Provided by: slapd_2.4.28-1.1ubuntu4_amd64 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