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

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


       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.

       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
	      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.

	      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

	      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

       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 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.

   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, this option
	      was the default.	In  all	 subsequence  releases,	 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 specified.

	      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.

       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).


       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.

	      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.)

	      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).

       # 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)

       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.


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

Linux				 4 March 2005			    EXPORTS(5)

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