Network Working Group S. Sahib
Internet-Draft Brave Software
Intended status: Best Current Practice S. Huque
Expires: 4 September 2025 Salesforce
P. Wouters
Aiven
E. Nygren
Akamai Technologies
T. Wicinski
Cox Communications
3 March 2025

Domain Control Validation using DNS
draft-ietf-dnsop-domain-verification-techniques-07

Abstract

Many application services on the Internet need to verify ownership or
control of a domain in the Domain Name System (DNS). The general
term for this process is "Domain Control Validation", and can be done
using a variety of methods such as email, HTTP/HTTPS, or the DNS
itself. This document focuses only on DNS-based methods, which
typically involve the Application Service Provider requesting a DNS
record with a specific format and content to be visible in the domain
to be verified. There is wide variation in the details of these
methods today. This document provides some best practices to avoid
known problems.

Discussion Venues

This note is to be removed before publishing as an RFC.

Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-dnsop/draft-ietf-dnsop-domain-
verification-techniques/.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on 4 September 2025.

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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Scope of Validation . . . . . . . . . . . . . . . . . . . . . 4
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. TXT Record based Validation . . . . . . . . . . . . . . . 5
4.1.1. Random Token . . . . . . . . . . . . . . . . . . . . 6
4.1.2. Token Metadata . . . . . . . . . . . . . . . . . . . 6
4.2. Validation Record Owner Name . . . . . . . . . . . . . . 7
4.3. Time-bound checking and Expiration . . . . . . . . . . . 7
4.4. TTL Considerations . . . . . . . . . . . . . . . . . . . 8
5. Delegated Domain Control Validation . . . . . . . . . . . . . 9
6. Supporting Multiple Intermediaries . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7.1. Token Guessing . . . . . . . . . . . . . . . . . . . . . 11
7.2. Service Confusion . . . . . . . . . . . . . . . . . . . . 11
7.3. Service Collision . . . . . . . . . . . . . . . . . . . . 11
7.4. Scope Confusion . . . . . . . . . . . . . . . . . . . . . 11
7.5. Authenticated Channels . . . . . . . . . . . . . . . . . 12
7.6. DNS Spoofing . . . . . . . . . . . . . . . . . . . . . . 12
7.7. DNSSEC Validation . . . . . . . . . . . . . . . . . . . . 12
7.8. Application Usage Enumeration . . . . . . . . . . . . . . 12
7.9. Public Suffixes . . . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13

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9.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Appendix . . . . . . . . . . . . . . . . . . . . . . 15
A.1. Common Pitfalls . . . . . . . . . . . . . . . . . . . . . 16
A.2. Domain Boundaries . . . . . . . . . . . . . . . . . . . . 17
A.3. Interactions with DNAME . . . . . . . . . . . . . . . . . 17
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction

Many Application Service Providers of internet services need domain
owners to prove that they control a particular DNS domain before the
Application Service Provider can operate services for or grant some
privilege to that domain. For instance, Certification Authorities
(CAs) ask requesters of TLS certificates to prove that they operate
the domain they are requesting the certificate for. Application
Service Providers generally allow for several different ways of
proving control of a domain. In practice, DNS-based methods take the
form of the Application Service Provider generating a random token
and asking the requester to create a DNS record containing this
random token and placing it at a location within the domain that the
Application Service Provider can query for. Generally only one time-
bound DNS record is sufficient for proving domain ownership.

This document recommends using TXT based domain control validation in
a way that is time-bounded and targeted to the specific application
service.

2. Conventions and Definitions

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.

* Application Service Provider: an internet-based provider of a
service, for e.g., a Certification Authority or a service that
allows for user-controlled websites. These services often require
a User to verify that they control a domain. The Application
Service Provider may be implementing a standard protocol for
domain validation (such as [RFC8555]) or they may have their own
specification.

