exports(5)                    File Formats Manual                   exports(5)

       exports - NFS server export table

       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 con-
       trol list.  The table is used by exportfs(8)  to  give  information  to

       The  file  format  is similar to the SunOS exports file. Each line con-
       tains an export point and a whitespace-separated list  of  clients  al-
       lowed 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

       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  ad-
              dress,  or  an  IPv6  address. IPv6 addresses must not be inside
              square brackets in /etc/exports lest they be confused with char-
              acter-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  subnet-
              works with 10 bits of host. IPv6 addresses must use a contiguous
              mask length and must not be inside square brackets to avoid con-
              fusion  with character-class wildcards. Wildcard characters gen-
              erally do not work on IP addresses, though they may work by  ac-
              cident 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 in-
              stance, *.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

              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 ig-

              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 crypto-
              graphic 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 re-
              spect 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  op-
              tions  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 all_squash.

   General Options
       exportfs understands the following export options:

       secure This option requires that requests originate on an Internet port
              less  than IPPORT_RESERVED (1024). This option is on by default.
              To turn it off, specify insecure.

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

       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,  ex-
              portfs  will issue a warning if neither sync nor async is speci-

              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 re-
              quests to be committed to disc with the one operation which  can
              improve performance.  If an NFS server received mainly small un-
              related requests, this behaviour could actually  reduce  perfor-
              mance,  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

              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 ex-
              ports.  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 set-
              ting "nohide" on B.

              With nohide the child filesystem  needs  to  be  explicitly  ex-
              ported.   With  crossmnt  it need not.  If a child of a crossmnt
              file is not explicitly exported, then it will be implicitly  ex-
              ported  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 filesys-
              tems of a parent are to be exported, then they must  be  explic-
              itly 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 circumstances.

              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 subtree_check.

              In order to perform this check, the server must include some in-
              formation about the location of the  file  in  the  "filehandle"
              that  is  given to the client.  This can cause problems with ac-
              cessing 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 di-
              rectories 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  nor-
              mally  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 re-
              names (e.g. /usr or /var) and for which  subdirectories  may  be
              exported,  should  probably  be exported with subtree checks en-

              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  prob-
              lems  than it is worth.  If you genuinely require subtree check-
              ing, 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  re-
              quire  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  re-
              quests 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 filesys-
              tem 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

              NFS needs to be able to identify each  filesystem  that  it  ex-
              ports.   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 de-

              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  punctua-

              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_NOT-
              SUPP,  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 dif-
              ferent 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 elsewhere.)

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

   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 anony-
       mous or nobody uid. This mode of operation (called `root squashing') is
       the default, and can be turned off with no_root_squash.

       By  default,  exportfs  chooses a uid and gid of 65534 for squashed ac-
       cess. 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  disk-
              less clients.

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

       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 exam-
              ple  section below, which maps all requests to uid 150 (which is
              supposedly that of user joe).

   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

       # 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  re-
       quests 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 sev-
       enth line exports a directory to both an IPv6 and an IPv4  subnet.  The
       eighth line demonstrates a character class wildcard match.

       /etc/exports /etc/exports.d

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

                               31 December 2009                     exports(5)
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