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SLAPD-META(5)									    SLAPD-META(5)

NAME
       slapd-meta - metadirectory backend

SYNOPSIS
       /etc/openldap/slapd.conf

DESCRIPTION
       The  meta backend to slapd(8) performs basic LDAP proxying with respect to a set of remote
       LDAP servers, called "targets".	The information contained in these servers  can  be  pre-
       sented as belonging to a single Directory Information Tree (DIT).

       A  basic knowledge of the functionality of the slapd-ldap(5) backend is recommended.  This
       backend has been designed as an enhancement of the ldap backend.  The two  backends  share
       many  features  (actually  they	also  share portions of code).	While the ldap backend is
       intended to proxy operations directed to a single  server,  the	meta  backend  is  mainly
       intended for proxying of multiple servers and possibly naming context masquerading.  These
       features, although useful in many scenarios, may result in  excessive  overhead	for  some
       applications,  so  its  use should be carefully considered.  In the examples section, some
       typical scenarios will be discussed.

EXAMPLES
       There are examples in various places in this document,  as  well  as  in  the  slapd/back-
       meta/data/ directory in the OpenLDAP source tree.

CONFIGURATION
       These  slapd.conf options apply to the META backend database.  That is, they must follow a
       "database meta" line and come before any subsequent "backend" or "database" lines.   Other
       database options are described in the slapd.conf(5) manual page.

       Note: as with the ldap backend, operational attributes related to entry creation/modifica-
       tion should not be used, as they would be passed to  the  target  servers,  generating  an
       error.	Moreover,  it makes little sense to use such attributes in proxying, as the proxy
       server doesn't actually store data, so it should have no  knowledge  of	such  attributes.
       While  code  to strip the modification attributes has been put in place (and #ifdef'd), it
       implies unmotivated overhead.  So it is strongly recommended to set
	      lastmod  off
       for every ldap and meta backend.

SPECIAL CONFIGURATION DIRECTIVES
       Target configuration starts with the "uri" directive.  All  the	configuration  directives
       that are not specific to targets should be defined first for clarity, including those that
       are common to all backends.  They are:

       default-target none
	      This directive forces the backend to reject all those operations that must  resolve
	      to  a  single  target in case none or multiple targets are selected.  They include:
	      add, delete, modify, modrdn; compare is not included, as well  as  bind  since,  as
	      they don't alter entries, in case of multiple matches an attempt is made to perform
	      the operation on any candidate target, with the constraint that at  most	one  must
	      succeed.	 This  directive  can also be used when processing targets to mark a spe-
	      cific target as default.

       dncache-ttl {forever|disabled|<ttl>}
	      This directive sets the time-to-live of the DN cache.  This caches the target  that
	      holds a given DN to speed up target selection in case multiple targets would result
	      from an uncached search; forever means cache never expires; disabled  means  no  DN
	      caching; otherwise a valid ( > 0 ) ttl in seconds is required.

TARGET SPECIFICATION
       Target specification starts with a "uri" directive:

       uri <protocol>://[<host>[:<port>]]/<naming context>
	      The  "server"  directive that was allowed in the LDAP backend (although deprecated)
	      has been discarded in the Meta  backend.	 The  <protocol>  part	can  be  anything
	      ldap_initialize(3) accepts ({ldap|ldaps|ldapi} and variants); <host> and <port> may
	      be omitted, defaulting to whatever is set in /etc/ldap.conf.  The <naming  context>
	      part  is	mandatory.   It  must end with one of the naming contexts defined for the
	      backend, e.g.:

	      suffix "dc=foo,dc=com"
	      uri    "ldap://x.foo.com/dc=x,dc=foo,dc=com"

       The <naming context> part doesn't need to be unique across the targets; it may also  match
       one  of	the  values  of the "suffix" directive.  Multiple URIs may be defined in a single
       argument.  The URIs must be separated by TABs (e.g. '\t'), and the  additional  URIs  must
       have  no  <naming  context> part.  This causes the underlying library to contact the first
       server of the list that responds.

