exports(5)							File Formats Manual							exports(5)

NAME

       exports - NFS server export table

DESCRIPTION

       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
       mountd(8).

       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 only.

       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 charac-
       ter 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 server.

   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 `/255.255.252.0' 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.

       wildcards
	      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.

       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.

       anonymous
	      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 net-
       groups), 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  pro-
	      tection),  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 specified.

       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 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 "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 appar-
	      ent 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 filesystem "B" is mounted on a parent "A", setting crossmnt on "A" has the same effect
	      as setting "nohide" on B.

       no_subtree_check
	      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 enabled.

	      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 nei-
	      ther option, exportfs will warn you that the change is pending.

       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 synonymous
	      auth_nlm, or secure_locks.

       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 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
	      exported.

       fsid=num|root|uuid
	      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 device).

	      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 punctuation.

	      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.

       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 must have a mountpoint here, though a different filesystem is not required;  so,  for  example,  mount  --bind
	      /path /path is sufficient.)

       replicas=path@host[+host][:path@host[+host]]
	      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 squash-
       ing') 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 or gids that might be equally
	      sensitive, such as user bin or group staff.

       no_root_squash
	      Turn off root squashing. This option is mainly useful for diskless clients.

       all_squash
	      Map all uids and gids to the anonymous user. Useful for NFS-exported public FTP directories, news spool directories, etc. The  oppo-
	      site option is no_all_squash, which is the default setting.

       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  sec-
	      tion 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

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)
       /srv/www        -sync,rw server @trusted @external(ro)
       /foo	       2001:db8:9:e54::/64(rw) 192.0.2.0/24(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.

FILES

       /etc/exports /etc/exports.d

SEE ALSO

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

								 31 December 2009							exports(5)