



Internet Engineering Task Force (IETF)                          S. Huque
Request for Comments: 8901                                       P. Aras
Category: Informational                                       Salesforce
ISSN: 2070-1721                                             J. Dickinson
                                                              Sinodun IT
                                                               J. Vcelak
                                                                     NS1
                                                               D. Blacka
                                                                Verisign
                                                          September 2020


                       Multi-Signer DNSSEC Models

Abstract

   Many enterprises today employ the service of multiple DNS providers
   to distribute their authoritative DNS service.  Deploying DNSSEC in
   such an environment may present some challenges, depending on the
   configuration and feature set in use.  In particular, when each DNS
   provider independently signs zone data with their own keys,
   additional key-management mechanisms are necessary.  This document
   presents deployment models that accommodate this scenario and
   describes these key-management requirements.  These models do not
   require any changes to the behavior of validating resolvers, nor do
   they impose the new key-management requirements on authoritative
   servers not involved in multi-signer configurations.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8901.

Copyright Notice

   Copyright (c) 2020 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
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   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.  Introduction and Motivation
   2.  Deployment Models
     2.1.  Multiple-Signer Models
       2.1.1.  Model 1: Common KSK Set, Unique ZSK Set per Provider
       2.1.2.  Model 2: Unique KSK Set and ZSK Set per Provider
   3.  Validating Resolver Behavior
   4.  Signing-Algorithm Considerations
   5.  Authenticated-Denial Considerations
     5.1.  Single Method
     5.2.  Mixing Methods
   6.  Key Rollover Considerations
     6.1.  Model 1: Common KSK, Unique ZSK per Provider
     6.2.  Model 2: Unique KSK and ZSK per Provider
   7.  Using Combined Signing Keys
   8.  Use of CDS and CDNSKEY
   9.  Key-Management-Mechanism Requirements
   10. DNS Response-Size Considerations
   11. IANA Considerations
   12. Security Considerations
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction and Motivation

   Many enterprises today employ the service of multiple Domain Name
   System (DNS) [RFC1034] [RFC1035] providers to distribute their
   authoritative DNS service.  This is primarily done for redundancy and
   availability, and it allows the DNS service to survive a complete,
   catastrophic failure of any single provider.  Additionally,
   enterprises or providers occasionally have requirements that preclude
   standard zone-transfer techniques [RFC1995][RFC5936]: either
   nonstandardized DNS features are in use that are incompatible with
   zone transfer, or operationally a provider must be able to (re-)sign
   DNS records using their own keys.  This document outlines some
   possible models of DNSSEC [RFC4033] [RFC4034] [RFC4035] deployment in
   such an environment.

   This document assumes a reasonable level of familiarity with DNS
   operations and protocol terms.  Much of the terminology is explained
   in further detail in "DNS Terminology" [RFC8499].

2.  Deployment Models

   If a zone owner can use standard zone-transfer techniques, then the
   presence of multiple providers does not require modifications to the
   normal deployment models.  In these deployments, there is a single
   signing entity (which may be the zone owner, one of the providers, or
   a separate entity), while the providers act as secondary
   authoritative servers for the zone.

   Occasionally, however, standard zone-transfer techniques cannot be
   used.  This could be due to the use of nonstandard DNS features or
   the operational requirements of a given provider (e.g., a provider
   that only supports "online signing").  In these scenarios, the
   multiple providers each act like primary servers, independently
   signing data received from the zone owner and serving it to DNS
   queriers.  This configuration presents some novel challenges and
   requirements.

2.1.  Multiple-Signer Models

   In this category of models, multiple providers each independently
   sign and serve the same zone.  The zone owner typically uses
   provider-specific APIs to update zone content identically at each of
   the providers and relies on the provider to perform signing of the
   data.  A key requirement here is to manage the contents of the DNSKEY
   and Delegation Signer (DS) RRsets in such a way that validating
   resolvers always have a viable path to authenticate the DNSSEC
   signature chain, no matter which provider is queried.  This
   requirement is achieved by having each provider import the public
   Zone Signing Keys (ZSKs) of all other providers into their DNSKEY
   RRsets.

