dnsop                                                        P. van Dijk
Internet-Draft                                                  PowerDNS
Updates: 4034, 4035, 5155 (if approved)                  13 January 2021
Intended status: Standards Track
Expires: 17 July 2021


                  NSEC(3) TTLs and NSEC Aggressive Use
                      draft-ietf-dnsop-nsec-ttl-00

Abstract

   Due to a combination of unfortunate wording in earlier documents,
   aggressive use of NSEC(3) records may deny names far beyond the
   intended lifetime of a denial.  This document changes the definition
   of the NSEC(3) TTL to correct that situation.  This document updates
   RFC 4034, RFC 4035, and RFC 5155.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 17 July 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
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.



van Dijk                  Expires 17 July 2021                  [Page 1]


Internet-Draft                  nsec-ttl                    January 2021


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  NSEC(3) TTL changes . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Updates to RFC4034  . . . . . . . . . . . . . . . . . . .   4
     3.2.  Updates to RFC4035  . . . . . . . . . . . . . . . . . . .   4
     3.3.  Updates to RFC5155  . . . . . . . . . . . . . . . . . . .   4
     3.4.  No updates to RFC8198 . . . . . . . . . . . . . . . . . .   5
   4.  Zone Operator Considerations  . . . . . . . . . . . . . . . .   5
     4.1.  A Note On Wildcards . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Implementation Status  . . . . . . . . . . . . . . .   6
   Appendix B.  Document history . . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

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 that the SOA TTL to be used in negative answers
   (NXDOMAIN, NoData NOERROR) is





van Dijk                  Expires 17 July 2021                  [Page 2]


Internet-Draft                  nsec-ttl                    January 2021


   |  the minimum of the MINIMUM field of the SOA record and the TTL of
   |  the SOA itself

   Thus, if the TTL of the SOA in the zone is lower than the SOA MINIMUM
   value (the last number in a SOA record), the negative TTL for that
   zone is lower than the SOA MINIMUM value.

   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(3) RRs should, per RFC4034, already be at the MINIMUM
   TTL, which means this advice would never actually change the TTL used
   for the NSEC(3) RRs.

   As a theoretical exercise, consider a TLD ".example" with a SOA 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(3) records in those negative responses have a 86400 TTL.  If a
   resolver were to use those NSEC3s aggressively, they would be
   considered valid for a day, instead of the intended 15 minutes.







van Dijk                  Expires 17 July 2021                  [Page 3]


Internet-Draft                  nsec-ttl                    January 2021


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(3) TTL changes

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 NSEC RR MUST have the same TTL value as the minimum 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].

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 value for any NSEC RR MUST be the same TTL value as the
   |  minimum 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].

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:





van Dijk                  Expires 17 July 2021                  [Page 4]


Internet-Draft                  nsec-ttl                    January 2021


   |  The NSEC3 RR MUST have the same TTL value as the minimum 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].

3.4.  No updates to RFC8198

   Instead of updating four documents, it would have been preferable to
   update it in one.  [RFC8198] says:

   |  With DNSSEC and aggressive use of DNSSEC-validated cache, the TTL
   |  of the NSEC/NSEC3 record and the SOA.MINIMUM field are the
   |  authoritative statement of how quickly a name can start working
   |  within a zone.

   Here, the SOA.MINIMUM field cannot be changed to "the minimum of the
   SOA.MINIMUM field and the SOA TTL" because the resolver may not have
   the SOA RRset in cache.  Because of that, this document cannot get
   away with updating just [RFC8198].  However, if authoritative servers
   follow the updates from this document, this should not make a
   difference, as the TTL of the NSEC/NSEC3 record is already set to the
   minimum value.

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 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(3) proof that shows
   that the wildcard expansion is legal.  Thus, changing the TTL of
   NSEC(3) 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(3) responses to be
   cached, for aggressive use purposes.  This document reduces the
   impact of that attack in cases where the NSEC(3) TTL is higher than
   the zone operator intended.





van Dijk                  Expires 17 July 2021                  [Page 5]


Internet-Draft                  nsec-ttl                    January 2021


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

   [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>.

   [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>.

8.  Informative References

   [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>.

   [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]






van Dijk                  Expires 17 July 2021                  [Page 6]


Internet-Draft                  nsec-ttl                    January 2021


   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) .

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

Acknowledgements

   Ralph Dolmans helpfully pointed out that fixing this in RFC8198 is
   only possible for negative (NXDOMAIN/NoData NOERROR) responses, and
   not for wildcard responses.  Warren Kumari gracefully acknowledged
   that the current behaviour of RFC8198, in context of the NSEC TTL
   defined in RFC4034, is not the intended behaviour.  Matthijs Mekking
   provided additional text explaining why this document cannot simply
   update RFC8198.  Vladimir Cunat pointed out that the effect wildcards
   should be made explicit.

Author's Address

   Peter van Dijk
   PowerDNS
   Den Haag
   Netherlands

   Email: peter.van.dijk@powerdns.com



van Dijk                  Expires 17 July 2021                  [Page 7]