public class DosAttackModule : IHttpModule  {      void IHttpModule.Dispose() { }       void IHttpModule.Init(HttpApplication context)      {          context.BeginRequest += new EventHandler(context_BeginRequest);      }       private static Dictionary
     
       _IpAdresses = new Dictionary
      
       ();      private static Stack
       
         _Banned = new Stack
        
         ();      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);      }       /// 
         
        /// 检查访问IP        /// 
        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       /// 
         
        /// 创建计时器，从_IpAddress减去一个请求。        /// 
        private static Timer CreateTimer()      {          Timer timer = GetTimer(REDUCTION_INTERVAL);          timer.Elapsed += new ElapsedEventHandler(TimerElapsed);          return timer;      }       /// 
         
        /// 创建定时器，消除一个禁止的IP地址        /// 
        /// 
                 private static Timer CreateBanningTimer()      {          Timer timer = GetTimer(RELEASE_INTERVAL);          timer.Elapsed += delegate { _Banned.Pop(); }; //消除一个禁止IP            return timer;      }       /// 
         
        /// 创建一个时间器，并启动它        /// 
        /// 
         以毫秒为单位的时间间隔        private static Timer GetTimer(int interval)      {          Timer timer = new Timer();          timer.Interval = interval;          timer.Start();           return timer;      }       /// 
         
        /// 减去从集合中的每个IP地址的请求        /// 
        private static void TimerElapsed(object sender, ElapsedEventArgs e)      {          foreach (string key in _IpAdresses.Keys)          {              _IpAdresses[key]--;              if (_IpAdresses[key] == 0)                  _IpAdresses.Remove(key);          }      }       #endregion   }
        
       
      
     

这个前提是客户端没有使用动态代理IP。