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       slapd-meta - metadirectory backend


       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 presented 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 scenar‐
       ios will be discussed.

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

       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/modification 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.

       Target configuration starts with the "uri" directive.  All the configu‐
       ration  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, mod‐
	      rdn; 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  specific	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 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://,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.	Multi‐
       ple  URIs  may be defined in a single argument.	The URIs must be sepa‐
       rated 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 giving  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.

	      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 backend's "rootdn" succeeds, and the bind is prop‐
	      agated to the target using the "pseudorootdn" DN.

       Note: cleartext credentials must be supplied here;  as  a  consequence,
       using the pseudorootdn/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" direc‐
	      tive  was introduced in the LDAP backend to allow suffix massag‐
	      ing while proxying.  It has  been	 obsoleted  by	the  rewriting
	      tools.  However, both for backward compatibility and for ease of
	      configuration 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).

       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 amaz‐
       ingly 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://,dc=foo,dc=com"
	      uri      "ldap://,dc=foo,dc=com"

       Operations directed to a specific target can be easily resolved because
       there  are no ambiguities.  The only operation that may resolve to mul‐
       tiple 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 configure our Meta backend as:

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

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

	      uri	    "ldap://,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  tar‐
       get'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

       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 context:

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

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

	      uri	    "ldap://,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 selection will  fail  unless  the	DN  is
       already	cached or a default target has been set.  Practical configura‐
       tions may result as a combination of all the above scenarios.

       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 considerations.  Two philosophies may be consid‐

       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 possi‐
       ble to imagine scenarios 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  supported	 by slapd except a special subtle case (please
       drop me a note if you can find other exceptions:	 <>).
       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	inter‐
       vention	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".

       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 behavior 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 (initially dedicated to the LDAP backend).

       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 performed 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
       pattern allows map resolution of substrings.  A map is a generic object
       that maps a substitution 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

	      `%' `{' [ <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 context 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 param‐
	      eter; 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 parsing routines; as a conse‐
       quence, 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  con‐
       text  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

       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 accessing 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 `%{$paramName}'.

       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. `'; 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_]+)(\\).*)"
       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://' '@'
	 rewriteRule '^cn=[m-z].*' 'ldap://' '@'
	 rewriteRule '.*'	   'ldap://' '@'

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

	 rewriteRule '^cn=root,.*' 'ldap://' '@'

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

	      default slapd configuration file

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


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