wdiff rfc9975v1.txt rfc9975.txt

Internet Engineering Task Force (IETF) P. Thomassen
Request for Comments: 9975 SSE - Secure Systems Engineering GmbH
Updates: 7344, 7477 May 2026
Category: Standards Track
ISSN: 2070-1721

Clarifications on CDS/CDNSKEY and CSYNC Consistency

Abstract

Maintenance of DNS delegations requires occasional changes of the DS
and NS record sets on the parent side of the delegation. For the
case of DS records, "Automating DNSSEC Delegation Trust Maintenance"
(RFC 7344) provides automation by allowing the child to publish CDS
and/or CDNSKEY records holding the prospective DS parameters that the
parent can ingest. Similarly, "Child-to-Parent Synchronization in
DNS" (RFC 7477) specifies CSYNC records to indicate a desired update
of the delegation's NS (and glue) records. Parent-side entities
(e.g., Registries and Registrars) can query these records from the
child and, after validation, use them to update the parent-side
Resource Record Sets (RRsets) of the delegation.

This document specifies under which conditions the target states
expressed via CDS/CDNSKEY and CSYNC records are considered
"consistent". Parent-side entities accepting such records from the
child have to ensure that update requests retrieved from different
authoritative nameservers satisfy these consistency requirements
before taking any action based on them.

This document updates RFCs 7344 and 7477.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in 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/rfc9975.

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 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
1.1. Requirements Notation
1.2. Terminology
2. Updates to RFCs 7344 and 7477
3. Processing Requirements
3.1. CDS and CDNSKEY
3.2. CSYNC
4. IANA Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Failure Scenarios due to Inconsistencies
A.1. DS Breakage due to Replication Lag
A.2. Escalation of Lame Delegation Takeover
A.3. Multi-Provider (Permanent Multi-Signer)
A.3.1. DS Breakage
A.3.2. NS Breakage
A.4. Bogus Provider Change (Temporary Multi-Signer)
Acknowledgments
Author's Address

1. Introduction

[RFC7344] automates DNS Security Extensions (DNSSEC) delegation trust
maintenance by having the child publish CDS and/or CDNSKEY records
that describe the prospective DS parameters. Similarly, [RFC7477]
specifies CSYNC records indicating a desired update of the
delegation's NS and associated glue records. Parent-side entities
(e.g., Registries and Registrars) can use these records to update the
corresponding records of the delegation.

For ingesting CSYNC records, Section 3.1 of [RFC7477] advocates that
Parental Agents limit queries to a single authoritative nameserver
(as done in normal resolution). The [RFC7344] (on CDS/CDNSKEY) has a
corresponding Section section (Section 6.1 of
[RFC7344] (CDS/CDNSKEY) [RFC7344]) that contains no
provision for how specifically queries for these records should be
done.

Retrieving records from just one authoritative server (e.g., by
directing queries towards a trusted validating resolver) works well
under ideal operating scenarios. However, problems may arise if
CDS/CDNSKEY/CSYNC record sets are inconsistent across authoritative
nameserver either because they are out of sync (e.g., during a Key
Signing Key (KSK) rollover) or because they are not controlled by the
same entity (e.g., in a multi-signer setup [RFC8901]).

In such cases, if CDS/CDNSKEY/CSYNC records are retrieved from one
nameserver only ("naively", without a consistency check), each
nameserver can unilaterally trigger an update of the delegation's DS
or NS record set.

For example, a single provider in a multi-signer setup may
(accidentally or maliciously) cause another provider's trust anchors
and/or nameservers to be removed from the delegation. This can occur
both when the multi-signer configuration is temporary (e.g., during a
provider change) and when it is permanent (e.g., for redundancy). In
any case, a single provider should not be in the position to remove
the
any other providers' provider's records from the delegation.

Similar breakage can occur when the delegation has lame nameservers,
where an attacker may illegitimately initialize a DS record set and
then manipulate the delegation's NS record set at will. More
detailed examples are given in Appendix A.

For a CDS/CDNSKEY/CSYNC consumer, it is generally impossible to
estimate the impact of a requested delegation update unless all of
the child's authoritative nameservers are inspected. At the same
time, applying an automated delegation update "MUST NOT break the
current delegation" (per [RFC7344], Section 4.1), i.e., it must not
hamper availability or validatability of the Child's resolution. As
part of a more holistic treatment of the problem space,
[DS-AUTOMATION] provides more-specific guidance on such safety
checks.

