NSEC and NSEC3 TTLs and NSEC Aggressive Use
draft-ietf-dnsop-nsec-ttl-04

dnsop                                                        P. van Dijk
Internet-Draft                                                  PowerDNS
Updates: 4034, 4035, 5155, 8198 (if approved)           18 February 2021
Intended status: Standards Track                                        
Expires: 22 August 2021

              NSEC and NSEC3 TTLs and NSEC Aggressive Use
                      draft-ietf-dnsop-nsec-ttl-04

Abstract

   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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 22 August 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  NSEC and NSEC3 TTL changes  . . . . . . . . . . . . . . . . .   4
     3.1.  Updates to RFC4034  . . . . . . . . . . . . . . . . . . .   4
     3.2.  Updates to RFC4035  . . . . . . . . . . . . . . . . . . .   5
     3.3.  Updates to RFC5155  . . . . . . . . . . . . . . . . . . .   5
     3.4.  Updates to RFC8198  . . . . . . . . . . . . . . . . . . .   6
   4.  Zone Operator Considerations  . . . . . . . . . . . . . . . .   6
     4.1.  A Note On Wildcards . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Implementation Status  . . . . . . . . . . . . . . .   7
   Appendix B.  Document history . . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   [RFC editor: please remove this block before publication.

   Earlier notes on this:

   *  https://indico.dns-oarc.net/event/29/sessions/98/#20181013
      (https://indico.dns-oarc.net/event/29/sessions/98/#20181013)

   *  https://lists.dns-oarc.net/pipermail/dns-operations/2018-April/
      thread.html#17420 (https://lists.dns-oarc.net/pipermail/dns-
      operations/2018-April/thread.html#17420)

   *  https://lists.dns-oarc.net/pipermail/dns-
      operations/2018-March/017416.html (https://lists.dns-
      oarc.net/pipermail/dns-operations/2018-March/017416.html)

   This document lives on GitHub (https://github.com/PowerDNS/draft-
   dnsop-nsec-ttl); 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):

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   |  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"

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   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.

2.  Conventions and Definitions

   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.

3.  NSEC and NSEC3 TTL changes

   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.

3.1.  Updates to RFC4034

   Where [RFC4034] says:

   |  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.

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3.2.  Updates to RFC4035

   Where [RFC4035] says:

   |  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.

3.3.  Updates to RFC5155

   Where [RFC5155] says:

   |  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.

   Where [RFC5155] says:

   |  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.

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3.4.  Updates to RFC8198

   [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.)

4.  Zone Operator Considerations

   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.

4.1.  A Note On Wildcards

   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.

5.  Security Considerations

   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.

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6.  IANA Considerations

   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.

7.  Normative References

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
              <https://www.rfc-editor.org/info/rfc2308>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
              <https://www.rfc-editor.org/info/rfc5155>.

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <https://www.rfc-editor.org/info/rfc8198>.

8.  Informative References

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Appendix A.  Implementation Status

   [RFC Editor: please remove this section before publication]

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   Implemented in PowerDNS Authoritative Server 4.3.0
   https://doc.powerdns.com/authoritative/dnssec/
   operational.html?highlight=ttl#some-notes-on-ttl-usage
   (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://mailarchive.ietf.org/arch/msg/dnsop/
   ga41J2PPUbmc21--dqf3i7_IY6M) https://gitlab.isc.org/isc-projects/
   bind9/-/merge_requests/4506 (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
   (https://gitlab.nic.cz/knot/knot-dns/-/merge_requests/1219) .

   Implemented in ldns, patch under review
   https://github.com/NLnetLabs/ldns/pull/118
   (https://github.com/NLnetLabs/ldns/pull/118)

   Implementation status is tracked at
   https://trac.ietf.org/trac/dnsop/wiki/draft-ietf-dnsop-nsec-ttl
   (https://trac.ietf.org/trac/dnsop/wiki/draft-ietf-dnsop-nsec-ttl)

Appendix B.  Document history

   [RFC editor: please remove this section before publication.]

   From draft-vandijk-dnsop-nsec-ttl-00 to draft-ietf-dnsop-nsec-ttl-00:

   *  document was adopted

   *  various minor editorial changes

   *  now also updates 4035

   *  use .example instead of .com for the example

   *  more words on 8198

   *  a note on wildcards

   From draft-ietf-dnsop-nsec-ttl-00 to draft-ietf-dnsop-nsec-ttl-01:

   *  various wording improvements

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   *  added Implementation note from Knot, expanded the BIND one with
      the GitLab MR URL

   *  reduced requirement level from MUST to SHOULD, like the original
      texts

   From draft-ietf-dnsop-nsec-ttl-01 to draft-ietf-dnsop-nsec-ttl-02:

   *  updated the second bit of wrong text in 5155

   From draft-ietf-dnsop-nsec-ttl-02 to draft-ietf-dnsop-nsec-ttl-03:

   *  document now updates resolver behaviour in 8198

   *  lots of extra text to clarify what behaviour goes where (thanks
      Paul Hoffman)

   *  replace 'any' with 'each' (thanks Duane)

   *  upgraded requirement level to MUST, plus a note on incremental
      signers

   From draft-ietf-dnsop-nsec-ttl-03 to draft-ietf-dnsop-nsec-ttl-04:

   *  the 'incremental signer exception' is now part of all relevant
      document updates

   *  added an explanation for the upgraded requirement level

Acknowledgements

   This document was made possible with the help of the following
   people:

   *  Ralph Dolmans

   *  Warren Kumari

   *  Matthijs Mekking

   *  Vladimir Cunat

   *  Matt Nordhoff

   *  Josh Soref

   *  Tim Wicinski

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   The author would like to explicitly thank Paul Hoffman for extensive
   reviews, text contributions, and help in navigating WG comments.

Author's Address

   Peter van Dijk
   PowerDNS
   Den Haag
   Netherlands

   Email: peter.van.dijk@powerdns.com

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