Support ipv6 for mDNS Server

[TA2k]

At the moment the mDNS server is not working if other service already offer mDNS service.
To allow multiple mDNS server for example in multi docker systems you have to support ipv6

Please add the option to enable mDNS server via ipv6
https://github.com/pion/ice/blob/c8227261a2b808446bf181475032c54c3b509f1c/mdns.go#L46C5-L46C5

Depends on
pion/mdns#69

[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))