源主页：
清华：https://mirrors.tuna.tsinghua.edu.cn/
科大：http://mirrors.ustc.edu.cn/
ubuntu：http://ports.ubuntu.com/
1.地址不要使用https的（尽管https也可以用浏览器打开，但apt-get时会失败）
2.ubuntu-ports里面有arm64的源，如果是PC机X86、X64使用ubuntu
3.xenial是ubuntu16.04（即发行版本)
security：仅修复漏洞，并且尽可能少的改变软件包的行为。低风险。
backports：backports的团队则认为最好的更新策略是security 策略加上新版本的软件（包括候选版本的）。但不会由Ubuntu security team审查和更新。
updates：修复严重但不影响系统安全运行的漏洞，这类补丁在经过QA人员记录和验证后才提供，和security那类一样低风险。
proposed：update类的测试部分，仅建议提供测试和反馈的人进行安装。
4.软件自由度
The four main repositories are:
Main - Canonical-supported free and open-source software.
Universe - Community-maintained free and open-source software.
Restricted - Proprietary drivers for devices.
Multiverse - Software restricted by copyright or legal issues.
main:完全的自由软件。
universe:ubuntu官方不提供支持与补丁，全靠社区支持。
restricted:不完全的自由软件。
muitiverse：非自由软件，完全不提供支持和补丁。
清华源：
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial main multiverse restricted universe deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-security main multiverse restricted universe deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-updates main multiverse restricted universe deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-backports main multiverse restricted universe deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial main multiverse restricted universe deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-security main multiverse restricted universe deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-updates main multiverse restricted universe deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-backports main multiverse restricted universe
科大源：
deb http://mirrors.ustc.edu.cn/ubuntu-ports/ xenial main multiverse restricted universe deb http://mirrors.ustc.edu.cn/ubuntu-ports/ xenial-security main multiverse restricted universe deb http://mirrors.ustc.edu.cn/ubuntu-ports/ xenial-updates main multiverse restricted universe deb http://mirrors.ustc.edu.cn/ubuntu-ports/ xenial-backports main multiverse restricted universe deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ xenial main multiverse restricted universe deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ xenial-security main multiverse restricted universe deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ xenial-updates main multiverse restricted universe deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ xenial-backports main multiverse restricted universe
ubuntu：
deb http://ports.ubuntu.com/ubuntu-ports/ xenial main multiverse restricted universe deb http://ports.ubuntu.com/ubuntu-ports/ xenial-security main multiverse restricted universe deb http://ports.ubuntu.com/ubuntu-ports/ xenial-updates main multiverse restricted universe deb http://ports.ubuntu.com/ubuntu-ports/ xenial-backports main multiverse restricted universe deb-src http://ports.ubuntu.com/ubuntu-ports/ xenial main multiverse restricted universe deb-src http://ports.ubuntu.com/ubuntu-ports/ xenial-security main multiverse restricted universe deb-src http://ports.ubuntu.com/ubuntu-ports/ xenial-updates main multiverse restricted universe deb-src http://ports.ubuntu.com/ubuntu-ports/ xenial-backports main multiverse restricted universe
