OpenLDAP Directory Server

OpenLDAP is a free, open source implementation of the Lightweight Directory Access Protocol (LDAP) developed by the OpenLDAP Project and distributed under the OpenLDAP Public License. The project provides a complete suite of LDAP software including slapd, the standalone LDAP daemon that serves as the directory server; slurpd, a standalone replication daemon (legacy, replaced by syncrepl in modern versions); and various client tools and libraries for interacting with LDAP directories. The slapd server supports multiple database backends including LMDB (Lightning Memory-Mapped Database), the default high-performance backend, as well as SQL, Berkeley DB, and dynamic objects. OpenLDAP implements LDAPv3 as specified in RFC 4510-4519, supporting features such as Simple Authentication and Security Layer (SASL) for flexible authentication mechanisms, Transport Layer Security (TLS) for encrypted connections, schema checking and enforcement, referrals, aliases, LDAP-based access control through access control lists (ACLs), and overlay modules that extend server functionality. The syncrepl replication protocol provides multi-master and master-slave replication with push and pull modes, enabling distributed directory deployments. Popular overlays include the password policy overlay for enforcing password quality rules, the memberOf overlay for maintaining reverse group membership, the audit overlay for change logging, and the unique attribute enforcement overlay. OpenLDAP serves as the directory backbone for many enterprise identity management systems.