code: func (l *LoadBalancer) Listen() error {
  	var err error
  	l.listener, err = net.Listen(l.frontend.AddrNetwork, l.frontend.Addr)
  	if err != nil {
  		return trace.ConvertSystemError(err)
  	}
  	l.Debugf("created listening socket")
  	return nil
  }
docs: // Listen creates a listener on the frontend addr