Change the representation of domain fronting in HTTP rendezvous.

Formerly, BrokerChannel represented the broker URL and possible domain
fronting as
	bc.url  *url.URL
        bc.Host string
That is, bc.url is the URL of the server which we contact directly, and
bc.Host is the Host header to use in the request. With no domain
fronting, bc.url points directly at the broker itself, and bc.Host is
blank. With domain fronting, we do the following reshuffling:
	if front != "" {
		bc.Host = bc.url.Host
		bc.url.Host = front
	}
That is, we alter bc.url to reflect that the server to which we send
requests directly is the CDN, not the broker, and store the broker's own
URL in the HTTP Host header.

The above representation was always confusing to me, because in my
mental model, we are always conceptually communicating with the broker;
but we may optionally be using a CDN proxy in the middle. The new
representation is
	bc.url   *url.URL
        bc.front string
bc.url is the URL of the broker itself, and never changes. bc.front is
the optional CDN front domain, and likewise never changes after
initialization. When domain fronting is in use, we do the swap in the
http.Request struct, not in BrokerChannel itself:
	if bc.front != "" {
		request.Host = request.URL.Host
		request.URL.Host = bc.front
	}

Compare to the representation in meek-client:

https://gitweb.torproject.org/pluggable-transports/meek.git/tree/meek-client/meek-client.go?h=v0.35.0#n94
	var options struct {
		URL       string
		Front     string
	}
https://gitweb.torproject.org/pluggable-transports/meek.git/tree/meek-client/meek-client.go?h=v0.35.0#n308
	if ok { // if front is set
		info.Host = info.URL.Host
		info.URL.Host = front
	}

client/lib/lib_test.go

@@ -172,14 +172,14 @@ func TestSnowflakeClient(t *testing.T) {
 
 	Convey("Dialers", t, func() {
 		Convey("Can construct WebRTCDialer.", func() {
-			broker := &BrokerChannel{Host: "test"}
+			broker := &BrokerChannel{front: "test"}
 			d := NewWebRTCDialer(broker, nil, 1)
 			So(d, ShouldNotBeNil)
 			So(d.BrokerChannel, ShouldNotBeNil)
-			So(d.BrokerChannel.Host, ShouldEqual, "test")
+			So(d.BrokerChannel.front, ShouldEqual, "test")
 		})
 		SkipConvey("WebRTCDialer can Catch a snowflake.", func() {
-			broker := &BrokerChannel{Host: "test"}
+			broker := &BrokerChannel{}
 			d := NewWebRTCDialer(broker, nil, 1)
 			conn, err := d.Catch()
 			So(conn, ShouldBeNil)
@@ -201,8 +201,8 @@ func TestSnowflakeClient(t *testing.T) {
 			b, err := NewBrokerChannel("http://test.broker", "", transport, false)
 			So(b.url, ShouldNotBeNil)
 			So(err, ShouldBeNil)
-			So(b.Host, ShouldResemble, "")
 			So(b.url.Host, ShouldResemble, "test.broker")
+			So(b.front, ShouldResemble, "")
 			So(b.transport, ShouldNotBeNil)
 		})
 
@@ -210,8 +210,8 @@ func TestSnowflakeClient(t *testing.T) {
 			b, err := NewBrokerChannel("http://test.broker", "front", transport, false)
 			So(b.url, ShouldNotBeNil)
 			So(err, ShouldBeNil)
-			So(b.Host, ShouldResemble, "test.broker")
-			So(b.url.Host, ShouldResemble, "front")
+			So(b.url.Host, ShouldResemble, "test.broker")
+			So(b.front, ShouldResemble, "front")
 			So(b.transport, ShouldNotBeNil)
 		})
 

client/lib/rendezvous.go

@@ -32,10 +32,8 @@ const (
 
 // Signalling Channel to the Broker.
 type BrokerChannel struct {
-	// The Host header to put in the HTTP request (optional and may be
-	// different from the host name in URL).
-	Host               string
 	url                *url.URL
+	front              string            // Optional front domain to replace url.Host in requests.
 	transport          http.RoundTripper // Used to make all requests.
 	keepLocalAddresses bool
 	NATType            string
@@ -61,14 +59,12 @@ func NewBrokerChannel(broker string, front string, transport http.RoundTripper,
 		return nil, err
 	}
 	log.Println("Rendezvous using Broker at:", broker)
-	bc := new(BrokerChannel)
-	bc.url = targetURL
-	if front != "" { // Optional front domain.
+	if front != "" {
 		log.Println("Domain fronting using:", front)
-		bc.Host = bc.url.Host
-		bc.url.Host = front
 	}
-
+	bc := new(BrokerChannel)
+	bc.url = targetURL
+	bc.front = front
 	bc.transport = transport
 	bc.keepLocalAddresses = keepLocalAddresses
 	bc.NATType = nat.NATUnknown
@@ -92,7 +88,7 @@ func limitedRead(r io.Reader, limit int64) ([]byte, error) {
 func (bc *BrokerChannel) Negotiate(offer *webrtc.SessionDescription) (
 	*webrtc.SessionDescription, error) {
 	log.Println("Negotiating via BrokerChannel...\nTarget URL: ",
-		bc.Host, "\nFront URL:  ", bc.url.Host)
+		bc.url.Host, "\nFront URL:  ", bc.front)
 	// Ideally, we could specify an `RTCIceTransportPolicy` that would handle
 	// this for us.  However, "public" was removed from the draft spec.
 	// See https://developer.mozilla.org/en-US/docs/Web/API/RTCConfiguration#RTCIceTransportPolicy_enum
@@ -126,8 +122,11 @@ func (bc *BrokerChannel) Negotiate(offer *webrtc.SessionDescription) (
 	if nil != err {
 		return nil, err
 	}
-	if "" != bc.Host { // Set true host if necessary.
-		request.Host = bc.Host
+	if bc.front != "" {
+		// Do domain fronting. Replace the domain in the URL's with the
+		// front, and store the original domain the HTTP Host header.
+		request.Host = request.URL.Host
+		request.URL.Host = bc.front
 	}
 	resp, err := bc.transport.RoundTrip(request)
 	if nil != err {