   These models can support DNSSEC even for the nonstandard features
   mentioned previously, if the DNS providers have the capability of
   signing the response data generated by those features.  Since these
   responses are often generated dynamically at query time, one method
   is for the provider to perform online signing (also known as on-the-
   fly signing).  However, another possible approach is to precompute
   all the possible response sets and associated signatures and then
   algorithmically determine at query time which response set and
   signature need to be returned.

   In the models presented, the function of coordinating the DNSKEY or
   DS RRset does not involve the providers communicating directly with
   each other.  Feedback from several commercial managed-DNS providers
   indicates that they may be unlikely to directly communicate, since
   they typically have a contractual relationship only with the zone
   owner.  However, if the parties involved are agreeable, it may be
   possible to devise a protocol mechanism by which the providers
   directly communicate to share keys.  Details of such a protocol are
   deferred to a future specification document, should there be
   interest.

   In the descriptions below, the Key Signing Key (KSK) and Zone Signing
   Key (ZSK) correspond to the definitions in [RFC8499], with the caveat
   that the KSK not only signs the zone apex DNSKEY RRset but also
   serves as the Secure Entry Point (SEP) into the zone.

2.1.1.  Model 1: Common KSK Set, Unique ZSK Set per Provider

   *  The zone owner holds the KSK set, manages the DS record set, and
      is responsible for signing the DNSKEY RRset and distributing it to
      the providers.

   *  Each provider has their own ZSK set, which is used to sign data in
      the zone.

   *  The providers have an API that the zone owner uses to query the
      ZSK public keys and insert a combined DNSKEY RRset that includes
      the ZSK sets of each provider and the KSK set, signed by the KSK.

   *  Note that even if the contents of the DNSKEY RRset do not change,
      the zone owner needs to periodically re-sign it as signature
      expiration approaches.  The provider API is also used to thus
      periodically redistribute the refreshed DNSKEY RRset.

   *  Key rollovers need coordinated participation of the zone owner to
      update the DNSKEY RRset (for KSK or ZSK) and the DS RRset (for
      KSK).

   *  (One specific variant of this model that may be interesting is a
      configuration in which there is only a single provider.  A
      possible use case for this is where the zone owner wants to
      outsource the signing and operation of their DNS zone to a single
      third-party provider but still control the KSK, so that they can
      authorize and/or revoke the use of specific zone signing keys.)

2.1.2.  Model 2: Unique KSK Set and ZSK Set per Provider

   *  Each provider has their own KSK and ZSK sets.

   *  Each provider offers an API that the zone owner uses to import the
      ZSK sets of the other providers into their DNSKEY RRset.

   *  The DNSKEY RRset is signed independently by each provider using
      their own KSK.

   *  The zone owner manages the DS RRset located in the parent zone.
      This is comprised of DS records corresponding to the KSKs of each
      provider.

   *  Key rollovers need coordinated participation of the zone owner to
      update the DS RRset (for KSK) and the DNSKEY RRset (for ZSK).

3.  Validating Resolver Behavior

   The central requirement for both of the multiple-signer models
   (Section 2.1) is to ensure that the ZSKs from all providers are
   present in each provider's apex DNSKEY RRset and vouched for by
   either the single KSK (in Model 1) or each provider's KSK (in Model
   2.)  If this is not done, the following situation can arise (assuming
   two providers, A and B):

   *  The validating resolver follows a referral (i.e., secure
      delegation) to the zone in question.

   *  It retrieves the zone's DNSKEY RRset from one of Provider A's
      nameservers, authenticates it against the parent DS RRset, and
      caches it.

   *  At some point in time, the resolver attempts to resolve a name in
      the zone while the DNSKEY RRset received from Provider A is still
      viable in its cache.

   *  It queries one of Provider B's nameservers to resolve the name and
      obtains a response that is signed by Provider B's ZSK, which it
      cannot authenticate because this ZSK is not present in its cached
      DNSKEY RRset for the zone that it received from Provider A.

   *  The resolver will not accept this response.  It may still be able
      to ultimately authenticate the name by querying other nameservers
      for the zone until it elicits a response from one of Provider A's
      nameservers.  But it has incurred the penalty of additional round
      trips with other nameservers, with the corresponding latency and
      processing costs.  The exact number of additional round trips
      depends on details of the resolver's nameserver-selection
      algorithm and the number of nameservers configured at Provider B.

