// tun2socks/main
flag.StringVar(&key.MulticastGroups, "multicast-groups", "", "Join these multicast groups ip1,ip2,...")

// tun2socks/engine/parse
func parseMulticastGroups(multicastGroupsString string) ([]net.IP, error) {
	if multicastGroupsString == "" {
		return []net.IP{}, nil
	}
	ipStrings := strings.Split(multicastGroupsString, ",")
	multicastGroups := []net.IP{}
	for _, ipString := range ipStrings {
		ip := net.ParseIP(ipString)
		if ip == nil {
			return nil, fmt.Errorf("unsupported ip format: %s", ipString)
		}
		if !ip.IsMulticast() {
			return nil, fmt.Errorf("invalid multicast address: %s", ipString)
		}
		multicastGroups = append(multicastGroups, ip)
	}
	return multicastGroups, nil
}

// tun2socks/engine/engine
var multicastGroups []net.IP = nil
if multicastGroups, err = parseMulticastGroups(k.MulticastGroups); err != nil {
    return err
}
if _defaultStack, err = core.CreateStack(&core.Config{
    LinkEndpoint:     _defaultDevice,
    TransportHandler: &mirror.Tunnel{},
    MulticastGroups:  multicastGroups,
    Options:          opts,
}); err != nil {
    return
}

func withMulticastGroups(nicID tcpip.NICID, multicastGroups []net.IP) option.Option {
	return func(s *stack.Stack) error {
        if multicastGroups == nil{
			return nil
		}
		s.AddProtocolAddress(
			nicID,
			tcpip.ProtocolAddress{
				Protocol: ipv4.ProtocolNumber,
				AddressWithPrefix: tcpip.AddressWithPrefix{
					Address:   "\x0A\x00\x00\x01",
					PrefixLen: 8,
				},
			},
			stack.AddressProperties{PEB: stack.CanBePrimaryEndpoint},
		)
		s.AddProtocolAddress(
			nicID,
			tcpip.ProtocolAddress{
				Protocol: ipv6.ProtocolNumber,
				AddressWithPrefix: tcpip.AddressWithPrefix{
					Address:   "\xfd\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01",
					PrefixLen: 8,
				},
			},
			stack.AddressProperties{PEB: stack.CanBePrimaryEndpoint},
		)
		for _, multicastGroup := range multicastGroups {
			if tcpIpAddr := multicastGroup.To4(); tcpIpAddr != nil {
				if err := s.JoinGroup(ipv4.ProtocolNumber, nicID, tcpip.Address(tcpIpAddr)); err != nil {
					return fmt.Errorf("join multicast groups: %s", err)
				}
			} else {
				tcpIpAddr := multicastGroup.To16()
				if err := s.JoinGroup(ipv6.ProtocolNumber, nicID, tcpip.Address(tcpIpAddr)); err != nil {
					return fmt.Errorf("join multicast groups: %s", err)
				}
			}
		}
		return nil
	}
}