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 no_wdelay
139 This option has no effect if async is also set. The NFS server
140 will normally delay committing a write request to disc slightly
141 if it suspects that another related write request may be in
142 progress or may arrive soon. This allows multiple write
143 requests to be committed to disc with the one operation which
144 can improve performance. If an NFS server received mainly small
145 unrelated requests, this behaviour could actually reduce perfor‐
146 mance, so no_wdelay is available to turn it off. The default
147 can be explicitly requested with the wdelay option.
148 nohide This option is based on the option of the same name provided in
150 IRIX NFS. Normally, if a server exports two filesystems one of
151 which is mounted on the other, then the client will have to
152 mount both filesystems explicitly to get access to them. If it
153 just mounts the parent, it will see an empty directory at the
154 place where the other filesystem is mounted. That filesystem is
155 "hidden".
156 Setting the nohide option on a filesystem causes it not to be
158 hidden, and an appropriately authorised client will be able to
159 move from the parent to that filesystem without noticing the
160 change.
161 However, some NFS clients do not cope well with this situation
163 as, for instance, it is then possible for two files in the one
164 apparent filesystem to have the same inode number.
165 The nohide option is currently only effective on single host
167 exports. It does not work reliably with netgroup, subnet, or
168 wildcard exports.
169 This option can be very useful in some situations, but it should
171 be used with due care, and only after confirming that the client
172 system copes with the situation effectively.
173 The option can be explicitly disabled for NFSv2 and NFSv3 with
175 hide.
176 This option is not relevant when NFSv4 is use. NFSv4 never
178 hides subordinate filesystems. Any filesystem that is exported
179 will be visible where expected when using NFSv4.
180 crossmnt
182 This option is similar to nohide but it makes it possible for
183 clients to access all filesystems mounted on a filesystem marked
184 with crossmnt. Thus when a child filesystem "B" is mounted on a
185 parent "A", setting crossmnt on "A" has a similar effect to set‐
186 ting "nohide" on B.
187 With nohide the child filesystem needs to be explicitly
189 exported. With crossmnt it need not. If a child of a crossmnt
190 file is not explicitly exported, then it will be implicitly
191 exported with the same export options as the parent, except for
192 fsid=. This makes it impossible to not export a child of a
193 crossmnt filesystem. If some but not all subordinate filesys‐
194 tems of a parent are to be exported, then they must be explic‐
195 itly exported and the parent should not have crossmnt set.
196 The nocrossmnt option can explictly disable crossmnt if it was
198 previously set. This is rarely useful.
199 no_subtree_check
201 This option disables subtree checking, which has mild security
202 implications, but can improve reliability in some circumstances.
203 If a subdirectory of a filesystem is exported, but the whole
205 filesystem isn't then whenever a NFS request arrives, the server
206 must check not only that the accessed file is in the appropriate
207 filesystem (which is easy) but also that it is in the exported
208 tree (which is harder). This check is called the subtree_check.
209 In order to perform this check, the server must include some
211 information about the location of the file in the "filehandle"
212 that is given to the client. This can cause problems with
213 accessing files that are renamed while a client has them open
214 (though in many simple cases it will still work).
215 subtree checking is also used to make sure that files inside
217 directories to which only root has access can only be accessed
218 if the filesystem is exported with no_root_squash (see below),
219 even if the file itself allows more general access.
220 As a general guide, a home directory filesystem, which is nor‐
222 mally exported at the root and may see lots of file renames,
223 should be exported with subtree checking disabled. A filesystem
224 which is mostly readonly, and at least doesn't see many file
225 renames (e.g. /usr or /var) and for which subdirectories may be
226 exported, should probably be exported with subtree checks
227 enabled.
228 The default of having subtree checks enabled, can be explicitly
230 requested with subtree_check.
231 From release 1.1.0 of nfs-utils onwards, the default will be
233 no_subtree_check as subtree_checking tends to cause more prob‐
234 lems than it is worth. If you genuinely require subtree check‐
235 ing, you should explicitly put that option in the exports file.
