exports(5) — Linux manual page


exports(5)                   File Formats Manual                  exports(5)

NAME         top

       exports - NFS server export table

DESCRIPTION         top

       The file /etc/exports contains a table of local physical file systems
       on an NFS server that are accessible to NFS clients.  The contents of
       the file are maintained by the server's system administrator.

       Each file system in this table has a list of options and an access
       control list.  The table is used by exportfs(8) to give information
       to mountd(8).

       The file format is similar to the SunOS exports file. Each line
       contains an export point and a whitespace-separated list of clients
       allowed to mount the file system at that point. Each listed client
       may be immediately followed by a parenthesized, comma-separated list
       of export options for that client. No whitespace is permitted between
       a client and its option list.

       Also, each line may have one or more specifications for default
       options after the path name, in the form of a dash ("-") followed by
       an option list. The option list is used for all subsequent exports on
       that line only.

       Blank lines are ignored.  A pound sign ("#") introduces a comment to
       the end of the line. Entries may be continued across newlines using a
       backslash. If an export name contains spaces it should be quoted
       using double quotes. You can also specify spaces or other unusual
       character in the export name using a backslash followed by the
       character code as three octal digits.

       To apply changes to this file, run exportfs -ra or restart the NFS

   Machine Name Formats
       NFS clients may be specified in a number of ways:

       single host
              You may specify a host either by an abbreviated name
              recognized be the resolver, the fully qualified domain name,
              an IPv4 address, or an IPv6 address. IPv6 addresses must not
              be inside square brackets in /etc/exports lest they be
              confused with character-class wildcard matches.

       IP networks
              You can also export directories to all hosts on an IP (sub-)
              network simultaneously. This is done by specifying an IP
              address and netmask pair as address/netmask where the netmask
              can be specified in dotted-decimal format, or as a contiguous
              mask length.  For example, either `/' or `/22'
              appended to the network base IPv4 address results in identical
              subnetworks with 10 bits of host. IPv6 addresses must use a
              contiguous mask length and must not be inside square brackets
              to avoid confusion with character-class wildcards. Wildcard
              characters generally do not work on IP addresses, though they
              may work by accident when reverse DNS lookups fail.

              Machine names may contain the wildcard characters * and ?, or
              may contain character class lists within [square brackets].
              This can be used to make the exports file more compact; for
              instance, *.cs.foo.edu matches all hosts in the domain
              cs.foo.edu.  As these characters also match the dots in a
              domain name, the given pattern will also match all hosts
              within any subdomain of cs.foo.edu.

              NIS netgroups may be given as @group.  Only the host part of
              each netgroup members is consider in checking for membership.
              Empty host parts or those containing a single dash (-) are

              This is specified by a single * character (not to be confused
              with the wildcard entry above) and will match all clients.

       If a client matches more than one of the specifications above, then
       the first match from the above list order takes precedence -
       regardless of the order they appear on the export line. However, if a
       client matches more than one of the same type of specification (e.g.
       two netgroups), then the first match from the order they appear on
       the export line takes precedence.

   RPCSEC_GSS security
       You may use the special strings "gss/krb5", "gss/krb5i", or
       "gss/krb5p" to restrict access to clients using rpcsec_gss security.
       However, this syntax is deprecated; on linux kernels since 2.6.23,
       you should instead use the "sec=" export option:

       sec=   The sec= option, followed by a colon-delimited list of
              security flavors, restricts the export to clients using those
              flavors.  Available security flavors include sys (the
              default--no cryptographic security), krb5 (authentication
              only), krb5i (integrity protection), and krb5p (privacy
              protection).  For the purposes of security flavor negotiation,
              order counts: preferred flavors should be listed first.  The
              order of the sec= option with respect to the other options
              does not matter, unless you want some options to be enforced
              differently depending on flavor.  In that case you may include
              multiple sec= options, and following options will be enforced
              only for access using flavors listed in the immediately
              preceding sec= option.  The only options that are permitted to
              vary in this way are ro, rw, no_root_squash, root_squash, and

   General Options
       exportfs understands the following export options:

       secure This option requires that requests not using gss originate on
              an Internet port less than IPPORT_RESERVED (1024). This option
              is on by default.  To turn it off, specify insecure.  (NOTE:
              older kernels (before upstream kernel version 4.17) enforced
              this requirement on gss requests as well.)

       rw     Allow both read and write requests on this NFS volume. The
              default is to disallow any request which changes the
              filesystem.  This can also be made explicit by using the ro

       async  This option allows the NFS server to violate the NFS protocol
              and reply to requests before any changes made by that request
              have been committed to stable storage (e.g. disc drive).

