Kamailio

Kamailio is an open-source SIP server and router used to build scalable VoIP and real-time communication infrastructure. As a SIP server, it routes, registers, and mediates requests between endpoints, gateways, and application servers. Kamailio can function as a front-end proxy, a registrar, a redirect server, or a combination, and is commonly used to implement large-scale SIP infrastructures, load balancing, and security features.

Kamailio's architecture is modular and written in C for performance. The core provides the SIP engine, while functionality is extended through modules for registration (Registrars), authentication (Auth), NAT traversal (NAT Helper), routing logic (Dispatcher, Route/Rewrite, Dialog), database access (MySQL, PostgreSQL, SQLite, NoSQL backends), presence, billing, and security. It can integrate with media servers via external components for RTP handling. Configuration is expressed in Kamailio's configuration language and, in newer versions, via the KEMI scripting interface for other languages.

History: Kamailio originated as a fork of the SIP Express Router (SER) project. The open-source line was known as OpenSER, then renamed Kamailio in 2008 and is maintained by the Kamailio project community.

Usage: It is deployed by service providers, enterprises, and carriers to implement SIP routing, load balancing, NAT traversal, presence, and security front-ends. Kamailio supports high availability, clustering, and horizontal scaling, and can sit in front of gateways, PBX systems, or media servers.

License: Kamailio is free and open-source software released under the GNU General Public License (GPL), with ongoing development by an international community.