Question

I’m trying to launch my client, but have an error. Server is already running on the same computer. So I’m using “localhost” with GetHostEntry:

 IPHostEntry ipHostInfo = System.Net.Dns.GetHostEntry("localhost");
 IPAddress ipAddress = ipHostInfo.AddressList[0];
 IPEndPoint remoteEP = new IPEndPoint(ipAddress, Port);

Sock = new Socket(remoteEP.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
Sock.BeginConnect(remoteEP, new AsyncCallback(ConnectCallback), Sock);

But I have this “no connection could be made because the target machine actively refused it”:

System.Net.Sockets.SocketException (0x80004005): No connection could be made because the target machine actively refused it [::1]:7777
   at System.Net.Sockets.Socket.EndConnect(IAsyncResult asyncResult)
   at NotifierClient.AsynchronousClient.ConnectCallback(IAsyncResult ar) in *** :line 156 

Line 156 is

client.EndConnect(ar);

What is the reason? Could it be because ipHostInfo.AddressList[0] is an IPv6? How then I can accept Ipv4 address?

Answer 1

You can use the AddressFamily property of the IPAddress class to tell whether the address is IPv4 or IPv6.

This way you can loop through the list of IPAddress-es returned and choose the first one that is an IPv4 address:

IPHostEntry ipHostInfo = System.Net.Dns.GetHostEntry("localhost");    

IPAddress ipAddress = null;
foreach(var addr in ipHostInfo.AddressList)
{
    if(addr.AddressFamily == AddressFamily.InterNetwork)        // this is IPv4
    {
         ipAddress = addr;
         break;
    }
}

// at this point, ipAddress is either going to be set to the first IPv4 address
//  or it is going to be null if no IPv4 address was found in the list
if(ipAddress == null)
    throw new Exception("Error finding an IPv4 address for localhost");

IPEndPoint remoteEP = new IPEndPoint(ipAddress, Port);

Sock = new Socket(remoteEP.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
Sock.BeginConnect(remoteEP, new AsyncCallback(ConnectCallback), Sock);