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exports(5)                                                          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  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-sepa-
       rated 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 con-
       tinued 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 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.

              NIS netgroups may be given as @group.  Only the host part of each netgroup members is consider in check-
              ing for membership.  Empty host parts or those containing a single dash (-) are ignored.

              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,  *
              matches  all  hosts in the domain  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 dot-
              ted-decimal format, or as a contiguous mask length.   For  example,  either  '/'  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.

   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 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  speci-

              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 explic-
              itly 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 filesys-
              tems 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 for NFSv2 and NFSv3 with hide.

              This option is not relevant when NFSv4 is use.  NFSv4 never hides subordinate filesystems.  Any filesys-
              tem that is exported will be visible where expected when using NFSv4.

              This option is similar to nohide but it makes it possible for clients to access all filesystems  mounted
              on a filesystem marked with crossmnt.  Thus when a child filesystem "B" is mounted on a parent "A", set-
              ting crossmnt on "A" has a similar effect to setting "nohide" on B.

              With nohide the child filesystem needs to be explicitly exported.  With crossmnt  it  need  not.   If  a
              child  of  a crossmnt file is not explicitly exported, then it will be implicitly exported with the same
              export options as the parent, except for fsid=.  This makes it impossible to not export  a  child  of  a
              crossmnt  filesystem.   If some but not all subordinate filesystems of a parent are to be exported, then
              they must be explicitly exported and the parent should not have crossmnt set.

              The nocrossmnt option can explictly disable crossmnt if it was previously set.  This is rarely useful.

              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

              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 read-
              only, 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  explic-
              itly  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 synonymous 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 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.

              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  filesys-
              tem (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 neces-
              sary 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 sup-
              ported 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_NOTSUPP, 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  different
              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 alter-
              natives. (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  squashing') is the default, and can be turned off with

       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 diskless 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 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, con-
              sider the export entry for /home/joe in the example 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  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

       # 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 subnet. The eighth line demonstrates a charac-
       ter class wildcard match.

       /etc/exports /etc/exports.d

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

                               31 December 2009                     exports(5)