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
114 Reply to requests only after the changes have been committed to stable
119 In releases of nfs-utils up to and including 1.0.0, this option was the
120 default. In all subsequence releases,
124 must be explicit requested if needed.
125 To help make system administrators aware of this change, 'exportfs'
126 will issue a warning if neither
133 This option has no effect if
135 is also set. The NFS server will normally delay committing a write request
136 to disc slightly if it suspects that another related write request may be in
137 progress or may arrive soon. This allows multiple write requests to
138 be committed to disc with the one operation which can improve
139 performance. If an NFS server received mainly small unrelated
140 requests, this behaviour could actually reduce performance, so
142 is available to turn it off.
143 The default can be explicitly requested with the
147 This option is based on the option of the same name provided in IRIX
148 NFS. Normally, if a server exports two filesystems one of which is
149 mounted on the other, then the client will have to mount both
150 filesystems explicitly to get access to them. If it just mounts the
151 parent, it will see an empty directory at the place where the other
152 filesystem is mounted. That filesystem is "hidden".
156 option on a filesystem causes it not to be hidden, and an
157 appropriately authorised client will be able to move from the parent to
158 that filesystem without noticing the change.
160 However, some NFS clients do not cope well with this situation as, for
161 instance, it is then possible for two files in the one apparent
162 filesystem to have the same inode number.
166 option is currently only effective on
168 exports. It does not work reliably with netgroup, subnet, or wildcard
171 This option can be very useful in some situations, but it should be
172 used with due care, and only after confirming that the client system
173 copes with the situation effectively.
175 The option can be explicitly disabled with
179 This option is similar to
181 but it makes it possible for clients to move from the filesystem marked
182 with crossmnt to exported filesystems mounted on it. Thus when a child
183 filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has
184 the same effect as setting "nohide" on B.
187 This option disables subtree checking, which has mild security
188 implications, but can improve reliability in some circumstances.
190 If a subdirectory of a filesystem is exported, but the whole
191 filesystem isn't then whenever a NFS request arrives, the server must
192 check not only that the accessed file is in the appropriate filesystem
193 (which is easy) but also that it is in the exported tree (which is
194 harder). This check is called the
197 In order to perform this check, the server must include some
198 information about the location of the file in the "filehandle" that is
199 given to the client. This can cause problems with accessing files that
200 are renamed while a client has them open (though in many simple cases
203 subtree checking is also used to make sure that files inside
204 directories to which only root has access can only be accessed if the
205 filesystem is exported with
207 (see below), even if the file itself allows more general access.
209 As a general guide, a home directory filesystem, which is normally
210 exported at the root and may see lots of file renames, should be
211 exported with subtree checking disabled. A filesystem which is mostly
212 readonly, and at least doesn't see many file renames (e.g. /usr or
213 /var) and for which subdirectories may be exported, should probably be
214 exported with subtree checks enabled.
216 The default of having subtree checks enabled, can be explicitly
220 From release 1.1.0 of nfs-utils onwards, the default will be
222 as subtree_checking tends to cause more problems than it is worth.
223 If you genuinely require subtree checking, you should explicitly put
226 file. If you put neither option,
228 will warn you that the change is pending.
234 This option (the two names are synonymous) tells the NFS server not to require authentication of
235 locking requests (i.e. requests which use the NLM protocol). Normally
236 the NFS server will require a lock request to hold a credential for a
237 user who has read access to the file. With this flag no access checks
240 Early NFS client implementations did not send credentials with lock
241 requests, and many current NFS clients still exist which are based on
242 the old implementations. Use this flag if you find that you can only
243 lock files which are world readable.
245 The default behaviour of requiring authentication for NLM requests can
246 be explicitly requested with either of the synonymous
252 On some specially patched kernels, and when exporting filesystems that
253 support ACLs, this option tells nfsd not to reveal ACLs to clients, so
254 they will see only a subset of actual permissions on the given file
255 system. This option is safe for filesystems used by NFSv2 clients and
256 old NFSv3 clients that perform access decisions locally. Current
257 NFSv3 clients use the ACCESS RPC to perform all access decisions on
258 the server. Note that the
260 option only has effect on kernels specially patched to support it, and
261 when exporting filesystems with ACL support. The default is to export
262 with ACL support (i.e. by default,
268 '''This makes everything below the directory inaccessible for the named
269 '''client. This is useful when you want to export a directory hierarchy to
270 '''a client, but exclude certain subdirectories. The client's view of a
271 '''directory flagged with noaccess is very limited; it is allowed to read
272 '''its attributes, and lookup `.' and `..'. These are also the only entries
273 '''returned by a readdir.
