[dns-operations] Forcing BIND to randomly expire records from cache ahead of time

Tim Wicinski tjw.ietf at gmail.com
Thu Jul 3 22:04:20 UTC 2014


Unbound has this feature, but its' a % of the TTL (oh they may of 
changed this).

You may be also interested in this idea which was floated during IETF, 
and not rejected, just a small sliver of useful customer base:


On 7/3/14 5:06 PM, Mark Pettit wrote:
> Hi, folks.
> I have an issue with BIND cache timeouts, and I was hoping someone else
> might have some idea how to fix this.
> Here's the situation: we have a large number of servers that do a huge
> number of DNS lookups at the top of every minute. The TTL for the
> records they're looking up is 3600.
> What we've noticed is that on a host with a recently-restarted copy of
> BIND, we see huge spikes in DNS latency every 61 minutes. This makes
> logical sense, given the behavior of the DNS lookups.
> What is more interesting is that on hosts that have been running BIND
> for a very long time (on the order of months), the spikiness is not visible.
> Our speculation is that over time, due to the interaction between the
> 3600 TTL and the "once every minute" lookup behavior, cache misses
> become randomly distributed throughout the hour, and don't cause the
> spiky behavior that is observed initially.
> One of our ideas to resolve this is to randomize the TTLs in the zone
> files, causing them to expire out of cache at different times, thus
> forcing more-rapid distribution of cache misses across the hour.
> However, this would involve some massive edits to our zone files, and
> isn't really ideal.
> What *would* be ideal would be if we could tell BIND to randomly expire
> some small percentage of cached entries ahead of the actual TTL
> expiration. This would serve the same purpose as assigning "random" TTLs
> to the actual records in the zone files.
> Does BIND have a config option like this? Has anyone else ever
> encountered this issue, and if so, how did you address it?
> Thanks for any advice, and I hope everyone has a fantastic Fourth of
> July weekend.
> Mark Pettit
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