code: func (a *AgentServer) ListenAndServe(addr utils.NetAddr) error {
  	l, err := net.Listen(addr.AddrNetwork, addr.Addr)
  	if err != nil {
  		return trace.Wrap(err)
  	}
  	a.listener = l
  	return a.Serve()
  }
docs: // ListenAndServe is similar http.ListenAndServe