This document specifies identifiers and challenges required to enable the Automated Certificate Management Environment (ACME) to issue certificates for IP addresses.

This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8738.

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The Automatic Certificate Management Environment (ACME) [RFC 8555] only defines challenges for validating control of DNS host name identifiers, which limits its use to being used for issuing certificates for DNS identifiers. In order to allow validation of IPv4 and IPv6 identifiers for inclusion in X.509 certificates, this document specifies how challenges defined in the original ACME specification and the TLS-ALPN extension specification [RFC 8737] can be used to validate IP identifiers.

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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

[RFC 8555] only defines the identifier type "dns", which is used to refer to fully qualified domain names. If an ACME server wishes to request proof that a user controls an IPv4 or IPv6 address, it MUST create an authorization with the identifier type "ip". The value field of the identifier MUST contain the textual form of the address as defined in Section 2.1 of RFC 1123 for IPv4 and in Section 4 of RFC 5952 for IPv6. An identifier for the IPv6 address 2001:db8::1 would be formatted like so:

IP identifiers MAY be used with the existing "http-01" (see Section 8.3 of RFC 8555) and "tls-alpn-01" (see Section 3 of RFC 8737). To use IP identifiers with these challenges, their initial DNS resolution step MUST be skipped, and the IP address used for validation MUST be the value of the identifier.

For the "http-01" challenge, the Host header field MUST be set to the IP address being used for validation per [RFC 7230]. The textual form of this address MUST be as defined in Section 2.1 of RFC 1123 for IPv4 and in Section 4 of RFC 5952 for IPv6.

For the "tls-alpn-01" challenge, the subjectAltName extension in the validation certificate MUST contain a single iPAddress that matches the address being validated. As [RFC 6066] does not permit IP addresses to be used in the Server Name Indication (SNI) extension HostName field, the server MUST instead use the IN-ADDR.ARPA [RFC 1034] or IP6.ARPA [RFC 3596] reverse mapping of the IP address as the HostName field value instead of the IP address string representation itself. For example, if the IP address being validated is 2001:db8::1, the SNI HostName field should contain "1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa".

The existing "dns-01" challenge MUST NOT be used to validate IP identifiers.

Per this document, a new type has been added to the "ACME Identifier Types" registry defined in Section 9.7.7 of RFC 8555 with Label "ip" and Reference "RFC 8738".

Per this document, two new entries have been added to the "ACME Validation Methods" registry defined in Section 9.7.8 of RFC 8555. These entries are defined below: Table 1

The extensions to ACME described in this document do not deviate from the broader threat model described in Section 10.1 of RFC 8555.

This document only specifies how an ACME server may validate that a certificate applicant controls an IP identifier at the time of validation. The CA may wish to perform additional checks not specified in this document. For example, if the CA believes an IP identifier belongs to an ISP or cloud service provider with short delegation periods, they may wish to impose additional restrictions on certificates requested for that identifier.

[RFC1034]

P.V. Mockapetris, "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.

[RFC1123]

R. Braden, "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.

[RFC2119]

S. Bradner, "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/info/rfc2119>.

[RFC3596]

S. Thomson, C. Huitema, V. Ksinant, and M. Souissi, "DNS Extensions to Support IP Version 6", STD 88, RFC 3596, DOI 10.17487/RFC3596, October 2003,
<https://www.rfc-editor.org/info/rfc3596>.

[RFC5952]

S. Kawamura, and M. Kawashima, "A Recommendation for IPv6 Address Text Representation", RFC 5952, DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.

[RFC6066]

D. Eastlake 3rd, "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[RFC7230]

R. Fielding, and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.

[RFC8174]

B. Leiba, "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/info/rfc8174>.

[RFC8555]

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

[RFC8737]

R.B. Shoemaker, "Automated Certificate Management Environment (ACME) TLS Application-Layer Protocol Negotiation (ALPN) Challenge Extension", RFC 8737, DOI 10.17487/RFC8737, February 2020,
<https://www.rfc-editor.org/info/rfc8737>.

The author would like to thank those who contributed to this document and offered editorial and technical input, especially Jacob Hoffman-Andrews and Daniel McCarney.

Roland Bracewell Shoemaker