guide/admin/overlays.sdf

# OpenLDAP: pkg/openldap-guide/admin/overlays.sdf,v 1.8.2.30 2010/04/15 18:25:42 quanah Exp
# Copyright 2007-2010 The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.

H1: Overlays

Overlays are software components that provide hooks to functions analogous to
those provided by backends, which can be stacked on top of the backend calls
and as callbacks on top of backend responses to alter their behavior.

Overlays may be compiled statically into {{slapd}}, or when module support
is enabled, they may be dynamically loaded. Most of the overlays
are only allowed to be configured on individual databases.

Some can be stacked on the {{EX:frontend}} as well, for global use. This means that
they can be executed after a request is parsed and validated, but right before the
appropriate database is selected. The main purpose is to affect operations
regardless of the database they will be handled by, and, in some cases,
to influence the selection of the database by massaging the request DN.

Essentially, overlays represent a means to:

    * customize the behavior of existing backends without changing the backend
      code and without requiring one to write a new custom backend with
      complete functionality
    * write functionality of general usefulness that can be applied to
      different backend types

When using {{slapd.conf}}(5), overlays that are configured before any other
databases are considered global, as mentioned above. In fact they are implicitly
stacked on top of the {{EX:frontend}} database. They can also be explicitly
configured as such:

>        database frontend
>        overlay <overlay name>

Overlays are usually documented by separate specific man pages in section 5;
the naming convention is

>        slapo-<overlay name>

All distributed core overlays have a man page. Feel free to contribute to any,
if you think there is anything missing in describing the behavior of the component
and the implications of all the related configuration directives.

Official overlays are located in

>        servers/slapd/overlays/

That directory also contains the file slapover.txt, which describes the
rationale of the overlay implementation, and may serve as a guideline for the
development of custom overlays.

Contribware overlays are located in

>        contrib/slapd-modules/<overlay name>/

along with other types of run-time loadable components; they are officially
distributed, but not maintained by the project.

All the current overlays in OpenLDAP are listed and described in detail in the
following sections.


H2: Access Logging


H3: Overview

This overlay can record accesses to a given backend database on another
database.

This allows all of the activity on a given database to be reviewed using arbitrary
LDAP queries, instead of just logging to local flat text files. Configuration
options are available for selecting a subset of operation types to log, and to
automatically prune older log records from the logging database. Log records
are stored with audit schema to assure their readability whether viewed as LDIF
or in raw form.

It is also used for {{SECT:delta-syncrepl replication}}

H3: Access Logging Configuration

The following is a basic example that implements Access Logging:

>        database bdb
>        suffix dc=example,dc=com
>        ...
>        overlay accesslog
>        logdb cn=log
>        logops writes reads
>        logold (objectclass=person)
>
>        database bdb
>        suffix cn=log
>        ...
>        index reqStart eq
>        access to *
>          by dn.base="cn=admin,dc=example,dc=com" read

The following is an example used for {{SECT:delta-syncrepl replication}}:

>        database hdb
>        suffix cn=accesslog
>        directory /usr/local/var/openldap-accesslog
>        rootdn cn=accesslog
>        index default eq
>        index entryCSN,objectClass,reqEnd,reqResult,reqStart

Accesslog overlay definitions for the primary db

>        database bdb
>        suffix dc=example,dc=com
>        ...
>        overlay accesslog
>        logdb cn=accesslog
>        logops writes
>        logsuccess TRUE
>        # scan the accesslog DB every day, and purge entries older than 7 days
>        logpurge 07+00:00 01+00:00

An example search result against {{B:cn=accesslog}} might look like:

>        [ghenry@suretec ghenry]# ldapsearch -x -b cn=accesslog
>        # extended LDIF
>        #
>        # LDAPv3
>        # base <cn=accesslog> with scope subtree
>        # filter: (objectclass=*)
>        # requesting: ALL
>        #
>
>        # accesslog
>        dn: cn=accesslog
>        objectClass: auditContainer
>        cn: accesslog
>
>        # 20080110163829.000004Z, accesslog
>        dn: reqStart=20080110163829.000004Z,cn=accesslog
>        objectClass: auditModify
>        reqStart: 20080110163829.000004Z
>        reqEnd: 20080110163829.000005Z
>        reqType: modify
>        reqSession: 196696
>        reqAuthzID: cn=admin,dc=suretecsystems,dc=com
>        reqDN: uid=suretec-46022f8$,ou=Users,dc=suretecsystems,dc=com
>        reqResult: 0
>        reqMod: sambaPwdCanChange:- ###CENSORED###
>        reqMod: sambaPwdCanChange:+ ###CENSORED###
>        reqMod: sambaNTPassword:- ###CENSORED###
>        reqMod: sambaNTPassword:+ ###CENSORED###
>        reqMod: sambaPwdLastSet:- ###CENSORED###
>        reqMod: sambaPwdLastSet:+ ###CENSORED###
>        reqMod: entryCSN:= 20080110163829.095157Z#000000#000#000000
>        reqMod: modifiersName:= cn=admin,dc=suretecsystems,dc=com
>        reqMod: modifyTimestamp:= 20080110163829Z
>
>        # search result
>        search: 2
>        result: 0 Success
>
>        # numResponses: 3
>        # numEntries: 2


H3: Further Information

{{slapo-accesslog(5)}} and the {{SECT:delta-syncrepl replication}} section.


H2: Audit Logging

The Audit Logging overlay can be used to record all changes on a given backend database to a specified log file.

H3: Overview

If the need arises whereby changes need to be logged as standard LDIF, then the auditlog overlay {{B:slapo-auditlog (5)}}
can be used. Full examples are available in the man page {{B:slapo-auditlog (5)}}

H3: Audit Logging Configuration

If the directory is running vi {{F:slapd.d}}, then the following LDIF could be used to add the overlay to the overlay list
in {{B:cn=config}} and set what file the {{TERM:LDIF}} gets logged to (adjust to suit)

>       dn: olcOverlay=auditlog,olcDatabase={1}hdb,cn=config
>       changetype: add
>       objectClass: olcOverlayConfig
>       objectClass: olcAuditLogConfig
>       olcOverlay: auditlog
>       olcAuditlogFile: /tmp/auditlog.ldif


In this example for testing, we are logging changes to {{F:/tmp/auditlog.ldif}}

A typical {{TERM:LDIF}} file created by {{B:slapo-auditlog(5)}} would look like:

