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.TH SLAPD-META 5 "RELEASEDATE" "OpenLDAP LDVERSION"
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.\" Copyright 1998-2020 The OpenLDAP Foundation, All Rights Reserved.
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.\" Copying restrictions apply.  See the COPYRIGHT file.
.\" Copyright 2001, Pierangelo Masarati, All rights reserved. <ando@sys-net.it>
.\" $OpenLDAP$
.\"
.\" Portions of this document should probably be moved to slapd-ldap(5)
.\" and maybe manual pages for librewrite.
.\"
.SH NAME
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slapd\-meta \- metadirectory backend to slapd
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.SH SYNOPSIS
ETCDIR/slapd.conf
.SH DESCRIPTION
The
.B meta
backend to
.BR 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).
.LP
A basic knowledge of the functionality of the
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.BR slapd\-ldap (5)
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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
.B ldap
backend is intended to proxy operations directed to a single server, the
.B 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.
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The proxy instance of
.BR 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.

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

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.SH EXAMPLES
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There are examples in various places in this document, as well as in the
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slapd/back\-meta/data/ directory in the OpenLDAP source tree.
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.SH CONFIGURATION
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These
.B slapd.conf
options apply to the META backend database.
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That is, they must follow a "database meta" line and come before any
subsequent "backend" or "database" lines.
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Other database options are described in the
.BR slapd.conf (5)
manual page.
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.LP
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Note: In early versions of back-ldap and back-meta it was recommended to always set
.LP
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.RS
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.nf
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lastmod  off
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.fi
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.RE
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.LP
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for 
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.B ldap
and
.B meta
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databases.
This was required because operational attributes related to entry creation 
and modification should not be proxied, as they could be mistakenly written
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to the target server(s), generating an error.
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The current implementation automatically sets lastmod to \fBoff\fP, 
so its use is redundant and should be omitted.
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.SH SPECIAL CONFIGURATION DIRECTIVES
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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:
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.TP
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.B conn\-ttl <time>
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This directive causes a cached connection to be dropped an recreated
after a given ttl, regardless of being idle or not.

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.TP
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.B default\-target none
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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
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target, with the constraint that at most one must succeed.
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This directive can also be used when processing targets to mark a
specific target as default.
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.TP
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.B dncache\-ttl {DISABLED|forever|<ttl>}
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This directive sets the time-to-live of the DN cache.
This caches the target that holds a given DN to speed up target
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selection in case multiple targets would result from an uncached
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search; forever means cache never expires; disabled means no DN
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caching; otherwise a valid ( > 0 ) ttl is required, in the format
illustrated for the 
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.B idle\-timeout
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directive.
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.TP
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.B onerr {CONTINUE|report|stop}
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This directive allows one to select the behavior in case an error is returned
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by one target during a search.
The default, \fBcontinue\fP, consists in continuing the operation, 
trying to return as much data as possible.
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If the value is set to \fBstop\fP, the search is terminated as soon 
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as an error is returned by one target, and the error is immediately 
propagated to the client.
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If the value is set to \fBreport\fP, 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.
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.TP
.B norefs <NO|yes>
If
.BR 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.

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.TP
.B noundeffilter <NO|yes>
If
.BR 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
.BR (!(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.

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.TP
.B 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 \fIunwillingToPerform\fP 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.

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.TP
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.B pseudoroot\-bind\-defer {YES|no}
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This directive, when set to 
.BR yes ,
causes the authentication to the remote servers with the pseudo-root
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identity (the identity defined in each
.B idassert-bind
directive) to be deferred until actually needed by subsequent operations.
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Otherwise, all binds as the rootdn are propagated to the targets.
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.TP
.B quarantine <interval>,<num>[;<interval>,<num>[...]]
Turns on quarantine of URIs that returned
.IR 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
.I interval
seconds elapsed since last attempt, for exactly
.I num
times; then use the next pattern.
If
.I num
for the last pattern is "\fB+\fP", it retries forever; otherwise, 
no more retries occur.
This directive must appear before any target specification;
it affects all targets with the same pattern.

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.TP
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.B rebind\-as\-user {NO|yes}
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If this option is given, the client's bind credentials are remembered
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for rebinds, when trying to re-establish a broken connection,
or when chasing a referral, if 
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.B chase\-referrals
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is set to
.IR yes .
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.TP
.B 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 \fIprotocol\-version\fP to 2.
If set before any target specification, it affects all targets, unless
overridden by any per-target directive.