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* Intermediary: an internet-based service that leverages the
services of other providers on behalf of a User. For example, an
Intermediary might be a service that allows for User-controlled
websites and in-turn needs to use a Certification Authority
provider to get TLS certificates for the User on behalf of the
website.

* Validation Record: the DNS record that is used to prove ownership
of a domain name ([RFC9499]). It typically contains an
unguessable value generated by the Application Service Provider
which serves as a challenge. The Application Service Provider
looks for the Validation Record in the zone of the domain name
being verified and checks if it contains the unguessable value.

* DNS Administrator: the owner or responsible party for the contents
of a domain in the DNS.

* User: the owner or operator of a domain in the DNS who needs to
prove ownership of that domain to an Application Service Provider,
working in coordination with their DNS Administrator.

* Random Token: a random value that uniquely identifies the DNS
domain control validation challenge, defined in Section 4.1.1.

3. Scope of Validation

For security reasons, it is crucial to understand the scope of the
domain name being validated. Both Application Service Providers and
the User need to clearly specify and understand whether the
validation request is for a single hostname, a wildcard (all
hostnames immediately under that domain), or for the entire domain
and subdomains rooted at that name. This is particularly important
in large multi-tenant enterprises, where an individual deployer of a
service may not necessarily have operational authority of an entire
domain.

In the case of X.509 certificate issuance, the certificate signing
request and associated challenge are clear about whether they are for
a single host or a wildcard domain. Unfortunately, the ACME
protocol's DNS-01 challenge mechanism ([RFC8555], Section 8.4) does
not differentiate these cases in the DNS Validation Record. In the
absence of this distinction, the DNS administrator tasked with
deploying the Validation Record may need to explicitly confirm the
details of the certificate issuance request to make sure the
certificate is not given broader authority than the User intended.

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In the more general case of an Internet application service granting
authority to a domain owner, again no existing DNS challenge scheme
makes this distinction today. New applications should consider
having different application names for different scopes, as
described. Regardless, services should very clearly indicate the
scope of the validation in their public documentation so that the
domain administrator can use this information to assess whether the
Validation Record is granting the appropriately scoped authority.

4. Recommendations

All Domain Control Validation mechanisms are implemented by a DNS
resource record with at least the following information:

1. A record name related to the domain name being validated, usually
constructed by prepending an application specific label.

2. One or more random tokens.

4.1. TXT Record based Validation

The RECOMMENDED method of doing DNS-based domain control validation
is to use DNS TXT records as the Validation Record. The name is
constructed as described in Section 4.2, and RDATA MUST contain at
least a Random Token (constructed as in Section 4.1.1). If there are
multiple RDATA strings for a record, the Application Service Provider
MUST treat them as a concatenated string. If metadata (see
Section 4.1.2) is not used, then the unique token generated as-above
can be placed as the only contents of the RDATA. For example:

_service-challenge.example.com. IN TXT "3419...3d206c4"

This again allows the Application Service Provider to query only for
application-specific records it needs, while giving flexibility to
the User adding the DNS record (i.e., they can be given permission to
only add records under a specific prefix by the DNS administrator).

Application Service Providers MUST validate that a random token in
the TXT record matches the one that they gave to the User for that
specific domain name. Whether or not multiple Validation Records can
exist for the same domain is up to the Application Service Provider's
application specification. In case there are multiple TXT records
for the specific domain name, the Application Service Provider MUST
confirm at least one record matches.

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4.1.1. Random Token

A unique token should be used in the challenge. It should be a
random value issued between parties (Application Service Provider to
User, Application Service Provider to Intermediary, or Intermediary
to User) with the following properties:

1. MUST have at least 128 bits of entropy.

2. base64url ([RFC4648], Section 5) encoded, base32 ([RFC4648],
Section 6) encoded, or base16 ([RFC4648], Section 8) encoded.

See [RFC4086] for additional information on randomness requirements.

