wdiff draft-ietf-uta-rfc6125bis-02.txt draft-ietf-uta-rfc6125bis-03.txt

Network Working Group P. Saint-Andre
Internet-Draft Mozilla
Obsoletes: 6125 (if approved) J. Hodges
Intended status: Standards Track Google
Expires: 12 March 16 April 2022 R. Salz
Akamai Technologies
8 September
13 October 2021

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)
draft-ietf-uta-rfc6125bis-02
draft-ietf-uta-rfc6125bis-03

Abstract

Many application technologies enable secure communication between two
entities by means of Transport Layer Security (TLS) with Internet
Public Key Infrastructure Using X.509 (PKIX) certificates. This
document specifies procedures for representing and verifying the
identity of application services in such interactions.

This document obsoletes RFC 6125.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the Using TLS in
Applications Working Group mailing list (uta@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/uta/.

Source for this draft and an issue tracker can be found at
https://github.com/richsalz/draft-ietf-uta-rfc6125bis.

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 12 March 16 April 2022.

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Please review these documents carefully, as they describe your rights
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Audience . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Changes since RFC 6125 . . . . . . . . . . . . . . . . . 4
1.4. How to Read This Document . . . . . . . . . . . . . . . . 5
1.5. 3
1.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
1.6. 4
1.4. Overview of Recommendations . . . . . . . . . . . . . . . 6
1.7. 4
1.5. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.7.1. 5
1.5.1. In Scope . . . . . . . . . . . . . . . . . . . . . . 7
1.7.2. 5
1.5.2. Out of Scope . . . . . . . . . . . . . . . . . . . . 7
1.8. 5
1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9 6
2. Naming of Application Services . . . . . . . . . . . . . . . 12
2.1. Naming Application Services . . . . . . . . . . . . . . . 12
2.2. DNS Domain Names . . . . . . . . . . . . . . . . . . . . 13
2.3. Subject Naming in PKIX Certificates . . . . . . . . . . . 14 9
3. Designing Application Protocols . . . . . . . . . . . . . . . 14 10
4. Representing Server Identity . . . . . . . . . . . . . . . . 15 11
4.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 15 11
4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 16 11
5. Requesting Server Certificates . . . . . . . . . . . . . . . 16 12
6. Verifying Service Identity . . . . . . . . . . . . . . . . . 17 13
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2. Constructing a List of Reference Identifiers . . . . . . 18
6.2.1. 13
6.1.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2.2. 13
6.1.2. Examples . . . . . . . . . . . . . . . . . . . . . . 20
6.3. 15
6.2. Preparing to Seek a Match . . . . . . . . . . . . . . . . 21
6.4. 15
6.3. Matching the DNS Domain Name Portion . . . . . . . . . . 22
6.4.1. Checking of Traditional Domain Names 16
6.4. Matching the Application Service Type Portion . . . . . . 17
6.5. Outcome . . 22
6.4.2. Checking of Internationalized Domain Names . . . . . 22
6.4.3. Checking of Wildcard Certificates . . . . . . . . . . 23

6.5. Matching the Application Service Type Portion . . . . . . 23
6.5.1. SRV-ID . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7.1. Wildcard Certificates . . 24
6.5.2. URI-ID . . . . . . . . . . . . . . . . 19
7.2. Internationalized Domain Names . . . . . . . 24
6.6. Outcome . . . . . . 19
7.3. Multiple Presented Identifiers . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . 24
6.6.1. Case #1: Match Found . . . . . . . . . . . . . . . 20
9. References . 24
6.6.2. Case #2: No Match Found, Pinned Certificate . . . . . 24
6.6.3. Case #3: No Match Found, No Pinned Certificate . . . 24
6.6.4. Fallback . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
7.1. Pinned Certificates . . . . . . . . . . . . . . . . . . . 25
7.2. Wildcard Certificates . . . . . . . . . . . . . . . . . . 26
7.3. Internationalized Domain Names . . . . . . . . . . . . . 26
7.4. Multiple Identifiers . . . . . . . . . . . . . . . . . . 26
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.1. Normative References . . . . . . . . . . . . . . . . . . 26
8.2. 20
9.2. Informative References . . . . . . . . . . . . . . . . . 27 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 30 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30 24

1. Introduction

1.1. Motivation

The visible face of the Internet largely consists of services that
employ a client-server architecture in which an interactive or
automated client communicates with an application service in order to
retrieve or upload information, communicate with other entities, or
access a broader network of services. service. When a
client communicates with an application service using Transport Layer
Security [TLS] or Datagram Transport Layer Security [DTLS], it references has
some notion of the server's identity (e.g., "the website at
example.com") while attempting to establish secure communication.
Likewise, during TLS negotiation, the server presents its notion of
the service's identity in the form of a public-key certificate that
was issued by a certification authority (CA) in the context of the
Internet Public Key Infrastructure using X.509 [PKIX]. Informally,
we can think of these identities as the client's "reference identity"
and the server's "presented identity" (these rough ideas (more formal definitions are defined more
precisely later in this document through the concept of particular
identifiers). In general, a
given later). A client needs to verify that the server's presented
identity matches its reference identity so it can authenticate the
communication.

Many application technologies adhere to the pattern just outlined.
Such protocols have traditionally specified their own rules

This document defines procedures for
representing and verifying application service identity.
Unfortunately, how clients do this divergence of approaches has caused some
confusion among certification authorities, application developers,
verification. It therefore implicitly defines requirements on other
parties, such as the CA's that issue certificates, the service
administrators requesting them, and protocol designers.

Therefore, to codify secure procedures for the implementation protocol designers defining
how things are named.

1.2. Changes since RFC 6125

This document revises and
deployment obsoletes [VERIFY] based on the decade of PKIX-based authentication, this document specifies
recommended procedures for representing and verifying application
service identity in certificates intended for use in application
protocols employing TLS.

1.2. Audience

The primary audience for this document consists of application
protocol designers, who can reference this document instead of
defining their own rules for the representation and verification of
application service identity. Secondarily, the audience consists of
certification authorities, service providers, and client developers
from technology communities that might reuse the recommendations in
this document when defining certificate issuance policies, generating
certificate signing requests, or writing software algorithms for
identity matching.

1.3. Changes since RFC 6125

This document revises and obsoletes [VERIFY] based on the decade of
experience
experience and changes since it was first published. The major
changes, in no particular order, include:

* All references have been updated to the current latest version.

* The TLS SNI extension is no longer new, it is commonplace.

* The only legal place for a certificate wildcard name is as the
left-most component in a domain name.

* It is no longer allowed to use the commonName RDN, known as "CN-
ID", CN-ID,
to represent the server identity; only the subjectAltNames
extension is used.

* References to the X.500 directory, the survey of prior art, and
the sample text in Appendix A have been removed.

1.4. How to Read This Document

This document is longer than the authors would have liked because it
was necessary to carefully define terminology, explain the underlying
concepts, define the scope, and specify recommended behavior for both
certification authorities and application software implementations.
The following sections are of special interest to various audiences:

* Protocol designers might want to first read the checklist in
Section 3.

* Certification authorities might want to first read the
recommendations for representation Detailed discussion of server identity in
Section 4.