       default-target [<target>]
	      The "default-target" directive can also be used during target specification.   With
	      no arguments it marks the current target as the default.	The optional number marks
	      target <target> as the default one, starting  from  1.   Target  <target>  must  be
	      defined.

       binddn <administrative DN for access control purposes>
	      This  directive,	as  in	the LDAP backend, allows to define the DN that is used to
	      query the target server for acl checking; it should have read access on the  target
	      server  to attributes used on the proxy for acl checking.  There is no risk of giv-
	      ing away such values; they are only used to check permissions.

       bindpw <password for access control purposes>
	      This directive sets the password for acl checking in  conjunction  with  the  above
	      mentioned "binddn" directive.

       rebind-as-user
	      If  this	option is given, the client's bind credentials are remembered for rebinds
	      when chasing referrals.

       pseudorootdn <substitute DN in case of rootdn bind>
	      This directive, if present, sets the DN that will be substituted to the bind DN  if
	      a  bind  with  the  backend's  "rootdn"  succeeds.  The true "rootdn" of the target
	      server ought not be used; an arbitrary administrative DN should used instead.

       pseudorootpw <substitute password in case of rootdn bind>
	      This directive sets the credential that will be used in case a bind with the  back-
	      end's  "rootdn" succeeds, and the bind is propagated to the target using the "pseu-
	      dorootdn" DN.

       Note: cleartext credentials must be supplied here; as a	consequence,  using  the  pseudo-
       rootdn/pseudorootpw directives is inherently unsafe.

       rewrite* ...
	      The rewrite options are described in the "REWRITING" section.

       suffixmassage <virtual naming context> <real naming context>
	      All  the	directives  starting  with "rewrite" refer to the rewrite engine that has
	      been added to slapd.  The "suffixmassage" directive  was	introduced  in	the  LDAP
	      backend  to  allow  suffix  massaging while proxying.  It has been obsoleted by the
	      rewriting tools.	However, both for backward compatibility and for ease of configu-
	      ration when simple suffix massage is required, it has been preserved.  It wraps the
	      basic rewriting instructions that perform suffix massaging.

       Note: this also fixes a flaw in suffix massaging, which operated on (case insensitive) DNs
       instead of normalized DNs, so "dc=foo, dc=com" would not match "dc=foo,dc=com".

       See the "REWRITING" section.

       map {attribute|objectclass} [<local name>|*] {<foreign name>|*}
	      This maps object classes and attributes as in the LDAP backend.  See slapd-ldap(5).

SCENARIOS
       A  powerful  (and  in some sense dangerous) rewrite engine has been added to both the LDAP
       and Meta backends.  While the former can gain limited beneficial  effects  from	rewriting
       stuff, the latter can become an amazingly powerful tool.

       Consider a couple of scenarios first.

       1)  Two directory servers share two levels of naming context; say "dc=a,dc=foo,dc=com" and
       "dc=b,dc=foo,dc=com".  Then, an unambiguous Meta database can be configured as:

	      database meta
	      suffix   "dc=foo,dc=com"
	      uri      "ldap://a.foo.com/dc=a,dc=foo,dc=com"
	      uri      "ldap://b.foo.com/dc=b,dc=foo,dc=com"

       Operations directed to a specific target can be easily resolved because there are no ambi-
       guities.   The  only  operation that may resolve to multiple targets is a search with base
       "dc=foo,dc=com" and scope at least "one", which results in spawning two	searches  to  the
       targets.

