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       slapd-meta - metadirectory backend to slapd




       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
       scenarios will be discussed.

       The  proxy instance of slapd(8) must contain schema information for the
       attributes and objectClasses used in filters, request DN  and  request-
       related data in general.  It should also contain schema information for
       the data returned by the proxied server.  It is the  responsibility  of
       the  proxy  administrator to keep the schema of the proxy lined up with
       that of the proxied server.

       Note: When  looping  back  to  the  same  instance  of  slapd(8),  each
       connection  requires  a  new thread; as a consequence, slapd(8) must be
       compiled with thread support, and the threads parameter may  need  some
       tuning; in those cases, unless the multiple target feature is required,
       one may consider  using  slapd-relay(5)  instead,  which  performs  the
       relayed operation internally and thus reuses the same connection.


       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:  In  early versions of back-ldap and back-meta it was recommended
       to always set

              lastmod  off

       for ldap and meta databases.  This  was  required  because  operational
       attributes  related  to  entry  creation and modification should not be
       proxied, as they could be mistakenly written to the  target  server(s),
       generating  an  error.   The  current implementation automatically sets
       lastmod to off, so its use is redundant and should be omitted.


       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:

       conn-ttl <time>
              This  directive  causes  a  cached  connection  to be dropped an
              recreated after a given ttl, regardless of being idle or not.

       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 specific target
              as default.

       dncache-ttl {DISABLED|forever|<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  is  required,  in  the
              format illustrated for the idle-timeout directive.

       onerr {CONTINUE|report|stop}
              This directive allows to select the behavior in case an error is
              returned by one target during a search.  The default,  continue,
              consists  in  continuing the operation, trying to return as much
              data as possible.  If the value is set to stop,  the  search  is
              terminated  as  soon  as an error is returned by one target, and
              the error is immediately propagated to the client.  If the value
              is  set  to report, the search is continuated to the end but, in
              case at least one target returned an error code, the first  non-
              success error code is returned.

       norefs <NO|yes>
              If  yes,  do not return search reference responses.  By default,
              they are returned unless request is LDAPv2.  If set  before  any
              target  specification, it affects all targets, unless overridden
              by any per-target directive.

       noundeffilter <NO|yes>
              If yes, return success instead  of  searching  if  a  filter  is
              undefined  or  contains  undefined  portions.   By  default, the
              search is propagated after  replacing  undefined  portions  with
              (!(objectClass=*)),  which  corresponds to the empty result set.
              If set before any target specification, it affects all  targets,
              unless overridden by any per-target directive.

       protocol-version {0,2,3}
              This  directive  indicates what protocol version must be used to
              contact the remote server.  If set to 0 (the default), the proxy
              uses the same protocol version used by the client, otherwise the
              requested    protocol    is    used.     The    proxy    returns
              unwillingToPerform if an operation that is incompatible with the
              requested protocol is  attempted.   If  set  before  any  target
              specification,  it affects all targets, unless overridden by any
              per-target directive.

       pseudoroot-bind-defer {YES|no}
              This directive, when set to yes, causes  the  authentication  to
              the  remote  servers with the pseudo-root identity (the identity
              defined in each idassert-bind directive) to  be  deferred  until
              actually  needed by subsequent operations.  Otherwise, all binds
              as the rootdn are propagated to the targets.

       quarantine <interval>,<num>[;<interval>,<num>[...]]
              Turns on quarantine of URIs that returned  LDAP_UNAVAILABLE,  so
              that  an  attempt  to  reconnect  only occurs at given intervals
              instead of any time a client requests an operation.  The pattern
              is:  retry  only  after  at least interval seconds elapsed since
              last attempt, for exactly num times; then use the next  pattern.
              If  num  for  the  last  pattern  is  "+",  it  retries forever;
              otherwise, no more retries occur.  This  directive  must  appear
              before any target specification; it affects all targets with the
              same pattern.

       rebind-as-user {NO|yes}
              If this option is  given,  the  client's  bind  credentials  are
              remembered  for  rebinds,  when  trying to re-establish a broken
              connection, or when chasing a referral,  if  chase-referrals  is
              set to yes.