              Using this option usually improves performance, but at the
              cost that an unclean server restart (i.e. a crash) can cause
              data to be lost or corrupted.

       sync   Reply to requests only after the changes have been committed
              to stable storage (see async above).

              In releases of nfs-utils up to and including 1.0.0, the async
              option was the default.  In all releases after 1.0.0, sync is
              the default, and async must be explicitly requested if needed.
              To help make system administrators aware of this change,
              exportfs will issue a warning if neither sync nor async is

              This option has no effect if async is also set.  The NFS
              server will normally delay committing a write request to disc
              slightly if it suspects that another related write request may
              be in progress or may arrive soon.  This allows multiple write
              requests to be committed to disc with the one operation which
              can improve performance.  If an NFS server received mainly
              small unrelated requests, this behaviour could actually reduce
              performance, so no_wdelay is available to turn it off.  The
              default can be explicitly requested with the wdelay option.

       nohide This option is based on the option of the same name provided
              in IRIX NFS.  Normally, if a server exports two filesystems
              one of which is mounted on the other, then the client will
              have to mount both filesystems explicitly to get access to
              them.  If it just mounts the parent, it will see an empty
              directory at the place where the other filesystem is mounted.
              That filesystem is "hidden".

              Setting the nohide option on a filesystem causes it not to be
              hidden, and an appropriately authorised client will be able to
              move from the parent to that filesystem without noticing the

              However, some NFS clients do not cope well with this situation
              as, for instance, it is then possible for two files in the one
              apparent filesystem to have the same inode number.

              The nohide option is currently only effective on single host
              exports.  It does not work reliably with netgroup, subnet, or
              wildcard exports.

              This option can be very useful in some situations, but it
              should be used with due care, and only after confirming that
              the client system copes with the situation effectively.

              The option can be explicitly disabled for NFSv2 and NFSv3 with

              This option is not relevant when NFSv4 is use.  NFSv4 never
              hides subordinate filesystems.  Any filesystem that is
              exported will be visible where expected when using NFSv4.

              This option is similar to nohide but it makes it possible for
              clients to access all filesystems mounted on a filesystem
              marked with crossmnt.  Thus when a child filesystem "B" is
              mounted on a parent "A", setting crossmnt on "A" has a similar
              effect to setting "nohide" on B.

              With nohide the child filesystem needs to be explicitly
              exported.  With crossmnt it need not.  If a child of a
              crossmnt file is not explicitly exported, then it will be
              implicitly exported with the same export options as the
              parent, except for fsid=.  This makes it impossible to not
              export a child of a crossmnt filesystem.  If some but not all
              subordinate filesystems of a parent are to be exported, then
              they must be explicitly exported and the parent should not
              have crossmnt set.

              The nocrossmnt option can explictly disable crossmnt if it was
              previously set.  This is rarely useful.

              This option disables subtree checking, which has mild security
              implications, but can improve reliability in some

              If a subdirectory of a filesystem is exported, but the whole
              filesystem isn't then whenever a NFS request arrives, the
              server must check not only that the accessed file is in the
              appropriate filesystem (which is easy) but also that it is in
              the exported tree (which is harder). This check is called the

              In order to perform this check, the server must include some
              information about the location of the file in the "filehandle"
              that is given to the client.  This can cause problems with
              accessing files that are renamed while a client has them open
              (though in many simple cases it will still work).

              subtree checking is also used to make sure that files inside
              directories to which only root has access can only be accessed
              if the filesystem is exported with no_root_squash (see below),
              even if the file itself allows more general access.

