Error fetching interface information: Device not found

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This article/section is a stub — probably a pile of half-sorted notes and assertions some of which may well be wrong, and not verified as a whole. Feel free to add or refine.

For example:

eth0: error fetching interface information: Device not found

Means that there is no device by that name. It frequently practically means it's not there anymore, as it was when you first used/configured it.

It often means that you're using a kernel with no support for the device, or you have it as a module and it isn't loaded (yet). This can have various causes for varying cases, e.g. depending on whether it's an ethernet card, wireless card, ppp, non-physical device.

You should know that udev persists the names of devices between boots. For example, if you had an eth0 and eth1 and pulled out one, you have a 50% chance that you pulled eth0 and you only have an eth1 left.

This is not like previous behaviour (which would assign on a first come, first serve basis, meaning you'ld be left with an eth0 in both cases). The persisting can be rather handy when you want a name to refer to a specific physical NIC and/or a specific cable.

udev's rules are in /etc/udev/rules.d/??-persistent-net.rules, where ?? is a number udev itself uses for ordering. The name is usually done by the device's MAC, and the file is pretty easily editable.