rproxy: remove every X-Forwarded- header from an untrusted peer (closes #325)

For a peer outside `SWRP_TRUSTED_PROXIES`, `rproxy` now removes every header whose name starts with `X-Forwarded-`, in any letter case, before it sets `X-Forwarded-For`, `X-Forwarded-Host` and `X-Forwarded-Proto`; `Forwarded` and `X-Real-IP` are removed as before. Until now only traefik's names were removed, so a client connecting directly could send `X-Forwarded-Ssl: on` or `X-Forwarded-Scheme: https` and make an app take a plain request for a secure one. A trusted peer's headers pass on unchanged. `SPEC.md` says so.

Model: opus-5-5

internal/rproxy/client.go

@@ -105,17 +105,17 @@ func setForwardedHeaders(in, out *http.Request, peer netip.Addr, trusted bool) {
 		return
 	}
 
-	// ReverseProxy has removed Forwarded and the three set below; these are the
-	// other headers in which traefik tells the app about the visitor and its
-	// request.
-	for _, name := range []string{
-		"X-Forwarded-Port", "X-Forwarded-Server", "X-Forwarded-Uri",
-		"X-Forwarded-Method", "X-Forwarded-Prefix", "X-Forwarded-Tls-Client-Cert",
-		"X-Forwarded-Tls-Client-Cert-Info", "X-Real-Ip",
-	} {
-		out.Header.Del(name)
+	// ReverseProxy has removed Forwarded and the three set below. Go's server
+	// gives header names in canonical form, so the prefix matches the name in
+	// any letter case.
+	for name := range out.Header {
+		if strings.HasPrefix(name, "X-Forwarded-") {
+			delete(out.Header, name)
+		}
 	}
 
+	out.Header.Del("X-Real-IP")
+
 	out.Header.Set("X-Forwarded-For", forwardedFor)
 	out.Header.Set("X-Forwarded-Host", in.Host)
 	out.Header.Set("X-Forwarded-Proto", "http")