Internet DRAFT - draft-friel-acme-subdomains
draft-friel-acme-subdomains
Network Working Group O. Friel
Internet-Draft R. Barnes
Intended status: Standards Track Cisco
Expires: April 28, 2022 T. Hollebeek
DigiCert
M. Richardson
Sandelman Software Works
October 25, 2021
ACME for Subdomains
draft-friel-acme-subdomains-06
Abstract
This document outlines how ACME can be used by a client to obtain a
certificate for a subdomain identifier from a certification
authority. The client has fulfilled a challenge against a parent
domain but does not need to fulfill a challenge against the explicit
subdomain as certificate policy allows issuance of the subdomain
certificate without explicit subdomain ownership proof.
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
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This Internet-Draft will expire on April 28, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. ACME Workflow and Identifier Requirements . . . . . . . . . . 4
4. ACME Issuance of Subdomain Certificates . . . . . . . . . . . 5
4.1. ACME Challenge Type . . . . . . . . . . . . . . . . . . . 6
4.2. Authorization Object . . . . . . . . . . . . . . . . . . 6
4.3. Pre-Authorization . . . . . . . . . . . . . . . . . . . . 7
4.4. New Orders . . . . . . . . . . . . . . . . . . . . . . . 8
4.5. Directory Object Metadata . . . . . . . . . . . . . . . . 10
5. Illustrative Call Flow . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6.1. Authorization Object Fields Registry . . . . . . . . . . 16
6.2. Directory Object Metadata Fields Registry . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7.1. ACME Server Policy Considerations . . . . . . . . . . . . 18
8. Informative References . . . . . . . . . . . . . . . . . . . 18
Appendix A. CA Browser Forum Baseline Requirements Extracts . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
ACME [RFC8555] defines a protocol that a certification authority (CA)
and an applicant can use to automate the process of domain name
ownership validation and X.509v3 (PKIX) [RFC5280] certificate
issuance. This document outlines how ACME can be used to issue
subdomain certificates, without requiring the ACME client to
explicitly fulfill an ownership challenge against the subdomain
identifiers - the ACME client need only fulfill an ownership
challenge against a parent domain identifier.
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.
The following terms are defined in DNS Terminology [RFC8499] and are
reproduced here:
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o Label: An ordered list of zero or more octets that makes up a
portion of a domain name. Using graph theory, a label identifies
one node in a portion of the graph of all possible domain names.
o Domain Name: An ordered list of one or more labels.
o Subdomain: "A domain is a subdomain of another domain if it is
contained within that domain. This relationship can be tested by
seeing if the subdomain's name ends with the containing domain's
name." (Quoted from [RFC1034], Section 3.1) For example, in the
host name "nnn.mmm.example.com", both "mmm.example.com" and
"nnn.mmm.example.com" are subdomains of "example.com". Note that
the comparisons here are done on whole labels; that is,
"ooo.example.com" is not a subdomain of "oo.example.com".
o Fully-Qualified Domain Name (FQDN): This is often just a clear way
of saying the same thing as "domain name of a node", as outlined
above. However, the term is ambiguous. Strictly speaking, a
fully-qualified domain name would include every label, including
the zero-length label of the root: such a name would be written
"www.example.net." (note the terminating dot). But, because every
name eventually shares the common root, names are often written
relative to the root (such as "www.example.net") and are still
called "fully qualified". This term first appeared in [RFC0819].
In this document, names are often written relative to the root.
The following terms are defined in the CA/Browser Forum Baseline
Requirements [CAB] version 1.7.1 and are reproduced here:
o Authorization Domain Name (ADN): The Domain Name used to obtain
authorization for certificate issuance for a given FQDN. The CA
may use the FQDN returned from a DNS CNAME lookup as the FQDN for
the purposes of domain validation. If the FQDN contains a
wildcard character, then the CA MUST remove all wildcard labels
from the left most portion of requested FQDN. The CA may prune
zero or more labels from left to right until encountering a Base
Domain Name and may use any one of the intermediate values for the
purpose of domain validation
o Base Domain Name: The portion of an applied-for FQDN that is the
first domain name node left of a registry-controlled or public
suffix plus the registry-controlled or public suffix (e.g.
