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 originate on an Internet port
113 less than IPPORT_RESERVED (1024). This option is on by default.
114 To turn it off, specify insecure.
115 rw Allow both read and write requests on this NFS volume. The
117 default is to disallow any request which changes the filesystem.
118 This can also be made explicit by using the ro option.
119 async This option allows the NFS server to violate the NFS protocol
121 and reply to requests before any changes made by that request
122 have been committed to stable storage (e.g. disc drive).
123 Using this option usually improves performance, but at the cost
125 that an unclean server restart (i.e. a crash) can cause data to
126 be lost or corrupted.
127 sync Reply to requests only after the changes have been committed to
130 stable storage (see async above).
131 In releases of nfs-utils up to and including 1.0.0, the async
133 option was the default. In all releases after 1.0.0, sync is
134 the default, and async must be explicitly requested if needed.
135 To help make system administrators aware of this change,
136 exportfs will issue a warning if neither sync nor async is spec‐
137 ified.
138 no_wdelay
140 This option has no effect if async is also set. The NFS server
141 will normally delay committing a write request to disc slightly
142 if it suspects that another related write request may be in
143 progress or may arrive soon. This allows multiple write
144 requests to be committed to disc with the one operation which
145 can improve performance. If an NFS server received mainly small
146 unrelated requests, this behaviour could actually reduce perfor‐
147 mance, so no_wdelay is available to turn it off. The default
148 can be explicitly requested with the wdelay option.
149 nohide This option is based on the option of the same name provided in
151 IRIX NFS. Normally, if a server exports two filesystems one of
152 which is mounted on the other, then the client will have to
153 mount both filesystems explicitly to get access to them. If it
154 just mounts the parent, it will see an empty directory at the
155 place where the other filesystem is mounted. That filesystem is
156 "hidden".
157 Setting the nohide option on a filesystem causes it not to be
159 hidden, and an appropriately authorised client will be able to
160 move from the parent to that filesystem without noticing the
161 change.
162 However, some NFS clients do not cope well with this situation
164 as, for instance, it is then possible for two files in the one
165 apparent filesystem to have the same inode number.
166 The nohide option is currently only effective on single host
168 exports. It does not work reliably with netgroup, subnet, or
169 wildcard exports.
170 This option can be very useful in some situations, but it should
172 be used with due care, and only after confirming that the client
173 system copes with the situation effectively.
174 The option can be explicitly disabled for NFSv2 and NFSv3 with
176 hide.
177 This option is not relevant when NFSv4 is use. NFSv4 never
179 hides subordinate filesystems. Any filesystem that is exported
180 will be visible where expected when using NFSv4.
181 crossmnt
183 This option is similar to nohide but it makes it possible for
184 clients to access all filesystems mounted on a filesystem marked
185 with crossmnt. Thus when a child filesystem "B" is mounted on a
186 parent "A", setting crossmnt on "A" has a similar effect to set‐
187 ting "nohide" on B.
188 With nohide the child filesystem needs to be explicitly
190 exported. With crossmnt it need not. If a child of a crossmnt
191 file is not explicitly exported, then it will be implicitly
192 exported with the same export options as the parent, except for
193 fsid=. This makes it impossible to not export a child of a
194 crossmnt filesystem. If some but not all subordinate filesys‐
195 tems of a parent are to be exported, then they must be explic‐
196 itly exported and the parent should not have crossmnt set.
197 The nocrossmnt option can explictly disable crossmnt if it was
199 previously set. This is rarely useful.
200 no_subtree_check
202 This option disables subtree checking, which has mild security
203 implications, but can improve reliability in some circumstances.
204 If a subdirectory of a filesystem is exported, but the whole
206 filesystem isn't then whenever a NFS request arrives, the server
207 must check not only that the accessed file is in the appropriate
208 filesystem (which is easy) but also that it is in the exported
209 tree (which is harder). This check is called the subtree_check.
210 In order to perform this check, the server must include some
212 information about the location of the file in the "filehandle"
213 that is given to the client. This can cause problems with
214 accessing files that are renamed while a client has them open
215 (though in many simple cases it will still work).
