The idea of a "default" interface for a particular physical transport is worth examining for coherency, I think. If you only have one adapter, then it's the one

The idea of a "default" interface for a particular physical transport is worth examining for coherency, I think. If you only have one adapter, then it's the one that shows up when you list your interfaces, so script to that end. If you have more than one, then trusting an ordering that can change on boot is unsafe. But don't worry - those of such a soberly considered persuasion can just use the configuration management they've definitely built and that necessitates such assumptions as "there's always an eth0" to bring it back.