Forum Discussion
You should check with your F5 sales engineers... am not sure how licensing works, but DNS related iRule events and commands might be present in LTM or LTM plus some DNS feature, but surely in GTM indeed.
https://devcentral.f5.com/wiki/iRules.DNS_REQUEST.ashx
With iRule dealing with DNS events, plus LTM pools and custom monitors, I imagine you could achieve that.
With GTM, you would either make GTM authoritative for your entire domain, so it intercepts all requests, or delegate some some subdomain where these "special DNS names" would live....then configure GTM to answer DNS requests for A records based on the health status of the web servers...
Note that pricing wise standalone GTM appliance is probably the same as the software addon module for LTM... so it would be cleaner to think of GTM is an independent appliance that handles your DNS requests and uses its knowledge of the LTM appliances virtual servers to answer accordingly....
Question: So, you have a DNS name that you need resolved to the IP of the web server that is UP? Why not put all those web servers in a pool behind the LTM, and let DNS resolve the name to the IP of the virtual server?