C# UDP client reading multicast IP (local interface), and sending unicast UDP to VPN

Question

I have a VPN client that works fine for sending unicast UDP packets over the VPN pipe but does not do that for multicast IP (UDP) packets (OpenVPN). So I thought I could write this little shim that would take the IP (port 3000) multicast and send them over the VPN as unicast.

I see the multicast packets arriving, but I see nothing arriving on the VPN side (WireShark is not helping, since unlike IpConfig /all) it does not see the VPN interface.).

I think my issue might be I am not really clear on the difference between BIND and CONNECT, and which interface (vpn or local) I should be binding the UDPClient to.

The command line is:

239.0.0.0 198.168.0.15 10.4.30.239 172.27.225.77
arg[0] = multicast from address
arg[1] = unicast to address
arg[2] = local physical ethernet assigned address
arg[3] = virtual address of computer on VPN network (client)

---

using System;
using System.Collections.Generic;
using System.Net;
using System.Net.Sockets;
using System.Threading.Tasks;

namespace MUTunnel
{
    public class EchoBot
    {
        protected UdpClient sender;
        protected UdpClient listener;

        protected const int listenPort = 3000;
        protected const int outPort = 3000;

        protected IPEndPoint inPoint;
        protected IPEndPoint outPoint;
        protected IPAddress listenAddress;
        protected IPAddress sendAddress;
        protected IPAddress localAddress;
        protected IPAddress vpnAddress;

        public EchoBot(string from, string to, string local, string vpn)
        {
            bool parsed = IPAddress.TryParse(from, out listenAddress);
            parsed = IPAddress.TryParse(to, out sendAddress) && parsed;
            parsed = IPAddress.TryParse(local, out localAddress) && parsed;
            parsed = IPAddress.TryParse(vpn, out vpnAddress) && parsed;
            if (!parsed)
            {
                System.Console.WriteLine("Usage: MUTunnel <source multicast IP> <dest unicast IP> <local host IP address> <vpn IP address>");
                Environment.Exit(1);
            }
            listener = new UdpClient();
            listener.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
            inPoint = new IPEndPoint(localAddress, listenPort);
            listener.Client.Bind(inPoint);
            listener.JoinMulticastGroup(listenAddress);

            sender = new UdpClient();
            sender.Ttl = 64;
            sender.AllowNatTraversal(true);
            //outPoint = new IPEndPoint(sendAddress, outPort);
            sender.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
            sender.Connect(sendAddress, outPort);
        }

        public void send(byte[] bytes)
        {
            sender.Send(bytes, bytes.Length);
        }
        public void loop()
        {
            bool done = false;
            try
            {
                while (!done)
                {
                    byte[] bytes = listener.Receive(ref inPoint);
                    Console.WriteLine("Received Multicast from  {0} : length {1}\n", listenAddress.ToString(), bytes.Length);
                    this.send(bytes);
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }
            finally
            {
                listener.Close();
                sender.Close();
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length != 4)
            {
                System.Console.WriteLine("Usage: MUTunnel <source multicast IP> <dest unicast IP> <local host IP address> <vpn IP address>");
                Environment.Exit(1);
            }
            EchoBot echoBot = new EchoBot(args[0], args[1], args[2], args[3]);
            Console.WriteLine("MUTunnel Waiting for messsages...");
            echoBot.loop();
        }
    }
}

The ipconfig /all reads as follows (when OpenVPN client is running)

P:\>ipconfig /all

Windows IP Configuration

   Host Name . . . . . . . . . . . . : YSG4206
   Primary Dns Suffix  . . . . . . . : draper.com
   Node Type . . . . . . . . . . . . : Hybrid
   IP Routing Enabled. . . . . . . . : No
   WINS Proxy Enabled. . . . . . . . : No
   DNS Suffix Search List. . . . . . : draper.com

Ethernet adapter Ethernet 3:

   Connection-specific DNS Suffix  . : draper.com
   Description . . . . . . . . . . . : Killer E2200 Gigabit Ethernet Controller
   Physical Address. . . . . . . . . : F8-B1-56-FF-8B-36
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes
   IPv4 Address. . . . . . . . . . . : 10.4.30.239(Preferred)
   Subnet Mask . . . . . . . . . . . : 255.255.254.0
   Lease Obtained. . . . . . . . . . : Monday, June 18, 2018 5:28:03 PM
   Lease Expires . . . . . . . . . . : Thursday, June 21, 2018 8:46:50 PM
   Default Gateway . . . . . . . . . : 10.4.31.254
   DHCP Server . . . . . . . . . . . : 140.102.100.111
   DNS Servers . . . . . . . . . . . : 10.10.20.11
                                       10.10.20.12
   NetBIOS over Tcpip. . . . . . . . : Enabled

Ethernet adapter Ethernet:

   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : TAP Adapter OAS NDIS 6.0
   Physical Address. . . . . . . . . : 00-FF-91-E7-8A-38
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes
   IPv4 Address. . . . . . . . . . . : 172.27.225.77(Preferred)
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . :
   NetBIOS over Tcpip. . . . . . . . : Enabled

P:\>

---

Update

Added sender.Ttl = 64; sender.AllowNatTraversal(true); also ran the RawCap on the client with the VPN interface selected. The Dump of packets does not show of the multicast packets that I am converting to unicast and trying to send to 129.168.0.15 coming to that interface.

just some other stuff. (see wireshark display of the .pccap file from RawCap.

Answer 1

I suspect you don't have anything listening to the other end of your unicast.

Unicast is the term used to describe communication where a piece of information is sent from one point to another point. In this case there is just one sender, and one receiver.

I added an additional listener to listen to the send endpoint and everything works and will print out the sent message.

Additional listener run on the computer with the ip: 198.168.0.15

IPAddress localAddress = IPAddress.Parse("198.168.0.15");
const int listenPort = 3000;

var listener = new UdpClient();
listener.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
var inPoint = new IPEndPoint(localAddress, listenPort);

listener.Client.Bind(inPoint);

while (true)
{
    byte[] bytes = listener.Receive(ref inPoint);
    Console.WriteLine("Received Message from  {0} : message {1}\n", localAddress.ToString(),Encoding.ASCII.GetString(bytes));
    Thread.Sleep(1000);
}