Provided by: slapd_2.4.25-1.1ubuntu4_i386 bug

NAME

       slapd-meta - metadirectory backend to slapd

SYNOPSIS

       /etc/ldap/slapd.conf

DESCRIPTION

       The  meta backend to slapd(8) performs basic LDAP proxying with respect
       to a set of remote LDAP servers,  called  "targets".   The  information
       contained  in  these  servers can be 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.

EXAMPLES

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

CONFIGURATION

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

       Note:  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.

SPECIAL CONFIGURATION DIRECTIVES

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

       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

       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
              succeeded.

       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
              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  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
              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 defined.

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

       idassert-bind    bindmethod=none|simple|sasl    [binddn=<simple    DN>]
              [credentials=<simple     password>]    [saslmech=<SASL    mech>]
              [secprops=<properties>] [realm=<realm>] [authcId=<authentication
              ID>]  [authzId=<authorization  ID>]  [authz={native|proxyauthz}]
              [mode=<mode>]     [flags=<flags>]     [starttls=no|yes|critical]
              [tls_cert=<file>]      [tls_key=<file>]      [tls_cacert=<file>]
              [tls_cacertdir=<path>]      [tls_reqcert=never|allow|try|demand]
              [tls_ciphersuite=<ciphers>]         [tls_protocol_min=<version>]
              [tls_crlcheck=none|peer|all]
              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

              none|simple|sasl

              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

              u:<user>

              [dn:]<DN>

              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

              override,[non-]prescriptive,proxy-authz-[non-]critical

              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

              [<d>d][<h>h][<m>m][<s>[s]]

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

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

       network-timeout <time>
              Sets  the  network  timeout  value after which poll(2)/select(2)
              following a connect(2) returns in  case  of  no  activity.   The
              value   is   in   seconds,  and  it  can  be  specified  as  for
              idle-timeout.   If  set  before  any  target  specification,  it
              affects   all  targets,  unless  overridden  by  any  per-target
              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
              draft-zeilenga-ldap-t-f  for  details).   If  set  to  discover,
              support is detected by reading the remote server's root DSE.  If
              set before any target specification,  it  affects  all  targets,
              unless overridden by any per-target directive.

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

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

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

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

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

SCENARIOS

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

       Consider a couple of scenarios first.

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

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

       Operations directed to a specific target can be easily resolved because
       there  are  no  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 context:

              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.

ACLs

       Note on ACLs: at present you may add whatever ACL rule you desire to to
       the  Meta  (and  LDAP)  backends.   However, the meaning of an ACL on a
       proxy  may  require  some  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 search)

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

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

REWRITING

       A string is rewritten according to a set of rules,  called  a  `rewrite
       context'.    The  rules  are  based  on  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).

Passes

       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' 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
       is:

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

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

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

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

       The operators are:

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

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

       &      variable assignment; <name> defines a variable  in  the  running
              operation  structure which can be dereferenced later; operator &
              assigns a variable in the rewrite  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
       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 `%{$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. `mail=someone@example.com'; note that the rule
       # in case of match stops rewriting; in case of error,
       # it is ignored.  In case we are mapping virtual
       # to real naming contexts, we also need to rewrite
       # regular DNs, because the definition of a bindDn
       # rewrite context overrides the default definition.
       rewriteContext bindDN
       rewriteRule "^mail=[^,]+@[^,]+$" "%{attr2dn(%0)}" ":@I"

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

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

       # 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 ...)

ACCESS CONTROL

       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.

PROXY CACHE OVERLAY

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

DEPRECATED STATEMENTS

       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.

FILES

       /etc/ldap/slapd.conf
              default slapd configuration file

SEE ALSO

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

AUTHOR

       Pierangelo Masarati, based on back-ldap by Howard Chu