1exports(5) File Formats Manual exports(5)
2
6 exports - NFS server export table
7
9 The file /etc/exports contains a table of local physical file systems
10 on an NFS server that are accessible to NFS clients. The contents of
11 the file are maintained by the server's system administrator.
12 Each file system in this table has a list of options and an access con‐
14 trol list. The table is used by exportfs(8) to give information to
15 mountd(8).
16 The file format is similar to the SunOS exports file. Each line con‐
18 tains an export point and a whitespace-separated list of clients
19 allowed to mount the file system at that point. Each listed client may
20 be immediately followed by a parenthesized, comma-separated list of
21 export options for that client. No whitespace is permitted between a
22 client and its option list.
23 Also, each line may have one or more specifications for default options
25 after the path name, in the form of a dash ("-") followed by an option
26 list. The option list is used for all subsequent exports on that line
27 only.
28 Blank lines are ignored. A pound sign ("#") introduces a comment to
30 the end of the line. Entries may be continued across newlines using a
31 backslash. If an export name contains spaces it should be quoted using
32 double quotes. You can also specify spaces or other unusual character
33 in the export name using a backslash followed by the character code as
34 three octal digits.
35 To apply changes to this file, run exportfs -ra or restart the NFS
37 server.
38 Machine Name Formats
40 NFS clients may be specified in a number of ways:
41 single host
43 You may specify a host either by an abbreviated name recognized
44 be the resolver, the fully qualified domain name, an IPv4
45 address, or an IPv6 address. IPv6 addresses must not be inside
46 square brackets in /etc/exports lest they be confused with char‐
47 acter-class wildcard matches.
48 IP networks
50 You can also export directories to all hosts on an IP (sub-)
51 network simultaneously. This is done by specifying an IP address
52 and netmask pair as address/netmask where the netmask can be
53 specified in dotted-decimal format, or as a contiguous mask
54 length. For example, either `/255.255.252.0' or `/22' appended
55 to the network base IPv4 address results in identical subnet‐
56 works with 10 bits of host. IPv6 addresses must use a contiguous
57 mask length and must not be inside square brackets to avoid con‐
58 fusion with character-class wildcards. Wildcard characters gen‐
59 erally do not work on IP addresses, though they may work by
60 accident when reverse DNS lookups fail.
61 wildcards
63 Machine names may contain the wildcard characters * and ?, or
64 may contain character class lists within [square brackets].
65 This can be used to make the exports file more compact; for
66 instance, *.cs.foo.edu matches all hosts in the domain
67 cs.foo.edu. As these characters also match the dots in a domain
68 name, the given pattern will also match all hosts within any
69 subdomain of cs.foo.edu.
70 netgroups
72 NIS netgroups may be given as @group. Only the host part of
73 each netgroup members is consider in checking for membership.
74 Empty host parts or those containing a single dash (-) are
75 ignored.
76 anonymous
78 This is specified by a single * character (not to be confused
79 with the wildcard entry above) and will match all clients.
80 If a client matches more than one of the specifications above, then the
82 first match from the above list order takes precedence - regardless of
83 the order they appear on the export line. However, if a client matches
84 more than one of the same type of specification (e.g. two netgroups),
85 then the first match from the order they appear on the export line
86 takes precedence.
87 RPCSEC_GSS security
89 You may use the special strings "gss/krb5", "gss/krb5i", or "gss/krb5p"
90 to restrict access to clients using rpcsec_gss security. However, this
91 syntax is deprecated; on linux kernels since 2.6.23, you should instead
92 use the "sec=" export option:
93 sec= The sec= option, followed by a colon-delimited list of security
95 flavors, restricts the export to clients using those flavors.
96 Available security flavors include sys (the default--no crypto‐
97 graphic security), krb5 (authentication only), krb5i (integrity
98 protection), and krb5p (privacy protection). For the purposes
99 of security flavor negotiation, order counts: preferred flavors
100 should be listed first. The order of the sec= option with
101 respect to the other options does not matter, unless you want
102 some options to be enforced differently depending on flavor. In
103 that case you may include multiple sec= options, and following
104 options will be enforced only for access using flavors listed in
105 the immediately preceding sec= option. The only options that
106 are permitted to vary in this way are ro, rw, no_root_squash,
107 root_squash, and all_squash.
