Flexisip
Modular C++ SIP server suite for VoIP, messaging, and conferencing
187 stars 83 forks last commit first released AGPL-3.0
Actively maintained
Last commit 18 Aug 2026.
Flexisip is a modular, scalable SIP server suite written in modern C++. It provides proxying, presence, conferencing, push gateway and B2BUA capabilities to build VoIP, messaging and real-time communication services.
Key Features
- Modular server suite: proxy, presence server, conference server, B2BUA and an account manager as separate components.
- Push Gateway: routes SIP notifications to mobile platforms and supports RFC 8599 and current APNs/Firebase requirements.
- Media and NAT handling: built-in media relay with ICE/STUN/TURN support and RTP/RTCP/SRTP handling for NAT traversal and secure media.
- Authentication and security: Digest, TLS client certificate authentication, TLS (OpenSSL) and optional OpenID Connect/OAuth support.
- Account management and remote config: REST API (FlexiAPI) for user/account lifecycle, dynamic per-user configuration, and a web admin platform for service management.
- Scalability and deployment: designed for high-availability cluster deployments, load balancing and low-footprint embedded targets (Raspberry Pi, IoT).
- Interoperability: compatible with other SIP systems and extensions; supports SIP transport over UDP/TCP/TLS and various SIP RFC extensions.
Use Cases
- Deploy a hosted SIP service with calling, messaging and conferencing features for mobile and desktop clients.
- Add push-notification support in front of legacy SIP platforms to deliver calls/messages reliably to smartphones.
- Embed a lightweight SIP proxy in edge/IoT devices (intercoms, access control) to add audio/video calling features.
Limitations and Considerations
- PSTN interconnection requires a separate trunk or gateway; Flexisip provides B2BUA functionality but does not supply PSTN lines or virtual numbers.
- Some optional features require additional native dependencies (for example MySQL for account DB clustering, Redis for registrar/cluster communication, XercesC for presence features), so build-time configuration impacts feature set.
- Dual-licensing (AGPLv3 or proprietary) may impose AGPL obligations for some deployments; evaluate licensing before commercial use.
Flexisip combines a full set of SIP server components with mobile-friendly features and small-footprint deployment options. It is targeted at teams building unified communication services, embedded SIP-enabled devices, and operators needing push-enabled SIP gateways.
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