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|Developer(s)||The OpenLDAP Project|
|Stable release||2.4.21 / December 20, 2009|
|Type||LDAP Directory service|
|License||OpenLDAP Public License|
OpenLDAP Software is a free, open source implementation of the Lightweight Directory Access Protocol (LDAP) developed by the OpenLDAP Project. It is released under its own BSD-style license called the OpenLDAP Public License. LDAP is a platform-independent protocol. Several common Linux distributions include OpenLDAP Software for LDAP support. The software also runs on BSD-variants, as well as AIX, Android, HP-UX, Mac OS X, Solaris, Microsoft Windows (NT and derivatives, e.g. 2000, XP, Vista, etc.), and z/OS.
Project history and core team
The OpenLDAP Project was started in 1998 by Kurt Zeilenga. The project started by cloning the LDAP reference source from the University Of Michigan where a long-running project had supported development and evolution of the LDAP protocol.
As of April, 2006, the OpenLDAP Project has three Core Team members: Howard Chu (Chief Architect), Pierangelo Masarati, and Kurt Zeilenga. There are numerous other important and active contributors including Luke Howard, Hallvard Furuseth, Quanah Gibson-Mount, and Gavin Henry.
Components of OpenLDAP Software
OpenLDAP Software has three main components:
- slapd - stand-alone LDAP daemon and associated overlays and tools,
- libraries implementing the LDAP protocol and ASN.1 Basic Encoding Rules (BER), and
- clients software: ldapsearch, ldapadd, ldapdelete, and others
Additionally, the OpenLDAP Project is home to a number of subprojects:
- JLDAP - LDAP class libraries for Java
- JDBC-LDAP - Java JDBC - LDAP Bridge driver
- ldapc++ - LDAP class libraries for C++
Historically the OpenLDAP server (slapd, the Standalone LDAP Daemon) architecture was split between a frontend which handles network access and protocol processing, and a backend which deals strictly with data storage. The architecture is modular and many different backends are available for interfacing to other technologies, not just traditional databases.
Note: In older (1.x) releases, the terms "backend" and "database" were often used interchangeably. To be precise, a "backend" is a class of storage interface, and a "database" is an instance of a backend. The slapd server can use arbitrarily many backends at once, and can have arbitrarily many instances of each backend (i.e., arbitrarily many databases) active at once.
Currently 16 different backends are provided in the OpenLDAP distribution, and various third parties are known to maintain other backends independently. The standard backends are loosely organized into three different categories:
- Data Storage backends - these actually store data
- back-bdb: the first transactional backend for OpenLDAP, built on BerkeleyDB
- back-hdb: a variant of back-bdb that is fully hierarchical and supports subtree renames
- back-ldif: built on plain text LDIF files
- back-ndb: a transactional backend built on MySQL's NDB cluster engine
- Proxy backends - these act as gateways to other data storage systems
- back-ldap: simple proxy to other LDAP servers
- back-meta: proxy with meta-directory features
- back-passwd: uses a Unix system's passwd and group data
- back-relay: internally redirects to other slapd backends
- back-sql: talks to arbitrary SQL databases
- Dynamic backends - these generate data on the fly
- back-config: slapd configuration via LDAP
- back-dnssrv: Locates LDAP servers via DNS
- back-monitor: slapd statistics via LDAP
- back-null: a sink/no-op backend, analogous to Unix /dev/null
- back-perl: invokes arbitrary perl modules in response to LDAP requests
- back-shell: invokes shell scripts for LDAP requests
- back-sock: forwards LDAP requests over IPC to arbitrary daemons
Some backends available in older OpenLDAP releases have been retired from use, most notably back-ldbm which was inherited from the original UMich code, and back-tcl which was similar to back-perl and back-shell.
In practice, backends like -perl, -shell, and -sock allow interfacing to any arbitrary programming language, thus providing limitless capabilities for customization and expansion. In effect the slapd server becomes an RPC engine with a compact, well-defined and ubiquitous API.
Ordinarily an LDAP request is received by the frontend, decoded, and then passed to a backend for processing. When the backend completes a request, it returns a result to the frontend, which then sends the result to the LDAP client. An overlay is a piece of code that can be inserted between the frontend and the backend. It is thus able to intercept requests and trigger other actions on them before the backend receives them, and it can also likewise act on the backend's results before they reach the frontend. Overlays have complete access to the slapd internal APIs, and so can invoke anything the frontend or other backends could perform. Multiple overlays can be used at once, forming a stack of modules between the frontend and the backend.
