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    }