Incomplete type bitmaps in NSEC(3) records and aggressive use of DNSSEC validated cache
Ruben van Staveren
ruben at verweg.com
Wed Sep 8 09:12:19 UTC 2021
Last month or so I saw two domains, postnl.nl <http://postnl.nl/> and minjenv.nl <http://minjenv.nl/>, return incomplete NSEC3 records where existing records where omitted from the Type Bit Maps.
This caused strange intermittent failures when a resolver was used that implements aggressive use of DNSSEC validated cache (RFC8198, 4 years old), e.g powerdns recursor 4.5.x.
e.g., the minjenv has a mx record, but it is not listed in the NSEC3 you’ll get if you query for the non existent A/AAAA record (only NS SOA RRSIG DNSKEY NSEC3PARAM) causing mail delivery failures until the TTL expires. postnl.nl <http://postnl.nl/> has A/AAAA, but the NSEC3 seen for a nonexistent query only has NS SOA MX TXT RRSIG DNSKEY NSEC3PARAM
It is not as such to contact the dns operators and persuade them to upgrade/fix their software used for DNSSEC signing, but more as should we do more analysis of this phenomenon and even have a dns flag day before even more resolvers and operators are going to implement RFC8198? There might be an issue by deliberately exploiting this and make websites/mail unreachable.
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