>       # add 1196797576 dc=suretecsystems,dc=com cn=admin,dc=suretecsystems,dc=com
>       dn: dc=suretecsystems,dc=com
>       changetype: add
>       objectClass: dcObject
>       objectClass: organization
>       dc: suretecsystems
>       o: Suretec Systems Ltd.
>       structuralObjectClass: organization
>       entryUUID: 1606f8f8-f06e-1029-8289-f0cc9d81e81a
>       creatorsName: cn=admin,dc=suretecsystems,dc=com
>       modifiersName: cn=admin,dc=suretecsystems,dc=com
>       createTimestamp: 20051123130912Z
>       modifyTimestamp: 20051123130912Z
>       entryCSN: 20051123130912.000000Z#000001#000#000000
>       auditContext: cn=accesslog
>       # end add 1196797576
>
>       # add 1196797577 dc=suretecsystems,dc=com cn=admin,dc=suretecsystems,dc=com
>       dn: ou=Groups,dc=suretecsystems,dc=com
>       changetype: add
>       objectClass: top
>       objectClass: organizationalUnit
>       ou: Groups
>       structuralObjectClass: organizationalUnit
>       entryUUID: 160aaa2a-f06e-1029-828a-f0cc9d81e81a
>       creatorsName: cn=admin,dc=suretecsystems,dc=com
>       modifiersName: cn=admin,dc=suretecsystems,dc=com
>       createTimestamp: 20051123130912Z
>       modifyTimestamp: 20051123130912Z
>       entryCSN: 20051123130912.000000Z#000002#000#000000
>       # end add 1196797577


H3: Further Information

{{:slapo-auditlog(5)}}


H2: Chaining


H3: Overview

The chain overlay provides basic chaining capability to the underlying
database.

What is chaining? It indicates the capability of a DSA to follow referrals on
behalf of the client, so that distributed systems are viewed as a single
virtual DSA by clients that are otherwise unable to "chase" (i.e. follow)
referrals by themselves.

The chain overlay is built on top of the ldap backend; it is compiled by
default when {{B:--enable-ldap}}.


H3: Chaining Configuration

In order to demonstrate how this overlay works, we shall discuss a typical
scenario which might be one master server and three Syncrepl slaves.

On each replica, add this near the top of the {{slapd.conf}}(5) file
(global), before any database definitions:

>        overlay                    chain
>        chain-uri                  "ldap://ldapmaster.example.com"
>        chain-idassert-bind        bindmethod="simple"
>                                   binddn="cn=Manager,dc=example,dc=com"
>                                   credentials="<secret>"
>                                   mode="self"
>        chain-tls                  start
>        chain-return-error         TRUE

Add this below your {{syncrepl}} statement:

>        updateref                  "ldap://ldapmaster.example.com/"

The {{B:chain-tls}} statement enables TLS from the slave to the ldap master.
The DITs are exactly the same between these machines, therefore whatever user
bound to the slave will also exist on the master. If that DN does not have
update privileges on the master, nothing will happen.

You will need to restart the slave after these {{slapd.conf}} changes.
Then, if you are using {{loglevel stats}} (256), you can monitor an
{{ldapmodify}} on the slave and the master. (If you're using {{cn=config}}
no restart is required.)

Now start an {{ldapmodify}} on the slave and watch the logs. You should expect
something like:

>        Sep  6 09:27:25 slave1 slapd[29274]: conn=11 fd=31 ACCEPT from IP=143.199.102.216:45181 (IP=143.199.102.216:389)
>        Sep  6 09:27:25 slave1 slapd[29274]: conn=11 op=0 STARTTLS
>        Sep  6 09:27:25 slave1 slapd[29274]: conn=11 op=0 RESULT oid= err=0 text=
>        Sep  6 09:27:25 slave1 slapd[29274]: conn=11 fd=31 TLS established tls_ssf=256 ssf=256
>        Sep  6 09:27:28 slave1 slapd[29274]: conn=11 op=1 BIND dn="uid=user1,ou=people,dc=example,dc=com" method=128
>        Sep  6 09:27:28 slave1 slapd[29274]: conn=11 op=1 BIND dn="uid=user1,ou=People,dc=example,dc=com" mech=SIMPLE ssf=0
>        Sep  6 09:27:28 slave1 slapd[29274]: conn=11 op=1 RESULT tag=97 err=0 text=
>        Sep  6 09:27:28 slave1 slapd[29274]: conn=11 op=2 MOD dn="uid=user1,ou=People,dc=example,dc=com"
>        Sep  6 09:27:28 slave1 slapd[29274]: conn=11 op=2 MOD attr=mail
>        Sep  6 09:27:28 slave1 slapd[29274]: conn=11 op=2 RESULT tag=103 err=0 text=
>        Sep  6 09:27:28 slave1 slapd[29274]: conn=11 op=3 UNBIND
>        Sep  6 09:27:28 slave1 slapd[29274]: conn=11 fd=31 closed
>        Sep  6 09:27:28 slave1 slapd[29274]: syncrepl_entry: LDAP_RES_SEARCH_ENTRY(LDAP_SYNC_MODIFY)
>        Sep  6 09:27:28 slave1 slapd[29274]: syncrepl_entry: be_search (0)
>        Sep  6 09:27:28 slave1 slapd[29274]: syncrepl_entry: uid=user1,ou=People,dc=example,dc=com
>        Sep  6 09:27:28 slave1 slapd[29274]: syncrepl_entry: be_modify (0)

And on the master you will see this:

>        Sep  6 09:23:57 ldapmaster slapd[2961]: conn=55902 op=3 PROXYAUTHZ dn="uid=user1,ou=people,dc=example,dc=com"
>        Sep  6 09:23:57 ldapmaster slapd[2961]: conn=55902 op=3 MOD dn="uid=user1,ou=People,dc=example,dc=com"
>        Sep  6 09:23:57 ldapmaster slapd[2961]: conn=55902 op=3 MOD attr=mail
>        Sep  6 09:23:57 ldapmaster slapd[2961]: conn=55902 op=3 RESULT tag=103 err=0 text=

Note: You can clearly see the PROXYAUTHZ line on the master, indicating the
proper identity assertion for the update on the master. Also note the slave
immediately receiving the Syncrepl update from the master.

H3: Handling Chaining Errors

By default, if chaining fails, the original referral is returned to the client
under the assumption that the client might want to try and follow the referral.

With the following directive however, if the chaining fails at the provider
side, the actual error is returned to the client.

