Mountain Lion introduces a small, yet significant change in how OS X clients communicate with Apple’s Software Update servers.
Mountain Lion clients by default now consult an HTTPS URL to check for updates on Apple’s servers.
This has the implication that you can no longer use DNS trickery with Mountain Lion clients to get them to access your internal SUS instead of Apple’s. Some organizations configured their internal DNS servers to resolve requests to swupdate.apple.com to an internal server.
The appeal of this approach (DNS hijacking) was that you did not need to touch any client machine to get it to use your internal Softwar Update server; any client using your network would “automagically” use your internal Software Update server.
HTTPS connections do certificate verification of the host. Since your internal SUS can’t offer a certificate proving it is Apple’s server (because it’s not!) a Mountain Lion client will refuse to talk to it. (It’s also possible that your internal SUS is not accepting HTTPS connections anyway, but even if you were to turn that on, it would not help.)
HTTPS prevents exactly the sort of DNS impersonation that made this hack work in the first place. Apple has closed that door.
In order to use an internal Software Update server (either Apple’s or Reposado) with Mountain Lion clients, the clients must be explicitly pointed to your internal server via MCX or by setting the preference using:
defaults write /Library/Preferences/com.apple.SoftwareUpdate CatalogURL <internalSUScatalogURL>