Internet-Draft ACME-SD October 2020
Tweedale Expires 24 April 2021 [Page]
Network Working Group
Intended Status:
Standards Track
F. Tweedale
Red Hat

Automated Certificate Management Environment (ACME) Service Discovery


This document specifies a service discovery mechanism that enables capable clients to locate an Automated Certificate Management Environment (ACME) server in their network environment.

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. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 24 April 2021.

Table of Contents

1. Introduction

Automatic Certificate Management Environment (ACME) [RFC8555] specifies a protocol by which a client may, in an automatable way, prove control of identifiers and obtain a certificate from an Certificate Authority (the ACME server). However, it did not specify a mechanism by which a client can locate a suitable ACME server. It is assumed that a client will be configured to use a particular ACME server, or else default to some well known, publicly accessible ACME service.

In some environments, such as corporate networks, it may be impossible for ACME clients to obtain certificates from a publicly accessible ACME servers, or an organisation may prefer clients to use a particular server. Explicitly configuring ACME clients to use a particular ACME server presents an administrative burden.

This document specifies a mechanism by which ACME clients can locate an ACME server using the Uniform Resource Identifier (URI) DNS resource record [RFC7553].

2. Terminology

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 URI Record

The URI resource record [RFC7553] facilitates client discovery of ACME server(s) for a given DNS parent domain name ("parent domain" having the meaning given in [RFC8552]). The owner name of the URI record SHALL be the parent domain with the label "_acme-server" prepended to it. The target of the URI record SHALL be the URI [RFC3986] of the directory resource of the target ACME server, enclosed in double quotes ("). For example:

_acme-server IN URI 10 1 ""

There MUST be exactly zero or one URI records for the "_acme-server" node.

4. Client Behaviour

4.1. When to Perform Service Discovery

If an ACME client provides for explicit configuration of an ACME server, and such configuration is provided, the client MUST use the configured ACME server and MUST NOT perform service discovery.

Otherwise, if an ACME client supports service discovery, in the absense of explicit configuration of an ACME server the client MAY attempt to locate an ACME server using the mechanisms specified in this document. A client MAY refuse to perform service discovery unless its configuration explicitly enables it.

4.2. Candidate Parent Domains

To perform service discovery, the ACME client needs a list of candidate parent domains. The client will query the associated URI records for the candidate parent domains.

If an ACME client provides for explicit configuration of parent domains to use for service discovery, and such configuration is provided, the candidate parent domains SHALL be the configured values.

Otherwise, there are a variety of ways an ACME client could choose candidate parent domains, including:

  • The host's fully-qualified domain name with one or more labels removed from the left.

  • The "search" domains from the host's DNS configuration.

  • The Kerberos [RFC4120] realm of the host.

  • The result of a PTR lookup on one of the host's non-loopback IP addresses, with one or more labels removed from the left.

An ACME client MAY use any or all of these or other suitable methods for identifying candidate parent domains. If multiple candidate parent domains are identified the client MUST establish an order of preference among them. If any candidate parent domain A is a subdomain of another candidate parent domain B, the client MUST preference A higher than B.

4.3. DNS URI Queries and Validation

Service discovery begins with the most preferred candidate parent domain.

The ACME client SHALL prepend the label "_acme-server" to the candidate parent domain name and query the DNS URI record for the resulting domain name. If any records are returned, the ACME client SHALL select exactly one of the target URIs. The client SHALL perform an HTTPS GET request for the target URI and SHALL attempt to parse the response body as an ACME directory object. If successful, service discovery has succeeded; the client SHALL use the target of the URI record as the ACME server, and MUST NOT process the remaining candidate parent domains.

Otherwise, service discovery for the current parent domain has failed. Either there is no "_acme-server" URI record under the parent domain, or the target URI value is not well formed, or the HTTP request failed, or the HTTP response is not a valid ACME directory object. In this case, the client MAY retry service discovery with the next most preferred candidate parent domain. The client MAY continue retrying until no candidate parent domains remain, or MAY give up earlier (e.g. after a fixed number of attempts).

If service discovery does not succeed, an ACME client MAY fall back to a default ACME server (e.g. a publicly accessible ACME server).