108 General Options
110 exportfs understands the following export options:
111 secure This option requires that requests not using gss originate on an
113 Internet port less than IPPORT_RESERVED (1024). This option is
114 on by default. To turn it off, specify insecure. (NOTE: older
115 kernels (before upstream kernel version 4.17) enforced this
116 requirement on gss requests as well.)
117 rw Allow both read and write requests on this NFS volume. The
119 default is to disallow any request which changes the filesystem.
120 This can also be made explicit by using the ro option.
121 async This option allows the NFS server to violate the NFS protocol
123 and reply to requests before any changes made by that request
124 have been committed to stable storage (e.g. disc drive).
125 Using this option usually improves performance, but at the cost
127 that an unclean server restart (i.e. a crash) can cause data to
128 be lost or corrupted.
129 sync Reply to requests only after the changes have been committed to
132 stable storage (see async above).
133 In releases of nfs-utils up to and including 1.0.0, the async
135 option was the default. In all releases after 1.0.0, sync is
136 the default, and async must be explicitly requested if needed.
137 To help make system administrators aware of this change,
138 exportfs will issue a warning if neither sync nor async is spec‐
139 ified.
140 no_wdelay
142 This option has no effect if async is also set. The NFS server
143 will normally delay committing a write request to disc slightly
144 if it suspects that another related write request may be in
145 progress or may arrive soon. This allows multiple write
146 requests to be committed to disc with the one operation which
147 can improve performance. If an NFS server received mainly small
148 unrelated requests, this behaviour could actually reduce perfor‐
149 mance, so no_wdelay is available to turn it off. The default
150 can be explicitly requested with the wdelay option.
151 nohide This option is based on the option of the same name provided in
153 IRIX NFS. Normally, if a server exports two filesystems one of
154 which is mounted on the other, then the client will have to
155 mount both filesystems explicitly to get access to them. If it
156 just mounts the parent, it will see an empty directory at the
157 place where the other filesystem is mounted. That filesystem is
158 "hidden".
159 Setting the nohide option on a filesystem causes it not to be
161 hidden, and an appropriately authorised client will be able to
162 move from the parent to that filesystem without noticing the
163 change.
164 However, some NFS clients do not cope well with this situation
166 as, for instance, it is then possible for two files in the one
167 apparent filesystem to have the same inode number.
168 The nohide option is currently only effective on single host
170 exports. It does not work reliably with netgroup, subnet, or
171 wildcard exports.
172 This option can be very useful in some situations, but it should
174 be used with due care, and only after confirming that the client
175 system copes with the situation effectively.
176 The option can be explicitly disabled for NFSv2 and NFSv3 with
178 hide.
179 This option is not relevant when NFSv4 is use. NFSv4 never
181 hides subordinate filesystems. Any filesystem that is exported
182 will be visible where expected when using NFSv4.
183 crossmnt
185 This option is similar to nohide but it makes it possible for
186 clients to access all filesystems mounted on a filesystem marked
187 with crossmnt. Thus when a child filesystem "B" is mounted on a
188 parent "A", setting crossmnt on "A" has a similar effect to set‐
189 ting "nohide" on B.
190 With nohide the child filesystem needs to be explicitly
192 exported. With crossmnt it need not. If a child of a crossmnt
193 file is not explicitly exported, then it will be implicitly
194 exported with the same export options as the parent, except for
195 fsid=. This makes it impossible to not export a child of a
196 crossmnt filesystem. If some but not all subordinate filesys‐
197 tems of a parent are to be exported, then they must be explic‐
198 itly exported and the parent should not have crossmnt set.
199 The nocrossmnt option can explictly disable crossmnt if it was
201 previously set. This is rarely useful.
202 no_subtree_check
204 This option disables subtree checking, which has mild security
205 implications, but can improve reliability in some circumstances.
206 If a subdirectory of a filesystem is exported, but the whole
208 filesystem isn't then whenever a NFS request arrives, the server
209 must check not only that the accessed file is in the appropriate
210 filesystem (which is easy) but also that it is in the exported
211 tree (which is harder). This check is called the subtree_check.
212 In order to perform this check, the server must include some
214 information about the location of the file in the "filehandle"
215 that is given to the client. This can cause problems with
216 accessing files that are renamed while a client has them open
217 (though in many simple cases it will still work).