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

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

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.SH TARGET SPECIFICATION
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Target specification starts with a "uri" directive:
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.TP
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.B uri <protocol>://[<host>]/<naming context> [...]
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The <protocol> part can be anything
.BR ldap_initialize (3)
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accepts ({ldap|ldaps|ldapi} and variants); the <host> may be
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omitted, defaulting to whatever is set in
.BR ldap.conf (5).
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The <naming context> part is \fImandatory\fP for the first URI,
but it \fImust be omitted\fP for subsequent ones, if any.
The naming context part must be within the naming context defined for the backend,
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e.g.:
.LP
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.RS
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.nf
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suffix "\fBdc=foo,dc=com\fP"
uri    "ldap://x.foo.com/dc=x,\fBdc=foo,dc=com\fP"
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.fi
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.RE
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.RS
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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.
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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
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to contact the first server of the list that responds.
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For example, if \fIl1.foo.com\fP and \fIl2.foo.com\fP are shadows
of the same server, the directive
.LP
.nf
suffix "\fBdc=foo,dc=com\fP"
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uri    "ldap://l1.foo.com/\fBdc=foo,dc=com\fP" "ldap://l2.foo.com/"
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.fi

.RE
.RS
causes \fIl2.foo.com\fP to be contacted whenever \fIl1.foo.com\fP
does not respond.
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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.
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.RE
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.TP
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.B acl\-authcDN "<administrative DN for access control purposes>"
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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.
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There is no risk of giving away such values; they are only used to
check permissions.
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.B The acl\-authcDN identity is by no means implicitly used by the proxy 
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.B when the client connects anonymously.
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.TP
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.B acl\-passwd <password>
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Password used with the
.B 
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acl\-authcDN
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above.
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.TP
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.B bind\-timeout <microseconds>
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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
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.BR bind\-timeout .
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The default value is used also for subsequent calls if
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.B bind\-timeout
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is not specified.
If set before any target specification, it affects all targets, unless
overridden by any per-target directive.

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

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.TP
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.B default\-target [<target>]
The "default\-target" directive can also be used during target specification.
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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.
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.TP
.B filter <pattern>
This directive allows specifying a
.BR regex (5)
pattern to indicate what search filter terms are actually served by a target.

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

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

.HP
.hy 0
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.B idassert\-bind
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.B bindmethod=none|simple|sasl [binddn=<simple DN>] [credentials=<simple password>]
.B [saslmech=<SASL mech>] [secprops=<properties>] [realm=<realm>]
.B [authcId=<authentication ID>] [authzId=<authorization ID>]
.B [authz={native|proxyauthz}] [mode=<mode>] [flags=<flags>]
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.B [starttls=no|yes|critical]
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.B [tls_cert=<file>]
.B [tls_key=<file>]
.B [tls_cacert=<file>]
.B [tls_cacertdir=<path>]
.B [tls_reqcert=never|allow|try|demand]
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.B [tls_cipher_suite=<ciphers>]
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.B [tls_protocol_min=<major>[.<minor>]]
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.B [tls_crlcheck=none|peer|all]
.RS
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Allows one to define the parameters of the authentication method that is
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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
.B proxyAuthz
privileges on a wide set of DNs, e.g.
.BR authzTo=dn.subtree:"" ,
and the remote server to have
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.B authz\-policy
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set to
.B to
or
.BR both .
See
.BR slapd.conf (5)
for details on these statements and for remarks and drawbacks about
their usage.
The supported bindmethods are

\fBnone|simple|sasl\fP

where
.B none
is the default, i.e. no \fIidentity assertion\fP is performed.

The authz parameter is used to instruct the SASL bind to exploit 
.B 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 
.B authzDN
or
.B authzID
parameters).
Otherwise, the default
.B proxyauthz
is used, i.e. the proxyAuthz control (Proxied Authorization, RFC 4370)
is added to all operations.

The supported modes are:

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

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

\fBu:<user>\fP

\fB[dn:]<DN>\fP

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

The default mode is 
.BR legacy ,
which implies that the proxy will either perform a simple bind as the
.I authcDN
or a SASL bind as the
.I 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
.IR authcDN
or a SASL bind as the
.IR authcID ,
unless restricted by
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.BR idassert\-authzFrom
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rules (see below), in which case the operation will fail;
eventually, it will assert some other identity according to
.BR <mode> .
Other identity assertion modes are
.BR anonymous
and
.BR self ,
which respectively mean that the 
.I empty 
or the 
.IR client 's 
identity
will be asserted;
.BR none ,
which means that no proxyAuthz control will be used, so the
.I authcDN
or the
.I authcID
identity will be asserted.
For all modes that require the use of the
.I proxyAuthz 
control, on the remote server the proxy identity must have appropriate 
.I authzTo
permissions, or the asserted identities must have appropriate
.I 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

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\fBoverride,[non\-]prescriptive,proxy\-authz\-[non\-]critical\fP
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When the 
.B 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
.B prescriptive
flag is used (the default), operations fail with
\fIinappropriateAuthentication\fP
for those identities whose assertion is not allowed by the
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.B idassert\-authzFrom
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patterns.
If the 
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.B non\-prescriptive
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flag is used, operations are performed anonymously for those identities 
whose assertion is not allowed by the
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.B idassert\-authzFrom
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patterns.

