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exports(5) - exports - NFS file systems being exported (for Kernel based NFS) - man 5 exports

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EXPORTS(5)                                                          EXPORTS(5)

       exports - NFS file(1,n) systems being exported (for Kernel based NFS)


       The  file(1,n)  /etc/exports serves as the access(2,5) control list for file(1,n) 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(1,n)
       server daemon nfsd(8).

       The file(1,n) 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(2,8) the file(1,n) 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(1,8)  the export name using a backslash followed by the character code as
       three octal digits.

   Machine Name Formats
       NFS clients may be specified in(1,8) a number of ways:

       single host(1,5)
              This is the most common format. You may specify a host(1,5) either by
              an abbreviated name recognized be the resolver(3,5), the fully quali-
              fied domain name, or an IP address.

              NIS netgroups may be given as @group.  Only  the  host(1,5)  part  of
              each  netgroup  members  is consider in(1,8) checking for membership.
              Empty host(1,5) 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(1,n) more compact; for instance,
              *  matches  all  hosts  in(1,8) the domain  As
              these characters also match the dots in(1,8) 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(1,8)  dotted-decimal  format,  or  as a contiguous mask
              length (for example, either `/' or  `/22'  appended
              to the network base address result in(1,8) identical subnetworks with
              10 bits of host(1,5)). Wildcard characters generally do not  work  on
              IP  addresses, though they may work by accident when reverse DNS
              lookups fail.

   RPCSEC_GSS security
       To restrict access(2,5) to an export using rpcsec_gss security, use the spe-
       cial string(3,n) "gss/krb5" as the client.  It is not possible to simultane-
       ously require rpcsec_gss and to make requirements on the IP address  of
       the client.

   General Options
       exportfs understands the following export options:

       secure This option requires that requests originate on an internet port
              less(1,3) than IPPORT_RESERVED (1024). This option is on by  default.
              To turn it off, specify insecure.

       rw     Allow  both  read(2,n,1 builtins)  and  write(1,2)  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(1,2,8)  is  the
              default,  and  async  must  be explicit requested if(3,n) needed.  To
              help make system adminstrators aware of this change,  'exportfs'
              will issue a warning if(3,n) neither sync(1,2,8) nor async is specified.

              This  option has no effect if(3,n) async is also set.  The NFS server
              will normally delay committing a write(1,2) request to disc  slightly
              if(3,n)  it  suspects  that  another  related write(1,2) request may be in(1,8)
              progress  or  may  arrive  soon.   This  allows  multiple  write(1,2)
              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(1,8)
              IRIX NFS.  Normally, if(3,n) a server exports two filesystems one  of
              which  is  mounted  on  the  other, then the client will have to
              mount(2,8) both filesystems explicitly to get access(2,5) 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(3x,7,3x curs_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(1,8) the one
              apparent filesystem to have the same inode number.

              The nohide option is currently only  effective  on  single  host(1,5)
              exports.   It  does  not work reliably with netgroup, subnet, or
              wildcard exports.

              This option can be very useful in(1,8) 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 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(1,n) is in(1,8) the appropriate
              filesystem  (which  is easy) but also that it is in(1,8) 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(1,n) in(1,8)  the  "filehandle"
              that  is  given  to  the  client.   This can cause problems with
              accessing files that are renamed while a client  has  them  open(2,3,n)
              (though in(1,8) 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(2,5) can only  be  accessed
              if(3,n)  the  filesystem is exported with no_root_squash (see below),
              even the file(1,n) 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(1,n) renames,
              should be exported with subtree checking disabled.  A filesystem
              which  is  mostly  readonly,  and at least doesn't see many file(1,n)
              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.


              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(2,n,1 builtins)  access(2,5)  to the file.  With this flag no access(2,5) checks will
              be performed.

              Early NFS client implementations did not send(2,n)  credentials  with
              lock  requests,  and  many current NFS clients still exist which
              are based on the old implementations.  Use this flag if(3,n) 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(3,n) it
              has successfully been  mounted.   If  no  path  is  given  (e.g.
              mountpoint  or  mp)  then  the export point must also be a mount(2,8)
              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(3,n), for example, the filesys-
              tem failed to mount(2,8) 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(1,n) handle and file(1,n) 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(1,n) handles for the
              shared  filesystem  thus  avoiding  stale  file(1,n)  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.

   User ID Mapping
       nfsd bases its access(2,5) control to files on the server machine on the uid
       and gid provided in(1,8) each NFS RPC request. The normal  behavior  a  user
       would expect is that she can access(2,5) her files on the server just as she
       would on a normal file(1,n) 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:

              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(1,3) 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(7,n,1 builtins) 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(1,8) the exam-
              ple  section below, which maps all requests to uid 150 (which is
              supposedly that of user joe).

       # sample /etc/exports file(1,n)
       /               master(5,8)(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(5,8)  and
       trusty.   In  addition to write(1,2) access(2,5), all uid squashing is turned off
       for host(1,5) 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(1,5) in(1,8) the world, executing all
       requests under the nobody account. The insecure option  in(1,8)  this  entry
       also  allows clients with NFS implementations that don't use a reserved
       port for NFS.


4.2 Berkeley Distribution       28 October 1999                     EXPORTS(5)

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