Re: [DNSOP] What is the purpose of NSEC3 "closest encloser" proofs?

Nick Johnson <> Fri, 09 October 2020 02:38 UTC

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From: Nick Johnson <>
Date: Fri, 09 Oct 2020 15:38:13 +1300
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To: Shumon Huque <>
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Subject: Re: [DNSOP] What is the purpose of NSEC3 "closest encloser" proofs?
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On Fri, Oct 9, 2020 at 1:59 PM Shumon Huque <> wrote:

> On Thu, Oct 8, 2020 at 7:46 PM Nick Johnson <nick=
>> wrote:
>> I'm reading RFC 5155, and I'm a bit puzzled by the requirement for
>> "closest encloser" proofs to prove nonexistence of a domain. Given that the
>> RFC requires generating NSEC3 records on empty non-terminals, isn't it
>> sufficient to examine a single NSEC3 record to prove nonexistence?
>> For example, if I want to prove the nonexistence of a.b.c.example, isn't
>> it sufficient to validate an NSEC3 record that covers that name and is one
>> level higher (eg, somehash.b.c.example)? Why do I need to prove the
>> closest-encloser with a second NSEC3 record?
>> -Nick Johnson
> The closest encloser proof actually *is* what proves that the name doesn't
> exist. But the other reason is that for NXDOMAIN proofs, you also need to
> prove that the name could not have been synthesized by a wildcard. The
> hypothetical wildcard that might have synthesized a response for the name
> is constructed by prepending the asterisk label to the closest encloser.
> Let's use your example and say 'a.b.c.example' doesn't exist in the zone
> example.
> Let's also say the longest ancestor of this name that actually does exist
> in the zone is 'c.example' (which could be an empty non-terminal or not --
> either way, it will have an NSEC3 record matching the hash of the name).
> The NXDOMAIN proof consists of:
> ### Closest Encloser proof:
> * the NSEC3 RR that matches the closest encloser name 'c.example'
> * the NSEC3 RR that covers the next closer name 'b.c.example'

Right; what I don't follow is why the second of those two proofs isn't
sufficient. Doesn't that alone prove that a.b.c.example doesn't exist?

> This proves that b.c.example does not exist. This automatically means that
> all names under it, including a.b.c.example, do not exist.
> ### Wildcard non existence proof:
> * the NSEC3 RR that covers the wildcard at the closest encloser, namely
> '*.c.example'.
> Shumon Huque