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TLD Zone Pipeline: Requirements And Design Principles

Document Type Active Internet-Draft (individual)
Authors Johan Stenstam , Jakob Schlyter
Last updated 2024-05-02
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DNSOP Working Group                                          J. Stenstam
Internet-Draft                           The Swedish Internet Foundation
Intended status: Standards Track                             J. Schlyter
Expires: 3 November 2024                                        Kirei AB
                                                              2 May 2024

         TLD Zone Pipeline: Requirements And Design Principles


   Today most TLD registries publish DNSSEC signed zones.  The sequence
   of steps from generating the unsigned zone, via DNSSEC signing and
   various types of verification is referred to as the "zone pipeline".

   The robustness and correctness of the zone pipeline is of crucial
   importance and the zone pipeline is one of the most critical parts of
   the operations of a TLD registry.

   The goal of this document is to describe the requirements that the
   .SE Registry choose in preparation for the implementation of a new
   zone pipeline.  The document also describes some of the design
   consequences that follow from the requirements.  Hence this document
   is intended to work as a guide for understanding the actual
   implementation, which is planned to be released as open source.

   TO BE REMOVED: This document is being collaborated on in Github at:
   (  The
   most recent working version of the document, open issues, etc. should
   all be available there.  The authors (gratefully) accept pull

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

   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."

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   This Internet-Draft will expire on 3 November 2024.

Copyright Notice

   Copyright (c) 2024 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 (
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   3
   2.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Basic Design Principles . . . . . . . . . . . . . . . . . . .   4
   5.  Interface to the Registry . . . . . . . . . . . . . . . . . .   5
   6.  Local Updates . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Ingress Verification  . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Requirements on ingress verification  . . . . . . . . . .   6
     7.2.  Examples of ingress verification checks . . . . . . . . .   6
   8.  Signing . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Key management  . . . . . . . . . . . . . . . . . . . . .   7
     8.2.  Zone signing  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Egress Verification . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Requirements on egress verification . . . . . . . . . . .   8
     9.2.  Examples of egress verification checks  . . . . . . . . .   8
   10. Distribution  . . . . . . . . . . . . . . . . . . . . . . . .   8
   11. Resulting Design Consequences . . . . . . . . . . . . . . . .   9
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   15. Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Change History (to be removed before publication)  .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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1.  Introduction

   Today most TLD registries publish DNSSEC signed zones.  This
   typically leads to a zone production "pipeline" that consists of
   several steps, including generation of the unsigned zone, signing of
   the zone and various types of verifications that the zone is correct
   before publication on the Internet.

   In some cases, including the .SE Registry, the zone pipeline was not
   the result of a clear requirements process, nor was it the result of
   a concious design and implementation.  Rather, it was the result of
   combining various tools in a mostly ad-hoc way that achieved the goal
   of moving the zone via signing and verifications towards publication.

   When a critical part of the operation of a TLD registry is the result
   of an ad-hoc process there are likely to be hidden risks.  That was
   the case for the .SE registry, which had a serious incident in
   February 2022.  Because of that incident, .SE decided to re-evaluate
   the requirements on the zone pipeline and then create a more robust
   implementation from scratch.

   This document aims to describe the requirements for zone production
   (also known as "zone pipeline") that the .SE Registry choose in
   preparation for the implementation of the new zone pipeline.  It is
   developed for the needs of the .SE and .NU TLD Registries, but the
   conclusions are intended to be generally applicable.

1.1.  Requirements Notation

   The key words "*MUST*", "*MUST NOT*", "*REQUIRED*", "*SHALL*",
   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.  Purpose

   A TLD Registry has a total responsibility towards society and the
   Internet community to ensure, at any given time, public access to
   correct versions of the DNS zones under their management.  In order
   to meet this commitment, three components are essentially required:

   *  Generation of unsigned zones from the Registry System.

   *  Quality control and signing of the zones.

   *  Publication of resulting signed zones.

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   The first step is handled by the Registry System.  The third step is
   handled by a combination of external providers of authoritative DNS
   service and in-house DNS service.  Both of these steps are out-of-
   scope for this document

   The sole purpose of this document is to provide a correct set of
   requirements for the second step, between zone generation in the
   Registry System and zone publication on the public Internet.

3.  Terminology

   Upstream: : Further up in the zone pipeline, i.e. in the direction of
   the Registry System.

   Downstream: : Further down the zone pipeline, i.e. towards the public

4.  Basic Design Principles

   A number of fundamental principles are defined for the design of the
   system.  The intention of these principles is to minimize the risk
   that the zone data (as generated by the Registry System) can somehow
   change at any stage through the zone pipeline.

   *  Critical path for zone data must be via proven and well-reviewed
      standard software.  This critical path is called the "zone

      Rationale: Using well-established software used by others in the
      industry reduces development needs for the Registry.  By not being
      critically dependent on self-developed software, the dependence on
      individuals is reduced.

