A template file for console-based CTF challenges that require interaction with a server in C#.

ConsoleApp.cs

using System.Net;
using System.Net.Sockets;
using System.Text;

namespace ConsoleApp1
{
    internal class Program
    {
        static void Main(string[] args)
        {
            // Change these 2
            IPAddress ip = IPAddress.Parse("10.10.180.121");
            int port = 19001;

            string responseText = "";
            int timeout = 2500; // ms
            string initialText = "";
            byte[] buffer = new byte[15000];
            using (Socket socket = new(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
            {
                socket.ReceiveTimeout = timeout;
                socket.SendTimeout = timeout;
                try
                {
                    var result = socket.BeginConnect(ip, port, null, null); // Error if an invalid IP
                    bool success = result.AsyncWaitHandle.WaitOne(timeout, true);
                    if (success)
                    {
                        if (!socket.Connected)
                        {
                            socket.Close();
                        }
                        if (initialText.Length != 0)
                        {
                            byte[] cmdBytes = Encoding.ASCII.GetBytes(initialText.ToCharArray());
                            socket.Send(cmdBytes, cmdBytes.Length, 0);
                        }
                        Thread.Sleep(2000); // Wait for all the data
                        int bytes = socket.Receive(buffer, buffer.Length, 0);
                        responseText += Encoding.ASCII.GetString(buffer, 0, bytes);

                        string oldResponse = "";
                        List<string> responseLines = new List<string>();
                        while (true)
                        {
                            // Show the input
                            Console.WriteLine("Received: " + responseText);
                            // Do any required parsing
                            responseLines = responseText.Split('\n').ToList();
                            // Remove invalid stuff
                            responseLines.Remove("Repeat the words 1000 times to get the flag");
                            responseLines.RemoveAll(x => x == "" || x == "> ");
                            string toReturn = "";
                            if (responseLines.Count == 1)
                            {
                                // We only have our input - Format it and return it
                                toReturn = responseLines[0];
                                toReturn = toReturn.Replace("> ", "");
                                oldResponse = toReturn;
                                toReturn += "\n"; // Else its not considered a newline when sending
                                Console.WriteLine("Sending back: " + toReturn.Replace("\n", ""));
                            }
                            else
                            {
                                // Something went wrong
                                Console.WriteLine("Woof");
                                Console.ReadLine();
                            }
                            // Send back the result
                            byte[] cmdBytes = Encoding.ASCII.GetBytes(toReturn.ToCharArray());
                            socket.Send(cmdBytes, cmdBytes.Length, 0);
                            // Wait for a response
                            Thread.Sleep(50);
                            bytes = socket.Receive(buffer, buffer.Length, 0);
                            responseText = Encoding.ASCII.GetString(buffer, 0, bytes);
                            // Split response into lines
                            responseLines = responseText.Split('\n').ToList();
                            // Remove invalid stuff
                            responseLines.RemoveAll(x => x == "" || x == "> ");

                            // If the response doesn''t contain any new lines, we didn't wait long enough
                            while (!responseLines.Any())
                            {
                                Thread.Sleep(50);
                                bytes = socket.Receive(buffer, buffer.Length, 0);
                                responseText = Encoding.ASCII.GetString(buffer, 0, bytes);
                                responseLines = responseText.Split('\n').ToList();
                                responseLines.RemoveAll(x => x == "" || x == "> ");
                            }
                        }
                    }
                    else
                    {
                        socket.Close();
                        Console.WriteLine("Closed");
                        Console.ReadLine();
                    }
                }
                catch (SocketException ex)
                {
                    if (ex.SocketErrorCode == SocketError.TimedOut)
                    {
                        // Could just mean that we''re using the wrong info to grab the banner
                        // Do nothing - A timeout response is handled later
                    }
                    else if (ex.SocketErrorCode == SocketError.ConnectionRefused)
                    {
                        Console.WriteLine("Connection refused");
                        Console.ReadLine();
                    }
                    // Connection reset by peer
                    else if (ex.SocketErrorCode == SocketError.ConnectionReset)
                    {
                        Console.WriteLine("Connection reset");
                        Console.ReadLine();
                    }
                    else
                    {
                        Console.WriteLine($"Error in BannerGrab with SocketErrorCode code: {ex.SocketErrorCode}");
                        Console.ReadLine();
                    }
                }
                catch (Exception ex)
                {
                    Console.WriteLine($"Error in General.BannerGrab ({ip}:{port} - {ex.Message})");
                    Console.ReadLine();
                }
            }
        }

    }
}