2 # Copyright 2007 The OpenLDAP Foundation, All Rights Reserved.
3 # COPYING RESTRICTIONS APPLY, see COPYRIGHT.
7 Overlays are software components that provide hooks to functions analogous to
8 those provided by backends, which can be stacked on top of the backend calls
9 and as callbacks on top of backend responses to alter their behavior.
11 Overlays may be compiled statically into slapd, or when module support
12 is enabled, they may be dynamically loaded. Most of the overlays
13 are only allowed to be configured on individual databases, but some
14 may also be configured globally.
16 Essentially they represent a means to:
18 * customize the behavior of existing backends without changing the backend
19 code and without requiring one to write a new custom backend with
20 complete functionality
21 * write functionality of general usefulness that can be applied to
22 different backend types
24 Overlays are usually documented by separate specific man pages in section 5;
25 the naming convention is
29 Not all distributed overlays have a man page yet. Feel free to contribute one,
30 if you think you well understood the behavior of the component and the
31 implications of all the related configuration directives.
33 Official overlays are located in
35 servers/slapd/overlays/
37 That directory also contains the file slapover.txt, which describes the
38 rationale of the overlay implementation, and may serve as guideline for the
39 development of custom overlays.
41 Contribware overlays are located in
43 contrib/slapd-modules/<overlay name>/
45 along with other types of run-time loadable components; they are officially
46 distributed, but not maintained by the project.
48 They can be stacked on the frontend as well; this means that they can be
49 executed after a request is parsed and validated, but right before the
50 appropriate database is selected. The main purpose is to affect operations
51 regardless of the database they will be handled by, and, in some cases,
52 to influence the selection of the database by massaging the request DN.
54 All the current overlays in 2.4 are listed and described in detail in the
63 This overlay can record accesses to a given backend database on another
67 H3: Access Logging Configuration
72 This overlay records changes on a given backend database to an LDIF log
79 H3: Audit Logging Configuration
87 The chain overlay provides basic chaining capability to the underlying
90 What is chaining? It indicates the capability of a DSA to follow referrals on
91 behalf of the client, so that distributed systems are viewed as a single
92 virtual DSA by clients that are otherwise unable to "chase" (i.e. follow)
93 referrals by themselves.
95 The chain overlay is built on top of the ldap backend; it is compiled by
96 default when --enable-ldap.
99 H3: Chaining Configuration
107 This overlay enforces a regular expression constraint on all values
108 of specified attributes. It is used to enforce a more rigorous
109 syntax when the underlying attribute syntax is too general.
112 H3: Constraint Configuration
115 H2: Dynamic Directory Services
120 This overlay supports dynamic objects, which have a limited life after
121 which they expire and are automatically deleted.
124 H3: Dynamic Directory Service Configuration
132 This overlay extends the Compare operation to detect
133 members of a dynamic group.
136 H3: Dynamic Group Configuration
144 This overlay allows expansion of dynamic groups and more.
147 H3: Dynamic List Configuration
150 H2: The Proxy Cache Engine
152 {{TERM:LDAP}} servers typically hold one or more subtrees of a
153 {{TERM:DIT}}. Replica (or shadow) servers hold shadow copies of
154 entries held by one or more master servers. Changes are propagated
155 from the master server to replica (slave) servers using LDAP Sync
156 replication. An LDAP cache is a special type of replica which holds
157 entries corresponding to search filters instead of subtrees.
161 The proxy cache extension of slapd is designed to improve the
162 responsiveness of the ldap and meta backends. It handles a search
164 by first determining whether it is contained in any cached search
165 filter. Contained requests are answered from the proxy cache's local
166 database. Other requests are passed on to the underlying ldap or
167 meta backend and processed as usual.
169 E.g. {{EX:(shoesize>=9)}} is contained in {{EX:(shoesize>=8)}} and
170 {{EX:(sn=Richardson)}} is contained in {{EX:(sn=Richards*)}}
172 Correct matching rules and syntaxes are used while comparing
173 assertions for query containment. To simplify the query containment
174 problem, a list of cacheable "templates" (defined below) is specified
175 at configuration time. A query is cached or answered only if it
176 belongs to one of these templates. The entries corresponding to
177 cached queries are stored in the proxy cache local database while
178 its associated meta information (filter, scope, base, attributes)
179 is stored in main memory.
181 A template is a prototype for generating LDAP search requests.
182 Templates are described by a prototype search filter and a list of
183 attributes which are required in queries generated from the template.
184 The representation for prototype filter is similar to {{REF:RFC4515}},
185 except that the assertion values are missing. Examples of prototype
186 filters are: (sn=),(&(sn=)(givenname=)) which are instantiated by
187 search filters (sn=Doe) and (&(sn=Doe)(givenname=John)) respectively.
189 The cache replacement policy removes the least recently used (LRU)
190 query and entries belonging to only that query. Queries are allowed
191 a maximum time to live (TTL) in the cache thus providing weak
192 consistency. A background task periodically checks the cache for
193 expired queries and removes them.
