code: func (s *Server) ListenAndServe(addr string) error {
  	ln, err := net.Listen("tcp4", addr)
  	if err != nil {
  		return err
  	}
  	if s.TCPKeepalive {
  		if tcpln, ok := ln.(*net.TCPListener); ok {
  			return s.Serve(tcpKeepaliveListener{
  				TCPListener:     tcpln,
  				keepalivePeriod: s.TCPKeepalivePeriod,
  			})
  		}
  	}
  	return s.Serve(ln)
  }
docs: // ListenAndServe serves HTTP requests from the given TCP4 addr.
  //
  // Pass custom listener to Serve if you need listening on non-TCP4 media
  // such as IPv6.
  //
  // Accepted connections are configured to enable TCP keep-alives.