Domain Control Validation using DNS
draft-ietf-dnsop-domain-verification-techniques-05
The information below is for an old version of the document.
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| Authors | Shivan Kaul Sahib , Shumon Huque , Paul Wouters , Erik Nygren | ||
| Last updated | 2024-07-08 (Latest revision 2024-03-03) | ||
| Replaces | draft-sahib-domain-verification-techniques | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-dnsop-domain-verification-techniques-05
Network Working Group S. Sahib
Internet-Draft Brave Software
Intended status: Best Current Practice S. Huque
Expires: 9 January 2025 Salesforce
P. Wouters
Aiven
E. Nygren
Akamai Technologies
8 July 2024
Domain Control Validation using DNS
draft-ietf-dnsop-domain-verification-techniques-05
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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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 9 January 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 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. Random Token . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Validation Record Domain Name . . . . . . . . . . . . . . 7
5.2.1. Scope Indication . . . . . . . . . . . . . . . . . . 8
5.2.2. CNAME Considerations . . . . . . . . . . . . . . . . 9
5.3. TXT Record . . . . . . . . . . . . . . . . . . . . . . . 9
5.3.1. Token Metadata . . . . . . . . . . . . . . . . . . . 10
5.3.2. Metadata For Expiry . . . . . . . . . . . . . . . . . 10
5.4. Delegated Domain Control Validation . . . . . . . . . . . 11
5.5. Domain Control Validation Supporting Multiple
Intermediaries . . . . . . . . . . . . . . . . . . . . . 12
5.6. Specification of Validation Records . . . . . . . . . . . 13
5.7. Time-bound checking . . . . . . . . . . . . . . . . . . . 13
5.8. TTL Considerations . . . . . . . . . . . . . . . . . . . 13
5.9. CNAME Records for Domain Control Validation . . . . . . . 14
5.9.1. Random Token in Domain Names . . . . . . . . . . . . 14
5.9.2. Random Token in CNAME Targets . . . . . . . . . . . . 15
5.10. Interactions with DNAME . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6.1. Public Suffixes . . . . . . . . . . . . . . . . . . . . . 16
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7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Appendix . . . . . . . . . . . . . . . . . . . . . . 20
A.1. Survey of Techniques . . . . . . . . . . . . . . . . . . 21
A.1.1. TXT based . . . . . . . . . . . . . . . . . . . . . . 21
A.1.2. CNAME based . . . . . . . . . . . . . . . . . . . . . 22
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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
temporary DNS record is sufficient for proving domain ownership.
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.
* 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
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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.
* 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.
* User: the owner or operator of a domain in the DNS who needs to
prove ownership of that domain to an Application Service Provider.
* Random Token: a random value that uniquely identifies the DNS
domain control validation challenge, defined in Section 5.1.
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.
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]).
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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.
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 utilized set of Application Service Providers.
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
protocol's DNS-01 challenge mechanism ([RFC8555], Section 8.4) does
not differentiate these cases in the DNS Validation Record. In the
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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. (The ACME protocol is addressing this in
[ACME-SCOPED-CHALLENGE].)
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 5.2.1. 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" ([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. See Appendix A.1.2.4 for cases where DNS records can be
used as constraints complementary to domain verification.
5. Recommendations
All Domain Control Validation mechanisms are implemented by a
resource record with:
1) A domain name related to the domain name being validated 2) A
Validation Record, either directly in RDATA or as the target of a
CNAME (or chain of CNAMEs)
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Both of these are issued to the User by either an Application Service
Provider or an Intermediary. An issued random token then needs to
exist in at least one of these to demonstrate the User has control
over the domain name in-question. Variations on this approach exist
to meet different uses.
5.1. Random Token
A unique token 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 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 a domain name, as
described in the rest of this section. Some methods of validation
may involve multiple independent random tokens.
5.2. Validation Record Domain Name
The RECOMMENDED format for a Validation Record's domain 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. "_foo-
challenge.example.com"). The prefixed "_" is used to avoid
collisions with existing hostnames.
