I installed a fresh Ubuntu system. Somehow - possibly through my own error - the backports repository got enabled. Then I did several upgrades. I noticed that this happened when networking suddenly stopped working, "Network Settings" now has an "(alpha)" in the title bar, "System Settings" ? "Network" now displays an error dialog saying "The system network services are not compatible with this version".

Now, I've disabled the backports repository, and I'd like to restore my system to its previously-functional state. My question is twofold:

1. How do I determine which packages were installed from backports?
2. Can I automatically re-install all those packages (and purge their configuration) to get back to a sensible state?

If the answer to 2 is "no" I can probably manually purge some things and reinstall, but it would be nice to have it handled automatically.

Update:

It wasn't an update that broke the network; it was apt-get install indicator-network, which installed something called "connman" and removed network-manager and network-manager-gnome.

Nevertheless I am leaving the question up, since I am still interested in how I can purge packages from a particular source after accidentally adding that source, and how I can determine which packages were installed from where.