* Service providers might want to first read the recommendations for
requesting pinning (configuring use of server certificates in Section 5.

* Software implementers might want to first read a certificate
that doesn't match the recommendations
for verification of server identity criteria in Section 6. this document) has been
removed.

* The sections on terminology (Section 1.8), naming of application
services (Section 2), document scope (Section 1.7), and the like
provide useful background information regarding the recommendations detailing different target audiences and guidelines that are contained in the above-referenced sections,
but are not absolutely necessary for a first reading of this
document.

1.5. which
sections to read (first) have been removed.

1.3. Applicability

This document does not supersede the rules for certificate issuance
or validation provided in specified by [PKIX]. Therefore, [PKIX] is authoritative
on any point that might also be discussed in this document.
Furthermore, [PKIX] That document also governs any
certificate-related topic on which this document is silent, including but not limited to silent. This
includes certificate syntax, certificate extensions such as name
constraints
and or extended key usage, and handling of certification
paths.

This document addresses only name forms in the leaf "end entity"
server certificate, certificate. It does not any address the name forms in the chain
of certificates used to validate a cetrificate, let alone creating or
checking the server certificate. Therefore, in validity of such a chain. In order to ensure proper
authentication, application clients applications need to verify the entire certification
path as per [PKIX].

This document also does not supersede the rules for verifying service
identity provided in specifications for existing application
protocols published prior to this document. However, the procedures
described here can be referenced by future specifications, including
updates to specifications for existing application protocols if the
relevant technology communities agree to do so.

1.6.

1.4. Overview of Recommendations

To orient the reader, this section provides an informational overview
of the recommendations contained in this document.

The previous version of this specification, [VERIFY], surveyed the
current practice from many IETF standards and tried to generalize
best practices. This document takes the lessons learned in the past
decade and codifies them as best practices.

For the primary audience of application protocol designers, this
document provides recommended procedures for the representation and
verification of application service identity within PKIX certificates
used in the context of TLS.

For the secondary audiences, in essence this document encourages
certification authorities, application service providers, and
application client developers to coalesce on the following practices:

* Stop including and checking strings that look like domain names in
the subject's Common Name. them. he rules are brief:

* Check Only check DNS domain names via the subjectAlternativeName
extension designed for that purpose: dNSName.

* Move toward including and checking Allow use of even more specific subjectAlternativeName extensions
where appropriate for using the
protocol (e.g., such as uniformResourceIdentifier and the
otherName form
SRVName). SRVName.

* Constrain and simplify the validation of wildcard certificates
(e.g., so that the wildcard can only be
the left-most component of a certificate containing an identifier for
"*.example.com").

1.7. domain name.

* Do not include or check strings that look like domain names in the
subject's Common Name.

1.5. Scope
1.7.1.

1.5.1. In Scope

This document applies only to service identities associated with
fully qualified DNS domain names,
FQDNs only to TLS and DTLS, and only to PKIX-based systems. As a result,

TLS uses the scenarios described in words client and server, where the
following section are out of scope for this specification (although
they might be addressed by future specifications).

1.7.2. Out of Scope

The following topics are out of scope for this specification:

* Client or end-user identities.

Certificates representing client or end-user identities (e.g., is the
rfc822Name identifier) can be used for mutual authentication
between entity
that initiates the connection. In many cases, this models common
practice, such as a client and server or between two clients, thus enabling
stronger client-server security or end-to-end security. However,
certification authorities, application developers, browser connecting to a Web origin. For the sake
of clarity, and service
operators have less experience with to follow the usage in [TLS] and related
specifications, we will continue to use to use the terms client certificates than with and
server certificates, thus giving us fewer models from which in this document. Note that these are TLS-layer roles, and
that the application protocol could support the TLS server making
requests to
generalize the TLS client after the TLS handshake; these is no
requirement that the roles at the application layer match the TLS-
layer.

At the time of this writing, other protocols such as [QUIC] and
Network Time Security ([NTS]) use TLS as a less solid basis service to do the initial
establishment of cryptographic key material. Such services MUST also
follow the rules specified here.

1.5.2. Out of Scope

The following topics are out of scope for defining best practices. this specification:

* Identifiers Security protocols other than fully qualified DNS domain names.

For example, this specification does not discuss IP addresses [TLS] or [DTLS] except as described
above.

* Keys or certificates employed outside the context of PKIX-based
systems.

* Client or end-user identities. Certificates representing client
identities other attributes within a certificate than that described above, such as rfc822Name,
are beyond the subjectAltName
extension. The scope of this document.

* Identifiers other than FQDNs. Identifiers such as IP address are
not discussed. In addition, the focus of this document is on
application service identities, not specific resources located at
such services. Therefore this document discusses Uniform Resource
Identifiers [URI] only as a way to communicate a DNS domain name
(via the URI "host" component or its equivalent), not as a way to communicate other
aspects of a service such as a specific resource (via the URI
"path" component) or parameters (via the URI "query" component).

* Security protocols other than [TLS] Certification authority policies. This includes items such as the
following:

- How to certify or [DTLS].

Although other secure, lower-layer protocols exist and even employ
PKIX certificates at times (e.g., IPsec [IPSEC]), their use cases
can differ from those of TLS-based validate FQDNs and DTLS-based application
technologies. Furthermore, application technologies have less
experience with IPsec than service types
(see [ACME] for some definition of this).

- Issuing certificates with TLS, thus making it more difficult
to gather feedback on proposed best practices.

* Keys or certificates employed outside the context of PKIX-based
systems.

Some deployed application technologies use a web of trust model
based on or similar to OpenPGP [OPENPGP], or use self-signed
certificates, or are deployed on networks that are not directly
connected to the public Internet and therefore cannot depend on
Certificate Revocation Lists (CRLs) additional identifiers such as IP
address or the Online Certificate
Status Protocol [OCSP] to check CA-issued certificates. However,
the method for binding a public key to an identifier in OpenPGP
differs essentially from the method in X.509, the data in self-
signed certificates has not been certified by a third party relative domain name, in any
way, and checking of CA-issued certificates via CRLs or OCSP is
critically important to maintaining the security of PKIX-based
systems. Attempting addition to define best practices for such
technologies would unduly complicate the rules defined in this
specification.

* Certification authority policies, such as: FQDNs.

- What types Types or "classes" of certificates to issue and whether to
apply different policies for them.

- Whether to issue certificates based on IP addresses (or some
other form, such as relative domain names) in addition to fully
qualified DNS domain names.

- Which identifiers to include (e.g., whether to include SRV-IDs
or URI-IDs as defined in the body of this specification).

- How to certify or validate fully qualified DNS domain names and
application service types.

- How to certify or validate other kinds of information that
might be included in a certificate (e.g., organization name).

* Resolution of DNS domain names. Although the process whereby a
client resolves the DNS domain name of an application service can
involve several steps (e.g., this is
true of resolutions that depend on DNS SRV resource records,
Naming Authority Pointer (NAPTR) DNS resource records [NAPTR], and
related technologies such as [S-NAPTR]), steps, for our purposes we care only about the
fact that the client needs to verify the identity of the entity
with which it communicates as a result of the resolution process.
Thus the resolution process itself is out of scope for this
specification.