216 subtree checking is also used to make sure that files inside
218 directories to which only root has access can only be accessed
219 if the filesystem is exported with no_root_squash (see below),
220 even if the file itself allows more general access.
221 As a general guide, a home directory filesystem, which is nor‐
223 mally exported at the root and may see lots of file renames,
224 should be exported with subtree checking disabled. A filesystem
225 which is mostly readonly, and at least doesn't see many file
226 renames (e.g. /usr or /var) and for which subdirectories may be
227 exported, should probably be exported with subtree checks
228 enabled.
229 The default of having subtree checks enabled, can be explicitly
231 requested with subtree_check.
232 From release 1.1.0 of nfs-utils onwards, the default will be
234 no_subtree_check as subtree_checking tends to cause more prob‐
235 lems than it is worth. If you genuinely require subtree check‐
236 ing, you should explicitly put that option in the exports file.
237 If you put neither option, exportfs will warn you that the
238 change is pending.
239 insecure_locks
242 no_auth_nlm
244 This option (the two names are synonymous) tells the NFS server
245 not to require authentication of locking requests (i.e. requests
246 which use the NLM protocol). Normally the NFS server will
247 require a lock request to hold a credential for a user who has
248 read access to the file. With this flag no access checks will
249 be performed.
250 Early NFS client implementations did not send credentials with
252 lock requests, and many current NFS clients still exist which
253 are based on the old implementations. Use this flag if you find
254 that you can only lock files which are world readable.
255 The default behaviour of requiring authentication for NLM
257 requests can be explicitly requested with either of the synony‐
258 mous auth_nlm, or secure_locks.
259 mountpoint=path
262 mp This option makes it possible to only export a directory if it
264 has successfully been mounted. If no path is given (e.g.
265 mountpoint or mp) then the export point must also be a mount
266 point. If it isn't then the export point is not exported. This
267 allows you to be sure that the directory underneath a mountpoint
268 will never be exported by accident if, for example, the filesys‐
269 tem failed to mount due to a disc error.
270 If a path is given (e.g. mountpoint=/path or mp=/path) then the
272 nominated path must be a mountpoint for the exportpoint to be
273 exported.
274 fsid=num|root|uuid
277 NFS needs to be able to identify each filesystem that it
278 exports. Normally it will use a UUID for the filesystem (if the
279 filesystem has such a thing) or the device number of the device
280 holding the filesystem (if the filesystem is stored on the
281 device).
282 As not all filesystems are stored on devices, and not all
284 filesystems have UUIDs, it is sometimes necessary to explicitly
285 tell NFS how to identify a filesystem. This is done with the
286 fsid= option.
287 For NFSv4, there is a distinguished filesystem which is the root
289 of all exported filesystem. This is specified with fsid=root or
290 fsid=0 both of which mean exactly the same thing.
291 Other filesystems can be identified with a small integer, or a
293 UUID which should contain 32 hex digits and arbitrary punctua‐
294 tion.
295 Linux kernels version 2.6.20 and earlier do not understand the
297 UUID setting so a small integer must be used if an fsid option
298 needs to be set for such kernels. Setting both a small number
299 and a UUID is supported so the same configuration can be made to
300 work on old and new kernels alike.
301 nordirplus
304 This option will disable READDIRPLUS request handling. When
305 set, READDIRPLUS requests from NFS clients return NFS3ERR_NOT‐
306 SUPP, and clients fall back on READDIR. This option affects
307 only NFSv3 clients.
308 refer=path@host[+host][:path@host[+host]]
310 A client referencing the export point will be directed to choose
311 from the given list an alternative location for the filesystem.
312 (Note that the server must have a mountpoint here, though a dif‐
313 ferent filesystem is not required; so, for example, mount --bind
314 /path /path is sufficient.)
315 replicas=path@host[+host][:path@host[+host]]
317 If the client asks for alternative locations for the export
318 point, it will be given this list of alternatives. (Note that
319 actual replication of the filesystem must be handled elsewhere.)
