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rfc:rfc6125

Internet Engineering Task Force (IETF) P. Saint-Andre Request for Comments: 6125 Cisco Category: Standards Track J. Hodges ISSN: 2070-1721 PayPal

                                                            March 2011
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)

Abstract

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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6125.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Saint-Andre & Hodges Standards Track [Page 1] RFC 6125 Service Identity March 2011

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2.  Audience . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.3.  How to Read This Document  . . . . . . . . . . . . . . . .  4
   1.4.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  5
   1.5.  Overview of Recommendations  . . . . . . . . . . . . . . .  5
   1.6.  Generalization from Current Technologies . . . . . . . . .  6
   1.7.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     1.7.1.  In Scope . . . . . . . . . . . . . . . . . . . . . . .  7
     1.7.2.  Out of Scope . . . . . . . . . . . . . . . . . . . . .  7
   1.8.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  9
 2.  Naming of Application Services . . . . . . . . . . . . . . . . 13
   2.1.  Naming Application Services  . . . . . . . . . . . . . . . 13
   2.2.  DNS Domain Names . . . . . . . . . . . . . . . . . . . . . 14
   2.3.  Subject Naming in PKIX Certificates  . . . . . . . . . . . 15
     2.3.1.  Implementation Notes . . . . . . . . . . . . . . . . . 17
 3.  Designing Application Protocols  . . . . . . . . . . . . . . . 18
 4.  Representing Server Identity . . . . . . . . . . . . . . . . . 18
   4.1.  Rules  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
 5.  Requesting Server Certificates . . . . . . . . . . . . . . . . 21
 6.  Verifying Service Identity . . . . . . . . . . . . . . . . . . 21
   6.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 21
   6.2.  Constructing a List of Reference Identifiers . . . . . . . 22
     6.2.1.  Rules  . . . . . . . . . . . . . . . . . . . . . . . . 22
     6.2.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . 24
   6.3.  Preparing to Seek a Match  . . . . . . . . . . . . . . . . 25
   6.4.  Matching the DNS Domain Name Portion . . . . . . . . . . . 26
     6.4.1.  Checking of Traditional Domain Names . . . . . . . . . 27
     6.4.2.  Checking of Internationalized Domain Names . . . . . . 27
     6.4.3.  Checking of Wildcard Certificates  . . . . . . . . . . 27
     6.4.4.  Checking of Common Names . . . . . . . . . . . . . . . 28
   6.5.  Matching the Application Service Type Portion  . . . . . . 28
     6.5.1.  SRV-ID . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.5.2.  URI-ID . . . . . . . . . . . . . . . . . . . . . . . . 29
   6.6.  Outcome  . . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.6.1.  Case #1: Match Found . . . . . . . . . . . . . . . . . 29
     6.6.2.  Case #2: No Match Found, Pinned Certificate  . . . . . 29
     6.6.3.  Case #3: No Match Found, No Pinned Certificate . . . . 30
     6.6.4.  Fallback . . . . . . . . . . . . . . . . . . . . . . . 30
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   7.1.  Pinned Certificates  . . . . . . . . . . . . . . . . . . . 30
   7.2.  Wildcard Certificates  . . . . . . . . . . . . . . . . . . 31
   7.3.  Internationalized Domain Names . . . . . . . . . . . . . . 32
   7.4.  Multiple Identifiers . . . . . . . . . . . . . . . . . . . 32
 8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 33

Saint-Andre & Hodges Standards Track [Page 2] RFC 6125 Service Identity March 2011

 9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 34
   10.2. Informative References . . . . . . . . . . . . . . . . . . 34
 Appendix A.  Sample Text . . . . . . . . . . . . . . . . . . . . . 40
 Appendix B.  Prior Art . . . . . . . . . . . . . . . . . . . . . . 42
   B.1.  IMAP, POP3, and ACAP (1999)  . . . . . . . . . . . . . . . 42
   B.2.  HTTP (2000)  . . . . . . . . . . . . . . . . . . . . . . . 43
   B.3.  LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 44
   B.4.  SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 47
   B.5.  XMPP (2004)  . . . . . . . . . . . . . . . . . . . . . . . 49
   B.6.  NNTP (2006)  . . . . . . . . . . . . . . . . . . . . . . . 50
   B.7.  NETCONF (2006/2009)  . . . . . . . . . . . . . . . . . . . 51
   B.8.  Syslog (2009)  . . . . . . . . . . . . . . . . . . . . . . 52
   B.9.  SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 54
   B.10. SNMP (2010)  . . . . . . . . . . . . . . . . . . . . . . . 55
   B.11. GIST (2010)  . . . . . . . . . . . . . . . . . . . . . . . 55

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.  When a client communicates
 with an application service using Transport Layer Security [TLS] or
 Datagram Transport Layer Security [DTLS], it references 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 are defined more
 precisely later in this document through the concept of particular
 identifiers).  In general, 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 for
 representing and verifying application service identity.
 Unfortunately, this divergence of approaches has caused some
 confusion among certification authorities, application developers,
 and protocol designers.

Saint-Andre & Hodges Standards Track [Page 3] RFC 6125 Service Identity March 2011

 Therefore, to codify secure procedures for the implementation and
 deployment 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. 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:
 o  Protocol designers might want to first read the checklist in
    Section 3.
 o  Certification authorities might want to first read the
    recommendations for representation of server identity in
    Section 4.
 o  Service providers might want to first read the recommendations for
    requesting of server certificates in Section 5.
 o  Software implementers might want to first read the recommendations
    for verification of server identity in Section 6.
 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
 and guidelines that are contained in the above-referenced sections,
 but are not absolutely necessary for a first reading of this
 document.

