Internet DRAFT - draft-ietf-dnsop-domain-verification-techniques
draft-ietf-dnsop-domain-verification-techniques
Network Working Group S. Sahib
Internet-Draft Brave Software
Intended status: Best Current Practice S. Huque
Expires: 4 September 2024 Salesforce
P. Wouters
Aiven
E. Nygren
Akamai Technologies
3 March 2024
Domain Control Validation using DNS
draft-ietf-dnsop-domain-verification-techniques-04
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
requester's domain. There is wide variation in the details of these
methods today. This document proposes 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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This Internet-Draft will expire on 4 September 2024.
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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Common Pitfalls . . . . . . . . . . . . . . . . . . . . . . . 4
4. Scope of Validation . . . . . . . . . . . . . . . . . . . . . 5
4.1. Domain Boundaries . . . . . . . . . . . . . . . . . . . . 6
5. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Validation Record Format . . . . . . . . . . . . . . . . 6
5.1.1. Name . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1.2. Scope Indication . . . . . . . . . . . . . . . . . . 7
5.1.3. Random Token . . . . . . . . . . . . . . . . . . . . 7
5.2. TXT Record . . . . . . . . . . . . . . . . . . . . . . . 8
5.2.1. Metadata For Expiry . . . . . . . . . . . . . . . . . 9
5.3. CNAME Records . . . . . . . . . . . . . . . . . . . . . . 9
5.3.1. CNAME Records for Domain Control Validation . . . . . 10
5.3.2. Delegated Domain Control Validation . . . . . . . . . 10
5.3.3. Domain Control Validation Supporting Multiple
Intermediaries . . . . . . . . . . . . . . . . . . . 11
5.4. Time-bound checking . . . . . . . . . . . . . . . . . . . 12
5.5. DNAME . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6.1. Public Suffixes . . . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 15
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Appendix A. Appendix . . . . . . . . . . . . . . . . . . . . . . 17
A.1. Survey of Techniques . . . . . . . . . . . . . . . . . . 17
A.1.1. TXT based . . . . . . . . . . . . . . . . . . . . . . 17
A.1.2. CNAME based . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Many providers of internet services need domain owners to prove that
they control a particular DNS domain before the 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. Providers generally allow for several different
ways of proving control of a domain. In practice, DNS-based methods
take the form of the 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 provider can
query for. Generally only one temporary DNS record is sufficient for
proving domain ownership, although sometimes the DNS record must be
kept in the zone to prove continued ownership of the domain.
This document describes pitfalls associated with some common
practices using DNS-based techniques deployed today, and recommends
using TXT based domain control validation in a way that is time-bound
and targeted to the service. The Appendix A includes a more detailed
survey of different methods used by a set of application service
providers.
Other techniques such as email or HTTP(S) based validation are out-
of-scope.
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.
* Validation record: the DNS record that is used to prove ownership
of a domain name ([RFC8499]). It typically contains an
unguessable value generated by the provider which serves as a
challenge. The provider looks for the validation record in the
zone of the domain name being verified and checks if it contains
the unguessable value.
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* 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 provider may be
implementing a standard protocol for domain validation (such as
[RFC8555]) or they may have their own specification.
* 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.
* User: the owner or operator of a domain in the DNS who needs to
prove ownership of that domain to a provider.
* Random Token: a random value that uniquely identifies the DNS
domain control validation challenge, defined in Section 5.1.3.
3. 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. This has a number of known operational issues.
If the domain owner uses multiple application services using this
technique, it will end up deploying a DNS TXT record "set" at the
domain name, containing one TXT 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.
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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 often does not work
reliably on the Internet today due to firewalls and middleboxes, and
also 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 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 create domain validation records at
the same domain 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
share the domain name.
This specification proposes the use of application-specific labels in
the domain validation record to address these issues.
4. 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 domain owner 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
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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 domain owner
intended.
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
below in Section 5.1.2. 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.
4.1. 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" ([RFC8499]) where care may need to be
taken when validating control. For example, there are security risks
if a 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. See Appendix A.1.2.4 for cases where DNS records can be
used as constraints complementary to domain verification.
5. Recommendations
5.1. Validation Record Format
5.1.1. Name
The RECOMMENDED format is application-specific underscore prefix
labels. Domain Control Validation records are constructed by the
provider by prepending the label "_<PROVIDER_RELEVANT_NAME>-
challenge" to the domain name being validated (e.g. "_foo-
challenge.example.com"). The prefixed "_" is used to avoid
collisions with existing hostnames.