   *  It may also be the case that a resolver is unable to provide an
      authenticated response, because it gave up after a certain number
      of retries or a certain amount of delay; or it is possible that
      downstream clients of the resolver that originated the query timed
      out waiting for a response.

   Hence, it is important that the DNSKEY RRset at each provider is
   maintained with the active ZSKs of all participating providers.  This
   ensures that resolvers can validate a response no matter which
   provider's nameservers it came from.

   Details of how the DNSKEY RRset itself is validated differ.  In Model
   1 (Section 2.1.1), one unique KSK managed by the zone owner signs an
   identical DNSKEY RRset deployed at each provider, and the signed DS
   record in the parent zone refers to this KSK.  In Model 2
   (Section 2.1.2), each provider has a distinct KSK and signs the
   DNSKEY RRset with it.  The zone owner deploys a DS RRset at the
   parent zone that contains multiple DS records, each referring to a
   distinct provider's KSK.  Hence, it does not matter which provider's
   nameservers the resolver obtains the DNSKEY RRset from; the signed DS
   record in each model can authenticate the associated KSK.

4.  Signing-Algorithm Considerations

   DNS providers participating in multi-signer models need to use a
   common DNSSEC signing algorithm (or a common set of algorithms if
   several are in use).  This is because the current specifications
   require that if there are multiple algorithms in the DNSKEY RRset,
   then RRsets in the zone need to be signed with at least one DNSKEY of
   each algorithm, as described in [RFC4035], Section 2.2.  If providers
   employ distinct signing algorithms, then this requirement cannot be
   satisfied.

5.  Authenticated-Denial Considerations

   Authenticated denial of existence enables a resolver to validate that
   a record does not exist.  For this purpose, an authoritative server
   presents, in a response to the resolver, signed NSEC (Section 3.1.3
   of [RFC4035]) or NSEC3 (Section 7.2 of [RFC5155]) records that
   provide cryptographic proof of this nonexistence.  The NSEC3 method
   enhances NSEC by providing opt-out for signing insecure delegations
   and also adds limited protection against zone-enumeration attacks.

   An authoritative server response carrying records for authenticated
   denial is always self-contained, and the receiving resolver doesn't
   need to send additional queries to complete the proof of denial.  For
   this reason, no rollover is needed when switching between NSEC and
   NSEC3 for a signed zone.

   Since authenticated-denial responses are self-contained, NSEC and
   NSEC3 can be used by different providers to serve the same zone.
   Doing so, however, defeats the protection against zone enumeration
   provided by NSEC3 (because an adversary can trivially enumerate the
   zone by just querying the providers that employ NSEC).  A better
   configuration involves multiple providers using different
   authenticated denial-of-existence mechanisms that all provide zone-
   enumeration defense, such as precomputed NSEC3, NSEC3 white lies
   [RFC7129], NSEC black lies [BLACKLIES], etc.  Note, however, that
   having multiple providers offering different authenticated-denial
   mechanisms may impact how effectively resolvers are able to make use
   of the caching of negative responses.

5.1.  Single Method

   Usually, the NSEC and NSEC3 methods are used exclusively (i.e., the
   methods are not used at the same time by different servers).  This
   configuration is preferred, because the behavior is well defined and
   closest to current operational practice.

5.2.  Mixing Methods

   Compliant resolvers should be able to validate zone data when
   different authoritative servers for the same zone respond with
   different authenticated-denial methods, because this is normally
   observed when NSEC and NSEC3 are being switched or when NSEC3PARAM is
   updated.

   Resolver software may, however, be designed to handle a single
   transition between two authenticated denial configurations more
   optimally than a permanent setup with mixed authenticated-denial
   methods.  This could make caching on the resolver side less
   efficient, and the authoritative servers may observe a higher number
   of queries.  This aspect should be considered especially in the
   context of "Aggressive Use of DNSSEC-Validated Cache" [RFC8198].

   In case all providers cannot be configured with the same
   authenticated-denial mechanism, it is recommended to limit the
   distinct configurations to the lowest number feasible.

   Note that NSEC3 configuration on all providers with different
   NSEC3PARAM values is considered a mixed setup.

