sudo gedit /etc/apt/apt.conf

Acquire::http::proxy "http://<a>172.18.0.1:3128/</a>";
Acquire::https::proxy "https://<a>172.18.0.1:3128/</a>";
Acquire::ftp::proxy "ftp://<a>172.18.0.1:3128/</a>/";
Acquire::socks::proxy "socks://<a>172.18.0.1:3128/</a>";

Acquire::http::proxy "http://username:password@your.network.proxy:port/";
Acquire::https::proxy "https://username:password@your.network.proxy:port/";
Acquire::ftp::proxy "ftp://username:password@your.network.proxy:port/";
Acquire::socks::proxy "socks://username:password@your.network.proxy:port/";