236 If you put neither option, exportfs will warn you that the
237 change is pending.
238 insecure_locks
241 no_auth_nlm
243 This option (the two names are synonymous) tells the NFS server
244 not to require authentication of locking requests (i.e. requests
245 which use the NLM protocol). Normally the NFS server will
246 require a lock request to hold a credential for a user who has
247 read access to the file. With this flag no access checks will
248 be performed.
249 Early NFS client implementations did not send credentials with
251 lock requests, and many current NFS clients still exist which
252 are based on the old implementations. Use this flag if you find
253 that you can only lock files which are world readable.
254 The default behaviour of requiring authentication for NLM
256 requests can be explicitly requested with either of the synony‐
257 mous auth_nlm, or secure_locks.
258 mountpoint=path
261 mp This option makes it possible to only export a directory if it
263 has successfully been mounted. If no path is given (e.g.
264 mountpoint or mp) then the export point must also be a mount
265 point. If it isn't then the export point is not exported. This
266 allows you to be sure that the directory underneath a mountpoint
267 will never be exported by accident if, for example, the filesys‐
268 tem failed to mount due to a disc error.
269 If a path is given (e.g. mountpoint=/path or mp=/path) then the
271 nominated path must be a mountpoint for the exportpoint to be
272 exported.
273 fsid=num|root|uuid
276 NFS needs to be able to identify each filesystem that it
277 exports. Normally it will use a UUID for the filesystem (if the
278 filesystem has such a thing) or the device number of the device
279 holding the filesystem (if the filesystem is stored on the
280 device).
281 As not all filesystems are stored on devices, and not all
283 filesystems have UUIDs, it is sometimes necessary to explicitly
284 tell NFS how to identify a filesystem. This is done with the
285 fsid= option.
286 For NFSv4, there is a distinguished filesystem which is the root
288 of all exported filesystem. This is specified with fsid=root or
289 fsid=0 both of which mean exactly the same thing.
290 Other filesystems can be identified with a small integer, or a
292 UUID which should contain 32 hex digits and arbitrary punctua‐
293 tion.
294 Linux kernels version 2.6.20 and earlier do not understand the
296 UUID setting so a small integer must be used if an fsid option
297 needs to be set for such kernels. Setting both a small number
298 and a UUID is supported so the same configuration can be made to
299 work on old and new kernels alike.
300 nordirplus
303 This option will disable READDIRPLUS request handling. When
304 set, READDIRPLUS requests from NFS clients return NFS3ERR_NOT‐
305 SUPP, and clients fall back on READDIR. This option affects
306 only NFSv3 clients.
307 refer=path@host[+host][:path@host[+host]]
309 A client referencing the export point will be directed to choose
310 from the given list an alternative location for the filesystem.
311 (Note that the server must have a mountpoint here, though a dif‐
312 ferent filesystem is not required; so, for example, mount --bind
313 /path /path is sufficient.)
314 replicas=path@host[+host][:path@host[+host]]
316 If the client asks for alternative locations for the export
317 point, it will be given this list of alternatives. (Note that
318 actual replication of the filesystem must be handled elsewhere.)
319 pnfs This option enables the use of the pNFS extension if the proto‐
322 col level is NFSv4.1 or higher, and the filesystem supports pNFS
323 exports. With pNFS clients can bypass the server and perform
324 I/O directly to storage devices. The default can be explicitly
325 requested with the no_pnfs option.
326 security_label
329 With this option set, clients using NFSv4.2 or higher will be
330 able to set and retrieve security labels (such as those used by
331 SELinux). This will only work if all clients use a consistent
332 security policy. Note that early kernels did not support this
333 export option, and instead enabled security labels by default.
334 User ID Mapping
337 nfsd bases its access control to files on the server machine on the uid
338 and gid provided in each NFS RPC request. The normal behavior a user
339 would expect is that she can access her files on the server just as she
340 would on a normal file system. This requires that the same uids and
341 gids are used on the client and the server machine. This is not always
342 true, nor is it always desirable.
