This tutorial, it's very simple lines code of apt.conf. Set a proxy server for the environments of Linux Mint. Works from Linux Mint 16 to Linux Mint 18.

First change directory and edit apt.conf:

cd /etc/apt/

sudo nano apt.conf

Add lines to apt.conf for all protocols:

Acquire::http::proxy "http://DOMAIN\USERNAME:PASSWORD@PROXYSERVER:PORT/";

Acquire::https::proxy "https://DOMAIN\USERNAME:PASSWORD@PROXYSERVER:PORT/";

Acquire::ftp::proxy "ftp://DOMAIN\USERNAME:PASSWORD@PROXYSERVER:PORT/";

CTRL+X for exit and press Y for save the file.

That's all.