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 draft-sahib-domain-verification-techniques-00 Abstract 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. 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/ShivanKaul/draft-sahib-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. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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 Acknowledgments 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", "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. 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. host.widgets.com 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: example.com. 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 ("example.com" 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 domain. 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 3.1.1.1. 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.". For example, if you wanted to prove domain ownership of "example.com", Let's Encrypt could ask you to create the DNS record: _acme-challenge.example.com. IN TXT "cE3A8qQpEzAIYq-T9DWNdLJ1_YRXamdxcjGTbzrOH5L" [RFC8555] (section 8.4) places requirements on the random value. 3.1.1.2. 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. 3.1.1.3. GitHub GitHub asks you to create a DNS TXT record under "_github-challenge- ORGANIZATION-", 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 3.2.1.1. 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 ".domainverify.googlehosted.com.". 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. 3.2.1.2. 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 "_.example.com", which would point to "_..acm- validations.aws." 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 [ACM-CNAME] AWS, ., "Option 1: DNS Validation", n.d., . [GITHUB-TXT] GitHub, ., "Verifying your organization's domain", n.d., . [GOOGLE-WORKSPACE-CNAME] Google, ., "CNAME record values", n.d., . [GOOGLE-WORKSPACE-TXT] Google, ., "TXT record values", n.d., . [LETSENCRYPT] 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, . Acknowledgments TODO Authors' Addresses Shivan Sahib Salesforce Email: shivankaulsahib@gmail.com Shumon Huque Salesforce Email: shuque@gmail.com