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5.1.2. Scope Indication
For applications that may apply more broadly than to a single host
name, the RECOMMENDED approach is to differentiate the application-
specific underscore prefix labels to also include the scope (see
#scope). In particular:
* "_<PROVIDER_RELEVANT_NAME>-host-challenge.example.com" applies
only to the specific host name of "example.com" and not to
anything underneath it.
* "_<PROVIDER_RELEVANT_NAME>-wildcard-challenge.example.com" applies
to all host names at the level immediately underneath
"example.com". For example, it would apply to "foo.example.com"
but not "example.com" nor "quux.bar.example.com"
* "_<PROVIDER_RELEVANT_NAME>-domain-challenge.example.com" applies
to the entire domain "example.com" as well as its subdomains. For
example, it would apply to all of "example.com",
"foo.example.com", and "quux.bar.example.com"
The application provider will normally know which of these scoped DNS
records to query based on the user's requested configuration. So
this does not typically result in multiple queries for different
possible scopes. If discovery of scope is needed for a specific
application as part of the domain control validation process, then
the scope could alternatively be encoded in a key value pair in the
record data.
Note that a proposed update to the ACME DNS challenge specification
[ACME-SCOPED-CHALLENGE] has incorporated this scope indication
format.
5.1.3. Random Token
A unique token used in the challenge. It should be a random value
issued between parties (provider to user, 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.
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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 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 the RDATA as described in the rest of
this section.
5.2. TXT Record
The RECOMMENDED method of doing DNS-based domain control validation
is to use DNS TXT records. The name is constructed as described in
Section 5.1.1, and RDATA MUST contain at least a Random Token
(constructed as in Section 5.1.3). If metadata (see Section 5.2.1)
is not used, then the unique token generated as-above can be placed
as the only contents of the RDATA. For example:
_foo-challenge.example.com. IN TXT "3419...3d206c4"
If a provider has an application-specific need to have multiple
validations for the same label, multiple prefixes can be used:
_feature1._foo-challenge.example.com. IN TXT "3419...3d206c4"
This again allows the 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). Whether or not multiple
validation records can exist for the same domain is up to the
provider's application specification.
Consumers of the provider services need to relay information from a
provider's website or APIs to their local DNS administrators. The
exact DNS record type, content and location is often not clear when
the DNS administrator receives the information, especially to
consumers who are not DNS experts. Providers SHOULD offer detailed
help pages, that are accessible without needing a login on the
provider website, as the DNS administrator often has no login account
on the provider service website. Similarly, for clarity, the exact
and full DNS record (including a Fully Qualified Domain Name) to be
added SHOULD be provided along with help instructions.
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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.
5.2.1. Metadata For Expiry
Providers MUST provide clear instructions on when a validation record
can be removed. These instructions SHOULD be encoded in the RDATA
via comma-separated ASCII key-value pairs [RFC1464], using the key
"expiry" to hold a time after which it is safe to remove the
validation record. If this key-value format is used, the
verification token should use the key "token". For example:
_foo-challenge.example.com. IN TXT "token=3419...3d206c4,expiry=2023-02-08T02:03:19+00:00"
When a expiry time is specified, the value of "expiry" SHALL be in
ISO 8601 format as specified in [RFC3339], Section 5.6.
Alternatively, if the record should never expire (for instance, if it
may be checked periodically by the provider) and should not be
removed, the key "expiry" SHALL be set to have value "never".
_foo-challenge.example.com. IN TXT "token=3419...3d206c4,expiry=never"
The "expiry" key MAY be omitted in cases where the provider has
clarified the record expiry policy out-of-band (Appendix A.1.1.3).
_foo-challenge.example.com. IN TXT "token=3419...3d206c4"
Note that this is semantically the same as:
_foo-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.
5.3. CNAME Records
CNAME records MAY be used instead of TXT records, either for
Delegated Domain Control Validation (Section 5.3.2) or where
specified by providers to support users who are unable to create TXT
records.
A provider supporting CNAME records MUST specify the use of an
underscore-prefixed label (e.g., _foo-<token> or even the less-
recommended _<token>) as a CNAME MUST NOT be placed at the same
domain name that is being validated. This is for the same reason
already cited in Section 3. CNAME records cannot co-exist with other
data, and there may already be other record types that exist at the
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domain name. Instead, as with the TXT record recommendation, a
provider specific label should be added as a subdomain of the domain
to be verified. This ensures that the CNAME does not collide with
other record types.
In practice, many providers that employ CNAMEs for domain control
validation today use a random subdomain label, which also works to
avoid collisions. But adding an provider-specific component in
addition (such as _foo-<RANDOM>-challenge) would make it easier for
the domain owner to keep track of why and for what service a
validation record has been deployed.