>        chain-return-error TRUE


H3: Read-Back of Chained Modifications

Occasionally, applications want to read back the data that they just wrote.
If a modification requested to a shadow server was silently chained to its
producer, an immediate read could result in receiving data not yet synchronized.
In those cases, clients should use the {{B:dontusecopy}} control to ensure
they are directed to the authoritative source for that piece of data.

This control usually causes a referral to the actual source of the data
to be returned.  However, when the {{slapo-chain(5)}} overlay is used,
it intercepts the referral being returned in response to the
{{B:dontusecopy}} control, and tries to fetch the requested data.


H3: Further Information

{{:slapo-chain(5)}}


H2: Constraints


H3: Overview

This overlay enforces a regular expression constraint on all values
of specified attributes during an LDAP modify request that contains add or modify
commands. It is used to enforce a more rigorous syntax when the underlying attribute
syntax is too general.


H3: Constraint Configuration

Configuration via {{slapd.conf}}(5) would look like:

>        overlay constraint
>        constraint_attribute mail regex ^[[:alnum:]]+@mydomain.com$
>        constraint_attribute title uri
>        ldap:///dc=catalog,dc=example,dc=com?title?sub?(objectClass=titleCatalog)

A specification like the above would reject any {{mail}} attribute which did not
look like {{<alpha-numeric string>@mydomain.com}}.

It would also reject any title attribute whose values were not listed in the
title attribute of any {{titleCatalog}} entries in the given scope.

An example for use with {{cn=config}}:

>       dn: olcOverlay=constraint,olcDatabase={1}hdb,cn=config
>       changetype: add
>       objectClass: olcOverlayConfig
>       objectClass: olcConstraintConfig
>       olcOverlay: constraint
>       olcConstraintAttribute: mail regex ^[[:alnum:]]+@mydomain.com$
>       olcConstraintAttribute: title uri ldap:///dc=catalog,dc=example,dc=com?title?sub?(objectClass=titleCatalog)


H3: Further Information

{{:slapo-constraint(5)}}


H2: Dynamic Directory Services


H3: Overview

The {{dds}} overlay to {{slapd}}(8) implements dynamic objects as per {{REF:RFC2589}}.
The name {{dds}} stands for Dynamic Directory Services. It allows to define
dynamic objects, characterized by the {{dynamicObject}} objectClass.

Dynamic objects have a limited lifetime, determined by a time-to-live (TTL)
that can be refreshed by means of a specific refresh extended operation. This
operation allows to set the Client Refresh Period (CRP), namely the period
between refreshes that is required to preserve the dynamic object from expiration.
The expiration time is computed by adding the requested TTL to the current time.
When dynamic objects reach the end of their lifetime without being further
refreshed, they are automatically {{deleted}}. There is no guarantee of immediate
deletion, so clients should not count on it.

H3: Dynamic Directory Service Configuration

A usage of dynamic objects might be to implement dynamic meetings; in this case,
all the participants to the meeting are allowed to refresh the meeting object,
but only the creator can delete it (otherwise it will be deleted when the TTL expires).

If we add the overlay to an example database, specifying a Max TTL of 1 day, a
min of 10 seconds, with a default TTL of 1 hour. We'll also specify an interval
of 120 (less than 60s might be too small) seconds between expiration checks and a
tolerance of 5 second (lifetime of a dynamic object will be {{entryTtl + tolerance}}).

>       overlay dds
>       dds-max-ttl     1d
>       dds-min-ttl     10s
>       dds-default-ttl 1h
>       dds-interval    120s
>       dds-tolerance   5s

and add an index:

>       entryExpireTimestamp

Creating a meeting is as simple as adding the following:

>       dn: cn=OpenLDAP Documentation Meeting,ou=Meetings,dc=example,dc=com
>       objectClass: groupOfNames
>       objectClass: dynamicObject
>       cn: OpenLDAP Documentation Meeting
>       member: uid=ghenry,ou=People,dc=example,dc=com
>       member: uid=hyc,ou=People,dc=example,dc=com

H4: Dynamic Directory Service ACLs

Allow users to start a meeting and to join it; restrict refresh to the {{member}};
restrict delete to the creator:

>       access to attrs=userPassword
>          by self write
>          by * read
>
>       access to dn.base="ou=Meetings,dc=example,dc=com"
>                 attrs=children
>            by users write
>
>       access to dn.onelevel="ou=Meetings,dc=example,dc=com"
>                 attrs=entry
>            by dnattr=creatorsName write
>            by * read
>
>       access to dn.onelevel="ou=Meetings,dc=example,dc=com"
>                 attrs=participant
>            by dnattr=creatorsName write
>            by users selfwrite
>            by * read
>
>       access to dn.onelevel="ou=Meetings,dc=example,dc=com"
>                 attrs=entryTtl
>            by dnattr=member manage
>            by * read

In simple terms, the user who created the {{OpenLDAP Documentation Meeting}} can add new attendees,
refresh the meeting using (basically complete control):

>       ldapexop -x -H ldap://ldaphost "refresh" "cn=OpenLDAP Documentation Meeting,ou=Meetings,dc=example,dc=com" "120" -D "uid=ghenry,ou=People,dc=example,dc=com" -W

Any user can join the meeting, but not add another attendee, but they can refresh the meeting. The ACLs above are quite straight forward to understand.


H3: Further Information

{{:slapo-dds(5)}}


H2: Dynamic Groups


H3: Overview

This overlay extends the Compare operation to detect
members of a dynamic group. This overlay is now deprecated
as all of its functions are available using the
{{SECT:Dynamic Lists}} overlay.


H3: Dynamic Group Configuration


H2: Dynamic Lists


H3: Overview

This overlay allows expansion of dynamic groups and lists. Instead of having the
group members or list attributes hard coded, this overlay allows us to define
an LDAP search whose results will make up the group or list.

H3: Dynamic List Configuration

This module can behave both as a dynamic list and dynamic group, depending on
the configuration. The syntax is as follows:

>       overlay dynlist
>       dynlist-attrset <group-oc> <URL-ad> [member-ad]

The parameters to the {{F:dynlist-attrset}} directive have the following meaning:
* {{F:<group-oc>}}: specifies which object class triggers the subsequent LDAP search.
Whenever an entry with this object class is retrieved, the search is performed.
* {{F:<URL-ad>}}: is the name of the attribute which holds the search URI. It
has to be a subtype of {{F:labeledURI}}. The attributes and values present in
the search result are added to the entry unless {{F:member-ad}} is used (see
below).
* {{F:member-ad}}: if present, changes the overlay behavior into a dynamic group.
Instead of inserting the results of the search in the entry, the distinguished name
of the results are added as values of this attribute.

