Internet Engineering Task Force (IETF) P. Thomassen

Internet Engineering Task Force (IETF) P. Thomassen
Request for Comments: 9615 deSEC, Secure Systems Engineering (SSE)
Updates: 7344, 8078 N. Wisiol
Category: Standards Track deSEC, Technische Universität Berlin
ISSN: 2070-1721 July 2024

Automatic DNSSEC Bootstrapping Using Authenticated Signals from the
Zone's Operator

Abstract

This document introduces an in-band method for DNS operators to
publish arbitrary information about the zones for which they are
authoritative, in an authenticated fashion and on a per-zone basis.
The mechanism allows managed DNS operators to securely announce
DNSSEC key parameters for zones under their management, including for
zones that are not currently securely delegated.

Whenever DS records are absent for a zone's delegation, this signal
enables the parent's registry or registrar to cryptographically
validate the CDS/CDNSKEY records found at the child's apex. The
parent can then provision DS records for the delegation without
resorting to out-of-band validation or weaker types of cross-checks
such as "Accept after Delay".

This document establishes the DS enrollment method described in
Section 4 of this document as the preferred method over those from
Section 3 of RFC 8078. It also updates RFC 7344.

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/rfc9615.

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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. Terminology
1.2. Requirements Notation
2. Updates to RFCs
3. Signaling
3.1. Chain of Trust
3.2. Signaling Names
4. Bootstrapping a DNSSEC Delegation
4.1. Signaling Consent to Act as the Child's Signer
4.1.1. Example
4.2. Validating CDS/CDNSKEY Records for DNSSEC Bootstrapping
4.2.1. Example
4.3. Triggers
4.4. Limitations
5. Operational Recommendations
5.1. Child DNS Operator
5.2. Parental Agent
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

Securing a DNS delegation for the first time requires that the
child's DNSSEC parameters be conveyed to the parent through some
trusted channel. While the communication conceptually has to occur
between the parent registry and the DNSSEC key holder, what that
means exactly and how communication is coordinated traditionally
depends on the relationship the child has with the parent.

A typical situation is that the key is held by the child DNS
operator; thus, the communication often involves this entity. In
addition, depending on the circumstances, it may also involve the
registrar, possibly via the registrant (for details, see Appendix A
of [RFC7344].

As observed in [RFC7344], these dependencies often result in a manual
process that is susceptible to mistakes and/or errors. In addition,
due to the annoyance factor of the process, involved parties may
avoid the process of getting a DS resource record set (RRset)
published in the first place.

To alleviate these problems, automated provisioning of DS records has
been specified in [RFC8078]. It is based on the parental agent
(registry or registrar) fetching DNSSEC key parameters from the CDS
and CDNSKEY records ([RFC7344]) located at the child zone's apex, and
validating them somehow. This validation can be done using the
child's existing DNSSEC chain of trust if the objective is to update
an existing DS RRset (such as during key rollover). However, when
bootstrapping a DNSSEC delegation, the child zone has no existing
DNSSEC validation path, so other means to ensure the CDS/CDNSKEY
records' legitimacy must be found.

Due to the lack of a comprehensive DNS-innate solution, either out-
of-band methods have been used so far to complete the chain of trust,
or cryptographic validation has been entirely dispensed with, in
exchange for weaker types of cross-checks such as "Accept after
Delay" (Section 3.3 of [RFC8078]). [RFC8078] does not define an in-
band validation method for enabling DNSSEC.

This document aims to close this gap by introducing an in-band method
for DNS operators to publish arbitrary information about the zones
for which they are authoritative, in an authenticated manner and on a
per-zone basis. The mechanism allows managed DNS operators to
securely announce DNSSEC key parameters for zones under their
management. The parent can then use this signal to cryptographically
validate the CDS/CDNSKEY RRsets found at an insecure child zone's
apex and, upon success, secure the delegation.

While applicable to the vast majority of domains, the protocol does
not support certain edge cases, such as excessively long child zone
names, or DNSSEC bootstrapping for domains with in-domain nameservers
only (see Section 4.4).