276 '''Convert absolute symbolic links (where the link contents start with a
277 '''slash) into relative links by prepending the necessary number of ../'s
278 '''to get from the directory containing the link to the root on the
279 '''server. This has subtle, perhaps questionable, semantics when the file
280 '''hierarchy is not mounted at its root.
283 '''Leave all symbolic link as they are. This is the default operation.
289 This option makes it possible to only export a directory if it has
290 successfully been mounted.
291 If no path is given (e.g.
292 .IR mountpoint " or " mp )
293 then the export point must also be a mount point. If it isn't then
294 the export point is not exported. This allows you to be sure that the
295 directory underneath a mountpoint will never be exported by accident
296 if, for example, the filesystem failed to mount due to a disc error.
298 If a path is given (e.g.
299 .IR mountpoint= "/path or " mp= /path)
300 then the nominated path must be a mountpoint for the exportpoint to be
305 This option forces the filesystem identification portion of the file
306 handle and file attributes used on the wire to be
308 instead of a number derived from the major and minor number of the
309 block device on which the filesystem is mounted. Any 32 bit number
310 can be used, but it must be unique amongst all the exported filesystems.
312 This can be useful for NFS failover, to ensure that both servers of
313 the failover pair use the same NFS file handles for the shared filesystem
314 thus avoiding stale file handles after failover.
316 Some Linux filesystems are not mounted on a block device; exporting
317 these via NFS requires the use of the
319 option (although that may still not be enough).
321 The value 0 has a special meaning when use with NFSv4. NFSv4 has a
322 concept of a root of the overall exported filesystem. The export point
323 exported with fsid=0 will be used as this root.
328 bases its access control to files on the server machine on the uid and
329 gid provided in each NFS RPC request. The normal behavior a user would
330 expect is that she can access her files on the server just as she would
331 on a normal file system. This requires that the same uids and gids are
332 used on the client and the server machine. This is not always true, nor
333 is it always desirable.
335 Very often, it is not desirable that the root user on a client machine
336 is also treated as root when accessing files on the NFS server. To this
337 end, uid 0 is normally mapped to a different id: the so-called
340 uid. This mode of operation (called `root squashing') is the default,
341 and can be turned off with
346 '''tries to obtain the anonymous uid and gid by looking up user
348 '''in the password file at startup time. If it isn't found, a uid and gid
350 chooses a uid and gid
351 of 65534 for squashed access. These values can also be overridden by
353 .IR anonuid " and " anongid
356 '''In addition to this,
358 '''lets you specify arbitrary uids and gids that should be mapped to user
360 Finally, you can map all user requests to the
361 anonymous uid by specifying the
362 .IR all_squash " option.
364 '''For the benefit of installations where uids differ between different
367 '''provides several mechanism to dynamically map server uids to client
368 '''uids and vice versa: static mapping files, NIS-based mapping, and
373 '''mapping is enabled with the
375 '''option, and uses the UGID RPC protocol. For this to work, you have to run
378 '''mapping daemon on the client host. It is the least secure of the three methods,
379 '''because by running
381 '''everybody can query the client host for a list of valid user names. You
382 '''can protect yourself by restricting access to
384 '''to valid hosts only. This can be done by entering the list of valid
389 '''file. The service name is
391 '''For a description of the file's syntax, please read
392 '''.IR hosts_access (5).
394 '''Static mapping is enabled by using the
396 '''option, which takes a file name as an argument that describes the mapping.