* User interface issues. In general, such issues are properly the
responsibility of client software developers and standards
development organizations dedicated to particular application
technologies (see, for example, [WSC-UI]).

1.8.

1.6. Terminology

Because many concepts related to "identity" are often too vague to be
actionable in application protocols, we define a set of more concrete
terms for use in this specification.

application service: A service on the Internet that enables
interactive and automated clients to connect for the purpose of
retrieving or uploading information, communicating with other
entities, or connecting to a broader network of services.

application service provider: An organization or individual that
hosts or deploys an application service.

application service type: A formal identifier for the application
protocol used to provide a particular kind of application service
at a domain; the application service type typically takes the form
of domain. This often apepars as a Uniform Resource Identifier URI scheme [URI] or a DNS
SRV Service [DNS-SRV].

automated client: A software agent or device that is not directly
controlled by a human user.

delegated domain: A domain name or host name that is explicitly
configured for communicating with the source domain, by either (a) the
human user controlling an interactive client or (b) a trusted
administrator. In case (a), one example of delegation is an
account setup that specifies the domain name of a particular host
to be used for retrieving information or connecting to a network,
which might be different from the server portion of the user's
account name (e.g., For example, a server at mailhost.example.com for
connecting to an IMAP server hosting an email address of
juliet@example.com). In case (b), one example of delegation is an
admin-configured host-to-address/address-to-host lookup table.
user@example.com.

derived domain: A domain name or host name that a client has derived
from the source domain in an automated fashion (e.g., by means of
a [DNS-SRV] lookup).

identifier: A particular instance of an identifier type that is
either presented by a server in a certificate or referenced by a
client for matching purposes.

identifier type: A formally defined category of identifier that can
be included in a certificate and therefore that can also be used
for matching purposes. For conciseness and convenience, we define
the following identifier types of interest, which are based on
those found in the PKIX specification [PKIX] and various PKIX
extensions.
extensions:

* DNS-ID = DNS-ID: a subjectAltName entry of type dNSName; see [PKIX] dNSName

* SRV-ID = SRV-ID: a subjectAltName entry of type otherName whose name
form is SRVName; see [SRVNAME]

* URI-ID = URI-ID: a subjectAltName entry of type
uniformResourceIdentifier whose value includes both (i) a
"scheme" and (ii) a "host" component (or its equivalent) that
matches the "reg-name" rule (where the quoted terms represent
the associated [ABNF] productions from [URI]); see [PKIX] [URI]) An entry which
does not have both the scheme and
[URI] host is not a valid URI-ID
and MUST be ignored.

interactive client: A software agent or device that is directly
controlled by a human user. (Other specifications related to
security and application protocols, such as [WSC-UI], often refer
to this entity user, commonly known as a "user agent".)

pinning: The act of establishing a cached name association between
the application service's certificate and one of the client's
reference identifiers, despite the fact that none of the presented
identifiers matches the given reference identifier. Pinning is
accomplished by allowing a human user to positively accept the
mismatch during an attempt to communicate with the application
service. Once a cached name association is established, the
certificate is said to be pinned to the reference identifier and
in future communication attempts the client simply verifies that
the service's presented certificate matches the pinned
certificate, as described under Section 6.6.2. (A similar
definition of "pinning" is provided in [WSC-UI].) agent."

PKIX: PKIX is a short name for the Internet Public Key
Infrastructure using X.509 defined in RFC 5280 [PKIX], which
comprises [PKIX]. That document
provides a profile of the X.509v3 certificate specifications and
X.509v2 certificate revocation list (CRL) specifications for use
in the Internet.

PKIX-based system: A software implementation or deployed service
that makes use of X.509v3 certificates and X.509v2 certificate
revocation lists (CRLs).

PKIX certificate: An X.509v3 certificate generated and employed in
the context of PKIX.

presented identifier: An identifier that is presented by a server to a
client within a PKIX certificate when the client attempts to
establish secure communication with the server; the server. The certificate
can include one or more presented identifiers of different types,
and if the server hosts more than one domain then the certificate
might present distinct identifiers for each domain.

reference identifier: An identifier, constructed from a source
domain and optionally an application service type, identifier used by the client for matching purposes when
examining presented identifiers. It is constructed from the
source domain, and optionally an application service type.

Relative Distinguished Name (RDN): The ASN.1-based construction
comprising a Relative Distinguished Name (RDN), which itself is a
building-block component of Distinguished Names. See [LDAP-DN],
Section 2.

source domain: The fully qualified DNS domain name FQDN that a client expects an application service
to present in the certificate
(e.g., "www.example.com"), certificate. This is typically input by a human
user, configured into a client, or provided by reference such as in a
hyperlink.
URL. The combination of a source domain and, optionally, an
application service type enables a client to construct one or more
reference identifiers.

subjectAltName entry: An identifier placed in a subjectAltName
extension.

subjectAltName extension: A standard PKIX certificate extension
[PKIX] enabling identifiers of various types to be bound to the
certificate subject.

subject name: In this specification, the term refers to the name of
a PKIX certificate's subject, encoded in a certificate's subject
field (see [PKIX], Section 4.1.2.6).

TLS client: An entity that assumes the role of a client

Security-related terms used in a
Transport Layer Security [TLS] negotiation. In this specification
we generally assume that the TLS client is an (interactive document, but not defined here or
automated) application client; however,
in application protocols
that enable server-to-server communication, the TLS client could
be a peer application service.

TLS server: An entity that assumes the role of a server in a
Transport Layer Security [TLS] negotiation; in this specification
we assume that the TLS server is an application service.

Most security-related terms in this document are to [PKIX] should be understood in the the sense defined in [SECTERMS]; such
[SECTERMS]. Such terms include, but are not
limited to, include "attack", "authentication", "authorization",
"certification authority", "certification path", "certificate",
"credential",
"identity", "self-signed certificate", "trust", "trust
anchor", "trust chain", "validate", and "verify".

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.

2. Naming of Application Services

This section discusses naming of application services on the
Internet, followed by a brief tutorial about subject naming in PKIX.

2.1. Naming Application Services

This specification assumes that the name of an application service is
based on a DNS domain name (e.g., "example.com") -- supplemented in
some circumstances by an application service type (e.g., "the IMAP
server at example.com").

From the perspective of the application client or user, some names
are direct because they are provided directly by a human user (e.g.,
via runtime input, prior configuration, or explicit acceptance of a
client communication attempt), whereas other names are indirect
because they are automatically resolved by the client based on user
input (e.g., a target name resolved from a source name using DNS SRV
or NAPTR records). This dimension matters most for certificate
consumption, specifically verification as discussed in this document.

From the perspective of the application service, some names are
unrestricted because they can be used in any type of service (e.g., a
certificate might be reused for both the HTTP service and the IMAP
service at example.com), whereas other names are restricted because
they can be used in only one type of service (e.g., a special-purpose
certificate that can be used only for an IMAP service). This
dimension matters most for certificate issuance.

Therefore, we can categorize the identifier types of interest as
follows:

* A DNS-ID is direct and unrestricted.

* An SRV-ID is typically indirect but can be direct, and is
restricted.

* A URI-ID is direct and restricted.

