3 .TH exports 5 "31 December 2009"
5 exports \- NFS server export table
9 contains a table of local physical file systems on an NFS server
10 that are accessible to NFS clients.
11 The contents of the file are maintained by the server's system
14 Each file system in this table has a list of options and an
18 to give information to
21 The file format is similar to the SunOS
23 file. Each line contains an export point and a whitespace-separated list
24 of clients allowed to mount the file system at that point. Each listed
25 client may be immediately followed by a parenthesized, comma-separated
26 list of export options for that client. No whitespace is permitted
27 between a client and its option list.
29 Also, each line may have one or more specifications for default options
30 after the path name, in the form of a dash ("\-") followed by an option
31 list. The option list is used for all subsequent exports on that line
34 Blank lines are ignored. A pound sign ("#") introduces a comment to the
35 end of the line. Entries may be continued across newlines using a
36 backslash. If an export name contains spaces it should be quoted using
37 double quotes. You can also specify spaces or other unusual character in
38 the export name using a backslash followed by the character code as three
41 To apply changes to this file, run
43 or restart the NFS server.
45 .SS Machine Name Formats
46 NFS clients may be specified in a number of ways:
48 This is the most common format. You may specify a host either by an
49 abbreviated name recognized be the resolver, the fully qualified domain
50 name, or an IP address.
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.
58 For example, either `/255.255.252.0' or `/22' appended
59 to the network base IPv4 address results in identical subnetworks with 10 bits of
60 host. Wildcard characters generally do not work on IP addresses, though they
61 may work by accident when reverse DNS lookups fail.
63 Machine names may contain the wildcard characters \fI*\fR and \fI?\fR.
64 This can be used to make the \fIexports\fR file more compact; for instance,
65 \fI*.cs.foo.edu\fR matches all hosts in the domain
66 \fIcs.foo.edu\fR. As these characters also match the dots in a domain
67 name, the given pattern will also match all hosts within any subdomain
70 NIS netgroups may be given as
72 Only the host part of each
73 netgroup members is consider in checking for membership. Empty host
74 parts or those containing a single dash (\-) are ignored.
76 This is specified by a single
78 character (not to be confused with the
80 entry above) and will match all clients.
83 '''This is a special ``hostname'' that identifies the given directory name
84 '''as the public root directory (see the section on WebNFS in
86 '''for a discussion of WebNFS and the public root handle). When using this
89 '''must be the only entry on this line, and must have no export options
90 '''associated with it. Note that this does
92 '''actually export the named directory; you still have to set the exports
93 '''options in a separate entry.
95 '''The public root path can also be specified by invoking
98 '''.B \-\-public\-root
99 '''option. Multiple specifications of a public root will be ignored.
101 If a client matches more than one of the specifications above, then
102 the first match from the above list order takes precedence - regardless of
103 the order they appear on the export line. However, if a client matches
104 more than one of the same type of specification (e.g. two netgroups),
105 then the first match from the order they appear on the export line takes
107 .SS RPCSEC_GSS security
108 You may use the special strings "gss/krb5", "gss/krb5i", or "gss/krb5p"
109 to restrict access to clients using rpcsec_gss security. However, this
110 syntax is deprecated; on linux kernels since 2.6.23, you should instead
111 use the "sec=" export option:
114 The sec= option, followed by a colon-delimited list of security flavors,
115 restricts the export to clients using those flavors. Available security
116 flavors include sys (the default--no cryptographic security), krb5
117 (authentication only), krb5i (integrity protection), and krb5p (privacy
118 protection). For the purposes of security flavor negotiation, order
119 counts: preferred flavors should be listed first. The order of the sec=
120 option with respect to the other options does not matter, unless you
121 want some options to be enforced differently depending on flavor.
122 In that case you may include multiple sec= options, and following options
123 will be enforced only for access using flavors listed in the immediately
124 preceding sec= option. The only options that are permitted to vary in
125 this way are ro, rw, no_root_squash, root_squash, and all_squash.
129 understands the following export options:
132 This option requires that requests originate on an Internet port less
133 than IPPORT_RESERVED (1024). This option is on by default. To turn it
138 Allow both read and write requests on this NFS volume. The
139 default is to disallow any request which changes the filesystem.
140 This can also be made explicit by using
145 This option allows the NFS server to violate the NFS protocol and
146 reply to requests before any changes made by that request have been
147 committed to stable storage (e.g. disc drive).
149 Using this option usually improves performance, but at the cost that
150 an unclean server restart (i.e. a crash) can cause data to be lost or
155 Reply to requests only after the changes have been committed to stable
160 In releases of nfs-utils up to and including 1.0.0, the
163 default. In all releases after 1.0.0,
167 must be explicitly requested if needed.
168 To help make system administrators aware of this change,
170 will issue a warning if neither
177 This option has no effect if
179 is also set. The NFS server will normally delay committing a write request
180 to disc slightly if it suspects that another related write request may be in
181 progress or may arrive soon. This allows multiple write requests to
182 be committed to disc with the one operation which can improve
183 performance. If an NFS server received mainly small unrelated
184 requests, this behaviour could actually reduce performance, so
186 is available to turn it off.
