title: How to check whether UPnP port 1900 and Bonjour port 5353 is blocked by the firewall
body: <p>I am listing the all the local area network devices using UPnP in C# .NET, I am using UPnP API's given by windows in C# .NET (by adding reference to UPnP COM Library). When UPnP Scan don't see any device then I have to check whether that port (1900) is blocked by the firewall, if yes I have to notify the user.</p>
  
  <p>Code to scan the UPnP Devices</p>
  
  <pre><code>UPnPDeviceFinder devFinder = new UPnPDeviceFinder();
  UPnPDevices devices = devFinder.FindByType("upnp:rootdevice", 0);
  Debug.WriteLine("Devices Count:=" + devices.Count);
  </code></pre>
  
  <p>I have connected multiple devices in the LAN and I am able to see those devices in the UPnP Scan. When I block 1900 port then devFinder.FindByType("upnp:rootdevice", 0); returns 0 devices. So I need to write some port scanning code which will tell me whether the port is open or close. As UPnP use UDP I am not getting any exception when I am trying to connect to the "239.255.255.250:1900" address.
  Below is the code snip which I have written</p>
  
  <pre><code>try
  {
    Socket server = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
    IPEndPoint iep = new IPEndPoint(IPAddress.Parse("239.255.255.250"), 1900);
  
    byte[] data = Encoding.ASCII.GetBytes("This is a test message");
    server.SendTo(data, iep);
    server.Close();
    Console.WriteLine("UPnP Port is open");
  }
  catch(SocketException ex)
  {
    Console.WriteLine("UPnP Port is blocked by firewall");
  }
  </code></pre>
  
  <p>I have blocked the 1900 port in firewall, so I was expecting the SocketException, but I never gets any exception, so I was not able to find whether port is open or blocked</p>