When implementing software, deploying services, and issuing
certificates for secure PKIX-based authentication, it is important to
keep these distinctions in mind. In particular, best practices
differ somewhat for application server implementations, application
client implementations, application service providers, and
certification authorities. Ideally, protocol specifications that
reference this document will specify which identifiers are mandatory-
to-implement by servers and clients, which identifiers ought to be
supported by certificate issuers, and which identifiers ought are to be
requested by application service providers. Because these
requirements differ across applications, it is impossible to
categorically stipulate universal rules (e.g., that all software
implementations, service providers, interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and certification authorities for only when, they appear in all application protocols need to use or support DNS-IDs
capitals, as a
baseline for the purpose of interoperability).

However, it is preferable that each application protocol will at
least define a baseline that applies to the community shown here.

2. Naming of software
developers, application service providers, and CAs actively using or
supporting Application Services

This document assumes that technology (one such community, the CA/Browser Forum,
has codified such a baseline for "Extended Validation Certificates"
in [EV-CERTS]).

2.2. DNS Domain Names

For the purposes of this specification, the name of an application service is (or is
based on) on a DNS domain name that (e.g., example.com) -- supplemented in
some circumstances by an application service type (e.g., "the IMAP
server at example.com"). The DNS name conforms to one of the
following forms:

1. A "traditional domain name", i.e., a fully qualified DNS domain
name or "FQDN" FQDN (see [DNS-CONCEPTS])
all of whose labels are "LDH labels" as described in [IDNA-DEFS].
Informally, such labels are constrained to [US-ASCII] letters,
digits, and the hyphen, with the hyphen prohibited in the first
character position. Additional qualifications apply (please refer (refer to
the above-
referenced above-referenced specifications for details), but they are
not relevant
to this specification. here.

2. An "internationalized domain name", i.e., a DNS domain name that
conforms to
includes at least one label containing appropriately encoded
Unicode code points outside the traditional US-ASCII range. That
is, it contains at least one U-label or A-label, but otherwise
may contain any mixture of NR-LDH labels, A-labels, or U-labels,
as described in [IDNA-DEFS] and the associated documents.

From the perspective of the application client or user, some names
are _direct_ because they are provided directly by a human user.
This includes runtime input, prior configuration, or explicit
acceptance of a client communication attempt. Other names are
_indirect_ because they are automatically resolved by the application
based on user input, such as a target name resolved from a source
name using DNS SRV or [NAPTR] records). The distinction matters most
for certificate consumption, specifically verification as discussed
in this document.

From the overall form perspective of a domain name (informally, dot-
separated letter-digit-hyphen labels) but includes at least one
label containing appropriately encoded Unicode code points
outside the traditional US-ASCII range. That is, it contains at
least one U-label or A-label, but otherwise may contain application service, some names are
_unrestricted_ because they can be used in any
mixture type of NR-LDH labels, A-labels, or U-labels, service, such
as described in
[IDNA-DEFS] a single certificate being used for both the HTTP and IMAP
services at the associated documents.

2.3. Subject Naming in PKIX Certificates

For our purposes, an application service host example.com. Other names are _restricted_
because they can only be identified by a name
or names carried in used for one or more type of service, such as a
special-purpose certificate that can only be used for an IMAP
service. This distinction matters most for certificate issuance.

We can categorize the following supported identifier types
within subjectAltName entries: as follows:

* A DNS-ID is direct and unrestricted.

* An SRV-ID is typically indirect but can be direct, and is
restricted.

* A URI-ID is direct and restricted.

It is important to keep these distinctions in mind, as best practices
for the deployment and use of the identifiers differ. As a further
example, cross-protocol attacks such as [ALPACA] are possibile when
two different protocol services use the same certificate. This can
be addressed by using restricted identifiers, or telling services to
not share certificates. Protocol specifications MUST specify which
identifiers are mandatory-to-implement and SHOULD provide operational
guidance when necessary.

The Common Name RDN MUST NOT be used to identify a service. Reasons
for this include:

* It is not strongly typed and therefore suffers from ambiguities in
interpretation.

* It can appear more than once in the Subject Name.

For similar reasons, other RDN's within the Subject Name MUST NOT be
used to identify a service.

3. Designing Application Protocols

This section provides guidelines for defines how protocol designers of application
protocols, in the form of should reference this
document, which MUST be a checklist to follow when reusing normative reference in their specification.
The technology MUST only use the
recommendations provided identifiers defined in this
document.

* Its specification MAY choose to allow only one of them.

If your the technology does not use DNS SRV records to resolve the DNS
domain names of application services then consider stating its specification MUST
state that SRV-ID as defined in this document is not supported. Note
that many existing application technologies use DNS SRV records to
resolve the DNS domain names of application services, but do not rely
on representations of those records in PKIX certificates by means of
SRV-IDs as defined in [SRVNAME].

If your the technology does not use use URIs URI's to identify application
services, then consider stating its specification MUST state that URI-ID as defined in
this document is not supported. Note that many existing application
technologies use URIs to identify application services, but do not
rely on representation of those URIs in PKIX certificates by means of
URI-IDs.

* If your

A technology disallows MAY disallow the use of the wildcard character in DNS domain
names,
names. If it does so, then state the specification MUST state that
wildcard certificates as defined in this document are not supported.

4. Representing Server Identity

This section provides rules and guidelines instructions for issuers of certificates.

4.1. Rules

When a certification authority issues a certificate based on the
fully qualified DNS domain name FQDN
at which the application service provider will provide the relevant
application, the following rules apply to the representation of
application service identities. The
reader needs to be aware Note that some of these rules are
cumulative and can interact in important ways that are illustrated
later in this document.

1. The certificate SHOULD MUST include a "DNS-ID" if possible as a baseline for
interoperability.

2. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type SRV-ID (e.g., this is true
of [XMPP]), then the certificate SHOULD include an SRV-ID.

3. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type URI-ID (e.g., this is true
of [SIP] as specified by [SIP-CERTS], but not true of [HTTP]
since [HTTP-TLS] does not describe usage of a URI-ID for HTTP
services), [SIP-CERTS]), then the certificate
SHOULD include a URI-ID. The scheme SHALL MUST be that of the protocol
associated with the application service type and the "host"
component (or its equivalent) SHALL MUST be the fully qualified DNS domain name FQDN of the service. A
The application protocol specification that reuses this one MUST specify which URI
schemes are to be considered acceptable in URI-IDs contained in PKIX certificates
used for the application protocol (e.g.,
"sip" sip but not "sips" sips or "tel" tel
for SIP as described in [SIP-SIPS],
or perhaps http and https for HTTP as might be described in a
future specification). [SIP-SIPS]).

4. The certificate MAY include other application-specific
identifiers for types that were defined before publication of
[SRVNAME] (e.g., XmppAddr for [XMPP]) or for which service names
or URI schemes do not exist; however, such application-specific
identifiers are not applicable to all application technologies
and therefore are out of scope for this specification.

5. The certificate MAY contain more than one DNS-ID, SRV-ID, or URI-
ID as further explained under Section 7.4. 7.3.

5. The certificate MAY include other application-specific
identifiers for compatibility with a deployed base. Such
identifiers are out of scope for this specification.