343 Very often, it is not desirable that the root user on a client machine
345 is also treated as root when accessing files on the NFS server. To this
346 end, uid 0 is normally mapped to a different id: the so-called anony‐
347 mous or nobody uid. This mode of operation (called `root squashing') is
348 the default, and can be turned off with no_root_squash.
349 By default, exportfs chooses a uid and gid of 65534 for squashed
351 access. These values can also be overridden by the anonuid and anongid
352 options. Finally, you can map all user requests to the anonymous uid
353 by specifying the all_squash option.
354 Here's the complete list of mapping options:
356 root_squash
358 Map requests from uid/gid 0 to the anonymous uid/gid. Note that
359 this does not apply to any other uids or gids that might be
360 equally sensitive, such as user bin or group staff.
361 no_root_squash
363 Turn off root squashing. This option is mainly useful for disk‐
364 less clients.
365 all_squash
367 Map all uids and gids to the anonymous user. Useful for NFS-
368 exported public FTP directories, news spool directories, etc.
369 The opposite option is no_all_squash, which is the default set‐
370 ting.
371 anonuid and anongid
373 These options explicitly set the uid and gid of the anonymous
374 account. This option is primarily useful for PC/NFS clients,
375 where you might want all requests appear to be from one user. As
376 an example, consider the export entry for /home/joe in the exam‐
377 ple section below, which maps all requests to uid 150 (which is
378 supposedly that of user joe).
379 Subdirectory Exports
382 Normally you should only export only the root of a filesystem. The NFS
383 server will also allow you to export a subdirectory of a filesystem,
384 however, this has drawbacks:
385 First, it may be possible for a malicious user to access files on the
387 filesystem outside of the exported subdirectory, by guessing filehan‐
388 dles for those other files. The only way to prevent this is by using
389 the no_subtree_check option, which can cause other problems.
390 Second, export options may not be enforced in the way that you would
392 expect. For example, the security_label option will not work on subdi‐
393 rectory exports, and if nested subdirectory exports change the secu‐
394 rity_label or sec= options, NFSv4 clients will normally see only the
395 options on the parent export. Also, where security options differ, a
396 malicious client may use filehandle-guessing attacks to access the
397 files from one subdirectory using the options from another.
398 Extra Export Tables
402 After reading /etc/exports exportfs reads files in the /etc/exports.d
403 directory as extra export tables. Only files ending in .exports are
404 considered. Files beginning with a dot are ignored. The format for
405 extra export tables is the same as /etc/exports
406
408 # sample /etc/exports file
409 / master(rw) trusty(rw,no_root_squash)
410 /projects proj*.local.domain(rw)
411 /usr *.local.domain(ro) @trusted(rw)
412 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
413 /pub *(ro,insecure,all_squash)
414 /srv/www -sync,rw server @trusted @external(ro)
415 /foo 2001:db8:9:e54::/64(rw) 192.0.2.0/24(rw)
416 /build buildhost[0-9].local.domain(rw)
417 The first line exports the entire filesystem to machines master and
419 trusty. In addition to write access, all uid squashing is turned off
420 for host trusty. The second and third entry show examples for wildcard
421 hostnames and netgroups (this is the entry `@trusted'). The fourth line
422 shows the entry for the PC/NFS client discussed above. Line 5 exports
423 the public FTP directory to every host in the world, executing all
424 requests under the nobody account. The insecure option in this entry
425 also allows clients with NFS implementations that don't use a reserved
426 port for NFS. The sixth line exports a directory read-write to the
427 machine 'server' as well as the `@trusted' netgroup, and read-only to
428 netgroup `@external', all three mounts with the `sync' option enabled.
429 The seventh line exports a directory to both an IPv6 and an IPv4 sub‐
430 net. The eighth line demonstrates a character class wildcard match.
431
433 /etc/exports /etc/exports.d
434
436 exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).
437 31 December 2009 exports(5)