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".
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An Application Service Provider may also specify prepending a random
token to the name, such as "<RANDOM_TOKEN>._<PROVIDER_RELEVANT_NAME>-
challenge". This can be done either as part of the challenge itself
(Section 5.9, to support multiple Intermediaries (Section 5.5), or to
make it harder for a third party to scan what Application Service
Providers are being used by a given domain name.
5.2.1. Scope Indication
For applications that may apply more broadly than to a single
hostname, the RECOMMENDED approach is to differentiate the
application-specific underscore prefix labels to also include the
scope (see Section 4). In particular:
* "_<PROVIDER_RELEVANT_NAME>-host-challenge.example.com" applies
only to the specific hostname of "example.com" and not to anything
underneath it.
* "_<PROVIDER_RELEVANT_NAME>-wildcard-challenge.example.com" applies
to all hostnames 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 Service 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 the ACME DNS challenge specification
[ACME-SCOPED-CHALLENGE] has incorporated this scope indication
format.
Application owners SHOULD consult the IANA "Underscored and Globally
Scoped DNS Node Names" registry [UNDERSCORE-REGISTRY] to confirm
there are no collisions with existing entries.
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5.2.2. CNAME Considerations
Any Validation Records that might include a CNAME MUST have a name
that is distinct from the domain name being validated, as a CNAME
MUST NOT be placed at the same domain name that is being validated.
The recommended format in Section 5.2 as well as others below all
have this property.
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 domain name. Instead, as with
the TXT record recommendation, an Application Service 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.
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. TXT Record
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 5.2, and RDATA MUST contain at
least a Random Token (constructed as in Section 5.1). If metadata
(see Section 5.3.1) 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"
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).
Whether or not multiple Validation Records can exist for the same
domain is up to the Application Service Provider's application
specification.
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.
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5.3.1. 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:
_foo-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:
_foo-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).
If an alternate syntax is used by the Application Service Provider
for token metadata, they MUST specify a grammar for it.
5.3.2. Metadata For Expiry
Application Service Providers MUST provide clear instructions on when
a Validation Record can be removed.
These instructions SHOULD be encoded in the RDATA as token metadata
(Section 5.3.1 using the key "expiry" to hold a time after which it
is safe to remove the Validation Record. 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.
A simpler variation of the expiry time is also ISO 8601 valid and can
also be specified, using the "full-date" format. For example:
_foo-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".
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_foo-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
(Appendix A.1.1.4).
_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.
5.4. Delegated Domain Control Validation
Delegated domain control validation lets a User delegate the domain
control validation process for their domain to an Intermediary
without having to hand over full DNS access. It is a variation of
the above TXT record validation (Section 5.3) 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 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. IN TXT "<provider-random-token>"
Such a setup is especially useful when the Application Service
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
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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.
See Appendix A.1.2.2 for examples.
5.5. 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 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> and following the requirements of
Section 5.1, specified as either base32 or base16 encoded. This
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 5.4).
The resulting record could either directly contain a TXT record or a
CNAME (as in Section 5.4). 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."
When performing validation, the Application Service Provider would
resolve the DNS name containing the appropriate identifier token.
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.
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5.6. Specification of Validation Records
Validation Records need to be securely relayed from an Application
Service Provider to a DNS administrator. Application Service
Providers and intermediaries SHOULD offer detailed and easily-
accessible help pages, keeping in mind that the DNS administrator
might not have a login account on the website of the Application
Service Provider or Intermediary. Similarly, for clarity, the exact
and full DNS record (including a Fully Qualified Domain Name and DNS
record type) to be added SHOULD be provided along with help
instructions. Where possible, APIs SHOULD be used to relay
instructions.
5.7. 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 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 secret are no guarantee
that the original owner is still in control of the domain, and a new
challenge needs to be issued.
One exception is if the record is being used as part of a delegated
domain control validation setup (Section 5.4); 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.
5.8. 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.
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
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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 Provider's verifiers MAY wish to either use
dedicated DNS resolvers configured with a low maximum negative
caching TTL or flush Validation Records from resolver caches prior to
issuing queries.
