Overview

Alt-Used tells the server that the client actually connected via an alternative service (a different endpoint) advertised by Alt-Svc. It is the request-direction counterpart to Alt-Svc.

The value is the host (and port) of the alternative service the client reached (e.g. `Alt-Used: alt.example.net:443`).

Details

When connecting to an alternative service, the request's Host header still names the original origin, but the actual TCP/QUIC connection goes to a different endpoint. Because of this mismatch it's hard for the server or an intermediate gateway to know which alternative endpoint a request arrived through — Alt-Used states it explicitly.

It serves two purposes. First, loop/misrouting prevention — a gateway can detect a cycle where it names itself as an alternative service. Second, operational observability — knowing which alternative endpoint actually receives traffic helps analyze HTTP/3 adoption and routing.

In practice a browser doesn't always send this header; it attaches it optionally when using an alternative service. Since the value is self-reported by the client, it must be validated outside the trust boundary, and essential routing decisions must not depend on it alone.

Syntax

Alt-Used: <host>[:<port>]

e.g. Alt-Used: alt.example.net:443

Notes

• Alt-Used informs the server of the mismatch where, on an alternative service, Host is the original origin but the actual connection is the alternative endpoint.
• It lets a gateway detect an infinite loop where it names itself as an alternative service.
• Browsers don't always send it (it's optional), and since it's client-supplied it needs validation outside the trust boundary.

Related headers

Specification