跨域登录，其他二级域读不到。

//在 mm.p.cn登录页面。//web.config<authentication mode="Forms">
<forms name=".ASPXAUTH" loginUrl="UserLogin.aspx"   path="/" protection="All" timeout="30" domain="p.cn"></forms>
</authentication>
//建立cookie
string username = "GUID=" + System.Guid.NewGuid() + ",SSID=" + Session.SessionID + ",USID=" + txtUserName.Text;
                FormsAuthenticationTicket tkt = new FormsAuthenticationTicket(1, username, DateTime.Now, DateTime.Now.AddHours(24), true, zid, FormsAuthentication.FormsCookiePath);
                string cookiestr = FormsAuthentication.Encrypt(tkt);
                HttpCookie ck = new HttpCookie(FormsAuthentication.FormsCookieName, cookiestr);
                //userCookie.Value = hashTicket;
                //userCookie.Expires = tichet.Expiration;
                //ck.Domain = FormsAuthentication.CookieDomain;
                ck.Path = FormsAuthentication.FormsCookiePath;
                Response.Cookies.Add(ck);
//在gg.p.cn下
//web.config
<authentication mode="Forms">
<forms name=".ASPXAUTH" loginUrl="mm.p.cn/UserLogin.aspx"   path="/" protection="All" timeout="30" domain="p.cn"></forms>
</authentication>
//读cookie
//方法一
 string cookieName = FormsAuthentication.FormsCookieName;
        HttpCookie authCookie = Request.Cookies[cookieName];
       //Common.common.alert(authCookie.Value);        if (authCookie != null)
        {
            try
            {
                FormsAuthenticationTicket authTicket = FormsAuthentication.Decrypt(authCookie.Value);                string MyCookie = authTicket.UserData.ToString();
                Common.common.alert(MyCookie);
            }
            catch (Exception ex)
            {
                return;
            }
        }//方法二
//判断用户是否登录
       if (Page.User.Identity.IsAuthenticated)
        {
            FormsIdentity identity = (FormsIdentity)HttpContext.Current.User.Identity;
            
       
            //不为空
            pnlLoggedin.Visible = true;
                        lblUserName.Text = identity.Name;
        }
方法一和二都得到那个cookie ,