DNSSEC bootstrapping is just one application of the generic signaling
mechanism specified in this document. Other applications might arise
in the future, such as publishing operational metadata or auxiliary
information that the DNS operator likes to make known (e.g., API
endpoints for third-party interaction).

Readers are expected to be familiar with DNSSEC [BCP237].

1.1. Terminology

This section defines the terminology used in this document.

CDS/CDNSKEY: This notation refers to CDS and/or CDNSKEY, i.e., one
or both.

Child: See Section 7 of [RFC9499].

Child DNS operator: The entity that maintains and publishes the zone
information for the child DNS.

Parent: See Section 7 of [RFC9499].

Parental agent: The entity that has the authority to insert DS
records into the parent zone on behalf of the child. (It could be
the registry, registrar, a reseller, or some other authorized
entity.)

Signaling domain: A domain name constructed by prepending the label
_signal to a hostname taken from a delegation's NS RRset. There
are as many signaling domains as there are distinct NS targets.

Signaling name: The labels that are prefixed to a signaling domain
in order to identify a signaling type and a child zone's name (see
Section 3.2).

Signaling record: A DNS record located at a signaling name under a
signaling domain. Signaling records are used by the child DNS
operator to publish information about the child.

Signaling type: A signal type identifier, such as _dsboot for DNSSEC
bootstrapping.

Signaling zone: The zone that is authoritative for a given signaling
record.

1.2. 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.

2. Updates to RFCs

The DS enrollment methods described in Section 3 of [RFC8078] are
less secure than the method described in Section 4 of this document.
Therefore, child DNS operators and parental agents wishing to use
CDS/CDNSKEY records for initial DS enrollment SHOULD support the
authentication protocol described here.

In order to facilitate publication of signaling records for the
purpose of DNSSEC bootstrapping (see Section 4.1), the first bullet
("Location") of Section 4.1 of [RFC7344] is removed.

3. Signaling

This section describes the general mechanism by which a child DNS
operator can publish an authenticated signal about a child zone.
Parental agents (or any other party) can then discover and process
the signal. Authenticity is ensured through standard DNSSEC
validation.

3.1. Chain of Trust

If a child DNS operator implements this specification, each signaling
zone MUST be signed and be validatable by the parental agent (i.e.,
have a valid publicly resolvable DNSSEC chain of trust). This is
typically achieved by securely delegating each signaling zone.

For example, when publishing a signal that relates to a child zone
with NS records ns1.example.net and ns2.example.org, the child DNS
operator needs to ensure that the parental agent has a valid DNSSEC
chain of trust for the zone(s) that are authoritative for the
signaling domains _signal.ns1.example.net and
_signal.ns2.example.org.

3.2. Signaling Names

To publish information about the child zone in an authenticated
fashion, the child DNS operator MUST publish one or more signaling
records at a signaling name under each signaling domain.

Signaling records MUST be accompanied by RRSIG records created with
the corresponding signaling zone's key(s). The type and contents of
these signaling records depend on the type of signal.

The signaling name identifies the child and the signaling type. It
is identical to the child name (with the final root label removed),
prefixed with a label containing the signaling type.

4. Bootstrapping a DNSSEC Delegation

When the child zone's CDS/CDNSKEY RRsets are used for setting up
initial trust, they need to be authenticated. This is achieved by
copublishing the child's CDS/CDNSKEY RRsets as an authenticated
signal as described in Section 3. The parent can discover and
validate it, thus transferring trust from the child DNS operator
nameservers' chain of trust to the child zone.

This protocol is not intended for updating an existing DS RRset. For
this purpose, the parental agent can validate the child's CDS/CDNSKEY
RRsets directly, using the chain of trust established by the existing
DS RRset (Section 4 of [RFC7344]).

