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 = ; //规定时间内访问的最大次数
      private const int REDUCTION_INTERVAL = ; // 1 秒（检查访问次数的时间段）
      private const int RELEASE_INTERVAL =  *  * ; // 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 = ;
              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] = ;
          }
          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] == )
                  _IpAdresses.Remove(key);
          }
      }

      #endregion

  }

这个前提是客户端没有使用动态代理IP。