1exports(5) File Formats Manual exports(5)
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6 exports - NFS server export table
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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 This is the most common format. You may specify a host either by
44 an abbreviated name recognized be the resolver, the fully quali‐
45 fied domain name, or an IP address.
46 netgroups
48 NIS netgroups may be given as @group. Only the host part of
49 each netgroup members is consider in checking for membership.
50 Empty host parts or those containing a single dash (-) are
51 ignored.
52 wildcards
54 Machine names may contain the wildcard characters * and ?. This
55 can be used to make the exports file more compact; for instance,
56 *.cs.foo.edu matches all hosts in the domain cs.foo.edu. As
57 these characters also match the dots in a domain name, the given
58 pattern will also match all hosts within any subdomain of
59 cs.foo.edu.
60 IP networks
62 You can also export directories to all hosts on an IP (sub-)
63 network simultaneously. This is done by specifying an IP address
64 and netmask pair as address/netmask where the netmask can be
65 specified in dotted-decimal format, or as a contiguous mask
66 length. For example, either `/255.255.252.0' or `/22' appended
67 to the network base IPv4 address results in identical subnet‐
68 works with 10 bits of host. Wildcard characters generally do not
69 work on IP addresses, though they may work by accident when
70 reverse DNS lookups fail.
71 RPCSEC_GSS security
73 You may use the special strings "gss/krb5", "gss/krb5i", or "gss/krb5p"
74 to restrict access to clients using rpcsec_gss security. However, this
75 syntax is deprecated; on linux kernels since 2.6.23, you should instead
76 use the "sec=" export option:
77 sec= The sec= option, followed by a colon-delimited list of security
79 flavors, restricts the export to clients using those flavors.
80 Available security flavors include sys (the default--no crypto‐
81 graphic security), krb5 (authentication only), krb5i (integrity
82 protection), and krb5p (privacy protection). For the purposes
83 of security flavor negotiation, order counts: preferred flavors
84 should be listed first. The order of the sec= option with
85 respect to the other options does not matter, unless you want
86 some options to be enforced differently depending on flavor. In
87 that case you may include multiple sec= options, and following
88 options will be enforced only for access using flavors listed in
89 the immediately preceding sec= option. The only options that
90 are permitted to vary in this way are ro, rw, no_root_squash,
91 root_squash, and all_squash.
92 General Options
94 exportfs understands the following export options:
95 secure This option requires that requests originate on an Internet port
97 less than IPPORT_RESERVED (1024). This option is on by default.
98 To turn it off, specify insecure.
99 rw Allow both read and write requests on this NFS volume. The
101 default is to disallow any request which changes the filesystem.
102 This can also be made explicit by using the ro option.
103 async This option allows the NFS server to violate the NFS protocol
105 and reply to requests before any changes made by that request
106 have been committed to stable storage (e.g. disc drive).
107 Using this option usually improves performance, but at the cost
109 that an unclean server restart (i.e. a crash) can cause data to
110 be lost or corrupted.
111 sync Reply to requests only after the changes have been committed to
114 stable storage (see async above).
115 In releases of nfs-utils up to and including 1.0.0, the async
117 option was the default. In all releases after 1.0.0, sync is
118 the default, and async must be explicitly requested if needed.
119 To help make system administrators aware of this change,
120 exportfs will issue a warning if neither sync nor async is spec‐
121 ified.
122 no_wdelay
124 This option has no effect if async is also set. The NFS server
125 will normally delay committing a write request to disc slightly
126 if it suspects that another related write request may be in
127 progress or may arrive soon. This allows multiple write
128 requests to be committed to disc with the one operation which
129 can improve performance. If an NFS server received mainly small
130 unrelated requests, this behaviour could actually reduce perfor‐
131 mance, so no_wdelay is available to turn it off. The default
132 can be explicitly requested with the wdelay option.
133 nohide This option is based on the option of the same name provided in
135 IRIX NFS. Normally, if a server exports two filesystems one of
136 which is mounted on the other, then the client will have to
137 mount both filesystems explicitly to get access to them. If it
138 just mounts the parent, it will see an empty directory at the
139 place where the other filesystem is mounted. That filesystem is
140 "hidden".
141 Setting the nohide option on a filesystem causes it not to be
143 hidden, and an appropriately authorised client will be able to
144 move from the parent to that filesystem without noticing the
145 change.
