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EXPORTS(5)		   Linux File Formats Manual		    EXPORTS(5)



NAME
       exports - NFS file systems being exported (for Kernel based NFS)

SYNOPSIS
       /etc/exports

DESCRIPTION
       The  file  /etc/exports serves as the access control list for file sys-
       tems which may be exported to NFS clients.  It is used  by  exportfs(8)
       to  give	 information  to  mountd(8)  and  to the kernel based NFS file
       server daemon nfsd(8).

       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.

       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.


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

       single host
	      This is the most common format. You may specify a host either by
	      an abbreviated name recognized be the resolver, the fully quali-
	      fied domain name, or an IP address.

       netgroups
	      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
	      ignored.

       wildcards
	      Machine names may contain the wildcard characters * and ?.  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.

       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 '/255.255.252.0' or  '/22'  appended
	      to the network base address result in identical subnetworks with
	      10 bits of host). Wildcard characters generally do not  work  on
	      IP  addresses, though they may work by accident when reverse DNS
	      lookups fail.


   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 might improve performance with version 2 only,
	      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).

       no_wdelay
	      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
	      "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
	      change.

	      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.

       crossmnt
	      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.

       subtree_check
	      This option enables subtree checking,  which  does  add  another
	      level  of	 security,  but	 can  be unreliability in some circum-
	      stances.

	      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
	      enabled.

	      This type of subtree checking is disabled by default.

       insecure_locks

       no_auth_nlm
	      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.

       no_acl On some specially patched kernels, and when  exporting  filesys-
	      tems  that  support  ACLs,  this option tells nfsd not to reveal
	      ACLs to clients, so they will see only a subset of  actual  per-
	      missions	on  the	 given	file  system.  This option is safe for
	      filesystems used by NFSv2 clients and  old  NFSv3	 clients  that
	      perform access decisions locally.	 Current NFSv3 clients use the
	      ACCESS RPC to perform all access decisions on the server.	  Note
	      that  the	 no_acl	 option	 only  has effect on kernels specially
	      patched to support it, and when exporting filesystems  with  ACL
	      support.	 The  default  is  to export with ACL support (i.e. by
	      default, no_acl is off).



       mountpoint=path

       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
	      nominted	path  must  be	a mountpoint for the exportpoint to be
	      exported.


       fsid=num
	      This option forces the filesystem identification portion of  the
	      file  handle  and	 file  attributes  used	 on the wire to be num
	      instead of a number derived from the major and minor  number  of
	      the block device on which the filesystem is mounted.  Any 32 bit
	      number can be used, but  it  must	 be  unique  amongst  all  the
	      exported filesystems.

	      This can be useful for NFS failover, to ensure that both servers
	      of the failover pair use the  same  NFS  file  handles  for  the
	      shared   filesystem  thus	 avoiding  stale  file	handles	 after
	      failover.

	      Some Linux filesystems  are  not	mounted	 on  a	block  device;
	      exporting	 these	via  NFS  requires  the use of the fsid option
	      (although that may still not be enough).

	      The value	 0 has a special meaning when use with	NFSv4.	 NFSv4
	      has  a concept of a root of the overall exported filesystem. The
	      export point exported with fsid=0 will be used as this root.


       refer=path@host[+host][:path@host[+host]]
	      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  currently	needs  to  have	 a  filesystem
	      mounted  here,  generally using mount --bind, although it is not
	      actually exported.)


       sec=flavor[:flavor]
	      The sec option, followed by a colon-delimited list  of  security
	      flavors,	restricts  the	export to clients using those flavors.
	      Available security flavors include:

	       none  (no cryptographic security)
	       sys   (no cryptographic security)
	       krb5  (authentication only)
	       krb5i (integrity protection)
	       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 depend-
	       ing 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.

   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:

       root_squash
	      Map requests from uid/gid 0 to the anonymous uid/gid. Note  that
	      this does not apply to any other uids that might be equally sen-
	      sitive, such as user bin.

       no_root_squash
	      Turn off root squashing. This option is mainly useful for	 disk-
	      less clients.

       all_squash
	      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-
	      ting.

       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).


EXAMPLE
       # 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)

       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.

FILES
       /etc/exports

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



Linux				 4 March 2005			    EXPORTS(5)
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