Outbound Proxy
An outbound proxy is the server a SIP client sends all its requests to, regardless of the destination domain. Learn when it is needed and how it is configured.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
An outbound proxy is a SIP server to which a client sends every request, regardless of the Request-URI, typically to pass through NAT, firewalls or a provider's edge.
Normally a SIP client sends a request to the host named in the Request-URI. An outbound proxy changes that: the client sends every request to one fixed server, and that server takes care of forwarding. RFC 3261 section 8.1.2 describes this behaviour, and RFC 5626 extends it so that clients behind NAT can keep a single connection open through which all traffic flows.
Outbound proxies are useful when the SIP domain and the actual server differ. A provider might give you the domain example.net for your account while asking you to send traffic to sip-edge.example.net. A company might route all SIP through a session border controller. In both cases the Domain stays the same while the outbound proxy points at the real entry point.
In SIPdia the Outbound proxy field is in the Advanced section of the account page. Leave it empty unless your provider or PBX documentation specifies one. When set, the Server and Domain fields still define your identity, but the connection goes to the proxy address.
Sources and further reading
- RFC 3261: SIP: Session Initiation Protocol, Section 8.1.2 Sending the Requestwww.rfc-editor.org
- RFC 5626: Managing Client-Initiated Connections in SIPwww.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.
