Issue #294: Use upstream hostname

This patch adds support for a 'host=dst' option on a route which will
trigger the proxy to use the hostname of the target host for the 
outgoing request instead of the one provided by incoming request.
This allows fabio to act as a reverse proxy for an external site.

proxy/http_integration_test.go

@@ -117,6 +117,45 @@ func TestProxyStripsPath(t *testing.T) {
 	}
 }
 
+//	TestProxyHost
+func TestProxyHost(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, r.Host)
+	}))
+
+	proxy := httptest.NewServer(&HTTPProxy{
+		Transport: &http.Transport{
+			Dial: func(network, addr string) (net.Conn, error) {
+				addr = server.URL[len("http://"):]
+				return net.Dial(network, addr)
+			},
+		},
+		Lookup: func(r *http.Request) *route.Target {
+			routes := "route add mock /hostdst http://a.com/ opts \"host=dst\"\n"
+			routes += "route add mock /hostunknown http://a.com/ opts \"host=garbble\"\n"
+			routes += "route add mock / http://a.com/"
+			tbl, _ := route.NewTable(routes)
+			return tbl.Lookup(r, "", route.Picker["rr"], route.Matcher["prefix"])
+		},
+	})
+	defer proxy.Close()
+
+	check := func(uri, host string) {
+		resp, body := mustGet(proxy.URL + uri)
+		if got, want := resp.StatusCode, http.StatusOK; got != want {
+			t.Fatalf("got status %d want %d", got, want)
+		}
+		if got, want := string(body), host; got != want {
+			t.Fatalf("got body %q want %q", got, want)
+		}
+	}
+
+	proxyHost := proxy.URL[len("http://"):]
+	t.Run("host eq dst", func(t *testing.T) { check("/hostdst", "a.com") })
+	t.Run("host is unknown", func(t *testing.T) { check("/hostunknown", proxyHost) })
+	t.Run("no host", func(t *testing.T) { check("/", proxyHost) })
+}
+
 func TestProxyLogOutput(t *testing.T) {
 	// build a format string from all log fields and one header field
 	fields := []string{"header.X-Foo:$header.X-Foo"}

proxy/http_proxy.go

@@ -100,6 +100,10 @@ func (p *HTTPProxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		targetURL.RawQuery = t.URL.RawQuery + "&" + r.URL.RawQuery
 	}
 
+	if t.Host == "dst" {
+		r.Host = targetURL.Host
+	}
+
 	// TODO(fs): The HasPrefix check seems redundant since the lookup function should
 	// TODO(fs): have found the target based on the prefix but there may be other
 	// TODO(fs): matchers which may have different rules. I'll keep this for

route/route.go

@@ -71,6 +71,7 @@ func (r *Route) addTarget(service string, targetURL *url.URL, fixedWeight float6
 	if r.Opts != nil {
 		t.StripPath = r.Opts["strip"]
 		t.TLSSkipVerify = r.Opts["tlsskipverify"] == "true"
+		t.Host = r.Opts["host"]
 	}
 
 	r.Targets = append(r.Targets, t)

route/target.go

@@ -21,6 +21,12 @@ type Target struct {
 	// TLS connections.
 	TLSSkipVerify bool
 
+	// Host signifies what the proxy will set the Host header to.
+	// The proxy does not modify the Host header by default.
+	// When Host is set to 'dst' the proxy will use the host name
+	// of the target host for the outgoing request.
+	Host string
+
 	// URL is the endpoint the service instance listens on
 	URL *url.URL