[dns-operations] Who Ignores TTLs ?

Patrick W. Gilmore patrick at ianai.net
Thu Feb 17 02:25:08 UTC 2011

On Feb 16, 2011, at 9:01 PM, Simon Lyall wrote:

> I keep seeing a persistent complaint that some DNS caching operators ignore TTLs or otherwise keep records for longer than the TTL would indicate.
> I suspect this might be an urban legend since most DNS caching software doesn't even offer this as an option last time I checked.
> Does anybody actually do this? Because it keep being brought up by some peopel as to why things like GSLB don't work.

The standard TTL of an Akamai hostname is 20 seconds.  If we change the hostname, traffic moves within seconds, and 90+% is moved within 10-15 minutes.  The few minutes could be OS, Browsers, and/or caching resolvers ignoring TTLs.  There are also some large files that take a while to download.  (This is for HTTP, streaming takes longer for reasons which should be obvious.)

We believe the final few percent are mostly browsers which never re-lookup a hostname.  I am not sure if we have data to prove this.  There are other Akamaites on this list who may have more info than I.

Either way, it is trivial to empirically prove the number of resolvers holding short TTL hostnames for more than a few minutes is pretty small.

Or, to turn the question on its head, if GSLB through DNS didn't work, most of the traffic on the Internet would be misdirected.  So those who say it does not work are confused - they're using it every day.


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