Therefore, this document specifies that parent-side entities need to
ensure that the updates indicated by CDS/CDNSKEY and CSYNC record
sets are plausibly consistent across the child's nameservers before
taking any action based on these records.

Readers are expected to be familiar with DNSSEC [RFC9364], in
particular, [RFC4033], [RFC4034], [RFC4035], [RFC7344], and
[RFC7477]. For an overview of related operational practices, refer
to [RFC6781] and [RFC8901].

1.1. Requirements Notation

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.

1.2. Terminology

Multi-provider setup: A constellation where several providers
independently operate authoritative DNS service for a domain,
usually for purposes of redundancy. This includes setups both
with and without DNSSEC.

Multi-signer setup: A multi-provider setup for a DNSSEC-enabled
domain with multiple independent signing entities [RFC8901]. Such
a setup may be permanent (for redundancy) or temporary (for
continuity of DNSSEC operation while changing the provider of a
domain that normally uses a single one).

Otherwise, the terminology in this document is as defined in
[RFC7344].

2. Updates to RFCs 7344 and 7477

Section 4.1 of [RFC7344] lists acceptance rules for CDS/CDNSKEY
records. This list is extended with the consistency requirements
defined in this document. This document does not modify any other
part of [RFC7344].

Sections 3.1 and 4.2 of [RFC7477] have logic for deciding from which
nameserver to query CSYNC information. This logic is replaced with
the CSYNC consistency requirements defined in this document.

3. Processing Requirements

Consistency requirements that apply equally to CDS/CDNSKEY and CSYNC
are listed first; type-specific consistency criteria are described in
separate subsections.

In order to determine plausible consistency of CDS/CDNSKEY or CSYNC
RRsets across the child's nameservers, the Parental Agent MUST fetch
all IP addresses for each nameserver hostname as listed in the
Child's delegation from the Parent using a validating resolver,
including any available glue records. Before acting on any CDS/
CDNSKEY or CSYNC record for the child, the Parental Agent MUST have
established plausible consistency by querying all of these IP
addresses for the record set(s) in question, as per the guidelines
spelled out in the following subsections.

In all cases, consistency is REQUIRED across received responses only.
(A NODATA response (see [RFC9499]) is a received response.)

When a response cannot be obtained from a given nameserver, the
Parental Agent SHOULD attempt to obtain it at a later time, before
concluding that the nameserver is permanently unreachable and
removing it from consideration. A configurable retry schedule is
RECOMMENDED to increase the likelihood of collecting data from all
nameservers. An exponential back-off schedule (e.g., 5, 10, 20, 40,
... minutes) provides a balance between faster task completion while
accommodating transient unreachability. To sidestep localized
routing issues, the Parental Agent MAY also attempt contacting the
nameserver from another network vantage point.

If an inconsistent state is encountered, the Parental Agent MUST
abort the operation. Specifically, it MUST NOT delete or alter any
existing RRset that would have been deleted or altered, and it MUST
NOT create any RRsets that would have been created had the
nameservers given consistent responses.

To accommodate transient inconsistencies (e.g., replication delays),
implementations MAY be configurable to undertake a retry of the full
process, repeating all queries (suggested default: enabled with
exponential back-off).

Any pending queries can immediately be dequeued when encountering a
response that confirms the status quo, either implicitly (NODATA) or
explicitly.
explicitly (via a response that matches the current delegation
state). This is because any subsequent responses could only confirm
that nothing needs to happen or give an inconsistent result in which
case nothing needs to happen. The parent may apply local policy in
determining whether the requested update is consistent or not with
the status quo, as illustrated in the type-specific sections below.
In any case, queries may be continued across all nameservers for
reporting purposes.

Existing requirements for ensuring integrity remain in effect. In
particular, DNSSEC signatures MUST be requested and validated for all
queries unless otherwise noted.

3.1. CDS and CDNSKEY

To retrieve

When retrieving a Child's CDS/CDNSKEY RRset for DNSSEC delegation
trust maintenance, the Parental Agent, knowing both the Child zone
name and its NS hostnames, MUST ascertain that queries are made
against all nameservers listed in the Child's delegation from the
Parent. The Parental Agent MUST also ensure that each key referenced
in any of the received answers is also referenced in all other
received responses or that responses consistently indicate a request
for removal of the entire DS RRset ([RFC8078], Section 6).

