| Internet-Draft | nsec-ttl | February 2021 |
| van Dijk | Expires 22 August 2021 | [Page] |
Due to a combination of unfortunate wording in earlier documents, aggressive use of NSEC and NSEC3 records may deny names far beyond the intended lifetime of a denial. This document changes the definition of the NSEC and NSEC3 TTL to correct that situation. This document updates RFC 4034, RFC 4035, RFC 5155, and RFC 8198.¶
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[RFC editor: please remove this block before publication.¶
Earlier notes on this:¶
This document lives on GitHub; proposed text and editorial changes are very much welcomed there, but any functional changes should always first be discussed on the IETF DNSOP WG mailing list.¶
]¶
[RFC2308] defines the TTL of the SOA record that must be returned in negative answers (NXDOMAIN or NODATA):¶
The TTL of this record is set from the minimum of the MINIMUM field of the SOA record and the TTL of the SOA itself, and indicates how long a resolver may cache the negative answer.¶
Thus, if the TTL of the SOA in the zone is lower than the SOA MINIMUM value (the last number in the SOA record), the authoritative server sends that lower value as the TTL of the returned SOA record. The resolver always uses the TTL of the returned SOA record when setting the negative TTL in its cache.¶
However, [RFC4034] section 4 has this unfortunate text:¶
The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL field. This is in the spirit of negative caching ([RFC2308]).¶
This text, while referring to RFC2308, can cause NSEC records to have much higher TTLs than the appropriate negative TTL for a zone. [RFC5155] contains equivalent text.¶
[RFC8198] section 5.4 tries to correct this:¶
Section 5 of [RFC2308] also states that a negative cache entry TTL is taken from the minimum of the SOA.MINIMUM field and SOA's TTL. This can be less than the TTL of an NSEC or NSEC3 record, since their TTL is equal to the SOA.MINIMUM field (see [RFC4035], Section 2.3 and [RFC5155], Section 3).¶
A resolver that supports aggressive use of NSEC and NSEC3 SHOULD reduce the TTL of NSEC and NSEC3 records to match the SOA.MINIMUM field in the authority section of a negative response, if SOA.MINIMUM is smaller.¶
But the NSEC and NSEC3 RRs should, according to RFC4034 and RFC5155, already be at the value of the MINIMUM field in the SOA. Thus, the advice from RFC8198 would not actually change the TTL used for the NSEC and NSEC3 RRs for authoritative servers that follow the RFCs.¶
As a theoretical exercise, consider a TLD named .example with a SOA record like this:¶
example. 900 IN SOA primary.example. hostmaster.example. 1 1800 900 604800 86400¶
The SOA record has a 900 second TTL, and a 86400 MINIMUM TTL. Negative responses from this zone have a 900 second TTL, but the NSEC or NSEC3 records in those negative responses have a 86400 TTL. If a resolver were to use those NSEC or NSEC3 records aggressively, they would be considered valid for a day, instead of the intended 15 minutes.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The existing texts in [RFC4034], [RFC4035], and [RFC5155] use the SHOULD requirement level, but they were written when [RFC4035] still said 'However, it seems prudent for resolvers to avoid blocking new authoritative data or synthesizing new data on their own'. [RFC8198] updated that text to contain 'DNSSEC-enabled validating resolvers SHOULD use wildcards and NSEC/NSEC3 resource records to generate positive and negative responses until the effective TTLs or signatures for those records expire'. This means that correctness of NSEC and NSEC3 records, and their TTLs, has become much more important. Because of that, the updates in this document upgrade the requirement level to MUST.¶
The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL field. This is in the spirit of negative caching ([RFC2308]).¶
This is updated to say:¶
The TTL of the NSEC RR that is returned MUST be the lesser of the MINIMUM field of the SOA record and the TTL of the SOA itself. This matches the definition of the TTL for negative responses in [RFC2308]. A signer MAY cause the TTL of the NSEC RR to have a deviating value after the SOA record has been updated, to allow for an incremental update of the NSEC chain.¶
The TTL value for any NSEC RR SHOULD be the same as the minimum TTL value field in the zone SOA RR.¶
This is updated to say:¶