"example.co.uk" or "example.com"). For FQDNs where the right-most
domain name node is a gTLD having ICANN Specification 13 in its
registry agreement, the gTLD itself may be used as the Base Domain
Name.
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o Certification Authority (CA): An organization that is responsible
for the creation, issuance, revocation, and management of
Certificates. The term applies equally to both Roots CAs and
Subordinate CAs
o Domain Namespace: The set of all possible Domain Names that are
subordinate to a single node in the Domain Name System
The following additional terms are used in this document:
o Certification Authority (CA): An organization that is responsible
for the creation, issuance, revocation, and management of
Certificates. The term applies equally to both Roots CAs and
Subordinate CAs
o CSR: Certificate Signing Request
o Parent Domain: a domain is a parent domain of a subdomain if it
contains that subdomain, as per the [RFC8499] definition of
subdomain. For example, for the host name "nnn.mmm.example.com",
both "mmm.example.com" and "example.com" are parent domains of
"nnn.mmm.example.com".
3. ACME Workflow and Identifier Requirements
A typical ACME workflow for issuance of certificates is as follows:
1. client POSTs a newOrder request that contains a set of
"identifiers"
2. server replies with a set of "authorizations" and a "finalize"
URI
3. client sends POST-as-GET requests to retrieve the
"authorizations", with the downloaded "authorization" object(s)
containing the "identifier" that the client must prove that they
control, and a set of associated "challenges", one of which the
the client must fulfil
4. client proves control over the "identifier" in the
"authorization" object by completing one of the specified
challenges, for example, by publishing a DNS TXT record
5. client POSTs a CSR to the "finalize" API
6. server replies with an updated order object that includes a
"certificate" URI
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7. client sends POST-as-GET request to the "certificate" URI to
download the certificate
ACME places the following restrictions on "identifiers":
o [RFC8555] section 7.1.3: The authorizations required are dictated
by server policy; there may not be a 1:1 relationship between the
order identifiers and the authorizations required.
o [RFC8555] section 7.1.4: the only type of "identifier" defined by
the ACME specification is an FQDN: "The only type of identifier
defined by this specification is a fully qualified domain name
(type: "dns"). The domain name MUST be encoded in the form in
which it would appear in a certificate."
o [RFC8555] section 7.4: the "identifier" in the CSR request must
match the "identifier" in the newOrder request: "The CSR MUST
indicate the exact same set of requested identifiers as the
initial newOrder request."
o [RFC8555] section 8.3: the "identifier", or FQDN, in the
"authorization" object must be used when fulfilling challenges via
HTTP: "Construct a URL by populating the URL template ... where
the domain field is set to the domain name being verified"
o [RFC8555] section 8.4: the "identifier", or FQDN, in the
"authorization" object must be used when fulfilling challenges via
DNS: "The client constructs the validation domain name by
prepending the label "_acme-challenge" to the domain name being
validated."
ACME does not mandate that the "identifier" in a newOrder request
matches the "identifier" in "authorization" objects.
4. ACME Issuance of Subdomain Certificates
As noted in the previous section, ACME does not mandate that the
"identifier" in a newOrder request matches the "identifier" in
"authorization" objects. This means that the ACME specification does
not preclude an ACME server processing newOrder requests and issuing
certificates for a subdomain without requiring a challenge to be
fulfilled against that explicit subdomain.
ACME server policy could allow issuance of certificates for a
subdomain to a client where the client only has to fulfill an
authorization challenge for a parent domain of that subdomain. This
allows a flow where a client proves ownership of, for example,
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"example.org" and then successfully obtains a certificate for
"sub.example.org".
ACME server policy is out of scope of this document, however some
commentary is provided in Section 7.1.
Clients need a mechanism to instruct the ACME server that they are
requesting authorization for a Domain Namespace subordinate to a
given ADN, as opposed to just requesting authorization for an
explicit ADN identifier. Clients need a mechanism to do this in both
newAuthz and newOrder requests. ACME servers need a mechanism to
indicate to clients that authorization objects are valid for an
entire Domain Namespace. These are described in this section.
