void Application_BeginRequest(Object sender, EventArgs e)
    {
        //这里把你的请求转发到站点B
    }

------解决方案--------------------

C# code

    void Application_BeginRequest(Object sender, EventArgs e)
    {
        HttpApplication application = sender as HttpApplication;

        HttpRequest request = application.Request;

        string domain = "http://www.b.com";
        string url = domain + request.Url.AbsolutePath + request.Url.Query;
        System.Net.HttpWebRequest webRequest = System.Net.WebRequest.Create(url) as System.Net.HttpWebRequest;

        if (request.HttpMethod == "GET")
        {
            //模拟个包

            webRequest.Accept = "*/*";
            webRequest.UserAgent = request.UserAgent;
            foreach(string head in request.Headers)
                webRequest.Headers.Add(head,request.Headers[head]);
            
            //还有其它的也加进去
        }
        else if (request.HttpMethod == "POST")
        { 
            //同理
        }
        
    }