6.  Key Rollover Considerations

   The multiple-signer (Section 2.1) models introduce some new
   requirements for DNSSEC key rollovers.  Since this process
   necessarily involves coordinated actions on the part of providers and
   the zone owner, one reasonable strategy is for the zone owner to
   initiate key-rollover operations.  But other operationally plausible
   models may also suit, such as a DNS provider initiating a key
   rollover and signaling their intent to the zone owner in some manner.
   The mechanism to communicate this intent could be some secure out-of-
   band channel that has been agreed upon, or the provider could offer
   an API function that could be periodically polled by the zone owner.

   For simplicity, the descriptions in this section assume two DNS
   providers.  They also assume that KSK rollovers employ the commonly
   used Double-Signature KSK rollover method and that ZSK rollovers
   employ the Pre-Publish ZSK rollover method, as described in detail in
   [RFC6781].  With minor modifications, they can be easily adapted to
   other models, such as Double-DS KSK rollover or Double-Signature ZSK
   rollover, if desired.  Key-use timing should follow the
   recommendations outlined in [RFC6781], but taking into account the
   additional operations needed by the multi-signer models.  For
   example, "time to propagate data to all the authoritative servers"
   now includes the time to import the new ZSKs into each provider.

6.1.  Model 1: Common KSK, Unique ZSK per Provider

   *  Key Signing Key Rollover: In this model, the two managed-DNS
      providers share a common KSK (public key) in their respective
      zones, and the zone owner has sole access to the private key
      portion of the KSK.  To initiate the rollover, the zone owner
      generates a new KSK and obtains the DNSKEY RRset of each DNS
      provider using their respective APIs.  The new KSK is added to
      each provider's DNSKEY RRset, and the RRset is re-signed with both
      the new and the old KSK.  This new DNSKEY RRset is then
      transferred to each provider.  The zone owner then updates the DS
      RRset in the parent zone to point to the new KSK and, after the
      necessary DS record TTL period has expired, proceeds with updating
      the DNSKEY RRset to remove the old KSK.

   *  Zone Signing Key Rollover: In this model, each DNS provider has
      separate Zone Signing Keys.  Each provider can choose to roll
      their ZSK independently by coordinating with the zone owner.
      Provider A would generate a new ZSK and communicate their intent
      to perform a rollover (note that Provider A cannot immediately
      insert this new ZSK into their DNSKEY RRset, because the RRset has
      to be signed by the zone owner).  The zone owner obtains the new
      ZSK from Provider A.  It then obtains the current DNSKEY RRset
      from each provider (including Provider A), inserts the new ZSK
      into each DNSKEY RRset, re-signs the DNSKEY RRset, and sends it
      back to each provider for deployment via their respective key-
      management APIs.  Once the necessary time period has elapsed
      (i.e., all zone data has been re-signed by the new ZSK and
      propagated to all authoritative servers for the zone, plus the
      maximum zone-TTL value of any of the data in the zone that has
      been signed by the old ZSK), Provider A and the zone owner can
      initiate the next phase of removing the old ZSK and re-signing the
      resulting new DNSKEY RRset.

6.2.  Model 2: Unique KSK and ZSK per Provider

   *  Key Signing Key Rollover: In Model 2, each managed-DNS provider
      has their own KSK.  A KSK roll for Provider A does not require any
      change in the DNSKEY RRset of Provider B but does require co-
      ordination with the zone owner in order to get the DS record set
      in the parent zone updated.  The KSK roll starts with Provider A
      generating a new KSK and including it in their DNSKEY RRSet.  The
      DNSKey RRset would then be signed by both the new and old KSK.
      The new KSK is communicated to the zone owner, after which the
      zone owner updates the DS RRset to replace the DS record for the
      old KSK with a DS record for the new KSK.  After the necessary DS
      RRset TTL period has elapsed, the old KSK can be removed from
      Provider A's DNSKEY RRset.

   *  Zone Signing Key Rollover: In Model 2, each managed-DNS provider
      has their own ZSK.  The ZSK roll for Provider A would start with
      them generating a new ZSK, including it in their DNSKEY RRset, and
      re-signing the new DNSKEY RRset with their KSK.  The new ZSK of
      Provider A would then be communicated to the zone owner, who would
      initiate the process of importing this ZSK into the DNSKEY RRsets
      of the other providers, using their respective APIs.  Before
      signing zone data with the new ZSK, Provider A should wait for the
      DNSKEY TTL plus the time to import the ZSK into Provider B, plus
      the time to propagate the DNSKEY RRset to all authoritative
      servers of both providers.  Once the necessary Pre-Publish key-
      rollover time periods have elapsed, Provider A and the zone owner
      can initiate the process of removing the old ZSK from the DNSKEY
      RRsets of all providers.