4.1. ACME Challenge Type
ACME for subdomains is restricted for use with "dns-01" challenges.
If a server policy allows a client to fulfill a challenge against a
parent ADN of a requested certificate FQDN identifier, then the
server MUST issue a "dns-01" challenge against that parent ADN.
4.2. Authorization Object
ACME [RFC8555] section 7.1.4 defines the authorization object. When
ACME server policy allows authorization for Domain Namespaces
subordinate to an ADN, the server indicates this by including the
"domainNamespace" flag in the authorization object for that ADN
identifier:
domainNamespace (optional, boolean): This field MUST be present
and true for authorizations where ACME server policy allows
certificates to be issued for any Domain Name in the Domain
Namespace subordinate to the ADN specified in the 'identifier'
field of the authorization object.
The following example shows an authorization object for the ADN
"example.org" where the authorization covers the Domain Namespace
subordinate to "example.org".
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{
"status": "valid",
"expires": "2015-03-01T14:09:07.99Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"url": "https://example.com/acme/chall/prV_B7yEyA4",
"type": "http-01",
"status": "valid",
"token": "DGyRejmCefe7v4NfDGDKfA",
"validated": "2014-12-01T12:05:58.16Z"
}
],
"domainNamespace": true
}
If the "domainNamespace" field is not included, then the assumed
default value is false.
4.3. Pre-Authorization
The standard ACME workflow has authorization objects created
reactively in response to a certificate order. ACME also allows for
pre-authorization, where clients obtain authorization for an
identifier proactively, outside of the context of a specific
issuance. With the ACME pre-authorization flow, a client can pre-
authorize for a parent ADN once, and then issue multiple newOrder
requests for certificates with identifiers in the Domain Namespace
subordinate to that ADN.
ACME [RFC8555] section 7.4.1 defines the "identifier" object for
newAuthz requests. One additional field for the "identifier" object
is defined:
domainNamespace (optional, boolean): An ACME client sets this flag
to indicate to the server that it is requesting an authorization
for the Domain Namespace subordinate to the specified ADN
identifier value
Clients include the flag in the "identifier" object of newAuthz
requests to indicate that they are requesting a Domain Namespace
authorization. In the following example newAuthz payload, the client
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is requesting pre-authorization for the Domain Namespace subordinate
to "example.org".
"payload": base64url({
"identifier": {
"type": "dns",
"value": "example.org",
"domainNamespace": true
}
})
If the server is willing to allow a single authorization for the
Domain Namespace, and there is not an existing authorization object
for the identifier, then it will create an authorization object and
include the "domainNamespace" flag with value of true. If the server
policy does not allow creation of Domain Namespace authorizations
subordinate to that ADN, the server can create an authorization
object for the indicated identifier, and include the
"domainNamespace" flag with value of false. In both scenarios,
handling of the pre-authorization follows the process documented in
ACME section 7.4.1.
4.4. New Orders
Clients need a mechanism to optionally indicate to servers whether or
not they are authorized to fulfill challenges against parent ADNs for
a given identifier FQDN. For example, if a client places an order
for an identifier "foo.bar.example.org", and is authorized to update
DNS TXT records against the parent ADNs "bar.example.org" or
"example.org", then the client needs a mechanism to indicate control
over the parent ADNs to the ACME server.
This can be achieved by adding an optional field "domainNamespace" to
the "identifiers" field in the order object:
domainNamespace (optional, string): This is the parent ADN of a
Domain Namespace that the requested identifier belongs to. The
client MUST have DNS control over the parent ADN.
This field specifies the ADN of the Domain Namespace that the client
has DNS control over, and is capable of fulfilling challenges
against. Based on server policy, the server can choose to issue a
challenge against any parent domain of the identifier in the Domain
Namespace up to and including the specified "domainNamespace", and
create a corresponding authorization object against the chosen
identifier.
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In the following example newOrder payload, the client requests a
certificate for identifier "foo.bar.example.org" and indicates that
it can fulfill a challenge against the parent ADN and the Domain
Namespace subordinate to "bar.example.org". The server can then
choose to issue a challenge against either "foo.bar.example.org" or
"bar.example.org" identifiers.