218 subtree checking is also used to make sure that files inside
220 directories to which only root has access can only be accessed
221 if the filesystem is exported with no_root_squash (see below),
222 even if the file itself allows more general access.
223 As a general guide, a home directory filesystem, which is nor‐
225 mally exported at the root and may see lots of file renames,
226 should be exported with subtree checking disabled. A filesystem
227 which is mostly readonly, and at least doesn't see many file
228 renames (e.g. /usr or /var) and for which subdirectories may be
229 exported, should probably be exported with subtree checks
230 enabled.
231 The default of having subtree checks enabled, can be explicitly
233 requested with subtree_check.
234 From release 1.1.0 of nfs-utils onwards, the default will be
236 no_subtree_check as subtree_checking tends to cause more prob‐
237 lems than it is worth. If you genuinely require subtree check‐
238 ing, you should explicitly put that option in the exports file.
239 If you put neither option, exportfs will warn you that the
240 change is pending.
241 insecure_locks
244 no_auth_nlm
246 This option (the two names are synonymous) tells the NFS server
247 not to require authentication of locking requests (i.e. requests
248 which use the NLM protocol). Normally the NFS server will
249 require a lock request to hold a credential for a user who has
250 read access to the file. With this flag no access checks will
251 be performed.
252 Early NFS client implementations did not send credentials with
254 lock requests, and many current NFS clients still exist which
255 are based on the old implementations. Use this flag if you find
256 that you can only lock files which are world readable.
257 The default behaviour of requiring authentication for NLM
259 requests can be explicitly requested with either of the synony‐
260 mous auth_nlm, or secure_locks.
261 mountpoint=path
264 mp This option makes it possible to only export a directory if it
266 has successfully been mounted. If no path is given (e.g.
267 mountpoint or mp) then the export point must also be a mount
268 point. If it isn't then the export point is not exported. This
269 allows you to be sure that the directory underneath a mountpoint
270 will never be exported by accident if, for example, the filesys‐
271 tem failed to mount due to a disc error.
272 If a path is given (e.g. mountpoint=/path or mp=/path) then the
274 nominated path must be a mountpoint for the exportpoint to be
275 exported.
276 fsid=num|root|uuid
279 NFS needs to be able to identify each filesystem that it
280 exports. Normally it will use a UUID for the filesystem (if the
281 filesystem has such a thing) or the device number of the device
282 holding the filesystem (if the filesystem is stored on the
283 device).
284 As not all filesystems are stored on devices, and not all
286 filesystems have UUIDs, it is sometimes necessary to explicitly
287 tell NFS how to identify a filesystem. This is done with the
288 fsid= option.
289 For NFSv4, there is a distinguished filesystem which is the root
291 of all exported filesystem. This is specified with fsid=root or
292 fsid=0 both of which mean exactly the same thing.
293 Other filesystems can be identified with a small integer, or a
295 UUID which should contain 32 hex digits and arbitrary punctua‐
296 tion.
297 Linux kernels version 2.6.20 and earlier do not understand the
299 UUID setting so a small integer must be used if an fsid option
300 needs to be set for such kernels. Setting both a small number
301 and a UUID is supported so the same configuration can be made to
302 work on old and new kernels alike.
303 nordirplus
306 This option will disable READDIRPLUS request handling. When
307 set, READDIRPLUS requests from NFS clients return NFS3ERR_NOT‐
308 SUPP, and clients fall back on READDIR. This option affects
309 only NFSv3 clients.
310 refer=path@host[+host][:path@host[+host]]
312 A client referencing the export point will be directed to choose
313 from the given list an alternative location for the filesystem.
314 (Note that the server must have a mountpoint here, though a dif‐
315 ferent filesystem is not required; so, for example, mount --bind
316 /path /path is sufficient.)
317 replicas=path@host[+host][:path@host[+host]]
319 If the client asks for alternative locations for the export
320 point, it will be given this list of alternatives. (Note that
321 actual replication of the filesystem must be handled elsewhere.)
322 pnfs This option enables the use of the pNFS extension if the proto‐
325 col level is NFSv4.1 or higher, and the filesystem supports pNFS
326 exports. With pNFS clients can bypass the server and perform
327 I/O directly to storage devices. The default can be explicitly
328 requested with the no_pnfs option.
