wdiff draft-ietf-dnsop-nsec-ttl-02.txt draft-ietf-dnsop-nsec-ttl-03.txt

dnsop P. van Dijk
Internet-Draft PowerDNS
Updates: 4034, 4035, 5155 5155, 8198 (if approved) 29 January 9 February 2021
Intended status: Standards Track
Expires: 2 13 August 2021

NSEC(3)

NSEC and NSEC3 TTLs and NSEC Aggressive Use
draft-ietf-dnsop-nsec-ttl-02
draft-ietf-dnsop-nsec-ttl-03

Abstract

Due to a combination of unfortunate wording in earlier documents,
aggressive use of NSEC(3) NSEC and NSEC3 records may deny names far beyond
the intended lifetime of a denial. This document changes the
definition of the NSEC(3) NSEC and NSEC3 TTL to correct that situation. This
document updates RFC 4034, RFC 4035, RFC 5155, and RFC 5155. 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 2 13 August 2021.

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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(3) NSEC and NSEC3 TTL changes . . . . . . . . . . . . . . . . . . . . . 4
3.1. Updates to RFC4034 . . . . . . . . . . . . . . . . . . . 4
3.2. Updates to RFC4035 . . . . . . . . . . . . . . . . . . . 4
3.3. Updates to RFC5155 . . . . . . . . . . . . . . . . . . . 4 5
3.4. No updates Updates to RFC8198 . . . . . . . . . . . . . . . . . . . 5
3.5. A note on incremental signers . . . . . . . . . . . . . . 6
4. Zone Operator Considerations . . . . . . . . . . . . . . . . 5 6
4.1. A Note On Wildcards . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6
8. Informative References . . . . . . . . . . . . . . . . . . . 7
Appendix A. Implementation Status . . . . . . . . . . . . . . . 7
Appendix B. Document history . . . . . . . . . . . . . . . . . . 7 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 9

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 of the SOA record that must be used returned in
negative answers (NXDOMAIN or NODATA) is 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 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 a the SOA record), the negative TTL for authoritative server
sends that
zone is lower than value as the TTL of the returned SOA MINIMUM value. 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(3) NSEC and NSEC3 RRs should, per RFC4034, according to RFC4034 and RFC5155,
already be at the value of the MINIMUM
TTL, which means this field in the SOA. Thus, the
advice from RFC8198 would never not actually change the TTL used for the NSEC(3) RRs.
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(3) NSEC
or NSEC3 records in those negative responses have a 86400 TTL. If a
resolver were to use those NSEC(3)s 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(3) NSEC and NSEC3 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 TTL of the NSEC RR SHOULD have the same TTL value as 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].

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:

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 SHOULD have the same TTL value as 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].

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:

3.4. No updates Updates to RFC8198

Instead

[RFC8198] section 5.4 (Consideration on TTL) is completely replaced
by the following text:

| The TTL value of updating three documents, it would have been preferable 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
update one. [RFC8198] says: this same value.
| With DNSSEC and
| A resolver that supports aggressive use of DNSSEC-validated cache, NSEC and NSEC3 MAY
| limit the TTL
| of the NSEC/NSEC3 record NSEC and NSEC3 records to the lesser of the
| SOA.MINIMUM field are and the
| authoritative statement TTL of how quickly the SOA in a name can start working response, if
| within present. It MAY also use a zone.

Here, the SOA.MINIMUM field cannot be changed previously cached SOA for a zone to "the minimum/lesser
| find these values.

(The third paragraph of the SOA.MINIMUM field original is removed, and the SOA TTL" because fourth
paragraph is updated to allow resolvers to also take the resolver may
not have lesser of
the SOA RRset TTL and SOA MINIMUM.)

3.5. A note on incremental signers

Some DNSSEC signer implementations might not (re-)sign whole zones in cache. Because
one go, instead spreading the work of that, this document
cannot get away with updating just [RFC8198]. However, if
authoritative servers follow the updates from this document, this
should inception/expiration
times over some period. Such implementations would not make a difference, as be able to
update all NSEC or NSEC3 records in the TTL of zone instantly either. To
aid these implementations, we additionally specify the NSEC/NSEC3 record is
already set following:

| If an implementation cannot update all NSEC or NSEC3 TTLs after a
| SOA change immediately, it MUST still attempt to do so as soon as
| possible during the minimum value. signing process.

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(3) NSEC and NSEC3 proof that
shows that the wildcard expansion is legal. Thus, changing the TTL
of
NSEC(3) 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(3) NSEC or NSEC3 responses to
be cached, for aggressive use purposes. This document reduces the
impact of that attack in cases where the NSEC(3) 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

[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>.

[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>.

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>.

[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>.

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

* 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

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

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

* updated the
current behaviour second bit of RFC8198, wrong text in 5155

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

* document now updates resolver behaviour in context 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

Acknowledgements

This document was made possible with the NSEC TTL defined in
RFC4034, is not help of the intended behaviour. following
people:

* Ralph Dolmans

* Warren Kumari

* 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 Nordhoff

* Josh Soref
provided helpful corrections as native speakers.

* 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