[dns-operations] Opinions sought .... have I come to the right place?

Stephan Lagerholm stephan.lagerholm at secure64.com
Thu Nov 7 15:33:19 UTC 2013

Keep in mind that most cache system are using Least Recent Used
Algorithm for their cache without any removal of expired records.  

So the reason that stuff gets thrown out is not because of TTL expiry,
but rather because the cache is full. 

I don't know your exact test setup, but that might be why you are seeing
so many resolvers not respecting your TTL.


I think that is one of the reason why pre-fetching is giving such low
gain. Every now and then you are going to prefetch something nobody
wants on the expense of throwing out something that somebody actually
later wanted. 





From: dns-operations-bounces at lists.dns-oarc.net
[mailto:dns-operations-bounces at lists.dns-oarc.net] On Behalf Of Wiley,
Sent: Thursday, November 07, 2013 9:18 AM
To: Edward Lewis; DNS Operations
Subject: Re: [dns-operations] Opinions sought .... have I come to the
right place?


Be careful about conclusions you may draw from your data.  


It may be helpful to remember that many large recursive implementations
are comprised of a non-trivial footprint of hosts who may not share a
cache across the network.  In this case where you may find a TTL
respected by a single host behind that recursive server VIP or even
across multiple nodes at a single site behind the VIP it is possible
that multiple successive queries land on dieffrent nodes with different


Glen Wiley


Sr. Engineer

The Hive, Verisign, Inc.


From: Edward Lewis <ed.lewis at neustar.biz>
Date: Thursday, November 7, 2013 9:52 AM
To: DNS Operations <dns-operations at mail.dns-oarc.net>
Cc: Edward Lewis <ed.lewis at neustar.biz>
Subject: [dns-operations] Opinions sought .... have I come to the right


I've been studying TTL settings off and on for a few weeks, trying to
decide what are appropriate numbers.

In the past we taught the trade-off as - longer TTLs will reduce queries
while shorter TTLs will enable agility.

In looking at a set of data with a long TTL - 6 days - over a period of
time I noticed that 0.005% of all queriers respected the TTL setting I
had.  I don't want to fork over details, so you can even say "0.005% +/-
5%" and in any case, it's small.  I'll admit by number here might be a
little bit of an undercount, still, it's little.

In experimenting with some recursive servers (and by no means an
exhaustive set), some code bases did adhere to the "rules" and some code
bases seem to ignore the "rules."  I say this to the extent that the
collective set of deployed tools out there pretty much are eating into
the "longer TTLs will reduce queries" part of the above trade-off.

I see that in the IETF there are drafts to pre-fetch expiring data sets
- which one can't argue with - but it makes, for an authoritative server
operator - even more uncertainty in planning TTLs.

And I'll throw in another factoid from history.  During DNSSEC workshops
eons ago, we found that is the TTLs got too low, DNSSEC had problems.
(Presumably because it took longer to fetch the chain than the TTL of
the queried data.)  Has anyone found a TTL to be too low for DNSSEC?

So, I'm turning to this list...what is a good range for TTLs?


Edward Lewis             
NeuStar                    You can leave a voice message at

Why is it that people who fear government monitoring of social media are

surprised to learn that I avoid contributing to social media?



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