RFC8738: Automated Certificate Management Environment (ACME) IP Identifier Validation Extension

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Internet Engineering Task Force (IETF)                    R.B. Shoemaker
Request for Comments: 8738                                          ISRG
Category: Standards Track                                  February 2020
ISSN: 2070-1721

   Automated Certificate Management Environment (ACME) IP Identifier
                          Validation Extension


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

Status of This Memo

   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

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
   2.  Terminology
   3.  IP Identifier
   4.  Identifier Validation Challenges
   5.  HTTP Challenge
   6.  TLS with Application-Layer Protocol Negotiation (TLS ALPN)
   7.  DNS Challenge
   8.  IANA Considerations
     8.1.  Identifier Types
     8.2.  Challenge Types
   9.  Security Considerations
     9.1.  Certification Authority (CA) Policy Considerations
   10. Normative References
   Author's Address

1.  Introduction

   The Automatic Certificate Management Environment (ACME) [RFC8555]
   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 [RFC8737] can
   be used to validate IP identifiers.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  IP Identifier

   [RFC8555] 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 [RFC1123] for IPv4 and in Section 4 of
   [RFC5952] for IPv6.

   An identifier for the IPv6 address 2001:db8::1 would be formatted
   like so:

   {"type": "ip", "value": "2001:db8::1"}

4.  Identifier Validation Challenges

   IP identifiers MAY be used with the existing "http-01" (see
   Section 8.3 of [RFC8555]) and "tls-alpn-01" (see Section 3 of
   [RFC8737]).  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.

5.  HTTP Challenge

   For the "http-01" challenge, the Host header field MUST be set to the
   IP address being used for validation per [RFC7230].  The textual form
   of this address MUST be as defined in Section 2.1 of [RFC1123] for
   IPv4 and in Section 4 of [RFC5952] for IPv6.

6.  TLS with Application-Layer Protocol Negotiation (TLS ALPN) Challenge

   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 [RFC6066] 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
   [RFC1034] or IP6.ARPA [RFC3596] 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."

7.  DNS Challenge

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

8.  IANA Considerations

8.1.  Identifier Types

   Per this document, a new type has been added to the "ACME Identifier
   Types" registry defined in Section 9.7.7 of [RFC8555] with Label "ip"
   and Reference "RFC 8738".

8.2.  Challenge Types

   Per this document, two new entries have been added to the "ACME
   Validation Methods" registry defined in Section 9.7.8 of [RFC8555].
   These entries are defined below:

           | Label       | Identifier Type | ACME | Reference |
           | http-01     | ip              | Y    | RFC 8738  |
           | tls-alpn-01 | ip              | Y    | RFC 8738  |

                                 Table 1

9.  Security Considerations

   The extensions to ACME described in this document do not deviate from
   the broader threat model described in Section 10.1 of [RFC8555].

9.1.  Certification Authority (CA) Policy Considerations

   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.

10.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", STD 88,
              RFC 3596, DOI 10.17487/RFC3596, October 2003,

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [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/info/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,

   [RFC8737]  Shoemaker, R.B., "Automated Certificate Management
              Environment (ACME) TLS Application-Layer Protocol
              Negotiation (ALPN) Challenge Extension", RFC 8737,
              DOI 10.17487/RFC8737, February 2020,


   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.

Author's Address

   Roland Bracewell Shoemaker
   Internet Security Research Group

   Email: roland@letsencrypt.org