I'm using my laptop sometimes behind a proxy and sometimes not. I've created an /etc/apt/apt.conf file, I've used it like that for the last 2 years:
when behind a proxy I add the line:
when I don't need proxy authentication, I remove or comment it out.
it worked so well like that, but now whenever I try to run any
apt-get command it tries to connect to the proxy server, even with these lines commented out, or with apt.conf doesn't even exist.