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 NIS netgroups may be given as
54 Only the host part of each
55 netgroup members is consider in checking for membership. Empty host
56 parts or those containing a single dash (\-) are ignored.
58 Machine names may contain the wildcard characters \fI*\fR and \fI?\fR.
59 This can be used to make the \fIexports\fR file more compact; for instance,
60 \fI*.cs.foo.edu\fR matches all hosts in the domain
61 \fIcs.foo.edu\fR. As these characters also match the dots in a domain
62 name, the given pattern will also match all hosts within any subdomain
65 You can also export directories to all hosts on an IP (sub-) network
66 simultaneously. This is done by specifying an IP address and netmask pair
69 where the netmask can be specified in dotted-decimal format, or as a
70 contiguous mask length.
71 For example, either `/255.255.252.0' or `/22' appended
72 to the network base IPv4 address results in identical subnetworks with 10 bits of
73 host. Wildcard characters generally do not work on IP addresses, though they
74 may work by accident when reverse DNS lookups fail.
77 '''This is a special ``hostname'' that identifies the given directory name
78 '''as the public root directory (see the section on WebNFS in
80 '''for a discussion of WebNFS and the public root handle). When using this
83 '''must be the only entry on this line, and must have no export options
84 '''associated with it. Note that this does
86 '''actually export the named directory; you still have to set the exports
87 '''options in a separate entry.
89 '''The public root path can also be specified by invoking
92 '''.B \-\-public\-root
93 '''option. Multiple specifications of a public root will be ignored.
95 .SS RPCSEC_GSS security
96 You may use the special strings "gss/krb5", "gss/krb5i", or "gss/krb5p"
97 to restrict access to clients using rpcsec_gss security. However, this
98 syntax is deprecated; on linux kernels since 2.6.23, you should instead
99 use the "sec=" export option:
102 The sec= option, followed by a colon-delimited list of security flavors,
103 restricts the export to clients using those flavors. Available security
104 flavors include sys (the default--no cryptographic security), krb5
105 (authentication only), krb5i (integrity protection), and krb5p (privacy
106 protection). For the purposes of security flavor negotiation, order
107 counts: preferred flavors should be listed first. The order of the sec=
108 option with respect to the other options does not matter, unless you
109 want some options to be enforced differently depending on flavor.
110 In that case you may include multiple sec= options, and following options
111 will be enforced only for access using flavors listed in the immediately
112 preceding sec= option. The only options that are permitted to vary in
113 this way are ro, rw, no_root_squash, root_squash, and all_squash.
117 understands the following export options:
120 This option requires that requests originate on an Internet port less
121 than IPPORT_RESERVED (1024). This option is on by default. To turn it
126 Allow both read and write requests on this NFS volume. The
127 default is to disallow any request which changes the filesystem.
128 This can also be made explicit by using
133 This option allows the NFS server to violate the NFS protocol and
134 reply to requests before any changes made by that request have been
135 committed to stable storage (e.g. disc drive).
137 Using this option usually improves performance, but at the cost that
138 an unclean server restart (i.e. a crash) can cause data to be lost or
143 Reply to requests only after the changes have been committed to stable
148 In releases of nfs-utils up to and including 1.0.0, the
151 default. In all releases after 1.0.0,
155 must be explicitly requested if needed.
156 To help make system administrators aware of this change,
158 will issue a warning if neither
165 This option has no effect if
167 is also set. The NFS server will normally delay committing a write request
168 to disc slightly if it suspects that another related write request may be in
169 progress or may arrive soon. This allows multiple write requests to
170 be committed to disc with the one operation which can improve
171 performance. If an NFS server received mainly small unrelated
172 requests, this behaviour could actually reduce performance, so
174 is available to turn it off.
175 The default can be explicitly requested with the
179 This option is based on the option of the same name provided in IRIX
180 NFS. Normally, if a server exports two filesystems one of which is
181 mounted on the other, then the client will have to mount both
182 filesystems explicitly to get access to them. If it just mounts the
183 parent, it will see an empty directory at the place where the other
184 filesystem is mounted. That filesystem is "hidden".
188 option on a filesystem causes it not to be hidden, and an
189 appropriately authorised client will be able to move from the parent to
190 that filesystem without noticing the change.
192 However, some NFS clients do not cope well with this situation as, for
193 instance, it is then possible for two files in the one apparent
194 filesystem to have the same inode number.
198 option is currently only effective on
200 exports. It does not work reliably with netgroup, subnet, or wildcard
203 This option can be very useful in some situations, but it should be
204 used with due care, and only after confirming that the client system
205 copes with the situation effectively.