"payload": base64url({
"identifiers": [
{ "type": "dns",
"value": "foo.bar.example.org",
"domainNamespace": "bar.example.org" }
],
"notBefore": "2016-01-01T00:04:00+04:00",
"notAfter": "2016-01-08T00:04:00+04:00"
})
In the following example newOrder payload, the client requests a
certificate for identifier "foo.bar.example.org" and indicates that
it can fulfill a challenge against the parent ADN and the Domain
Namespace subordinate to "example.org". The server can then choose
to issue a challenge against any one of "foo.bar.example.org",
"bar.example.org" or "example.org" identifiers.
"payload": base64url({
"identifiers": [
{ "type": "dns",
"value": "foo.bar.example.org",
"domainNamespace": "example.org" }
],
"notBefore": "2016-01-01T00:04:00+04:00",
"notAfter": "2016-01-08T00:04:00+04:00"
})
If the client is unable to fulfill authorizations against parent
ADNs, the client should not include the "domainNamespace" field.
Server newOrder handling generally follows the process documented
ACME section 7.4. If the server is willing to allow Domain Namespace
authorizations for the ADN specified in "domainNamespace", then it
creates an authorization object against that ADN and includes the
"domainNamespace" flag with a value of true. If the server policy
does not allow creation of Domain Namespace authorizations against
that ADN, then it can create an authorization object for the
indicated identifier value, and include the "domainNamespace" flag
with value of false.
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4.5. Directory Object Metadata
An ACME server can advertise support for authorization of Domain
Namespaces by including the following boolean flag in its "ACME
Directory Metadata Fields" registry:
domainNamespace (optional, bool): Indicates if an ACME server
supports authorization of Domain Namespaces.
If not specified, then no default value is assumed. If an ACME
server supports authorization of Domain Namespaces, it can indicate
this by including this field with a value of "true".
5. Illustrative Call Flow
The call flow illustrated here uses the ACME pre-authorization flow
using DNS-based proof of ownership.
+--------+ +------+ +-----+
| Client | | ACME | | DNS |
+--------+ +------+ +-----+
| | |
STEP 1: Pre-Authorization of parent domain
| | |
| POST /newAuthz | |
| "example.org" | |
|--------------------------->| |
| | |
| 201 authorizations | |
|<---------------------------| |
| | |
| Publish DNS TXT | |
| "example.org" | |
|--------------------------------------->|
| | |
| POST /challenge | |
|--------------------------->| |
| | Verify |
| |---------->|
| 200 status=valid | |
|<---------------------------| |
| | |
| Delete DNS TXT | |
| "example.org" | |
|--------------------------------------->|
| | |
STEP 2: Place order for sub1.example.org
| | |
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| POST /newOrder | |
| "sub1.example.org" | |
|--------------------------->| |
| | |
| 201 status=ready | |
|<---------------------------| |
| | |
| POST /finalize | |
| CSR SAN "sub1.example.org" | |
|--------------------------->| |
| | |
| 200 OK status=valid | |
|<---------------------------| |
| | |
| POST /certificate | |
|--------------------------->| |
| | |
| 200 OK | |
| PEM SAN "sub1.example.org" | |
|<---------------------------| |
| | |
STEP 3: Place order for sub2.example.org
| | |
| POST /newOrder | |
| "sub2.example.org" | |
|--------------------------->| |
| | |
| 201 status=ready | |
|<---------------------------| |
| | |
| POST /finalize | |
| CSR SAN "sub2.example.org" | |
|--------------------------->| |
| | |
| 200 OK status=valid | |
|<---------------------------| |
| | |
| POST /certificate | |
|--------------------------->| |
| | |
| 200 OK | |
| PEM SAN "sub2.example.org" | |
|<---------------------------| |
o STEP 1: Pre-authorization of Domain Namespace
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The client sends a newAuthz request for the parent ADN of the
Domain Namespace including the "domainNamespace" flag in the
identifier object.
