# $OpenLDAP$ # Copyright 2007 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, but some may also be configured globally. Essentially they 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 Overlays are usually documented by separate specific man pages in section 5; the naming convention is > slapo- 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 > servers/slapd/overlays/ 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// 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. H2: Access Logging H3: Overview This overlay can record accesses to a given backend database on another database. H3: Access Logging Configuration H2: Audit Logging This overlay records changes on a given backend database to an LDIF log file. H3: Overview H3: Audit Logging Configuration 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 --enable-ldap. H3: Chaining Configuration H2: Constraints H3: 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. H3: Constraint Configuration H2: Dynamic Directory Services H3: Overview This overlay supports dynamic objects, which have a limited life after which they expire and are automatically deleted. H3: Dynamic Directory Service Configuration H2: Dynamic Groups H3: Overview This overlay extends the Compare operation to detect members of a dynamic group. H3: Dynamic Group Configuration H2: Dynamic Lists H3: Overview This overlay allows expansion of dynamic groups and more. H3: Dynamic List Configuration H2: Reverse Group Membership Maintenance H3: Member Of Configuration 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 This directive enables proxy caching and sets general cache parameters. The parameter specifies which underlying database is to be used to hold cached entries. It should be set to {{EX:bdb}} or {{EX:hdb}}. The parameter specifies the total number of entries which may be held in the cache. The parameter specifies the total number of attribute sets (as specified by the {{EX:proxyAttrSet}} directive) that may be defined. The parameter specifies the maximum number of entries in a cacheable query. The specifies the consistency check period (in seconds). In each period, queries with expired TTLs are removed. H4: Defining attribute sets > proxyAttrset Used to associate a set of attributes to an index. Each attribute set is associated with an index number from 0 to -1. These indices are used by the proxyTemplate directive to define cacheable templates. H4: Specifying cacheable templates > proxyTemplate Specifies a cacheable template and the "time to live" (in sec) for queries belonging to the template. A template is described by its prototype filter string and set of required attributes identified by . 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/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 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 "&" ) H2: Password Policies H3: Overview This overlay provides a variety of password control mechanisms, e.g. password aging, password reuse and duplication control, mandatory password resets, etc. H3: Password Policy Configuration 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. H3: Referential Integrity Configuration H2: Return Code H3: Overview This overlay is useful to test the behavior of clients when server-generated erroneous and/or unusual responses occur. H3: Return Code Configuration H2: Rewrite/Remap H3: Overview It performs basic DN/data rewrite and objectClass/attributeType mapping. H3: Rewrite/Remap Configuration H2: Sync Provider H3: Overview This overlay implements the provider-side support for syncrepl replication, including persistent search functionality H3: Sync Provider Configuration H2: Translucent Proxy H3: 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. H3: Translucent Proxy Configuration 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 H2: Value Sorting H3: Overview This overlay can be used to enforce a specific order for the values of an attribute when it is returned in a search. H3: Value Sorting Configuration H2: Overlay Stacking H3: Overview H3: Example Scenarios H4: Samba