Here is an example which will allow us to have an email alias which automatically
expands to all user's emails according to our LDAP filter:

In {{slapd.conf}}(5):

>       overlay dynlist
>       dynlist-attrset nisMailAlias labeledURI

This means that whenever an entry which has the {{F:nisMailAlias}} object class is
retrieved, the search specified in the {{F:labeledURI}} attribute is performed.

Let's say we have this entry in our directory:

>       cn=all,ou=aliases,dc=example,dc=com
>       cn: all
>       objectClass: nisMailAlias
>       labeledURI: ldap:///ou=People,dc=example,dc=com?mail?one?(objectClass=inetOrgPerson)

If this entry is retrieved, the search specified in {{F:labeledURI}} will be
performed and the results will be added to the entry just as if they have always
been there. In this case, the search filter selects all entries directly
under {{F:ou=People}} that have the {{F:inetOrgPerson}} object class and retrieves
the {{F:mail}} attribute, if it exists.

This is what gets added to the entry when we have two users under {{F:ou=People}}
that match the filter:
!import "allmail-en.png"; align="center"; title="Dynamic list for email aliases"
FT[align="Center"] Figure X.Y: Dynamic List for all emails

The configuration for a dynamic group is similar. Let's see an example which would
automatically populate an {{F:allusers}} group with all the user accounts in the
directory.

In {{F:slapd.conf}}(5):

>       include /path/to/dyngroup.schema
>       ...
>       overlay dynlist
>       dynlist-attrset groupOfURLs labeledURI member
+
+Note: We must include the {{F:dyngroup.schema}} file that defines the
+{{F:groupOfURLs}} objectClass used in this example.

Let's apply it to the following entry:

>       cn=allusers,ou=group,dc=example,dc=com
>       cn: all
>       objectClass: groupOfURLs
>       labeledURI: ldap:///ou=people,dc=example,dc=com??one?(objectClass=inetOrgPerson)

The behavior is similar to the dynamic list configuration we had before:
whenever an entry with the {{F:groupOfURLs}} object class is retrieved, the
search specified in the {{F:labeledURI}} attribute is performed. But this time,
only the distinguished names of the results are added, and as values of the
{{F:member}} attribute.

This is what we get:
!import "allusersgroup-en.png"; align="center"; title="Dynamic group for all users"
FT[align="Center"] Figure X.Y: Dynamic Group for all users

Note that a side effect of this scheme of dynamic groups is that the members
need to be specified as full DNs. So, if you are planning in using this for
{{F:posixGroup}}s, be sure to use RFC2307bis and some attribute which can hold
distinguished names. The {{F:memberUid}} attribute used in the {{F:posixGroup}}
object class can hold only names, not DNs, and is therefore not suitable for
dynamic groups.


H3: Further Information

{{:slapo-dynlist(5)}}


H2: Reverse Group Membership Maintenance

H3: Overview

In some scenarios, it may be desirable for a client to be able to determine
which groups an entry is a member of, without performing an additional search.
Examples of this are applications using the {{TERM:DIT}} for access control
based on group authorization.

The {{B:memberof}} overlay updates an attribute (by default {{B:memberOf}}) whenever
changes occur to the membership attribute (by default {{B:member}}) of entries of the
objectclass (by default {{B:groupOfNames}}) configured to trigger updates.

Thus, it provides maintenance of the list of groups an entry is a member of,
when usual maintenance of groups is done by modifying the members on the group
entry.

H3: Member Of Configuration

The typical use of this overlay requires just enabling the overlay for a
specific database. For example, with the following minimal slapd.conf:

>        include /usr/share/openldap/schema/core.schema
>        include /usr/share/openldap/schema/cosine.schema
>
>        authz-regexp "gidNumber=0\\\+uidNumber=0,cn=peercred,cn=external,cn=auth"
>                "cn=Manager,dc=example,dc=com"
>        database        bdb
>        suffix          "dc=example,dc=com"
>        rootdn          "cn=Manager,dc=example,dc=com"
>        rootpw          secret
>        directory       /var/lib/ldap2.4
>        checkpoint 256 5
>        index   objectClass   eq
>        index   uid           eq,sub
>
>        overlay memberof

adding the following ldif:

>        cat memberof.ldif
>        dn: dc=example,dc=com
>        objectclass: domain
>        dc: example
>
>        dn: ou=Group,dc=example,dc=com
>        objectclass: organizationalUnit
>        ou: Group
>
>        dn: ou=People,dc=example,dc=com
>        objectclass: organizationalUnit
>        ou: People
>
>        dn: uid=test1,ou=People,dc=example,dc=com
>        objectclass: account
>        uid: test1
>
>        dn: cn=testgroup,ou=Group,dc=example,dc=com
>        objectclass: groupOfNames
>        cn: testgroup
>        member: uid=test1,ou=People,dc=example,dc=com

Results in the following output from a search on the test1 user:

> # ldapsearch -LL -Y EXTERNAL -H ldapi:/// "(uid=test1)" -b dc=example,dc=com memberOf
> SASL/EXTERNAL authentication started
> SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
> SASL SSF: 0
> version: 1
>
> dn: uid=test1,ou=People,dc=example,dc=com
> memberOf: cn=testgroup,ou=Group,dc=example,dc=com

Note that the {{B:memberOf}} attribute is an operational attribute, so it must be
requested explicitly.


H3: Further Information

{{:slapo-memberof(5)}}


H2: The Proxy Cache Engine

{{TERM:LDAP}} servers typically hold one or more subtrees of a
{{TERM:DIT}}. Replica (or shadow) servers hold shadow copies of
entries held by one or more master servers.  Changes are propagated
from the master server to replica (slave) servers using LDAP Sync
replication.  An LDAP cache is a special type of replica which holds
entries corresponding to search filters instead of subtrees.

H3: Overview

The proxy cache extension of slapd is designed to improve the
responsiveness of the ldap and meta backends. It handles a search
request (query)
by first determining whether it is contained in any cached search
filter. Contained requests are answered from the proxy cache's local
database. Other requests are passed on to the underlying ldap or
meta backend and processed as usual.

