Question

Using C# Winforms I am trying to automatically detect the local machines IP address through which it can connect to a particular remote DNS/IP address.

One senario is running over a VPN, with the remote address being 10.8.0.1 and local address being 10.8.0.6, netmask of 255.255.255.252

Iterating through the local addresses and checking if the remote and local are on the same subnet obviously fails and I am unsure of how else to do this.

Answer 1

Here is some sample code that should get you the information you’re looking for. It creates a UDP socket and calls Connect() on it (effectively a NOOP), and then checks the local address.

static EndPoint GetLocalEndPointFor(IPAddress remote)
{
    using (Socket s = new Socket(remote.AddressFamily,
                                 SocketType.Dgram,
                                 ProtocolType.IP))
    {
        // Just picked a random port, you could make this application
        // specific if you want, but I don't think it really matters
        s.Connect(new IPEndPoint(remote, 35353));

        return s.LocalEndPoint;
    }
}

static void Main(string[] args)
{
    IPAddress remoteAddress = IPAddress.Parse("10.8.0.1");
    IPEndPoint localEndPoint = GetLocalEndPointFor(remoteAddress) as IPEndPoint;

    if (localEndPoint == null)
        Console.WriteLine("Couldn't find local address");
    else
        Console.WriteLine(localEndPoint.Address);

    Console.ReadKey();
}

Note that this is effectively an implementation of this answer, but in C#.