   *  Standardized protocols shall be used as far as possible.

      Rationale: Individual components must be replaceable as easily as

   *  Consequences of inaccuracies in custom software must be limited as
      far as possible and must never affect published zone data.

      Rationale: Obvious opportunities for risk minimization of a
      critical system within the business.

   *  Verification, signing and publication of the zone must be able to
      take place independently and without dependence on infrastructure
      outside the operating facility.

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      Rationale: The ability to always maintain and publish an updated
      zone is the most important responsibility of the Registry.  To
      ensure the ability to always maintain this ability the zone
      production must be self-contained.

5.  Interface to the Registry

   Interface to the Registry System must be done according to
   standardized protocols.  This requirement has the following

   *  Zone data MUST be retrieved from the Registry System using AXFR
      and IXFR [RFC5936].

   *  The request for publication of new zone data MUST be notified with
      DNS NOTIFY [RFC1995].

   *  Zone data published by the Registry System MUST contain a checksum
      in the form of ZONEMD [RFC8976].

6.  Local Updates

   During normal operation, no changes to the zone data retrieved from
   the Registry may take place.  However, there may be situations where
   the Registry is not reachable (nor is it expected to be reachable
   within a reasonable time) and where local updates of zone data must
   be able to be carried out.  This can for example, be redirection of
   socially important infrastructure.

   In a crisis situation (emergency operation), zone updates must be
   able to take place locally.  Updates that take place in this way are
   introduced into regular systems as soon as possible.  Return to
   normal operation may only take place after all changes made during
   emergency operation have been introduced into the regular system.

   Local updates must be applied using a strict and traceable method.
   It must be clear at all times whether local updates have been
   applied, what these are and who requested them.

   In cases where local updates have taken place, ZONEMD MUST be

   N.B.  Local updates are an extraordinary measure and must not be used
   except in emergency situations.  Procedures for who may request these
   are decided by the Internet Foundation's crisis management.

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7.  Ingress Verification

   Before signing, a number of checks must be performed on the zone
   contents.  The reason why checking must take place before signing is
   to ensure that the zone being signed is always correct and can thus
   continue to be re-signed in the event of problems upstream.  The
   exact checks to be carried out are set out in a separate
   specification and are subject to local policy.

7.1.  Requirements on ingress verification

   *  The ingress verification MUST prevent an updated incorrect zone
      from being signed.  An already approved previous version of the
      zone MUST continue to be signed until a new zone is approved.

   *  New zone controls MUST be able to be added without the component
      for ingress verification of zone data needing to be redesigned

   *  The interface from the zone pipeline to the ingress verification
      function MUST be DNS AXFR.  This means that all controls must
      logically sit to the side and not be part of the critical path.
      I.e. the verification code is not part of the zone pipeline.

   *  All zone controls, self-developed or imported, must have local
      ownership within the organization.

7.2.  Examples of ingress verification checks

   *  Check that the zone data is complete.

   *  Check that the ZONEMD, which has been generated by the registry
      system, is correct

   *  Check that delegation information for the zone itself is correct.

   *  Check that the delta (i.e. the difference) between the current
      zone and the previous version is within approved limits.

   *  Check that certain crucial records are present and correct (the
      zone's SOA record is one example).

8.  Signing

   The task of the signing step is to keep an approved and received zone
   signed for an arbitrary length of time until a new zone is received
   from upstream (i.e. from Registry via Ingress Verification).

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8.1.  Key management

   The following requirements apply to the management of cryptographic
   keys for signing zone data:

   *  The key material must be stored and used in an HSM that meets the
      security requirements set by the CISO.

   *  The interface for accessing the key material SHALL be PKCS#11.

   *  All keys must, to facilitate replication between different signing
      entities, be generated in advance.In order to facilitate
      replication between different signing entities, all keys must be
      generated in advance.

   *  Exchange of KSK can be initiated automatically or manually, and is
      automatically terminated when the DS record in the parent zone is
      updated (after according to an appropriate safety margin).

   *  Changing the KSK may only be completed if the DS record in the
      parent zone is updated.

   *  Replacement of ZSK MUST be done automatically.

8.2.  Zone signing

   The following requirements apply to signing zone data:

   *  Signing MUST be done using key material via PKCS#11.

   *  The signing function MUST support algorithm rollover, e.g,. from
      RSA/SHA-256 to ECC/P-256/SHA-256.

   *  Signing MUST be done with either NSEC or NSEC3 semantics.

   *  If NSEC3 salt is used, the salt MUST be periodically changed

   *  Change of DNSSEC signing semantics from NSEC to NSEC3 and vice
      versa MUST be possible automatically.

   *  Previous signatures that are valid within a configurable time
      window SHALL be reused when re-signing, in order to reduce the
      rate of change on the zone.

   *  The signing function SHALL recreate the ZONEMD RR per [RFC8976]
      after the zone has been signed.

