14.04版本，安装扩展报错。

Err http://us.archive.ubuntu.com/ubuntu/ trusty-updates/main libc6 amd64 2.19-0ubuntu6.5
  Could not resolve 'us.archive.ubuntu.com'
Err http://us.archive.ubuntu.com/ubuntu/ trusty-updates/main libgcrypt11 amd64 1.5.3-2ubuntu4.1
  Could not resolve 'us.archive.ubuntu.com'
Err http://us.archive.ubuntu.com/ubuntu/ trusty/main libasan0 amd64 4.8.2-19ubuntu1
  Could not resolve 'us.archive.ubuntu.com'
Err http://us.archive.ubuntu.com/ubuntu/ trusty/main libatomic1 amd64 4.8.2-19ubuntu1
  Could not resolve 'us.archive.ubuntu.com'
Err http://us.archive.ubuntu.com/ubuntu/ trusty/main libgmp10 amd64 2:5.1.3+dfsg-1ubuntu1
  Could not resolve 'us.archive.ubuntu.com'
Err http://us.archive.ubuntu.com/ubuntu/ trusty/main libisl10 amd64 0.12.2-1
  Could not resolve 'us.archive.ubuntu.com'
Err http://security.ubuntu.com/ubuntu/ trusty-security/main libgcrypt11 amd64 1.5.3-2ubuntu4.1
  Could not resolve 'security.ubuntu.com'

求解答，之前下载其他三个软件都是没问题的，难道是断网了？