Reason
Ubuntu的国外源经常访问不上，可以通过设置国内镜像源来提高访问速度。 sudo vim /etc/apt/sources.list 将文件内容修改为如下：
deb http://cn.archive.ubuntu.com/ubuntu/ willy main restricted universe multiverse deb http://cn.archive.ubuntu.com/ubuntu/ willy-security main restricted universe multiverse deb http://cn.archive.ubuntu.com/ubuntu/ willy-updates main restricted universe multiverse deb http://cn.archive.ubuntu.com/ubuntu/ willy-backports main restricted universe multiverse ##测试版源 deb http://cn.archive.ubuntu.com/ubuntu/ willy-proposed main restricted universe multiverse # 源码 deb-src http://cn.archive.ubuntu.com/ubuntu/ willy main restricted universe multiverse deb-src http://cn.archive.ubuntu.com/ubuntu/ willy-security main restricted universe multiverse deb-src http://cn.archive.ubuntu.com/ubuntu/ willy-updates main restricted universe multiverse deb-src http://cn.archive.ubuntu.com/ubuntu/ willy-backports main restricted universe multiverse ##测试版源 deb-src http://cn.archive.ubuntu.com/ubuntu/ willy-proposed main restricted universe multiverse
