Automated Certificate Management Environment (ACME) DNS Labeled With ACME Account ID Challenge

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Table of Contents

1. Introduction

The dns-01 challenge specified in section 8.4 of [RFC8555] uses a single DNS authorization label (_acme-challenge) for domain validation. This single-label approach creates a limitation in domain validation: each domain can only delegate its validation to one ACME client at a time. Since delegation requires the use of CNAME records, of which only one can exist per DNS name, operators are forced to choose a single ACME challenge solver for their domain name.¶

This limitation becomes particularly problematic in modern deployment architectures. In multi-region deployments, separate availability zones serve the same content while avoiding cross-zone dependencies. These zones need to independently obtain and manage certificates for the same domain name. Similarly, during zero-downtime migrations, two different infrastructure setups may coexist for extended periods, with both requiring access to valid certificates. Other use cases include multi-CDN deployments and the provision of backup certificates for use when an active certificate must be quickly revoked.¶

This document specifies a new challenge type: dns-account-01, which addresses these operational needs. The dns-account-01 challenge incorporates the ACME account URL into the DNS validation record name, allowing multiple independent ACME clients to perform domain validation concurrently. Since these authorization labels depend on the ACME account KID ([RFC8555], Section 7.3), operators can generate and configure the necessary DNS records in advance.¶

This RFC does not deprecate the dns-01 challenge specified in [RFC8555]. The ability to complete the dns-account-01 challenge requires ACME server operators to deploy new code, making adoption of this challenge an opt-in process.¶

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. DNS-ACCOUNT-01 Challenge

The dns-account-01 challenge allows a client to prove control of a domain name by provisioning a TXT resource record containing a designated value for a specific validation domain name. It leverages the ACME account URL to construct a unique but stable validation domain name. The ACME server validates control of the domain name by performing one or more DNS queries to this validation domain name, following CNAME records, to arrive at one or more TXT resource record. The ACME server verifies that the contents of one or more of these TXT record(s) match the digest value of the key authorization that is constructed from the token value provided in the challenge.¶

{
    "type": "dns-account-01",
    "url": "https://example.com/acme/chall/i00MGYwLWIx",
    "status": "pending",
    "token": "ODE4OWY4NTktYjhmYS00YmY1LTk5MDgtZTFjYTZmNjZlYTUx"
}

_ujmmovf2vn55tgye._acme-challenge.example.org 300 IN TXT "LoqXcYV8...jxAjEuX0.9jg46WB3...fm21mqTI"

POST /acme/chall/Rg5dV14Gh1Q
Host: example.com
Content-Type: application/jose+json

{
  "protected": base64url({
    "alg": "ES256",
    "kid": "https://example.com/acme/acct/ExampleAccount",
    "nonce": "SS2sSl1PtspvFZ08kNtzKd",
    "url": "https://example.com/acme/chall/Rg5dV14Gh1Q"
  }),
  "payload": base64url({}),
  "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}

4. Security Considerations

The same security considerations apply for the integrity of authorizations ([RFC8555], Section 10.2) and DNS security ([RFC8555], Section 11.2) as in the original specification for dns-01.¶

To allow for seamless account key rollover without the label changing, the dynamic part of the label depends on the ACME account and not the account key. This allows for long-lived labels, without the security considerations of keeping the account key static.¶

In terms of the construction of the account label prepended to the domain name, there is no need for a cryptographic hash. The goal is to simply create a long-lived and statistically distinct label of minimal size. SHA-256 was chosen due to its existing use in the dns-01 challenge ([RFC8555], Section 8.1).¶

The first 10 bytes were picked as a tradeoff: the value needs to be short enough to stay lower than the size limits for DNS ([RFC1035], Section 2.3.4), long enough to provide sufficient probability of collision avoidance across ACME accounts, and just the right size to have Base32 require no padding. As the algorithm is used for a uniform distribution of inputs, and not for integrity, we do not consider the trimming a security issue.¶

5. IANA Considerations

label: dns-account-01
identifier-type: dns
ACME: Y
Reference: This document

6. References

Acknowledgments

Authors' Addresses