195 The Proxy Cache paper
196 ({{URL:http://www.openldap.org/pub/kapurva/proxycaching.pdf}}) provides
197 design and implementation details.
200 H3: Proxy Cache Configuration
202 The cache configuration specific directives described below must
203 appear after a {{EX:overlay proxycache}} directive within a
204 {{EX:"database meta"}} or {{EX:database ldap}} section of
205 the server's {{slapd.conf}}(5) file.
207 H4: Setting cache parameters
209 > proxyCache <DB> <maxentries> <nattrsets> <entrylimit> <period>
211 This directive enables proxy caching and sets general cache
212 parameters. The <DB> parameter specifies which underlying database
213 is to be used to hold cached entries. It should be set to
214 {{EX:bdb}} or {{EX:hdb}}. The <maxentries> parameter specifies the
215 total number of entries which may be held in the cache. The
216 <nattrsets> parameter specifies the total number of attribute sets
217 (as specified by the {{EX:proxyAttrSet}} directive) that may be
218 defined. The <entrylimit> parameter specifies the maximum number of
219 entries in a cacheable query. The <period> specifies the consistency
220 check period (in seconds). In each period, queries with expired
223 H4: Defining attribute sets
225 > proxyAttrset <index> <attrs...>
227 Used to associate a set of attributes to an index. Each attribute
228 set is associated with an index number from 0 to <numattrsets>-1.
229 These indices are used by the proxyTemplate directive to define
232 H4: Specifying cacheable templates
234 > proxyTemplate <prototype_string> <attrset_index> <TTL>
236 Specifies a cacheable template and the "time to live" (in sec) <TTL>
237 for queries belonging to the template. A template is described by
238 its prototype filter string and set of required attributes identified
244 An example {{slapd.conf}}(5) database section for a caching server
245 which proxies for the {{EX:"dc=example,dc=com"}} subtree held
246 at server {{EX:ldap.example.com}}.
249 > suffix "dc=example,dc=com"
250 > rootdn "dc=example,dc=com"
251 > uri ldap://ldap.example.com/dc=example%2cdc=com
253 > proxycache bdb 100000 1 1000 100
254 > proxyAttrset 0 mail postaladdress telephonenumber
255 > proxyTemplate (sn=) 0 3600
256 > proxyTemplate (&(sn=)(givenName=)) 0 3600
257 > proxyTemplate (&(departmentNumber=)(secretary=*)) 0 3600
260 > directory ./testrun/db.2.a
261 > index objectClass eq
262 > index cn,sn,uid,mail pres,eq,sub
265 H5: Cacheable Queries
267 A LDAP search query is cacheable when its filter matches one of the
268 templates as defined in the "proxyTemplate" statements and when it references
269 only the attributes specified in the corresponding attribute set.
270 In the example above the attribute set number 0 defines that only the
271 attributes: {{EX:mail postaladdress telephonenumber}} are cached for the following
276 > Filter: (&(sn=Richard*)(givenName=jack))
277 > Attrs: mail telephoneNumber
279 is cacheable, because it matches the template {{EX:(&(sn=)(givenName=))}} and its
280 attributes are contained in proxyAttrset 0.
282 > Filter: (&(sn=Richard*)(telephoneNumber))
285 is not cacheable, because the filter does not match the template,
286 nor is the attribute givenName stored in the cache
288 > Filter: (|(sn=Richard*)(givenName=jack))
289 > Attrs: mail telephoneNumber
291 is not cacheable, because the filter does not match the template ( logical
292 OR "|" condition instead of logical AND "&" )
295 H2: Password Policies
300 This overlay provides a variety of password control mechanisms,
301 e.g. password aging, password reuse and duplication control, mandatory
302 password resets, etc.
305 H3: Password Policy Configuration
308 H2: Referential Integrity
313 This overlay can be used with a backend database such as slapd-bdb (5)
314 to maintain the cohesiveness of a schema which utilizes reference
318 H3: Referential Integrity Configuration
326 This overlay is useful to test the behavior of clients when
327 server-generated erroneous and/or unusual responses occur.
330 H3: Return Code Configuration
338 It performs basic DN/data rewrite and
339 objectClass/attributeType mapping.
342 H3: Rewrite/Remap Configuration
350 This overlay implements the provider-side support for syncrepl
351 replication, including persistent search functionality
354 H3: Sync Provider Configuration
357 H2: Translucent Proxy
362 This overlay can be used with a backend database such as slapd-bdb (5)
363 to create a "translucent proxy".
365 Content of entries retrieved from a remote LDAP server can be partially
366 overridden by the database.
369 H3: Translucent Proxy Configuration
372 H2: Attribute Uniqueness
377 This overlay can be used with a backend database such as slapd-bdb (5)
378 to enforce the uniqueness of some or all attributes within a subtree.
381 H3: Attribute Uniqueness Configuration
389 This overlay can be used to enforce a specific order for the values
390 of an attribute when it is returned in a search.
393 H3: Value Sorting Configuration
402 H3: Example Scenarios