Provided by: nfs-kernel-server_2.6.4-3ubuntu5_amd64 bug

NAME

       exports - NFS server export table

DESCRIPTION

       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 control list.  The
       table is used by exportfs(8) to give information to mountd(8).

       The file format is similar to the SunOS exports file. Each line contains 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 only.

       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 server.

   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 character-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 `/255.255.252.0' or  `/22'  appended
              to  the  network base IPv4 address results in identical subnetworks with 10 bits of
              host. IPv6 addresses must use a contiguous mask  length  and  must  not  be  inside
              square  brackets  to  avoid  confusion  with  character-class  wildcards.  Wildcard
              characters generally do not work on IP addresses, though they may work by  accident
              when reverse DNS lookups fail.

       wildcards
              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.

       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.

       anonymous
              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  cryptographic  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.

   Transport layer security
       The Linux NFS server allows the use of RPC-with-TLS (RFC  9289)  to  protect  RPC  traffic
       between  itself and its clients.  Alternately, administrators can secure NFS traffic using
       a VPN, or an ssh tunnel or similar mechanism, in a way that is transparent to the server.

       To enable the use of RPC-with-TLS, the server's administrator must install  and  configure
       tlshd  to  handle  transport  layer  security  handshake  requests  from the local kernel.
       Clients can then choose to use RPC-with-TLS or they may continue operating without it.

       Administrators may require the  use  of  RPC-with-TLS  to  protect  access  to  individual
       exports.   This  is particularly useful when using non-cryptographic security flavors such
       as sec=sys.  The xprtsec=  option,  followed  by  an  unordered  colon-delimited  list  of
       security  policies, can restrict access to the export to only clients that have negotiated
       transport-layer security.  Currently supported transport layer security policies include:

       none   The server permits clients to access the export without the use of transport  layer
              security.

       tls    The  server  permits  clients  that have negotiated an RPC-with-TLS session without
              peer authentication (confidentiality only) to access the export.  Clients  are  not
              required to offer an x.509 certificate when establishing a transport layer security
              session.

       mtls   The server permits clients that have negotiated an RPC-with-TLS session  with  peer
              authentication to access the export.  The server requires clients to offer an x.509
              certificate when establishing a transport layer security session.

       If RPC-with-TLS is configured and enabled and the xprtsec= option is  not  specified,  the
       default  setting  for  an  export is xprtsec=none:tls:mtls.  With this setting, the server
       permits clients to use any transport layer security mechanism or none at all to access the
       export.

   General Options
       exportfs understands the following export options:

       secure This option requires that requests not using gss originate on an Internet port less
              than IPPORT_RESERVED (1024). This option is on by default.  To turn it off, specify
              insecure.  (NOTE: older kernels (before upstream kernel version 4.17) enforced this
              requirement on gss requests as well.)

       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.

       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 performance, 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 for NFSv2 and NFSv3 with hide.

              This  option  is  not  relevant  when  NFSv4 is use.  NFSv4 never hides subordinate
              filesystems.  Any filesystem that is exported will be visible where  expected  when
              using NFSv4.

       crossmnt
              This option is similar to nohide but it makes it possible for clients to access all
              filesystems mounted on a filesystem  marked  with  crossmnt.   Thus  when  a  child
              filesystem  "B"  is  mounted on a parent "A", setting crossmnt on "A" has a similar
              effect to setting "nohide" on B.

              With nohide the child filesystem needs to be explicitly exported.  With crossmnt it
              need  not.   If a child of a crossmnt file is not explicitly exported, then it will
              be implicitly exported with the same export  options  as  the  parent,  except  for
              fsid=.   This  makes  it impossible to not export a child of a crossmnt filesystem.
              If some but not all subordinate filesystems of a parent are to  be  exported,  then
              they must be explicitly exported and the parent should not have crossmnt set.

              The  nocrossmnt  option  can  explictly  disable crossmnt if it was previously set.
              This is rarely useful.

       no_subtree_check
              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 normally 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.

              From release 1.1.0 of nfs-utils onwards, the default will  be  no_subtree_check  as
              subtree_checking  tends  to cause more problems than it is worth.  If you genuinely
              require subtree checking, you should explicitly put  that  option  in  the  exports
              file.   If  you  put  neither  option,  exportfs  will  warn you that the change is
              pending.

