How to Debug Network Errors (Specifically ICMP)

Question

I have a server/client type app, Wireshark shows that the client has sent a packet to the server, the server had given the expected response but shows a ICMP Destination port unreachable error.

I'm using a function that was on the MDSN website which has worked for me before.

EDIT: To update I have checked that the packet is being sent after the phone has started listening, I have tried other ports. There is no socket exception so i'm just looking for the best way to go about debugging network errors.

Any ideas?

public string Receive()
    {
        string response = "Operation Timeout";



        // We are receiving over an established socket connection
        if (udpSocket != null)
        {
            // Create SocketAsyncEventArgs context object
            SocketAsyncEventArgs socketEventArg = new SocketAsyncEventArgs();
            socketEventArg.RemoteEndPoint = new DnsEndPoint(SERVER, RECIVEPORT);

            // Setup the buffer to receive the data
            socketEventArg.SetBuffer(new Byte[MAX_BUFFER_SIZE], 0, MAX_BUFFER_SIZE);

            // Inline event handler for the Completed event.
            // Note: This even handler was implemented inline in order to make this method self-contained.
            socketEventArg.Completed += new EventHandler<SocketAsyncEventArgs>(delegate(object s, SocketAsyncEventArgs e)
            {
                if (e.SocketError == SocketError.Success)
                {
                    // Retrieve the data from the buffer
                    response = Encoding.UTF8.GetString(e.Buffer, e.Offset, e.BytesTransferred);
                    response = response.Trim('\0');
                }
                else
                {
                    response = e.SocketError.ToString();

                }

                _clientDone.Set();
            });

            // Sets the state of the event to nonsignaled, causing threads to block
            _clientDone.Reset();

            // Make an asynchronous Receive request over the socket
            Debug.WriteLine("Listening now:" + DateTime.Now.Second + ":" + DateTime.Now.Millisecond);
            try
            {
                Debug.WriteLine("No socket exception");
                udpSocket.ReceiveFromAsync(socketEventArg);
            }
            catch (SocketException e)
            {
                Debug.WriteLine(e.SocketErrorCode);
            }

            // Block the UI thread for a maximum of TIMEOUT_MILLISECONDS milliseconds.
            // If no response comes back within this time then proceed
            _clientDone.WaitOne(TIMEOUT_MILLISECONDS);
        }
        else
        {
            response = "Socket is not initialized";
        }

        return response;
    }

Seth Noble Seth Noble · Accepted Answer · 2012-02-13T22:54:24

"ICMP Destination port unreachable" means that there was no application bound to the port you were sending to. Make sure that your sendto() is targeting to correct IP address and port number. Also check that your listener is calling bind() on INADDR_ANY and the correct port. A common mistake is to forget to convert the port number to network byte order (big-endian). See htons().

How to Debug Network Errors (Specifically ICMP)

1 Answers