真正的解决方法如下：

新建文件 apt_proxy_conf(文件名任意)

内容如下：

Acquire::http::proxy "http://127.0.0.1:8000/";
Acquire::ftp::proxy "ftp://127.0.0.1:8000/";
Acquire::https::proxy "https://127.0.0.1:8000/";

往后就可以在apt-get之后加上 -c apt_proxy_conf 来执行。

更改源的步骤如下：

sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup

sudo vi /etc/apt/sources.list

sudo apt-get update

附录为Ubuntu 12.04在sohu的mirror（/etc/apt/source.list）：

deb http://mirrors.sohu.com/ubuntu/ precise main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-security main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-updates main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-proposed main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-security main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-updates main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-proposed main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse