Internet DRAFT - draft-schwartz-dnsop-dnssec-strict-mode
draft-schwartz-dnsop-dnssec-strict-mode
dnsop B. Schwartz
Internet-Draft Google LLC
Intended status: Standards Track 22 February 2021
Expires: 26 August 2021
DNSSEC Strict Mode
draft-schwartz-dnsop-dnssec-strict-mode-00
Abstract
Currently, the DNSSEC security of a zone is limited by the strength
of its weakest signature algorithm. DNSSEC Strict Mode makes zones
as secure as their strongest algorithm instead.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the mailing list
(dnsop@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/dnsop/.
Source for this draft and an issue tracker can be found at
https://github.com/bemasc/dnssec-strict-mode.
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
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This Internet-Draft will expire on 26 August 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. Conventions and Definitions . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. DNSSEC validation behavior . . . . . . . . . . . . . . . 2
2.2. Algorithm trust levels . . . . . . . . . . . . . . . . . 3
3. The DNSSEC Strict Mode flag . . . . . . . . . . . . . . . . . 4
4. Operational Considerations . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. 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.
2. Background
2.1. DNSSEC validation behavior
According to [RFC6840] Section 5.4, when validators (i.e. resolvers)
are checking DNSSEC signatures:
a resolver SHOULD accept any valid RRSIG as sufficient, and only
determine that an RRset is Bogus if all RRSIGs fail validation.
[RFC6840] Section 5.11 clarifies further:
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Validators SHOULD accept any single valid path. They SHOULD NOT
insist that all algorithms signaled in the DS RRset work, and they
MUST NOT insist that all algorithms signaled in the DNSKEY RRset
work. A validator MAY have a configuration option to perform a
signature completeness test to support troubleshooting.
Thus, validators are required to walk through the set of RRSIGs,
checking each one that they are able until they find one that matches
or run out.
Some implementations do offer an option to enforce signature
completeness, e.g. Unbound's "harden-algo-downgrade" option
[Unbound], but most validating resolvers appear to follow the
standards guidance on this point. Validators' tolerance for invalid
paths is important due to transient inconsistencies during certain
kinds of zone maintenance (e.g. Pre-Publish Key Rollover, [RFC6781]
Section 4.1.1.1).
2.2. Algorithm trust levels
From the viewpoint of any single party, each DNSSEC Algorithm (i.e.
signature algorithm) can be assigned some level of perceived strength
or confidence. The party might be a zone owner, considering which
algorithms to use, or a validator, consider which algorithms to
implement. Either way, the party can safely include algorithms in
which they have maximal confidence (i.e. viewed as secure), and
safely exclude algorithms in which they have no confidence (i.e.
viewed as worthless).
Under the current DNSSEC validation behavior, a zone is only as
secure as the weakest algorithm implemented by both the signer and
the validator. If there is at least one algorithm that all parties
agree offers maximum strength, this is not a problem. Otherwise, we
have a dilemma. Each party is faced with two options:
* Use/implement only their most preferred algorithms, at the cost of
achieving no security with counterparties who distrust those
algorithms.
* Use/implement a wide range of algorithms, at the cost of weaker
security for counterparties who also implement a wide range of
algorithms.
In practice, zone owners typically select a small number of
algorithms, and validators typically support a wide range. This
arrangement often works well, but can fail for a variety of reasons:
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* When a new, stronger algorithm is introduced but is not yet widely
implemented, zone owners must continue to sign with older, weaker
algorithms, typically for many years, until nearly all validators
are updated.
* National crypto standards are often highly trusted by some
parties, and viewed with suspicion by others.
* Quantum computing has the potential to further confuse the
landscape of signature algorithm confidence. Under the present
standards, parties might be required to trust a novel postquantum
algorithm of uncertain strength or remain vulnerable to quantum
attack.
This specification resolves these dilemmas by providing zones with
the security level of their strongest selected algorithm, instead of
the weakest.
3. The DNSSEC Strict Mode flag
The DNSSEC Strict Mode flag appears in bit $N of the DNSKEY flags
field. If this flag is set, all records in the zone MUST be signed
correctly under this key's specified Algorithm. A validator that
receives a Strict Mode DNSKEY with a supported Algorithm SHOULD
reject as Bogus any RRSet that lacks a valid RRSIG with this
Algorithm. If there are multiple Strict Mode keys for the zone,
validators SHOULD validate signatures under each of their Algorithms.
4. Operational Considerations
Once a zone is signed, enabling Strict Mode can be done using any
ordinary key rollover procedure ([RFC6781] Section 4.1), to a new
DNSKEY that contains the Strict Mode flag. When signing a zone for
the first time, or adding a new Algorithm, care must be taken to
fully sign the zone before enabling Strict Mode.
By making it safe to use a wider range of DNSSEC Algorithms, this
specification could encourage larger RRSIG RRSets, and hence larger
responses.
When a zone has multiple Strict Mode keys, validators will check them
all, likely increasing CPU usage.
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5. Security Considerations
This specification enables the safe use of signature algorithms with
intermediate or indeterminate security. It does not protect against
weak Digest Types in DS records (especially "second preimage"
attacks).
A zone that adds signatures under a less secure algorithm, relying on
a strong Strict Mode algorithm for security, will weaken security for
validators that have not implemented support for Strict Mode. Zone
owners should use caution when relying on Strict Mode until Strict
Mode is widely supported in validators.
6. IANA Considerations
IANA is instructed to add this allocation to the DNSKEY RR Flags
registry:
+========+=============+=================+
| Number | Description | Reference |
+========+=============+=================+
| $N | STRICT | (This document) |
+--------+-------------+-----------------+
Table 1
7. References
7.1. Normative References
[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/rfc/rfc2119>.
[RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
DOI 10.17487/RFC6840, February 2013,
<https://www.rfc-editor.org/rfc/rfc6840>.
[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/rfc/rfc8174>.
7.2. Informative References
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[RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
Operational Practices, Version 2", RFC 6781,
DOI 10.17487/RFC6781, December 2012,
<https://www.rfc-editor.org/rfc/rfc6781>.
[Unbound] "unbound.conf", n.d.,
<https://nlnetlabs.nl/documentation/unbound/
unbound.conf/>.
Acknowledgments
TODO acknowledge.
Author's Address
Benjamin M. Schwartz
Google LLC
Email: bemasc@google.com
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