207 The option can be explicitly disabled with
211 This option is similar to
213 but it makes it possible for clients to move from the filesystem marked
214 with crossmnt to exported filesystems mounted on it. Thus when a child
215 filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has
216 the same effect as setting "nohide" on B.
219 This option disables subtree checking, which has mild security
220 implications, but can improve reliability in some circumstances.
222 If a subdirectory of a filesystem is exported, but the whole
223 filesystem isn't then whenever a NFS request arrives, the server must
224 check not only that the accessed file is in the appropriate filesystem
225 (which is easy) but also that it is in the exported tree (which is
226 harder). This check is called the
229 In order to perform this check, the server must include some
230 information about the location of the file in the "filehandle" that is
231 given to the client. This can cause problems with accessing files that
232 are renamed while a client has them open (though in many simple cases
235 subtree checking is also used to make sure that files inside
236 directories to which only root has access can only be accessed if the
237 filesystem is exported with
239 (see below), even if the file itself allows more general access.
241 As a general guide, a home directory filesystem, which is normally
242 exported at the root and may see lots of file renames, should be
243 exported with subtree checking disabled. A filesystem which is mostly
244 readonly, and at least doesn't see many file renames (e.g. /usr or
245 /var) and for which subdirectories may be exported, should probably be
246 exported with subtree checks enabled.
248 The default of having subtree checks enabled, can be explicitly
252 From release 1.1.0 of nfs-utils onwards, the default will be
254 as subtree_checking tends to cause more problems than it is worth.
255 If you genuinely require subtree checking, you should explicitly put
258 file. If you put neither option,
260 will warn you that the change is pending.
266 This option (the two names are synonymous) tells the NFS server not to require authentication of
267 locking requests (i.e. requests which use the NLM protocol). Normally
268 the NFS server will require a lock request to hold a credential for a
269 user who has read access to the file. With this flag no access checks
272 Early NFS client implementations did not send credentials with lock
273 requests, and many current NFS clients still exist which are based on
274 the old implementations. Use this flag if you find that you can only
275 lock files which are world readable.
277 The default behaviour of requiring authentication for NLM requests can
278 be explicitly requested with either of the synonymous
284 On some specially patched kernels, and when exporting filesystems that
285 support ACLs, this option tells
287 not to reveal ACLs to clients, so
288 they will see only a subset of actual permissions on the given file
289 system. This option is safe for filesystems used by NFSv2 clients and
290 old NFSv3 clients that perform access decisions locally. Current
291 NFSv3 clients use the ACCESS RPC to perform all access decisions on
292 the server. Note that the
294 option only has effect on kernels specially patched to support it, and
295 when exporting filesystems with ACL support. The default is to export
296 with ACL support (i.e. by default,
302 '''This makes everything below the directory inaccessible for the named
303 '''client. This is useful when you want to export a directory hierarchy to
304 '''a client, but exclude certain subdirectories. The client's view of a
305 '''directory flagged with noaccess is very limited; it is allowed to read
306 '''its attributes, and lookup `.' and `..'. These are also the only entries
307 '''returned by a readdir.
310 '''Convert absolute symbolic links (where the link contents start with a
311 '''slash) into relative links by prepending the necessary number of ../'s
312 '''to get from the directory containing the link to the root on the
313 '''server. This has subtle, perhaps questionable, semantics when the file
314 '''hierarchy is not mounted at its root.
317 '''Leave all symbolic link as they are. This is the default operation.
323 This option makes it possible to only export a directory if it has
324 successfully been mounted.
325 If no path is given (e.g.
326 .IR mountpoint " or " mp )
327 then the export point must also be a mount point. If it isn't then
328 the export point is not exported. This allows you to be sure that the
329 directory underneath a mountpoint will never be exported by accident
330 if, for example, the filesystem failed to mount due to a disc error.
332 If a path is given (e.g.
333 .IR mountpoint= "/path or " mp= /path)
334 then the nominated path must be a mountpoint for the exportpoint to be
338 .IR fsid= num|root|uuid
339 NFS needs to be able to identify each filesystem that it exports.
340 Normally it will use a UUID for the filesystem (if the filesystem has
341 such a thing) or the device number of the device holding the
342 filesystem (if the filesystem is stored on the device).
344 As not all filesystems are stored on devices, and not all filesystems
345 have UUIDs, it is sometimes necessary to explicitly tell NFS how to
346 identify a filesystem. This is done with the
350 For NFSv4, there is a distinguished filesystem which is the root of
351 all exported filesystem. This is specified with
355 both of which mean exactly the same thing.
