Issue
Earlier, I was at my university and accessing the internet through a proxy server i.e. proxy.iiit.ac.in. But, currently I am trying to run sudo apt-get update from my home internet (which as no proxy server) and I am getting the following error message.
I have removed the system and terminal proxies, and I have also removed proxies from apt.conf in /etc/apt/. What is it that's still causing the problem?
Solution
It looks like you have inherited the information from the DHCP server.
Check the APT file (you have already done it) under /etc/apt/apt.conf and remove any Acquire::XXX::proxy where XXX is http/ https / ftp
In addition to this, check /etc/environment for proxy information.
Last but not least, try to renew your DHCP information, it can eventually help: sudo dhclient -r
One last word, are you using the same BASH shell process you were using at the university or you opened a new one? Because, it could have cached information.
Answered By - Maurizio Benedetti