Internet DRAFT - draft-ietf-dnsop-nsec-ttl
draft-ietf-dnsop-nsec-ttl
dnsop P. van Dijk
Internet-Draft PowerDNS
Updates: 4034, 4035, 5155, 8198 (if approved) 20 May 2021
Intended status: Standards Track
Expires: 21 November 2021
NSEC and NSEC3 TTLs and NSEC Aggressive Use
draft-ietf-dnsop-nsec-ttl-05
Abstract
Due to a combination of unfortunate wording in earlier documents,
aggressive use of NSEC and NSEC3 records may deny the existence of
names far beyond the intended lifetime of a denial. This document
changes the definition of the NSEC and NSEC3 TTL (Time To Live) 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
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 21 November 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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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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 . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Implementation Status . . . . . . . . . . . . . . . 8
Appendix B. Document history . . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
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)
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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 (Start Of Authority) 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.
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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.
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:
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| 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]. Because some signers incrementally update the NSEC
| chain, a transient inconsistency between the observed and expected
| TTL MAY exist.
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]. Because some signers incrementally update the NSEC
| chain, a transient inconsistency between the observed and expected
| TTL MAY exist.
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]. Because some signers incrementally update the NSEC3
| chain, a transient inconsistency between the observed and expected
| TTL MAY exist.
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:
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| 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. Because some signers incrementally update the NSEC3
| chain, a transient inconsistency between the observed and expected
| TTL MAY exist.
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 and 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.
Note that some signers might use the SOA TTL or MINIMUM as a default
for other values, such as the TTL for DNSKEY records. Operators
should consult documentation before changing values.
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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 or 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 delay 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.
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
[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>.
[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>.
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[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>.
Appendix A. Implementation Status
[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
(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
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* 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
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
From draft-ietf-dnsop-nsec-ttl-04 to draft-ietf-dnsop-nsec-ttl-05:
* various minor rewordings (from IESG review, and things I spotted
while handling IESG review comments)
* added a note on the secondary impact of changing the SOA TTL and/
or MINIMUM (IESG review)
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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
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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