Receive from UDP Socket and send data back

Question

I'm trying to write a console application that takes a request (size is to be 18 bytes), and then send something (size of 7 bytes) back to the client. I for the life of me can't seem to get this to work. I can receive the data fine, but the data I send back never gets to the client.

Here is my code

 static void Main(string[] args)
 {
        // Data to return
        byte[] ret = { 0xfe, 0xfd, 0x09, 0x00, 0x00, 0x00, 0x00 };

        // tell the user that we are waiting
        Console.WriteLine("Waiting for UDP Connection...");

        // Create a new socket to listen from
        Socket Skt = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
        Skt.EnableBroadcast = true;
        Skt.Bind(new IPEndPoint(IPAddress.Loopback, 27900));

        try
        {
            // Blocks until a message returns on this socket from a remote host.
            Byte[] receiveBytes = new byte[48];
            IPEndPoint sender = new IPEndPoint(IPAddress.Any, 0);
            EndPoint senderRemote = (EndPoint)sender;

            Skt.ReceiveFrom(receiveBytes, ref senderRemote);
            string returnData = Encoding.UTF8.GetString(receiveBytes).Trim();

            Console.WriteLine("This is the message you received " + returnData.ToString());

            // Sent return data
            int sent = Skt.SendTo(ret, senderRemote);
            Console.WriteLine("Sent {0} bytes back", sent);
            Skt.Close();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }

        Console.ReadLine();
    }

Anyone give me some pointers please?

Answer 1

Here is the sample code i have modified and you can see you can receive and send from this sample. Method Test is acting as client which can be a different process now i have made it in different thread for simulation.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Sockets;
using System.Net;
using System.Threading;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            // Data to return
        byte[] ret = { 0xfe, 0xfd, 0x09, 0x00, 0x00, 0x00, 0x00 };

        // tell the user that we are waiting
        Console.WriteLine("Waiting for UDP Connection...");

        // Create a new socket to listen from
        Socket Skt = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
        Skt.EnableBroadcast = true;
        Skt.Bind(new IPEndPoint(IPAddress.Loopback, 27900));

        try
        {
            // Blocks until a message returns on this socket from a remote host.
            Byte[] receiveBytes = new byte[48];
            IPEndPoint sender = new IPEndPoint(IPAddress.Any, 0);
            EndPoint senderRemote = (EndPoint)sender;

            Thread thr = new Thread(new ThreadStart(Test));
            thr.Start();
            Skt.ReceiveFrom(receiveBytes, ref senderRemote);
            string returnData = Encoding.UTF8.GetString(receiveBytes).Trim();

            Console.WriteLine("This is the message you received " + returnData.ToString());

            // Sent return data
            int sent = Skt.SendTo(ret, senderRemote);
            Console.WriteLine("Sent {0} bytes back", sent);
            Skt.Close();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }

        Console.ReadLine();

        }



        public static void Test()
        {
            byte[] ret = { 0xfe, 0xfd, 0x09, 0x00, 0x00, 0x00, 0x00 };
            Socket Skt = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            Skt.EnableBroadcast = true;
            IPEndPoint test=new IPEndPoint(IPAddress.Loopback, 27900);

            int sent = Skt.SendTo(ret, test);
            try
            {
                // Blocks until a message returns on this socket from a remote host.
                Byte[] receiveBytes = new byte[48];
                IPEndPoint sender = new IPEndPoint(IPAddress.Any, 0);
                EndPoint senderRemote = (EndPoint)sender;

                Skt.ReceiveFrom(receiveBytes, ref senderRemote);
                string returnData = Encoding.UTF8.GetString(receiveBytes).Trim();

                Console.WriteLine("This is the message you received " + returnData.ToString());

                // Sent return data
                //int sent = Skt.SendTo(ret, senderRemote);
                Console.WriteLine("Sent {0} bytes back", sent);
                Skt.Close();



            }
            catch (Exception ex)
            {
            }
        }

    }
}

Answer 2

Without also seeing your client code it's difficult to be sure where the problem might lie. I can give you a working solution using the network library networkcomms.net though. The code for the server would be as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using NetworkCommsDotNet;

namespace UPDServer
{
    class Program
    {
        static void Main(string[] args)
        {
            NetworkComms.AppendGlobalIncomingPacketHandler<string>("Message", (packetHeader, connection, incomingString) => 
            {
                Console.WriteLine("This is the message you received " + incomingString);
                connection.SendObject("Message", incomingString + " relayed by server.");
            });

            UDPConnection.StartListening(true);

            Console.WriteLine("Server ready. Press any key to shutdown server.");
            Console.ReadKey(true);
            NetworkComms.Shutdown();
        }
    }
}

And for the client:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using NetworkCommsDotNet;

namespace UDPClient
{
    class Program
    {
        static void Main(string[] args)
        {
            string messageToSend = "This is a message To Send";
            string messageFromServer = UDPConnection.GetConnection(new ConnectionInfo("127.0.0.1", 10000), UDPOptions.None).SendReceiveObject<string>("Message", "Message", 2000, messageToSend);
            Console.WriteLine("Server said '{0}'.", messageFromServer);

            Console.WriteLine("Press any key to exit.");
            Console.ReadKey(true);
            NetworkComms.Shutdown();
        }
    }
}

You will obviously need to download the NetworkCommsDotNet DLL from the website so that you can add it in the 'using NetworkCommsDotNet' reference. Also see the server IP address in the client example is currently "127.0.0.1", this should work if you run both the server and client on the same machine. For more information checkout the getting started or how to create a client server application articles.