187 The default can be explicitly requested with the
191 This option is based on the option of the same name provided in IRIX
192 NFS. Normally, if a server exports two filesystems one of which is
193 mounted on the other, then the client will have to mount both
194 filesystems explicitly to get access to them. If it just mounts the
195 parent, it will see an empty directory at the place where the other
196 filesystem is mounted. That filesystem is "hidden".
200 option on a filesystem causes it not to be hidden, and an
201 appropriately authorised client will be able to move from the parent to
202 that filesystem without noticing the change.
204 However, some NFS clients do not cope well with this situation as, for
205 instance, it is then possible for two files in the one apparent
206 filesystem to have the same inode number.
210 option is currently only effective on
212 exports. It does not work reliably with netgroup, subnet, or wildcard
215 This option can be very useful in some situations, but it should be
216 used with due care, and only after confirming that the client system
217 copes with the situation effectively.
219 The option can be explicitly disabled with
223 This option is similar to
225 but it makes it possible for clients to move from the filesystem marked
226 with crossmnt to exported filesystems mounted on it. Thus when a child
227 filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has
228 the same effect as setting "nohide" on B.
231 This option disables subtree checking, which has mild security
232 implications, but can improve reliability in some circumstances.
234 If a subdirectory of a filesystem is exported, but the whole
235 filesystem isn't then whenever a NFS request arrives, the server must
236 check not only that the accessed file is in the appropriate filesystem
237 (which is easy) but also that it is in the exported tree (which is
238 harder). This check is called the
241 In order to perform this check, the server must include some
242 information about the location of the file in the "filehandle" that is
243 given to the client. This can cause problems with accessing files that
244 are renamed while a client has them open (though in many simple cases
247 subtree checking is also used to make sure that files inside
248 directories to which only root has access can only be accessed if the
249 filesystem is exported with
251 (see below), even if the file itself allows more general access.
253 As a general guide, a home directory filesystem, which is normally
254 exported at the root and may see lots of file renames, should be
255 exported with subtree checking disabled. A filesystem which is mostly
256 readonly, and at least doesn't see many file renames (e.g. /usr or
257 /var) and for which subdirectories may be exported, should probably be
258 exported with subtree checks enabled.
260 The default of having subtree checks enabled, can be explicitly
264 From release 1.1.0 of nfs-utils onwards, the default will be
266 as subtree_checking tends to cause more problems than it is worth.
267 If you genuinely require subtree checking, you should explicitly put
270 file. If you put neither option,
272 will warn you that the change is pending.
278 This option (the two names are synonymous) tells the NFS server not to require authentication of
279 locking requests (i.e. requests which use the NLM protocol). Normally
280 the NFS server will require a lock request to hold a credential for a
281 user who has read access to the file. With this flag no access checks
284 Early NFS client implementations did not send credentials with lock
285 requests, and many current NFS clients still exist which are based on
286 the old implementations. Use this flag if you find that you can only
287 lock files which are world readable.
289 The default behaviour of requiring authentication for NLM requests can
290 be explicitly requested with either of the synonymous
296 On some specially patched kernels, and when exporting filesystems that
297 support ACLs, this option tells
299 not to reveal ACLs to clients, so
300 they will see only a subset of actual permissions on the given file
301 system. This option is safe for filesystems used by NFSv2 clients and
302 old NFSv3 clients that perform access decisions locally. Current
303 NFSv3 clients use the ACCESS RPC to perform all access decisions on
304 the server. Note that the
306 option only has effect on kernels specially patched to support it, and
307 when exporting filesystems with ACL support. The default is to export
308 with ACL support (i.e. by default,
314 '''This makes everything below the directory inaccessible for the named
315 '''client. This is useful when you want to export a directory hierarchy to
316 '''a client, but exclude certain subdirectories. The client's view of a
317 '''directory flagged with noaccess is very limited; it is allowed to read
318 '''its attributes, and lookup `.' and `..'. These are also the only entries
319 '''returned by a readdir.
322 '''Convert absolute symbolic links (where the link contents start with a
323 '''slash) into relative links by prepending the necessary number of ../'s
324 '''to get from the directory containing the link to the root on the
325 '''server. This has subtle, perhaps questionable, semantics when the file
326 '''hierarchy is not mounted at its root.
329 '''Leave all symbolic link as they are. This is the default operation.
335 This option makes it possible to only export a directory if it has
336 successfully been mounted.
337 If no path is given (e.g.
338 .IR mountpoint " or " mp )
339 then the export point must also be a mount point. If it isn't then
340 the export point is not exported. This allows you to be sure that the
341 directory underneath a mountpoint will never be exported by accident
342 if, for example, the filesystem failed to mount due to a disc error.
344 If a path is given (e.g.
345 .IR mountpoint= "/path or " mp= /path)
346 then the nominated path must be a mountpoint for the exportpoint to be
350 .IR fsid= num|root|uuid
351 NFS needs to be able to identify each filesystem that it exports.
352 Normally it will use a UUID for the filesystem (if the filesystem has
353 such a thing) or the device number of the device holding the
354 filesystem (if the filesystem is stored on the device).
