Apple Breaks Ethernet in OS X El Capitan
It’s startling that Apple can break core system functionality over a weekend. This issue is with OS X El Capitan (not Yosemite):
Ars Technica: OS X blacklist accidentally disables Ethernet in El Capitan.
This blacklist isn't updated through the Mac App Store like purchased apps or OS X itself. Rather, it uses a silent auto-update mechanism that executes in the background even if you haven't enabled normal automatic updates
For the fix, see the instructions at Apple: If the Ethernet connection on your Mac stopped working recently. Oddly, the instructions do not mention the OS X version at all.