void Application_AuthenticateRequest(Object sender, EventArgs e)
    {
        string userData = "Domain Users";
        string userName = string.Empty;
        //HttpContext _context = HttpContext.Current;
        //System.Security.Principal.IPrincipal _princ = HttpContext.Current.User;
        if (HttpContext.Current.User != null)
            userName = HttpContext.Current.User.Identity.Name;

        if (userName.StartsWith(@"Имя_домена\"))
            userName = userName.Split('\\').GetValue(1).ToString();

            FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(
                1,
                userName,
                DateTime.Now,
                DateTime.Now.AddMinutes(30),
                false,
                userData,
                FormsAuthentication.FormsCookiePath);
            string encTicket = FormsAuthentication.Encrypt(ticket);

            Response.Cookies.Add(new HttpCookie("OU", encTicket));
            Response.Redirect(ConfigurationManager.AppSettings.Get("RedirectUrl"));        
            
        
    }

<authentication mode="Windows"/>
<machineKey validationKey="357356792679345184568256876535689056434617489465" 
                decryptionKey="357356792679345184568256876535689056434617489465" 
                validation="SHA1"/>

void Application_AuthenticateRequest(Object sender, EventArgs e)
    {
        bool cookieExists = false;
        HttpCookie authCookie = null;
        authCookie = Request.Cookies.Get(FormsAuthentication.FormsCookieName);
        if (authCookie != null)
            cookieExists = true;

        if (!cookieExists)
            return;
        
        FormsAuthenticationTicket winAuthTicket = 
            FormsAuthentication.Decrypt(authCookie.Value);
        
        string[] roles = winAuthTicket.UserData.Split(';');        
        FormsIdentity formsId = new FormsIdentity(winAuthTicket);
        GenericPrincipal princ = new GenericPrincipal(formsId, roles);
        HttpContext.Current.User = princ;

        User u = ToDoList.Lib.User.GetUser(HttpContext.Current.User.Identity.Name);
        if (u == null)
        {
            FormsAuthentication.SignOut();
            FormsAuthentication.RedirectToLoginPage();            
        }
    }

<authentication mode="Forms">
      <forms name="OU" loginUrl="Default.aspx" protection="All" timeout="30" path="/" requireSSL="false" slidingExpiration="true">
      </forms>
		</authentication>
    <machineKey validationKey="357356792679345184568256876535689056434617489465" 
                decryptionKey="357356792679345184568256876535689056434617489465" 
                validation="SHA1"/>

Response.Write(userName)

<authentication mode="Windows"/>

<identity impersonate="true"/>