Saint-Andre & Hodges Standards Track [Page 4] RFC 6125 Service Identity March 2011

1.4. Applicability

 This document does not supersede the rules for certificate issuance
 or validation provided in [PKIX].  Therefore, [PKIX] is authoritative
 on any point that might also be discussed in this document.
 Furthermore, [PKIX] also governs any certificate-related topic on
 which this document is silent, including but not limited to
 certificate syntax, certificate extensions such as name constraints
 and extended key usage, and handling of certification paths.
 This document addresses only name forms in the leaf "end entity"
 server certificate, not any name forms in the chain of certificates
 used to validate the server certificate.  Therefore, in order to
 ensure proper authentication, application clients need to verify the
 entire certification path 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, such as those excerpted
 under Appendix B.  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.

1.5. Overview of Recommendations

 To orient the reader, this section provides an informational overview
 of the recommendations contained in this document.
 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:
 o  Move away from including and checking strings that look like
    domain names in the subject's Common Name.
 o  Move toward including and checking DNS domain names via the
    subjectAlternativeName extension designed for that purpose:
    dNSName.

Saint-Andre & Hodges Standards Track [Page 5] RFC 6125 Service Identity March 2011

 o  Move toward including and checking even more specific
    subjectAlternativeName extensions where appropriate for using the
    protocol (e.g., uniformResourceIdentifier and the otherName form
    SRVName).
 o  Move away from the issuance of so-called wildcard certificates
    (e.g., a certificate containing an identifier for
    "*.example.com").
 These suggestions are not entirely consistent with all practices that
 are currently followed by certification authorities, client
 developers, and service providers.  However, they reflect the best
 aspects of current practices and are expected to become more widely
 adopted in the coming years.

1.6. Generalization from Current Technologies

 This document attempts to generalize best practices from the many
 application technologies that currently use PKIX certificates with
 TLS.  Such technologies include, but are not limited to:
 o  The Internet Message Access Protocol [IMAP] and the Post Office
    Protocol [POP3]; see also [USINGTLS]
 o  The Hypertext Transfer Protocol [HTTP]; see also [HTTP-TLS]
 o  The Lightweight Directory Access Protocol [LDAP]; see also
    [LDAP-AUTH] and its predecessor [LDAP-TLS]
 o  The Simple Mail Transfer Protocol [SMTP]; see also [SMTP-AUTH] and
    [SMTP-TLS]
 o  The Extensible Messaging and Presence Protocol [XMPP]; see also
    [XMPP-OLD]
 o  The Network News Transfer Protocol [NNTP]; see also [NNTP-TLS]
 o  The NETCONF Configuration Protocol [NETCONF]; see also
    [NETCONF-SSH] and [NETCONF-TLS]
 o  The Syslog Protocol [SYSLOG]; see also [SYSLOG-TLS] and
    [SYSLOG-DTLS]
 o  The Session Initiation Protocol [SIP]; see also [SIP-CERTS]
 o  The Simple Network Management Protocol [SNMP]; see also [SNMP-TLS]
 o  The General Internet Signalling Transport [GIST]

Saint-Andre & Hodges Standards Track [Page 6] RFC 6125 Service Identity March 2011

 However, as noted, this document does not supersede the rules for
 verifying service identity provided in specifications for those
 application protocols.

1.7. Scope

1.7.1. In Scope

 This document applies only to service identities associated with
 fully qualified DNS domain names, only to TLS and DTLS (or the older
 Secure Sockets Layer (SSL) technology), and only to PKIX-based
 systems.  As a result, the scenarios described in 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:
 o  Client or end-user identities.
    Certificates representing client or end-user identities (e.g., the
    rfc822Name identifier) can be used for mutual authentication
    between a client and server or between two clients, thus enabling
    stronger client-server security or end-to-end security.  However,
    certification authorities, application developers, and service
    operators have less experience with client certificates than with
    server certificates, thus giving us fewer models from which to
    generalize and a less solid basis for defining best practices.
 o  Identifiers other than fully qualified DNS domain names.
    Some certification authorities issue server certificates based on
    IP addresses, but preliminary evidence indicates that such
    certificates are a very small percentage (less than 1%) of issued
    certificates.  Furthermore, IP addresses are not necessarily
    reliable identifiers for application services because of the
    existence of private internets [PRIVATE], host mobility, multiple
    interfaces on a given host, Network Address Translators (NATs)
    resulting in different addresses for a host from different
    locations on the network, the practice of grouping many hosts
    together behind a single IP address, etc.  Most fundamentally,
    most users find DNS domain names much easier to work with than IP
    addresses, which is why the domain name system was designed in the
    first place.  We prefer to define best practices for the much more
    common use case and not to complicate the rules in this
    specification.

Saint-Andre & Hodges Standards Track [Page 7] RFC 6125 Service Identity March 2011

    Furthermore, we focus here 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).
    We also do not discuss attributes unrelated to DNS domain names,
    such as those defined in [X.520] and other such specifications
    (e.g., organizational attributes, geographical attributes, company
    logos, and the like).
 o  Security protocols other than [TLS], [DTLS], or the older Secure
    Sockets Layer (SSL) technology.
    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 and DTLS-based application
    technologies.  Furthermore, application technologies have less
    experience with IPsec than with TLS, thus making it more difficult
    to gather feedback on proposed best practices.
 o  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) 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 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 to define best practices for such
    technologies would unduly complicate the rules defined in this
    specification.
 o  Certification authority policies, such as:
  • What types or "classes" of certificates to issue and whether to

apply different policies for them (e.g., allow the wildcard

       character in certificates issued to individuals who have
       provided proof of identity but do not allow the wildcard
       character in "Extended Validation" certificates [EV-CERTS]).

