Additionally, how do I determine which repo is the source of a package, either installed or available?However, that will show you the repository of the latest version available of that package, you may have more repositories for the same package with older versions.Yeah I know, that conclusion sounded like a long shot to me at the time too.To test my “name server theory”, the only alternative DNS servers of which I am aware belong to Google.As “apt” had been updating, upgrading, downloading, and installing programs without issue prior to this massive failure, my first thought was to wait, as there might well be a problem from Unbuntu’s side with their servers.
Although it hasn't yet been released at the time of this writing, it's already possible to upgrade a 15.10 system to the development version of 16.04.
You should never upgrade a production system without first testing all of your deployed software and services against the upgrade in a staging environment.
Keep in mind that libraries, languages, and system services may have changed substantially.
This may be useful for testing both the upgrade process and the features of 16.04 itself in advance of the official release date.
This guide will explain the process for systems including (but not limited to) Digital Ocean Droplets running Ubuntu 15.10.