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

}