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

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

The identity associated to this directive is also used for privileged
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operations whenever \fBidassert\-bind\fP is defined and \fBacl\-bind\fP
is not.  See \fBacl\-bind\fP for details.
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.RE

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.TP
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.B idle\-timeout <time>
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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.
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If set before any target specification, it affects all targets, unless
overridden by any per-target directive.

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

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.TP
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.B map "{attribute|objectclass} [<local name>|*] {<foreign name>|*}"
This maps object classes and attributes as in the LDAP backend.
See
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.BR slapd\-ldap (5).
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.TP
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.B network\-timeout <time>
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Sets the network timeout value after which
.BR poll (2)/ select (2) 
following a 
.BR connect (2) 
returns in case of no activity.
The value is in seconds, and it can be specified as for
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.BR idle\-timeout .
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If set before any target specification, it affects all targets, unless
overridden by any per-target directive.

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.TP
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.B 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,
.BR 3
times);
the global value can be overridden by redefinitions inside each target
specification.
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.TP
.B rewrite* ...
The rewrite options are described in the "REWRITING" section.
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.TP
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.B subtree\-{exclude|include} "<rule>"
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This directive allows one to indicate what subtrees are actually served
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by a target.
The syntax of the supported rules is

\fB<rule>: [dn[.<style>]:]<pattern>\fP

\fB<style>: subtree|children|regex\fP

When \fB<style>\fP is either \fBsubtree\fP or \fBchildren\fP
the \fB<pattern>\fP is a DN that must be within the naming context
served by the target.
When \fB<style>\fP is \fBregex\fP the \fB<pattern>\fP is a
.BR regex (5)
pattern.
If the \fBdn.<style>:\fP prefix is omitted, \fBdn.subtree:\fP
is implicitly assumed for backward compatibility.

In the
.B subtree\-exclude
form if the \fIrequest DN\fP 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 \fIrequest DN\fP.
When the request is a search, also the \fIscope\fP is considered.

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

.LP
.RS
.nf
    |  match  | exclude |
    +---------+---------+-------------------+
    |    T    |    T    | not candidate     |
    |    F    |    T    | continue checking |
    +---------+---------+-------------------+
    |    T    |    F    | candidate         |
    |    F    |    F    | not candidate     |
    +---------+---------+-------------------+
.fi

.RE
.RS
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There may be multiple occurrences of the
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.B subtree\-exclude
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or
.B subtree\-include
directive for each of the targets, but they are mutually exclusive.
.RE
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.TP
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.B suffixmassage "<virtual naming context>" "<real naming context>"
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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
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massaging.  See the "REWRITING" section for a detailed list 
of the rewrite rules it implies.
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.TP
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.B t\-f\-support {NO|yes|discover}
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enable if the remote server supports absolute filters
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(see \fIRFC 4526\fP for details).
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If set to
.BR 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.

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

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

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

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

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.SH SCENARIOS
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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.
.LP
Consider a couple of scenarios first.
.LP
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:
.LP
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.nf
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database meta
suffix   "\fBdc=foo,dc=com\fP"
uri      "ldap://a.foo.com/dc=a,\fBdc=foo,dc=com\fP"
uri      "ldap://b.foo.com/dc=b,\fBdc=foo,dc=com\fP"
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.fi
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.RE
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.LP
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.
.LP
2a) Two directory servers don't share any portion of naming context,
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but they'd present as a single DIT
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[Caveat: uniqueness of (massaged) entries among the two servers is
assumed; integrity checks risk to incur in excessive overhead and have
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not been implemented].
Say we have "dc=bar,dc=org" and "o=Foo,c=US",
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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:
.LP
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.nf
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database      meta
suffix        "dc=foo,dc=com"

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

uri           "ldap://b.foo.com/\fBdc=b,dc=foo,dc=com\fP"
suffixmassage "\fBdc=b,dc=foo,dc=com\fP" "o=Foo,c=US"
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.fi
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.RE
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.LP
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.
.LP
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).
.LP
Finally, if the scope is "sub" the incoming base is replaced
by each target's unmassaged naming context, and the scope
is not altered.
.LP
2b) Consider the above reported scenario with the two servers
sharing the same naming context:
.LP
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.nf
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database      meta
suffix        "\fBdc=foo,dc=com\fP"

