之前网上提供那个废了，至少上海电信的网络访问都不行，国外源到国内更新太慢，慢的都可以母鸡下蛋再生母鸡了。搜索并验证了清华大学的源，可以正常使用。
修改 sources.list 文件
sudo nano /etc/apt/sources.list
把官方源前面加上#号注释掉，最下面加上下面的清华大学源列表
清华大学源
deb https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch main contrib non-free
deb https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch-updates main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch-updates main contrib non-free
deb https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch-backports main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch-backports main contrib non-free
deb https://mirrors.tuna.tsinghua.edu.cn/debian-security/ stretch/updates main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu.cn/debian-security/ stretch/updates main contrib non-free
更新
sudo apt update
sudo apt upgrade
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原文链接：https://blog.csdn.net/no1xium/article/details/107476202
另一个号称是阿里云的，备用，暂时没测试：
deb http://mirrors.aliyun.com/ubuntu-ports/ xenial main
deb-src http://mirrors.aliyun.com/ubuntu-ports/ xenial main
deb http://mirrors.aliyun.com/ubuntu-ports/ xenial-updates main
deb-src http://mirrors.aliyun.com/ubuntu-ports/ xenial-updates main
deb http://mirrors.aliyun.com/ubuntu-ports/ xenial universe
deb-src http://mirrors.aliyun.com/ubuntu-ports/ xenial universe
deb http://mirrors.aliyun.com/ubuntu-ports/ xenial-updates universe
deb-src http://mirrors.aliyun.com/ubuntu-ports/ xenial-updates universe
deb http://mirrors.aliyun.com/ubuntu-ports/ xenial-security main
deb-src http://mirrors.aliyun.com/ubuntu-ports/ xenial-security main
deb http://mirrors.aliyun.com/ubuntu-ports/ xenial-security universe
deb-src http://mirrors.aliyun.com/ubuntu-ports/ xenial-security universe
