Solving "configuration not set for eth0" on Gentoo Linux

Recently I updated some of my configuration files on my Gentoo box. When I brought it back up, X had died and eth0 was failing with "configuration not set for eth0".

After about an hour of searching for why eth0 wasn't coming up, I paused and stepped back from it. I realized that my USB hub wasn't lit up, meaning that USB wasn't being recognized. Since I use a USB dongle for my Wireless adapter, it made me realize that it wasn't working either.

I found out that hotplug, which controls when you plug in a new device in Linux while the system is running, has a complement coldplug which registers USB devices when the system has started up. Appararently up until recently hotplug also registered devices at startup. But the configuration change "fixed" that, and since I didn't have coldplug registered to come up at boot, the USB devices weren't being recognized.

So, I emerged coldplug (luckily I had it locally!) and added it to the boot runlevel. I restarted, and whammo! I had eth0 again.

So, in summary, if eth0 is not being recognized, make sure your system actually sees the device, whether it's a pci (with lspci) or USB.