| Internet-Draft | Domain Verification Techniques | April 2021 |
| Sahib & Huque | Expires 17 October 2021 | [Page] |
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 surveys various techniques in wide use today, the pros and cons of each, and possible improvements.¶
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.¶
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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.¶
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.¶
Provider: an internet-based provider of a service, for e.g., Let's Encrypt provides a certificate authority service or GitHub provides code-hosting services. These services often require a user to verify that they control a domain.¶
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. 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 verification, and they often do not follow the key-value pair model. Even so, the rdata portion of the DNS TXT record has to contain the value being used to verify the domain. 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.¶
The same domain name can have multiple distinct TXT records (a TXT Record Set).¶
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 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.¶
[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 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.¶
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 typically uses a randomized label prepended to the domain name being verified.¶
[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.¶
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>.example.com, which would point to _<random-token2>.<random-token3>.acm-validations.aws.¶
Note that if there are more than 5 CNAMEs being chained, then this method does not work.¶
ACME and GitHub have a suffix of _PROVIDER_NAME-challenge in the Name field of the TXT record challenge. For ACME, the full Host is _acme-challenge.<YOUR_DOMAIN>, while for GitHub it is _github-challenge-ORGANIZATION-<YOUR_DOMAIN>. Both these patterns are useful for doing targeted domain verification, as discussed in section (#targeted-domain-verification) because if the provider knows what it is looking for (domain in the case of ACME, organization name + domain in case of GitHub) it can specifically do a DNS query for that TXT record, as opposed to having to do a TXT query for the apex.¶
ACME does the same name construction for CNAME records.¶
One pattern that quite a few providers follow (Dropbox, Atlassian) is constructing the rdata of the TXT DNS record in the form of PROVIDER-SERVICE-domain-verification= followed by the random value being checked for. This is in accordance with [RFC1464] which mandates that attributes must be stored as key-value pairs.¶
The TXT record being used for domain verification is most commonly placed at the domain name being verified. For example, if example.com is being verified, then the DNS TXT record will have example.com in the Name section.¶
If many services are attempting to verify the domain name, many distinct TXT records end up being placed at that name. There is no way to surgically query only the TXT record for a specific service, resulting in extra work for a verifying service to sift through the records for its own domain verification record. In addition, since DNS Resource Record sets are treated atomically, all TXT records must be returned to the querier, which leads to a bloating of DNS responses. This could cause truncation and expensive retrying over TCP.¶
A better method is to place the TXT record at a subdomain of the domain being verified that is specially reserved for use by the application service in question.¶
TODO¶
After domain verification is done, there is no need for the TXT or CNAME record to continue to exist as the existence of the domain verifying 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 verifying DNS record once the verification is done. However, despite this, some services ask the record to exist in perpetuity [ATLASSIAN-VERIFY].¶
DNSSEC [RFC4033] should be employed by the domain owner to protect against domain name spoofing.¶
This document has no IANA actions.¶
TODO¶