Note that some DNS implementations permit the deployment of CNAME
records co-existing with other record types. These implementations
are in violation of the DNS protocol. Furthermore, they can cause
resolution failures in unpredictable ways depending on the behavior
of DNS resolvers, the order in which query types for the name are
processed etc. In short, they cannot work reliably and these
implementations should be fixed.
5.3.1. CNAME Records for Domain Control Validation
A provider may specify using CNAME records instead of TXT records for
Domain Control Validation. In this case, the target of the CNAME
would contain the base16-encoded (or base32-encoded) random token
followed by a suffix specified by the provider. For example:
_foo-challenge.example.com. IN CNAME <random-token>.dcv.provider.example.
5.3.2. Delegated Domain Control Validation
Separately, CNAME records also enable delegated domain control
validation, which lets the user delegate the domain control
validation process for their domain to an intermediary without having
to hand over full DNS access. The intermediary gives the user a
CNAME record to add for the domain and provider being validated that
points to the intermediary's DNS, where the actual validation TXT
record is placed. The record name and base16-encoded (or
base32-encoded) random tokens are generated as in Section 5.1. For
example:
_foo-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. TXT "<provider-random-token>"
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Such a setup is especially useful when the provider wants to
periodically re-issue the challenge. CNAMEs allow automating the
renewal process by letting the intermediary place the random token in
their DNS 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
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.
See Appendix A.1.2.2 for examples.
5.3.3. Domain Control Validation Supporting Multiple Intermediaries
There are use-cases where a user may wish to simultaneously use
multiple intermediaries or multiple independent accounts with a
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, providers may support prefixing the challenge with a
label containing an unique account identifier of the form
_<identifier-token> and following the requirements of Section 5.1.3,
specified as either base32 or base16 encoded. This identifier token
should be stable over time and would be provided to the user by the
provider, or by an intermediary in the case where domain validation
is delegated (Section 5.3.2).
The resulting record could either directly contain a TXT record or a
CNAME (as in Section 5.3.2). For example:
_<identifier-token>._foo-challenge.example.com. IN TXT "3419...3d206c4"
or
_<identifier-token>._foo-challenge.example.com. IN CNAME "<intermediary-random-token>.dcv.intermediary.example."
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When performing validation, the provider would resolve the DNS name
containing the appropriate identifier token.
5.4. Time-bound checking
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 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).
One exception is if the record is being used as part of a delegated
domain control validation setup (Section 5.3.2); 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.
5.5. DNAME
Domain control validation in the presence of a DNAME [RFC6672] is
theoretically possible. 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.
6. Security Considerations
A malicious service that promises to deliver something after domain
control validation could surreptitiously ask another service provider
to start processing or sending mail for the target domain and then
present the victim domain administrator with this DNS TXT record
pretending to be for their service. Once the administrator 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 replay
this without the DNS administrator noticing this. Both the provider
and the service being authenticated and authorized should be
unambiguous from the TXT record owner name and RDATA content to
prevent malicious services from misleading the domain owner into
certifying a different provider or service.
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Amiguity of scope introduces risks, as described in Section 4.
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 host name, 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
attacker includes the attribute to mislead the DNS administrator.
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.
A domain owner SHOULD sign their DNS zone using DNSSEC [RFC9364] to
protect validation records against DNS spoofing attacks.
DNSSEC validation SHOULD be performed by service providers that
verify validation records they have requested to be deployed. If no
DNSSEC support is detected for the domain owner zone, or if DNSSEC
validation cannot be performed, 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, service providers MAY perform multiple
queries spread out over a longer time period to reduce the chance of
receiving spoofed DNS answers.
6.1. Public Suffixes
As discussed above in Section 4.1, 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.
Operators of public suffix domains which are in the "PRIVATE"
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 public suffix domains
to demonstrate control over their domain, such as to be added to the
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Public Suffix List (Appendix A.1.1.4) 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:
* Providers SHOULD NOT allow verification of ownership for domains
which are public suffixes in the "ICANN" division. For example,
"_foo-challenge.co.uk" would not be allowed.
* 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.
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. 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>.
[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>.
[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>.
[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>.
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[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/rfc/rfc9364>.
8.2. Informative References
[ACM-CNAME]
AWS, "Option 1: DNS Validation", n.d.,
<https://docs.aws.amazon.com/acm/latest/userguide/dns-
validation.html>.
[ACME-SCOPED-CHALLENGE]
Chariton, A. A., Omidi, A. A., Kasten, J., Loukos, F., and
S. A. Janikowski, "ACME Scoped DNS Challenges", 2024,
<https://datatracker.ietf.org/doc/draft-ietf-acme-scoped-
dns-challenges/>.
