ubuntu 14.04:
http://mirrors.aliyun.com/ubuntu-releases/14.04/
ubuntu 16.04:
http://mirrors.aliyun.com/ubuntu-releases/16.04/
ubuntu 18.04:
http://mirrors.aliyun.com/ubuntu-releases/18.04/
ubuntu 14.04:
http://mirrors.aliyun.com/ubuntu-releases/14.04/
ubuntu 16.04:
http://mirrors.aliyun.com/ubuntu-releases/16.04/
ubuntu 18.04:
http://mirrors.aliyun.com/ubuntu-releases/18.04/