source: binary-improvements/7dtd-server-fixes/src/NetConnections/Servers/Telnet/Telnet.cs@ 188

using System;
using System.Collections.Generic;
using System.Net;
using System.Net.Sockets;
using System.Threading;

namespace AllocsFixes.NetConnections.Servers.Telnet
{
        public class Telnet : IServer
        {
                private const int MAX_LOGIN_ATTEMPTS = 10;
                private const int BLOCK_TIME_SECONDS = 10;

                private class LoginAttempts
                {
                        private int count = 0;
                        private DateTime lastAttempt = new DateTime (0);

                        public bool LogAttempt ()
                        {
                                lastAttempt = DateTime.Now;
                                count++;
                                return count < MAX_LOGIN_ATTEMPTS;
                        }

                        public bool IsBanned ()
                        {
                                if ((DateTime.Now - lastAttempt).TotalSeconds > BLOCK_TIME_SECONDS)
                                        count = 0;
                                return count >= MAX_LOGIN_ATTEMPTS;
                        }
                }

                private TcpListener listener = null;
                private bool authEnabled = false;
                private List<TelnetConnection> connections = new List<TelnetConnection> ();
                private Dictionary<int, LoginAttempts> loginAttemptsPerIP = new Dictionary<int, LoginAttempts> ();

                public Telnet ()
                {
                        try {
                                if (!GamePrefs.GetBool (EnumGamePrefs.TelnetEnabled)) {
                                        return;
                                }

                                int port = GamePrefs.GetInt (EnumGamePrefs.TelnetPort);

                                authEnabled = GamePrefs.GetString (EnumGamePrefs.TelnetPassword).Length != 0;
                                if (authEnabled)
                                        listener = new TcpListener (IPAddress.Any, port);
                                else
                                        listener = new TcpListener (IPAddress.Loopback, port);

                                listener.Start ();
                                listener.BeginAcceptTcpClient (new AsyncCallback (AcceptClient), null);

                                NetTelnetServer.RegisterServer (this);

                                Log.Out ("Started Telnet on " + port);
                        } catch (Exception e) {
                                Log.Out ("Error in Telnet.ctor: " + e);
                        }
                }

                public bool RegisterFailedLogin (int addressHash)
                {
                        lock (loginAttemptsPerIP) {
                                LoginAttempts la = loginAttemptsPerIP [addressHash];
                                return la.LogAttempt ();
                        }
                }

                private void AcceptClient (IAsyncResult asyncResult)
                {
                        if (listener.Server.IsBound) {
                                TcpClient client = listener.EndAcceptTcpClient (asyncResult);

                                EndPoint endpoint = client.Client.RemoteEndPoint;
                                int addressHash = -1;
                                if (endpoint is IPEndPoint) {
                                        addressHash = ((IPEndPoint)endpoint).Address.GetHashCode ();
                                        //Log.Out ("Hash: " + endpointAddressHash);
                                } else {
                                        Log.Out ("EndPoint is not an IPEndPoint but: " + endpoint.GetType ().ToString ());
                                }

                                lock (loginAttemptsPerIP) {
                                        LoginAttempts la = null;
                                        if (loginAttemptsPerIP.ContainsKey(addressHash))
                                                la = loginAttemptsPerIP [addressHash];
                                        if (la == null) {
                                                la = new LoginAttempts ();
                                                loginAttemptsPerIP [addressHash] = la;
                                        }
                                        if (!la.IsBanned ()) {
                                                TelnetConnection con = new TelnetConnection (this, client, authEnabled);
                                                connections.Add (con);
                                        } else {
                                                client.Close ();
                                                Log.Out ("Telnet connection not accepted for too many login attempts: " + endpoint);
                                        }
                                }
                                listener.BeginAcceptTcpClient (new AsyncCallback (AcceptClient), null);
                        }
                }

                public void Disconnect ()
                {
                        try {
                                if (listener != null) {
                                        listener.Stop ();
                                        listener = null;
                                }
                                foreach (TelnetConnection c in connections) {
                                        c.Close ();
                                }
                        } catch (Exception e) {
                                Log.Out ("Error in Telnet.Disconnect: " + e);
                        }
                }

                private void RemoveClosedConnections ()
                {
                        try {
                                List<TelnetConnection> toRemove = new List<TelnetConnection> ();
                                foreach (TelnetConnection c in connections) {
                                        if (c.IsClosed ())
                                                toRemove.Add (c);
                                }
                                foreach (TelnetConnection c in toRemove) {
                                        connections.Remove (c);
                                }
                        } catch (Exception e) {
                                Log.Out ("Error in Telnet.RemoveClosedConnections: " + e);
                        }
                }

                public void WriteToClient (string line)
                {
                        if (line == null) {
                                return;
                        }
                        RemoveClosedConnections ();
                        foreach (TelnetConnection c in connections) {
                                if (c.IsAuthenticated ())
                                        c.WriteLine (line);
                        }
                }

                public void WriteToClient_Single (string line, IConnection client)
                {
                        if (line == null) {
                                return;
                        }
                        RemoveClosedConnections ();
                        foreach (TelnetConnection con in connections) {
                                if (con == client) {
                                        if (con.IsAuthenticated ())
                                                con.WriteLine (line);
                                }
                        }
                }

        }
}