In other words, CDS/CDNSKEY records at the Child zone apex must be
fetched directly from each reachable authoritative server as
determined by the delegation's NS record set. When a key is
referenced in a CDS record set but not the CDNSKEY record set (or
vice versa), or returned by one nameserver but is missing from at
least one other nameserver's answer, the CDS/CDNSKEY state MUST be
considered inconsistent. Similarly, the state MUST be considered
inconsistent if there is a CDS or CDNSKEY response that indicates a
removal request for the DS RRset whereas another response indicates
no change (NODATA) or a DS update.

As an example of local policy, the parent may restrict the choice of
hash digest types used when publishing a DS RRset (notwithstanding
the requirements specified in [DS-IANA]). (The set of keys
referenced in the DS RRset is not up to local policy. Only if all
keys from the CDS/CDNSKEY RRsets are included is the DS RRset
considered consistent.)

During initial DS provisioning (DNSSEC bootstrapping), conventional
DNSSEC validation for CDS/CDNSKEY responses is not (yet) available;
in this case, authenticated bootstrapping [RFC9615] should be used.

3.2. CSYNC

A CSYNC-based workflow generally consists of:

1. querying the CSYNC (and possibly SOA) record to determine which
data records shall be synchronized from child to parent, and

2. querying for these data records (e.g., NS) before placing them in
the parent zone.

If the below conditions are not met during these steps, the CSYNC
state MUST be considered inconsistent.

When querying the CSYNC record, the Parental Agent MUST ascertain
that queries are made against all nameservers listed in the Child's
delegation from the Parent and ensure that the record's immediate
flag and type bitmap are equal across received responses.

The CSYNC record's SOA serial field and soaminimum flag might
legitimately differ across nameservers (such as in multi-provider
setups); thus, equality is not required across responses. Instead,
for a given response, processing of these values MUST occur with
respect to the SOA record as obtained from the same nameserver. If
the resulting per-nameserver assessments of whether the update is
permissible do not all agree, the CSYNC state MUST be considered
inconsistent.

Further, when retrieving the data record sets as indicated in the
CSYNC record (such as NS or A/AAAA records), the Parental Agent MUST
ascertain that all queries are made against all nameservers from
which a CSYNC record was received and ensure that all of them return
responses with equal rdata sets (including cases where all are
empty).

As an example of local policy, the parent may choose to accept glue
records only for in-domain or sibling NS hostnames [RFC9499].

Other CSYNC processing rules from Section 3 of [RFC7477] remain in
place without modification. For example, when the NS type flag is
present, associated NS processing has to occur before potential glue
updates to ensure that glue addresses match the right set of
nameservers. Also, when the type bitmap contains the A/AAAA flags,
corresponding address queries are only to be sent for NS hostnames
that are in bailiwick, while out-of-bailiwick NS records are ignored.
Refer to Sections 3.2.2 and 4.3 of [RFC7477] for more details.

CSYNC-based updates may cause validation or even insecure resolution
to break (e.g., by changing the delegation to a set of nameservers
that do not serve required DNSKEY records or do not know the zone at
all). Parental Agents SHOULD check that CSYNC-based updates, if
applied, do not break the delegation.

4. IANA Considerations

This document has no IANA actions.

5. Security Considerations

The level of rigor mandated by this document is needed to prevent
publication of half-baked DS or delegation NS RRsets (authorized only
under an insufficient subset of authoritative nameservers), ensuring
that a single operator cannot unilaterally modify the delegation (add
or remove trust anchors or nameservers) when other operators are
present. This applies both when the setup is intentional and when it
is unintentional (such as in the case of lame-delegation hijacking).

As a consequence, the delegation's records can only be modified when
zones are synchronized across operators, unanimously reflecting the
domain owner's intentions. Both availability and integrity of the
domain's DNS service benefit from this policy.

In order to resolve situations in which consensus about child zone
contents cannot be reached (e.g., because one of the nameserver
operators is uncooperative), Parental Agents SHOULD continue to
accept DS and NS/glue update requests from the domain owner via an
authenticated out-of-band channel (such as EPP Extensible Provisioning
Protocol (EPP) [RFC5730]), irrespective of the adoption of automated
delegation maintenance. Availability of such an interface also
enables recovery from a situation where the private key is no longer
available for signing the CDS/CDNSKEY or CSYNC records in the child
zone.

