I took down my DHCP server to change some stuff, and after bringing it back up (down for 15 mins or so), I noticed that four other Ubuntu 12.04 servers set to use DHCP were unreachable. Running ifconfig on them returned just lo, so the eth0 and eth1 had been disabled.
I'm assuming the DHCP server missing caused them to shutdown the interface? Not sure why, they've been assigned static dhcp ips in dnsmasq. How do I set Ubuntu to never give up trying for a dhcp address on a connected interface?