tcpProxy.cs

using System;
using System.Net;
using System.Net.Sockets;
using System.Threading.Tasks;

const int grpcPort = 5000;
const int proxyPort = 3000;


var tcp1 = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
tcp1.Bind(new IPEndPoint(IPAddress.Loopback, proxyPort));
tcp1.Listen();

var pipe = true;

Task.Factory.StartNew(() => {
    while (true) {
        var source = tcp1.Accept();
        var target = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        target.Connect(new IPEndPoint(IPAddress.Loopback, grpcPort));
        var sourceStream = new NetworkStream(source);
        var targetStream = new NetworkStream(target);

        Task.Factory.StartNew(() => {
            var buffer = new byte[1024];
            while (true) {
                var read = sourceStream.Read(buffer);
                if (pipe) {
                    targetStream.Write(buffer, 0, read);
                }
            }
        });

        Task.Factory.StartNew(() => {
            var buffer = new byte[1024];
            while (true) {
                var read = targetStream.Read(buffer);
                if (pipe) {
                    sourceStream.Write(buffer, 0, read);
                }
            }
        });
    }
});

while (true) {
    Console.CursorLeft = 0;
    Console.Write("TCP proxy " + (pipe ? "on" : "off") + ". Press any key to toggle. ");
    Console.ReadKey(true);
    pipe = !pipe;
}