When implementing Direct Access you will have to allow name resolution for ISATAP addresses as by default this is not enabled.
You can do this by doing the following change on all your DNS servers.
- Open RegEdit.exe
- Navigate to HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesDNSParameters
- Right-Click modify on the GlobalQueryBlockList value.
- Remove ISATAP from list
- Restart the DNS service