C# 通过 HTTPModule 防范 DOS

  public class DosAttackModule : IHttpModule
    {
        void IHttpModule.Dispose() { }

        void IHttpModule.Init(HttpApplication context)
        {
            context.BeginRequest += new EventHandler(context_BeginRequest);
        }

        private static Dictionary<string, short> _IpAdresses = new Dictionary<string, short>();
        private static Stack<string> _Banned = new Stack<string>();
        private static Timer _Timer = CreateTimer();
        private static Timer _BannedTimer = CreateBanningTimer();

        private const int BANNED_REQUESTS = 1; //规定时间内访问的最大次数  
        private const int REDUCTION_INTERVAL = 1000; // 1 秒（检查访问次数的时间段）  
        private const int RELEASE_INTERVAL = 5 * 60 * 1000; // 5 分钟（清除一个禁止IP的时间段） 

        private void context_BeginRequest(object sender, EventArgs e)
        {
            string ip = HttpContext.Current.Request.UserHostAddress;
            if (_Banned.Contains(ip))
            {
                HttpContext.Current.Response.StatusCode = 403;
                HttpContext.Current.Response.End();
            }

            CheckIpAddress(ip);
        }

        /// <summary>  
        /// 检查访问IP  
        /// </summary>  
        private static void CheckIpAddress(string ip)
        {
            if (!_IpAdresses.ContainsKey(ip)) //如果没有当前访问IP的记录就将访问次数设为1  
            {
                _IpAdresses[ip] = 1;
            }
            else if (_IpAdresses[ip] == BANNED_REQUESTS) //如果当前IP访问次数等于规定时间段的最大访问次数就拉于“黑名单”  
            {
                _Banned.Push(ip);
                _IpAdresses.Remove(ip);
            }
            else //正常访问就加次数 1  
            {
                _IpAdresses[ip]++;
            }
        }

        #region Timers

        /// <summary>  
        /// 创建计时器，从_IpAddress减去一个请求。  
        /// </summary>  
        private static Timer CreateTimer()
        {
            Timer timer = GetTimer(REDUCTION_INTERVAL);
            timer.Elapsed += new ElapsedEventHandler(TimerElapsed);
            return timer;
        }

        /// <summary>  
        /// 创建定时器，消除一个禁止的IP地址  
        /// </summary>  
        /// <returns></returns>  
        private static Timer CreateBanningTimer()
        {
            Timer timer = GetTimer(RELEASE_INTERVAL);
            timer.Elapsed += delegate { _Banned.Pop(); }; //消除一个禁止IP  
            return timer;
        }

        /// <summary>  
        /// 创建一个时间器，并启动它  
        /// </summary>  
        /// <param name="interval">以毫秒为单位的时间间隔</param>  
        private static Timer GetTimer(int interval)
        {
            Timer timer = new Timer();
            timer.Interval = interval;
            timer.Start();

            return timer;
        }

        /// <summary>  
        /// 减去从集合中的每个IP地址的请求  
        /// </summary>  
        private static void TimerElapsed(object sender, ElapsedEventArgs e)
        {
            foreach (string key in _IpAdresses.Keys)
            {
                _IpAdresses[key]--;
                if (_IpAdresses[key] == 0)
                    _IpAdresses.Remove(key);
            }
        }

        #endregion

    }