Configure proxy for APT

Create and edit the following file:

sudo nano /etc/apt/apt.conf.d/99proxy

Add the following configuration

Acquire::http::Proxy "http://USERNAME:PASSWORD@SERVER:PORT";
Acquire::https::Proxy "http://USERNAME:PASSWORD@SERVER:PORT";

example:

Acquire::http::proxy "http://lgn:pwd@192.168.1.254:8080/";
Acquire::https::proxy "http://lgn:pwd@192.168.1.254:8080/";

Save and exit.

References