Saint-Andre & Hodges Standards Track [Page 8] RFC 6125 Service Identity March 2011

  • 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).

 o  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]), 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.
 o  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. 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.

Saint-Andre & Hodges Standards Track [Page 9] RFC 6125 Service Identity March 2011

 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 a Uniform Resource Identifier scheme [URI] or a DNS SRV Service
    [DNS-SRV].
 attribute-type-and-value pair:  A colloquial name for the ASN.1-based
    construction comprising a Relative Distinguished Name (RDN), which
    itself is a building-block component of Distinguished Names.  See
    Section 2 of [LDAP-DN].
 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., 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.
 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.
  • CN-ID = a Relative Distinguished Name (RDN) in the certificate

subject field that contains one and only one attribute-type-

       and-value pair of type Common Name (CN), where the value
       matches the overall form of a domain name (informally, dot-
       separated letter-digit-hyphen labels); see [PKIX] and also
       [LDAP-SCHEMA]

Saint-Andre & Hodges Standards Track [Page 10] RFC 6125 Service Identity March 2011

  • DNS-ID = a subjectAltName entry of type dNSName; see [PKIX]
  • SRV-ID = a subjectAltName entry of type otherName whose name

form is SRVName; see [SRVNAME]

  • 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] and
       [URI]
 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 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].)
 PKIX:  PKIX is a short name for the Internet Public Key
    Infrastructure using X.509 defined in RFC 5280 [PKIX], which
    comprises 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 certificate
    can include one or more presented identifiers of different types,

Saint-Andre & Hodges Standards Track [Page 11] RFC 6125 Service Identity March 2011

    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, used by the
    client for matching purposes when examining presented identifiers.
 source domain:  The fully qualified DNS domain name that a client
    expects an application service to present in the certificate
    (e.g., "www.example.com"), typically input by a human user,
    configured into a client, or provided by reference such as in a
    hyperlink.  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 -- in addition to, or in place of, identifiers
    that may be embedded within or provided as a certificate's subject
    field.
 subject field:  The subject field of a PKIX certificate identifies
    the entity associated with the public key stored in the subject
    public key field (see Section 4.1.2.6 of [PKIX]).
 subject name:  In an overall sense, a subject's name(s) can be
    represented by or in the subject field, the subjectAltName
    extension, or both (see [PKIX] for details).  More specifically,
    the term often refers to the name of a PKIX certificate's subject,
    encoded as the X.501 type Name and conveyed in a certificate's
    subject field (see Section 4.1.2.6 of [PKIX]).
 TLS client:  An entity that assumes the role of a client in a
    Transport Layer Security [TLS] negotiation.  In this specification
    we generally assume that the TLS client is an (interactive 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.

Saint-Andre & Hodges Standards Track [Page 12] RFC 6125 Service Identity March 2011

 Most security-related terms in this document are to be understood in
 the sense defined in [SECTERMS]; such terms include, but are not
 limited to, "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 RFC
 2119 [KEYWORDS].

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:
 o  A CN-ID is direct and unrestricted.
 o  A DNS-ID is direct and unrestricted.

Saint-Andre & Hodges Standards Track [Page 13] RFC 6125 Service Identity March 2011

 o  An SRV-ID can be either direct or (more typically) indirect, and
    is restricted.
 o  A URI-ID is direct and restricted.
 We summarize this taxonomy in the following table.
 +-----------+-----------+---------------+
 |           |  Direct   |  Restricted   |
 +-----------+-----------+---------------+
 |  CN-ID    |  Yes      |  No           |
 +-----------+-----------+---------------+
 |  DNS-ID   |  Yes      |  No           |
 +-----------+-----------+---------------+
 |  SRV-ID   |  Either   |  Yes          |
 +-----------+-----------+---------------+
 |  URI-ID   |  Yes      |  Yes          |
 +-----------+-----------+---------------+
 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 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, and certification authorities for
 all application protocols need to use or support DNS-IDs 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 of software
 developers, application service providers, and CAs actively using or
 supporting 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) a DNS domain name that conforms to one of
 the following forms:

Saint-Andre & Hodges Standards Track [Page 14] RFC 6125 Service Identity March 2011

 1.  A "traditional domain name", i.e., a fully qualified DNS domain
     name or "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 to the above-
     referenced specifications for details), but they are not relevant
     to this specification.
 2.  An "internationalized domain name", i.e., a DNS domain name that
     conforms to the overall form 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 any
     mixture of NR-LDH labels, A-labels, or U-labels, as described in
     [IDNA-DEFS] and the associated documents.