4.2. Examples

Consider a simple website at "www.example.com", www.example.com, which is not
discoverable via DNS SRV lookups. Because HTTP does not specify the
use of URIs in server certificates, a certificate for this service
might include only a DNS-ID of "www.example.com". www.example.com.

Consider an IMAP-accessible email server at the host
"mail.example.net" mail.example.net
servicing email addresses of the form
"user@example.net" user@example.net and
discoverable via DNS SRV lookups on the application service name of "example.net".
example.net. A certificate for this service might include SRV-IDs of "_imap.example.net"
_imap.example.net and
"_imaps.example.net" _imaps.example.net (see [EMAIL-SRV]) along with
DNS-IDs of
"example.net" example.net and "mail.example.net". mail.example.net.

Consider a SIP-accessible voice-over-IP (VoIP) server at the host
"voice.example.edu"
voice.example.edu servicing SIP addresses of the form
"user@voice.example.edu"
user@voice.example.edu and identified by a URI of
<sip:voice.example.edu>. A certificate for this service would
include a URI-ID of "sip:voice.example.edu" sip:voice.example.edu (see [SIP-CERTS]) along
with a DNS-ID of "voice.example.edu". voice.example.edu.

Consider an XMPP-compatible instant messaging (IM) server at the host
"im.example.org"
im.example.org servicing IM addresses of the form
"user@im.example.org" user@im.example.org
and discoverable via DNS SRV lookups on the
"im.example.org" im.example.org domain. A
certificate for this service might include SRV-IDs of "_xmpp-client.im.example.org" _xmpp-
client.im.example.org and "_xmpp-
server.im.example.org" _xmpp-server.im.example.org (see [XMPP]), a
DNS-ID of "im.example.org",
and im.example.org. For backward compatibility, it may also
have an XMPP-specific "XmppAddr" XmppAddr of "im.example.org" im.example.org (see [XMPP]).

5. Requesting Server Certificates

This section provides rules and guidelines instructions for service providers regarding
the information to include in certificate signing requests (CSRs).
In general, service providers are encouraged to SHOULD request certificates that
include all of the identifier types that are required or recommended
for the application service type that will be secured using the
certificate to be issued.

If the certificate might be used for any type of application service,
then the service provider is encouraged to request a certificate that
includes only a DNS-ID.

If the certificate will be used for only a single type of application
service, then the service provider is encouraged to SHOULD request a certificate that
includes a DNS-ID and, if appropriate for the application service
type, an SRV-ID or URI-ID that limits the deployment scope of the
certificate to only the defined application service type.

If the certificate might be used for any type of application service,
then the service provider SHOULD to request a certificate that
includes only a DNS-ID. Again, because of multi-protocol attacks
this practice is discouraged; this can be mitigated by providing only
one service on a host.

If a service provider offering multiple offersmultiple application service types
(e.g., a World Wide Web service, an email service, and an instant
messaging service)
wishes to limit the applicability of certificates using SRV-IDs or URI-IDs, then the service provider is encouraged to SRV-IDs or
URI-IDs, they SHOULD request multiple certificates, i.e., one certificate per application
service type. Conversely, the service provider is discouraged from
requesting rather than a
single certificate containing multiple SRV-IDs or URI-
IDs identifying URI-IDs each
identifying ia different application service type. This
guideline rule does
not apply to application service type "bundles" that
are used to identify manifold
distinct access methods to the same underlying application (e.g., such as an
email application with access methods denoted by the application
service types of "imap", "imaps",
"pop3", "pop3s", imap, imaps, pop3, pop3s, and "submission" submission as
described in [EMAIL-SRV]). [EMAIL-SRV].

6. Verifying Service Identity

This section provides rules and guidelines for implementers of
application client software regarding algorithms for verification of
application service identity.

6.1. Overview

At a high level, the client verifies the application service's
identity by performing the actions listed below (which are defined in
the following subsections of this document): actions:

1. The client constructs a list of acceptable reference identifiers
based on the source domain and, optionally, the type of service
to which the client is connecting.

2. The server provides its identifiers in the form of a PKIX
certificate.

3. The client checks each of its reference identifiers against the
presented identifiers for the purpose of finding a match.

4. When
checking a reference identifier against a presented identifier,
the client matches the source domain of the identifiers and,
optionally, their application service type.

Naturally, in addition to checking identifiers, a client might
complete should
perform further checks checks, such as expiration and revocation, to ensure
that the server is authorized to provide the requested service. However, such Such
checking is not a matter of verifying the application service
identity presented in a certificate, however, and therefore methods for doing
so (e.g., consulting
local policy information) are therefore out of scope for this document.

6.2.

6.1. Constructing a List of Reference Identifiers

6.2.1.

6.1.1. Rules

The client MUST construct a list of acceptable reference identifiers,
and MUST do so independently of the identifiers presented by the
service.

The inputs used by the client to construct its list of reference
identifiers might be a URI that a user has typed into an interface
(e.g., an HTTPS URL for a website), configured account information
(e.g., the domain name of a particular host or URI used for retrieving information or connecting to a network, email, which might be
different from the DNS domain name portion of a username), a
hyperlink in a web page that triggers a browser to retrieve a media
object or script, or some other combination of information that can
yield a source domain and an application service type.

The client might need to extract the source domain and application
service type from the input(s) it has received. The extracted data
MUST include only information that can be securely parsed out of the
inputs (e.g.,
inputs, such as parsing the fully qualified DNS domain name FQDN out of the "host" component (or its equivalent) of a URI or
deriving the application service type from the scheme of a URI) or information
that is derived in a manner not subject to subversion by network
attackers (e.g., URI.
Other possibilities include pulling the data from a delegated domain
that is explicitly established via client or system configuration,
resolving the data via [DNSSEC], or obtaining the data from a third-party third-
party domain mapping service in which a human user has explicitly
placed trust and with which the client communicates over a connection
or association that provides both mutual authentication and integrity
checking).
checking. These considerations apply only to extraction of the
source domain from the inputs; naturally, inputs. Naturally, if the inputs themselves
are invalid or corrupt (e.g., a user has clicked a link provided by a
malicious entity in a phishing attack), then the client might end up
communicating with an unexpected application service.

For example, given an input URI of <sips:alice@example.net>, a client
would derive the application service type "sip" sip from the scheme and
parse the domain name "example.net" example.net from the host component.

Each reference identifier in the list SHOULD MUST be based on the source
domain and SHOULD MUST NOT be based on a derived domain (e.g., such as a host name
or domain
name discovered through DNS resolution of the source
domain). domain. This
rule is important because only a match between the user inputs and a
presented identifier enables the client to be sure that the
certificate can legitimately be used to secure the client's
communication with the server. There is only one scenario in which
it is acceptable for an interactive client to override the
recommendation in this rule and therefore communicate with a domain
name other than the source domain: because a human user has "pinned"
the application service's certificate to the alternative domain name
as further discussed under Section 6.6.4 and Section 7.1. In this
case, the inputs used by the client to construct its list of
reference identifiers might include more than one fully qualified DNS
domain name, i.e., both (a) the source domain and (b) the alternative
domain contained in the pinned certificate.