Base32 encoding or hexadecimal base16 encoding are RECOMMENDED to be
specified when the random token would exist in a DNS label such as in
a CNAME target. This is because base64 relies on mixed case (and DNS
is case-insensitive as clarified in [RFC4343]) and because some
base64 characters ("/", "+", and "=") may not be permitted by
implementations that limit allowed characters to those allowed in
hostnames. If base32 is used, it SHOULD be specified in way that
safely omits the trailing padding ("="). Note that DNS labels are
limited to 63 octets which limits how large such a token may be.

This random token is placed in either the RDATA or an owner name, as
described in the rest of this section. Some methods of validation
may involve multiple independent random tokens.

4.1.2. Token Metadata

It may be desirable to associate metadata with the token in a
Validation Record. When specified, metadata SHOULD be encoded in the
RDATA via space-separated ASCII key-value pairs [RFC1464], with the
key "token" prefixing the random token. For example:

_service-challenge.example.com. IN TXT "token=3419...3d206c4"

If there are multiple tokens required, each one MUST be in a separate
RR to allow them to match up with any additional attributes. For
example:

_service-challenge.example.com. IN TXT "token=3419...3d206c4 attr=bar"
IN TXT "token=5454...45dc45a attr=quux"

The token MUST be the first element in the key-value list. If the
TXT record RDATA is not prefixed with token= then [RFC1464] encoding
MUST NOT be assumed (as this might split the trailing "==" or "=" at
the end of base64 encoding).

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If an alternate syntax is used by the Application Service Provider
for token metadata, they MUST specify a grammar for it.

4.2. Validation Record Owner Name

The RECOMMENDED format for a Validation Record's owner name is
application-specific underscore prefix labels. Domain Control
Validation Records are constructed by the Application Service
Provider by prepending the label "_<PROVIDER_RELEVANT_NAME>-
challenge" to the domain name being validated (e.g. "_service-
challenge.example.com"). The prefix "_" is used to avoid collisions
with existing hostnames and to prevent the owner name from being a
valid hostname.

If an Application Service Provider has an application-specific need
to have multiple validations for the same label, multiple prefixes
can be used, such as "_<FEATURE>._<PROVIDER_RELEVANT_NAME>-
challenge".

Application owners SHOULD utilize the IANA "Underscored and Globally
Scoped DNS Node Names" registry [UNDERSCORE-REGISTRY] and avoid using
underscore labels that already exist in the registry.

As a simplification, some applications may decide to omit the
"-challenge" suffix and use just "_<PROVIDER_RELEVANT_NAME>" as the
label.

4.3. Time-bound checking and Expiration

After domain control validation is completed, there is typically no
need for the TXT or CNAME record to continue to exist as the presence
of the domain validation DNS record for a service only implies that a
User with access to the service also has DNS control of the domain at
the time the code was generated. It should be safe to remove the
validation DNS record once the validation is done and the Application
Service Provider doing the validation should specify how long the
validation will take (i.e., after how much time can the validation
DNS record be deleted).

Some Application Service Providers currently require the Validation
Record to remain in the zone indefinitely for periodic revalidation
purposes. This practice should be discouraged. Subsequent
validation actions using an already disclosed token are no guarantee
that the original owner is still in control of the domain, and a new
challenge needs to be issued.

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One exception is if the record is being used as part of a delegated
domain control validation setup (Section 5); in that case, the CNAME
record that points to the actual validation TXT record cannot be
removed as long as the User is still relying on the Intermediary.

Application Service Providers MUST provide clear instructions on how
long the challenge token is valid for, and thus when a Validation
Record can be removed.

These instructions MAY be encoded in the RDATA as token metadata
(Section 4.1.2 using the key "expiry" to hold a time after which it
is safe to remove the Validation Record. For example:

_service-challenge.example.com. IN TXT "token=3419...3d206c4 expiry=2023-02-08T02:03:19+00:00"

When an expiry time is specified, the value of "expiry" SHALL be in
ISO 8601 format as specified in [RFC3339], Section 5.6.