The TTL of the NSEC RR that is returned MUST be the lesser of the MINIMUM field of the SOA record and the TTL of the SOA itself. This matches the definition of the TTL for negative responses in [RFC2308]. A signer MAY cause the TTL of the NSEC RR to have a deviating value after the SOA record has been updated, to allow for an incremental update of the NSEC chain.¶
The NSEC3 RR SHOULD have the same TTL value as the SOA minimum TTL field. This is in the spirit of negative caching [RFC2308].¶
This is updated to say:¶
The TTL of the NSEC3 RR that is returned MUST be the lesser of the MINIMUM field of the SOA record and the TTL of the SOA itself. This matches the definition of the TTL for negative responses in [RFC2308]. A signer MAY cause the TTL of the NSEC RR to have a deviating value after the SOA record has been updated, to allow for an incremental update of the NSEC chain.¶
o The TTL value for any NSEC3 RR SHOULD be the same as the minimum TTL value field in the zone SOA RR.¶
This is updated to say:¶
o The TTL value for each NSEC3 RR MUST be the lesser of the MINIMUM field of the zone SOA RR and the TTL of the zone SOA RR itself. A signer MAY cause the TTL of the NSEC RR to have a deviating value after the SOA record has been updated, to allow for an incremental update of the NSEC chain.¶
[RFC8198] section 5.4 (Consideration on TTL) is completely replaced by the following text:¶
The TTL value of negative information is especially important, because newly added domain names cannot be used while the negative information is effective.¶
Section 5 of [RFC2308] suggests a maximum default negative cache TTL value of 3 hours (10800). It is RECOMMENDED that validating resolvers limit the maximum effective TTL value of negative responses (NSEC/NSEC3 RRs) to this same value.¶
A resolver that supports aggressive use of NSEC and NSEC3 MAY limit the TTL of NSEC and NSEC3 records to the lesser of the SOA.MINIMUM field and the TTL of the SOA in a response, if present. It MAY also use a previously cached SOA for a zone to find these values.¶
(The third paragraph of the original is removed, and the fourth paragraph is updated to allow resolvers to also take the lesser of the SOA TTL and SOA MINIMUM.)¶
If signers & DNS servers for a zone cannot immediately be updated to conform to this document, zone operators are encouraged to consider setting their SOA record TTL and the SOA MINIMUM field to the same value. That way, the TTL used for aggressive NSEC and NSEC3 use matches the SOA TTL for negative responses.¶
Validating resolvers consider an expanded wildcard valid for the wildcard's TTL, capped by the TTLs of the NSEC and NSEC3 proof that shows that the wildcard expansion is legal. Thus, changing the TTL of NSEC or NSEC3 records (explicitly, or by implementation of this document, implicitly) might affect (shorten) the lifetime of wildcards.¶
An attacker can prevent future records from appearing in a cache by seeding the cache with queries that cause NSEC or NSEC3 responses to be cached, for aggressive use purposes. This document reduces the impact of that attack in cases where the NSEC or NSEC3 TTL is higher than the zone operator intended.¶
IANA is requested to add a reference to this document in the "Resource Record (RR) TYPEs" subregistry of the "Domain Name System (DNS) Parameters" registry, for the NSEC and NSEC3 types.¶
[RFC Editor: please remove this section before publication]¶
Implemented in PowerDNS Authoritative Server 4.3.0 https://doc.powerdns.com/authoritative/dnssec/operational.html?highlight=ttl#some-notes-on-ttl-usage .¶
Implemented in BIND 9.16 and up, to be released early 2021 https://mailarchive.ietf.org/arch/msg/dnsop/ga41J2PPUbmc21--dqf3i7_IY6M https://gitlab.isc.org/isc-projects/bind9/-/merge_requests/4506 .¶
Implemented in Knot DNS 3.1, to be released in 2021 https://gitlab.nic.cz/knot/knot-dns/-/merge_requests/1219 .¶
Implemented in ldns, patch under review https://github.com/NLnetLabs/ldns/pull/118¶
Implementation status is tracked at https://trac.ietf.org/trac/dnsop/wiki/draft-ietf-dnsop-nsec-ttl¶
[RFC editor: please remove this section before publication.]¶
From draft-vandijk-dnsop-nsec-ttl-00 to draft-ietf-dnsop-nsec-ttl-00:¶
From draft-ietf-dnsop-nsec-ttl-00 to draft-ietf-dnsop-nsec-ttl-01:¶
From draft-ietf-dnsop-nsec-ttl-01 to draft-ietf-dnsop-nsec-ttl-02:¶
From draft-ietf-dnsop-nsec-ttl-02 to draft-ietf-dnsop-nsec-ttl-03:¶
From draft-ietf-dnsop-nsec-ttl-03 to draft-ietf-dnsop-nsec-ttl-04:¶
This document was made possible with the help of the following people:¶
The author would like to explicitly thank Paul Hoffman for extensive reviews, text contributions, and help in navigating WG comments.¶