329 security_label
332 With this option set, clients using NFSv4.2 or higher will be
333 able to set and retrieve security labels (such as those used by
334 SELinux). This will only work if all clients use a consistent
335 security policy. Note that early kernels did not support this
336 export option, and instead enabled security labels by default.
337 User ID Mapping
340 nfsd bases its access control to files on the server machine on the uid
341 and gid provided in each NFS RPC request. The normal behavior a user
342 would expect is that she can access her files on the server just as she
343 would on a normal file system. This requires that the same uids and
344 gids are used on the client and the server machine. This is not always
345 true, nor is it always desirable.
346 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 anony‐
350 mous or nobody uid. This mode of operation (called `root squashing') is
351 the default, and can be turned off with no_root_squash.
352 By default, exportfs chooses a uid and gid of 65534 for squashed
354 access. These values can also be overridden by the anonuid and anongid
355 options. Finally, you can map all user requests to the anonymous uid
356 by specifying the all_squash option.
357 Here's the complete list of mapping options:
359 root_squash
361 Map requests from uid/gid 0 to the anonymous uid/gid. Note that
362 this does not apply to any other uids or gids that might be
363 equally sensitive, such as user bin or group staff.
364 no_root_squash
366 Turn off root squashing. This option is mainly useful for disk‐
367 less clients.
368 all_squash
370 Map all uids and gids to the anonymous user. Useful for NFS-
371 exported public FTP directories, news spool directories, etc.
372 The opposite option is no_all_squash, which is the default set‐
373 ting.
374 anonuid and anongid
376 These options explicitly set the uid and gid of the anonymous
377 account. This option is primarily useful for PC/NFS clients,
378 where you might want all requests appear to be from one user. As
379 an example, consider the export entry for /home/joe in the exam‐
380 ple section below, which maps all requests to uid 150 (which is
381 supposedly that of user joe).
382 Subdirectory Exports
385 Normally you should only export only the root of a filesystem. The NFS
386 server will also allow you to export a subdirectory of a filesystem,
387 however, this has drawbacks:
388 First, it may be possible for a malicious user to access files on the
390 filesystem outside of the exported subdirectory, by guessing filehan‐
391 dles for those other files. The only way to prevent this is by using
392 the no_subtree_check option, which can cause other problems.
393 Second, export options may not be enforced in the way that you would
395 expect. For example, the security_label option will not work on subdi‐
396 rectory exports, and if nested subdirectory exports change the secu‐
397 rity_label or sec= options, NFSv4 clients will normally see only the
398 options on the parent export. Also, where security options differ, a
399 malicious client may use filehandle-guessing attacks to access the
400 files from one subdirectory using the options from another.
401 Extra Export Tables
405 After reading /etc/exports exportfs reads files in the /etc/exports.d
406 directory as extra export tables. Only files ending in .exports are
407 considered. Files beginning with a dot are ignored. The format for
408 extra export tables is the same as /etc/exports
409
411 # sample /etc/exports file
412 / master(rw) trusty(rw,no_root_squash)
413 /projects proj*.local.domain(rw)
414 /usr *.local.domain(ro) @trusted(rw)
415 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
416 /pub *(ro,insecure,all_squash)
417 /srv/www -sync,rw server @trusted @external(ro)
418 /foo 2001:db8:9:e54::/64(rw) 192.0.2.0/24(rw)
419 /build buildhost[0-9].local.domain(rw)
420 The first line exports the entire filesystem to machines master and
422 trusty. In addition to write access, all uid squashing is turned off
423 for host trusty. The second and third entry show examples for wildcard
424 hostnames and netgroups (this is the entry `@trusted'). The fourth line
425 shows the entry for the PC/NFS client discussed above. Line 5 exports
426 the public FTP directory to every host in the world, executing all
427 requests under the nobody account. The insecure option in this entry
428 also allows clients with NFS implementations that don't use a reserved
429 port for NFS. The sixth line exports a directory read-write to the
430 machine 'server' as well as the `@trusted' netgroup, and read-only to
431 netgroup `@external', all three mounts with the `sync' option enabled.
432 The seventh line exports a directory to both an IPv6 and an IPv4 sub‐
433 net. The eighth line demonstrates a character class wildcard match.
434
436 /etc/exports /etc/exports.d
437
439 exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).
440 31 December 2009 exports(5)