146 However, some NFS clients do not cope well with this situation
148 as, for instance, it is then possible for two files in the one
149 apparent filesystem to have the same inode number.
150 The nohide option is currently only effective on single host
152 exports. It does not work reliably with netgroup, subnet, or
153 wildcard exports.
154 This option can be very useful in some situations, but it should
156 be used with due care, and only after confirming that the client
157 system copes with the situation effectively.
158 The option can be explicitly disabled with hide.
160 crossmnt
162 This option is similar to nohide but it makes it possible for
163 clients to move from the filesystem marked with crossmnt to
164 exported filesystems mounted on it. Thus when a child filesys‐
165 tem "B" is mounted on a parent "A", setting crossmnt on "A" has
166 the same effect as setting "nohide" on B.
167 no_subtree_check
169 This option disables subtree checking, which has mild security
170 implications, but can improve reliability in some circumstances.
171 If a subdirectory of a filesystem is exported, but the whole
173 filesystem isn't then whenever a NFS request arrives, the server
174 must check not only that the accessed file is in the appropriate
175 filesystem (which is easy) but also that it is in the exported
176 tree (which is harder). This check is called the subtree_check.
177 In order to perform this check, the server must include some
179 information about the location of the file in the "filehandle"
180 that is given to the client. This can cause problems with
181 accessing files that are renamed while a client has them open
182 (though in many simple cases it will still work).
183 subtree checking is also used to make sure that files inside
185 directories to which only root has access can only be accessed
186 if the filesystem is exported with no_root_squash (see below),
187 even if the file itself allows more general access.
188 As a general guide, a home directory filesystem, which is nor‐
190 mally exported at the root and may see lots of file renames,
191 should be exported with subtree checking disabled. A filesystem
192 which is mostly readonly, and at least doesn't see many file
193 renames (e.g. /usr or /var) and for which subdirectories may be
194 exported, should probably be exported with subtree checks
195 enabled.
196 The default of having subtree checks enabled, can be explicitly
198 requested with subtree_check.
199 From release 1.1.0 of nfs-utils onwards, the default will be
201 no_subtree_check as subtree_checking tends to cause more prob‐
202 lems than it is worth. If you genuinely require subtree check‐
203 ing, you should explicitly put that option in the exports file.
204 If you put neither option, exportfs will warn you that the
205 change is pending.
206 insecure_locks
209 no_auth_nlm
211 This option (the two names are synonymous) tells the NFS server
212 not to require authentication of locking requests (i.e. requests
213 which use the NLM protocol). Normally the NFS server will
214 require a lock request to hold a credential for a user who has
215 read access to the file. With this flag no access checks will
216 be performed.
217 Early NFS client implementations did not send credentials with
219 lock requests, and many current NFS clients still exist which
220 are based on the old implementations. Use this flag if you find
221 that you can only lock files which are world readable.
222 The default behaviour of requiring authentication for NLM
224 requests can be explicitly requested with either of the synony‐
225 mous auth_nlm, or secure_locks.
226 no_acl On some specially patched kernels, and when exporting filesys‐
228 tems that support ACLs, this option tells nfsd not to reveal
229 ACLs to clients, so they will see only a subset of actual per‐
230 missions on the given file system. This option is safe for
231 filesystems used by NFSv2 clients and old NFSv3 clients that
232 perform access decisions locally. Current NFSv3 clients use the
233 ACCESS RPC to perform all access decisions on the server. Note
234 that the no_acl option only has effect on kernels specially
235 patched to support it, and when exporting filesystems with ACL
236 support. The default is to export with ACL support (i.e. by
237 default, no_acl is off).
238 mountpoint=path
242 mp This option makes it possible to only export a directory if it
244 has successfully been mounted. If no path is given (e.g.
245 mountpoint or mp) then the export point must also be a mount
246 point. If it isn't then the export point is not exported. This
247 allows you to be sure that the directory underneath a mountpoint
248 will never be exported by accident if, for example, the filesys‐
249 tem failed to mount due to a disc error.
250 If a path is given (e.g. mountpoint=/path or mp=/path) then the
252 nominated path must be a mountpoint for the exportpoint to be
253 exported.
254 fsid=num|root|uuid
257 NFS needs to be able to identify each filesystem that it
258 exports. Normally it will use a UUID for the filesystem (if the
259 filesystem has such a thing) or the device number of the device
260 holding the filesystem (if the filesystem is stored on the
261 device).
