exports(5)                    File Formats Manual                   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 con-
       trol list.  The table is used by exportfs(8)  to  give  information  to

       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
              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 char-
              acter-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 `/' or `/22' appended
              to the network base IPv4 address results  in  identical  subnet-
              works with 10 bits of host. IPv6 addresses must use a contiguous
              mask length and must not be inside square brackets to avoid con-
              fusion  with character-class wildcards. Wildcard characters gen-
              erally do not work on IP addresses,  though  they  may  work  by
              accident when reverse DNS lookups fail.

              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.

              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

              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  netgroups),
       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 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, 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 spec-

              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.


       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.)

   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).

   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

       # 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  sub-
       net. The eighth line demonstrates a character class wildcard match.

       /etc/exports /etc/exports.d

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

                               31 December 2009                     exports(5)
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