PHP Hostname not correct
My first guess would be /etc/hosts which can override DNS (if it exists for the host) and substitute almost entirely for forward DNS when it's missing. For whatever reason this host (dbl) was in /etc/hosts and then it wasn't.
Secondarily, let's say /etc/resolv.conf lost the search option for the local domain, so that dbl.foo.com resolves but dbl does not... also a possibility.