Overlays provide a simple means to augment the functionality of a database without requiring that an entirely new backend be written, and allow new functionalities to be added in compact, easily debuggable and maintainable modules. Since the introduction of the overlay feature in OpenLDAP 2.2 many new overlays have been contributed from the OpenLDAP community.
Currently there are 21 overlays in the core OpenLDAP distribution, with another 12 overlays in the user-contributed code section, and more awaiting approval for inclusion.
- The core overlays include:
- accesslog: log server activity in another LDAP database, for LDAP-accessible logging
- auditlog: log server activity in a flat text file
- chain: intercept referrals and chain them instead; code is part of back-ldap
- collect: implement X.500-style collective attributes (aka Netscape Class Of Service)
- constraint: restrict the acceptable values for particular attributes
- dds: dynamic data service - short-lived, self-expiring entries
- deref: return information about entries referenced in a given search result
- dyngroup: simple dynamic group support
- dynlist: more sophisticated dynamic group support plus more
- memberof: support for memberOf and similar backlink attributes
- pcache: cache search results, mainly to improve performance for proxied servers
- ppolicy: LDAP Password Policy - password quality, expiration, etc.
- refint: referential integrity
- retcode: set predetermined return codes for various operations; used for client debugging
- rwm: rewrite module, for various alterations of LDAP data
- seqmod: serialize writes to individual entries
- sssvlv: Server Side Sorting and Virtual List Views
- syncprov: Syncrepl Provider, implements the master side of a replication agreement
- translucent: Semi-transparent pass-through, for locally augmenting data on a proxied server
- unique: for enforcing uniqueness of attribute values within a tree
- valsort: maintain various sort orders for values of an attribute
- The contrib overlays include:
- addpartial: receive Add requests and turn them into Modifies if the target entry already exists
- allop: returns all operational attributes, for clients that don't know how to request them
- autogroup: dynamically managed static groups
- cloak: hide attributes unless explicitly requested in a search
- denyop: reject arbitrarily configured requests
- lastmod: maintain the timestamp of the last change within a tree
- nops: filter out redundant modifies
- nssov: Answer NSS and PAM requests directly in slapd, replaces nss-ldap and pam-ldap.
- proxyOld: support an obsolete encoding of ProxyAuthz used by Sun et al.
- smbk5pwd: Maintain Samba and Kerberos passwords
- trace: Log every LDAP request and response
- usn: Update Sequence Numbers (as in Microsoft AD, not yet released)
OpenLDAP also supports SLAPI, the plugin architecture used by Sun and Netscape/Fedora/Red Hat. In current releases, the SLAPI framework is implemented inside a slapd overlay. While many plugins written for Sun/Netscape/Fedora/Red Hat are compatible with OpenLDAP, very few members of the OpenLDAP community use SLAPI.
The major (functional) releases of OpenLDAP Software include:
- OpenLDAP Version 1 was a general clean-up of the last release from the University of Michigan project (release 3.3), and consolidation of additional changes.
- OpenLDAP Version 2.0, released in August, 2000, included major enhancements including LDAP version 3 (LDAPv3) support, Internet Protocol version 6 (IPv6) support, and numerous other enhancements.
- OpenLDAP Version 2.1, released in June, 2002, included the transactional database backend (based on Berkeley Database or BDB), Simple Authentication and Security Layer (SASL) support, and Meta, Monitor, and Virtual experimental backends.
- OpenLDAP Version 2.2, released in December, 2003, included the LDAP "sync" Engine with replication support (syncrepl), the overlay interface, and numerous database and RFC-related functional enhancements.
- OpenLDAP Version 2.3, released in June, 2005, included the Configuration Backend (dynamic configuration), additional overlays including RFC-compliant Password Policy software, and numerous additional enhancements.
- OpenLDAP Version 2.4, released in October, 2007, introduced N-way MultiMaster replication, Stand-by master, and the ability to delete and modify Schema elements on the fly, plus many more.
- The OpenLDAP website
- The OpenLDAP Project
- The OpenLDAP Foundation
- OpenLDAP Software
- OpenLDAP Public License
- Using libldap, A tutorial on the OpenLDAP client API
- An OpenLDAP Update article by Marty Heyman 09/13/2007cs:OpenLDAP