4.1. Signaling Consent to Act as the Child's Signer

To confirm its willingness to act as the child's delegated signer and
authenticate the child's CDS/CDNSKEY RRsets, the child DNS operator
MUST copublish them at the corresponding signaling name under each
signaling domain, excluding those that would fall within the child
domain (Section 3.2). For simplicity, the child DNS operator MAY
also copublish the child's CDS/CDNSKEY RRsets under signaling domains
within the child domain, although those signaling domains are not
used for validation (Section 4.2).

Unlike the CDS/CDNSKEY RRsets at the child's apex, a signaling RRset
MUST be signed with the corresponding signaling zone's key(s). Its
contents MUST be identical to the corresponding RRset published at
the child's apex.

Existing use of CDS/CDNSKEY records was specified at the child apex
only (Section 4.1 of [RFC7344]). This protocol extends the use of
these record types to non-apex owner names for the purpose of DNSSEC
bootstrapping. To exclude the possibility of semantic collision,
there MUST NOT be a zone cut at a signaling name.

4.1.1. Example

For the purposes of bootstrapping the child zone example.co.uk with
NS records ns1.example.net, ns2.example.org, and ns3.example.co.uk,
the required signaling domains are _signal.ns1.example.net and
_signal.ns2.example.org.

In the zones containing these domains, the child DNS operator
authenticates the CDS/CDNSKEY RRsets found at the child's apex by
copublishing them as CDS/CDNSKEY RRsets at the names:

_dsboot.example.co.uk._signal.ns1.example.net
_dsboot.example.co.uk._signal.ns2.example.org

These RRsets are signed with DNSSEC just like any other zone data.

Publication of signaling records under the in-domain name
_signal.ns3.example.co.uk is not required.

4.2. Validating CDS/CDNSKEY Records for DNSSEC Bootstrapping

To validate a child's CDS/CDNSKEY RRset for DNSSEC bootstrapping, the
parental agent, knowing both the child zone name and its NS
hostnames, MUST execute the following steps:

Step 1: verify that the child has no DS records published at the
parent and that at least one of its nameservers is outside
the child domain;

Step 2: query the CDS/CDNSKEY RRset at the child zone apex directly
from each of the authoritative servers as determined by the
delegation's (parent-side) NS RRset, without caching;

Step 3: query the CDS/CDNSKEY RRset located at the signaling name
under each signaling domain (except those falling within the
child domain) using a trusted DNS resolver and enforce
DNSSEC validation;

Step 4: check (separately by record type) that all RRsets retrieved
in Steps 2 and 3 have equal contents;

If the above steps succeed without error, the CDS/CDNSKEY RRsets are
successfully verified, and the parental agent can proceed with the
publication of the DS RRset under the precautions described in
Section 5 of [RFC8078].

The parental agent MUST abort the procedure if an error condition
occurs, in particular:

* in Step 1: the child is already securely delegated or has in-
domain nameservers only;

* in Step 2: any failure during the retrieval of the CDS/CDNSKEY
RRset located at the child apex from any of the authoritative
nameservers;

* in Step 3: any failure to retrieve the CDS/CDNSKEY RRsets located
at the signaling name under any signaling domain, including
failure of DNSSEC validation, or unauthenticated data (AD bit not
set);

* in Step 4: inconsistent responses (for at least one of the types),
including an RRset that is empty in one of Steps 2 or 3, but non-
empty in the other.

4.2.1. Example

To verify the CDS/CDNSKEY RRsets for the child example.co.uk, the
parental agent (assuming that the child delegation's NS records are
ns1.example.net, ns2.example.org, and ns3.example.co.uk)

1. checks that the child domain is not yet securely delegated;

2. queries the CDS/CDNSKEY RRsets for example.co.uk directly from
ns1.example.net, ns2.example.org, and ns3.example.co.uk (without
caching);

3. queries and validates the CDS/CDNSKEY RRsets located at (see
Section 3.2; ns3.example.co.uk is ignored because it is in-
domain)

_dsboot.example.co.uk._signal.ns1.example.net
_dsboot.example.co.uk._signal.ns2.example.org

4. checks that the CDS/CDNSKEY RRsets retrieved in Steps 2 and 3
agree across responses.

