Source File
/etc/apt/sources.list
deb http://cn.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ precise-proposed main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ precise-proposed main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
update command
sudo apt-get update