Recently, I have a problem where when I tried to update Ubuntu package via apt-get, it shows HTTP 401 proxy error related.
Just a note, I’m running VM using my office network which has an enterprise web proxy servers.
From this site;
APT configuration file method
This method uses the apt.conf file which is found in your /etc/apt/ directory. This method is useful if you only want apt-get (and not other applications) to use a http-proxy permanently.
On some installations there will be no apt-conf file set up. This procedure will either edit an existing apt-conf file or create a new apt-conf file.
gksudo gedit /etc/apt/apt.conf
Add this line to your /etc/apt/apt.conf file (substitute your details for yourproxyaddress and proxyport).
Save the apt.conf file.