Optimizing this AntiDos HttpModule

I've written this module (using a tutorial on the web I can't find now to stop unusual requests from clients. It's working as I've tested it on a local system.

1. Is the logic fine enough?

2. Another problem is that it counts requests for non-aspx resources (images, css, ...), but it shouldn't. How can I filter request for aspx pages ?

This is the module code:

public class AntiDosModule : IHttpModule
{
    const string blockKey = "IsBlocked";
    const string reqsKey = "Requests";


    public void Dispose()
    { }

    public void Init(HttpApplication context)
    {
        context.BeginRequest += ValidateRequest;
    }

    private static void ValidateRequest(object sender, EventArgs e)
    {
        // get configs from web.config
        AntiDosModuleSettings setting = AntiDosModuleSettings.GetConfig();

        int blockDuration = setting.IPBlockDuration;

        // time window in which request are counted e.g 1000 request in 1 minute
        int validationTimeWindow = setting.ValidationTimeWindow;

        // max requests count in specified time window e.g 1000 request in 1 minute
        int maxValidRequests = setting.MaxValidRequests;

        string masterKey = setting.MasterKey;

        HttpContextBase context = new HttpContextWrapper(HttpContext.Current);

        CacheManager cacheMgr = new CacheManager(context, masterKey);

        // is client IP blocked
        bool IsBlocked = (bool)cacheMgr.GetItem<Boolean>(blockKey);

        if (IsBlocked)
        {
            context.Response.End();
        }


        // number of requests sent by client till now
        IPReqsHint hint = cacheMgr.GetItem<IPReqsHint>(reqsKey) ?? new IPReqsHint();

        if (hint.HintCount > maxValidRequests)
        {
            // block client IP
            cacheMgr.AddItem(blockKey, true, blockDuration);

            context.Response.End();

        }


        hint.HintCount++;

        if (hint.HintCount == 1)
        {
            cacheMgr.AddItem(reqsKey, hint, validationTimeWindow);
        }

    }

}

internal class IPReqsHint
{

    public int HintCount { get; set; }

    public IPReqsHint()
    {
        HintCount = 0;
    }
}

and this is the CacheManager class:

public class CacheManager
{

    HttpContextBase context;

    string masterKey;

    public CacheManager(HttpContextBase context, string masterKey)
    {
        this.context = context;
        this.masterKey = masterKey;
    }

    public void AddItem(string key, object value, int duration)
    {
        string finalKey = GenerateFinalKey(key);

        context.Cache.Add(
            finalKey,
            value,
            null,
            DateTime.Now.AddSeconds(duration),
            System.Web.Caching.Cache.NoSlidingExpiration,
            System.Web.Caching.CacheItemPriority.Normal,
            null);
    }

    public T GetItem<T>(string key)
    {
        string finalKey = GenerateFinalKey(key);

        var obj = context.Cache[finalKey] ?? default(T);

        return (T)obj;
    }

    string GenerateFinalKey(string key)
    {
        return masterKey + "-" + context.Request.UserHostAddress + "-" + key;
    }
}