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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  pre‐
       sented as belonging to a single Directory Information Tree (DIT).

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

       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 opera‐
       tion 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  spe‐
              cific 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  opera‐
              tion,  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 unwill‐
              ingToPerform 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 direc‐
              tive) 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 proto‐
              col-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://x.foo.com/dc=x,dc=foo,dc=com"

              The <naming context> part doesn't need to be unique across the targets; it may also
              match one of the values of the "suffix" directive.  Multiple URIs may be defined in
              a single 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  l1.foo.com  and
              l2.foo.com are shadows of the same server, the directive

              suffix "dc=foo,dc=com"
              uri    "ldap://l1.foo.com/dc=foo,dc=com" "ldap://l2.foo.com/"

              causes  l2.foo.com  to  be contacted whenever l1.foo.com 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 suc‐

       acl-authcDN <administrative DN for access control purposes>
              DN which is used to query the target server for acl checking, as in the LDAP  back‐
              end;  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 anonymously.

       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 per‐
              formed 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 directive.

       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

       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 consid‐
              ered 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 performed.

              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 operations.

              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 directive.

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

       Operations directed to a specific target can be  easily  resolved  because  there  are  no
       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://a.bar.com/dc=a,dc=foo,dc=com"
              suffixmassage "dc=a,dc=foo,dc=com" "dc=bar,dc=org"

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

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

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

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

       2b)  Consider  the  above  reported  scenario with the two servers sharing the same naming

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

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

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

       All the previous considerations hold, except that now there is  no  way  to  unambiguously
       resolve  a  DN.   In  this  case,  all  the  operations that require an unambiguous target
       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 considered:

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

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

       Of  course  the  latter  sounds  unreasonable,  but  it is not.  It is possible to imagine
       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: <http://www.openldap.org/its/>).  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

       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 ''extended'')

       `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'

       `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 is:

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

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

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

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

       The operators are:

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

       |      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

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

       client -> server:

              (default)            if defined and no specific context
                                   is available
              bindDN               bind
              searchBase           search
              searchFilter         search
              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

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

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

       # A search filter containing `uid=' is rewritten only
       # if an appropriate DN is bound.
       # To do this, in the first rule the bound DN is
       # dereferenced, while the filter is decomposed in a
       # prefix, in the value of the `uid=<arg>' AVA, and
       # in a suffix. A tag `<>' is appended to the DN.
       # If the DN refers to an entry in the `ou=admin' subtree,
       # the filter is rewritten OR-ing the `uid=<arg>' with
       # `cn=<arg>'; otherwise it is left as is. This could be
       # useful, for instance, to allow apache's auth_ldap-1.4
       # module to authenticate users with both `uid' and
       # `cn', but only if the request comes from a possible
       # `cn=Web auth,ou=admin,dc=home,dc=net' user.
       rewriteContext searchFilter
       rewriteRule "(.*\\()uid=([a-z0-9_]+)(\\).*)"
       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://ldap1.my.org/%0' ':@'
         rewriteRule '^cn=[m-z].*' 'ldap://ldap2.my.org/%0' ':@'
         rewriteRule '.*'          'ldap://ldap3.my.org/%0' ':@'

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

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

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

       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 used.

       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), re_format(7).

       Pierangelo Masarati, based on back-ldap by Howard Chu

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

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