320 pnfs This option enables the use of the pNFS extension if the proto‐
323 col level is NFSv4.1 or higher, and the filesystem supports pNFS
324 exports. With pNFS clients can bypass the server and perform
325 I/O directly to storage devices. The default can be explicitly
326 requested with the no_pnfs option.
327 security_label
330 With this option set, clients using NFSv4.2 or higher will be
331 able to set and retrieve security labels (such as those used by
332 SELinux). This will only work if all clients use a consistent
333 security policy. Note that early kernels did not support this
334 export option, and instead enabled security labels by default.
335 User ID Mapping
338 nfsd bases its access control to files on the server machine on the uid
339 and gid provided in each NFS RPC request. The normal behavior a user
340 would expect is that she can access her files on the server just as she
341 would on a normal file system. This requires that the same uids and
342 gids are used on the client and the server machine. This is not always
343 true, nor is it always desirable.
344 Very often, it is not desirable that the root user on a client machine
346 is also treated as root when accessing files on the NFS server. To this
347 end, uid 0 is normally mapped to a different id: the so-called anony‐
348 mous or nobody uid. This mode of operation (called `root squashing') is
349 the default, and can be turned off with no_root_squash.
350 By default, exportfs chooses a uid and gid of 65534 for squashed
352 access. These values can also be overridden by the anonuid and anongid
353 options. Finally, you can map all user requests to the anonymous uid
354 by specifying the all_squash option.
355 Here's the complete list of mapping options:
357 root_squash
359 Map requests from uid/gid 0 to the anonymous uid/gid. Note that
360 this does not apply to any other uids or gids that might be
361 equally sensitive, such as user bin or group staff.
362 no_root_squash
364 Turn off root squashing. This option is mainly useful for disk‐
365 less clients.
366 all_squash
368 Map all uids and gids to the anonymous user. Useful for NFS-
369 exported public FTP directories, news spool directories, etc.
370 The opposite option is no_all_squash, which is the default set‐
371 ting.
372 anonuid and anongid
374 These options explicitly set the uid and gid of the anonymous
375 account. This option is primarily useful for PC/NFS clients,
376 where you might want all requests appear to be from one user. As
377 an example, consider the export entry for /home/joe in the exam‐
378 ple section below, which maps all requests to uid 150 (which is
379 supposedly that of user joe).
380 Extra Export Tables
382 After reading /etc/exports exportfs reads files in the /etc/exports.d
383 directory as extra export tables. Only files ending in .exports are
384 considered. Files beginning with a dot are ignored. The format for
385 extra export tables is the same as /etc/exports
386
388 # sample /etc/exports file
389 / master(rw) trusty(rw,no_root_squash)
390 /projects proj*.local.domain(rw)
391 /usr *.local.domain(ro) @trusted(rw)
392 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
393 /pub *(ro,insecure,all_squash)
394 /srv/www -sync,rw server @trusted @external(ro)
395 /foo 2001:db8:9:e54::/64(rw) 192.0.2.0/24(rw)
396 /build buildhost[0-9].local.domain(rw)
397 The first line exports the entire filesystem to machines master and
399 trusty. In addition to write access, all uid squashing is turned off
400 for host trusty. The second and third entry show examples for wildcard
401 hostnames and netgroups (this is the entry `@trusted'). The fourth line
402 shows the entry for the PC/NFS client discussed above. Line 5 exports
403 the public FTP directory to every host in the world, executing all
404 requests under the nobody account. The insecure option in this entry
405 also allows clients with NFS implementations that don't use a reserved
406 port for NFS. The sixth line exports a directory read-write to the
407 machine 'server' as well as the `@trusted' netgroup, and read-only to
408 netgroup `@external', all three mounts with the `sync' option enabled.
409 The seventh line exports a directory to both an IPv6 and an IPv4 sub‐
410 net. The eighth line demonstrates a character class wildcard match.
411
413 /etc/exports /etc/exports.d
414
416 exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).
417 31 December 2009 exports(5)