[Tfug] Recommended DNS TTL values

[Matt Jacob]

Hello friends,

Before anyone makes an accusation that this topic is OT, please sit down 
and be quiet for a moment. Consider the fact that BIND is still the most 
popular DNS server currently in use and that BIND runs on Unixes. DNS, 
therefore, is inherently a Unix-like topic. And since we all use DNS 
every day on a variety of Unixes, whatever I just said in the previous 
four lines has already become moot. Anyway, onto more pressing issues...

What are "reasonable" numbers for DNS TTL values? It seems like I 
usually set up DNS and forget about it until I need to change something 
or until something breaks. Lo and behold, I'm moving to a new mail 
server soon, and the issue has come up again (mostly because I want a 
clean transition, but also because it makes for a fun educational argument).

So, I've got some thoughts. Currently, my TTL is 86400 for all record 
types across all my domains. For the purpose of switching to a new host, 
I usually lower my TTL to 300 for the cutover and then bring it back up 
again once everything is working. The problem is that 24 hours is a long 
time to wait before changes become active (i.e., I want to use my new 
mail server NOW!). I'm considering changing my standard TTL to 10800, 
which seems fairly common and is much more reasonable for making 
changes. Why wait 24 hours when you only have to wait 3 hours, right?

Is that too low of a value for day-to-day use? In some cases, it 
probably doesn't even matter, e.g., when dealing with DNS servers that 
don't honor TTL values and hold onto records for way longer than they 
should (a la Comcast and other cable providers). Burning up DNS query 
bandwidth isn't really an issue for me since I use third-party DNS 
servers through my hosting provider.

What's a happy medium? Is 10800 good, or will I burden the internet with 
my ultra-popular sites? (100 uniques a day is considered high-traffic, 
right...?) :-P What does everyone else do?

Matt