UPnP Multicast: missing answers from M-SEARCH (Discovery)

Yeah, I solved the problem! Small mistake, big impact:

My program is sending the M-SEARCH on port 1900 which is bound to the UPnP multicast group. Because i bound the LocalEndPoint to the same port, the UPnP devices answers with unicast to port 1900. On iOS it worked, because my program was the only service bound to this port. But on the PC, i found several services bound to port 1900 (found with "netstat -p UDP -a"). So the unicast messages from the UPnP devices was absorbed by one of the other services.

The solution: I bound the LocalEndPoint to a free port (e.g. 60000), and now it works fine!

IPEndPoint LocalEndPoint = new IPEndPoint(IPAddress.Any, 60000);

On create of local endpoint use port 0 (zero) to bind a free port not using a fixed port. Another point discovered. Binding IPAddress.Any or IPAddress.Loopback get responses from Microsoft (local?) system where as binding to one of the LAN address(es) get responses from the local net. Getting first IPV4 address can be done like this:

IPAddress localNetwork = Dns.GetHostAddresses(Environment.GetEnvironmentVariable("COMPUTERNAME")).Where(ia => (ia.AddressFamily == AddressFamily.InterNetwork)).First();

For posterity: setting all these options above is unnecessary for M-SEARCH and might even be counter-productive:

UdpSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);UdpSocket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.AddMembership, new MulticastOption(MulticastEndPoint.Address, IPAddress.Any));UdpSocket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastTimeToLive, 2);UdpSocket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastLoopback, true);

So don't do it.