Internet Engineering Task Force (IETF) O. Gudmundsson

Internet Engineering Task Force (IETF) O. Gudmundsson
Request for Comments: 8078 CloudFlare
Updates: 7344 P. Wouters
Category: Standards Track Red Hat
ISSN: 2070-1721 March 2017

Managing DS Records from the Parent via CDS/CDNSKEY

Abstract

RFC 7344 specifies how DNS trust can be maintained across key
rollovers in-band between parent and child. This document elevates
RFC 7344 from Informational to Standards Track. It also adds a
method for initial trust setup and removal of a secure entry point.

Changing a domain's DNSSEC status can be a complicated matter
involving multiple unrelated parties. Some of these parties, such as
the DNS operator, might not even be known by all the organizations
involved. The inability to disable DNSSEC via in-band signaling is
seen as a problem or liability that prevents some DNSSEC adoption at
a large scale. This document adds a method for in-band signaling of
these DNSSEC status changes.

This document describes reasonable policies to ease deployment of the
initial acceptance of new secure entry points (DS records).

It is preferable that operators collaborate on the transfer or move
of a domain. The best method is to perform a Key Signing Key (KSK)
plus Zone Signing Key (ZSK) rollover. If that is not possible, the
method using an unsigned intermediate state described in this
document can be used to move the domain between two parties. This
leaves the domain temporarily unsigned and vulnerable to DNS
spoofing, but that is preferred over the alternative of validation
failures due to a mismatched DS and DNSKEY record.

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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
http://www.rfc-editor.org/info/rfc8078.

Copyright Notice

Copyright (c) 2017 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
(http://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 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.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Introducing a DS Record . . . . . . . . . . . . . . . . . 3
1.2. Removing a DS Record . . . . . . . . . . . . . . . . . . 4
1.3. Notation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. The Three Uses of CDS . . . . . . . . . . . . . . . . . . . . 5
2.1. The Meaning of the CDS RRset . . . . . . . . . . . . . . 5
3. Enabling DNSSEC via CDS/CDNSKEY . . . . . . . . . . . . . . . 6
3.1. Accept Policy via Authenticated Channel . . . . . . . . . 6
3.2. Accept with Extra Checks . . . . . . . . . . . . . . . . 6
3.3. Accept after Delay . . . . . . . . . . . . . . . . . . . 7
3.4. Accept with Challenge . . . . . . . . . . . . . . . . . . 7
3.5. Accept from Inception . . . . . . . . . . . . . . . . . . 7
4. DNSSEC Delete Algorithm . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Promoting RFC 7344 to Standards Track . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10

1. Introduction

CDS (Child DS) and CDNSKEY (Child DNSKEY) [RFC7344] records are used
to signal changes in secure entry points. This is one method to
maintain delegations that can be used when the DNS operator has no
other way to inform the parent that changes are needed. This
document elevates [RFC7344] from Informational to Standards Track.

In addition, [RFC7344] lacks two different options for full automated
operation to be possible. It does not define a method for the
initial trust establishment, leaving it open to each parent to come
up with an acceptance policy. Additionally, [RFC7344] does not
provide a "delete" signal for the child to inform the parent that the
DNSSEC security for its domain must be removed.

1.1. Introducing a DS Record

Automated insertion of DS records has been limited for many zones by
the requirement that all changes pass through a "Registry" of the
child zone's parent. This has significantly hindered deployment of
DNSSEC at a large scale for DNS hosters, as the child zone owner is
often not aware or able to update DNS records such as the DS record.

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This document describes a few possible methods for the parent to
accept a request by the child to add a DS record to its zone. These
methods have different security properties that address different
deployment scenarios, all resulting in an automated method of trust
introduction.

1.2. Removing a DS Record

This document introduces the delete option for both CDS and CDNSKEY,
allowing a child to signal to the parent to turn off DNSSEC. When a
domain is moved from one DNS operator to another, sometimes it is
necessary to turn off DNSSEC to facilitate the change of DNS
operator. Common scenarios include:

1. Alternative to doing a proper DNSSEC algorithm rollover due to
operational limitations such as software limitations.

2. Moving from a DNSSEC operator to a non-DNSSEC-capable operator.

3. Moving to an operator that cannot or does not want to do a proper
DNSSEC rollover.

4. When moving between two DNS operators that use disjoint sets of
algorithms to sign the zone, an algorithm rollover cannot be
performed.

5. The domain holder no longer wants DNSSEC enabled.

The lack of a "remove my DNSSEC" option is cited as a reason why some
operators cannot deploy DNSSEC, as this is seen as an operational
risk.

Turning off DNSSEC reduces the security of the domain and thus should
only be done carefully, and that decision should be fully under the
child domain's control.

