apt-get Behind a proxy server that requires authentication

Create a new file in the /etc/apt/apt.conf.d

for example: create the file 80proxy with

sudo nano /etc/apt/apt.conf.d/80proxy

and add the following lines to configure the proxy setting:

Acquire::http::Proxy "http://<username>:<password>@<proxy IP>:<port>";

Acquire::https::Proxy "http://<username>:<password>@<proxy IP>:<port>";

Acquire::ftp::Proxy "http://<username>:<password>@<proxy IP>:<port>";