In NOC A I have a web server and an authoritative DNS server.
In NOC B I have a backup web server.
The authoritative DNS server has 2 WAN links on it. If one WAN goes down, it serves out the IP address of the secondary WAN connection.
How can I configure my DNS so that if a) the DNS servers dies or b) or both WAN links go down, our domain is resolved to NOC B and the backup web server?