1.3. Notation

Signaling can happen via CDS or CDNSKEY records. The only
differences between the two records are how information is
represented and who calculates the DS digest. For clarity, this
document uses the term "CDS" to mean "either CDS or CDNSKEY".

When this document uses the word "parent", it implies an entity that
is authorized to insert DS records into the parent zone on behalf of
the child domain. Which entity this exactly is does not matter. It

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could be the Registrar or Reseller that the child domain was
purchased from. It could be the Registry that the domain is
registered in when allowed. Or it could be some other entity.

1.4. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2. The Three Uses of CDS

In general, there are three operations that a domain wants to
instruct its parent to perform:

1. Enable DNSSEC validation, i.e., place an initial DS Resource
Record Set (RRset) in the parent.

2. Roll over the KSK. This means updating the DS records in the
parent to reflect the new set of KSKs at the child. This could
be an ADD operation, a DELETE operation on one or more records
while keeping at least one DS RR, or a full REPLACE operation.

3. Turn off DNSSEC validation, i.e., delete all the DS records.

KSK rollover is covered in [RFC7344]. It is considered the safest
use case of a CDS/CDNSKEY record as it makes no change to the trust
relationship between parent and child. Introduction and removal of
DS records are defined in this document. As these CDS/CDNSKEY use
cases create or end the trust relationship between the parent and
child, these use cases should be carefully implemented and monitored.

2.1. The Meaning of the CDS RRset

The semantic meaning of publishing a CDS RRset is interpreted to
mean:

Publishing a CDS or CDNSKEY record signals to the parent that the
child desires that the corresponding DS records be synchronized.
Every parent or parental agent should have an acceptance policy of
these records for the three different use cases involved: Initial
DS publication, Key rollover, and Returning to Insecure.

In short, the CDS RRset is an instruction to the parent to modify the
DS RRset if the CDS and DS Resets differ.

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The acceptance policy for CDS in the rollover case is "seeing"
according to [RFC7344]. The acceptance policy in the Delete case is
seeing a (validly signed) CDS RRset with the delete operation
specified in this document.

3. Enabling DNSSEC via CDS/CDNSKEY

There are number of different models for managing initial trust, but
in the general case, the child wants to enable global validation. As
long as the child is insecure, DNS answers can be forged. The goal
is to promote the child from insecure to secure as soon as reasonably
possible by the parent. This means that the period from the child's
publication of CDS/CDNSKEY RRset to the parent publishing the
synchronized DS RRset should be as short as possible.

One important use case is how a third-party DNS operator can upload
its DNSSEC information to the parent, so the parent can publish a DS
record for the child. In this case, there is a possibility of
setting up some kind of authentication mechanism and submission
mechanism that is outside the scope of this document.

Below are some policies that parents can use. These policies assume
that the notifications can be verified or authenticated.

3.1. Accept Policy via Authenticated Channel

In this case, the parent is notified via authenticated channel UI/API
that a CDS/CDNSKEY RRset exists. In the case of a CDS RRset, the
parent retrieves the CDS RRset and inserts the corresponding DS RRset
as requested. In the case of CDNSKEY, the parent retrieves the
CDNSKEY RRset and calculates the DS record(s). Parents may limit the
DS record type based on local policy. Parents SHOULD NOT refuse CDS/
CDNSKEY updates that do not (yet) have a matching DNSKEY in the child
zone. This will allow the child to pre-publish a spare (and
potentially offline) DNSKEY.

3.2. Accept with Extra Checks

In this case, the parent checks that the source of the notification
is allowed to request the DS insertion. The checks could include
whether this is a trusted entity, whether the nameservers correspond
to the requester, whether there have been any changes in registration
in the last few days, etc. The parent can also send a notification
requesting a confirmation, for example, by sending email to the
registrant requesting a confirmation. The end result is that the CDS
RRset is accepted at the end of the checks or when the out-of-band
confirmation is received. Any extra checks should have proper rate
limiting in place to prevent abuse.

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3.3. Accept after Delay

In this case, if the parent deems the request valid, it starts
monitoring the CDS RRset at the child nameservers over a period of
time to make sure nothing changes. After some time or after a number
of checks, preferably from different vantage points in the network,
the parent accepts the CDS RRset as a valid signal to update its DS
RRset for this child.

3.4. Accept with Challenge

In this case, the parent instructs the requester to insert some
record into the child domain to prove it has the ability to do so
(i.e., it is the operator of the zone). This method imposes a new
task on the parent to monitor the child zone to see if the challenge
has been added to the zone. The parent should verify that the
challenge is published by all the child's nameservers and should test
for this challenge from various diverse network locations to increase
the security of this method as much as possible.