2.3. Subject Naming in PKIX Certificates

 In theory, the Internet Public Key Infrastructure using X.509 [PKIX]
 employs the global directory service model defined in [X.500] and
 [X.501].  Under that model, information is held in a directory
 information base (DIB) and entries in the DIB are organized in a
 hierarchy called the directory information tree (DIT).  An object or
 alias entry in that hierarchy consists of a set of attributes (each
 of which has a defined type and one or more values) and is uniquely
 identified by a Distinguished Name (DN).  The DN of an entry is
 constructed by combining the Relative Distinguished Names of its
 superior entries in the tree (all the way down to the root of the
 DIT) with one or more specially nominated attributes of the entry
 itself (which together comprise the Relative Distinguished Name (RDN)
 of the entry, so-called because it is relative to the Distinguished
 Names of the superior entries in the tree).  The entry closest to the
 root is sometimes referred to as the "most significant" entry, and
 the entry farthest from the root is sometimes referred to as the
 "least significant" entry.  An RDN is a set (i.e., an unordered
 group) of attribute-type-and-value pairs (see also [LDAP-DN]), each
 of which asserts some attribute about the entry.
 In practice, the certificates used in [X.509] and [PKIX] borrow key
 concepts from X.500 and X.501 (e.g., DNs and RDNs) to identify
 entities, but such certificates are not necessarily part of a global
 directory information base.  Specifically, the subject field of a
 PKIX certificate is an X.501 type Name that "identifies the entity
 associated with the public key stored in the subject public key
 field" (see Section 4.1.2.6 of [PKIX]).  However, it is perfectly
 acceptable for the subject field to be empty, as long as the

Saint-Andre & Hodges Standards Track [Page 15] RFC 6125 Service Identity March 2011

 certificate contains a subject alternative name ("subjectAltName")
 extension that includes at least one subjectAltName entry, because
 the subjectAltName extension allows various identities to be bound to
 the subject (see Section 4.2.1.6 of [PKIX]).  The subjectAltName
 extension itself is a sequence of typed entries, where each type is a
 distinct kind of identifier.
 For our purposes, an application service can be identified by a name
 or names carried in the subject field (i.e., a CN-ID) and/or in one
 of the following identifier types within subjectAltName entries:
 o  DNS-ID
 o  SRV-ID
 o  URI-ID
 Existing certificates often use a CN-ID in the subject field to
 represent a fully qualified DNS domain name; for example, consider
 the following three subject names, where the attribute of type Common
 Name contains a string whose form matches that of a fully qualified
 DNS domain name ("im.example.org", "mail.example.net", and
 "www.example.com", respectively):
    CN=im.example.org,O=Example Org,C=GB
    C=CA,O=Example Internetworking,CN=mail.example.net
    O=Examples-R-Us,CN=www.example.com,C=US
 However, the Common Name is not strongly typed because a Common Name
 might contain a human-friendly string for the service, rather than a
 string whose form matches that of a fully qualified DNS domain name
 (a certificate with such a single Common Name will typically have at
 least one subjectAltName entry containing the fully qualified DNS
 domain name):
    CN=A Free Chat Service,O=Example Org,C=GB
 Or, a certificate's subject might contain both a CN-ID as well as
 another common name attribute containing a human-friendly string:
    CN=A Free Chat Service,CN=im.example.org,O=Example Org,C=GB
 In general, this specification recommends and prefers use of
 subjectAltName entries (DNS-ID, SRV-ID, URI-ID, etc.) over use of the
 subject field (CN-ID) where possible, as more completely described in
 the following sections.  However, specifications that reuse this one

Saint-Andre & Hodges Standards Track [Page 16] RFC 6125 Service Identity March 2011

 can legitimately encourage continued support for the CN-ID identifier
 type if they have good reasons to do so, such as backward
 compatibility with deployed infrastructure (see, for example,
 [EV-CERTS]).

2.3.1. Implementation Notes

 Confusion sometimes arises from different renderings or encodings of
 the hierarchical information contained in a certificate.
 Certificates are binary objects and are encoded using the
 Distinguished Encoding Rules (DER) specified in [X.690].  However,
 some implementations generate displayable (a.k.a. printable)
 renderings of the certificate issuer, subject field, and
 subjectAltName extension, and these renderings convert the DER-
 encoded sequences into a "string representation" before being
 displayed.  Because a certificate subject field (of type Name
 [X.509], the same as for a Distinguished Name (DN) [X.501]) is an
 ordered sequence, order is typically preserved in subject string
 representations, although the two most prevalent subject (and DN)
 string representations differ in employing left-to-right vs. right-
 to-left ordering.  However, because a Relative Distinguished Name
 (RDN) is an unordered group of attribute-type-and-value pairs, the
 string representation of an RDN can differ from the canonical DER
 encoding (and the order of attribute-type-and-value pairs can differ
 in the RDN string representations or display orders provided by
 various implementations).  Furthermore, various specifications refer
 to the order of RDNs in DNs or certificate subject fields using
 terminology that is implicitly related to an information hierarchy
 (which may or may not actually exist), such as "most specific" vs.
 "least specific", "left-most" vs. "right-most", "first" vs. "last",
 or "most significant" vs. "least significant" (see, for example,
 [LDAP-DN]).
 To reduce confusion, in this specification we avoid such terms and
 instead use the terms provided under Section 1.8; in particular, we
 do not use the term "(most specific) Common Name field in the subject
 field" from [HTTP-TLS] and instead state that a CN-ID is a Relative
 Distinguished Name (RDN) in the certificate subject containing one
 and only one attribute-type-and-value pair of type Common Name (thus
 removing the possibility that an RDN might contain multiple AVAs
 (Attribute Value Assertions) of type CN, one of which could be
 considered "most specific").
 Finally, although theoretically some consider the order of RDNs
 within a subject field to have meaning, in practice that rule is
 often not observed.  An AVA of type CN is considered to be valid at
 any position within the subject field.

