Delegated http-01 Validation in ACME Protocol

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Table of Contents

1. Introduction

The ACME protocol provides mechanisms for validating domain control using several challenge types, including http-01 and dns-01. While dns-01 challenges allow for delegation using DNS CNAME records, http-01 challenges require direct hosting of challenge responses on the domain being validated. This document introduces a mechanism to delegate http-01 validation requests to a designated server. By leveraging DNS records to redirect HTTP validation, this proposal enables http-01 validation while addressing challenges associated with validating domains hosted on corporate networks, on load balancers, or behind firewalls. Delegated HTTP validation combines the following benefits from the existing dns-01 and http-01 validation methods.¶

• Centralized Management: Enables centralized management of challenge responses, reducing the complexity of managing multiple domains.¶

• Reduced Exposure: Eliminates the need for direct access to the primary web server for validation.¶

• Performance: Reduces latency compared to dns-01 challenges by allowing synchronous validation.¶

• Security: Avoids risks associated with exposing DNS API credentials.¶

• Scalability: Enables parallelized validation of domains on distributed load balancers.¶

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. Delegated HTTP Validation

This proposal extends the http-01 challenge type to support delegation using a DNS record, similar to the approach used for dns-01 challenges. The process for validating example.com is as follows:¶

1. DNS Configuration: The domain owner creates a DNS record for _acme-challenge.example.com pointing to a server capable of serving HTTP validation requests. For example:¶

   _acme-challenge.example.com. CNAME validation-server-example.net. ¶

2. Challenge File Provisioning: The designated server hosts the challenge file at the path /.well-known/acme-challenge/{token}, containing the domain authorization string.¶

3. Validation Request: The ACME server attempts validation in the following order:¶

   • http://_acme-challenge.example.com/.well-known/acme-challenge/{token}¶

   • http://example.com/.well-known/acme-challenge/{token}¶

   An ACME server MUST try the undelegated URL if the delegated URL fails.¶

4. Validation Response: The designated server responds with the authorization string, which the ACME server verifies against the expected value.¶

5. Challenge Completion: If the validation succeeds, the challenge status is updated to “valid.” If it fails, the status is updated to “invalid,” with diagnostic information provided.¶

4. Security Considerations

• Relies on the security of both DNS and the delegated server.¶

5. IANA Considerations

TODO: https://github.com/jmcrawford45/draft-crawford-acme-delegated-http/issues/1¶

6. Normative References

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[RFC8555]

Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, <https://www.rfc-editor.org/rfc/rfc8555>.

Acknowledgments

TODO acknowledge.¶

Authors' Addresses