7.  Using Combined Signing Keys

   A Combined Signing Key (CSK) is one in which the same key serves the
   purposes of both being the secure entry point (SEP) key for the zone
   and signing all the zone data, including the DNSKEY RRset (i.e.,
   there is no KSK/ZSK split).

   Model 1 is not compatible with CSKs because the zone owner would then
   hold the sole signing key, and providers would not be able to sign
   their own zone data.

   Model 2 can accommodate CSKs without issue.  In this case, any or all
   of the providers could employ a CSK.  The DS record in the parent
   zone would reference the provider's CSK instead of KSK, and the
   public CSK would need to be imported into the DNSKEY RRsets of all of
   the other providers.  A CSK key rollover for such a provider would
   involve the following: The provider generates a new CSK, installs the
   new CSK into the DNSKEY RRset, and signs it with both the old and new
   CSKs.  The new CSK is communicated to the zone owner.  The zone owner
   exports this CSK into the other provider's DNSKEY RRsets and replaces
   the DS record referencing the old CSK with one referencing the new
   one in the parent DS RRset.  Once all the zone data has been re-
   signed with the new CSK, the old CSK is removed from the DNSKEY
   RRset, and the latter is re-signed with only the new CSK.  Finally,
   the old CSK is removed from the DNSKEY RRsets of the other providers.

8.  Use of CDS and CDNSKEY

   CDS and CDNSKEY records [RFC7344][RFC8078] are used to facilitate
   automated updates of DNSSEC secure-entry-point keys between parent
   and child zones.  Multi-signer DNSSEC configurations can support
   this, too.  In Model 1, CDS/CDNSKEY changes are centralized at the
   zone owner.  However, the zone owner will still need to push down
   updated signed CDNS/DNSKEY RRsets to the providers via the key-
   management mechanism.  In Model 2, the key-management mechanism needs
   to support cross-importation of the CDS/CDNSKEY records, so that a
   common view of the RRset can be constructed at each provider and is
   visible to the parent zone attempting to update the DS RRset.

9.  Key-Management-Mechanism Requirements

   Managed-DNS providers typically have their own proprietary zone
   configuration and data-management APIs, commonly utilizing HTTPS and
   Representational State Transfer (REST) interfaces.  So, rather than
   outlining a new API for key management here, we describe the specific
   functions that the provider API needs to support in order to enable
   the multi-signer models.  The zone owner is expected to use these API
   functions to perform key-management tasks.  Other mechanisms that can
   partly offer these functions, if supported by the providers, include
   the DNS UPDATE protocol [RFC2136] and Extensible Provisioning
   Protocol (EPP) [RFC5731].

   *  The API must offer a way to query the current DNSKEY RRset of the
      provider.

   *  For Model 1, the API must offer a way to import a signed DNSKEY
      RRset and replace the current one at the provider.  Additionally,
      if CDS/CDNSKEY is supported, the API must also offer a way to
      import a signed CDS/CDNSKEY RRset.

   *  For Model 2, the API must offer a way to import a DNSKEY record
      from an external provider into the current DNSKEY RRset.
      Additionally, if CDS/CDNSKEY is supported, the API must offer a
      mechanism to import individual CDS/CDNSKEY records from an
      external provider.

   In Model 2, once initially bootstrapped with each other's zone-
   signing keys via these API mechanisms, providers could, if desired,
   periodically query each other's DNSKEY RRsets, authenticate their
   signatures, and automatically import or withdraw ZSKs in the keyset
   as key-rollover events happen.

