Automated Certificate Management Environment (ACME) IP Identifier Validation Extension
Internet Security Research Group
roland@letsencrypt.org
Security
ACME Working Group
acme
pki
This document specifies identifiers and challenges required to enable the Automated Certificate Management Environment (ACME) to issue certificates for IP addresses.
Introduction
The Automatic Certificate Management Environment (ACME) 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 can be used to validate IP identifiers.
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
when, and only when, they appear in all capitals,
as shown here.
IP Identifier
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 for IPv4 and in
for IPv6.
An identifier for the IPv6 address 2001:db8::1 would be formatted
like so:
Identifier Validation Challenges
IP identifiers MAY be used with the existing "http-01"
(see ) and
"tls-alpn-01" (see ). 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.
HTTP Challenge
For the "http-01" challenge, the Host header field
MUST be set to the IP address being used for validation
per . The textual form of this
address MUST be as defined in for IPv4 and in for IPv6.
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 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 or IP6.ARPA
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".
DNS Challenge
The existing "dns-01" challenge MUST NOT be used to validate IP identifiers.
IANA Considerations
Identifier Types
Per this document, a new type has been added to the "ACME Identifier Types"
registry defined in with Label "ip" and Reference
"RFC 8738".
Challenge Types
Per this document, two new entries have been added to the "ACME Validation Methods"
registry defined in . These entries are defined below:
Label |
Identifier Type |
ACME |
Reference |
http-01 |
ip |
Y |
RFC 8738 |
tls-alpn-01 |
ip |
Y |
RFC 8738 |
Security Considerations
The extensions to ACME described in this document do not deviate from
the broader threat model described in .
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.
Normative References
Automated Certificate Management Environment (ACME) TLS
Application-Layer Protocol Negotiation (ALPN) Challenge Extension
Acknowledgments
The author would like to thank those who contributed to this document
and offered editorial and technical input, especially
and .