What is the different between internal DNS and external DNS?

What are the risks of managing internal DNS records via an external DNS service?

• Security through obscurity is considered bad. I understand that internal topology is revealed and reconnaissance becomes easy if I expose my internal DNS information to the world.  However, if proper ACLs and firewalls are in place to restrict hostile ingress traffic, what are the arguments for not exposing internal DNS records(apart from defense-in-depth)? Note: I am assuming that the DNS records themselves are protected against tampering/hijacking.

• Answer:

  Your caveat assumption is a very bad one. DNS is not protected from tampering nor hijacking, nor man-in-the-middle (MITM) attacks. So the best way to break into your internal networks are to redirect employee requests for internal resources to hostile servers that engage in credential stealing and/or social engineering. Giving an attacker knowledge of server names and their (likely RFC 1918) addresses is of much less concern in comparison.