To make your standard internal DNS accept the overrides, you will need to define it as the authoritative DNSs for the sites monitored. The way to do this varies from DNS to DNS. As an example, this is how you would do it using Microsoft DNS:
In your Microsoft DNS, right-click the DNS itself, select properties and click on “Forwarders”. Add a forwarder named “facebook.com” and point it to the IP address of the Unbound DNS. Do the same for “twitter.com”. This will make sure that the Microsoft DNS resolves all domains (other than Facebook and Twitter) by going external, but for Facebook and Twitter it will ask the Unbound DNS instead.
Wait for the TTL to expire in the DNS (30 seconds or so) and then try pinging www.facebook.com – this should resolve to the Composable Architecture Platform appliance IP. Then ping login.facebook.com, and you should get the external address.