Mac OS X firewall vs. DHCP
I’ve been having some issues with Mac OS X not being able to configure itself with DHCP. It looks like the firewall was blocking DHCP responses. Quick solution was to turn off the firewall. Longer-term solution may be that I need to reinstall configd and mDNSResponder from the latest Mac OS X update, so that they are signed correctly, so that the firewall trusts them again.