Holmium (the designate/pdns server) will be providing both internal and external dns service. It has a public IP and a public name (currently labs-ns2.wikimedia.org) which is good for the public part.
We're going to start with it serving internal IPs for internal labs instance names, e.g. util-abogott.testlabs.wikimedia.org. Instances could contact it via the public IP if necessary, but it would be nicer to do it via an internal IP so that we can tell the difference between queries.
...or... we just need two different dns servers entirely. I'm currently a bit confused.