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9.  Egress Verification

   After signing, several checks must be performed on the zone contents.
   Apart from the obvious validation of generated DNSSEC signatures it
   is also important to ensure that the signing step only added DNSSEC-
   information without in any way modifying the unsigned data.

9.1.  Requirements on egress verification

   *  All generated DNSSEC signatures (RRSIG records) MUST be validated.

   *  The NSEC (or NSEC3) chain MUST be verified to be complete.

   *  The non-DNSSEC content of the signed zone MUST be provably
      identical to the corresponding unsigned zone that entered the
      signing step.

9.2.  Examples of egress verification checks

   *  Verify the zone with other software than was used for signing.

   *  Remove all data added by the signing step and check the ZONEMD
      value over that data with the ZONEMD from before the signing.

10.  Distribution

   The following requirements apply to distribution of the signed zone:

   *  The signed zone MUST be retrieved from signing and egress
      verification to the distribution points with AXFR (not IXFR).

      It must be AXFR, as otherwise it is not possible to switch from
      one upstream to another in the distribution points (the IXFR from
      one upstream will not apply cleanly to a zone from another

   *  The signed zone SHALL be distributed to the designated
      authoritative name server services using AXFR/IXFR.

   *  To reduce convergence time towards the public Internet, the signed
      zone MUST be distributed with IXFR as far as possible.

   *  At least two complete zone publishing chains MUST be operational
      and always active.

      This is a policy decision, based on the strong intent to avoid all
      single points of failure, including in the signing operation.

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   *  Choice of zone publishing chain is an active configuration choice
      in each distribution point and MUST always be the same for all
      distribution points.

   *  The signed zone from the selected zone publishing chain MUST be
      retrieved by all distribution points in all operating facilities.

11.  Resulting Design Consequences

   *  The requirement on being able to prove that no unsigned data has
      been modified during signing is most efficiently fullfilled by
      computing the ZONEMD checksum on the unsigned data after signing
      (i.e. the signed zone modulo the DNSSEC related records DNSKEY,
      RRSIG.  NSEC, NSEC3, NSEC3PARAM, apex CDS and CDNSKEY) and
      comparing that to the ZONEMD checksum for the corresponding
      unsigned zone.

   *  The ZONEMD checksums need to be stored outside the zone pipeline,
      indexed by the unsigned zone that each checksum corresponds to.

   *  The signed zone may (and will) change the SOA Serial independently
      of the unsigned zone.  For this reason the ZONEMD checksums can
      not be stored using the SOA Serial as the index.  Therefore a
      separate, unique, identifier is attached to each new version of
      the zone as a TXT record.  A UUID is used as the identifier.

   *  Each unsigned zone MUST have a ZONEMD and a UUID index to store it
      under.  Therefore changes to the unsigned zone via the local
      update facility MUST update the UUID in addition to any other
      change that is executed.

   *  The Registry runs multiple, parallel zone pipelines for the same
      zone with the requirement to be able to switch which pipeline is
      "active" at any time.  As the local update facility is responsible
      for updating the UUID if a local change is needed, the same UUID
      will identify the exact same zone in all pipelines.

   *  The requirement that the zone pipeline only consists of proven and
      widely used software forces all local and custom software
      (including various tests and verification modules) to be located
      outside the zone pipeline.  This cause a need for a component
      inside the zone pipeline with the ability to call an external
      "verifier" for verifications.  At present only one such component
      is known (the authoritative nameserver NSD with its "verify:"
      attribute), but we hope that there will be more alternatives in
      the future.

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12.  Security Considerations

   The correct generation and DNSSEC signing of a TLD zone is a matter
   of significant importance.  As such requirements and methods for
   verification of correctness is an important matter that has
   previously not been much publically discussed.

   As a requirements specification this document aims to make this topic
   more public and visible with the intent of improving the correctness
   of the requirements via public review.

13.  IANA Considerations


14.  Acknowledgements

   We appreciate suggestions and helpful comments from a number of
   people, including but not limited to:

   *  Stefan Ubbingk, SIDN

   *  Calvin Browne, DNS Pty, Ltd

   *  David Blacka, Verisign

   *  Pawel Kowalik, DENIC

15.  Normative References

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              DOI 10.17487/RFC1995, August 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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   [RFC8976]  Wessels, D., Barber, P., Weinberg, M., Kumari, W., and W.
              Hardaker, "Message Digest for DNS Zones", RFC 8976,
              DOI 10.17487/RFC8976, February 2021,

Appendix A.  Change History (to be removed before publication)

   *  draft-johani-tld-zone-pipeline-00

      Initial public draft.

   *  draft-johani-tld-zone-pipeline-01

      Security and IANA Considerations sections added.  Minor typos

   *  draft-johani-tld-zone-pipeline-02

      Several typos fixed.  Further clarifications and explanations.

Authors' Addresses

   Johan Stenstam
   The Swedish Internet Foundation

   Jakob Schlyter
   Kirei AB

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