Using the combination of fully qualified DNS domain name(s) FQDN(s) and application service type, the
client constructs a MUST construct its list of reference identifiers in accordance
with the following rules:

* The list SHOULD include a DNS-ID. A reference identifier of type
DNS-ID can be directly constructed from a fully qualified DNS
domain name FQDN that is (a)
contained in or securely derived from the
inputs (i.e., the source domain), inputs, or (b)
explicitly associated with the source domain by means of user configuration (i.e., a
derived domain).
configuration.

* If a server for the application service type is typically
discovered by means of DNS SRV records, then the list SHOULD
include an SRV-ID.

* If a server for the application service type is typically
associated with a URI for security purposes (i.e., a formal
protocol document specifies the use of URIs in server
certificates), then the list SHOULD include a URI-ID.

Which identifier types a client includes in its list of reference
identifiers
identifiers, and their priority, is a matter of local policy. For
example, in certain
deployment environments, a client that is built to connect only to a particular kind
of service (e.g., only IM services) might be configured to accept as valid only certificates
that include an SRV-
ID SRV-ID for that application service type; in this case, the client would
include only SRV-IDs matching the application service type in its
list of reference identifiers (not, for example, DNS-IDs). type. By
contrast, a more lenient client (even one client, even if built to connect only to a
particular kind of service) service, might include both SRV-IDs and DNS-IDs in
its list of reference identifiers.

6.2.2.

6.1.2. Examples

A web browser that is connecting via HTTPS to the website at
"www.example.com"
www.example.com would have a single reference identifier: a DNS-ID of "www.example.com".
www.example.com.

A mail user agent that is connecting via IMAPS to the email service
at "example.net" example.net (resolved as "mail.example.net") mail.example.net) might have three
reference identifiers: an SRV-ID of "_imaps.example.net" _imaps.example.net (see
[EMAIL-SRV]), and DNS-IDs of "example.net" example.net and "mail.example.net".
(A legacy mail.example.net. An
email user agent would agentthat does not support [EMAIL-SRV] and
therefore would probably be
explicitly configured to connect to
"mail.example.net", mail.example.net, whereas an SRV-aware SRV-
aware user agent would derive
"example.net" example.net from an email address of
the form "user@example.net" user@example.net but might also accept "mail.example.net" mail.example.net as
the DNS domain name portion of reference identifiers for the service.) service.

A voice-over-IP (VoIP) user agent that is connecting via SIP to the
voice service at "voice.example.edu" voice.example.edu might have only one reference
identifier: a URI-ID of "sip:voice.example.edu" sip:voice.example.edu (see [SIP-CERTS]).

An instant messaging (IM) client that is connecting via XMPP to the
IM service at "im.example.org" im.example.org might have three reference identifiers:
an SRV-ID of "_xmpp-client.im.example.org" _xmpp-client.im.example.org (see [XMPP]), a DNS-ID of "im.example.org",
im.example.org, and an XMPP-specific "XmppAddr" XmppAddr of
"im.example.org" im.example.org (see
[XMPP]).

6.3.

6.2. Preparing to Seek a Match

Once the client has constructed its list of reference identifiers and
has received the server's presented identifiers in the form of a PKIX
certificate, the client checks its reference identifiers against the
presented identifiers for the purpose of finding a match. The search
fails if the client exhausts its list of reference identifiers
without finding a match. The search succeeds if any presented
identifier matches one of the reference identifiers, at which point
the client SHOULD stop the search.

Before applying the comparison rules provided in the following
sections, the client might need to split the reference identifier
into its DNS domain name portion and its application service type
portion, as follows:

* A DNS-ID reference identifier of type DNS-ID does not include an
application service type portion and thus can MUST be used directly as the DNS
domain name for comparison purposes. As an example, a
DNS-ID of "www.example.com" would result in a DNS domain name
portion of "www.example.com". and there is no application service type.

* For a an SRV-ID reference identifier of type SRV-ID, identifier, the DNS domain name portion is
the Name and the application service type portion is the Service. As an
For example, an SRV-ID of "_imaps.example.net"
would be split into _imaps.example.net has a DNS domain name
portion of "example.net" example.net and an application service type portion of "imaps" (mapping
imaps, which maps to an the IMAP application protocol of IMAP as explained in [EMAIL-SRV]).
[EMAIL-SRV].

* For a reference identifier of type URI-ID, the DNS domain name
portion is the "reg-name" part of the "host" component (or its
equivalent) and the
application service type portion is the
application service type associated with the scheme name matching
the [ABNF] "scheme" rule from [URI] (not including the ':'
separator). As previously mentioned, this document specifies that
a URI-ID always contains a "host" component (or its equivalent)
containing a "reg-name". (Matching scheme, as defind above.
Matching only the "reg-name" rule from [URI] limits verification
to DNS domain names, thereby differentiating a URI-ID from a
uniformResourceIdentifier entry that contains an IP address or a
mere host name, or that does not contain a "host" component at all.)
all. Furthermore, note that extraction of the "reg-name" might
necessitate normalization of the URI (as explained in [URI]). As an For
example, a URI-ID of
"sip:voice.example.edu" sip:voice.example.edu would be split into a
DNS domain name portion of "voice.example.edu" voice.example.edu and an application
service type of
"sip" sip (associated with an application protocol of
SIP as explained in [SIP-CERTS]).

Detailed comparison rules for matching

A client MUST match the DNS domain name portion name, and if an application service type portion of
is present it MUST also match the reference identifier service type as well. These are
provided in the following sections.

6.4.
described below.

6.3. Matching the DNS Domain Name Portion

The

This section describes how the client MUST match must determine if the the
presented DNS domain name portion of a reference
identifier according to the following rules (and SHOULD also check matches the application service type as described under Section 6.5). reference DNS name. The rules differ
depending on whether the domain to be checked is a "traditional
domain name" or an "internationalized domain name" (as defined under
Section 2.2). Furthermore, to meet the needs of 2). For clients that support presented identifiers names containing the wildcard
character "*", we define this section also specifies a supplemental rule for
such "wildcard certificates".

6.4.1. Checking This section uses the description of Traditional Domain Names
labels and domain names in [DNS-CONCEPTS].

If the DNS domain name portion of a reference identifier is a
"traditional domain name", then matching of the reference identifier
against the presented identifier is MUST be performed by comparing the
set of domain name labels using a case-insensitive ASCII comparison,
as clarified by [DNS-CASE] (e.g., "WWW.Example.Com" [DNS-CASE]. For example, WWW.Example.Com would be
lower-cased to "www.example.com" www.example.com for comparison purposes). purposes. Each label
MUST match in order for the names to be considered to match, except
as supplemented by the rule about checking of wildcard labels
(Section 6.4.3).

6.4.2. Checking of Internationalized Domain Names given
below.

If the DNS domain name portion of a reference identifier is an
internationalized domain name, then an implementation the client MUST convert any
U-labels [IDNA-DEFS] in the domain name to A-labels before checking
the domain name. In accordance with [IDNA-PROTO], A-labels MUST be
compared as case-insensitive ASCII. Each label MUST match in order
for the domain names to be considered to match, except as
supplemented by the rule about checking of wildcard labels
(Section 6.4.3; but see also Section 7.2 regarding wildcards in
internationalized domain names).