A simpler variation of the expiry time is also ISO 8601 valid and can
also be specified, using the "full-date" format. For example:

_service-challenge.example.com. IN TXT "token=3419...3d206c4 expiry=2023-02-08"

Alternatively, if the record should never expire (for instance, if it
may be checked periodically by the Application Service Provider) and
should not be removed, the key "expiry" SHALL be set to have value
"never".

_service-challenge.example.com. IN TXT "token=3419...3d206c4 expiry=never"

The "expiry" key MAY be omitted in cases where the Application
Service Provider has clarified the record expiry policy out-of-band.

_service-challenge.example.com. IN TXT "token=3419...3d206c4"

Note that this is semantically the same as:

_service-challenge.example.com. IN TXT "3419...3d206c4"

The User SHOULD de-provision the resource record provisioned for DNS-
based domain control validation once it is no longer required.

4.4. TTL Considerations

The TTL [RFC1034] for Validation Records SHOULD be short to allow
recovering from potential misconfigurations. These records will not
be polled frequently so caching or resolver load will not be an
issue.

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The Application Service Provider looking up a Validation Record may
have to wait for up to the SOA minimum TTL (negative caching TTL) of
the enclosing zone for the record to become visible, if it has been
previously queried. If the application User wants to make the
Validation Record visible more quickly they may need to work with the
DNS administrator to see if they are willing to lower the SOA minimum
TTL (which has implications across the entire zone).

Application Service Providers' verifiers MAY wish to use dedicated
DNS resolvers configured with a low maximum negative caching TTL,
flush Validation Records from resolver caches prior to issuing
queries or just directly query authoritative name servers to avoid
caching.

5. Delegated Domain Control Validation

Delegated domain control validation lets a User delegate the domain
control validation process for their domain to an Intermediary
without granting the Intermediary the ability to make changes to
their domain or zone configuration. It is a variation of TXT record
validation (Section 4.1) that indirectly inserts a CNAME record prior
to the TXT record.

The Intermediary gives the User a CNAME record to add for the domain
and Application Service Provider being validated that points to the
Intermediary's domain, where the actual validation TXT record is
placed. The record name and base16-encoded (or base32-encoded)
random tokens are generated as in Section 4.1.1. For example:

_service-challenge.example.com. IN CNAME <intermediary-random-token>.dcv.intermediary.example.

The Intermediary then adds the actual Validation Record in a domain
they control:

<intermediary-random-token>.dcv.intermediary.example. IN TXT "<provider-random-token>"

Such a setup is especially useful when the Application Service
Provider wants to periodically re-issue the challenge with a new
provider random token. CNAMEs allow automating the renewal process
by letting the Intermediary place the random token in their DNS zone
instead of needing continuous write access to the User's DNS.

Importantly, the CNAME record target also contains a random token
issued by the Intermediary to the User (preferably over a secure
channel) which proves to the Intermediary that example.com is
controlled by the User. The Intermediary must keep an association of
Users and domain names to the associated Intermediary-random-tokens.
Without a linkage validated by the Intermediary during provisioning

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and renewal there is the risk that an attacker could leverage a
"dangling CNAME" to perform a "subdomain takeover" attack
([SUBDOMAIN-TAKEOVER]).

When a User stops using the Intermediary they should remove the
domain control validation CNAME in addition to any other records they
have associated with the Intermediary.

6. Supporting Multiple Intermediaries

There are use-cases where a User may wish to simultaneously use
multiple intermediaries or multiple independent accounts with an
Application Service Provider. For example, a hostname may be using a
"multi-CDN" where the hostname simultaneously uses multiple Content
Delivery Network (CDN) providers.

To support this, Application Service Providers may support prefixing
the challenge with a label containing an unique account identifier of
the form _<identifier-token>. The identifier token is encoded in
base32 or base16, and if the identifier is sensitive in nature, it
should be run through a truncated hashing algorithm first. The
identifier token should be stable over time and would be provided to
the User by the Application Service Provider, or by an Intermediary
in the case where domain validation is delegated (Section 5).

