Network Working Group                                           S. Sahib
Internet-Draft                                            Brave Software
Intended status: Best Current Practice                          S. Huque
Expires: 11 January 2024                                      Salesforce
                                                              P. Wouters
                                                                   Aiven
                                                            10 July 2023


                  Domain Control Validation using DNS
           draft-ietf-dnsop-domain-verification-techniques-02

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/.

   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."




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   This Internet-Draft will expire on 11 January 2024.

Copyright Notice

   Copyright (c) 2023 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 . . . . . . . . . . . . . . . . . . . . .   4
   5.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Validation Record Format  . . . . . . . . . . . . . . . .   5
       5.1.1.  Name  . . . . . . . . . . . . . . . . . . . . . . . .   5
       5.1.2.  Random Token  . . . . . . . . . . . . . . . . . . . .   5
     5.2.  TXT Record  . . . . . . . . . . . . . . . . . . . . . . .   6
       5.2.1.  Metadata For Expiry . . . . . . . . . . . . . . . . .   6
     5.3.  CNAME Record  . . . . . . . . . . . . . . . . . . . . . .   7
       5.3.1.  Delegated Domain Control Validation . . . . . . . . .   7
     5.4.  Time-bound checking . . . . . . . . . . . . . . . . . . .   8
     5.5.  DNAME . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Appendix . . . . . . . . . . . . . . . . . . . . . .  11
     A.1.  Survey of Techniques  . . . . . . . . . . . . . . . . . .  12
       A.1.1.  TXT based . . . . . . . . . . . . . . . . . . . . . .  12
       A.1.2.  CNAME based . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14









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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,
   certificate 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 which 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.  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 being verified
      and checks if it contains the unguessable value.

   *  Provider: an internet-based provider of a service, for e.g., a
      Certificate Authority or a service that allows for user-controlled
      websites.  These services often require a user to verify that they
      control a domain.

   *  Random Token: a random value that uniquely identifies the DNS
      domain control validation challenge, defined in Section 5.1.2.




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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 surgically 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 response
   is large enough that it does not fit into a single DNS UDP packet
   (UDP being the most common DNS transport today), this may result in
   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 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 place 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.

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 or for the entire domain
   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.






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   In the case of X.509 certificate issuance, the request is clear about
   whether it is for a single host or a wildcard domain.  Unfortunately,
   the ACME protocol's DNS challenge mechanism ([DNS-01]) does not
   appear to 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.  These 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.

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.

5.1.2.  Random Token

   A unique token used in the challenge.  It should be a random value
   with the following properties:

   1.  MUST have at least 128 bits of entropy.

   2.  base64url ([RFC4648], Section 5) encoded.

   See [RFC4086] for additional information on randomness requirements.

   This random token is placed in the RDATA as described in the rest of
   this section.







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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 5.1.2).  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
   implementation.

   Consumers of the provider services need to relay information from a
   provider's website 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.

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"

   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" can 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 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 Record

   CNAME records MAY be used instead of TXT records, but they should 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 domain name.  Instead, as with the TXT record
   recommendation, an application 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
   application services that employ CNAMEs today use a random subdomain
   label, which also works to avoid collisions.  But adding an
   application specific component makes 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.  Delegated Domain Control Validation

   CNAME records 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 provides the user with 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 random tokens are generated as in Section 5.1.
   For example:



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_foo-challenge.example.com.  IN   CNAME  "<random-token-given-by-intermediary>.intermediarydns.com"

   The intermediary then adds the actual validation record:

<random-token-given-by-intermediary>.intermediarydns.com. TXT "<random-token-given-by-service>"

   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 also contains a random token which
   proves to the intermediary that example.com is controlled by the
   user.

   See Appendix A.1.2.3 for examples.

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 5.3.1); in that case, the
   CNAME record that points to the actual validation TXT record cannot
   be removed.

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.







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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.

   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.

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>.







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   [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>.

   [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>.

   [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]
              Google, "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>.

   [LETSENCRYPT-90-DAYS-RENEWAL]
              Let's Encrypt, "Why ninety-day lifetimes for
              certificates?", 2015,
              <https://letsencrypt.org/2015/11/09/why-90-days.html>.

   [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>.

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

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

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

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 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"

   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.






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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.  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.1.2.  CNAME based

A.1.2.1.  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.1.2.2.  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.1.2.3.  Delegated Domain Control Validation

A.1.2.3.1.  Cloudflare

   In order to be issued a TLS cert from a Certificate 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



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   to be replaced with a new one.  Doing this manually is error-prone.
   Content Delivery Networks like Cloudflare offer to automate this
   process using a CNAME record in the user's DNS that points to the
   validation record in Cloudflare's zone [CLOUDFLARE-DELEGATED].

A.1.2.3.2.  AWS Certificate Manager (ACM)

   AWS Certificate Manager [ACM-CNAME] allows delegated domain control
   validation Section 5.3.1.  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.1.2.4.  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.

Authors' Addresses

   Shivan Sahib
   Brave Software
   Email: shivankaulsahib@gmail.com


   Shumon Huque
   Salesforce
   Email: shuque@gmail.com





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
















































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