Saint-Andre & Hodges Standards Track [Page 17] RFC 6125 Service Identity March 2011

3. Designing Application Protocols

 This section provides guidelines for designers of application
 protocols, in the form of a checklist to follow when reusing the
 recommendations provided in this document.
 o  Does your technology use DNS SRV records to resolve the DNS domain
    names of application services?  If so, consider recommending or
    requiring support for the SRV-ID identifier type in PKIX
    certificates issued and used in your technology community.  (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].)
 o  Does your technology use URIs to identify application services?
    If so, consider recommending or requiring support for the URI-ID
    identifier type.  (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.)
 o  Does your technology need to use DNS domain names in the Common
    Name of certificates for the sake of backward compatibility?  If
    so, consider recommending support for the CN-ID identifier type as
    a fallback.
 o  Does your technology need to allow the wildcard character in DNS
    domain names?  If so, consider recommending support for wildcard
    certificates, and specify exactly where the wildcard character is
    allowed to occur (e.g., only the complete left-most label of a DNS
    domain name).
 Sample text is provided under Appendix A.

4. Representing Server Identity

 This section provides rules and guidelines for issuers of
 certificates.

4.1. Rules

 When a certification authority issues a certificate based on the
 fully qualified DNS domain name at which the application service
 provider will provide the relevant application, the following rules
 apply to the representation of application service identities.  The

Saint-Andre & Hodges Standards Track [Page 18] RFC 6125 Service Identity March 2011

 reader needs to be aware that some of these rules are cumulative and
 can interact in important ways that are illustrated later in this
 document.
 1.  The certificate SHOULD 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), then the certificate SHOULD include a URI-ID.  The
     scheme SHALL be that of the protocol associated with the
     application service type and the "host" component (or its
     equivalent) SHALL be the fully qualified DNS domain name of the
     service.  A 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" but not "sips" or "tel" for SIP as described in [SIP-SIPS],
     or perhaps http and https for HTTP as might be described in a
     future specification).
 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.  Even though many deployed clients still check for the CN-ID
     within the certificate subject field, certification authorities
     are encouraged to migrate away from issuing certificates that
     represent the server's fully qualified DNS domain name in a
     CN-ID.  Therefore, the certificate SHOULD NOT include a CN-ID
     unless the certification authority issues the certificate in
     accordance with a specification that reuses this one and that
     explicitly encourages continued support for the CN-ID identifier
     type in the context of a given application technology.
 6.  The certificate MAY contain more than one DNS-ID, SRV-ID, or
     URI-ID but SHOULD NOT contain more than one CN-ID, as further
     explained under Section 7.4.

Saint-Andre & Hodges Standards Track [Page 19] RFC 6125 Service Identity March 2011

 7.  Unless a specification that reuses this one allows continued
     support for the wildcard character '*', the DNS domain name
     portion of a presented identifier SHOULD NOT contain the wildcard
     character, whether as the complete left-most label within the
     identifier (following the description of labels and domain names
     in [DNS-CONCEPTS], e.g., "*.example.com") or as a fragment
     thereof (e.g., *oo.example.com, f*o.example.com, or
     fo*.example.com).  A more detailed discussion of so-called
     "wildcard certificates" is provided under Section 7.2.

4.2. Examples

 Consider a simple website at "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".  It might also
 include a CN-ID of "www.example.com" for backward compatibility with
 deployed infrastructure.
 Consider an IMAP-accessible email server at the host
 "mail.example.net" servicing email addresses of the form
 "user@example.net" and discoverable via DNS SRV lookups on the
 application service name of "example.net".  A certificate for this
 service might include SRV-IDs of "_imap.example.net" and
 "_imaps.example.net" (see [EMAIL-SRV]) along with DNS-IDs of
 "example.net" and "mail.example.net".  It might also include CN-IDs
 of "example.net" and "mail.example.net" for backward compatibility
 with deployed infrastructure.
 Consider a SIP-accessible voice-over-IP (VoIP) server at the host
 "voice.example.edu" servicing SIP addresses of the form
 "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" (see [SIP-CERTS]) along with a
 DNS-ID of "voice.example.edu".  It might also include a CN-ID of
 "voice.example.edu" for backward compatibility with deployed
 infrastructure.
 Consider an XMPP-compatible instant messaging (IM) server at the host
 "im.example.org" servicing IM addresses of the form
 "user@im.example.org" and discoverable via DNS SRV lookups on the
 "im.example.org" domain.  A certificate for this service might
 include SRV-IDs of "_xmpp-client.im.example.org" and
 "_xmpp-server.im.example.org" (see [XMPP]), a DNS-ID of
 "im.example.org", and an XMPP-specific "XmppAddr" of "im.example.org"
 (see [XMPP]).  It might also include a CN-ID of "im.example.org" for
 backward compatibility with deployed infrastructure.

Saint-Andre & Hodges Standards Track [Page 20] RFC 6125 Service Identity March 2011

5. Requesting Server Certificates

 This section provides rules and guidelines for service providers
 regarding the information to include in certificate signing requests
 (CSRs).
 In general, service providers are encouraged to 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 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 a service provider offering multiple 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
 request multiple certificates, i.e., one certificate per application
 service type.  Conversely, the service provider is discouraged from
 requesting a single certificate containing multiple SRV-IDs or URI-
 IDs identifying each different application service type.  This
 guideline does not apply to application service type "bundles" that
 are used to identify manifold distinct access methods to the same
 underlying application (e.g., an email application with access
 methods denoted by the application service types of "imap", "imaps",
 "pop3", "pop3s", and "submission" as described in [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):

Saint-Andre & Hodges Standards Track [Page 21] RFC 6125 Service Identity March 2011

 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 further checks to ensure that the server is authorized to
 provide the requested service.  However, such checking is not a
 matter of verifying the application service identity presented in a
 certificate, and therefore methods for doing so (e.g., consulting
 local policy information) are out of scope for this document.

