HTTP Proxy Requests: Absolute URLs vs. the Host Header

Keep the absolute request target and Host field consistent when making an HTTP/1.1 request through an explicit proxy.

Which host does this HTTP/1.1 proxy request mean?

GET http://first.example/path HTTP/1.1
Host: second.example

The message is inconsistent. For an absolute-form request sent to a proxy, RFC 9112 requires the forwarded Host value to come from the request target. Forwarding Host: first.example is the expected behavior.

For a development-site test, keep the resource hostname coherent in the URL and HTTP authority. Supply the desired name-to-address mapping at the hop responsible for resolution. A conflicting Host override is not a portable backend-selection instruction.

HTTPS differs. After an ordinary CONNECT tunnel opens, TLS and the inner HTTP exchange occur inside it. A non-intercepting proxy cannot generally rewrite that encrypted Host field. CONNECT destination, TLS name and certificate identity remain separate roles.

Deleting Via, Referer or Cache-Control does not reconcile the two authorities. Record the request form and the point where you observed the header before diagnosing a proxy rewrite.

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