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
   control list.  The table is used by exportfs(8) to give information  to

   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

   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 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
          default 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,
          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

          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 with hide.

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

          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
          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

          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

          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

          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.

          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 elsewhere.)

   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 no_root_squash.

   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).

   Extra Export Tables
   After  reading  /etc/exports exportfs reads files under /etc/exports.d.
   directory as extra export tables.  exportfs regards only a  file  which
   name  is ended with .exports and not started with .  as an extra export
   file. A file which name is not met this condition is just 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
   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.


   /etc/exports /etc/exports.d


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

                           31 December 2009                     exports(5)


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