6.2. Constructing a List of Reference Identifiers

6.2.1. 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, 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., parsing the fully qualified DNS domain name 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., pulling the data from a delegated domain that is
 explicitly established via client or system configuration, resolving

Saint-Andre & Hodges Standards Track [Page 22] RFC 6125 Service Identity March 2011

 the data via [DNSSEC], or obtaining the data from a 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).  These considerations apply only to extraction of the
 source domain from the 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.
    Example: Given an input URI of <sips:alice@example.net>, a client
    would derive the application service type "sip" from the "scheme"
    and parse the domain name "example.net" from the "host" component
    (or its equivalent).
 Each reference identifier in the list SHOULD be based on the source
 domain and SHOULD NOT be based on a derived domain (e.g., a host name
 or domain name discovered through DNS resolution of the source
 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) and
 application service type, the client constructs a list of reference
 identifiers in accordance with the following rules:
 o  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 that is (a) contained in or securely derived from the
    inputs (i.e., the source domain), or (b) explicitly associated
    with the source domain by means of user configuration (i.e., a
    derived domain).
 o  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.

Saint-Andre & Hodges Standards Track [Page 23] RFC 6125 Service Identity March 2011

 o  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.
 o  The list MAY include a CN-ID, mainly for the sake of backward
    compatibility with deployed infrastructure.
 Which identifier types a client includes in its list of reference
 identifiers 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 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).  By
 contrast, a more lenient client (even one built to connect only to a
 particular kind of service) might include both SRV-IDs and DNS-IDs in
 its list of reference identifiers.
    Implementation Note: It is highly likely that implementers of
    client software will need to support CN-IDs for the foreseeable
    future, because certificates containing CN-IDs are so widely
    deployed.  Implementers are advised to monitor the state of the
    art with regard to certificate issuance policies and migrate away
    from support CN-IDs in the future if possible.
    Implementation Note: The client does not need to construct the
    foregoing identifiers in the actual formats found in a certificate
    (e.g., as ASN.1 types); it only needs to construct the functional
    equivalent of such identifiers for matching purposes.
    Security Warning: A client MUST NOT construct a reference
    identifier corresponding to Relative Distinguished Names (RDNs)
    other than those of type Common Name and MUST NOT check for RDNs
    other than those of type Common Name in the presented identifiers.

6.2.2. Examples

 A web browser that is connecting via HTTPS to the website at
 "www.example.com" might have two reference identifiers: a DNS-ID of
 "www.example.com" and, as a fallback, a CN-ID of "www.example.com".
 A mail user agent that is connecting via IMAPS to the email service
 at "example.net" (resolved as "mail.example.net") might have five
 reference identifiers: an SRV-ID of "_imaps.example.net" (see
 [EMAIL-SRV]), DNS-IDs of "example.net" and "mail.example.net", and,
 as a fallback, CN-IDs of "example.net" and "mail.example.net".  (A

Saint-Andre & Hodges Standards Track [Page 24] RFC 6125 Service Identity March 2011

 legacy email user agent would not support [EMAIL-SRV] and therefore
 would probably be explicitly configured to connect to
 "mail.example.net", whereas an SRV-aware user agent would derive
 "example.net" from an email address of the form "user@example.net"
 but might also accept "mail.example.net" as the DNS domain name
 portion of reference identifiers for the service.)
 A voice-over-IP (VoIP) user agent that is connecting via SIP to the
 voice service at "voice.example.edu" might have only one reference
 identifier: a URI-ID of "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" might have three reference
 identifiers: an SRV-ID of "_xmpp-client.im.example.org" (see [XMPP]),
 a DNS-ID of "im.example.org", and an XMPP-specific "XmppAddr" of
 "im.example.org" (see [XMPP]).

6.3. 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.
    Implementation Note: A client might be configured to perform
    multiple searches, i.e., to match more than one reference
    identifier.  Although such behavior is not forbidden by this
    specification, rules for matching multiple reference identifiers
    are a matter for implementation or future specification.
    Security Warning: A client MUST NOT seek a match for a reference
    identifier of CN-ID if the presented identifiers include a DNS-ID,
    SRV-ID, URI-ID, or any application-specific identifier types
    supported by the client.
 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:
 o  A reference identifier of type DNS-ID does not include an
    application service type portion and thus can be used directly as
    the DNS domain name for comparison purposes.  As an example, a

Saint-Andre & Hodges Standards Track [Page 25] RFC 6125 Service Identity March 2011

    DNS-ID of "www.example.com" would result in a DNS domain name
    portion of "www.example.com".
 o  A reference identifier of type CN-ID also does not include an
    application service type portion and thus can be used directly as
    the DNS domain name for comparison purposes.  As previously
    mentioned, this document specifies that a CN-ID always contains a
    string whose form matches that of a DNS domain name (thus
    differentiating a CN-ID from a Common Name containing a human-
    friendly name).
 o  For a reference identifier of type SRV-ID, the DNS domain name
    portion is the Name and the application service type portion is
    the Service.  As an example, an SRV-ID of "_imaps.example.net"
    would be split into a DNS domain name portion of "example.net" and
    an application service type portion of "imaps" (mapping to an
    application protocol of IMAP as explained in [EMAIL-SRV]).
 o  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 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.)  Furthermore, note that
    extraction of the "reg-name" might necessitate normalization of
    the URI (as explained in [URI]).  As an example, a URI-ID of "sip:
    voice.example.edu" would be split into a DNS domain name portion
    of "voice.example.edu" and an application service type of "sip"
    (associated with an application protocol of SIP as explained in
    [SIP-CERTS]).
 Detailed comparison rules for matching the DNS domain name portion
 and application service type portion of the reference identifier are
 provided in the following sections.

