[cabf_validation] Adding Support for ACME Scoped DNS Challenges

Tim Hollebeek tim.hollebeek at digicert.com
Thu Feb 22 19:31:31 UTC 2024

Yeah, I remember pointing out at the ACME working group a few times that the BRs would have to be changed to allow two labels (but also suggesting this is fine, I think two labels is actually superior and advocated that for the draft).  


The approach #2 that Wayne is suggesting here is the one I suggested at the mike line, but I think I like #1 better.  It’s easier to just say “do it the RFC way” instead of allowing two labels without any guidance on how or why.


The question does come up whether the draft is mature enough to adopt right now … IMO the answer might be yes, as I seem to recall the document was pretty close to working group last call.




From: Validation <validation-bounces at cabforum.org> On Behalf Of Wayne Thayer via Validation
Sent: Thursday, February 22, 2024 2:04 PM
To: CABforum3 <validation at cabforum.org>
Subject: [cabf_validation] Adding Support for ACME Scoped DNS Challenges


There has been an effort underway for some time in the IETF ACME working group that resolves a significant hurdle to ACME adoption for some Applicants. The decision has been made to implement this in a way that requires a BR change.



It is common for Cloud Service Providers (CSPs) to ask their customers to delegate domain control to the CSP via a CNAME record that points to a domain name controlled by the CSP [1]. CSPs in general, and Fastly in particular, have found that Applicants often request certificates for the same domain name from multiple CAs. Because (unlike TXT records) only a single CNAME record is permitted for a particular FQDN, and because RFC 8555 requires the use of "_acme-challenge" as the DNS validation prefix, Applicants are unable to automate issuance via ACME dns-01 in this scenario.



A new ACME challenge originally called dns-account-01 was proposed back in 2022. Last week, the fourth draft was published [2]. The scope of this draft has expanded to include two new challenges, but I believe that the more relevant change is that the Authorization Domain Name (ADN) is now prefixed with TWO labels instead of one. My understanding is that this change was made to align with the work being done to standardize domain verification techniques [3] in the dnsop working group. Unfortunately, I think it's reasonable to interpret BR as only permitting a single label to be prepended to an ADN: "an Authorization Domain Name that is prefixed with a Domain Label that begins with an underscore character."



I would like to remove this barrier to automation as soon as possible, and prior to RFC publication. I can see two ways to accomplish this:

1. Add the current draft spec for dns-account-01 to the BRs as a new validation method. There is precedent for supporting draft versions of ACME validation methods in the BRs ( originally referenced a draft RFC)

2. Tweak the existing language to allow one or two labels to be prepended to an ADN.


I would appreciate everyone's feedback on how best to approach this and any concerns that you may have.






[1] Note that Michael Slaughter is working on a ballot that will clarify the requirements for DNS delegation to CAs: https://github.com/slghtr-says/servercert/pull/1 <https://url.avanan.click/v2/___https:/github.com/slghtr-says/servercert/pull/1___.YXAzOmRpZ2ljZXJ0OmE6bzo5ODRmZWNhNGYxMDRjMzhlMTM2NmZjNTNiM2IyZDRkNDo2OmVlMDM6Y2MzMDJjZjM1NWY4NjQ0NDMzMGFiYzA2OTFjN2FiZjczYzUzY2FiZTU1N2Y2YTY4YjRmYTcwMzY5MTM1NzM2NTpoOkY> 

[2] https://datatracker.ietf.org/doc/draft-ietf-acme-scoped-dns-challenges/ <https://url.avanan.click/v2/___https:/datatracker.ietf.org/doc/draft-ietf-acme-scoped-dns-challenges/___.YXAzOmRpZ2ljZXJ0OmE6bzo5ODRmZWNhNGYxMDRjMzhlMTM2NmZjNTNiM2IyZDRkNDo2OmQxNzk6MTQzMzEzOGUwMzQ2MWNlZTY0N2UyOWVlMjM1OGQwNmM2MTcxYTJmNDE2ZjM3NDhkMjhiZjgzNTM2YzkzNDM3ZTpoOkY>  

[3] https://datatracker.ietf.org/doc/draft-ietf-dnsop-domain-verification-techniques/ <https://url.avanan.click/v2/___https:/datatracker.ietf.org/doc/draft-ietf-dnsop-domain-verification-techniques/___.YXAzOmRpZ2ljZXJ0OmE6bzo5ODRmZWNhNGYxMDRjMzhlMTM2NmZjNTNiM2IyZDRkNDo2OmE1OWY6NTA4NGUzZDQ2NTllMzAwYWI3NDg1MDAwYTI4ZWU2Y2U4YTZhMDY4YTQyYjRkZWYzZmQ3MzBjZTZmYjA5OWMzYzpoOkY> 

This is issue 486: https://github.com/cabforum/servercert/issues/486 <https://url.avanan.click/v2/___https:/github.com/cabforum/servercert/issues/486___.YXAzOmRpZ2ljZXJ0OmE6bzo5ODRmZWNhNGYxMDRjMzhlMTM2NmZjNTNiM2IyZDRkNDo2OjA0MmQ6MTFlMzQyOTZiZDgzNWRlZTk4NmY5M2Y1YmYzYTI0ZTEyNTQ0NWM3NGU0ZWM2OWMzNTJlNjZmMTk0MDNhMmZjZDpoOkY> 

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