Restore Forwarded header in reverse proxy Rewrite mode - #575
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The migration from ReverseProxy.Director to Rewrite in ebe6fed (cloudfoundry#573) restored X-Forwarded-For, X-Forwarded-Host, and X-Forwarded-Proto, but missed the RFC 7239 Forwarded header. Rewrite mode strips Forwarded, X-Forwarded-For, X-Forwarded-Host, and X-Forwarded-Proto from the outbound request before calling the Rewrite func. Director never stripped any of these, so client-set Forwarded headers now get dropped. Copy Forwarded from the inbound request verbatim, preserving multiple header values, to match the previous pass-through behavior. Add regression tests covering single value, multiple values, and the absent-header case.
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Summary
The standard RFC 7239
Forwardedheader was being silently dropped by gorouter and never reaching backend apps.The cause is the switch from the deprecated
ReverseProxy.DirectortoReverseProxy.Rewritein ebe6fed (#573), done for Go 1.26 staticcheck compliance. Go'sRewritemode stripsForwarded,X-Forwarded-For,X-Forwarded-Host, andX-Forwarded-Protofrom the outbound request before it calls the rewrite func (seeoutreq.Header.Del("Forwarded")innet/http/httputil/reverseproxy.go).Directormode never stripped any of these. That commit put back the threeX-Forwarded-*headers but missedForwarded, so client-setForwardedheaders stopped gettingforwarded.
This change copies
Forwardedfrom the inbound request to the outbound request as-is, keeping multiple header values intact per RFC 7239. It follows the same restore logic already used for theX-Forwarded-*headers inproxy/proxy.go. It also adds regression tests for a single value, multiple values, and the case where the client sends no header.net/http/httputilReverseProxy.Rewritestrips forwarding headers before calling the rewrite funcBackward Compatibility
Breaking Change? No
This puts back the pass-through behavior that existed before ebe6fed (#573), so it returns gorouter to how it used to work rather than adding new behavior. It is a plain verbatim pass-through with no new config, applies right away to all deployments, and needs no opt-in.