       2a) Two directory servers don't share any portion of naming context, but they'd present as
       a single DIT [Caveat: uniqueness of (massaged) entries among the two servers  is  assumed;
       integrity  checks risk to incur in excessive overhead and have not been implemented].  Say
       we have "dc=bar,dc=org" and "o=Foo,c=US", and we'd like them  to  appear  as  branches  of
       "dc=foo,dc=com",  say "dc=a,dc=foo,dc=com" and "dc=b,dc=foo,dc=com".  Then we need to con-
       figure our Meta backend as:

	      database	    meta
	      suffix	    "dc=foo,dc=com"

	      uri	    "ldap://a.bar.com/dc=a,dc=foo,dc=com"
	      suffixmassage "dc=a,dc=foo,dc=com" "dc=bar,dc=org"

	      uri	    "ldap://b.foo.com/dc=b,dc=foo,dc=com"
	      suffixmassage "dc=b,dc=foo,dc=com" "o=Foo,c=US"

       Again, operations can be resolved without ambiguity, although some rewriting is	required.
       Notice that the virtual naming context of each target is a branch of the database's naming
       context; it is rewritten back and forth when operations are performed towards  the  target
       servers.  What "back and forth" means will be clarified later.

       When a search with base "dc=foo,dc=com" is attempted, if the scope is "base" it fails with
       "no such object"; in fact, the common root of the two targets (prior  to  massaging)  does
       not  exist.   If  the scope is "one", both targets are contacted with the base replaced by
       each target's base; the scope is derated to "base".  In general, a scope "one"  search  is
       honored,  and the scope is derated, only when the incoming base is at most one level lower
       of a target's naming context (prior to massaging).

       Finally, if the scope is "sub" the incoming base is replaced by each  target's  unmassaged
       naming context, and the scope is not altered.

       2b) Consider the above reported scenario with the two servers sharing the same naming con-
       text:

	      database	    meta
	      suffix	    "dc=foo,dc=com"

	      uri	    "ldap://a.bar.com/dc=foo,dc=com"
	      suffixmassage "dc=foo,dc=com" "dc=bar,dc=org"

	      uri	    "ldap://b.foo.com/dc=foo,dc=com"
	      suffixmassage "dc=foo,dc=com" "o=Foo,c=US"

       All the previous considerations hold, except that now there is  no  way	to  unambiguously
       resolve	a DN.  In this case, all the operations that require an unambiguous target selec-
       tion will fail unless the DN is already cached or a default target has been set.   Practi-
       cal configurations may result as a combination of all the above scenarios.

ACLs
       Note  on  ACLs:	at  present  you may add whatever ACL rule you desire to to the Meta (and
       LDAP) backends.	However, the meaning of an ACL on a proxy  may	require  some  considera-
       tions.  Two philosophies may be considered:

       a)  the	remote server dictates the permissions; the proxy simply passes back what it gets
       from the remote server.

       b) the remote server unveils "everything"; the proxy is responsible  for  protecting  data
       from unauthorized access.

       Of  course  the latter sounds unreasonable, but it is not.  It is possible to imagine sce-
       narios in which a remote host discloses data that can be  considered  "public"  inside  an
       intranet,  and a proxy that connects it to the internet may impose additional constraints.
       To this purpose, the proxy should be able to comply with all  the  ACL  matching  criteria
       that  the  server  supports.   This has been achieved with regard to all the criteria sup-
       ported by slapd except a special subtle case (please drop me a note if you can find  other
       exceptions: <ando@openldap.org>).  The rule

	      access to dn="<dn>" attr=<attr>
		     by dnattr=<dnattr> read
		     by * none

       cannot  be matched iff the attribute that is being requested, <attr>, is NOT <dnattr>, and
       the attribute that determines membership, <dnattr>, has not  been  requested  (e.g.  in	a
       search)

       In  fact  this  ACL  is resolved by slapd using the portion of entry it retrieved from the
       remote server without requiring any further  intervention  of  the  backend,  so,  if  the
       <dnattr>  attribute  has  not  been  fetched,  the  match  cannot  be assessed because the
       attribute is not present, not because no value matches the requirement!

       Note on ACLs and attribute mapping:  ACLs  are  applied	to  the  mapped  attributes;  for
       instance,  if  the attribute locally known as "foo" is mapped to "bar" on a remote server,
       then local ACLs apply to attribute "foo" and are totally unaware of its remote name.   The
       remote server will check permissions for "bar", and the local server will possibly enforce
       additional restrictions to "foo".