4.4. ACME Operations

An ACME client MAY record (cache) the URI of the ACME server located via service discovery and MAY use the cached server for new account and new order operations, without performing service discovery each time.

When storing data about accounts and orders, ACME clients SHOULD record the URI of the actual ACME server used. When retrieving or revoking certificates or performing account operations, the client SHOULD use the recorded URI to contact the ACME server and SHOULD NOT perform service discovery.

When renewing or replacing a certificate, if the recorded ACME server cannot be contacted or fails to issue a certificate, a client MAY perform service discovery to attempt to locate an alternative ACME server that may be able to issue the certificate.

5. IANA Considerations

5.1. Underscored Node Name for ACME Service Discovery

Per RFC 8552, please add the following entry to the "Underscored and Globally Scoped DNS Node Names" registry:

   | RR Type | _NODE NAME   | Reference       |
   | URI     | _acme-server | {this document} |

6. Security Considerations

6.1. TLS and Certificate Validation

Use of TLS is REQUIRED by the ACME specification [RFC8555]. X.509 [RFC5280] supports the Uniform Resource Identifier name type in the Subject Alternative Name extension, but this name type is not widely supported by TLS clients or certificates. HTTP Over TLS [RFC2818] does not describe the use of a URI-ID for HTTP services. Therefore when an ACME server was located via service discovery its certificate MUST be validated according to both RFC 5280 and [RFC6125] and MUST match the host from the target URI against the dNSName (if the host is a reg-name) or iPAddress (if the host is an IP address) value(s) in the Subject Alternative Name extension. The client SHOULD NOT use a URI-ID when validating the server's certificate.

6.2. Parent Domain Selection

An attacker who is able to influence an ACME client's candidate parent domains can influence which ACME server the client uses, or cause service discovery to fail. The attacker could use this capability to perform a denial of service against the ACME client (i.e. the client cannot acquire or renew a certificate), or against parties that validate certificates issued to the client (because they do not trust the issuing CA or because the certificate is invalid in some way), or against a target ACME server (by directing many clients to it). ACME client implementers should carefully consider which methods of determining the parent domain(s) are appropriate for their use cases, and the security implications of their chosen methods.

An ACME client may form candidate parent domains by removing one or more labels from the left side of some other DNS name (e.g. the host name of the client's machine). If too many labels are removed, the ACME client could perform DNS queries in zones outside the control of the organisation that operates the ACME client. As a result, the ACME client could locate and use an ACME server that the organisation does not intend.

To mitigate this risk, it is RECOMMENDED that clients limit the amount of label pruning that occurs. It is not possible to make a concrete recommendation that is suitable for all environments. Implementers must consider what is appropriate for their use cases and environments. The candidate parent domain ordering requirements also mitigate this risk.

6.3. DNS Security

Without ACME service discovery, an ACME client must be configured or hard-coded to use a particular ACME server, specified as the HTTPS URI of the server's directory resource. Typically the host will be a DNS name rather than an IP address, and one or more DNS queries are necessary to resolve the host's DNS name to an IP address.

When service discovery is used, the URI of the ACME server is obtained from a DNS URI record. If an attacker is able to spoof the _acme-server URI record for a candidate parent domain name, the attacker could cause service discovery to fail or could direct the client to an ACME server of the attacker's choosing. This could constitute a denial of service attack against the client, against parties that validate certificates issued to the client, or against the target server.

Therefore it is RECOMMENDED that URI records used for ACME service discovery be secured using DNSSEC. It is RECOMMENDED that ACME clients make DNS URI queries via DNSSEC-validating stub or recursive resolvers.

Some methods of candidate parent domain selection may involve DNS queries. For example, a client could query PTR records to find a host name, from which it derives a candidate parent domain. Implementers must consider the security of DNS data used for parent domain selection.

7. Normative References

Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, , <>.
Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, , <>.
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <>.
Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, , <>.

8. Informative References

Faltstrom, P. and O. Kolkman, "The Uniform Resource Identifier (URI) DNS Resource Record", RFC 7553, DOI 10.17487/RFC7553, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, , <>.
Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, DOI 10.17487/RFC4120, , <>.

Author's Address

Fraser Tweedale
Red Hat