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.

       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.

              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.

   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).

              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 enables subtree checking,  which  does  add  another
              level  of  security,  but  can  be unreliability in some circum-

              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

              This type of subtree checking is disabled by default.


              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
              nominted  path  must  be  a mountpoint for the exportpoint to be

              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

              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.

              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.)

              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:

              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.

              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)

       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.


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

Linux                            4 March 2005                       EXPORTS(5)