357 Other filesystems can be identified with a small integer, or a UUID
358 which should contain 32 hex digits and arbitrary punctuation.
360 Linux kernels version 2.6.20 and earlier do not understand the UUID
361 setting so a small integer must be used if an fsid option needs to be
362 set for such kernels. Setting both a small number and a UUID is
363 supported so the same configuration can be made to work on old and new
367 .IR refer= path@host[+host][:path@host[+host]]
368 A client referencing the export point will be directed to choose from
369 the given list an alternative location for the filesystem.
370 (Note that the server must have a mountpoint here, though a different
371 filesystem is not required; so, for example,
372 .IR "mount --bind" " /path /path"
375 .IR replicas= path@host[+host][:path@host[+host]]
376 If the client asks for alternative locations for the export point, it
377 will be given this list of alternatives. (Note that actual replication
378 of the filesystem must be handled elsewhere.)
383 bases its access control to files on the server machine on the uid and
384 gid provided in each NFS RPC request. The normal behavior a user would
385 expect is that she can access her files on the server just as she would
386 on a normal file system. This requires that the same uids and gids are
387 used on the client and the server machine. This is not always true, nor
388 is it always desirable.
390 Very often, it is not desirable that the root user on a client machine
391 is also treated as root when accessing files on the NFS server. To this
392 end, uid 0 is normally mapped to a different id: the so-called
395 uid. This mode of operation (called `root squashing') is the default,
396 and can be turned off with
401 '''tries to obtain the anonymous uid and gid by looking up user
403 '''in the password file at startup time. If it isn't found, a uid and gid
405 chooses a uid and gid
406 of 65534 for squashed access. These values can also be overridden by
408 .IR anonuid " and " anongid
411 '''In addition to this,
413 '''lets you specify arbitrary uids and gids that should be mapped to user
415 Finally, you can map all user requests to the
416 anonymous uid by specifying the
417 .IR all_squash " option.
419 Here's the complete list of mapping options:
422 Map requests from uid/gid 0 to the anonymous uid/gid. Note that this does
423 not apply to any other uids or gids that might be equally sensitive, such as
430 Turn off root squashing. This option is mainly useful for diskless clients.
433 Map all uids and gids to the anonymous user. Useful for NFS-exported
434 public FTP directories, news spool directories, etc. The opposite option
437 which is the default setting.
439 .IR anonuid " and " anongid
440 These options explicitly set the uid and gid of the anonymous account.
441 This option is primarily useful for PC/NFS clients, where you might want
442 all requests appear to be from one user. As an example, consider the
445 in the example section below, which maps all requests to uid 150 (which
446 is supposedly that of user joe).
447 .SS Extra Export Tables
453 directory as extra export tables.
455 regards only a file which name is ended with
460 as an extra export file. A file which name
461 is not met this condition is just ignored.
462 The format for extra export tables is the same as
470 # sample /etc/exports file
471 / master(rw) trusty(rw,no_root_squash)
472 /projects proj*.local.domain(rw)
473 /usr *.local.domain(ro) @trusted(rw)
474 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
475 /pub *(ro,insecure,all_squash)
476 /srv/www \-sync,rw server @trusted @external(ro)
477 '''/pub/private (noaccess)
480 The first line exports the entire filesystem to machines master and trusty.
481 In addition to write access, all uid squashing is turned off for host
482 trusty. The second and third entry show examples for wildcard hostnames
483 and netgroups (this is the entry `@trusted'). The fourth line shows the
484 entry for the PC/NFS client discussed above. Line 5 exports the
485 public FTP directory to every host in the world, executing all requests
486 under the nobody account. The
488 option in this entry also allows clients with NFS implementations that
489 don't use a reserved port for NFS.
490 The sixth line exports a directory read-write to the machine 'server'
491 as well as the `@trusted' netgroup, and read-only to netgroup `@external',
492 all three mounts with the `sync' option enabled.
493 ''' The last line denies all NFS clients
494 '''access to the private directory.
496 '''Unlike other NFS server implementations, this
498 '''allows you to export both a directory and a subdirectory thereof to
499 '''the same host, for instance
500 '''.IR /usr " and " /usr/X11R6 .
501 '''In this case, the mount options of the most specific entry apply. For
502 '''instance, when a user on the client host accesses a file in
504 '''the mount options given in the
506 '''entry apply. This is also true when the latter is a wildcard or netgroup
518 '''An error parsing the file is reported using syslogd(8) as level NOTICE from
523 '''is started up. Any unknown
524 '''host is reported at that time, but often not all hosts are not yet known
527 '''at boot time, thus as hosts are found they are reported