Network Working Group                                           S. Sahib
Internet-Draft                                                  S. Huque
Intended status: Informational                                Salesforce
Expires: 11 September 2021                                 10 March 2021

           Survey of Domain Verification Techniques using DNS


   Verification of ownership of domains in the Domain Name System (DNS)
   [RFC1034] [RFC1035] often relies on adding or editing DNS records
   within the domain.  This document lays out the various techniques and
   the pros and cons of each.

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Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
   3.  Verification Techniques
     3.1.  TXT based
       3.1.1.  Examples
     3.2.  CNAME based
       3.2.1.  Examples
   4.  Recommendations
     4.1.  TXT vs CNAME
     4.2.  TXT recommendations
     4.3.  CNAME recommendations
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Authors' Addresses

1.  Introduction

   Many providers on the internet need users to prove that they control
   a particular domain before granting them some sort of privilege
   associated with that domain.  For instance, certificate authorities
   like Let's Encrypt [LETSENCRYPT] ask requesters of TLS certificates
   to prove that they operate the domain they're requesting the
   certificate for.  Providers generally allow for several different
   ways of proving domain control, some of which include manipulating
   DNS records.  This document focuses on DNS techniques for domain
   verification; other techniques (such as email or HTML verification)
   are out-of-scope.

   In practice, DNS-based verification often looks like the provider
   generating a random value and asking the requester to create a DNS
   record containing this random value and placing it at a location that
   the provider can query for.  Generally only one temporary DNS record
   is sufficient for proving domain ownership.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

3.  Verification Techniques

3.1.  TXT based

   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   TXT   "printer=lpr5"

   In practice, there is wide variation in the content of DNS TXT
   records used for domain verification, and they often do not follow
   the key-value pair model.

   The same domain name can have multiple distinct TXT records (a TXT
   Record Set).

   TXT record-based DNS domain verification is usually the default
   option for DNS verification.  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 value exists.  For example, this is what a DNS TXT
   verification record could look like:   IN   TXT   "foo-verification=bar"

   Here, the value "bar" for the attribute "foo-verification" serves as
   the randomly-generated TXT value being added to prove ownership of
   the domain to Foo provider.  The value is usually a randomly-
   generated 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.  The generated token typically expires in a
   few days.  The TXT record is usually placed at the domain being
   verified ("" in the example above).  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

   One drawback of this method is that the TXT record is typically
   placed at the domain name being verified.  If many services are
   attempting to verify the domain name, many distinct TXT records end
   up being placed at that name.  Since DNS Resource Record sets are
   treated atomically, all TXT records must be returned to the querier,
   increasing the size of the response.  There is no way to surgically
   query only the TXT record for a specific service.

3.1.1.  Examples  Let's Encrypt

   Let's Encrypt [LETSENCRYPT] has a challenge type "DNS-01" that lets a
   user prove domain ownership in accordance with the ACME protocol
   [RFC8555].  In this challenge, Let's Encrypt asks you to create a TXT
   record with a randomly-generated token at "_acme-
   challenge.<YOUR_DOMAIN>".  For example, if you wanted to prove domain
   ownership of "", Let's Encrypt could ask you to create the
   DNS record:  IN  TXT "cE3A8qQpEzAIYq-T9DWNdLJ1_YRXamdxcjGTbzrOH5L"

   [RFC8555] (section 8.4) places requirements on the random value.  Google Workspace

   [GOOGLE-WORKSPACE-TXT] asks the user to sign in with their
   administrative account and obtain their verification 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 neme
   being verified.  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.

3.2.  CNAME based

   Less commonly than TXT record verification, service providers also
   provide the ability to verify domain ownership via CNAME records.
   This is used in case the user cannot create TXT records.  One common
   reason is that the domain name may already have CNAME record that
   aliases it to a 3rd-party target domain.  CNAMEs have a technical
   restriction that no other record types can be placed along side them
   at the same domain name ([RFC1034], Section 3.6.2).. The CNAME based
   domain verification method teypically uses a randomized label
   prepended to the domain name being verified.

3.2.1.  Examples  Google

   [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 "".

   To verify a subdomain, the unique 12-character string is appended
   with the subdomain name for "Host" field for e.g.
   JLKDER712AFP.subdomain where subdomain is the subdomain being
   verified.  AWS Certificate Manager (ACM)

   To get issued a certificate by AWS Certificate Manager (ACM), you can
   create a CNAME record to verify domain ownership [ACM-CNAME].  The
   record name for the CNAME looks like "_<random-token1>",
   which would point to "_<random-token2>.<random-token3>.acm-"

   Note that if there are more than 5 CNAMEs being chained, then this
   method does not work.

4.  Recommendations

4.1.  TXT vs CNAME

4.2.  TXT recommendations

4.3.  CNAME recommendations

5.  Security Considerations

   DNSSEC [RFC4033] should be employed by the domain owner to protect
   against domain name spoofing.

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  Normative References

   [RFC1034]  Mockapetris, P.V., "Domain names - concepts and
              facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034,
              November 1987, <>.

   [RFC1035]  Mockapetris, P.V., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC1464]  Rosenbaum, R., "Using the Domain Name System To Store
              Arbitrary String Attributes", RFC 1464,
              DOI 10.17487/RFC1464, May 1993,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

7.2.  Informative References

              AWS, ., "Option 1: DNS Validation", n.d.,

              GitHub, ., "Verifying your organization's domain", n.d.,

              Google, ., "CNAME record values", n.d.,

              Google, ., "TXT record values", n.d.,

              Let's Encrypt, ., "Challenge Types: DNS-01 challenge",
              2020, <

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,



Authors' Addresses

   Shivan Sahib


   Shumon Huque