The resulting record could either directly contain a TXT record or a
CNAME (as in Section 5). For example:

_<identifier-token>._service-challenge.example.com. IN TXT "3419...3d206c4"

_<identifier-token>._service-challenge.example.com. IN CNAME <intermediary-random-token>.dcv.intermediary.example.

When performing validation, the Application Service Provider would
resolve the DNS name containing the appropriate identifier token.

The ACME protocol has incorporated this method to specify DNS account
specific challenages in [ACME-DNS-ACCOUNT-ID].

Application Service Providers may wish to always prepend the
_<identifier-token> to make it harder for third parties to scan, even
absent supporting multiple intermediaries. The _<identifier-token>
MUST start with an underscore so as to not be a valid hostname.

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7. Security Considerations

7.1. Token Guessing

If token values aren't long enough or lack adequate entropy there's a
risk that a malicious actor could produce a token that could be
confused with an application-specific underscore prefix label.

7.2. Service Confusion

A malicious Application Service Provider that promises to deliver
something after domain control validation could surreptitiously ask
another Application Service Provider to start processing or sending
mail for the target domain and then present the victim User with this
DNS TXT record pretending to be for their service. Once the User has
added the DNS TXT record, instead of getting their service, their
domain is now certifying another service of which they are not aware
they are now a consumer. If services use a clear description and
name attribution in the required DNS TXT record, this can be avoided.
For example, by requiring a DNS TXT record at _vendorname.example.com
instead of at example.com, a malicious service could no longer
forward a challenge from a different service without the User
noticing. Both the Application Service Provider and the service
being authenticated and authorized should be unambiguous from the
Validation Record to prevent malicious services from misleading the
domain owner into certifying a different provider or service.

7.3. Service Collision

As a corollary to Section 7.2, if the Validation Record is not well-
scoped and unambiguous with respect to the Application Service
Provider, it could be used to authorize use of another Application
Service Provider or service in addition to the original Application
Service Provider or service.

7.4. Scope Confusion

Ambiguity of scope introduces risks, as described in Section 3.
Distinguishing the scope in the application-specific label, along
with good documentation, should help make it clear to DNS
administrators whether the record applies to a single hostname, a
wildcard, or an entire domain. Always using this indication rather
than having a default scope reduces ambiguity, especially for
protocols that may have used a shared application-specific label for
different scopes in the past. While it would also have been possible
to include the scope in as an attribute in the TXT record, that has
more potential for ambiguity and misleading an operator, such as if
an implementation ignores attribute it doesn't recognize but an

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attacker includes the attribute to mislead the DNS administrator.

7.5. Authenticated Channels

Application Service Providers and intermediaries should use
authenticated channels to convey instructions and random tokens to
Users. Otherwise, an attacker in the middle could alter the
instructions, potentially allowing the attacker to provision the
service instead of the User.

7.6. DNS Spoofing

A domain owner SHOULD sign their DNS zone using DNSSEC [RFC9364] to
protect Validation Records against DNS spoofing attacks.

7.7. DNSSEC Validation

DNSSEC validation SHOULD be performed by Application Service
Providers that verify Validation Records they have requested to be
deployed. If no DNSSEC support is detected for the domain being
validated, or if DNSSEC validation cannot be performed, Application
Service Providers SHOULD attempt to query and confirm the Validation
Record by matching responses from multiple DNS resolvers on
unpredictable geographically diverse IP addresses to reduce an
attacker's ability to complete a challenge by spoofing DNS.
Alternatively, Application Service Providers MAY perform multiple
queries spread out over a longer time period to reduce the chance of
receiving spoofed DNS answers.

7.8. Application Usage Enumeration

The presence of a Validation Record with a predictable domain name
(either as a TXT record for the exact domain name where control is
being validated or with a well-known label) can allow attackers to
enumerate the utilized set of Application Service Providers.