REWRITING
       A string is rewritten according to a set of rules, called a `rewrite context'.  The  rules
       are  based  on  Regular	Expressions (POSIX regex) with substring matching; extensions are
       planned to allow basic variable substitution and map resolution of substrings.  The behav-
       ior of pattern matching/substitution can be altered by a set of flags.

       The underlying concept is to build a lightweight rewrite module for the slapd server (ini-
       tially dedicated to the LDAP backend).

Passes
       An incoming string is matched agains a set of rules.  Rules are made of a match pattern, a
       substitution  pattern  and  a set of actions.  In case of match a string rewriting is per-
       formed according to the substitution pattern that allows to refer to substrings matched in
       the  incoming  string.  The actions, if any, are finally performed.  The substitution pat-
       tern allows map resolution of substrings.  A map is a generic object that maps a substitu-
       tion pattern to a value.

Pattern Matching Flags
       `C'    honors case in matching (default is case insensitive)

       `R'    use POSIX Basic Regular Expressions (default is Extended)

Action Flags
       `:'    apply the rule once only (default is recursive)

       `@'    stop applying rules in case of match.

       `#'    stop  current  operation	if  the rule matches, and issue an `unwilling to perform'
	      error.

       `G{n}' jump n rules back and forth (watch for loops!).  Note that `G{1}'  is  implicit  in
	      every rule.

       `I'    ignores  errors  in  rule;  this means, in case of error, e.g. issued by a map, the
	      error is treated as a missed match.  The `unwilling to perform' is not overridden.

       The ordering of the flags is significant.  For instance: `IG{2}' means ignore  errors  and
       jump  two  lines  ahead	both  in  case of match and in case of error, while `G{2}I' means
       ignore errors, but jump thwo lines ahead only in case of match.

       More flags (mainly Action Flags) will be added as needed.

Pattern matching:
       See regex(7).

Substitution Pattern Syntax:
       Everything starting with `%' requires substitution;

       the only obvious exception is `%%', which is left as is;

       the basic substitution is `%d', where `d' is a digit; 0 means the whole string, while  1-9
       is a submatch, as discussed in regex(7);

       a `%' followed by a `{' invokes an advanced substitution.  The pattern is:

	      `%' `{' [ <op> ] <name> `(' <substitution> `)' `}'

       where <name> must be a legal name for the map, i.e.

	      <name> ::= [a-z][a-z0-9]* (case insensitive)
	      <op> ::= `>' `|' `&' `&&' `*' `**' `$'

       and  <substitution>  must  be  a legal substitution pattern, with no limits on the nesting
       level.

       The operators are:

       >      sub context invocation; <name> must be a legal,  already	defined  rewrite  context
	      name

       |      external	command invocation; <name> must refer to a legal, already defined command
	      name (NOT IMPL.)

       &      variable assignment; <name> defines a variable in the running  operation	structure
	      which  can be dereferenced later; operator & assigns a variable in the rewrite con-
	      text scope; operator && assigns a variable that scopes the entire session, e.g. its
	      value can be derefenced later by other rewrite contexts

       *      variable	dereferencing;	<name>	must  refer  to  a  variable  that is defined and
	      assigned for the running operation; operator * dereferences a variable scoping  the
	      rewrite  context;  operator  **  dereferences a variable scoping the whole session,
	      e.g. the value is passed across rewrite contexts

       $      parameter dereferencing; <name> must refer to an existing parameter; the idea is to
	      make some run-time parameters set by the system available to the rewrite engine, as
	      the client host name, the bind DN if any, constant parameters initialized at config
	      time,  and  so  on; no parameter is currently set by either back-ldap or back-meta,
	      but constant parameters can be defined in  the  configuration  file  by  using  the
	      rewriteParam directive.

