linux里的源，简单理解就是你用 apt-get 命令去下载安装软件时，系统去哪里找这个软件。去的那个位置就是源。
linux默认的源是国外的，下载速度比较慢，可以修改为国内的一些好的源地址，例如网易的、阿里的。
源是以一个文件的形式保存的，在/etc/apt/sources.list
修改源就是修改这个文本文件，把下面的国内源替换这个文件里的内容就好了。
网易源：
deb http://mirrors.163.com/ubuntu/ intrepid main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ intrepid-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ intrepid-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ intrepid-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ intrepid-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid-backports main restricted universe multiverse
阿里云源（从阿里云的ECS主机上copy下来的，应该挺靠谱的）：
deb http://mirrors.aliyun.com/ubuntu precise main restricted
deb http://mirrors.aliyuncs.com/ubuntu precise main restricted
deb-src http://mirrors.aliyun.com/ubuntu precise main restricted
deb-src http://mirrors.aliyuncs.com/ubuntu precise main restricted
deb http://mirrors.aliyun.com/ubuntu precise-updates main restricted
deb http://mirrors.aliyuncs.com/ubuntu precise-updates main restricted
deb-src http://mirrors.aliyun.com/ubuntu precise-updates main restricted
deb-src http://mirrors.aliyuncs.com/ubuntu precise-updates main restricted
deb http://mirrors.aliyun.com/ubuntu precise universe
deb http://mirrors.aliyuncs.com/ubuntu precise universe
deb-src http://mirrors.aliyun.com/ubuntu precise universe
deb-src http://mirrors.aliyuncs.com/ubuntu precise universe
deb http://mirrors.aliyun.com/ubuntu precise-updates universe
deb http://mirrors.aliyuncs.com/ubuntu precise-updates universe
deb-src http://mirrors.aliyun.com/ubuntu precise-updates universe
deb-src http://mirrors.aliyuncs.com/ubuntu precise-updates universe
deb http://mirrors.aliyun.com/ubuntu precise multiverse
deb http://mirrors.aliyuncs.com/ubuntu precise multiverse
deb-src http://mirrors.aliyun.com/ubuntu precise multiverse
deb-src http://mirrors.aliyuncs.com/ubuntu precise multiverse
deb http://mirrors.aliyun.com/ubuntu precise-updates multiverse
deb http://mirrors.aliyuncs.com/ubuntu precise-updates multiverse
deb-src http://mirrors.aliyun.com/ubuntu precise-updates multiverse
deb-src http://mirrors.aliyuncs.com/ubuntu precise-updates multiverse
deb http://mirrors.aliyun.com/ubuntu precise-backports main restricted universe multiverse
deb http://mirrors.aliyuncs.com/ubuntu precise-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu precise-backports main restricted universe multiverse
deb-src http://mirrors.aliyuncs.com/ubuntu precise-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu precise-security main restricted
deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
deb http://security.ubuntu.com/ubuntu precise-security universe
deb-src http://security.ubuntu.com/ubuntu precise-security universe
deb http://security.ubuntu.com/ubuntu precise-security multiverse
deb-src http://security.ubuntu.com/ubuntu precise-security multiverse