3.5. Accept from Inception

If a parent is adding a new child domain that is not currently
delegated at all, it could use the child CDS/CDNSKEY RRset to
immediately publish a DS RRset along with the new NS RRset. This
would ensure that the new child domain is never active in an insecure
state.

4. DNSSEC Delete Algorithm

This document defines the previously reserved DNS Security Algorithm
Number of value 0 in the context of CDS and CDNSKEY records to mean
that the entire DS RRset at the parent must be removed. The value 0
remains reserved for the DS and DNSKEY records.

No DNSSEC validator can treat algorithm 0 as a valid signature
algorithm. If a validator sees a DNSKEY or DS record with this
algorithm value, it must treat it as unknown. Accordingly, the zone
is treated as unsigned unless there are other algorithms present. In
general, the value 0 should never be used in the context of DNSKEY
and DS records.

The CERT record [RFC4398] defines the value 0 similarly to mean the
algorithm in the CERT record is not defined in DNSSEC.

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The contents of the CDS or CDNSKEY RRset MUST contain one RR and only
contain the exact fields as shown below.

CDS 0 0 0 0

CDNSKEY 0 3 0 0

The keying material payload is represented by a single 0. This
record is signed in the same way as regular CDS/CDNSKEY RRsets are
signed.

Strictly speaking, the CDS record could be "CDS X 0 X 0" as only the
DNSKEY algorithm is what signals the DELETE operation, but for
clarity, the "0 0 0 0" notation is mandated -- this is not a
definition of DS digest algorithm 0. The same argument applies to
"CDNSKEY 0 3 0 0"; the value 3 in the second field is mandated by
[RFC4034], Section 2.1.2.

Once the parent has verified the CDS/CDNSKEY RRset and it has passed
other acceptance tests, the parent MUST remove the DS RRset. After
waiting a sufficient amount of time -- depending on the parental TTLs
-- the child can start the process of turning off DNSSEC.

5. Security Considerations

Turning off DNSSEC reduces the security of the domain and thus should
only be done as a last resort in preventing DNSSEC validation errors
due to mismatched DS and DNSKEY records.

Users should keep in mind that re-establishing trust in delegation
can be hard and takes time. Before deciding to complete the rollover
via an unsigned state, all other options should be considered first.

A parent SHOULD ensure that when it is allowing a child to become
securely delegated, it has a reasonable assurance that the CDS/
CDNSKEY RRset used to bootstrap the security is visible from a
geographically and topologically diverse view. It SHOULD also ensure
that the zone validates correctly if the parent publishes the DS
record. A parent zone might also consider sending an email to its
contact addresses to give the child zone a warning that security will
be enabled after a certain amount of wait time -- thus allowing a
child administrator to cancel the request.

This document describes a few possible acceptance criteria for the
initial trust establishment. Due to a large variety of legal
frameworks surrounding parent domains (Top-Level Domain (TLDs) in
particular), this document cannot give a definitive list of valid
acceptance criteria. Parental zones should look at the listed

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methods and pick the most secure method possible within their legal
and technical scenario, possibly further securing the acceptance
criteria, as long as the deployed method still enables a fully
automated method for non-direct parties such as third-party DNS
hosters.

6. IANA Considerations

IANA has assigned entry number 0 in the "DNS Security Algorithm
Numbers" registry as follows:

+--------+--------------+----------+----------+---------+-----------+
| Number | Description | Mnemonic | Zone | Trans. | Reference |
| | | | Signing | Sec. | |
+--------+--------------+----------+----------+---------+-----------+
| 0 | Delete DS | DELETE | N | N | [RFC4034] |
| | | | | | [RFC4398] |
| | | | | | [RFC8078] |
+--------+--------------+----------+----------+---------+-----------+

6.1. Promoting RFC 7344 to Standards Track

Experience has shown that CDS and CDNSKEY are useful in the
deployment of DNSSEC. [RFC7344] was published as Informational; this
document elevates RFC 7344 to Standards Track.

7. References

7.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.

[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,
<http://www.rfc-editor.org/info/rfc4034>.

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

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7.2. Informative References

[RFC4398] Josefsson, S., "Storing Certificates in the Domain Name
System (DNS)", RFC 4398, DOI 10.17487/RFC4398, March 2006,
<http://www.rfc-editor.org/info/rfc4398>.

Acknowledgments

We thank a number of people that have provided feedback and useful
comments including Bob Harold, John Levine, Dan York, Shane Kerr,
Jacques Latour, and especially Matthijs Mekking.

Authors' Addresses

Olafur Gudmundsson
CloudFlare

Email: [email protected]

Paul Wouters
Red Hat

Email: [email protected]

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