262 As not all filesystems are stored on devices, and not all
264 filesystems have UUIDs, it is sometimes necessary to explicitly
265 tell NFS how to identify a filesystem. This is done with the
266 fsid= option.
267 For NFSv4, there is a distinguished filesystem which is the root
269 of all exported filesystem. This is specified with fsid=root or
270 fsid=0 both of which mean exactly the same thing.
271 Other filesystems can be identified with a small integer, or a
273 UUID which should contain 32 hex digits and arbitrary punctua‐
274 tion.
275 Linux kernels version 2.6.20 and earlier do not understand the
277 UUID setting so a small integer must be used if an fsid option
278 needs to be set for such kernels. Setting both a small number
279 and a UUID is supported so the same configuration can be made to
280 work on old and new kernels alike.
281 refer=path@host[+host][:path@host[+host]]
284 A client referencing the export point will be directed to choose
285 from the given list an alternative location for the filesystem.
286 (Note that the server must have a mountpoint here, though a dif‐
287 ferent filesystem is not required; so, for example, mount --bind
288 /path /path is sufficient.)
289 replicas=path@host[+host][:path@host[+host]]
291 If the client asks for alternative locations for the export
292 point, it will be given this list of alternatives. (Note that
293 actual replication of the filesystem must be handled elsewhere.)
294 User ID Mapping
297 nfsd bases its access control to files on the server machine on the uid
298 and gid provided in each NFS RPC request. The normal behavior a user
299 would expect is that she can access her files on the server just as she
300 would on a normal file system. This requires that the same uids and
301 gids are used on the client and the server machine. This is not always
302 true, nor is it always desirable.
303 Very often, it is not desirable that the root user on a client machine
305 is also treated as root when accessing files on the NFS server. To this
306 end, uid 0 is normally mapped to a different id: the so-called anony‐
307 mous or nobody uid. This mode of operation (called `root squashing') is
308 the default, and can be turned off with no_root_squash.
309 By default, exportfs chooses a uid and gid of 65534 for squashed
311 access. These values can also be overridden by the anonuid and anongid
312 options. Finally, you can map all user requests to the anonymous uid
313 by specifying the all_squash option.
314 Here's the complete list of mapping options:
316 root_squash
318 Map requests from uid/gid 0 to the anonymous uid/gid. Note that
319 this does not apply to any other uids or gids that might be
320 equally sensitive, such as user bin or group staff.
321 no_root_squash
323 Turn off root squashing. This option is mainly useful for disk‐
324 less clients.
325 all_squash
327 Map all uids and gids to the anonymous user. Useful for NFS-
328 exported public FTP directories, news spool directories, etc.
329 The opposite option is no_all_squash, which is the default set‐
330 ting.
331 anonuid and anongid
333 These options explicitly set the uid and gid of the anonymous
334 account. This option is primarily useful for PC/NFS clients,
335 where you might want all requests appear to be from one user. As
336 an example, consider the export entry for /home/joe in the exam‐
337 ple section below, which maps all requests to uid 150 (which is
338 supposedly that of user joe).
339
341 # sample /etc/exports file
342 / master(rw) trusty(rw,no_root_squash)
343 /projects proj*.local.domain(rw)
344 /usr *.local.domain(ro) @trusted(rw)
345 /home/joe pc001(rw,all_squash,anonuid=150,anongid=100)
346 /pub *(ro,insecure,all_squash)
347 /srv/www -sync,rw server @trusted @external(ro)
348 The first line exports the entire filesystem to machines master and
350 trusty. In addition to write access, all uid squashing is turned off
351 for host trusty. The second and third entry show examples for wildcard
352 hostnames and netgroups (this is the entry `@trusted'). The fourth line
353 shows the entry for the PC/NFS client discussed above. Line 5 exports
354 the public FTP directory to every host in the world, executing all
355 requests under the nobody account. The insecure option in this entry
356 also allows clients with NFS implementations that don't use a reserved
357 port for NFS. The sixth line exports a directory read-write to the
358 machine 'server' as well as the `@trusted' netgroup, and read-only to
359 netgroup `@external', all three mounts with the `sync' option enabled.
360
362 /etc/exports
363
365 exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).
366 31 December 2009 exports(5)