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

[Wessels, Duane]

Hey Mark,

I don't think BIND has such a feature, but you'd be better off asking on the BIND users list.

Maybe you could benefit from some kind of script that does "pre-lookups" in advance of the
job that runs at the top of each minute.  i.e., spend seconds 1-59 doing the lookups slowly
so that at second "0" most of the names should be fresh in the cache.

DW

On Jul 3, 2014, at 2:06 PM, Mark Pettit <mark at pettit.org> 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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