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exports

EXPORTS(5)                                                          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. How-
              ever, these wildcard characters do  not  match  the  dots  in  a
              domain name, so the above pattern does not include hosts such as
              a.b.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 usually improves performance, but at the  cost
              that  an unclean server restart (i.e. a crash) can cause data to
              be lost or corrupted.

              In releases of nfs-utils upto and including 1.0.0,  this  option
              was  the  default.   In  this  and  future releases, sync is the
              default, and async must be explicit  requested  if  needed.   To
              help  make system adminstrators 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 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.

       no_subtree_check
              This  option  disables subtree checking, which has mild security
              implications, but can improve reliability is 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 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.

              The  default of having subtree checks enabled, can be explicitly
              requested with subtree_check.


       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.


   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 -2 (i.e. 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 anony-
       mous 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
              sensitive, 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



4.2 Berkeley Distribution       28 October 1999                     EXPORTS(5)