SIP Proxy
A SIP proxy receives SIP requests and forwards them toward their destination, making routing decisions and often enforcing authentication along the way.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
A SIP proxy is an intermediary server that receives SIP requests and forwards them toward the destination, applying routing rules and authentication without handling the media.
A SIP proxy sits between endpoints and routes requests. When an INVITE arrives, the proxy decides where it should go next, perhaps by looking up the registrar's bindings, applying a dial plan, or consulting DNS, and forwards it. Responses travel back along the same path. RFC 3261 section 16 defines proxy behaviour in detail.
Proxies work on signaling only. The audio normally flows directly between the endpoints or through a separate media server, so a proxy can handle many calls without touching any RTP.
A proxy may be stateless, forwarding each message independently, or stateful, tracking each transaction so it can retransmit, fork a request to several devices, or cancel branches that are not answered. Proxies also commonly challenge requests for authentication, insert Record-Route headers so that later messages in the call follow the same path, and enforce policy such as which users may call which destinations.
In small deployments the proxy, registrar and media handling are all inside one PBX. In larger ones, dedicated proxies such as Kamailio or OpenSIPS front a pool of media servers. From the softphone's point of view, the server address you configure is usually the proxy.
Sources and further reading
- RFC 3261: SIP: Session Initiation Protocol, Section 16 Proxy Behaviorwww.rfc-editor.org
This article is published by SpaGreen Creative, the maker of SIPdia. Product names mentioned belong to their owners. Where it describes other software, treat the steps as general guidance and check the official documentation linked above.
