rproxy: remove forwarded headers spelt with underscores from an untrusted peer (closes #334)

For a peer outside `SWRP_TRUSTED_PROXIES`, `rproxy` now reads each header name with `_` as `-` and in any letter case, and removes the header when that name starts with `X-Forwarded-` or is `Forwarded` or `X-Real-IP`. Before, `X_Forwarded_Ssl: on` or `X_Real_IP` from a client connecting directly reached the app unchanged, and an app behind CGI or FastCGI reads them as the dash-spelt headers. A trusted peer's headers pass on as before. `SPEC.md` says so.

Model: opus-5-5

SPEC.md

@@ -2326,7 +2326,8 @@ but for these headers:
   claims and are replaced. `X-Forwarded-For` names the peer, `X-Forwarded-Host`
   the host it asked for, and `X-Forwarded-Proto` plain `http`. Every other
   header whose name starts with `X-Forwarded-` is removed, and so are
-  `Forwarded` and `X-Real-IP`.
+  `Forwarded` and `X-Real-IP`. A name spelt with `_` for `-`, such as
+  `X_Forwarded_Ssl`, counts as the same name.
 - `X-Request-ID`, which carries the fresh id.
 - `X-Client-ASN` and `X-Client-Country`, which carry what `waf` answered. A
   header it did not give is left out. Any of the two the visitor sent is

internal/rproxy/client.go

@@ -105,17 +105,16 @@ func setForwardedHeaders(in, out *http.Request, peer netip.Addr, trusted bool) {
 		return
 	}
 
-	// ReverseProxy has removed Forwarded and the three set below. Go's server
+	// An app behind CGI or FastCGI reads X_Forwarded_Ssl as X-Forwarded-Ssl, so
-	// gives header names in canonical form, so the prefix matches the name in
+	// a name is matched with each _ read as - and in any letter case.
-	// any letter case.
 	for name := range out.Header {
-		if strings.HasPrefix(name, "X-Forwarded-") {
+		spelt := strings.ToLower(strings.ReplaceAll(name, "_", "-"))
+		if strings.HasPrefix(spelt, "x-forwarded-") ||
+			spelt == "forwarded" || spelt == "x-real-ip" {
 			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")

internal/rproxy/rproxy_test.go

@@ -561,6 +561,7 @@ func TestAPeerNotTrustedHasItsForwardedHeadersReplaced(t *testing.T) {
 		"X-Forwarded-Tls-Client-Cert-Info", "X-Real-Ip",
 		"X-Forwarded-Scheme", "X-Forwarded-Ssl", "X-Forwarded-Protocol",
 		"X-Forwarded-Path", "X-Forwarded-Client-Cert",
+		"X_Forwarded_Ssl", "X_Forwarded_Proto", "X_Real_IP",
 	}
 
 	standIn := startWAF(t, allow)
@@ -628,6 +629,7 @@ func TestATrustedProxysForwardedHeadersPassOn(t *testing.T) {
 		forwardedProtoHeader: "https",
 		"X-Real-Ip":          "198.51.100.1",
 		"X-Forwarded-Ssl":    "on",
+		"X_Forwarded_Ssl":    "on",
 	}
 
 	req := newRequest(t, http.MethodGet, addr, "/", http.NoBody)