6.4. Matching the DNS Domain Name Portion

 The client MUST match the DNS domain name portion of a reference
 identifier according to the following rules (and SHOULD also check
 the application service type as described under Section 6.5).  The
 rules differ depending on whether the domain to be checked is a
 "traditional domain name" or an "internationalized domain name" (as

Saint-Andre & Hodges Standards Track [Page 26] RFC 6125 Service Identity March 2011

 defined under Section 2.2).  Furthermore, to meet the needs of
 clients that support presented identifiers containing the wildcard
 character '*', we define a supplemental rule for so-called "wildcard
 certificates".  Finally, we also specify the circumstances under
 which it is acceptable to check the "CN-ID" identifier type.

6.4.1. Checking of Traditional Domain Names

 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 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" would be lower-cased
 to "www.example.com" for comparison 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

 If the DNS domain name portion of a reference identifier is an
 internationalized domain name, then an implementation 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 of Wildcard Certificates

 A client employing this specification's rules MAY match the reference
 identifier against a presented identifier whose DNS domain name
 portion contains the wildcard character '*' as part or all of a label
 (following the description of labels and domain names in
 [DNS-CONCEPTS]).
 For information regarding the security characteristics of wildcard
 certificates, see Section 7.2.
 If a client matches the reference identifier against a presented
 identifier whose DNS domain name portion contains the wildcard
 character '*', the following rules apply:
 1.  The client SHOULD NOT attempt to match a presented identifier in
     which the wildcard character comprises a label other than the
     left-most label (e.g., do not match bar.*.example.net).

Saint-Andre & Hodges Standards Track [Page 27] RFC 6125 Service Identity March 2011

 2.  If the wildcard character is the only character of the left-most
     label in the presented identifier, the client SHOULD NOT compare
     against anything but the left-most label of the reference
     identifier (e.g., *.example.com would match foo.example.com but
     not bar.foo.example.com or example.com).
 3.  The client MAY match a presented identifier in which the wildcard
     character is not the only character of the label (e.g.,
     baz*.example.net and *baz.example.net and b*z.example.net would
     be taken to match baz1.example.net and foobaz.example.net and
     buzz.example.net, respectively).  However, the client SHOULD NOT
     attempt to match a presented identifier where the wildcard
     character is embedded within an A-label or U-label [IDNA-DEFS] of
     an internationalized domain name [IDNA-PROTO].

6.4.4. Checking of Common Names

 As noted, a client MUST NOT seek a match for a reference identifier
 of CN-ID if the presented identifiers include a DNS-ID, SRV-ID,
 URI-ID, or any application-specific identifier types supported by the
 client.
 Therefore, if and only if the presented identifiers do not include a
 DNS-ID, SRV-ID, URI-ID, or any application-specific identifier types
 supported by the client, then the client MAY as a last resort check
 for a string whose form matches that of a fully qualified DNS domain
 name in a Common Name field of the subject field (i.e., a CN-ID).  If
 the client chooses to compare a reference identifier of type CN-ID
 against that string, it MUST follow the comparison rules for the DNS
 domain name portion of an identifier of type DNS-ID, SRV-ID, or
 URI-ID, as described under Section 6.4.1, Section 6.4.2, and
 Section 6.4.3.

6.5. 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 the application service type portion as described in
 the following subsections.
    Implementation Note: An identifier of type SRV-ID or URI-ID
    provides 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

Saint-Andre & Hodges Standards Track [Page 28] RFC 6125 Service Identity March 2011

    reference identifiers includes an SRV-ID of "_xmpp-
    client.im.example.org" and a DNS-ID of "apps.example.net", the
    client would check (a) the combination of an application service
    type of "xmpp-client" and a DNS domain name of "im.example.org"
    and (b) a DNS domain name of "apps.example.net".  However, the
    client would not check (c) the combination of an application
    service type of "xmpp-client" and a DNS domain name of
    "apps.example.net" because it does not have an SRV-ID of "_xmpp-
    client.apps.example.net" in its list of reference identifiers.

6.5.1. SRV-ID

 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 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. 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 identity check has
 succeeded.

Saint-Andre & Hodges Standards Track [Page 29] RFC 6125 Service Identity March 2011

6.6.3. Case #3: No Match Found, No Pinned Certificate

 If the client does not find a presented identifier matching any of
 the reference identifiers and the client has not previously pinned
 the certificate to one of the reference identifiers in the list it
 constructed for this communication attempt, then the client MUST
 proceed as described under Section 6.6.4.

6.6.4. Fallback

 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
 users from inadvertently bypassing security protections in hostile
 situations.
    Security Warning: Some interactive clients give advanced users the
    option of proceeding with acceptance despite the identity
    mismatch, thereby "pinning" the certificate to one of the
    reference identifiers in the list constructed by the client for
    this communication attempt.  Although this behavior can be
    appropriate in certain specialized circumstances, in general it
    ought to be exposed only to advanced users.  Even then 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 to proceed anyway, by forcing the
    user to view the entire certification path and only then allowing
    the user to pin the certificate (on a temporary or permanent
    basis, at the user's option).
 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

Saint-Andre & Hodges Standards Track [Page 30] RFC 6125 Service Identity March 2011

 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 the user or associated by the client).

