code: func gratuitousARPReply(ip net.IP, mac net.HardwareAddr) (*arpMessage, error) {
  	if ip.To4() == nil {
  		return nil, fmt.Errorf("%q is not an IPv4 address", ip)
  	}
  	if len(mac) != hwLen {
  		return nil, fmt.Errorf("%q is not an Ethernet MAC address", mac)
  	}
  
  	m := &arpMessage{
  		arpHeader{
  			1,           // Ethernet
  			0x0800,      // IPv4
  			hwLen,       // 48-bit MAC Address
  			net.IPv4len, // 32-bit IPv4 Address
  			opARPReply,  // ARP Reply
  		},
  		mac,
  		ip.To4(),
  		ethernetBroadcast,
  		net.IPv4bcast,
  	}
  
  	return m, nil
  }
docs: // gratuitousARPReply returns an ARP message that contains a gratuitous ARP
  // reply from the specified sender.