              As a general guide, a home directory filesystem, which is
              normally exported at the root and may see lots of file
              renames, should be exported with subtree checking disabled.  A
              filesystem which is mostly readonly, and at least doesn't see
              many file renames (e.g. /usr or /var) and for which
              subdirectories may be exported, should probably be exported
              with subtree checks enabled.

              The default of having subtree checks enabled, can be
              explicitly requested with subtree_check.

              From release 1.1.0 of nfs-utils onwards, the default will be
              no_subtree_check as subtree_checking tends to cause more
              problems than it is worth.  If you genuinely require subtree
              checking, you should explicitly put that option in the exports
              file.  If you put neither option, exportfs will warn you that
              the change is pending.


              This option (the two names are synonymous) tells the NFS
              server not to require authentication of locking requests (i.e.
              requests which use the NLM protocol).  Normally the NFS server
              will require a lock request to hold a credential for a user
              who has read access to the file.  With this flag no access
              checks will be performed.

              Early NFS client implementations did not send credentials with
              lock requests, and many current NFS clients still exist which
              are based on the old implementations.  Use this flag if you
              find that you can only lock files which are world readable.

              The default behaviour of requiring authentication for NLM
              requests can be explicitly requested with either of the
              synonymous auth_nlm, or secure_locks.


       mp     This option makes it possible to only export a directory if it
              has successfully been mounted.  If no path is given (e.g.
              mountpoint or mp) then the export point must also be a mount
              point.  If it isn't then the export point is not exported.
              This allows you to be sure that the directory underneath a
              mountpoint will never be exported by accident if, for example,
              the filesystem failed to mount due to a disc error.

              If a path is given (e.g.  mountpoint=/path or mp=/path) then
              the nominated path must be a mountpoint for the exportpoint to
              be exported.

              NFS needs to be able to identify each filesystem that it
              exports.  Normally it will use a UUID for the filesystem (if
              the filesystem has such a thing) or the device number of the
              device holding the filesystem (if the filesystem is stored on
              the device).

              As not all filesystems are stored on devices, and not all
              filesystems have UUIDs, it is sometimes necessary to
              explicitly tell NFS how to identify a filesystem.  This is
              done with the fsid= option.

              For NFSv4, there is a distinguished filesystem which is the
              root of all exported filesystem.  This is specified with
              fsid=root or fsid=0 both of which mean exactly the same thing.

              Other filesystems can be identified with a small integer, or a
              UUID which should contain 32 hex digits and arbitrary

              Linux kernels version 2.6.20 and earlier do not understand the
              UUID setting so a small integer must be used if an fsid option
              needs to be set for such kernels.  Setting both a small number
              and a UUID is supported so the same configuration can be made
              to work on old and new kernels alike.

              This option will disable READDIRPLUS request handling.  When
              set, READDIRPLUS requests from NFS clients return
              NFS3ERR_NOTSUPP, and clients fall back on READDIR.  This
              option affects only NFSv3 clients.

              A client referencing the export point will be directed to
              choose from the given list an alternative location for the
              filesystem.  (Note that the server must have a mountpoint
              here, though a different filesystem is not required; so, for
              example, mount --bind /path /path is sufficient.)

              If the client asks for alternative locations for the export
              point, it will be given this list of alternatives. (Note that
              actual replication of the filesystem must be handled

       pnfs   This option enables the use of the pNFS extension if the
              protocol level is NFSv4.1 or higher, and the filesystem
              supports pNFS exports.  With pNFS clients can bypass the
              server and perform I/O directly to storage devices. The
              default can be explicitly requested with the no_pnfs option.

              With this option set, clients using NFSv4.2 or higher will be
              able to set and retrieve security labels (such as those used
              by SELinux).  This will only work if all clients use a
              consistent security policy.  Note that early kernels did not
              support this export option, and instead enabled security
              labels by default.

   User ID Mapping
       nfsd bases its access control to files on the server machine on the
       uid and gid provided in each NFS RPC request. The normal behavior a
       user would expect is that she can access her files on the server just
       as she would on a normal file system. This requires that the same
       uids and gids are used on the client and the server machine. This is
       not always true, nor is it always desirable.