[AKAMAI-DELEGATED]
Akamai Technologies, "Onboard a secure by default
property", 2023, <https://techdocs.akamai.com/property-
mgr/reference/onboard-a-secure-by-default-property>.
[ATLASSIAN-VERIFY]
Atlassian, "Verify over DNS", n.d.,
<https://support.atlassian.com/user-management/docs/
verify-a-domain-to-manage-accounts/#Verify-over-DNS>.
[AVOID-FRAGMENTATION]
Fujiwara, K. and P. Vixie, "Fragmentation Avoidance in
DNS", 2023, <https://datatracker.ietf.org/doc/draft-ietf-
dnsop-avoid-fragmentation/>.
[CLOUDFLARE-DELEGATED]
Cloudflare, "Auto-renew TLS certificates with DCV
Delegation", 2023,
<https://blog.cloudflare.com/introducing-dcv-delegation/>.
[DNS-01] Let's Encrypt, "Challenge Types: DNS-01 challenge", 2020,
<https://letsencrypt.org/docs/challenge-types/#dns-
01-challenge>.
[DOCUSIGN-CNAME]
DocuSign Admin for Organization Management, "Claim a
Domain", n.d., <https://support.docusign.com/s/document-it
em?rsc_301=&bundleId=rrf1583359212854&topicId=gso158335914
1256_1.html>.
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[GITHUB-TXT]
GitHub, "Verifying your organization's domain", n.d.,
<https://docs.github.com/en/github/setting-up-and-
managing-organizations-and-teams/verifying-your-
organizations-domain>.
[GOOGLE-WORKSPACE-CNAME]
Google, "CNAME record values", n.d.,
<https://support.google.com/a/answer/112038>.
[GOOGLE-WORKSPACE-TXT]
Google, "TXT record values", n.d.,
<https://support.google.com/a/answer/2716802>.
[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>.
[LETSENCRYPT-90-DAYS-RENEWAL]
Let's Encrypt, "Why ninety-day lifetimes for
certificates?", 2015,
<https://letsencrypt.org/2015/11/09/why-90-days.html>.
[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>.
[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>.
[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>.
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[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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/rfc/rfc8499>.
[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>.
[RFC8659] Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
"DNS Certification Authority Authorization (CAA) Resource
Record", RFC 8659, DOI 10.17487/RFC8659, November 2019,
<https://www.rfc-editor.org/rfc/rfc8659>.
[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>.
[SUBDOMAIN-TAKEOVER]
Mozilla, "Subdomain takeovers", n.d.,
<https://developer.mozilla.org/en-US/docs/Web/Security/
Subdomain_takeovers>.
Appendix A. Appendix
A survey of several different methods deployed today for DNS based
domain control validation follows.
A.1. Survey of Techniques
A.1.1. TXT based
TXT records is usually the default option for domain control
validation. The service provider asks the user to add a DNS TXT
record (perhaps through their domain host or DNS provider) at the
domain with a certain value. Then the service provider does a DNS
TXT query for the domain being verified and checks that the correct
value is present. For example, this is what a DNS TXT record could
look like for a provider Foo:
example.com. IN TXT "237943648324687364"
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Here, the value "237943648324687364" serves as the randomly-generated
TXT value being added to prove ownership of the domain to Foo
provider. Note that in this construction provider Foo would have to
query for all TXT records at "example.com" to get the validating
record. Although the original DNS protocol specifications did not
associate any semantics with the DNS TXT record, [RFC1464] describes
how to use them to store attributes in the form of ASCII text key-
value pairs for a particular domain. In practice, there is wide
variation in the content of DNS TXT records used for domain control
validation, and they often do not follow the key-value pair model.
Even so, the RDATA [RFC1034] portion of the DNS TXT record has to
contain the value being used to verify the domain. The value is
usually a Random Token in order to guarantee that the entity who
requested that the domain be verified (i.e. the person managing the
account at Foo provider) is the one who has (direct or delegated)
access to DNS records for the domain. After a TXT record has been
added, the service provider will usually take some time to verify
that the DNS TXT record with the expected token exists for the
domain. The generated token typically expires in a few days.
Some providers use a prefix of _PROVIDER_NAME-challenge in the Name
field of the TXT record challenge. For ACME, the full Host is _acme-
challenge.<YOUR_DOMAIN>. Such patterns are useful for doing targeted
domain control validation. The ACME protocol ([RFC8555]) has a
challenge type DNS-01 that lets a user prove domain ownership. In
this challenge, an implementing CA asks you to create a TXT record
with a randomly-generated token at _acme-challenge.<YOUR_DOMAIN>:
_acme-challenge.example.com. IN TXT "cE3A8qQpEzAIYq-T9DWNdLJ1_YRXamdxcjGTbzrOH5L"
[RFC8555] (section 8.4) places requirements on the Random Token.