       Substitution  escaping  has been delegated to the `%' symbol, which is used instead of `\'
       in string substitution patterns because `\' is already escaped by slapd's low level  pars-
       ing  routines;  as  a  consequence,  regex(7)  escaping	requires  two  `\'  symbols, e.g.
       `.*\.foo\.bar' must be written as `.*\\.foo\\.bar'.

Rewrite context:
       A rewrite context is a set of rules which are applied in sequence.  The basic idea  is  to
       have an application initialize a rewrite engine (think of Apache's mod_rewrite ...) with a
       set of rewrite contexts; when string rewriting is required, one	invokes  the  appropriate
       rewrite	context  with  the  input string and obtains the newly rewritten one if no errors
       occur.

       Each basic server operation is associated to a rewrite context; they are  divided  in  two
       main groups: client -> server and server -> client rewriting.

       client -> server:

	      (default)      if defined and no specific context
			     is available
	      bindDn	     bind
	      searchBase     search
	      searchFilter   search
	      compareDn      compare
	      addDn	     add
	      modifyDn	     modify
	      modrDn	     modrdn
	      newSuperiorDn  modrdn
	      deleteDn	     delete

       server -> client:

	      searchResult   search (only if defined; no default;
			     acts on DN and DN-syntax attributes
			     of search results)
	      matchedDn      all ops (only if defined; no default;
			     NOT IMPL. except in search)

Basic configuration syntax
       rewriteEngine { on | off }
	      If  `on',  the requested rewriting is performed; if `off', no rewriting takes place
	      (an easy way to stop rewriting without altering too much the configuration file).

       rewriteContext <context name> [ alias <aliased context name> ]
	      <Context name> is the name that identifies the context, i.e. the name used  by  the
	      application to refer to the set of rules it contains.  It is used also to reference
	      sub contexts in string rewriting.  A context may aliase another one.  In this  case
	      the  alias context contains no rule, and any reference to it will result in access-
	      ing the aliased one.

       rewriteRule <regex pattern> <substitution pattern> [ <flags> ]
	      Determines how a tring can be rewritten if a  pattern  is  matched.   Examples  are
	      reported below.

Additional configuration syntax:
       rewriteMap <map name> <map type> [ <map attrs> ]
	      Allows  to  define  a  map that transforms substring rewriting into something else.
	      The map is referenced inside the substitution pattern of a rule.

       rewriteParam <param name> <param value>
	      Sets a value with global scope, that can be dereferenced by the command  `%{$param-
	      Name}'.

       rewriteMaxPasses <number of passes>
	      Sets the maximum number of total rewriting passes that can be performed in a single
	      rewrite operation (to avoid loops).

Configuration examples:
       # set to `off' to disable rewriting
       rewriteEngine on

       # Everything defined here goes into the `default' context.
       # This rule changes the naming context of anything sent
       # to `dc=home,dc=net' to `dc=OpenLDAP, dc=org'

       rewriteRule "(.*)dc=home,[ ]?dc=net"
		   "%1dc=OpenLDAP, dc=org"  ":"

       # since a pretty/normalized DN does not include spaces
       # after rdn separators, e.g. `,', this rule suffices:

       rewriteRule "(.*)dc=home,dc=net"
		   "%1dc=OpenLDAP,dc=org"  ":"

       # Start a new context (ends input of the previous one).
       # This rule adds blanks between DN parts if not present.
       rewriteContext  addBlanks
       rewriteRule     "(.*),([^ ].*)" "%1, %2"

       # This one eats blanks
       rewriteContext  eatBlanks
       rewriteRule     "(.*),[ ](.*)" "%1,%2"

       # Here control goes back to the default rewrite
       # context; rules are appended to the existing ones.
       # anything that gets here is piped into rule `addBlanks'
       rewriteContext  default
       rewriteRule     ".*" "%{>addBlanks(%0)}" ":"

