Jetty Transparent Proxy - add Userinfo

Question

I've successfully created a transparent proxy using jetty 9 with the following servlet code:

public class ProxyRewriteServlet extends ProxyServlet.Transparent {
   private static final String HOST = "192.168.1.20" // 'hidden' server
   private static final int PORT = 28000 // the port the server is listening to

   @Override
   protected URI rewriteURI(HttpServletRequest request) {
       URIBuilder builder = new URIBuilder()
        .setScheme("http").setHost(HOST).setPort(PORT)
        .setPath(request.getRequestURI());

      return builder.build();
   }

The proxy forwards to a well-known server (192.168.1.20) which requires basic authentication. A direct access to the server with the address http://admin:[email protected]/path works. An access over the proxy http://admin:[email protected]/path works as well.

However, accessing over http://proxy.from.above/path does not work because of the authentication. Therefore I complement rewriteURI() with the following:

builder.setUserInfo("admin", "password"); 

Unfortunately, this does not have any effect and the server asks the browser to enter the credentials.

Is there a proper way to inject the user info at the proxy? Or do they necessarily need to come from the client?