7.9. Public Suffixes

As discussed above in Appendix A.2, there are risks in allowing
control to be demonstrated over domains which are "public suffixes"
(such as ".co.uk" or ".com"). The volunteer-managed Public Suffix
List ([PSL]) is one mechanism that can be used. It includes two
"divisions" ([PSL-DIVISIONS]) covering both registry-owned public
suffixes (the "ICANN" division) and a "PRIVATE" division covering
domains submitted by the domain owner.

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Operators of domains which are in the "PRIVATE" public suffix
division often provide multi-tenant services such as dynamic DNS, web
hosting, and CDN services. As such, they sometimes allow their sub-
tenants to provision names as subdomains of their public suffix.
There are use-cases that require operators of domains in the public
suffix list to demonstrate control over their domain, such as to be
added to the Public Suffix List, or to provision a wildcard
certificate. At the same time, if an operator of such a domain
allows its customers or tenants to create names starting with an
underscore ("_") then it opens up substantial risk to the domain
operator for attackers to provision services on their domain.

Whether or not it is appropriate to allow domain verification on a
public suffix will depend on the application. In the general case:

* Application Service Providers SHOULD NOT allow verification of
ownership for domains which are public suffixes in the "ICANN"
division. For example, "_service-challenge.co.uk" would not be
allowed.

* Application Service Providers MAY allow verification of ownership
for domains which are public suffixes in the "PRIVATE" division,
although it would be preferable to apply additional safety checks
in this case.

8. IANA Considerations

This document has no IANA actions.

9. References

9.1. Normative References

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

[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/rfc/rfc2119>.

[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/rfc/rfc3339>.

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[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.

[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/rfc/rfc8174>.

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

9.2. Informative References

[ACME-DNS-ACCOUNT-ID]
Chariton, A. A., Omidi, A. A., Kasten, J., Loukos, F., and
S. A. Janikowski, "ACME DNS Labeled with Account ID
Challenge", 2024, <https://datatracker.ietf.org/doc/draft-
ietf-acme-dns-account-label/>.

[AVOID-FRAGMENTATION]
Fujiwara, K. and P. Vixie, "Fragmentation Avoidance in
DNS", 2023, <https://datatracker.ietf.org/doc/draft-ietf-
dnsop-avoid-fragmentation/>.

[I-D.draft-tjw-dbound2-problem-statement]
Wicinski, T., "Domain Boundaries 2.0 Problem Statement",
Work in Progress, Internet-Draft, draft-tjw-dbound2-
problem-statement-01, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-tjw-dbound2-
problem-statement-01>.

[PSL] Mozilla Foundation, "Public Suffix List", 2022,
<https://publicsuffix.org/>.

[PSL-DIVISIONS]
Frakes, J., "Public Suffix List format", 2022,
<https://github.com/publicsuffix/list/wiki/
Format#divisions>.

[RFC1464] Rosenbaum, R., "Using the Domain Name System To Store
Arbitrary String Attributes", RFC 1464,
DOI 10.17487/RFC1464, May 1993,
<https://www.rfc-editor.org/rfc/rfc1464>.

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[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.

[RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<https://www.rfc-editor.org/rfc/rfc4343>.

[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
<https://www.rfc-editor.org/rfc/rfc6672>.

[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/rfc/rfc8555>.

[RFC9210] Kristoff, J. and D. Wessels, "DNS Transport over TCP -
Operational Requirements", BCP 235, RFC 9210,
DOI 10.17487/RFC9210, March 2022,
<https://www.rfc-editor.org/rfc/rfc9210>.

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

[SUBDOMAIN-TAKEOVER]
Mozilla, "Subdomain takeovers", n.d.,
<https://developer.mozilla.org/en-US/docs/Web/Security/
Subdomain_takeovers>.

