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Comment by bigdubs | Hacker News Reader

What are the downsides of a small TTL? Why wouldn't a low TTL be default?

If your TTL is very low, you end up creating/handling a lot more DNS traffic, because your records are flushed from the cache more often and have to be re-retrieved. Also, many public DNS servers (e.g. your ISP's, or Google's) set a minimum TTL on all records (overwriting any lower value) to minimize DNS traffic/requests. Setting your TTL to an hour is fairly standard, but some DNS hosts (especially old ones) will leave the defaults set to something like 48 hours for no real reason.

Very few resolvers break DNS TTLs in that way anymore. Google certainly honors TTLs down to at least 30 seconds. I'm not aware of any major ISPs that get this wrong anymore either.

This hasn't been a significant problem in years. When I execute a DNS change on a record with a 30 second TTL, I expect to see 95+% of the traffic move within a couple of minutes. The things that tend to get it wrong these days are applications that don't honor the TTL instead of resolvers, but browsers generally get it right.

madsushi is talking about servers, not resolvers.

The primary downside to a small TTL is cost. For many DNS providers you are allotted a number of requests per month that DNS will resolve. On particularly popular websites having a ton of DNS requests can cause the cost of DNS services to inflate significantly.

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