       session-tracking-request {NO|yes}
              Adds session tracking control for all requests.  The client's IP
              and hostname, and the identity associated to  each  request,  if
              known, are sent to the remote server for informational purposes.
              This directive is incompatible with setting protocol-version  to
              2.   If  set  before  any  target  specification, it affects all
              targets, unless overridden by any per-target directive.

       single-conn {NO|yes}
              Discards current cached connection when the client rebinds.

       use-temporary-conn {NO|yes}
              when  set  to  yes,  create  a  temporary  connection   whenever
              competing  with  other threads for a shared one; otherwise, wait
              until the shared connection is available.


       Target specification starts with a "uri" directive:

       uri <protocol>://[<host>]/<naming context> [...]
              The <protocol> part can be anything  ldap_initialize(3)  accepts
              ({ldap|ldaps|ldapi}  and  variants);  the <host> may be omitted,
              defaulting to whatever is  set  in  ldap.conf(5).   The  <naming
              context>  part  is  mandatory  for the first URI, but it must be
              omitted for subsequent ones, if any.  The  naming  context  part
              must be within the naming context 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.  Multiple  URIs  may  be  defined  in  a  single  URI
              statement.   The  additional URIs must be separate arguments and
              must not have  any  <naming  context>  part.   This  causes  the
              underlying  library to contact the first server of the list that
              responds.  For example, if and are shadows
              of the same server, the directive

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

              causes  to be contacted whenever does not
              respond.  In that case, the URI list is  internally  rearranged,
              by   moving  unavailable  URIs  to  the  end,  so  that  further
              connection attempts occur with respect  to  the  last  URI  that

       acl-authcDN <administrative DN for access control purposes>
              DN which is used to query the target server for acl checking, as
              in the LDAP backend; it is supposed to 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.  The acl-authcDN identity is by no means
              implicitly  used  by  the  proxy  when   the   client   connects

       acl-passwd <password>
              Password used with the acl-authcDN above.

       bind-timeout <microseconds>
              This  directive  defines the timeout, in microseconds, used when
              polling for response after an asynchronous bind connection.  The
              initial  call  to  ldap_result(3)  is performed with a trade-off
              timeout of 100000 us; if that results  in  a  timeout  exceeded,
              subsequent  calls use the value provided with bind-timeout.  The
              default value is used also for subsequent calls if  bind-timeout
              is  not  specified.   If set before any target specification, it
              affects  all  targets,  unless  overridden  by  any   per-target

       chase-referrals {YES|no}
              enable/disable automatic referral chasing, which is delegated to
              the underlying libldap, with rebinding eventually  performed  if
              the  rebind-as-user  directive is used.  The default is to chase
              referrals.  If set before any target specification,  it  affects
              all targets, unless overridden by any per-target directive.

       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.

       idassert-authzFrom <authz-regexp>
              if  defined,  selects  what  local  identities are authorized to
              exploit the identity  assertion  feature.   The  string  <authz-
              regexp>  follows  the rules defined for the authzFrom attribute.
              See slapd.conf(5), section related to authz-policy, for  details
              on the syntax of this field.

       idassert-bind    bindmethod=none|simple|sasl    [binddn=<simple    DN>]
              [credentials=<simple    password>]    [saslmech=<SASL     mech>]
              [secprops=<properties>] [realm=<realm>] [authcId=<authentication
              ID>]  [authzId=<authorization  ID>]  [authz={native|proxyauthz}]
              [mode=<mode>]     [flags=<flags>]     [starttls=no|yes|critical]
              [tls_cert=<file>]      [tls_key=<file>]      [tls_cacert=<file>]
              [tls_cacertdir=<path>]      [tls_reqcert=never|allow|try|demand]
              [tls_ciphersuite=<ciphers>]         [tls_protocol_min=<version>]
              Allows  to  define  the  parameters of the authentication method
              that is internally used by the proxy  to  authorize  connections
              that are authenticated by other databases.  The identity defined
              by this directive, according to the properties associated to the
              authentication  method,  is  supposed to have auth access on the
              target server to attributes used on the proxy for authentication
              and  authorization,  and  to  be allowed to authorize the users.
              This requires to have proxyAuthz privileges on  a  wide  set  of
              DNs,  e.g.  authzTo=dn.subtree:"", and the remote server to have
              authz-policy set to to or both.  See slapd.conf(5)  for  details
              on  these  statements  and for remarks and drawbacks about their
              usage.  The supported bindmethods are