5.9. CNAME Records for Domain Control Validation
CNAME records MAY be used instead of TXT records where specified by
Application Service Providers to support Users who are unable to
create TXT records. Two forms of this are common: including the
challenge token in the domain name itself, or including the challenge
token as a part of the CNAME target. This approach has a number of
limitatations relative to using TXT records.
5.9.1. Random Token in Domain Names
Application Service Providers MAY include the random token in a
domain name that is related to the domain name being validated. An
Application Service Provider including the random token in a domain
name MUST specify the use of an underscore-prefixed label (e.g.,
_<token>._foo or _foo-<token>). The resource record is then a CNAME
to a domain name specified by the Application Service Provider. The
Application Service Provider uses the presence of a resource record
with this domain name to perform the validation, validating the both
presence of the record as well as the CNAME target. For example:
_<random-token>._foo-challenge.example.com. IN CNAME "dcv.provider.example."
In practice, many Application Service Providers that employ CNAMEs
for domain control validation today use an entirely random subdomain
label which works to avoid accidential collisions, but which could
allow for a malicious Application Service Provider to smuggle
instructions from some other Application Service Provider. Adding an
provider-specific component in addition (such as _<token>._foo-
challenge or _foo-<token>-challenge) make it easier for the domain
owner to keep track of why and for what service a Validation Record
has been deployed.
Since the random token exists entirely in the challenge, it is not
possible to delegate Domain Control Validation challenges of this
form to Intermediaries in a way that allows the Intermediary to
refresh the challenge over time.
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5.9.2. Random Token in CNAME Targets
An Application Service 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
Application Service Provider. For example:
_foo-challenge.example.com. IN CNAME "<random-token>.dcv.provider.example."
The Application Service Provider then validates that the target of
the CNAME matches the token provided. This approach has similar
properties to TXT records (Section 5.3) but does not allow for
additional attributes such as expiry to be added.
As mentioned in Section 5.2.2, the domain name of the Validation
Record MUST be distinct from the domain name being validated.
5.10. Interactions with 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 Application
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 Application Service 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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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.
Ambiguity 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 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
attacker includes the attribute to mislead the DNS administrator.
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.
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 Application 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, 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.
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.
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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
Public Suffix List (Appendix A.1.1.5) 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, "_foo-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.
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>.
[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>.
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[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>.
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>.
[ATPROTO-TXT]
Bluesky, "DNS TXT Method", n.d.,
<https://atproto.com/specs/handle#dns-txt-method>.
[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>.
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[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>.
[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>.
[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>.
[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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[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>.
[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>.
[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
A survey of several different methods deployed today for DNS based
domain control validation follows.
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A.1. Survey of Techniques
A.1.1. TXT based
TXT records is usually the default option for domain control
validation. The Application 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 Application 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 an Application Service Provider
Foo:
example.com. IN TXT "237943648324687364"
Here, the value "237943648324687364" serves as the randomly-generated
TXT value being added to prove ownership of the domain to Foo
Application Service Provider. Note that in this construction
Application Service 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 Application Service Provider
Foo) is the one who has (direct or delegated) access to DNS records
for the domain. After a TXT record has been added, the Application
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 Application Service 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.
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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.
A.1.1.3. The AT Protocol
The Authenticated Transfer (AT) Protocol supports DNS TXT records for
resolving social media "handles" (human-readable identifiers) to the
User's persistent account identifier [ATPROTO-TXT]. For example,
this is how the handle bsky.app would be resolved:
_atproto.bsky.app. IN TXT "did=did:plc:z72i7hdynmk6r22z27h6tvur"
A.1.1.4. 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.1.1.5. 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
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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
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 5.4. 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.
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* <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.
Appendix B. Acknowledgments
Thank you to Tim Wicinski, John Levine, Daniel Kahn Gillmor, Amir
Omidi, Tuomo Soini, 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
Erik Nygren
Akamai Technologies
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Email: erik+ietf@nygren.org
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