       # Rewrite the search base  according to `default' rules.
       rewriteContext  searchBase alias default

       # Search results with OpenLDAP DN are rewritten back with
       # `dc=home,dc=net' naming context, with spaces eaten.
       rewriteContext  searchResult
       rewriteRule     "(.*[^ ]?)[ ]?dc=OpenLDAP,[ ]?dc=org"
		       "%{>eatBlanks(%1)}dc=home,dc=net"    ":"

       # Bind with email instead of full DN: we first need
       # an ldap map that turns attributes into a DN (the
       # argument used when invoking the map is appended to
       # the URI and acts as the filter portion)
       rewriteMap ldap attr2dn "ldap://host/dc=my,dc=org?dn?sub"

       # Then we need to detect DN made up of a single email,
       # e.g. `mail=someone@example.com'; note that the rule
       # in case of match stops rewriting; in case of error,
       # it is ignored.  In case we are mapping virtual
       # to real naming contexts, we also need to rewrite
       # regular DNs, because the definition of a bindDn
       # rewrite context overrides the default definition.
       rewriteContext bindDn
       rewriteRule "^mail=[^,]+@[^,]+$" "%{attr2dn(%0)}" "@I"

       # This is a rather sophisticated example. It massages a
       # search filter in case who performs the search has
       # administrative privileges.  First we need to keep
       # track of the bind DN of the incoming request, which is
       # stored in a variable called `binddn' with session scope,
       # and left in place to allow regular binding:
       rewriteContext  bindDn
       rewriteRule     ".+" "%{&&binddn(%0)}%0" ":"

       # A search filter containing `uid=' is rewritten only
       # if an appropriate DN is bound.
       # To do this, in the first rule the bound DN is
       # dereferenced, while the filter is decomposed in a
       # prefix, in the value of the `uid=<arg>' AVA, and
       # in a suffix. A tag `<>' is appended to the DN.
       # If the DN refers to an entry in the `ou=admin' subtree,
       # the filter is rewritten OR-ing the `uid=<arg>' with
       # `cn=<arg>'; otherwise it is left as is. This could be
       # useful, for instance, to allow apache's auth_ldap-1.4
       # module to authenticate users with both `uid' and
       # `cn', but only if the request comes from a possible
       # `cn=Web auth,ou=admin,dc=home,dc=net' user.
       rewriteContext searchFilter
       rewriteRule "(.*\\()uid=([a-z0-9_]+)(\\).*)"
	 "%{**binddn}<>%{&prefix(%1)}%{&arg(%2)}%{&suffix(%3)}"
	 ":I"
       rewriteRule "[^,]+,ou=admin,dc=home,dc=net"
	 "%{*prefix}|(uid=%{*arg})(cn=%{*arg})%{*suffix}" "@I"
       rewriteRule ".*<>" "%{*prefix}uid=%{*arg}%{*suffix}" ":"

LDAP Proxy resolution (a possible evolution of slapd-ldap(5)):
       In case the rewritten  DN  is  an  LDAP	URI,  the  operation  is  initiated  towards  the
       host[:port] indicated in the uri, if it does not refer to the local server.  E.g.:

	 rewriteRule '^cn=root,.*' '%0' 		    'G{3}'
	 rewriteRule '^cn=[a-l].*' 'ldap://ldap1.my.org/%0' '@'
	 rewriteRule '^cn=[m-z].*' 'ldap://ldap2.my.org/%0' '@'
	 rewriteRule '.*'	   'ldap://ldap3.my.org/%0' '@'

       (Rule 1 is simply there to illustrate the `G{n}' action; it could have been written:

	 rewriteRule '^cn=root,.*' 'ldap://ldap3.my.org/%0' '@'

       with the advantage of saving one rewrite pass ...)

FILES
       /etc/openldap/slapd.conf
	      default slapd configuration file

SEE ALSO
       slapd.conf(5), slapd-ldap(5), slapd(8), regex(7).

OpenLDAP 2.1.X				   RELEASEDATE				    SLAPD-META(5)
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