Question

I’m working on a system to send data between peers on a network. One app is written in Java for the Android. The other app is written in C# on the PC.

I wrote code in Java on the Android to send UDP datagrams. And I wrote C# code to both send and receive datagrams. I tried to send messages from Android to PC. I could see the message in WireShark but not in my program. So, I put my program on a second PC. I succeeded in sending a message from my PC to the second one. But when I tried to send a message from seond PC back to mine it failed. I could see it in WireShark on my PC but not my application. Im at a loss for what to try next. Do you have any suggestions? Why would the UDP packet be visible in WireShark but not my application?

Here is my code.

//C# code on PC
//Sender
sending_socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, rotocolType.Udp);
send_to_address = IPAddress.Parse(strIPAddress);
sending_end_point = new IPEndPoint(send_to_address, intPort);
sending_socket.EnableBroadcast = true;
byte[] bytMessage = Encoding.ASCII.GetBytes(strMessage);
sending_socket.SendTo(bytMessage, sending_end_point);



//Listener
UdpClient listener = new UdpClient(listenPort);
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, listenPort);
byte[] bytReceiveDataByteArray;
try
{
   listener.EnableBroadcast = true;

   while (isRunning)
   {
      //listen for data from sender
      bytReceiveDataByteArray = listener.Receive(ref groupEP);
      //Fire an event to send the data to the hosting code
      if (DataReceived != null)
      {
         DataReceivedEventArgs e = new DataReceivedEventArgs(bytReceiveDataByteArray);
         DataReceived(this, e);
      }
   }
}


//Java code on Android
DatagramSocket socket = new DatagramSocket();
InetAddress serverIP = InetAddress.getByName(strIpAddress);
byte[] outData = (strMsg).getBytes();
DatagramPacket out = new DatagramPacket(outData,outData.length, serverIP,50005);
socket.send(out);
socket.close();

Thanks,

Mike

Answer 1

If I understand the problem correct when your program runs on a specific PC (lets call it “Windows1”) it never receives the UDP packets. It will not receive them from Java Android or from C# code running on a different PC ( lets call it “Windows2”).

However when you run your program on “Windows2” it DOES receive messages from “Windows1”. Sounds like you have the firewall enabled on “Windows1” and do not have an exception for UDP port 50005. On “Windows2” your firewall is turned off or has the exception for 50005 and this is why it receives messages from “Windows1”.

Note that since UDP is not connection oriented, firewall errors will not cause the usual connection timed out error. The messages are just dropped and you never get an error.