https://blog.wongcw.com/2018/12/01/%e5%85%8d%e8%b4%b9%e4%bd%bf%e7%94%a8cname%e7%89%88cloudflare-%e6%9e%81%e5%ae%a2%e6%97%8f%e6%8f%90%e4%be%9b/