              where none  is  the  default,  i.e.  no  identity  assertion  is

              The authz parameter is used to instruct the SASL bind to exploit
              native SASL authorization, if available; since  connections  are
              cached,  this  should only be used when authorizing with a fixed
              identity (e.g. by means of the authzDN or  authzID  parameters).
              Otherwise,  the  default proxyauthz is used, i.e. the proxyAuthz
              control (Proxied  Authorization,  RFC  4370)  is  added  to  all

              The supported modes are:

              <mode> := {legacy|anonymous|none|self}

              If <mode> is not present, and authzId is given, the proxy always
              authorizes that identity.  <authorization ID> can be



              The former is supposed to  be  expanded  by  the  remote  server
              according to the authz rules; see slapd.conf(5) for details.  In
              the latter case, whether or not the dn: prefix is  present,  the
              string must pass DN validation and normalization.

              The  default  mode  is legacy, which implies that the proxy will
              either perform a simple bind as the authcDN or a  SASL  bind  as
              the  authcID  and  assert  the  client's identity when it is not
              anonymous.  Direct binds are always proxied.   The  other  modes
              imply that the proxy will always either perform a simple bind as
              the authcDN or a SASL bind as the authcID, unless restricted  by
              idassert-authzFrom   rules   (see  below),  in  which  case  the
              operation will fail;  eventually,  it  will  assert  some  other
              identity  according  to  <mode>.  Other identity assertion modes
              are anonymous and self, which respectively mean that  the  empty
              or  the  client's  identity  will be asserted; none, which means
              that no proxyAuthz control will be used, so the authcDN  or  the
              authcID  identity  will be asserted.  For all modes that require
              the use of the proxyAuthz control,  on  the  remote  server  the
              proxy identity must have appropriate authzTo permissions, or the
              asserted identities must have appropriate authzFrom permissions.
              Note,  however,  that  the ID assertion feature is mostly useful
              when the asserted identities do not exist on the remote  server.

              Flags can be


              When  the  override flag is used, identity assertion takes place
              even when the database is authorizing for the  identity  of  the
              client,  i.e. after binding with the provided identity, and thus
              authenticating it, the proxy  performs  the  identity  assertion
              using the configured identity and authentication method.

              When  the  prescriptive  flag  is used (the default), operations
              fail with inappropriateAuthentication for those identities whose
              assertion is not allowed by the idassert-authzFrom patterns.  If
              the non-prescriptive flag  is  used,  operations  are  performed
              anonymously  for those identities whose assertion is not allowed
              by the idassert-authzFrom patterns.

              When the proxy-authz-non-critical flag is  used  (the  default),
              the  proxyAuthz  control is not marked as critical, in violation
              of RFC 4370.  Use of proxy-authz-critical is recommended.

              The TLS settings default to the  same  as  the  main  slapd  TLS
              settings, except for tls_reqcert which defaults to "demand".

              The  identity  associated  to  this  directive  is also used for
              privileged operations  whenever  idassert-bind  is  defined  and
              acl-bind is not.  See acl-bind for details.

       idle-timeout <time>
              This  directive  causes  a  cached  connection  to be dropped an
              recreated after it has been idle for the  specified  time.   The
              value can be specified as


              where  <d>,  <h>,  <m> and <s> are respectively treated as days,
              hours,  minutes  and  seconds.   If  set   before   any   target
              specification,  it affects all targets, unless overridden by any
              per-target directive.

