SLAPD-META(5)                 File Formats Manual                SLAPD-META(5)

       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.

       client-pr {accept-unsolicited|DISABLE|<size>}
              This feature allows to use RFC 2696 Paged Results control when
              performing search operations with a specific target,
              irrespective of the client's request.  When set to a numeric
              value, Paged Results control is always used with size as the
              page size.  When set to accept-unsolicited, unsolicited Paged
              Results control responses are accepted and honored for
              compatibility with broken remote DSAs.  The client is not
              exposed to paged results handling between slapd-meta(5) and the
              remote servers.  By default (disabled), Paged Results control is
              not used and responses are not accepted.  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.

       filter <pattern>
              This directive allows specifying a regex(5) pattern to indicate
              what search filter terms are actually served by a target.

              In a search request, if the search filter matches the pattern
              the target is considered while fulfilling the request; otherwise
              the target is ignored. There may be multiple occurrences of the
              filter directive for each target.

       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=<major>[.<minor>]]
              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.

       keepalive <idle>:<probes>:<interval>
              The keepalive parameter sets the values of idle, probes, and
              interval used to check whether a socket is alive; idle is the
              number of seconds a connection needs to remain idle before TCP
              starts sending keepalive probes; probes is the maximum number of
              keepalive probes TCP should send before dropping the connection;
              interval is interval in seconds between individual keepalive
              probes.  Only some systems support the customization of these
              values; the keepalive parameter is ignored otherwise, and
              system-wide settings are used.

       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.

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

       subtree-{exclude|include} <rule>
              This directive allows to indicate what subtrees are actually
              served by a target.  The syntax of the supported rules is

              <rule>: [dn[.<style>]:]<pattern>

              <style>: subtree|children|regex

              When <style> is either subtree or children the <pattern> is a DN
              that must be within the naming context served by the target.
              When <style> is regex the <pattern> is a regex(5) pattern.  If
              the dn.<style>: prefix is omitted, dn.subtree: is implicitly
              assumed for backward compatibility.

              In the subtree-exclude form if the request DN matches at least
              one rule, the target is not considered while fulfilling the
              request; otherwise, the target is considered based on the value
              of the request DN.  When the request is a search, also the scope
              is considered.

              In the subtree-include form if the request DN matches at least
              one rule, the target is considered while fulfilling the request;
              otherwise the target is ignored.

                  |  match  | exclude |
                  |    T    |    T    | not candidate     |
                  |    F    |    T    | continue checking |
                  |    T    |    F    | candidate         |
                  |    F    |    F    | not candidate     |

              There may be multiple occurrences of the subtree-exclude or
              subtree-include directive for each of the targets, but they are
              mutually exclusive.

       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 RFC
              4526 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 file an ITS 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.

       The following statements have been deprecated and should no longer be

       pseudorootdn <substitute DN in case of rootdn bind>
              Use idassert-bind instead.

       pseudorootpw <substitute password in case of rootdn bind>
              Use idassert-bind instead.

              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

OpenLDAP 2.4.40                   2014/09/20                     SLAPD-META(5)