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Versions: 00                                                            
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
   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 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
   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.

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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