       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 synonymous auth_nlm, or secure_locks.

       mountpoint=path

       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 filesystem 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 exported.

       fsid=num|root|uuid
              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
              device).

              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 punctuation.

              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.

       nordirplus
              This option will disable  READDIRPLUS  request  handling.   When  set,  READDIRPLUS
              requests from NFS clients return NFS3ERR_NOTSUPP, and clients fall back on READDIR.
              This option affects only NFSv3 clients.

       refer=path@host[+host][:path@host[+host]]
              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 different filesystem is not required;  so,  for  example,
              mount --bind /path /path is sufficient.)

       replicas=path@host[+host][:path@host[+host]]
              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.)

       pnfs   This  option enables the use of the pNFS extension if the protocol level is NFSv4.1
              or higher, and the filesystem supports pNFS exports.  With pNFS clients can  bypass
              the  server  and  perform  I/O  directly  to  storage  devices.  The default can be
              explicitly requested with the no_pnfs option.

       security_label
              With this option set, clients using NFSv4.2 or higher  will  be  able  to  set  and
              retrieve  security  labels (such as those used by SELinux).  This will only work if
              all clients use a consistent security policy.  Note  that  early  kernels  did  not
              support this export option, and instead enabled security labels by default.

       reexport=auto-fsidnum|predefined-fsidnum
              This  option  helps  when  a NFS share is re-exported. Since the NFS server needs a
              unique identifier for each exported filesystem and a NFS share cannot provide such,
              usually  a manual fsid is needed.  As soon crossmnt is used manually assigning fsid
              won't work anymore. This is where this option becomes handy. It will  automatically
              assign  a  numerical  fsid  to exported NFS shares. The fsid and path relations are
              stored in a SQLite  database.  If  auto-fsidnum  is  selected,  the  fsid  is  also
              autmatically  allocated.  predefined-fsidnum assumes pre-allocated fsid numbers and
              will just look them up.  This option depends also on the kernel, you will  need  at
              least  kernel  version 5.19.  Since reexport= can automatically allocate and assign
              numerical fsids, it is no longer possible to have numerical fsids in other  exports
              as soon this option is used in at least one export entry.

              The  association  between  fsid  numbers  and paths is stored in a SQLite database.
              Don't edit or remove the database  unless  you  know  exactly  what  you're  doing.
              predefined-fsidnum  is useful when you have used auto-fsidnum before and don't want
              further entries stored.

   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 anonymous 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:

       root_squash
              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.

       no_root_squash
              Turn off root squashing. This option is mainly useful for diskless 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 setting.

       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
              example section below, which maps all requests to uid 150 (which is supposedly that
              of user joe).

   Subdirectory Exports
       Normally  you  should only export only the root of a filesystem.  The NFS server will also
       allow you to export a subdirectory of a filesystem, however, this has drawbacks:

       First, it may be possible for a malicious user to access files on the  filesystem  outside
       of the exported subdirectory, by guessing filehandles for those other files.  The only way
       to prevent this is by using the no_subtree_check option, which can cause other problems.

       Second, export options may not be enforced in the way that you would expect.  For example,
       the   security_label  option  will  not  work  on  subdirectory  exports,  and  if  nested
       subdirectory exports change  the  security_label  or  sec=  options,  NFSv4  clients  will
       normally  see only the options on the parent export.  Also, where security options differ,
       a malicious client may use filehandle-guessing  attacks  to  access  the  files  from  one
       subdirectory using the options from another.

   Extra Export Tables
       After  reading  /etc/exports exportfs reads files in the /etc/exports.d directory as extra
       export tables.  Only files ending in .exports are considered.  Files beginning with a  dot
       are ignored.  The format for extra export tables is the same as /etc/exports

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)
       /srv/www        -sync,rw server @trusted @external(ro)
       /foo            2001:db8:9:e54::/64(rw) 192.0.2.0/24(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 subnet. The eighth  line  demonstrates  a  character
       class wildcard match.

FILES

       /etc/exports /etc/exports.d

SEE ALSO

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

                                         31 December 2009                              exports(5)