Unable to establish TCP connection when enabling internet sharing (Phone acting as hotspot)

Question

I have developed a small .NET Desktop application which hosts a TCP service. In addition, it sends UDP Multicasts to anounce that service.
Service consumer is a Phone 8.1 app, which listens for these UDP multicasts and when it receives such a multicast, it creates a TCP connection to the service.
The following snippet shows how the Phone 8.1 app listens for UDP packets and then connects via TCP
const string UdpPort = "42";
const string TcpPort = "43";
const string MultiCastIp = "224.5.6.7";
const int TimeoutForServerLookup = 10000;
//-- listen for UDP multicasts
HostName hostNameOfServer = null;
var udp = new DatagramSocket();
var clientFound = false;
udp.MessageReceived += delegate(DatagramSocket sender, DatagramSocketMessageReceivedEventArgs args)
try
hostNameOfServer = args.RemoteAddress;
catch (Exception ex)
s_Log.Error("Error in handling UDP packet", ex);
await udp.BindServiceNameAsync(UdpPort);
udp.JoinMulticastGroup(new HostName(MultiCastIp));
var waitedTimeInMs = 0;
const int delayInMs = 250;
while (waitedTimeInMs < TimeoutForServerLookup && hostNameOfServer == null)
waitedTimeInMs += delayInMs;
await Task.Delay(TimeSpan.FromMilliseconds(delayInMs));
udp.Dispose();
//-- we now have the adress of the TCP server (hostNameOfServer)
//-- connect now to the server. this works when being in a "normal" WLAN, does not work when the phone is the hotspot
var socket = new StreamSocket();
await socket.ConnectAsync(hostNameOfServer, TcpPort);
This works well when the Desktop computer and the Windows Phone 8.1 are within the same WLAN. For some customers it is important that this also works when the Windows Phone 8.1 acts as hotspot (by enabling internet sharing on the phone) and the desktop computer
(or Laptop) connects itself to this WLAN. In this scenario the Phone 8.1 still receives the UDP packets, but when trying to connect via TCP, the connection attempt fails.
Any ideas how to get rid of this problem?

Answer 1

Have you tracked down if the phone actually sends the TCP connection to the desktop? I'm feeling that this is a firewall issue that shows itself when the firewall zone changes (public/private/domain).
Matt Small - Microsoft Escalation Engineer - Forum Moderator
If my reply answers your question, please mark this post as answered.
NOTE: If I ask for code, please provide something that I can drop directly into a project and run (including XAML), or an actual application project. I'm trying to help a lot of people, so I don't have time to figure out weird snippets with undefined
objects and unknown namespaces.