Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (Debian-5.0)
Apropos / Subsearch:
optional field

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.

       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
              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.

   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, this option
              was the default.  In  all  subsequence  releases,  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 specified.

              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.

       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
              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).

       # 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)

       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.


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

Linux                            4 March 2005                       EXPORTS(5)