E.g. {{EX:(shoesize>=9)}} is contained in {{EX:(shoesize>=8)}} and
{{EX:(sn=Richardson)}} is contained in {{EX:(sn=Richards*)}}

Correct matching rules and syntaxes are used while comparing
assertions for query containment. To simplify the query containment
problem, a list of cacheable "templates" (defined below) is specified
at configuration time. A query is cached or answered only if it
belongs to one of these templates. The entries corresponding to
cached queries are stored in the proxy cache local database while
its associated meta information (filter, scope, base, attributes)
is stored in main memory.

A template is a prototype for generating LDAP search requests.
Templates are described by a prototype search filter and a list of
attributes which are required in queries generated from the template.
The representation for prototype filter is similar to {{REF:RFC4515}},
except that the assertion values are missing. Examples of prototype
filters are: (sn=),(&(sn=)(givenname=)) which are instantiated by
search filters (sn=Doe) and (&(sn=Doe)(givenname=John)) respectively.

The cache replacement policy removes the least recently used (LRU)
query and entries belonging to only that query. Queries are allowed
a maximum time to live (TTL) in the cache thus providing weak
consistency. A background task periodically checks the cache for
expired queries and removes them.

The Proxy Cache paper
({{URL:http://www.openldap.org/pub/kapurva/proxycaching.pdf}}) provides
design and implementation details.


H3: Proxy Cache Configuration

The cache configuration specific directives described below must
appear after a {{EX:overlay proxycache}} directive within a
{{EX:"database meta"}} or {{EX:database ldap}} section of
the server's {{slapd.conf}}(5) file.

H4: Setting cache parameters

> proxyCache <DB> <maxentries> <nattrsets> <entrylimit> <period>

This directive enables proxy caching and sets general cache
parameters.  The <DB> parameter specifies which underlying database
is to be used to hold cached entries.  It should be set to
{{EX:bdb}} or {{EX:hdb}}.  The <maxentries> parameter specifies the
total number of entries which may be held in the cache.  The
<nattrsets> parameter specifies the total number of attribute sets
(as specified by the {{EX:proxyAttrSet}} directive) that may be
defined.  The <entrylimit> parameter specifies the maximum number of
entries in a cacheable query.  The <period> specifies the consistency
check period (in seconds).  In each period, queries with expired
TTLs are removed.

H4: Defining attribute sets

> proxyAttrset <index> <attrs...>

Used to associate a set of attributes to an index. Each attribute
set is associated with an index number from 0 to <numattrsets>-1.
These indices are used by the proxyTemplate directive to define
cacheable templates.

H4: Specifying cacheable templates

> proxyTemplate <prototype_string> <attrset_index> <TTL>

Specifies a cacheable template and the "time to live" (in sec) <TTL>
for queries belonging to the template. A template is described by
its prototype filter string and set of required attributes identified
by <attrset_index>.


H4: Example

An example {{slapd.conf}}(5) database section for a caching server
which proxies for the {{EX:"dc=example,dc=com"}} subtree held
at server {{EX:ldap.example.com}}.

>	database 	ldap
>	suffix 		"dc=example,dc=com"
>	rootdn 		"dc=example,dc=com"
>	uri    		ldap://ldap.example.com/
>	overlay proxycache
>	proxycache    bdb 100000 1 1000 100
>	proxyAttrset  0 mail postaladdress telephonenumber
>	proxyTemplate (sn=) 0 3600
>	proxyTemplate (&(sn=)(givenName=)) 0 3600
>	proxyTemplate (&(departmentNumber=)(secretary=*)) 0 3600
>
>	cachesize 20
>	directory ./testrun/db.2.a
>	index       objectClass eq
>	index       cn,sn,uid,mail  pres,eq,sub


H5: Cacheable Queries

A LDAP search query is cacheable when its filter matches one of the
templates as defined in the "proxyTemplate" statements and when it references
only the attributes specified in the corresponding attribute set.
In the example above the attribute set number 0 defines that only the
attributes: {{EX:mail postaladdress telephonenumber}} are cached for the following
proxyTemplates.

H5: Examples:

>	Filter: (&(sn=Richard*)(givenName=jack))
>	Attrs: mail telephoneNumber

    is cacheable, because it matches the template {{EX:(&(sn=)(givenName=))}} and its
    attributes are contained in proxyAttrset 0.

>	Filter: (&(sn=Richard*)(telephoneNumber))
>	Attrs: givenName

    is not cacheable, because the filter does not match the template,
    nor is the attribute givenName stored in the cache

>	Filter: (|(sn=Richard*)(givenName=jack))
>	Attrs: mail telephoneNumber

    is not cacheable, because the filter does not match the template ( logical
    OR "|" condition instead of logical AND "&" )


H3: Further Information

{{:slapo-pcache(5)}}


H2: Password Policies


H3: Overview

This overlay follows the specifications contained in the draft RFC titled
draft-behera-ldap-password-policy-09. While the draft itself is expired, it has
been implemented in several directory servers, including slapd. Nonetheless,
it is important to note that it is a draft, meaning that it is subject to change
and is a work-in-progress.

The key abilities of the password policy overlay are as follows:

* Enforce a minimum length for new passwords
* Make sure passwords are not changed too frequently
* Cause passwords to expire, provide warnings before they need to be changed, and allow a fixed number of 'grace' logins to allow them to be changed after they have expired
* Maintain a history of passwords to prevent password re-use
* Prevent password guessing by locking a password for a specified period of time after repeated authentication failures
* Force a password to be changed at the next authentication
* Set an administrative lock on an account
* Support multiple password policies on a default or a per-object basis.
* Perform arbitrary quality checks using an external loadable module. This is a non-standard extension of the draft RFC.


H3: Password Policy Configuration

Instantiate the module in the database where it will be used, after adding the
new ppolicy schema and loading the ppolicy module. The following example shows
the ppolicy module being added to the database that handles the naming
context "dc=example,dc=com". In this example we are also specifying the DN of
a policy object to use if none other is specified in a user's object.

