NSEC(3) TTLs and NSEC Aggressive Use
draft-ietf-dnsop-nsec-ttl-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9077.
|
|
|---|---|---|---|
| Author | Peter van Dijk | ||
| Last updated | 2021-01-24 (Latest revision 2021-01-13) | ||
| Replaces | draft-vandijk-dnsop-nsec-ttl | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 9077 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-dnsop-nsec-ttl-01
dnsop P. van Dijk
Internet-Draft PowerDNS
Updates: 4034, 4035, 5155 (if approved) 24 January 2021
Intended status: Standards Track
Expires: 28 July 2021
NSEC(3) TTLs and NSEC Aggressive Use
draft-ietf-dnsop-nsec-ttl-01
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
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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 28 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
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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(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 . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
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 or NODATA) is
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| 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 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(3) records in those negative responses have a 86400 TTL. If a
resolver were to use those NSEC(3)s aggressively, they would be
considered valid for a day, instead of the intended 15 minutes.
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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 SHOULD have the same TTL value as 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].
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 SHOULD be the same TTL value as 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].
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:
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| The NSEC3 RR SHOULD have the same TTL value as 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].
3.4. No updates to RFC8198
Instead of updating three documents, it would have been preferable to
update 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/lesser
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.
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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>.
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]
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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) .
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
* 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
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Acknowledgements
Ralph Dolmans helpfully pointed out that fixing this in RFC8198 is
only possible for negative (NXDOMAIN/NODATA) 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 on wildcards
should be made explicit. Paul Hoffman, Matt Nordhoff, and Josh Soref
provided helpful corrections as native speakers.
Author's Address
Peter van Dijk
PowerDNS
Den Haag
Netherlands
Email: peter.van.dijk@powerdns.com
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