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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
       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 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
	      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 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
	      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 spec-

	      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 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
	      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 filesys-
	      tem "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 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
	      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 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
	      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  synony-
	      mous 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
	      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 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.)

   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
       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 disk-
	      less 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  set-

       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
       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  sub-
       net. 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)