[UNDERSCORE-REGISTRY]
IANA, "Underscored and Globally Scoped DNS Node Name",
n.d., <https://www.iana.org/assignments/dns-parameters/
dns-parameters.xhtml#underscored-globally-scoped-dns-node-
names>.

Appendix A. Appendix

Placeholder for things to put into appendix.

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A.1. Common Pitfalls

A very common but unfortunate technique in use today is to employ a
DNS TXT record and placing it at the exact domain name whose control
is being validated (e.g., often the zone apex). This has a number of
known operational issues. If the User has multiple application
services employing this technique, it will end up with multiple DNS
TXT records having the same owner name; one record for each of the
services.

Since DNS resource record sets are treated atomically, a query for
the Validation Record will return all TXT records in the response.
There is no way for the verifier to specifically query only the TXT
record that is pertinent to their application service. The verifier
must obtain the aggregate response and search through it to find the
specific record it is interested in.

Additionally, placing many such TXT records at the same name
increases the size of the DNS response. If the size of the UDP
response (UDP being the most common DNS transport today) is large
enough that it does not fit into the Path MTU of the network path,
this may result in IP fragmentation, which can be unreliable due to
firewalls and middleboxes is vulnerable to various attacks
([AVOID-FRAGMENTATION]). Depending on message size limits configured
or being negotiated, it may alternatively cause the DNS server to
"truncate" the UDP response and force the DNS client to re-try the
query over TCP in order to get the full response. Not all networks
properly transport DNS over TCP and some DNS software mistakenly
believe TCP support is optional ([RFC9210]).

Other possible issues may occur. If a TXT record (or any other
record type) is designed to be placed at the same domain name that is
being validated, it may not be possible to do so if that name already
has a CNAME record. This is because CNAME records cannot co-exist
with other (non-DNSSEC) records at the same name. This situation
cannot occur at the apex of a DNS zone, but can at a name deeper
within the zone.

When multiple distinct services specify placing Validation Records at
the same owner name, there is no way to delegate an application
specific domain Validation Record to a third party. Furthermore,
even without delegation, an organization may have a shared DNS zone
where they need to provide record level permissions to the specific
division within the organization that is responsible for the
application in question. This can't be done if all applications
expect to find validation records at the same name.

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A.2. Domain Boundaries

The hierarchical structure of domain names do not necessarily define
boundaries of ownership and administrative control (e.g., as
discussed in [I-D.draft-tjw-dbound2-problem-statement]). Some domain
names are "public suffixes" ([RFC9499]) where care may need to be
taken when validating control. For example, there are security risks
if an Application Service Provider can be tricked into believing that
an attacker has control over ".co.uk" or ".com". The volunteer-
managed Public Suffix List [PSL] is one mechanism available today
that can be useful for identifying public suffixes.

Future specifications may provide better mechanisms or
recommendations for defining domain boundaries or for enabling
organizational administrators to place constraints on domains and
subdomains.

A.3. Interactions with DNAME

Domain control validation in the presence of a DNAME [RFC6672] is
possible with caveats. Since a DNAME record redirects the entire
subtree of names underneath the owner of the DNAME, it is not
possible to place a Validation Record under the DNAME owner itself.
It would have to be placed under the DNAME target name, since any
lookups for a name under the DNAME owner will be redirected to the
corresponding name under the DNAME target.

Appendix B. Acknowledgments

Thank you to Tim Wicinski, John Levine, Daniel Kahn Gillmor, Amir
Omidi, Tuomo Soini, Ben Kaduk and many others for their feedback and
suggestions on this document.

Authors' Addresses

Shivan Sahib
Brave Software
Email: shivankaulsahib@gmail.com

Shumon Huque
Salesforce
Email: shuque@gmail.com

Paul Wouters
Aiven
Email: paul.wouters@aiven.io

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Erik Nygren
Akamai Technologies
Email: erik+ietf@nygren.org

Tim Wicinski
Cox Communications
Email: tjw.ietf@gmail.com

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