root@ubuntu:/etc/apt# apt-get update

Err: http://mirrors.aliyun.com/ubuntu trusty InRelease

  Could not resolve 'mirrors.aliyun.com'

Err: http://mirrors.aliyun.com/ubuntu trusty-backports InRelease

  Could not resolve 'mirrors.aliyun.com'

Err: http://mirrors.aliyun.com/ubuntu trusty-proposed InRelease

  Could not resolve 'mirrors.aliyun.com'

Err: http://mirrors.aliyun.com/ubuntu trusty-security InRelease

  Could not resolve 'mirrors.aliyun.com'

Err: http://mirrors.aliyun.com/ubuntu trusty-updates InRelease

  Could not resolve 'mirrors.aliyun.com'

Reading package lists... Done

W: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/trusty/InRelease  Could not resolve 'mirrors.aliyun.com'

W: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/trusty-backports/InRelease  Could not resolve 'mirrors.aliyun.com'

W: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/trusty-proposed/InRelease  Could not resolve 'mirrors.aliyun.com'

W: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/trusty-security/InRelease  Could not resolve 'mirrors.aliyun.com'

W: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/trusty-updates/InRelease  Could not resolve 'mirrors.aliyun.com'

W: Some index files failed to download. They have been ignored, or old ones used instead.

vim /etc/systemd/resolved.conf