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

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   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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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