A.1.1.1. Let's Encrypt
The ACME example in Appendix A.1.1 is implemented by Let's Encrypt
[DNS-01].
A.1.1.2. Google Workspace
[GOOGLE-WORKSPACE-TXT] asks the user to sign in with their
administrative account and obtain their token as part of the setup
process for Google Workspace. The verification token is a
68-character string that begins with "google-site-verification=",
followed by 43 characters. Google recommends a TTL of 3600 seconds.
The owner name of the TXT record is the domain or subdomain name
being verified.
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A.1.1.3. GitHub
GitHub asks you to create a DNS TXT record under _github-challenge-
ORGANIZATION.<YOUR_DOMAIN>, where ORGANIZATION stands for the GitHub
organization name [GITHUB-TXT]. The code is a numeric code that
expires in 7 days.
A.1.1.4. Public Suffix List
The Public Suffix List ([PSL]) asks for owners of private domains to
authenticate by creating a TXT record containing the pull request URL
for adding the domain to the Public Suffix List. For example, to
authenticate "example.com" submitted under pull request 100, a
requestor would add:
_psl.example.com. IN TXT "https://github.com/publicsuffix/list/pull/100"
A.1.2. CNAME based
A.1.2.1. CNAME for Domain Control Validation
A.1.2.1.1. DocuSign
[DOCUSIGN-CNAME] asks the user to add a CNAME record with the "Host
Name" set to be a 32-digit random value pointing to
verifydomain.docusign.net..
A.1.2.1.2. Google Workspace
[GOOGLE-WORKSPACE-CNAME] lets you specify a CNAME record for
verifying domain ownership. The user gets a unique 12-character
string that is added as "Host", with TTL 3600 (or default) and
Destination an 86-character string beginning with "gv-" and ending
with ".domainverify.googlehosted.com.".
A.1.2.2. Delegated Domain Control Validation
A.1.2.2.1. Content Delivery Networks (CDNs): Akamai and Cloudflare
In order to be issued a TLS cert from a Certification Authority like
Let’s Encrypt, the requester needs to prove that they control the
domain. Typically, this is done via the [DNS-01] challenge. Let’s
Encrypt only issues certs with a 90 day validity period for security
reasons [LETSENCRYPT-90-DAYS-RENEWAL]. This means that after 90
days, the DNS-01 challenge has to be re-done and the random token has
to be replaced with a new one. Doing this manually is error-prone.
Content Delivery Networks like Akamai and Cloudflare offer to
automate this process using a CNAME record in the user's DNS that
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points to the validation record in the CDN's zone ([AKAMAI-DELEGATED]
and [CLOUDFLARE-DELEGATED]).
A.1.2.2.2. AWS Certificate Manager (ACM)
AWS Certificate Manager [ACM-CNAME] allows delegated domain control
validation Section 5.3.2. The record name for the CNAME looks like:
`_<random-token1>.example.com. IN CNAME _<random-token2>.acm-validations.aws.`
The CNAME points to:
`_<random-token2>.acm-validations.aws. IN TXT <random-token3>`
Here, the random tokens are used for the following:
* <random-token1>: Unique sub-domain, so there's no clashes when
looking up the validation record.
* <random-token2>: Proves to ACM that the requester controls the DNS
for the requested domain.
* <random-token3>: The actual token being verified.
Note that if there are more than 5 CNAMEs being chained, then this
method does not work.
A.1.2.3. Atlassian
Some services ask the DNS record to exist in perpetuity
[ATLASSIAN-VERIFY]. If the record is removed, the user gets a
limited amount of time to re-add it before they lose domain
validation status.
A.1.2.4. Constraints on Domains and Subdomains
A.1.2.4.1. CAA records
While the ACME protocol ([RFC8555]) specifies a way to demonstrate
ownership over a given domain, Certification Authorities are required
to use it in-conjunction with [RFC8659] that specifies CAA records.
CAA allows a domain owner to apply policy across a domain and its
subdomains to limit which Certification Authorities may issue
certificates.
Authors' Addresses
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Shivan Sahib
Brave Software
Email: shivankaulsahib@gmail.com
Shumon Huque
Salesforce
Email: shuque@gmail.com
Paul Wouters
Aiven
Email: paul.wouters@aiven.io
Erik Nygren
Akamai Technologies
Email: erik+ietf@nygren.org
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