If all of these steps succeed, the parental agent can proceed to
publish a DS RRset as indicated by the validated CDS/CDNSKEY RRset.

As in-domain signaling names do not have a chain of trust at
bootstrapping time, the parental agent does not consider them during
validation. Consequently, if all NS hostnames are in-domain,
validation cannot be completed and DS records are not published.

4.3. Triggers

Parental agents SHOULD trigger the procedure described in Section 4.2
once one of the following conditions is fulfilled:

* The parental agent receives a new or updated NS RRset for a child;

* The parental agent receives a notification indicating that the
child wishes to have its CDS/CDNSKEY RRset processed;

* The parental agent encounters a signaling record during a
proactive, opportunistic scan (e.g., daily queries of signaling
records for some or all of its delegations);

* The parental agent encounters a signaling record during an NSEC
walk or when parsing a signaling zone (e.g., when made available
via AXFR by the child DNS operator);

* Any other condition deemed appropriate by local policy.

Timer-based trigger mechanisms (such as scans) exhibit undesirable
properties with respect to processing delay and scaling; on-demand
triggers (like notifications) are preferable. Whenever possible,
child DNS operators and parental agents are thus encouraged to use
them, reducing both delays and the amount of scanning traffic.

Most types of discovery (such as daily scans of delegations) are
based directly on the delegation's NS RRset. In this case, these NS
names can be used as is by the bootstrapping algorithm (Section 4.2)
for querying signaling records.

Some discovery methods, however, do not imply reliable knowledge of
the delegation's NS RRset. For example, when discovering signaling
names by performing an NSEC walk or zone transfer of a signaling
zone, the parental agent MUST NOT assume that a nameserver under
whose signaling domain a signaling record appears is actually
authoritative for the corresponding child.

Instead, whenever a list of "bootstrappable domains" is obtained by
means other than directly from the parent, the parental agent MUST
ascertain that the delegation actually contains the nameserver
hostname seen during discovery, and ensure that signaling-record
queries are only made against the proper set of nameservers as listed
in the child's delegation from the parent.

4.4. Limitations

As a consequence of Step 3 in Section 4.2, DS bootstrapping does not
work for fully in-domain delegations, as no preexisting chain of
trust to the child domain is available during bootstrapping. (As a
workaround, one can add an out-of-domain nameserver to the initial NS
RRset and remove it once bootstrapping is completed. Automation for
this is available via CSYNC records, see [RFC7477].)

Fully qualified signaling names must by valid DNS names. Label count
and length requirements for DNS names (Section 3.1 of [RFC1035])
imply that the protocol does not work for unusually long child domain
names or NS hostnames.

5. Operational Recommendations

5.1. Child DNS Operator

It is possible to add CDS/CDNSKEY records and corresponding signaling
records to a zone without the domain owner's explicit knowledge. To
spare domain owners from being caught off guard by the ensuing DS
changes, child DNS operators following this practice are advised to
make that transparent, such as by informing the domain owner during
zone creation (e.g., in a GUI) or by notifying them via email.

When transferring a zone to another DNS operator, the old and new
child DNS operators need to cooperate to achieve a smooth transition,
e.g., by using the multi-signer protocols described in [RFC8901]. If
all else fails, the domain owner might have to request the removal of
all DS records and have the transfer performed insecurely (see
[INSEC]).

Signaling domains SHOULD be delegated as standalone zones, so that
the signaling zone's apex coincides with the signaling domain (such
as _signal.ns1.example.net). While it is permissible for the
signaling domain to be contained in a signaling zone of fewer labels
(such as example.net), a zone cut ensures that bootstrapping
activities do not require modifications of the zone containing the
nameserver hostname.

Once a child DNS operator determines that specific signaling record
sets have been processed (e.g., by seeing the result in the parent
zone), they are advised to remove them. This will reduce the size of
the signaling zone and facilitate more efficient bulk processing
(such as via zone transfers).

