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

Nick Johnson <nick@ethereum.org> Thu, 08 October 2020 23:46 UTC

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From: Nick Johnson <nick@ethereum.org>
Date: Fri, 9 Oct 2020 12:46:10 +1300
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Subject: [DNSOP] What is the purpose of NSEC3 "closest encloser" proofs?
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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