Issue #304: Add X-Forwarded-Prefix header

This patch adds the X-Forwarded-Prefix header to
upstream requests which contains the unmodified
url path from the original request.

Fixes #304

proxy/http_headers.go

@@ -66,6 +66,10 @@ func addHeaders(r *http.Request, cfg config.Proxy) error {
 		r.Header.Set("X-Forwarded-Port", localPort(r))
 	}
 
+	if r.URL != nil && r.Header.Get("X-Forwarded-Prefix") == "" {
+		r.Header.Set("X-Forwarded-Prefix", r.URL.Path)
+	}
+
 	fwd := r.Header.Get("Forwarded")
 	if fwd == "" {
 		fwd = "for=" + remoteIP + "; proto=" + proto

proxy/http_headers_test.go

@@ -3,6 +3,7 @@ package proxy
 import (
 	"crypto/tls"
 	"net/http"
+	"net/url"
 	"testing"
 
 	"github.com/fabiolb/fabio/config"
@@ -25,13 +26,14 @@ func TestAddHeaders(t *testing.T) {
 		},
 
 		{"http request",
-			&http.Request{RemoteAddr: "1.2.3.4:5555"},
+			&http.Request{URL: &url.URL{Path: "/foo"}, RemoteAddr: "1.2.3.4:5555"},
 			config.Proxy{},
 			http.Header{
-				"Forwarded":         []string{"for=1.2.3.4; proto=http"},
-				"X-Forwarded-Proto": []string{"http"},
-				"X-Forwarded-Port":  []string{"80"},
-				"X-Real-Ip":         []string{"1.2.3.4"},
+				"Forwarded":          []string{"for=1.2.3.4; proto=http"},
+				"X-Forwarded-Proto":  []string{"http"},
+				"X-Forwarded-Port":   []string{"80"},
+				"X-Forwarded-Prefix": []string{"/foo"},
+				"X-Real-Ip":          []string{"1.2.3.4"},
 			},
 			"",
 		},