356 As not all filesystems are stored on devices, and not all filesystems
357 have UUIDs, it is sometimes necessary to explicitly tell NFS how to
358 identify a filesystem. This is done with the
362 For NFSv4, there is a distinguished filesystem which is the root of
363 all exported filesystem. This is specified with
367 both of which mean exactly the same thing.
369 Other filesystems can be identified with a small integer, or a UUID
370 which should contain 32 hex digits and arbitrary punctuation.
372 Linux kernels version 2.6.20 and earlier do not understand the UUID
373 setting so a small integer must be used if an fsid option needs to be
374 set for such kernels. Setting both a small number and a UUID is
375 supported so the same configuration can be made to work on old and new
379 .IR refer= path@host[+host][:path@host[+host]]
380 A client referencing the export point will be directed to choose from
381 the given list an alternative location for the filesystem.
382 (Note that the server must have a mountpoint here, though a different
383 filesystem is not required; so, for example,
384 .IR "mount --bind" " /path /path"
387 .IR replicas= path@host[+host][:path@host[+host]]
388 If the client asks for alternative locations for the export point, it
389 will be given this list of alternatives. (Note that actual replication
390 of the filesystem must be handled elsewhere.)
395 bases its access control to files on the server machine on the uid and
396 gid provided in each NFS RPC request. The normal behavior a user would
397 expect is that she can access her files on the server just as she would
398 on a normal file system. This requires that the same uids and gids are
399 used on the client and the server machine. This is not always true, nor
400 is it always desirable.
402 Very often, it is not desirable that the root user on a client machine
403 is also treated as root when accessing files on the NFS server. To this
404 end, uid 0 is normally mapped to a different id: the so-called
407 uid. This mode of operation (called `root squashing') is the default,
408 and can be turned off with
413 '''tries to obtain the anonymous uid and gid by looking up user
415 '''in the password file at startup time. If it isn't found, a uid and gid
417 chooses a uid and gid
418 of 65534 for squashed access. These values can also be overridden by
420 .IR anonuid " and " anongid
423 '''In addition to this,
425 '''lets you specify arbitrary uids and gids that should be mapped to user
427 Finally, you can map all user requests to the
428 anonymous uid by specifying the
429 .IR all_squash " option.
431 Here's the complete list of mapping options:
434 Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
435 not apply to any other uids or gids that might be equally sensitive, such as
442 Turn off root squashing. This option is mainly useful for diskless clients.
445 Map all uids and gids to the anonymous user. Useful for NFS-exported
446 public FTP directories, news spool directories, etc. The opposite option
449 which is the default setting.
451 .IR anonuid " and " anongid
452 These options explicitly set the uid and gid of the anonymous account.
453 This option is primarily useful for PC/NFS clients, where you might want
454 all requests appear to be from one user. As an example, consider the
457 in the example section below, which maps all requests to uid 150 (which
458 is supposedly that of user joe).
459 .SS Extra Export Tables
465 directory as extra export tables.
467 regards only a file which name is ended with
472 as an extra export file. A file which name
473 is not met this condition is just ignored.
474 The format for extra export tables is the same as
482 # sample /etc/exports file
483 / master(rw) trusty(rw,no_root_squash)
484 /projects proj*.local.domain(rw)
485 /usr *.local.domain(ro) @trusted(rw)
486 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
487 /pub *(ro,insecure,all_squash)
488 /srv/www \-sync,rw server @trusted @external(ro)
489 '''/pub/private (noaccess)
492 The first line exports the entire filesystem to machines master and trusty.
493 In addition to write access, all uid squashing is turned off for host
494 trusty. The second and third entry show examples for wildcard hostnames
495 and netgroups (this is the entry `@trusted'). The fourth line shows the
496 entry for the PC/NFS client discussed above. Line 5 exports the
497 public FTP directory to every host in the world, executing all requests
498 under the nobody account. The
500 option in this entry also allows clients with NFS implementations that
501 don't use a reserved port for NFS.
502 The sixth line exports a directory read-write to the machine 'server'
503 as well as the `@trusted' netgroup, and read-only to netgroup `@external',
504 all three mounts with the `sync' option enabled.
505 ''' The last line denies all NFS clients
506 '''access to the private directory.
508 '''Unlike other NFS server implementations, this
510 '''allows you to export both a directory and a subdirectory thereof to
511 '''the same host, for instance
512 '''.IR /usr " and " /usr/X11R6 .
513 '''In this case, the mount options of the most specific entry apply. For
514 '''instance, when a user on the client host accesses a file in
516 '''the mount options given in the
518 '''entry apply. This is also true when the latter is a wildcard or netgroup
530 '''An error parsing the file is reported using syslogd(8) as level NOTICE from
535 '''is started up. Any unknown
536 '''host is reported at that time, but often not all hosts are not yet known
539 '''at boot time, thus as hosts are found they are reported
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