asp.net user identity returns application pool identity

Question

I have a web server farm with 2 windows 2008 r2 server nlb configured.

I am trying to get the identity of the current logged in user but instead I get the application pool user. Not sure what I'm doing wrong. any ideas?

here's the code I'm using

   protected void Page_Load(object sender, EventArgs e)
   {
       Response.Write(User.Identity.Name);
   }

web.config

<system.web>
    <authentication mode="Windows" />        
    <authorization>
        <deny users="?" />            
    </authorization>
    <compilation debug="true" targetFramework="4.0" />
</system.web>

In IIS I have windows authentication enabled and all other authentication disabled.