Re: [DNSOP] How Slack didn't turn on DNSSEC

Mark Andrews <marka@isc.org> Thu, 09 December 2021 02:04 UTC

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From: Mark Andrews <marka@isc.org>
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Date: Thu, 09 Dec 2021 13:04:09 +1100
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To: Philip Homburg <pch-dnsop-4@u-1.phicoh.com>
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Subject: Re: [DNSOP] How Slack didn't turn on DNSSEC
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> On 9 Dec 2021, at 00:36, Philip Homburg <pch-dnsop-4@u-1.phicoh.com> wrote:
> 
>> Also stop hiding this
>> breakage. Knot and unbound ignore the NSEC records which trigger
>> this when synthesising.  All it does is push the problem down the
>> road and makes it harder for others to do proper synthesis based
>> on the records returned.
> 
> I did some tests with unbound (version 1.13.1-1 on Debian Bullseye). 
> 
> For types other than 'A', the behavior is quite simple: if both
> DNSSEC validation (auto-trust-anchor-file) and aggressive-nsec are enabled
> then unbound will synthesize NODATA based on a cached NSEC record.
> Both are off by default.
> 
> For A records the situation is more complex. If qname-minimisation is off,
> then the same applies to A records. However if qname-minimisation is on (and
> it is on the default) then unbound will internally generate A record
> queries. So the A record will be cached before the NSEC record. 
> 
> So in the case of Slack, anybody who enabled both DNSSEC validation and
> aggressive-nsec would probably not have seen a failure due to the
> broken NSEC records because qname-minimisation is on by default.

Actually they still can.  Just wait until the A response expires while 
there are still cached NSEC present.  This will happen naturally with a
big enough client pool.  Incorrect negative proofs lead to intermittent
“incorrect” responses with aggressive synthesis.

Mark
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka@isc.org