Comment 10 for bug 1308378

OK, I think I know what you are talking about now. (When you wrote "The proposed branch adds upgrade) preinst rule for resolvconf < 1.64" I thought you meant that you were going to update resolvconf 1.63 in Precise via precise-updates. This threw me off.)

In 12.04 the NetworkManager-controlled nameserver listened locally at 127.0.0.1 so resolving worked even if /etc/resolv.conf was absent.

In 12.10 and later the NetworkManager-controlled nameserver listens at 127.0.1.1. If /etc/resolv.conf is absent then resolving fails.

If you are running 12.04 using the NetworkManager-controlled nameserver and /etc/resolv.conf is absent and you upgrade to 14.04 then resolving no longer works and you are sad.