10. 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, but some may also be configured globally.

Essentially they represent a means to:

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

        slapo-<overlay name>

Not all distributed overlays have a man page yet. Feel free to contribute one, if you think you well understood the behavior of the component and the implications of all the related configuration directives.

Official overlays are located in


That directory also contains the file slapover.txt, which describes the rationale of the overlay implementation, and may serve as 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.

They can be stacked on the frontend as well; 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.

All the current overlays in 2.4 are listed and described in detail in the following sections.

10.1. Access Logging

10.1.1. Overview

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

10.1.2. Access Logging Configuration

10.2. Audit Logging

This overlay records changes on a given backend database to an LDIF log file.

10.2.1. Overview

10.2.2. Audit Logging Configuration

10.3. Chaining

10.3.1. 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 --enable-ldap.

10.3.2. 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 file (global), before any database definitions:

        overlay                    chain
        chain-uri                  "ldap://ldapmaster.example.com"
        chain-idassert-bind        bindmethod="simple"
        chain-tls                  start
        chain-return-error         TRUE
        updateref                  "ldap://ldapmaster.example.com/"

The 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 changes. Then, if you are using loglevel 256, you can monitor an ldapmodify on the slave and the master.

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= (IP=
        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.

10.4. Constraints

10.4.1. Overview

This overlay enforces a regular expression constraint on all values of specified attributes. It is used to enforce a more rigorous syntax when the underlying attribute syntax is too general.

10.4.2. Constraint Configuration

10.5. Dynamic Directory Services

10.5.1. Overview

This overlay supports dynamic objects, which have a limited life after which they expire and are automatically deleted.

10.5.2. Dynamic Directory Service Configuration

10.6. Dynamic Groups

10.6.1. 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 Dynamic Lists overlay.

10.6.2. Dynamic Group Configuration

10.7. Dynamic Lists

10.7.1. Overview

This overlay allows expansion of dynamic groups and more.

10.7.2. Dynamic List Configuration

10.8. Reverse Group Membership Maintenance

10.8.1. Member Of Configuration

10.9. The Proxy Cache Engine

LDAP servers typically hold one or more subtrees of a 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.

10.9.1. 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. (shoesize>=9) is contained in (shoesize>=8) and (sn=Richardson) is contained in (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 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 (http://www.openldap.org/pub/kapurva/proxycaching.pdf) provides design and implementation details.

10.9.2. Proxy Cache Configuration

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

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

        database        ldap
        suffix          "dc=example,dc=com"
        rootdn          "dc=example,dc=com"
        uri             ldap://ldap.example.com/dc=example%2cdc=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 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: mail postaladdress telephonenumber are cached for the following proxyTemplates. Examples:
        Filter: (&(sn=Richard*)(givenName=jack))
        Attrs: mail telephoneNumber

is cacheable, because it matches the template (&(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 "&" )

10.10. Password Policies

10.10.1. Overview

This overlay provides a variety of password control mechanisms, e.g. password aging, password reuse and duplication control, mandatory password resets, etc.

10.10.2. Password Policy Configuration

10.11. Referential Integrity

10.11.1. 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.

10.11.2. Referential Integrity Configuration

10.12. Return Code

10.12.1. Overview

This overlay is useful to test the behavior of clients when server-generated erroneous and/or unusual responses occur.

10.12.2. Return Code Configuration

10.13. Rewrite/Remap

10.13.1. Overview

It performs basic DN/data rewrite and objectClass/attributeType mapping.

10.13.2. Rewrite/Remap Configuration

10.14. Sync Provider

10.14.1. Overview

This overlay implements the provider-side support for syncrepl replication, including persistent search functionality

10.14.2. Sync Provider Configuration

10.15. Translucent Proxy

10.15.1. Overview

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

Content of entries retrieved from a remote LDAP server can be partially overridden by the database.

10.15.2. Translucent Proxy Configuration

10.16. Attribute Uniqueness

10.16.1. 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.

10.16.2. Attribute Uniqueness Configuration

10.17. Value Sorting

10.17.1. Overview

This overlay can be used to enforce a specific order for the values of an attribute when it is returned in a search.

10.17.2. Value Sorting Configuration

10.18. Overlay Stacking

10.18.1. Overview

10.18.2. Example Scenarios Samba