       Very often, it is not desirable that the root user on a client
       machine is also treated as root when accessing files on the NFS
       server. To this end, uid 0 is normally mapped to a different id: the
       so-called anonymous or nobody uid. This mode of operation (called
       `root squashing') is the default, and can be turned off with

       By default, exportfs chooses a uid and gid of 65534 for squashed
       access. These values can also be overridden by the anonuid and
       anongid options.  Finally, you can map all user requests to the
       anonymous uid by specifying the all_squash option.

       Here's the complete list of mapping options:

              Map requests from uid/gid 0 to the anonymous uid/gid. Note
              that this does not apply to any other uids or gids that might
              be equally sensitive, such as user bin or group staff.

              Turn off root squashing. This option is mainly useful for
              diskless clients.

              Map all uids and gids to the anonymous user. Useful for NFS-
              exported public FTP directories, news spool directories, etc.
              The opposite option is no_all_squash, which is the default

       anonuid and anongid
              These options explicitly set the uid and gid of the anonymous
              account.  This option is primarily useful for PC/NFS clients,
              where you might want all requests appear to be from one user.
              As an example, consider the export entry for /home/joe in the
              example section below, which maps all requests to uid 150
              (which is supposedly that of user joe).

   Subdirectory Exports
       Normally you should only export only the root of a filesystem.  The
       NFS server will also allow you to export a subdirectory of a
       filesystem, however, this has drawbacks:

       First, it may be possible for a malicious user to access files on the
       filesystem outside of the exported subdirectory, by guessing
       filehandles for those other files.  The only way to prevent this is
       by using the no_subtree_check option, which can cause other problems.

       Second, export options may not be enforced in the way that you would
       expect.  For example, the security_label option will not work on
       subdirectory exports, and if nested subdirectory exports change the
       security_label or sec= options, NFSv4 clients will normally see only
       the options on the parent export.  Also, where security options
       differ, a malicious client may use filehandle-guessing attacks to
       access the files from one subdirectory using the options from

   Extra Export Tables
       After reading /etc/exports exportfs reads files in the /etc/exports.d
       directory as extra export tables.  Only files ending in .exports are
       considered.  Files beginning with a dot are ignored.  The format for
       extra export tables is the same as /etc/exports

EXAMPLE         top

       # sample /etc/exports file
       /               master(rw) trusty(rw,no_root_squash)
       /projects       proj*.local.domain(rw)
       /usr            *.local.domain(ro) @trusted(rw)
       /home/joe       pc001(rw,all_squash,anonuid=150,anongid=100)
       /pub            *(ro,insecure,all_squash)
       /srv/www        -sync,rw server @trusted @external(ro)
       /foo            2001:db8:9:e54::/64(rw)
       /build          buildhost[0-9].local.domain(rw)

       The first line exports the entire filesystem to machines master and
       trusty.  In addition to write access, all uid squashing is turned off
       for host trusty. The second and third entry show examples for
       wildcard hostnames and netgroups (this is the entry `@trusted'). The
       fourth line shows the entry for the PC/NFS client discussed above.
       Line 5 exports the public FTP directory to every host in the world,
       executing all requests under the nobody account. The insecure option
       in this entry also allows clients with NFS implementations that don't
       use a reserved port for NFS.  The sixth line exports a directory
       read-write to the machine 'server' as well as the `@trusted'
       netgroup, and read-only to netgroup `@external', all three mounts
       with the `sync' option enabled. The seventh line exports a directory
       to both an IPv6 and an IPv4 subnet. The eighth line demonstrates a
       character class wildcard match.

FILES         top

       /etc/exports /etc/exports.d

SEE ALSO         top

       exportfs(8), netgroup(5), mountd(8), nfsd(8), showmount(8).

COLOPHON         top

       This page is part of the nfs-utils (NFS utilities) project.
       Information about the project can be found at 
       ⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩.  If you have a bug
       report for this manual page, see
       ⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩.  This page was
       obtained from the project's upstream Git repository
       ⟨http://git.linux-nfs.org/?p=steved/nfs-utils.git;a=summary⟩ on
       2020-08-13.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-08-07.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

                              31 December 2009                    exports(5)

Pages that refer to this page: nfs(5)nfsd(7)exportfs(8)mountd(8)nfsd(8)nfsref(8)rpc.mountd(8)rpc.nfsd(8)