I recently attempted to apt-get upgrade an Ubuntu 17.04 VM that I hadn't upgraded in a while, and got all kinds of errors like:
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
It took me a surprising amount of time to discover the solution. Apparently this typically happens when a mirror has some problem that makes the updates unreadable, and I ran into lots of forums explaining how to fix that. Unfortunately, that wasn't my problem. My problem was a lot more legitimate.
Ubuntu 17.04 reached EOL in mid January of 2018, and so us.archive.ubuntu.com and security.ubuntu.com don't host updates any longer.
Ha!
Well, the solution is simple. Edit /etc/apt/sources.list and replace us.archive.ubuntu.com and security.ubuntu.com with old-releases.ubuntu.com. Then rerun your apt-get update and apt-get upgrade again.
It guess I never ran into the problem before because other than 17.04, I'm only running LTS releases. 16.04 is still supported, and the other versions are so old that I had to make that change immediately after installation so apt-get update would work at all. I guess I never run into a case where it had been working and stopped.
You learn something new every day.