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

       network-timeout <time>
              Sets  the  network  timeout  value after which poll(2)/select(2)
              following a connect(2) returns in  case  of  no  activity.   The
              value   is   in   seconds,  and  it  can  be  specified  as  for
              idle-timeout.   If  set  before  any  target  specification,  it
              affects   all  targets,  unless  overridden  by  any  per-target

       nretries {forever|never|<nretries>}
              This directive defines how many times a bind should  be  retried
              in case of temporary failure in contacting a target.  If defined
              before any target specification, it applies to all  targets  (by
              default,  3  times);  the  global  value  can  be  overridden by
              redefinitions inside each target specification.

       pseudorootdn <substitute DN in case of rootdn bind>
              Deprecated; use idassert-bind instead.

       pseudorootpw <substitute password in case of rootdn bind>
              Deprecated; use idassert-bind instead.

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

       subtree-exclude <DN>
              This directive instructs back-meta to ignore the current  target
              for  operations whose requestDN is subordinate to DN.  There may
              be multiple occurrences of  the  subtree-exclude  directive  for
              each of the targets.

       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  configuration  when  simple  suffix  massage   is
              required,  it  has been preserved.  It wraps the basic rewriting
              instructions that perform suffix massaging.  See the "REWRITING"
              section for a detailed list of the rewrite rules it implies.

       t-f-support {NO|yes|discover}
              enable  if  the  remote  server  supports  absolute filters (see
              draft-zeilenga-ldap-t-f  for  details).   If  set  to  discover,
              support is detected by reading the remote server's root DSE.  If
              set before any target specification,  it  affects  all  targets,
              unless overridden by any per-target directive.

       timeout [<op>=]<val> [...]
              This directive allows to set per-operation timeouts.  Operations
              can be

              <op> ::= bind, add, delete, modrdn, modify, compare, search

              The overall duration  of  the  search  operation  is  controlled
              either  by  the  timelimit  parameter or by server-side enforced
              time limits (see  timelimit  and  limits  in  slapd.conf(5)  for
              details).   This  timeout parameter controls how long the target
              can be irresponsive before the operation is aborted.  Timeout is
              meaningless  for  the  remaining operations, unbind and abandon,
              which do not imply any response, while it is not yet implemented
              in  currently supported extended operations.  If no operation is
              specified, the timeout val affects all supported operations.  If
              specified  before  any target definition, it affects all targets
              unless overridden by per-target directives.

              Note: if the timeout is exceeded,  the  operation  is  cancelled
              (according  to  the  cancel  directive);  the  protocol does not
              provide any means to rollback operations, so the client will not
              be  notified  about  the  result  of  the  operation,  which may
              eventually succeeded or not.  In case the  timeout  is  exceeded
              during  a bind operation, the connection is destroyed, according
              to RFC4511.

       tls {[try-]start|[try-]propagate}
              execute the StartTLS extended operation when the  connection  is
              initialized;  only works if the URI directive protocol scheme is
              not ldaps://.  propagate issues the StartTLS operation  only  if
              the  original  connection  did.   The  try- prefix instructs the
              proxy to continue operations if the StartTLS  operation  failed;
              its  use  is  highly  deprecated.   If  set  before  any  target
              specification, it affects all targets, unless overridden by  any
              per-target directive.


       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://,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
       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 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
       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 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
       configurations 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

       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  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>" attrs=<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".


       A  string  is  rewritten according to a set of rules, called a `rewrite
       context'.   The  rules  are  based  on  POSIX  (''extended'')   regular
       expressions   (regex)   with   substring   matching;   basic   variable
       substitution and map resolution of substrings is  allowed  by  specific
       mechanisms   detailed  in  the  following.   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 against a set of rules.  Rules are made
       of a regex match pattern, a substitution pattern and a set of  actions,
       described  by  a  set of flags.  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.  The flags are divided  in  "Pattern  matching  Flags"  and
       "Action Flags"; the former alter the regex match pattern behavior while
       the latter alter the action that is taken after substitution.

Pattern Matching Flags

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

       `R'    use   POSIX   ''basic''   regular   expressions   (default    is

       `M{n}' allow  no more than n recursive passes for a specific rule; does
              not alter the max total count of passes, so it can only  enforce
              a stricter limit for a specific rule.

