No idea where to begin tracking this down :-(. After approximately six
days of uptime with a 4.12.0 kernel, the DNS resolver simply quits
working. All query attempts come back with "no DNS servers can be
reached". External hosts can still query the BIND server running on
the host with the non-functional resolver.
Restarting "named" has no effect. Rebooting the machine restores
functionality. So far, this is completely reproducible.
Everything worked fine with 4.11.0. Going to try a 4.13-rc4 kernel.