Question

I am attempting to build a simple C# TCP proxy for my business so I can block certain websites from my employees. All is well except I am having trouble seeing what website the user is trying to visit… I can see that the user has connected to my proxy server so I know I am getting connections but the OnRecieve callback isn’t even firing. Am I reading from the socket wrong?

Here is my code:

internal class AsyncState
{
    public const int BufferSize = 4096;
    public byte[] Buffer = new byte[AsyncState.BufferSize];
    public Socket Socket;
    public StringBuilder Content = new StringBuilder();
}

private void OnLoad(object sender, EventArgs e)
{
    IPAddress[] addressCollection = Dns.GetHostAddresses(Dns.GetHostName());
    foreach (IPAddress ipAddress in addressCollection)
    {
        if (ipAddress.AddressFamily == AddressFamily.InterNetwork)
        {
            localEndPoint = new IPEndPoint(ipAddress, 8080);
            Console.WriteLine("Local IP address found... " + localEndPoint.ToString());
            break;
        }
    }

    isListening = true;

    thread = new Thread(new ThreadStart(
        delegate()
        {
            serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            serverSocket.Bind(localEndPoint);
            serverSocket.Listen(10);

            while (isListening)
            {
                resetEvent.Reset();

                Console.WriteLine("Waiting for clients...");
                serverSocket.BeginAccept(new AsyncCallback(OnAccept), serverSocket);

                resetEvent.WaitOne();
                }
            }));

        thread.Start();
    }
}

private void OnAccept(IAsyncResult result)
{
    resetEvent.Set();

    Socket clientSocket = (result.AsyncState as Socket).EndAccept(result);
    Console.WriteLine("Client has connected... " + clientSocket.RemoteEndPoint.ToString());

    AsyncState state = new AsyncState();
    state.Socket = clientSocket;
    state.Socket.BeginReceive(state.Buffer, 0, AsyncState.BufferSize, SocketFlags.None, new AsyncCallback(OnRecieve), state);
}

private void OnRecieve(IAsyncResult result)
{
    AsyncState state = result.AsyncState as AsyncState;

    int totalRead = state.Socket.EndReceive(result);
    if (totalRead > 0)
    {
        state.Content.Append(Encoding.ASCII.GetString(state.Buffer, 0, totalRead));
        state.Socket.BeginReceive(state.Buffer, 0, AsyncState.BufferSize, SocketFlags.None, new AsyncCallback(OnRecieve), state);
    }
    else
    {
        if (state.Content.Length > 1)
            Console.WriteLine("Message recieved from client... " + state.Content.ToString());

        state.Socket.Close();
    }
}

Answer 1

Building a well working proxy is no simple task as you will have to understand and handle HTTP etc. in both directions…

I would recommend to either use an existing library for that OR some configurable proxy…

• http://www.mentalis.org/soft/projects/proxy/ (with source)
• http://sourceforge.net/p/portfusion/home/PortFusion/ (with source)
• http://www.wingate.com/
• http://www.squid-cache.org/

REMARK:

I don’t know in which jurisdiction you are but using such technology without knowledge/consent of employees can in some places be a problem…

Another point: Instead of using such methods I would tell the employee to stop abusing the internet connection of the company 1-3 times and if that doesn’t work I would rather fire that person… such employees is not only abusing the internet connection of the company but in worstcase is putting the company at risk (virus/trojan etc.) and also defrauding the company (if he does this in work hours)…