https://blog.wongcw.com/2019/01/29/nginx-%e4%b8%8d%e5%8f%97cdn-%e6%9c%8d%e5%8b%99%e5%bd%b1%e9%9f%bf%e7%8d%b2%e5%8f%96%e8%a8%aa%e5%ae%a2%e7%9c%9f%e5%af%a6ip/