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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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