5.2. Parental Agent

In order to ensure timely DNSSEC bootstrapping of insecure domains,
stalemate situations due to mismatch of stale cached records (Step 4
of Section 4.2) need to be avoided. It is thus RECOMMENDED that
queries into signaling domains be performed with an (initially) empty
resolver cache, or that some other method for retrieving fresh data
from authoritative servers be used.

It is also RECOMMENDED that QNAME minimization [RFC9156] be used when
resolving queries for signaling records to guard against certain
attacks (see Section 6).

6. Security Considerations

The DNSSEC bootstrapping method introduced in this document is based
on the approaches described in Section 3 of [RFC8078], but adds
authentication to the CDS/CDNSKEY concept. Its security level is
therefore strictly higher than that of existing approaches described
in that document (e.g., "Accept after Delay"). Apart from this
general improvement, the same Security Considerations apply as in
[RFC8078].

The level of rigor in Section 4.2 is needed to prevent publication of
an ill-conceived DS RRset (authorized only under a subset of NS
hostnames). This ensures, for example, that an operator in a multi-
homed setup cannot enable DNSSEC unless all other operators agree.

In any case, as the child DNS operator has authoritative knowledge of
the child's CDS/CDNSKEY records, it can readily detect fraudulent
provisioning of DS records.

In order to prevent the parents of nameserver hostnames from becoming
a single point of failure for a delegation (both in terms of
resolution availability and for the trust model of this protocol),
diversifying the path from the root to the child's nameserver
hostnames is advisable. For example, different and independently
operated TLDs may be used for each one.

If QNAME minimization [RFC9156] is not used when querying for
signaling records, an upstream parent of a signaling domain will see
those CDS/CDNSKEY queries and could respond with an authoritative
answer signed with its own key, instead of sending the referral.
Enabling QNAME minimization reduces the attack surface for such
forgery.

7. IANA Considerations

IANA has added the following entries to the "Underscored and Globally
Scoped DNS Node Names" registry [RFC8552]:

+=========+============+===========+
| RR Type | _NODE NAME | Reference |
+=========+============+===========+
| CDS | _signal | RFC 9615 |
+---------+------------+-----------+
| CDNSKEY | _signal | RFC 9615 |
+---------+------------+-----------+

Table 1

8. References

8.1. Normative References

[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>.

[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>.

[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>.

[RFC8552] Crocker, D., "Scoped Interpretation of DNS Resource
Records through "Underscored" Naming of Attribute Leaves",
BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,
<https://www.rfc-editor.org/info/rfc8552>.

[RFC9156] Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
Name Minimisation to Improve Privacy", RFC 9156,
DOI 10.17487/RFC9156, November 2021,
<https://www.rfc-editor.org/info/rfc9156>.

[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>.

8.2. Informative References

[BCP237] Best Current Practice 237,
<https://www.rfc-editor.org/info/bcp237>.
At the time of writing, this BCP comprises the following:

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

[INSEC] Hardaker, W., "Intentionally Temporarily Degraded or
Insecure", Work in Progress, Internet-Draft, draft-
hardaker-dnsop-intentionally-temporary-insec-01, 21
October 2021, <https://datatracker.ietf.org/doc/html/
draft-hardaker-dnsop-intentionally-temporary-insec-01>.

[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>.

Acknowledgements

Thanks to Brian Dickson, Ondřej Caletka, John R. Levine, Christian
Elmerot, Oli Schacher, Donald Eastlake, Libor Peltan, Warren Kumari,
Scott Rose, Linda Dunbar, Tim Wicinski, Paul Wouters, Paul Hoffman,
Peter Yee, Benson Muite, Roman Danyliw, Éric Vyncke, and Joe Abley
for reviewing draft proposals and offering comments and suggestions.

Thanks also to Steve Crocker, Hugo Salgado, and Ulrich Wisser for
early-stage brainstorming.

Authors' Addresses

Peter Thomassen
deSEC, Secure Systems Engineering (SSE)
Berlin
Germany
Email: [email protected]

Nils Wisiol
deSEC, Technische Universität Berlin
Berlin
Germany
Email: [email protected]