP directories, news spool directories, etc. The opposite option
430 which is the default setting.
433 '''This option turns on dynamic uid/gid mapping. Each uid in an NFS request
434 '''will be translated to the equivalent server uid, and each uid in an
435 '''NFS reply will be mapped the other way round. This option requires that
437 '''runs on the client host. The default setting is
438 '''.IR map_identity ,
439 '''which leaves all uids untouched. The normal squash options apply regardless
440 '''of whether dynamic mapping is requested or not.
443 '''This option enables static mapping. It specifies the name of the file
444 '''that describes the uid/gid mapping, e.g.
446 '''.IR map_static=/etc/nfs/foobar.map
448 '''The file's format looks like this
452 '''# Mapping for client foobar:
454 '''uid 0-99 - # squash these
455 '''uid 100-500 1000 # map 100-500 to 1000-1400
456 '''gid 0-49 - # squash these
457 '''gid 50-100 700 # map 50-100 to 700-750
461 '''This option enables NIS-based uid/gid mapping. For instance, when
462 '''the server encounters the uid 123 on the server, it will obtain the
463 '''login name associated with it, and contact the NFS client's NIS server
464 '''to obtain the uid the client associates with the name.
466 '''In order to do this, the NFS server must know the client's NIS domain.
467 '''This is specified as an argument to the
471 '''.I map_nis=foo.com
473 '''Note that it may not be sufficient to simply specify the NIS domain
474 '''here; you may have to take additional actions before
476 '''is actually able to contact the server. If your distribution uses
477 '''the NYS library, you can specify one or more NIS servers for the
478 '''client's domain in
479 '''.IR /etc/yp.conf .
480 '''If you are using a different NIS library, you may have to obtain a
483 '''daemon that can be configured via
486 .IR anonuid " and " anongid
487 These options explicitly set the uid and gid of the anonymous account.
488 This option is primarily useful for PC/NFS clients, where you might want
489 all requests appear to be from one user. As an example, consider the
492 in the example section below, which maps all requests to uid 150 (which
493 is supposedly that of user joe).
499 # sample /etc/exports file
500 / master(rw) trusty(rw,no_root_squash)
501 /projects proj*.local.domain(rw)
502 /usr *.local.domain(ro) @trusted(rw)
503 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
504 /pub (ro,insecure,all_squash)
505 '''/pub/private (noaccess)
508 The first line exports the entire filesystem to machines master and trusty.
509 In addition to write access, all uid squashing is turned off for host
510 trusty. The second and third entry show examples for wildcard hostnames
511 and netgroups (this is the entry `@trusted'). The fourth line shows the
512 entry for the PC/NFS client discussed above. Line 5 exports the
513 public FTP directory to every host in the world, executing all requests
514 under the nobody account. The
516 option in this entry also allows clients with NFS implementations that
517 don't use a reserved port for NFS.
518 ''' The last line denies all NFS clients
519 '''access to the private directory.
521 '''Unlike other NFS server implementations, this
523 '''allows you to export both a directory and a subdirectory thereof to
524 '''the same host, for instance
525 '''.IR /usr " and " /usr/X11R6 .
526 '''In this case, the mount options of the most specific entry apply. For
527 '''instance, when a user on the client host accesses a file in
529 '''the mount options given in the
531 '''entry apply. This is also true when the latter is a wildcard or netgroup
542 '''An error parsing the file is reported using syslogd(8) as level NOTICE from
543 '''a DAEMON whenever nfsd(8) or mountd(8) is started up. Any unknown
544 '''host is reported at that time, but often not all hosts are not yet known
545 '''to named(8) at boot time, thus as hosts are found they are reported
546 '''with the same syslogd(8) parameters.
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