10.  DNS Response-Size Considerations

   The multi-signer models result in larger DNSKEY RRsets, so the size
   of a response to a query for the DNSKEY RRset will be larger.  The
   actual size increase depends on multiple factors: DNSKEY algorithm
   and keysize choices, the number of providers, whether additional keys
   are prepublished, how many simultaneous key rollovers are in
   progress, etc.  Newer elliptic-curve algorithms produce keys small
   enough that the responses will typically be far below the common
   Internet-path MTU.  Thus, operational concerns related to IP
   fragmentation or truncation and TCP fallback are unlikely to be
   encountered.  In any case, DNS operators need to ensure that they can
   emit and process large DNS UDP responses when necessary, and a future
   migration to alternative transports like DNS over TLS [RFC7858] or
   DNS over HTTPS [RFC8484] may make this topic moot.

11.  IANA Considerations

   This document has no IANA actions.

12.  Security Considerations

   The multi-signer models necessarily involve third-party providers
   holding the private keys that sign the zone-owner's data.  Obviously,
   this means that the zone owner has decided to place a great deal of
   trust in these providers.  By contrast, the more traditional model in
   which the zone owner runs a hidden master and uses the zone-transfer
   protocol with the providers is arguably more secure, because only the
   zone owner holds the private signing keys, and the third-party
   providers cannot serve bogus data without detection by validating
   resolvers.

   The zone-key import and export APIs required by these models need to
   be strongly authenticated to prevent tampering of key material by
   malicious third parties.  Many providers today offer REST/HTTPS APIs
   that utilize a number of client-authentication mechanisms (username/
   password, API keys etc) and whose HTTPS layer provides transport
   security and server authentication.  Multifactor authentication could
   be used to further strengthen security.  If DNS protocol mechanisms
   like UPDATE are being used for key insertion and deletion, they
   should similarly be strongly authenticated -- e.g., by employing
   Transaction Signatures (TSIG) [RFC2845].  Key generation and other
   general security-related operations should follow the guidance
   specified in [RFC6781].

13.  References

13.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
              <https://www.rfc-editor.org/info/rfc2845>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [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>.

   [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>.

   [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>.

   [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/info/rfc6781>.

   [RFC7344]  Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
              DNSSEC Delegation Trust Maintenance", RFC 7344,
              DOI 10.17487/RFC7344, September 2014,
              <https://www.rfc-editor.org/info/rfc7344>.

   [RFC8078]  Gudmundsson, O. and P. Wouters, "Managing DS Records from
              the Parent via CDS/CDNSKEY", RFC 8078,
              DOI 10.17487/RFC8078, March 2017,
              <https://www.rfc-editor.org/info/rfc8078>.

   [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>.

13.2.  Informative References

   [BLACKLIES]
              Valsorda, F. and O. Gudmundsson, "Compact DNSSEC Denial of
              Existence or Black Lies", Work in Progress, Internet-
              Draft, draft-valsorda-dnsop-black-lies-00, 21 March 2016,
              <https://tools.ietf.org/html/draft-valsorda-dnsop-black-
              lies-00>.

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              DOI 10.17487/RFC1995, August 1996,
              <https://www.rfc-editor.org/info/rfc1995>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731,
              DOI 10.17487/RFC5731, August 2009,
              <https://www.rfc-editor.org/info/rfc5731>.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
              <https://www.rfc-editor.org/info/rfc5936>.

   [RFC7129]  Gieben, R. and W. Mekking, "Authenticated Denial of
              Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
              February 2014, <https://www.rfc-editor.org/info/rfc7129>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

Acknowledgments

   The initial version of this document benefited from discussions with
   and review from Duane Wessels.  Additional helpful comments were
   provided by Steve Crocker, Ulrich Wisser, Tony Finch, Olafur
   Gudmundsson, Matthijs Mekking, Daniel Migault, and Ben Kaduk.

Authors' Addresses

   Shumon Huque
   Salesforce
   415 Mission Street, 3rd Floor
   San Francisco, CA 94105
   United States of America

   Email: shuque@gmail.com


   Pallavi Aras
   Salesforce
   415 Mission Street, 3rd Floor
   San Francisco, CA 94105
   United States of America

   Email: paras@salesforce.com


   John Dickinson
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford
   OX4 4GA
   United Kingdom

   Email: jad@sinodun.com


   Jan Vcelak
   NS1
   55 Broad Street, 19th Floor
   New York, NY 10004
   United States of America

   Email: jvcelak@ns1.com


   David Blacka
   Verisign
   12061 Bluemont Way
   Reston, VA 20190
   United States of America

   Email: davidb@verisign.com