Action Flags

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

       `@'    stop  applying rules in case of match; the current rule is still
              applied recursively; combine with `:' to apply the current  rule
              only once and then stop.

       `#'    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.

       `U{n}' uses  n  as  return  code if the rule matches; the flag does not
              alter the recursive  behavior  of  the  rule,  so,  to  have  it
              performed  only  once,  it must be used in combination with `:',
              e.g.   `:U{16}'  returns  the  value  `16'  after  exactly   one
              execution   of   the   rule,  if  the  pattern  matches.   As  a
              consequence, its behavior is equivalent to `@', with the  return
              code  set to n; or, in other words, `@' is equivalent to `U{0}'.
              By convention, the freely available codes are above 16 included;
              the others are reserved.

       The  ordering  of  the flags can be 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 two lines
       ahead only in case of match.

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

Pattern matching:

       See regex(7) and/or re_format(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;

       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 dereferenced 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  parsing  routines;  as  a
       consequence,  regex   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

       client -> server:

              (default)            if defined and no specific context
                                   is available
              bindDN               bind
              searchBase           search
              searchFilter         search
              searchFilterAttrDN   search
              compareDN            compare
              compareAttrDN        compare AVA
              addDN                add
              addAttrDN            add AVA
              modifyDN             modify
              modifyAttrDN         modify AVA
              modrDN               modrdn
              newSuperiorDN        modrdn
              deleteDN             delete
              exopPasswdDN         password modify extended operation DN if proxy

       server -> client:

              searchResult         search (only if defined; no default;
                                   acts on DN and DN-syntax attributes
                                   of search results)
              searchAttrDN         search AVA
              matchedDN            all ops (only if applicable)

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 alias 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 match pattern> <substitution pattern> [ <flags> ]
              Determines  how  a  string  can  be  rewritten  if  a pattern is
              matched.  Examples are reported below.

Additional configuration syntax:

       rewriteMap <map type> <map name> [ <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> [<number of passes per rule>]
              Sets  the  maximum  number of total rewriting passes that can be
              performed in a single rewrite operation  (to  avoid  loops).   A
              safe  default  is  set  to 100; note that reaching this limit is
              still treated as a success; recursive  invocation  of  rules  is
              simply   interrupted.    The  count  applies  to  the  rewriting
              operation as a whole, not to any single rule; an  optional  per-
              rule  limit  can  be  set.   This limit is overridden by setting
              specific per-rule limits with the `M{n}' flag.

Configuration examples:

       # set to `off' to disable rewriting
       rewriteEngine on

       # the rules the "suffixmassage" directive implies
       rewriteEngine on
       # all dataflow from client to server referring to DNs
       rewriteContext default
       rewriteRule "(.*)<virtualnamingcontext>$" "%1<realnamingcontext>" ":"
       # empty filter rule
       rewriteContext searchFilter
       # all dataflow from server to client
       rewriteContext searchResult
       rewriteRule "(.*)<realnamingcontext>$" "%1<virtualnamingcontext>" ":"
       rewriteContext searchAttrDN alias searchResult
       rewriteContext matchedDN alias searchResult

       # 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}" ":"

       # This example shows how to strip unwanted DN-valued
       # attribute values from a search result; the first rule
       # matches DN values below "ou=People,dc=example,dc=com";
       # in case of match the rewriting exits successfully.
       # The second rule matches everything else and causes
       # the value to be rejected.
       rewriteContext searchResult
       rewriteRule ".*,ou=People,dc=example,dc=com" "%0" ":@"
       rewriteRule ".*" "" "#"

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 ...)


       The  meta  backend  does  not  honor  all ACL semantics as described in
       slapd.access(5).  In general,  access  checking  is  delegated  to  the
       remote  server(s).  Only read (=r) access to the entry pseudo-attribute
       and to the other attribute values of the entries returned by the search
       operation is honored, which is performed by the frontend.


       The  proxy  cache  overlay  allows  caching  of  LDAP  search  requests
       (queries) in a local database.  See slapo-pcache(5) for details.


              default slapd configuration file


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


       Pierangelo Masarati, based on back-ldap by Howard Chu