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

uri           "ldap://b.foo.com/\fBdc=foo,dc=com\fP"
suffixmassage "\fBdc=foo,dc=com\fP" "o=Foo,c=US"
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.RE
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.LP
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
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selection will fail unless the DN is already cached or a default
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target has been set.
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Practical configurations may result as a combination of all the
above scenarios.
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.SH ACLs
Note on ACLs: at present you may add whatever ACL rule you desire
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to the Meta (and LDAP) backends.
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However, the meaning of an ACL on a proxy may require some
considerations.
Two philosophies may be considered:
.LP
a) the remote server dictates the permissions; the proxy simply passes
back what it gets from the remote server.
.LP
b) the remote server unveils "everything"; the proxy is responsible
for protecting data from unauthorized access.
.LP
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
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slapd except a special subtle case (please file an ITS if you can
find other exceptions: <http://www.openldap.org/its/>).
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The rule
.LP
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.nf
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access to dn="<dn>" attrs=<attr>
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       by dnattr=<dnattr> read
       by * none
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.fi
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.RE
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.LP
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)
.LP
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!
.LP
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".
.\"
.\" If this section is moved, also update the reference in
.\" libraries/librewrite/RATIONALE.
.\"
.SH REWRITING
A string is rewritten according to a set of rules, called a `rewrite
context'.
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The rules are based on POSIX (''extended'') regular expressions (regex)
with substring matching; basic variable substitution and map resolution 
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of substrings is allowed by specific mechanisms detailed in the following.
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The behavior of pattern matching/substitution can be altered by a set
of flags.
.LP
The underlying concept is to build a lightweight rewrite module
for the slapd server (initially dedicated to the LDAP backend).
.SH Passes
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An incoming string is matched against a set of rules.
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Rules are made of a regex match pattern, a substitution pattern
and a set of actions, described by a set of flags.
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In case of match a string rewriting is performed according to the
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substitution pattern that allows one to refer to substrings matched in the
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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.
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The flags are divided in "Pattern matching Flags" and "Action Flags";
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the former alter the regex match pattern behavior while the latter
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alter the action that is taken after substitution.
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.SH "Pattern Matching Flags"
.TP
.B `C'
honors case in matching (default is case insensitive)
.TP
.B `R'
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use POSIX ''basic'' regular expressions (default is ''extended'')
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.TP
.B `M{n}'
allow no more than
.B 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.
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.SH "Action Flags"
.TP
.B `:'
apply the rule once only (default is recursive)
.TP
.B `@'
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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.
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.TP
.B `#'
stop current operation if the rule matches, and issue an `unwilling to
perform' error.
.TP
.B `G{n}'
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jump
.B n
rules back and forth (watch for loops!).
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Note that `G{1}' is implicit in every rule.
.TP
.B `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.
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.TP
.B `U{n}'
uses
.B
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.
.B `: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
.BR n ;
or, in other words, `@' is equivalent to `U{0}'.
By convention, the freely available codes are above 16 included;
the others are reserved.
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The ordering of the flags can be significant.
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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
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errors, but jump two lines ahead only in case of match.
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.LP
More flags (mainly Action Flags) will be added as needed.
.SH "Pattern matching:"
See
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.BR regex (7)
and/or
.BR re_format (7).
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.SH "Substitution Pattern Syntax:"
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Everything starting with `%' requires substitution;
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.LP
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the only obvious exception is `%%', which is left as is;
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.LP
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the basic substitution is `%d', where `d' is a digit;
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0 means the whole string, while 1-9 is a submatch;
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.LP
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a `%' followed by a `{' invokes an advanced substitution.
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The pattern is:
.LP
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.RS
`%' `{' [ <op> ] <name> `(' <substitution> `)' `}'
.RE
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.LP
where <name> must be a legal name for the map, i.e.
.LP
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.nf
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<name> ::= [a-z][a-z0-9]* (case insensitive)
<op> ::= `>' `|' `&' `&&' `*' `**' `$'
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.fi
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.RE
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.LP
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and <substitution> must be a legal substitution
pattern, with no limits on the nesting level.
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.LP
The operators are:
.TP
.B >
sub context invocation; <name> must be a legal, already defined
rewrite context name
.TP
.B |
external command invocation; <name> must refer to a legal, already
defined command name (NOT IMPL.)
.TP
.B &
variable assignment; <name> defines a variable in the running
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operation structure which can be dereferenced later; operator
.B &
assigns a variable in the rewrite context scope; operator
.B &&
assigns a variable that scopes the entire session, e.g. its value
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can be dereferenced later by other rewrite contexts
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.TP
.B *
variable dereferencing; <name> must refer to a variable that is
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defined and assigned for the running operation; operator
.B *
dereferences a variable scoping the rewrite context; operator
.B **
dereferences a variable scoping the whole session, e.g. the value
is passed across rewrite contexts
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.TP
.B $
parameter dereferencing; <name> must refer to an existing parameter;
the idea is to make some run-time parameters set by the system
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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 
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