>       database bdb
>       suffix "dc=example,dc=com"
>       [...additional database configuration directives go here...]
>
>       overlay ppolicy
>       ppolicy_default "cn=default,ou=policies,dc=example,dc=com"


Now we need a container for the policy objects. In our example the password
policy objects are going to be placed in a section of the tree called
"ou=policies,dc=example,dc=com":

>       dn: ou=policies,dc=example,dc=com
>       objectClass: organizationalUnit
>       objectClass: top
>       ou: policies


The default policy object that we are creating defines the following policies:

* The user is allowed to change his own password. Note that the directory ACLs for this attribute can also affect this ability (pwdAllowUserChange: TRUE).
* The name of the password attribute is "userPassword" (pwdAttribute: userPassword). Note that this is the only value that is accepted by OpenLDAP for this attribute.
* The server will check the syntax of the password. If the server is unable to check the syntax (i.e., it was hashed or otherwise encoded by the client) it will return an error refusing the password (pwdCheckQuality: 2).
* When a client includes the Password Policy Request control with a bind request, the server will respond with a password expiration warning if it is going to expire in ten minutes or less (pwdExpireWarning: 600). The warnings themselves are returned in a Password Policy Response control.
* When the password for a DN has expired, the server will allow five additional "grace" logins (pwdGraceAuthNLimit: 5).
* The server will maintain a history of the last five passwords that were used for a DN (pwdInHistory: 5).
* The server will lock the account after the maximum number of failed bind attempts has been exceeded (pwdLockout: TRUE).
* When the server has locked an account, the server will keep it locked until an administrator unlocks it (pwdLockoutDuration: 0)
* The server will reset its failed bind count after a period of 30 seconds.
* Passwords will not expire (pwdMaxAge: 0).
* Passwords can be changed as often as desired (pwdMinAge: 0).
* Passwords must be at least 5 characters in length (pwdMinLength: 5).
* The password does not need to be changed at the first bind or when the administrator has reset the password (pwdMustChange: FALSE)
* The current password does not need to be included with password change requests (pwdSafeModify: FALSE)
* The server will only allow five failed binds in a row for a particular DN (pwdMaxFailure: 5).


The actual policy would be:

>       dn: cn=default,ou=policies,dc=example,dc=com
>       cn: default
>       objectClass: pwdPolicy
>       objectClass: person
>       objectClass: top
>       pwdAllowUserChange: TRUE
>       pwdAttribute: userPassword
>       pwdCheckQuality: 2
>       pwdExpireWarning: 600
>       pwdFailureCountInterval: 30
>       pwdGraceAuthNLimit: 5
>       pwdInHistory: 5
>       pwdLockout: TRUE
>       pwdLockoutDuration: 0
>       pwdMaxAge: 0
>       pwdMaxFailure: 5
>       pwdMinAge: 0
>       pwdMinLength: 5
>       pwdMustChange: FALSE
>       pwdSafeModify: FALSE
>       sn: dummy value

You can create additional policy objects as needed.


There are two ways password policy can be applied to individual objects:

1. The pwdPolicySubentry in a user's object - If a user's object has a
pwdPolicySubEntry attribute specifying the DN of a policy object, then
the policy defined by that object is applied.

2. Default password policy - If there is no specific pwdPolicySubentry set
for an object, and the password policy module was configured with the DN of a
default policy object and if that object exists, then the policy defined in
that object is applied.

Please see {{slapo-ppolicy(5)}} for complete explanations of features and discussion of
 "Password Management Issues" at {{URL:http://www.symas.com/blog/?page_id=66}}


H3: Further Information

{{:slapo-ppolicy(5)}}


H2: Referential Integrity


H3: Overview

This overlay can be used with a backend database such as slapd-bdb(5)
to maintain the cohesiveness of a schema which utilizes reference
attributes.

Whenever a {{modrdn}} or {{delete}} is performed, that is, when an entry's DN
is renamed or an entry is removed, the server will search the directory for
references to this DN (in selected attributes: see below) and update them
accordingly. If it was a {{delete}} operation, the reference is deleted. If it
was a {{modrdn}} operation, then the reference is updated with the new DN.

For example, a very common administration task is to maintain group membership
lists, specially when users are removed from the directory. When an
user account is deleted or renamed, all groups this user is a member of have to be
updated. LDAP administrators usually have scripts for that. But we can use the
{{F:refint}} overlay to automate this task. In this example, if the user is
removed from the directory, the overlay will take care to remove the user from
all the groups he/she was a member of. No more scripting for this.

H3: Referential Integrity Configuration

The configuration for this overlay is as follows:

>       overlay refint
>       refint_attributes <attribute [attribute ...]>
>       refint_nothing <string>

* {{F:refint_attributes}}: this parameter specifies a space separated list of
attributes which will have the referential integrity maintained. When an entry is
removed or has its DN renamed, the server will do an internal search for any of the
{{F:refint_attributes}} that point to the affected DN and update them accordingly. IMPORTANT:
the attributes listed here must have the {{F:distinguishedName}} syntax, that is,
hold DNs as values.
* {{F:refint_nothing}}: some times, while trying to maintain the referential
integrity, the server has to remove the last attribute of its kind from an
entry. This may be prohibited by the schema: for example, the
{{F:groupOfNames}} object class requires at least one member. In these cases,
the server will add the attribute value specified in {{F:refint_nothing}}
to the entry.

To illustrate this overlay, we will use the group membership scenario.

In {{F:slapd.conf}}:

>       overlay refint
>       refint_attributes member
>       refint_nothing "cn=admin,dc=example,dc=com"

This configuration tells the overlay to maintain the referential integrity of the {{F:member}}
attribute. This attribute is used in the {{F:groupOfNames}} object class which always needs
a member, so we add the {{F:refint_nothing}} directive to fill in the group with a standard
member should all the members vanish.

If we have the following group membership, the refint overlay will
automatically remove {{F:john}} from the group if his entry is removed from the
directory:

!import "refint.png"; align="center"; title="Group membership"
FT[align="Center"] Figure X.Y: Maintaining referential integrity in groups

Notice that if we rename ({{F:modrdn}}) the {{F:john}} entry to, say, {{F:jsmith}}, the refint
overlay will also rename the reference in the {{F:member}} attribute, so the group membership
stays correct.

If we removed all users from the directory who are a member of this group, then the end result
would be a single member in the group: {{F:cn=admin,dc=example,dc=com}}. This is the
{{F:refint_nothing}} parameter kicking into action so that the schema is not violated.


H3: Further Information

{{:slapo-refint(5)}}


H2: Return Code


H3: Overview

This overlay is useful to test the behavior of clients when
server-generated erroneous and/or unusual responses occur,
for example; error codes, referrals, excessive response times and so on.

This would be classed as a debugging tool whilst developing client software
or additional Overlays.

For detailed information, please see the {{slapo-retcode(5)}} man page.


H3: Return Code Configuration

The retcode overlay utilizes the "return code" schema described in the man page.
This schema is specifically designed for use with this overlay and is not intended
to be used otherwise.

Note: The necessary schema is loaded automatically by the overlay.