397 '''NIS-based mapping queries the client's NIS server to obtain a mapping from
398 '''user and group names on the server host to user and group names on the
401 Here's the complete list of mapping options:
404 Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
405 not apply to any other uids or gids that might be equally sensitive, such as
412 Turn off root squashing. This option is mainly useful for diskless clients.
414 '''.IR squash_uids " and " squash_gids
415 '''This option specifies a list of uids or gids that should be subject to
416 '''anonymous mapping. A valid list of ids looks like this:
418 '''.IR squash_uids=0-15,20,25-50
420 '''Usually, your squash lists will look a lot simpler.
423 Map all uids and gids to the anonymous user. Useful for NFS-exported
424 public FTP directories, news spool directories, etc. The opposite option
427 which is the default setting.
430 '''This option turns on dynamic uid/gid mapping. Each uid in an NFS request
431 '''will be translated to the equivalent server uid, and each uid in an
432 '''NFS reply will be mapped the other way round. This option requires that
434 '''runs on the client host. The default setting is
435 '''.IR map_identity ,
436 '''which leaves all uids untouched. The normal squash options apply regardless
437 '''of whether dynamic mapping is requested or not.
440 '''This option enables static mapping. It specifies the name of the file
441 '''that describes the uid/gid mapping, e.g.
443 '''.IR map_static=/etc/nfs/foobar.map
445 '''The file's format looks like this
449 '''# Mapping for client foobar:
451 '''uid 0-99 - # squash these
452 '''uid 100-500 1000 # map 100-500 to 1000-1400
453 '''gid 0-49 - # squash these
454 '''gid 50-100 700 # map 50-100 to 700-750
458 '''This option enables NIS-based uid/gid mapping. For instance, when
459 '''the server encounters the uid 123 on the server, it will obtain the
460 '''login name associated with it, and contact the NFS client's NIS server
461 '''to obtain the uid the client associates with the name.
463 '''In order to do this, the NFS server must know the client's NIS domain.
464 '''This is specified as an argument to the
468 '''.I map_nis=foo.com
470 '''Note that it may not be sufficient to simply specify the NIS domain
471 '''here; you may have to take additional actions before
473 '''is actually able to contact the server. If your distribution uses
474 '''the NYS library, you can specify one or more NIS servers for the
475 '''client's domain in
476 '''.IR /etc/yp.conf .
477 '''If you are using a different NIS library, you may have to obtain a
480 '''daemon that can be configured via
483 .IR anonuid " and " anongid
484 These options explicitly set the uid and gid of the anonymous account.
485 This option is primarily useful for PC/NFS clients, where you might want
486 all requests appear to be from one user. As an example, consider the
489 in the example section below, which maps all requests to uid 150 (which
490 is supposedly that of user joe).
496 # sample /etc/exports file
497 / master(rw) trusty(rw,no_root_squash)
498 /projects proj*.local.domain(rw)
499 /usr *.local.domain(ro) @trusted(rw)
500 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
501 /pub (ro,insecure,all_squash)
502 '''/pub/private (noaccess)
505 The first line exports the entire filesystem to machines master and trusty.
506 In addition to write access, all uid squashing is turned off for host
507 trusty. The second and third entry show examples for wildcard hostnames
508 and netgroups (this is the entry `@trusted'). The fourth line shows the
509 entry for the PC/NFS client discussed above. Line 5 exports the
510 public FTP directory to every host in the world, executing all requests
511 under the nobody account. The
513 option in this entry also allows clients with NFS implementations that
514 don't use a reserved port for NFS.
515 ''' The last line denies all NFS clients
516 '''access to the private directory.
518 '''Unlike other NFS server implementations, this
520 '''allows you to export both a directory and a subdirectory thereof to
521 '''the same host, for instance
522 '''.IR /usr " and " /usr/X11R6 .
523 '''In this case, the mount options of the most specific entry apply. For
524 '''instance, when a user on the client host accesses a file in
526 '''the mount options given in the
528 '''entry apply. This is also true when the latter is a wildcard or netgroup
539 '''An error parsing the file is reported using syslogd(8) as level NOTICE from
540 '''a DAEMON whenever nfsd(8) or mountd(8) is started up. Any unknown
541 '''host is reported at that time, but often not all hosts are not yet known
542 '''to named(8) at boot time, thus as hosts are found they are reported
543 '''with the same syslogd(8) parameters.
projects / nfs-utils.git / blob