Is it feasible to set my domain's TTL to a very low value (a couple of minutes) so that, if my main host goes down for an extended period of time, I can update the DNS records in my main host to point to my secondary host's IP, thus directing new visitors — once the DNS propogation is finished — to my secondary host's servers? Or is this not even possible, perhaps even stupid? I'm just trying to figure out an inexpensive way to prevent downtime.
If it is possible, how low is too low for a TTL?
Best Answer
Low TTLs only work so far, as some DNS resolvers don't honor TTLs below certain, completely arbitrary levels. Even if you set your TTLs to 5 minutes, it can still take up to 48 hours for the change to fully flush from DNS caches on the Internet. What makes this problem worse is that Malware authors use this very trick to change where their command-and-control servers are kept (this is called "fast flux DNS" link), so a countermeasure for this is to not honor TTLs below a certain level.
It is possible to do what you want, and by the standards it should be completely workable, but it is not reliable.