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
27 > slapo-<overlay name>
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. This overlay is now deprecated
134 as all of its functions are available using the
135 {{SECT:Dynamic Lists}} overlay.
138 H3: Dynamic Group Configuration
146 This overlay allows expansion of dynamic groups and more.
149 H3: Dynamic List Configuration
152 H2: Reverse Group Membership Maintenance
155 H3: Member Of Configuration
158 H2: The Proxy Cache Engine
160 {{TERM:LDAP}} servers typically hold one or more subtrees of a
161 {{TERM:DIT}}. Replica (or shadow) servers hold shadow copies of
162 entries held by one or more master servers. Changes are propagated
163 from the master server to replica (slave) servers using LDAP Sync
164 replication. An LDAP cache is a special type of replica which holds
165 entries corresponding to search filters instead of subtrees.
169 The proxy cache extension of slapd is designed to improve the
170 responsiveness of the ldap and meta backends. It handles a search
172 by first determining whether it is contained in any cached search
173 filter. Contained requests are answered from the proxy cache's local
174 database. Other requests are passed on to the underlying ldap or
175 meta backend and processed as usual.
177 E.g. {{EX:(shoesize>=9)}} is contained in {{EX:(shoesize>=8)}} and
178 {{EX:(sn=Richardson)}} is contained in {{EX:(sn=Richards*)}}
180 Correct matching rules and syntaxes are used while comparing
181 assertions for query containment. To simplify the query containment
182 problem, a list of cacheable "templates" (defined below) is specified
183 at configuration time. A query is cached or answered only if it
184 belongs to one of these templates. The entries corresponding to
185 cached queries are stored in the proxy cache local database while
186 its associated meta information (filter, scope, base, attributes)
187 is stored in main memory.
189 A template is a prototype for generating LDAP search requests.
190 Templates are described by a prototype search filter and a list of
191 attributes which are required in queries generated from the template.
192 The representation for prototype filter is similar to {{REF:RFC4515}},
193 except that the assertion values are missing. Examples of prototype
194 filters are: (sn=),(&(sn=)(givenname=)) which are instantiated by
195 search filters (sn=Doe) and (&(sn=Doe)(givenname=John)) respectively.
197 The cache replacement policy removes the least recently used (LRU)
198 query and entries belonging to only that query. Queries are allowed
199 a maximum time to live (TTL) in the cache thus providing weak
200 consistency. A background task periodically checks the cache for
201 expired queries and removes them.
203 The Proxy Cache paper
204 ({{URL:http://www.openldap.org/pub/kapurva/proxycaching.pdf}}) provides
205 design and implementation details.
208 H3: Proxy Cache Configuration
210 The cache configuration specific directives described below must
211 appear after a {{EX:overlay proxycache}} directive within a
212 {{EX:"database meta"}} or {{EX:database ldap}} section of
213 the server's {{slapd.conf}}(5) file.
215 H4: Setting cache parameters
217 > proxyCache <DB> <maxentries> <nattrsets> <entrylimit> <period>
219 This directive enables proxy caching and sets general cache
220 parameters. The <DB> parameter specifies which underlying database
221 is to be used to hold cached entries. It should be set to
222 {{EX:bdb}} or {{EX:hdb}}. The <maxentries> parameter specifies the
223 total number of entries which may be held in the cache. The
224 <nattrsets> parameter specifies the total number of attribute sets
225 (as specified by the {{EX:proxyAttrSet}} directive) that may be
226 defined. The <entrylimit> parameter specifies the maximum number of
227 entries in a cacheable query. The <period> specifies the consistency
228 check period (in seconds). In each period, queries with expired
231 H4: Defining attribute sets
233 > proxyAttrset <index> <attrs...>
235 Used to associate a set of attributes to an index. Each attribute
236 set is associated with an index number from 0 to <numattrsets>-1.
237 These indices are used by the proxyTemplate directive to define
240 H4: Specifying cacheable templates
242 > proxyTemplate <prototype_string> <attrset_index> <TTL>
244 Specifies a cacheable template and the "time to live" (in sec) <TTL>
245 for queries belonging to the template. A template is described by
246 its prototype filter string and set of required attributes identified
252 An example {{slapd.conf}}(5) database section for a caching server
253 which proxies for the {{EX:"dc=example,dc=com"}} subtree held
254 at server {{EX:ldap.example.com}}.
257 > suffix "dc=example,dc=com"
258 > rootdn "dc=example,dc=com"
259 > uri ldap://ldap.example.com/dc=example%2cdc=com
261 > proxycache bdb 100000 1 1000 100
262 > proxyAttrset 0 mail postaladdress telephonenumber
263 > proxyTemplate (sn=) 0 3600
264 > proxyTemplate (&(sn=)(givenName=)) 0 3600
265 > proxyTemplate (&(departmentNumber=)(secretary=*)) 0 3600
268 > directory ./testrun/db.2.a
269 > index objectClass eq
270 > index cn,sn,uid,mail pres,eq,sub
273 H5: Cacheable Queries
275 A LDAP search query is cacheable when its filter matches one of the
276 templates as defined in the "proxyTemplate" statements and when it references
277 only the attributes specified in the corresponding attribute set.
278 In the example above the attribute set number 0 defines that only the
279 attributes: {{EX:mail postaladdress telephonenumber}} are cached for the following
284 > Filter: (&(sn=Richard*)(givenName=jack))
285 > Attrs: mail telephoneNumber
287 is cacheable, because it matches the template {{EX:(&(sn=)(givenName=))}} and its
288 attributes are contained in proxyAttrset 0.
290 > Filter: (&(sn=Richard*)(telephoneNumber))
293 is not cacheable, because the filter does not match the template,
294 nor is the attribute givenName stored in the cache
296 > Filter: (|(sn=Richard*)(givenName=jack))
297 > Attrs: mail telephoneNumber
299 is not cacheable, because the filter does not match the template ( logical
300 OR "|" condition instead of logical AND "&" )
303 H2: Password Policies
308 This overlay provides a variety of password control mechanisms,
309 e.g. password aging, password reuse and duplication control, mandatory
310 password resets, etc.
313 H3: Password Policy Configuration
316 H2: Referential Integrity
321 This overlay can be used with a backend database such as slapd-bdb (5)
322 to maintain the cohesiveness of a schema which utilizes reference
326 H3: Referential Integrity Configuration
334 This overlay is useful to test the behavior of clients when
335 server-generated erroneous and/or unusual responses occur.
338 H3: Return Code Configuration
346 It performs basic DN/data rewrite and
347 objectClass/attributeType mapping.
350 H3: Rewrite/Remap Configuration
358 This overlay implements the provider-side support for syncrepl
359 replication, including persistent search functionality
362 H3: Sync Provider Configuration
365 H2: Translucent Proxy
370 This overlay can be used with a backend database such as slapd-bdb (5)
371 to create a "translucent proxy".
373 Content of entries retrieved from a remote LDAP server can be partially
374 overridden by the database.
377 H3: Translucent Proxy Configuration
380 H2: Attribute Uniqueness
385 This overlay can be used with a backend database such as slapd-bdb (5)
386 to enforce the uniqueness of some or all attributes within a subtree.
389 H3: Attribute Uniqueness Configuration
397 This overlay can be used to enforce a specific order for the values
398 of an attribute when it is returned in a search.
401 H3: Value Sorting Configuration
410 H3: Example Scenarios