How to get *internet* IP?

Question

Imagine a situation, I have PC with two lan cards, one is connected to internet another is connected to local network, how can I detect IP which is connected to internet with C# ?

Answer 1

Try this:

static IPAddress getInternetIPAddress()
{
    try
    {
        IPAddress[] addresses = Dns.GetHostAddresses(Dns.GetHostName());
        IPAddress gateway = IPAddress.Parse(getInternetGateway());
        return findMatch(addresses, gateway);
    }
    catch (FormatException e) { return null; }
}

static string getInternetGateway()
{
    using (Process tracert = new Process())
    {
        ProcessStartInfo startInfo = tracert.StartInfo;
        startInfo.FileName = "tracert.exe";
        startInfo.Arguments = "-h 1 208.77.188.166"; // www.example.com
        startInfo.UseShellExecute = false;
        startInfo.RedirectStandardOutput = true;
        tracert.Start();

        using (StreamReader reader = tracert.StandardOutput)
        {
            string line = "";
            for (int i = 0; i < 9; ++i)
                line = reader.ReadLine();
            line = line.Trim();
            return line.Substring(line.LastIndexOf(' ') + 1);
        }
    }
}

static IPAddress findMatch(IPAddress[] addresses, IPAddress gateway)
{
    byte[] gatewayBytes = gateway.GetAddressBytes();
    foreach (IPAddress ip in addresses)
    {
        byte[] ipBytes = ip.GetAddressBytes();
        if (ipBytes[0] == gatewayBytes[0]
            && ipBytes[1] == gatewayBytes[1]
            && ipBytes[2] == gatewayBytes[2])
        {
            return ip;
        }
    }
    return null;
}

Note that this implementation of findMatch() relies on class C matching. If you want to support class B matching, just omit the check for ipBytes[2] == gatewayBytes[2].

Edit History:

• Updated to use www.example.com.
• Updated to include getInternetIPAddress(), to show how to use the other methods.
• Updated to catch FormatException if getInternetGateway() failed to parse the gateway IP. (This can happen if the gateway router is configured such that it doesn't respond to traceroute requests.)
• Cited Brian Rasmussen's comment.
• Updated to use the IP for www.example.com, so that it works even when the DNS server is down.