<authentication mode="Forms">
      <forms loginUrl="~/Login.aspx" name=".ASPXFORMSAUTH" enableCrossAppRedirects="true">
      </forms>
    </authentication>
    <authorization>
      <deny users="?" />
    </authorization>

------解决方案--------------------
web.config 的location节点可以限制是否已经登录过的

------解决方案--------------------
iframe嵌套页面判断
basepage判断sesson
forms验证
public class BasePage : System.Web.UI.Page 
 { 
 public BasePage() 
 {  }
 protected override void OnInit(EventArgs O) 
 { 
 if (base.Session["UserId"] == null || base.Session["UserId"].ToString().Equals("")) 
 { 
 Response.Redirect("~/Error.aspx"); 
 } 
 } 
  
 } 
<authentication mode="Forms">
 <forms loginUrl="/login.aspx" defaultUrl="~/a.aspx" />
 </authentication>