View source for RFC8901



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 [[RFC7841|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 [[BCP78|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
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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.

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

== 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]].

== 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.

=== 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.

==== 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.)

==== 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).

== 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.

== 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.

== 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.

=== 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.

=== 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.

== 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.

=== 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.

=== 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.

== 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.

== 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.

== 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",
           [[STD13|STD 13]], [[RFC1034|RFC 1034]], DOI 10.17487/RFC1034, November 1987,
           <https://www.rfc-editor.org/info/rfc1034>.

[[RFC1035]]  Mockapetris, P., "Domain names - implementation and
           specification", [[STD13|STD 13]], [[RFC1035|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)", [[RFC2845|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",
           [[RFC4033|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",
           [[RFC4034|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", [[RFC4035|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", [[RFC5155|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", [[RFC6781|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", [[RFC7344|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", [[RFC8078|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", [[RFC8198|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", [[RFC1995|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)",
           [[RFC2136|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", [[STD69|STD 69]], [[RFC5731|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)", [[RFC5936|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", [[RFC7129|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)", [[RFC7858|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)", [[RFC8484|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", [[BCP219|BCP 219]], [[RFC8499|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

[[Category:Informational]]