good url validation.

cmd/agent/agent.go

@@ -222,9 +222,17 @@ func main() {
 	if len(args) != 1 {
 		printHelp("")
 	}
-	validateString(args[0], "wsUrl", `^wss?://[a-zA-Z0-9]*(:[0-9]+)?$`)
 
-	wsURL := args[0] + "/agent/" + id
+	wsURL := args[0]
+	url, err := url.Parse(wsURL)
+	if err != nil {
+		printHelp(fmt.Sprintf("Invalid URL %s", wsURL))
+	}
+	if url.Path != "" && url.Path != "/" {
+		printHelp(fmt.Sprintf("Only a base URL without path may be specified: %s", wsURL))
+	}
+
+	wsURL += "/agent/" + id
 
 	dialer := websocket.Dialer{
 		Proxy:            http.ProxyFromEnvironment,