An example configuration might be:

>       overlay         retcode
>       retcode-parent  "ou=RetCodes,dc=example,dc=com"
>       include         ./retcode.conf
>
>       retcode-item    "cn=Unsolicited"                0x00 unsolicited="0"
>       retcode-item    "cn=Notice of Disconnect"       0x00 unsolicited="1.3.6.1.4.1.1466.20036"
>       retcode-item    "cn=Pre-disconnect"             0x34 flags="pre-disconnect"
>       retcode-item    "cn=Post-disconnect"            0x34 flags="post-disconnect"

Note: {{retcode.conf}} can be found in the openldap source at: {{F:tests/data/retcode.conf}}

An excerpt of a {{F:retcode.conf}} would be something like:

>       retcode-item    "cn=success"                            0x00
>
>       retcode-item    "cn=success w/ delay"                   0x00    sleeptime=2
>
>       retcode-item    "cn=operationsError"                    0x01
>       retcode-item    "cn=protocolError"                      0x02
>       retcode-item    "cn=timeLimitExceeded"                  0x03    op=search
>       retcode-item    "cn=sizeLimitExceeded"                  0x04    op=search
>       retcode-item    "cn=compareFalse"                       0x05    op=compare
>       retcode-item    "cn=compareTrue"                        0x06    op=compare
>       retcode-item    "cn=authMethodNotSupported"             0x07
>       retcode-item    "cn=strongAuthNotSupported"             0x07    text="same as authMethodNotSupported"
>       retcode-item    "cn=strongAuthRequired"                 0x08
>       retcode-item    "cn=strongerAuthRequired"               0x08    text="same as strongAuthRequired"

Please see {{F:tests/data/retcode.conf}} for a complete {{F:retcode.conf}}


H3: Further Information

{{:slapo-retcode(5)}}


H2: Rewrite/Remap


H3: Overview

It performs basic DN/data rewrite and objectClass/attributeType mapping. Its
usage is mostly intended to provide virtual views of existing data either
remotely, in conjunction with the proxy backend described in {{slapd-ldap(5)}},
or locally, in conjunction with the relay backend described in {{slapd-relay(5)}}.

This overlay is extremely configurable and advanced, therefore recommended
reading is the {{slapo-rwm(5)}} man page.


H3: Rewrite/Remap Configuration


H3: Further Information

{{:slapo-rwm(5)}}


H2: Sync Provider


H3: Overview

This overlay implements the provider-side support for the LDAP Content Synchronization
({{REF:RFC4533}}) as well as syncrepl replication support, including persistent search functionality.

H3: Sync Provider Configuration

There is very little configuration needed for this overlay, in fact for many situations merely loading
the overlay will suffice.

However, because the overlay creates a contextCSN attribute in the root entry of the database which is
updated for every write operation performed against the database and only updated in memory, it is
recommended to configure a checkpoint so that the contextCSN is written into the underlying database to
minimize recovery time after an unclean shutdown:

>       overlay syncprov
>       syncprov-checkpoint 100 10

For every 100 operations or 10 minutes, which ever is sooner, the contextCSN will be checkpointed.

The four configuration directives available are {{B:syncprov-checkpoint}}, {{B:syncprov-sessionlog}},
{{B:syncprov-nopresent}} and {{B:syncprov-reloadhint}} which are covered in the man page discussing
various other scenarios where this overlay can be used.

H3: Further Information

The {{:slapo-syncprov(5)}} man page and the {{SECT:Configuring the different replication types}} section


H2: Translucent Proxy


H3: Overview

This overlay can be used with a backend database such as {{:slapd-bdb}}(5)
to create a "translucent proxy".

Entries retrieved from a remote LDAP server may have some or all attributes
overridden, or new attributes added, by entries in the local database before
being presented to the client.

A search operation is first populated with entries from the remote LDAP server,
the attributes of which are then overridden with any attributes defined in the
local database. Local overrides may be populated with the add, modify, and
modrdn operations, the use of which is restricted to the root user of the
translucent local database.

A compare operation will perform a comparison with attributes defined in the
local database record (if any) before any comparison is made with data in the
remote database.


H3: Translucent Proxy Configuration

There are various options available with this overlay, but for this example we
will demonstrate adding new attributes to a remote entry and also searching
against these newly added local attributes. For more information about overriding remote
entries and search configuration, please see {{:slapo-translucent(5)}}

Note: The Translucent Proxy overlay will disable schema checking in the local
database, so that an entry consisting of overlay attributes need not adhere
 to the complete schema.

First we configure the overlay in the normal manner:

>       include     /usr/local/etc/openldap/schema/core.schema
>       include     /usr/local/etc/openldap/schema/cosine.schema
>       include     /usr/local/etc/openldap/schema/nis.schema
>       include     /usr/local/etc/openldap/schema/inetorgperson.schema
>
>       pidfile     ./slapd.pid
>       argsfile    ./slapd.args
>
>       database    bdb
>       suffix      "dc=suretecsystems,dc=com"
>       rootdn      "cn=trans,dc=suretecsystems,dc=com"
>       rootpw      secret
>       directory   ./openldap-data
>
>       index       objectClass eq
>
>       overlay     translucent
>       translucent_local carLicense
>
>       uri         ldap://192.168.X.X:389
>       lastmod     off
>       acl-bind    binddn="cn=admin,dc=suretecsystems,dc=com" credentials="blahblah"

You will notice the overlay directive and a directive to say what attribute we
want to be able to search against in the local database. We must also load the
ldap backend which will connect to the remote directory server.

Now we take an example LDAP group:

>       # itsupport, Groups, suretecsystems.com
>       dn: cn=itsupport,ou=Groups,dc=suretecsystems,dc=com
>       objectClass: posixGroup
>       objectClass: sambaGroupMapping
>       cn: itsupport
>       gidNumber: 1000
>       sambaSID: S-1-5-21-XXX
>       sambaGroupType: 2
>       displayName: itsupport
>       memberUid: ghenry
>       memberUid: joebloggs

and create an LDIF file we can use to add our data to the local database, using
 some pretty strange choices of new attributes for demonstration purposes:

>       [ghenry@suretec test_configs]$ cat test-translucent-add.ldif
>       dn: cn=itsupport,ou=Groups,dc=suretecsystems,dc=com
>       businessCategory: frontend-override
>       carLicense: LIVID
>       employeeType: special
>       departmentNumber: 9999999
>       roomNumber: 41L-535

Searching against the proxy gives:

>       [ghenry@suretec test_configs]$ ldapsearch -x -H ldap://127.0.0.1:9001 "(cn=itsupport)"
>       # itsupport, Groups, OxObjects, suretecsystems.com
>       dn: cn=itsupport,ou=Groups,ou=OxObjects,dc=suretecsystems,dc=com
>       objectClass: posixGroup
>       objectClass: sambaGroupMapping
>       cn: itsupport
>       gidNumber: 1003
>       SAMBASID: S-1-5-21-XXX
>       SAMBAGROUPTYPE: 2
>       displayName: itsupport
>       memberUid: ghenry
>       memberUid: joebloggs
>       roomNumber: 41L-535
>       departmentNumber: 9999999
>       employeeType: special
>       carLicense: LIVID
>       businessCategory: frontend-override

Here we can see that the 5 new attributes are added to the remote entry before
being returned to the our client.

