Centos：
https://wiki.centos.org/Download
http://mirrors.aliyun.com
http://mirrors.sohu.com
http://mirrors.163.com
https://mirrors.tuna.tsinghua.edu.cn/centos/
Ubuntu
http://cdimage.ubuntu.com/releases/18.04.1/release/?_ga=2.56783850.1533668672.1544323446-1412352718.1543052421