http2/h2i: add flag to dial one ip:port but SNI/:authority like it's another

Change-Id: I35049c37558ddedb9056278c9878f3e3fd6f7f76
Reviewed-on: https://go-review.googlesource.com/53640
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Tom Bergan <tombergan@google.com>

http2/h2i/h2i.go

@@ -45,6 +45,7 @@ var (
 	flagNextProto = flag.String("nextproto", "h2,h2-14", "Comma-separated list of NPN/ALPN protocol names to negotiate.")
 	flagInsecure  = flag.Bool("insecure", false, "Whether to skip TLS cert validation")
 	flagSettings  = flag.String("settings", "empty", "comma-separated list of KEY=value settings for the initial SETTINGS frame. The magic value 'empty' sends an empty initial settings frame, and the magic value 'omit' causes no initial settings frame to be sent.")
+	flagDial      = flag.String("dial", "", "optional ip:port to dial, to connect to a host:port but use a different SNI name (including a SNI name without DNS)")
 )
 
 type command struct {
@@ -147,11 +148,14 @@ func (app *h2i) Main() error {
 		InsecureSkipVerify: *flagInsecure,
 	}
 
-	hostAndPort := withPort(app.host)
+	hostAndPort := *flagDial
+	if hostAndPort == "" {
+		hostAndPort = withPort(app.host)
+	}
 	log.Printf("Connecting to %s ...", hostAndPort)
 	tc, err := tls.Dial("tcp", hostAndPort, cfg)
 	if err != nil {
-		return fmt.Errorf("Error dialing %s: %v", withPort(app.host), err)
+		return fmt.Errorf("Error dialing %s: %v", hostAndPort, err)
 	}
 	log.Printf("Connected to %v", tc.RemoteAddr())
 	defer tc.Close()