POST /acme/new-authz HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "uQpSjlRb4vQVCjVYAyyUWg",
"url": "https://example.com/acme/new-authz"
}),
"payload": base64url({
"identifier": {
"type": "dns",
"value": "example.org",
"domainNamespace": true
}
}),
"signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
}
The server creates and returns an authorization object for the
identifier including the "domainNamespace" flag. The object is
initially in "pending" state. Once the client completes the
challenge, the server will transition the authorization object and
associated challenge object status to "valid".
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{
"status": "pending",
"expires": "2015-03-01T14:09:07.99Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"url": "https://example.com/acme/chall/prV_B7yEyA4",
"type": "http-01",
"status": "pending",
"token": "DGyRejmCefe7v4NfDGDKfA",
"validated": "2014-12-01T12:05:58.16Z"
}
],
"domainNamespace": true
}
o STEP 2: The client places a newOrder for "sub1.example.org"
The client sends a newOrder request to the server and includes the
subdomain identifier. Note that the identifier is in the Domain
Namespace that has been pre-authorised in step 1. The client does
not need to include the "domainNamespace" field in the
"identifier" object as it has already pre-authorized the Domain
Namespace.
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POST /acme/new-order HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://example.com/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{ "type": "dns", "value": "sub1.example.org" }
],
"notBefore": "2016-01-01T00:04:00+04:00",
"notAfter": "2016-01-08T00:04:00+04:00"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
As an authorization object already exists for the parent ADN of the
Domain Namespace, the server replies with an order object with a
status of "valid" that includes a link to the existing "valid"
authorization object.
HTTP/1.1 201 Created
Replay-Nonce: MYAuvOpaoIiywTezizk5vw
Link: <https://example.com/acme/directory>;rel="index"
Location: https://example.com/acme/order/TOlocE8rfgo
{
"status": "valid",
"expires": "2016-01-05T14:09:07.99Z",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z",
"identifiers": [
{ "type": "dns", "value": "sub1.example.org" }
],
"authorizations": [
"https://example.com/acme/authz/PAniVnsZcis"
],
"finalize": "https://example.com/acme/order/TOlocrfgo/finalize"
}
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The client can proceed to finalize the order and download the
certificate for "sub1.example.org".
o STEP 3: The client places a newOrder for "sub2.example.org"
The client sends a newOrder request to the server and includes the
subdomain identifier. Note that the identifier is in the Domain
Namespace that has been pre-authorised in step 1. The client does
not need to include the "domainNamespace" field in the
"identifier" object as it has already pre-authorized the Domain
Namespace.
POST /acme/new-order HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://example.com/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{ "type": "dns", "value": "sub2.example.org" }
],
"notBefore": "2016-01-01T00:04:00+04:00",
"notAfter": "2016-01-08T00:04:00+04:00"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
As an authorization object already exists for the parent ADN of the
Domain Namespace, the server replies with an order object with a
status of "valid" that includes a link to the existing "valid"
authorization object.
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HTTP/1.1 201 Created
Replay-Nonce: MYAuvOpaoIiywTezizk5vw
Link: <https://example.com/acme/directory>;rel="index"
Location: https://example.com/acme/order/TOlocE8rfgo
{
"status": "valid",
"expires": "2016-01-05T14:09:07.99Z",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z",
"identifiers": [
{ "type": "dns", "value": "sub1.example.org" }
],
"authorizations": [
"https://example.com/acme/authz/PAniVnsZcis"
],
"finalize": "https://example.com/acme/order/ROni7rdde/finalize"
}
The client can proceed to finalize the order and download the
certificate for "sub2.example.org".
6. IANA Considerations
6.1. Authorization Object Fields Registry
The following field is added to the "ACME Authorization Object
Fields" registry defined in ACME [RFC8555].
+-----------------+------------+--------------+-----------+
| Field Name | Field Type | Configurable | Reference |
+-----------------+------------+--------------+-----------+
| domainNamespace | boolean | false | RFC XXXX |
+-----------------+------------+--------------+-----------+
6.2. Directory Object Metadata Fields Registry
The following field is added to the "ACME Directory Metadata Fields"
registry defined in ACME [RFC8555].