6.4.3. Checking about checking of Wildcard Certificates

A wildcard labels given
below.

If the technology specification supports wildcards, then the client MAY
MUST match the reference identifier against a presented identifier
whose DNS domain name portion contains the wildcard character "*" in
a label (following the description of labels and
domain names in [DNS-CONCEPTS]), provided these requirements are met:

1. There is only one wildcard character.

2. The wildcard character appears only as the content of the left-
most label.

3. The wildcard character is

If the requirements are not embedded in an A-label or U-label
[IDNA-DEFS] of an internationalized domain name [IDNA-PROTO]. met, the presented identifier is invalid
and MUST be ignored.

A wildcard in a presented identifier can only match exactly one label
in a reference identifier. Note that this is not the same as DNS
wildcard matching, where the "*" label always matches at least one
whole label and sometimes more. See [DNS-CONCEPTS], Section 4.3.3
and [DNS-WILDCARDS].

For information regarding the security characteristics of wildcard
certificates, see Section 7.2.

6.5. 7.1.

6.4. Matching the Application Service Type Portion

When a client checks identifiers of type SRV-ID and URI-ID, it MUST
check not only the DNS domain name portion of the identifier but also
the application service type portion.

The client does this by
splitting the identifier into the DNS domain name portion and the
application service type portion (as described under Section 6.3),
then checking both the DNS domain name portion (as described under
Section 6.4) and rules for matching the application service type portion as described in deopend on
whether the following subsections.

Implementation Note: An identifier of type is an SRV-ID or URI-ID provides a URI-ID.

These identifiers provide an application service type portion to be
checked, but that portion is combined only with the DNS domain name
portion of the SRV-ID or URI-ID itself. For example, if a client's
list of reference identifiers includes an SRV-ID of "_xmpp-client.im.example.org" _xmpp-
client.im.example.org and a DNS-ID of "apps.example.net", apps.example.net, the client
would check (a) both the combination of an application service type of "xmpp-client"
xmpp-client and a DNS domain name of "im.example.org" im.example.org and (b) a DNS domain
name of
"apps.example.net". apps.example.net. However, the client would not check (c) the
combination of an application service type of "xmpp-client" xmpp-client and a DNS
domain name of "apps.example.net" apps.example.net because it does not have an SRV-ID of "_xmpp-client.apps.example.net"
_xmpp-client.apps.example.net in its list of reference identifiers.

6.5.1. SRV-ID

The

If the identifier is an SRV-ID, then the application service name portion of an SRV-ID (e.g., "imaps")
MUST be matched in a case-insensitive manner, in accordance with
[DNS-SRV]. Note that the "_" _ character is prepended to the service
identifier in DNS SRV records and in SRV-IDs (per [SRVNAME]), and
thus does not need to be included in any comparison.

6.5.2. URI-ID

The

If the identifier is a URI-ID, then the scheme name portion of a URI-ID (e.g., "sip") MUST be
matched in a case-insensitive manner, in accordance with [URI]. Note
that the
":" : character is a separator between the scheme name and the
rest of the URI, and thus does not need to be included in any
comparison.

6.6.

6.5. Outcome

The outcome of the matching procedure is one of the following cases.

6.6.1. Case #1: Match Found

If the client has found a presented identifier that matches a
reference identifier, then the service identity check has succeeded.
In this case, the client MUST use the matched reference identifier as
the validated identity of the application service.

6.6.2. Case #2: No Match Found, Pinned Certificate

If the client does not find a presented identifier matching any of
the reference identifiers but the client has previously pinned the
application service's certificate to one of the reference identifiers
in the list it constructed for this communication attempt (as
"pinning" is explained under Section 1.8), and the presented
certificate matches the pinned certificate (including the context as
described under Section 7.1), then the service validated identity check has
succeeded.

6.6.3. Case #3: No Match Found, No Pinned Certificate of the application service.

If the client does not find a presented identifier matching any of
the reference identifiers and then the client has MUST proceed as described
as follows.

If the client is an automated application not previously pinned directly controlled by
a human user, then it SHOULD terminate the communication attempt with
a bad certificate to one of the reference identifiers in error and log the list it
constructed for error appropriately. The
application MAY provide a configuration setting to disable this communication attempt, then the client
behavior, but it MUST
proceed as described under Section 6.6.4.

6.6.4. Fallback enable it by default.

If the client is an interactive client that is directly controlled by
a human user, then it SHOULD inform the user of the identity mismatch
and automatically terminate the communication attempt with a bad
certificate error; this behavior is preferable because it prevents error in order to prevent users from inadvertently
bypassing security protections in hostile situations.

Some interactive clients MAY give advanced users the option of
proceeding with acceptance despite the identity mismatch. Although
this behavior can be appropriate in certain specialized
circumstances, it needs to be handled with extreme caution, for
example by first encouraging even an advanced user to terminate the
communication attempt and, if the advanced user chooses they choose to proceed anyway, by
forcing the user to view the entire certification path before
proceeding.

Otherwise, if the client is an automated application not directly
controlled by a human user, then it SHOULD terminate the
communication attempt with a bad certificate error and log the error
appropriately. An automated application MAY provide a configuration
setting that disables this behavior, but MUST enable the behavior by
default.

7. Security Considerations

7.1. Pinned Certificates

As defined under Section 1.8, a certificate is said to be "pinned" to
a DNS domain name when a user has explicitly chosen to associate a
service's certificate with that DNS domain name despite the fact that
the certificate contains some other DNS domain name (e.g., the user
has explicitly approved "apps.example.net" as a domain associated
with a source domain of "example.com").

The cached name association
MUST take account of both the certificate presented and the context
in which it was accepted or configured (where the "context" includes
the chain of certificates from the presented certificate to the trust
anchor, the source domain, the application service type, the
service's derived domain and port number, and any other relevant
information provided by MAY also present the user or associated by with the client).

7.2. ability to accept
the presented certificate as valid for subsequent connections. Such
ad-hoc "pinning" SHOULD NOT restrict future connections to just the
pinned certificate. Local policy that statically enforces a given
certificate for a given peer is best made available only as prior
configuration, rather than a just-in-time override for a failed
connection.

7. Security Considerations

7.1. Wildcard Certificates

Wildcard certificates, those that have an identifier with "*" as the
left-most DNS label, automatically vouch for any single-label host
names within their domain, but not multiple levels of domains. This
can be convenient for administrators but also poses the risk of
vouching for rogue or buggy hosts. See for example [Defeating-SSL]
(beginning at slide 91) and [HTTPSbytes] (slides 38-40).

Protection against a wildcard that identifies a so-called "public
suffix" (e.g., "*.co.uk" public suffix
[Public-Suffix], such as *.co.uk or "*.com") *.com, is beyond the scope of
this document.

7.3.

7.2. Internationalized Domain Names

Allowing internationalized domain names can lead to the inclusion of visually similar (so-called "confusable") characters in certificates;
for
characters, also referred to as "confusables", being included within
certificates. For discussion, see for example [IDNA-DEFS].

7.4. [IDNA-DEFS],
Section 4.4 and [UTS-39].

