Question

I have two lan cards installed in my pc. One is for internet connection and another for share the internet to client machines. I am getting my IP with this code:

IPHostEntry HosyEntry = Dns.GetHostEntry((Dns.GetHostName()));
foreach (IPAddress ip in HosyEntry.AddressList)
{
    trackingIp = ip.ToString();
    textBox1.Text += trackingIp + ",";
}

How can I find which one my internet connecting IP (I dont want to do it by text processing)?

Answer 1

Ok. I wrote 2 methods.

First method is faster but require to use a socket.
It tries to connect to remote host using each local IP.

Second methos is slower and did not use sockets. It connects to remote host (get response, waste some traffic) and look for local IP in the active connections table.

Code is draft so double check it.

namespace ConsoleApplication1
{
    using System;
    using System.Collections.Generic;
    using System.Net;
    using System.Net.NetworkInformation;
    using System.Net.Sockets;

    class Program
    {
        public static List<IPAddress> GetInternetIPAddressUsingSocket(string internetHostName, int port)
        {
            // get remote host  IP. Will throw SocketExeption on invalid hosts
            IPHostEntry remoteHostEntry = Dns.GetHostEntry(internetHostName);
            IPEndPoint remoteEndpoint = new IPEndPoint(remoteHostEntry.AddressList[0], port);

            // Get all locals IP
            IPHostEntry hostEntry = Dns.GetHostEntry(Dns.GetHostName());

            var internetIPs = new List<IPAddress>();
            // try to connect using each IP             
            foreach (IPAddress ip in hostEntry.AddressList) {
                IPEndPoint localEndpoint = new IPEndPoint(ip, 80);

                bool endpointIsOK = true;
                try {
                    using (Socket socket = new Socket(localEndpoint.Address.AddressFamily, SocketType.Stream, ProtocolType.Tcp)) {
                        socket.Connect(remoteEndpoint);
                    }
                }
                catch (Exception) {
                    endpointIsOK = false;
                }

                if (endpointIsOK) {
                    internetIPs.Add(ip); // or you can return first IP that was found as single result.
                }
            }

            return internetIPs;
        }

        public static HashSet <IPAddress> GetInternetIPAddress(string internetHostName)
        {
            // get remote IPs
            IPHostEntry remoteMachineHostEntry = Dns.GetHostEntry(internetHostName);

            var internetIPs = new HashSet<IPAddress>();

            // connect and search for local IP
            WebRequest request = HttpWebRequest.Create("http://" + internetHostName);
            using (WebResponse response = request.GetResponse()) {
                IPGlobalProperties properties = IPGlobalProperties.GetIPGlobalProperties();
                TcpConnectionInformation[] connections = properties.GetActiveTcpConnections();
                response.Close();

                foreach (TcpConnectionInformation t in connections) {
                    if (t.State != TcpState.Established) {
                        continue;
                    }

                    bool isEndpointFound = false;
                    foreach (IPAddress ip in remoteMachineHostEntry.AddressList) {
                        if (ip.Equals(t.RemoteEndPoint.Address)) {
                            isEndpointFound = true;
                            break;
                        }
                    }

                    if (isEndpointFound) {
                        internetIPs.Add(t.LocalEndPoint.Address);
                    }
                }
            }

            return internetIPs;
        }


        static void Main(string[] args)
        {

            List<IPAddress> internetIP = GetInternetIPAddressUsingSocket("google.com", 80);
            foreach (IPAddress ip in internetIP) {
                Console.WriteLine(ip);
            }

            Console.WriteLine("======");

            HashSet<IPAddress> internetIP2 = GetInternetIPAddress("google.com");
            foreach (IPAddress ip in internetIP2) {
                Console.WriteLine(ip);
            }

            Console.WriteLine("Press any key");
            Console.ReadKey(true);
        }
    }
}