Kamailio is an open source SIP server, the piece of software that receives, authenticates and steers the signaling messages of a voice over IP network. Its story begins in Berlin on 2 September 2001, at the Fraunhofer FOKUS research institute, with a project called SIP Express Router, usually shortened to SER, which was released under a free license a year later.
The Kamailio name only arrived in July 2008, replacing OpenSER, a 2005 fork of SER that had to be renamed because of a trademark dispute. Development is now carried by a community of developers and companies grouped under The Kamailio SIP Server Project, and the code is distributed under the GPL, version 2 or later. The newest branch, 6.1, reached version 6.1.4 on 20 August 2026.
What Kamailio is for
In a VoIP network, some software carries the voice while other software handles signaling: who is calling whom, where a phone can be reached and which path a call should take. Kamailio belongs to the second group. It can act as a registrar that remembers where each device is reachable, as a proxy relaying SIP requests to the right destination, as a load balancer in front of a pool of servers, or as a SIP firewall and edge controller. It never transcodes or mixes audio. When media streams need help crossing NAT, it is paired with a relay such as RTPEngine.
Its behavior is described in a configuration file that reads like a small program. Every incoming request travels through routing blocks in which the administrator decides, line by line, what to check, rewrite or forward. According to the project’s features page, more than 150 modules come with the source tree, from least cost routing and presence to WebRTC and the IMS building blocks used for VoLTE. A mechanism called KEMI also lets you write that logic in Lua, Python or JavaScript instead of the native scripting language.
Comparing Kamailio with OpenSIPS, the other server from the same family, takes a short history lesson. In 2005 three developers, Daniel-Constantin Mierla, Bogdan-Andrei Iancu and Elena-Ramona Modroiu, started OpenSER from the SER code. In the summer of 2008 the project split. OpenSER became Kamailio, while Bogdan-Andrei Iancu launched OpenSIPS and described it as the true continuation of OpenSER. Kamailio then went the other way and merged its code back with SER, a reunion agreed in November 2008 and completed in December 2012. The two servers therefore share roots, module names and a similar syntax, but their code bases have been drifting apart for more than fifteen years, and a configuration written for one will not run unchanged on the other.
Typical deployments put it at the edge of a telephony provider, in front of several Asterisk or FreeSWITCH servers, or ahead of gateways to the traditional phone network. The project’s website quotes more than 5,000 call setups per second for stateless load balancing.
Who is it for?
Kamailio is aimed at carriers, internet telephony service providers, integrators building communications platforms and network teams that need a SIP entry point able to absorb heavy traffic.
It is not a ready-made phone system. There is no graphical administration interface, no voicemail and no interactive voice response, and it does not process audio. Deploying it requires a good grasp of SIP, network address translation and scripting, and a single logic error in the routing file can stop every call. A small business that just wants a PBX will be better served by FreeSWITCH or Yate, which Kamailio can later sit in front of once traffic grows.
Download Kamailio safely
The only source worth trusting is kamailio.org. Its download page points to the source tarballs, to a signed package repository for Debian and Ubuntu, to RPM packages for CentOS, Fedora and openSUSE, and to the official kamailio/kamailio repository on GitHub for those who build from source. The 6.0 branch is still maintained alongside it and received version 6.0.8 on 16 September 2026.
Everything is free. Companies sell training and support, but the software exists in a single open source edition. Since it usually faces the internet and attracts toll fraud attempts, avoid repackaged builds from forums and apply security fixes as soon as they are announced.
Frequently asked questions
Are Kamailio and OpenSIPS the same software?
No. Both descend from SER and OpenSER, but they have been developed separately since the summer of 2008. Kamailio merged the SER code back in and accepts Lua, Python or JavaScript scripts through KEMI. Their configuration files are not interchangeable.
Can Kamailio replace a PBX?
Not on its own. It routes and controls SIP signaling, but it offers no voicemail, conferencing or voice menus. It is usually placed in front of a media server such as FreeSWITCH, which provides those features.
Does Kamailio run on Windows?
No, the official website offers no Windows version. The packages target Linux distributions, and building from source requires a Unix-like environment.
