[dns-operations] TTL=0

bert hubert bert.hubert at powerdns.com
Sun Jan 20 22:54:35 UTC 2019

On Sun, Jan 20, 2019 at 05:34:45PM -0500, Andrew Sullivan wrote:
> > The reason I am asking this question here is not just for academic
> > interest. We have a very real problem in our network at the moment that
> > hinges on who is right. I consider you all to be 'expert witnesses' whose
> > testimony I can potentially use to wave in front of a vendor and prove they
> > are wrong. Hence why I am being really picky in wanting a definitive
> > answer, if there is one.
> I don't understand how any problem in a network could hinge on this,
> but I am sometimes dull of imagination.

This typically happens when (non-open source) vendors refuse to fix
something, or more often, when they actually *can't* fix something (because
they are running on code they don't control or have no idea how to change).

Like a cornered cat, they then try to prove there is no bug and that someone
else should fix it for them.

We've been dragged into a few of such conflicts and it rarely ends well -
even if you end up proving that a vendor is in the wrong, you then find
they _can't_ fix the problem.

The problem is that in some cases you can't make a hard and fast ruling if
something is "compliant".  In addition, something might still be compliant
but also be something you shouldn't do - like handing out TTL=0 answers on
an authoritative server, or conversely, as a DNS resolver, dropping TTL=0

Because of such areas of confusion & vendors that can't or won't fix their
stuff, a number of PowerDNS users run with custom Lua scripts to work around
broken load balancers or strange DNS implementations hiding in storage
fail-over solutions.

But the ground truth is - if you have to do battle with vendors over corner
cases, the problem is rarely with the DNS standard.  It is more likely the
constellation of vendors that is the problem.


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