6. References

6.1. 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/info/rfc2119>.

[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>.

[RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,
<https://www.rfc-editor.org/info/rfc5730>.

[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>.

[RFC7477] Hardaker, W., "Child-to-Parent Synchronization in DNS",
RFC 7477, DOI 10.17487/RFC7477, March 2015,
<https://www.rfc-editor.org/info/rfc7477>.

[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>.

[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/info/rfc8174>.

[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/info/rfc9364>.

[RFC9615] Thomassen, P. and N. Wisiol, "Automatic DNSSEC
Bootstrapping Using Authenticated Signals from the Zone's
Operator", RFC 9615, DOI 10.17487/RFC9615, July 2024,
<https://www.rfc-editor.org/info/rfc9615>.

6.2. Informative References

[DS-IANA] IANA, "DNSSEC Delegation Signer (DS) Resource Record (RR)
Type Digest Algorithms",
<https://www.iana.org/assignments/ds-rr-types>.

[DS-AUTOMATION]
Sheng, S. and P. Thomassen, "Operational Recommendations
for DNSSEC Delegation Signer (DS) Automation", Work in
Progress, Internet-Draft, draft-ietf-dnsop-ds-automation-
05, 17 April 2026, <https://datatracker.ietf.org/doc/html/
draft-ietf-dnsop-ds-automation-05>.

[DS-IANA] IANA, "DNSSEC Delegation Signer (DS) Resource Record (RR)
Type Digest Algorithms",
<https://www.iana.org/assignments/ds-rr-types>.

[LAME1] Akiwate, G., Jonker, M., Sommese, R., Foster, I., Voelker,
G. M., Savage, S., and K. Claffy, "Unresolved Issues:
Prevalence, Persistence, and Perils of Lame Delegations",
IMC '20: Proceedings of the ACM Internet Measurement
Conference, pp. 281-294, DOI 10.1145/3419394.3423623, 27
October 2020, <https://doi.org/10.1145/3419394.3423623>.

[LAME2] Akiwate, G., Savage, S., Voelker, G. M., and K. Claffy,
"Risky BIZness: risks derived from registrar name
management", IMC '21: Proceedings of the 21st ACM Internet
Measurement Conference, pp. 673-686,
DOI 10.1145/3487552.3487816, 2 November 2021,
<https://doi.org/10.1145/3487552.3487816>.

[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>.

[RFC8901] Huque, S., Aras, P., Dickinson, J., Vcelak, J., and D.
Blacka, "Multi-Signer DNSSEC Models", RFC 8901,
DOI 10.17487/RFC8901, September 2020,
<https://www.rfc-editor.org/info/rfc8901>.

[RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<https://www.rfc-editor.org/info/rfc9499>.

Appendix A. Failure Scenarios due to Inconsistencies

The following scenarios are informative examples of how things can go
wrong when consistency is not enforced by the parent during
CDS/CDNSKEY/CSYNC processing. Other scenarios that cause similar (or
perhaps even more) harm may exist.

The common feature of these scenarios is that if one nameserver steps
out of line and the parent is not careful, DNS resolution and/or
validation will break down. When several DNS providers are involved,
this undermines the very guarantees of operator independence that
multi-provider configurations are intended to provide.

A.1. DS Breakage due to Replication Lag

If an authoritative nameserver is lagging behind during a key
rollover, the parent may see different CDS/CDNSKEY RRsets depending
on the nameserver contacted. This may cause old and new DS RRsets to
be deployed in an alternating fashion and without the awareness of
the zone maintainer, who may then inadvertently break the chain of
trust by prematurely removing a DNSKEY still referenced by a (stale)
CDS/CDNSKEY RRset.

While foreseen in Section 6.2 of [RFC7344], the solution specified
there requires parents to keep state on CDS/CDNSKEY RRsets. This
document achieves the same without this burden and, in case the
parent reports consistency errors downstream, can also help detection
of the child-side replication issue by the operator.

A.2. Escalation of Lame Delegation Takeover

A delegation may include a nonexistent NS hostname, for example, due
to a typo or the nameserver's domain registration having expired.
(Re-)registering such a non-resolvable nameserver domain allows a
third party to run authoritative DNS service for all domains
delegated to that NS hostname, serving responses different from the
legitimate ones.

