Support ipv6 for mDNS Server

[TA2k]

Support ipv6 for mDNS Server

[edaniels]

mdns/v2 now supports IPv6 so it can get into ICE now! Shouldn't be too difficult but we'll need to explicitly configure it

[edaniels]

@TA2k do you want to take a shot at this?

It may be as simple as in mdns.go:

func createMulticastDNS(n transport.Net, networkTypes []NetworkType, mDNSMode MulticastDNSMode, mDNSName string, log logging.LeveledLogger) (*mdns.Conn, MulticastDNSMode, error) {
	if mDNSMode == MulticastDNSModeDisabled {
		return nil, mDNSMode, nil
	}

	var wantV4, wantV6 bool
	if len(networkTypes) == 0 {
		wantV4 = true
		wantV6 = true
	}
	for _, nt := range networkTypes {
		switch nt {
		case NetworkTypeTCP4, NetworkTypeUDP4:
			wantV4 = true
		case NetworkTypeTCP6, NetworkTypeUDP6:
			wantV6 = true
		}
	}
	if !(wantV4 || wantV6) {
		return nil, mDNSMode, errMulticastDNSNoUsableNetworkType
	}

	var multicastPktConnV4 *ipv4.PacketConn
	if wantV4 {
		addr4, mdnsErr := n.ResolveUDPAddr("udp4", mdns.DefaultAddressIPv4)
		if mdnsErr != nil {
			return nil, mDNSMode, mdnsErr
		}

		l4, mdnsErr := n.ListenUDP("udp4", addr4)
		if mdnsErr != nil {
			// If ICE fails to start MulticastDNS server just warn the user and continue
			log.Errorf("Failed to enable mDNS, continuing in mDNS disabled mode: (%s)", mdnsErr)
			return nil, MulticastDNSModeDisabled, nil
		}

		multicastPktConnV4 = ipv4.NewPacketConn(l4)
	}

	var multicastPktConnV6 *ipv6.PacketConn
	if wantV6 {
		addr6, mdnsErr := n.ResolveUDPAddr("udp6", mdns.DefaultAddressIPv6)
		if mdnsErr != nil {
			return nil, mDNSMode, mdnsErr
		}

		l6, mdnsErr := n.ListenUDP("udp6", addr6)
		if mdnsErr != nil {
			// If ICE fails to start MulticastDNS server just warn the user and continue
			log.Errorf("Failed to enable mDNS, continuing in mDNS disabled mode: (%s)", mdnsErr)
			return nil, MulticastDNSModeDisabled, nil
		}

		multicastPktConnV6 = ipv6.NewPacketConn(l6)
	}

	switch mDNSMode {
	case MulticastDNSModeQueryOnly:
		conn, err := mdns.Server(multicastPktConnV4, multicastPktConnV6, &mdns.Config{})
		return conn, mDNSMode, err
	case MulticastDNSModeQueryAndGather:
		conn, err := mdns.Server(multicastPktConnV4, multicastPktConnV6, &mdns.Config{
			LocalNames: []string{mDNSName},
		})
		return conn, mDNSMode, err
	default:
		return nil, mDNSMode, nil
	}
}

but it needs some testing around interfaces that have supported ipv6 addresses https://tools.ietf.org/html/rfc8445#section-5.1.1.1 (link-local ones which are common are not allowed for security reasons of exposing the zone (I think))