Because we have configured a local attribute to search against:

>       overlay     translucent
>       translucent_local carLicense

we can also search for that to return the completely fabricated entry:

>       ldapsearch -x -H ldap://127.0.0.1:9001 (carLicense=LIVID)

This is an extremely feature because you can then extend a remote directory server
locally and also search against the local entries.

Note: Because the translucent overlay does not perform any DN rewrites, the local
 and remote database instances must have the same suffix. Other configurations
will probably fail with No Such Object and other errors

H3: Further Information

{{:slapo-translucent(5)}}


H2: Attribute Uniqueness


H3: Overview

This overlay can be used with a backend database such as {{slapd-bdb(5)}}
to enforce the uniqueness of some or all attributes within a subtree.


H3: Attribute Uniqueness Configuration

This overlay is only effective on new data from the point the overlay is enabled. To
check uniqueness for existing data, you can export and import your data again via the
LDAP Add operation, which will not be suitable for large amounts of data, unlike {{B:slapcat}}.

For the following example, if uniqueness were enforced for the {{B:mail}} attribute,
the subtree would be searched for any other records which also have a {{B:mail}} attribute
containing the same value presented with an {{B:add}}, {{B:modify}} or {{B:modrdn}} operation
which are unique within the configured scope. If any are found, the request is rejected.

Note:  If no attributes are specified, for example {{B:ldap:///??sub?}}, then the URI applies to all non-operational attributes. However,
the keyword {{B:ignore}} can be specified to exclude certain non-operational attributes.

To search at the base dn of the current backend database ensuring uniqueness of the {{B:mail}}
attribute, we simply add the following configuration:

>       overlay unique
>       unique_uri ldap:///?mail?sub?

For an existing entry of:

>       dn: cn=gavin,dc=suretecsystems,dc=com
>       objectClass: top
>       objectClass: inetorgperson
>       cn: gavin
>       sn: henry
>       mail: ghenry@suretecsystems.com

and we then try to add a new entry of:

>       dn: cn=robert,dc=suretecsystems,dc=com
>       objectClass: top
>       objectClass: inetorgperson
>       cn: robert
>       sn: jones
>       mail: ghenry@suretecsystems.com

would result in an error like so:

>       adding new entry "cn=robert,dc=example,dc=com"
>       ldap_add: Constraint violation (19)
>               additional info: some attributes not unique

The overlay can have multiple URIs specified within a domain, allowing complex
selections of objects and also have multiple {{B:unique_uri}} statements or
{{B:olcUniqueURI}} attributes which will create independent domains.

For more information and details about the {{B:strict}} and {{B:ignore}} keywords,
please see the {{:slapo-unique(5)}} man page.

H3: Further Information

{{:slapo-unique(5)}}


H2: Value Sorting


H3: Overview

The Value Sorting overlay can be used with a backend database to sort the
values of specific multi-valued attributes within a subtree. The sorting occurs
whenever the attributes are returned in a search response.

H3: Value Sorting Configuration

Sorting can be specified in ascending or descending order, using either numeric
or alphanumeric sort methods. Additionally, a "weighted" sort can be specified,
 which uses a numeric weight prepended to the attribute values.

The weighted sort is always performed in ascending order, but may be combined
with the other methods for values that all have equal weights. The weight is
specified by prepending an integer weight {<weight>} in front of each value
of the attribute for which weighted sorting is desired. This weighting factor
is stripped off and never returned in search results.

Here are a few examples:

>       loglevel    sync stats
>
>       database    hdb
>       suffix      "dc=suretecsystems,dc=com"
>       directory   /usr/local/var/openldap-data
>
>       ......
>
>       overlay valsort
>       valsort-attr memberUid ou=Groups,dc=suretecsystems,dc=com alpha-ascend

For example, ascend:

>       # sharedemail, Groups, suretecsystems.com
>       dn: cn=sharedemail,ou=Groups,dc=suretecsystems,dc=com
>       objectClass: posixGroup
>       objectClass: top
>       cn: sharedemail
>       gidNumber: 517
>       memberUid: admin
>       memberUid: dovecot
>       memberUid: laura
>       memberUid: suretec

For weighted, we change our data to:

>       # sharedemail, Groups, suretecsystems.com
>       dn: cn=sharedemail,ou=Groups,dc=suretecsystems,dc=com
>       objectClass: posixGroup
>       objectClass: top
>       cn: sharedemail
>       gidNumber: 517
>       memberUid: {4}admin
>       memberUid: {2}dovecot
>       memberUid: {1}laura
>       memberUid: {3}suretec

and change the config to:

>       overlay valsort
>       valsort-attr memberUid ou=Groups,dc=suretecsystems,dc=com weighted

Searching now results in:

>       # sharedemail, Groups, OxObjects, suretecsystems.com
>       dn: cn=sharedemail,ou=Groups,ou=OxObjects,dc=suretecsystems,dc=com
>       objectClass: posixGroup
>       objectClass: top
>       cn: sharedemail
>       gidNumber: 517
>       memberUid: laura
>       memberUid: dovecot
>       memberUid: suretec
>       memberUid: admin


H3: Further Information

{{:slapo-valsort(5)}}


H2: Overlay Stacking


H3: Overview

Overlays can be stacked, which means that more than one overlay
can be instantiated for each database, or for the {{EX:frontend}}.
As a consequence, each overlays function is called, if defined,
when overlay execution is invoked.
Multiple overlays are executed in reverse order (as a stack)
with respect to their definition in slapd.conf (5), or with respect
to their ordering in the config database, as documented in slapd-config (5).


H3: Example Scenarios


H4: Samba