This strategy for hijacking (at least part of the) DNS traffic and
spoofing responses is not new but is surprisingly common [LAME1]
[LAME2]. It is also known that DNSSEC reduces the impact of such an
attack, as validating resolvers will reject illegitimate responses
due to lack of signatures consistent with the delegation's DS
records.

On the other hand, if the delegation is not protected by DNSSEC, the
rogue nameserver is not only able to serve unauthorized responses
without detection: it is even possible for the attacker to escalate
the nameserver takeover to a full domain takeover.

In particular, the rogue nameserver can publish CDS/CDNSKEY records.
If those are processed by the parent without ensuring consistency
with other authoritative nameservers, the delegation will, with some
patience, get secured with the attacker's DNSSEC keys. Of course, as
the parent's query (or sometimes queries) need to hit the attacker's
nameserver, this requires some statistical luck, but, eventually, it
will succeed. As responses served by the remaining legitimate
nameservers are not signed with these keys, validating resolvers will
start rejecting them.

Once DNSSEC is established, the attacker can use CSYNC to remove
other nameservers from the delegation at will (and potentially add
new ones under their control) or change glue records to point to the
attacker's nameservers. This enables the attacker to position itself
as the only party providing authoritative DNS service for the victim
domain, significantly augmenting the attack's impact.

A.3. Multi-Provider (Permanent Multi-Signer)

A.3.1. DS Breakage

While performing a key rollover and adjusting the corresponding CDS/
CDNSKEY records, a provider could accidentally publish CDS/CDNSKEY
records that only include its own keys.

When the parent happens to retrieve the records from a nameserver
controlled by this provider, the other providers' DS records would be
removed from the delegation. As a result, the zone is broken (at
least for some queries).

A.3.2. NS Breakage

A similar scenario affects the CSYNC record, which is used to update
the delegation's NS record set at the parent. For example, the issue
occurs when a provider accidentally includes only their own set of
hostnames in the local NS record set or publishes an otherwise flawed
NS record set.

If the parent then observes a CSYNC signal and fetches the flawed NS
record set without ensuring consistency across nameservers, the
delegation may be updated in a way that breaks resolution or silently
reduces the multi-provider setup to a single-provider setup.

A.4. Bogus Provider Change (Temporary Multi-Signer)

Transferring DNS service for a domain name from one (signing) DNS
provider to another, without going insecure, necessitates a brief
period during which the domain is operated in multi-signer mode.
First, the providers include each other's signing keys as DNSKEY and
CDS/CDNSKEY records in their own copy of the zone. Once the parent
learns about the updated CDS/CDNSKEY record set at the old provider,
the delegation's DS record set is updated. Then, after waiting for
cache expiration, the new provider's NS hostnames can be added to the
zone's NS record set so that queries start balancing across both
providers. To conclude the handover, the old provider is removed by
inverting these steps with swapped roles.

The multi-signer phase of this process breaks when the new provider,
perhaps unaware of the situation and its intricacies, fails to
include the old provider's keys in the DNSKEY (and CDS/CDNSKEY)
record sets. One obvious consequence is that whenever the resolver
happens to retrieve the DNSKEY record set from the new provider, the
old provider's RRSIGs no longer validate, causing SERVFAIL to be
returned.

However, an even worse consequence can occur when the parent performs
their next CDS/CDNSKEY scan. The incorrect CDS/CDNSKEY record set is
fetched from the new provider and used to update the delegation's DS
record set. As a result, the old provider (who still appears in the
delegation) is prematurely removed from the domain's DNSSEC chain of
trust. The new DS record set authenticates the new provider's
DNSKEYs only, and DNSSEC validation fails for all answers served by
the old provider.

Acknowledgments

In order of first contribution or review: Viktor Dukhovni, Wes
Hardaker, Libor Peltan, Oli Schacher, David Blacka, Charlie Kaufman,
Michael Bauland, Patrick Mevzek, Joe Abley, Ondřej Caletka, Ondřej
Surý, Mohamed Boucadair, Vijay Gurbani, Gorry Fairhurst, Paul
Wouters, Andy Newton, Mike Bishop, and Warren Kumari.

Author's Address

Peter Thomassen
SSE - Secure Systems Engineering GmbH
Hauptstraße 3
10827 Berlin
Germany
Email: pth@systemsecurity.com