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+-----------------+------------+-----------+
| Field Name | Field Type | Reference |
+-----------------+------------+-----------+
| domainNamespace | boolean | RFC XXXX |
+-----------------+------------+-----------+
7. Security Considerations
This document documents enhancements to ACME [RFC8555] that optimize
the protocol flows for issuance of certificates for subdomains. The
underlying goal of ACME for Subdomains remains the same as that of
ACME: managing certificates that attest to identifier/key bindings
for these subdomains. Thus, ACME for Subdomains has the same two
security goals as ACME:
1. Only an entity that controls an identifier can get an
authorization for that identifier
2. Once authorized, an account key's authorizations cannot be
improperly used by another account
ACME for Subdomains makes no changes to:
o account or account key management
o ACME channel establishment, security mechanisms or threat model
o Validation channel establishment, security mechanisms or threat
model
Therefore, all Security Considerations in ACME in the following areas
are equally applicable to ACME for Subdomains:
o Threat Model
o Integrity of Authorizations
o Denial-of-Service Considerations
o Server-Side Request Forgery
o CA Policy Considerations
Some additional comments on ACME server policy are given in the
following section.
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7.1. ACME Server Policy Considerations
The ACME for Subdomains and the ACME specifications do not mandate
any specific ACME server or CA policies, or any specific use cases
for issuance of certificates. For example, an ACME server could be
used:
o to issue Web PKI certificates where the ACME server must comply
with CA/Browser Forum [CAB] Baseline Requirements.
o as a Private CA for issuance of certificates within an
organisation. The organisation could enforce whatever policies
they desire on the ACME server.
o for issuance of IoT device certificates. There are currently no
IoT device certificate policies that are generally enforced across
the industry. Organizations issuing IoT device certificates can
enforce whatever policies they desire on the ACME server.
ACME server policy could specify whether:
o issuance of subdomain certificates is allowed based on proof of
ownership of a parent domain
o issuance of subdomain certificates is allowed, but only for a
specific set of parent domains
o whether DNS based proof of ownership, or HTTP based proof of
ownership, or both, are allowed
ACME server policy specification is explicitly out of scope of this
document. For reference, extracts from CA/Browser Forum Baseline
Requirements are given in the appendices.
8. Informative References
[CAB] CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates", n.d.,
<https://cabforum.org/wp-content/uploads/CA-Browser-Forum-
BR-1.7.1.pdf>.
[RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for
Internet User Applications", RFC 819,
DOI 10.17487/RFC0819, August 1982,
<https://www.rfc-editor.org/info/rfc819>.
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Internet-Draft ACME-SUBDOMAINS October 2021
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[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/info/rfc2119>.
[RFC5280] 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, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[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/info/rfc8555>.
Appendix A. CA Browser Forum Baseline Requirements Extracts
The CA/Browser Forum Baseline Requirements [CAB] allow issuance of
subdomain certificates where authorization is only required for a
parent domain. Baseline Requirements version 1.7.1 states:
o Section: "1.6.1 Definitions": Authorization Domain Name: The
Domain Name used to obtain authorization for certificate issuance
for a given FQDN. The CA may use the FQDN returned from a DNS
CNAME lookup as the FQDN for the purposes of domain validation.
If the FQDN contains a wildcard character, then the CA MUST remove
all wildcard labels from the left most portion of requested FQDN.
The CA may prune zero or more labels from left to right until
encountering a Base Domain Name and may use any one of the
intermediate values for the purpose of domain validation.
o Section: "3.2.2.4.6 Agreed-Upon Change to Website": Once the FQDN
has been validated using this method, the CA MAY also issue
Certificates for other FQDNs that end with all the labels of the
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validated FQDN. This method is suitable for validating Wildcard
Domain Names.
o Section: "3.2.2.4.7 DNS Change": Once the FQDN has been validated
using this method, the CA MAY also issue Certificates for other
FQDNs that end with all the labels of the validated FQDN. This
method is suitable for validating Wildcard Domain Names.
Authors' Addresses
Owen Friel
Cisco
Email: ofriel@cisco.com
Richard Barnes
Cisco
Email: rlb@ipv.sx
Tim Hollebeek
DigiCert
Email: tim.hollebeek@digicert.com
Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
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