7.3. Multiple Presented Identifiers

A given application service might be addressed by multiple DNS domain
names for a variety of reasons, and a given deployment might service
multiple domains or protocols. The client SHOULD use the TLS Extensions such as TLS Server
Name Identification (SNI) extension as (SNI), discussed in [TLS], Section 4.4.2.2. 4.4.2.2, and
Application Layer Protocol Negotiation (ALPN), discussed in [ALPN],
provide a way for the application to indicate the desired identifier
and protocol to the server, which can be used to select the most
appropriate certificate.

To accommodate the workaround that was needed before the development
of the SNI extension, this specification allows multiple DNS-IDs,
SRV-IDs, or URI-IDs in a certificate.

8. IANA Considerations

This document has no actions for IANA.

9. References

8.1.

9.1. Normative References

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

[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://doi.org/10.17487/RFC2782>.
<https://www.rfc-editor.org/rfc/rfc2782>.

[DNS-WILDCARDS]
Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
<https://doi.org/10.17487/RFC4592>.
<https://www.rfc-editor.org/rfc/rfc4592>.

[IDNA-DEFS]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://doi.org/10.17487/RFC5890>.
<https://www.rfc-editor.org/rfc/rfc5890>.

[IDNA-PROTO]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://doi.org/10.17487/RFC5891>.
<https://www.rfc-editor.org/rfc/rfc5891>.

[LDAP-DN] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names",
RFC 4514, DOI 10.17487/RFC4514, June 2006,
<https://doi.org/10.17487/RFC4514>.
<https://www.rfc-editor.org/rfc/rfc4514>.

[PKIX] 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://doi.org/10.17487/RFC5280>.
<https://www.rfc-editor.org/rfc/rfc5280>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://doi.org/10.17487/RFC2119>.
<https://www.rfc-editor.org/rfc/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://doi.org/10.17487/RFC8174>. <https://www.rfc-editor.org/rfc/rfc8174>.

[SRVNAME] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, DOI 10.17487/RFC4985, August 2007,
<https://doi.org/10.17487/RFC4985>.
<https://www.rfc-editor.org/rfc/rfc4985>.

[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://doi.org/10.17487/RFC3986>.

8.2.
<https://www.rfc-editor.org/rfc/rfc3986>.

9.2. Informative References

[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://doi.org/10.17487/RFC5234>.
<https://www.rfc-editor.org/rfc/rfc5234>.

[ACME] 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/rfc/rfc8555>.

[ALPACA] Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D.,
Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel,
"ALPACA: Application Layer Protocol Confusion - Analyzing
and Mitigating Cracks in TLS Authentication", September
2021, <https://alpaca-attack.com/ALPACA.pdf>.

[ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.

[Defeating-SSL]
Marlinspike, M., "New Tricks for Defeating SSL in
Practice", BlackHat DC, February 2009,
<http://www.blackhat.com/presentations/bh-dc-
09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-
SSL.pdf>.

[DNS-CASE] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<https://doi.org/10.17487/RFC4343>.
<https://www.rfc-editor.org/rfc/rfc4343>.

[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://doi.org/10.17487/RFC4033>.
<https://www.rfc-editor.org/rfc/rfc4033>.

[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://doi.org/10.17487/RFC6347>. <https://www.rfc-editor.org/rfc/rfc6347>.

[EMAIL-SRV]
Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<https://doi.org/10.17487/RFC6186>.

[EV-CERTS] CA/Browser Forum, "Guidelines For The Issuance And
Management Of Extended Validation Certificates", October
2009, <http://www.cabforum.org/Guidelines_v1_2.pdf>.

[HTTP] 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,
<https://doi.org/10.17487/RFC7230>.

[HTTP-TLS] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://doi.org/10.17487/RFC2818>.
<https://www.rfc-editor.org/rfc/rfc6186>.

[HTTPSbytes]
Sokol, J. and R. Hansen, "HTTPS Can Byte Me", BlackHat Abu
Dhabi, November 2010, <https://media.blackhat.com/bh-ad-
10/Hansen/Blackhat-AD-2010-Hansen-Sokol-HTTPS-Can-Byte-Me-
slides.pdf>.

[IPSEC] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://doi.org/10.17487/RFC4301>.

[NAPTR] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, DOI 10.17487/RFC3403, October 2002,
<https://doi.org/10.17487/RFC3403>.

[OCSP] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://doi.org/10.17487/RFC6960>.

[OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw,
<https://www.rfc-editor.org/rfc/rfc3403>.

[NTS] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
Thayer, "OpenPGP Message Format",
Sundblad, "Network Time Security for the Network Time
Protocol", RFC 4880, 8915, DOI 10.17487/RFC4880, November 2007,
<https://doi.org/10.17487/RFC4880>.

[S-NAPTR] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs 10.17487/RFC8915, September 2020,
<https://www.rfc-editor.org/rfc/rfc8915>.

[Public-Suffix]
"Public Suffix List", 2020, <https://publicsuffix.org>.

[QUIC] Thomson, M., Ed. and the Dynamic Delegation
Discovery Service (DDDS)", S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 3958, 9001, DOI 10.17487/RFC3958,
January 2005, <https://doi.org/10.17487/RFC3958>. 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/rfc/rfc9001>.

[SECTERMS] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://doi.org/10.17487/RFC4949>.
<https://www.rfc-editor.org/rfc/rfc4949>.

[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://doi.org/10.17487/RFC3261>.
<https://www.rfc-editor.org/rfc/rfc3261>.

[SIP-CERTS]
Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, DOI 10.17487/RFC5922, June 2010,
<https://doi.org/10.17487/RFC5922>.
<https://www.rfc-editor.org/rfc/rfc5922>.

[SIP-SIPS] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630,
DOI 10.17487/RFC5630, October 2009,
<https://doi.org/10.17487/RFC5630>.
<https://www.rfc-editor.org/rfc/rfc5630>.

[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://doi.org/10.17487/RFC8446>.
<https://www.rfc-editor.org/rfc/rfc8446>.

[US-ASCII] American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.

[UTS-39] Davis, M. and M. Suignard, "Unicode Security Mechanisms",
n.d., <https://unicode.org/reports/tr39>.

[VERIFY] 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, March
2011, <https://doi.org/10.17487/RFC6125>. <https://www.rfc-editor.org/rfc/rfc6125>.

[WSC-UI] Saldhana, A. and T. Roessler, "Web Security Context: User
Interface Guidelines", World Wide Web Consortium LastCall
WD-wsc-ui-20100309, March August 2010,
<http://www.w3.org/TR/2010/WD-wsc-ui-20100309>.
<https://www.w3.org/TR/2010/REC-wsc-ui-20100812/>.

[XMPP] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://doi.org/10.17487/RFC6120>. <https://www.rfc-editor.org/rfc/rfc6120>.

Acknowledgements

We gratefully acknowledge everyone who contributed to the previous
version of this document, [VERIFY]. Thanks also to Carsten Bormann
for converting the previous document to Markdown so that we could
more easily use Martin Thomson's i-d-template software.

Authors' Addresses

Peter Saint-Andre
Mozilla
United States of America

Email: stpeter@mozilla.com

Jeff Hodges
Google
United States of America

Email: jdhodges@google.com

Rich Salz
Akamai Technologies
United States of America

Email: rsalz@akamai.com