Getting the name of the current Windows user returning “IIS APPPOOL/Sitename”

string user = System.Security.Principal.WindowsIdentity.GetCurrent().Name.ToString();

1. Enable Windows authentication
2. Disable anonymous authentication
3. Add <identity impersonate="true"> in web.config

 <system.web>
    ...
    <authentication mode="Windows"/>
    <identity impersonate="true"/>
    ...
 </system.web>