7.2. Wildcard Certificates

 This document states that the wildcard character '*' SHOULD NOT be
 included in presented identifiers but MAY be checked by application
 clients (mainly for the sake of backward compatibility with deployed
 infrastructure).  As a result, the rules provided in this document
 are more restrictive than the rules for many existing application
 technologies (such as those excerpted under Appendix B).  Several
 security considerations justify tightening the rules:
 o  Wildcard certificates automatically vouch for any and all host
    names within their domain.  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).
 o  Specifications for existing application technologies are not clear
    or consistent about the allowable location of the wildcard
    character, such as whether it can be:
  • only the complete left-most label (e.g., *.example.com)
  • some fragment of the left-most label (e.g., fo*.example.com,

f*o.example.com, or *oo.example.com)

  • all or part of a label other than the left-most label (e.g.,

www.*.example.com or www.foo*.example.com)

  • all or part of a label that identifies a so-called "public

suffix" (e.g., *.co.uk or *.com)

  • included more than once in a given label (e.g.,

f*b*r.example.com

  • included as all or part of more than one label (e.g.,
    • .*.example.com)
    These ambiguities might introduce exploitable differences in
    identity checking behavior among client implementations and
    necessitate overly complex and inefficient identity checking
    algorithms.

Saint-Andre & Hodges Standards Track [Page 31] RFC 6125 Service Identity March 2011

 o  There is no specification that defines how the wildcard character
    may be embedded within the A-labels or U-labels [IDNA-DEFS] of an
    internationalized domain name [IDNA-PROTO]; as a result,
    implementations are strongly discouraged from including or
    attempting to check for the wildcard character embedded within the
    A-labels or U-labels of an internationalized domain name (e.g.,
    "xn--kcry6tjko*.example.org").  Note, however, that a presented
    domain name identifier MAY contain the wildcard character as long
    as that character occupies the entire left-most label position,
    where all of the remaining labels are valid NR-LDH labels,
    A-labels, or U-labels (e.g., "*.xn--kcry6tjko.example.org").
 Notwithstanding the foregoing security considerations, specifications
 that reuse this one can legitimately encourage continued support for
 the wildcard character if they have good reasons to do so, such as
 backward compatibility with deployed infrastructure (see, for
 example, [EV-CERTS]).

7.3. Internationalized Domain Names

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

7.4. Multiple 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 (e.g., in so-called "virtual hosting" environments).
 In the default TLS handshake exchange, the client is not able to
 indicate the DNS domain name with which it wants to communicate, and
 the TLS server returns only one certificate for itself.  Absent an
 extension to TLS, a typical workaround used to facilitate mapping an
 application service to multiple DNS domain names is to embed all of
 the domain names into a single certificate.
 A more recent approach, formally specified in [TLS-EXT], is for the
 client to use the TLS "Server Name Indication" (SNI) extension when
 sending the client_hello message, stipulating the DNS domain name it
 desires or expects of the service.  The service can then return the
 appropriate certificate in its Certificate message, and that
 certificate can represent a single DNS domain name.
 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; however, it explicitly
 discourages multiple CN-IDs.  Although it would be preferable to
 forbid multiple CN-IDs entirely, there are several reasons at this

Saint-Andre & Hodges Standards Track [Page 32] RFC 6125 Service Identity March 2011

 time why this specification states that they SHOULD NOT (instead of
 MUST NOT) be included:
 o  At least one significant technology community of interest
    explicitly allows multiple CN-IDs [EV-CERTS].
 o  At least one significant certification authority is known to issue
    certificates containing multiple CN-IDs.
 o  Many service providers often deem inclusion of multiple CN-IDs
    necessary in virtual hosting environments because at least one
    widely deployed operating system does not yet support the SNI
    extension.
 It is hoped that the recommendation regarding multiple CN-IDs can be
 further tightened in the future.

8. Contributors

 The following individuals made important contributions to the text of
 this document: Shumon Huque, RL 'Bob' Morgan, and Kurt Zeilenga.

9. Acknowledgements

 The editors and contributors wish to thank the following individuals
 for their feedback and suggestions: Bernard Aboba, Richard Barnes,
 Uri Blumenthal, Nelson Bolyard, Kaspar Brand, Anthony Bryan, Scott
 Cantor, Wan-Teh Chang, Bil Corry, Dave Cridland, Dave Crocker, Cyrus
 Daboo, Charles Gardiner, Philip Guenther, Phillip Hallam-Baker, Bruno
 Harbulot, Wes Hardaker, David Harrington, Paul Hoffman, Love
 Hornquist Astrand, Henry Hotz, Russ Housley, Jeffrey Hutzelman,
 Cullen Jennings, Simon Josefsson, Geoff Keating, John Klensin, Scott
 Lawrence, Matt McCutchen, Alexey Melnikov, Subramanian Moonesamy,
 Eddy Nigg, Ludwig Nussel, Joe Orton, Tom Petch, Yngve N. Pettersen,
 Tim Polk, Robert Relyea, Eric Rescorla, Pete Resnick, Martin Rex, Joe
 Salowey, Stefan Santesson, Jim Schaad, Rob Stradling, Michael
 Stroeder, Andrew Sullivan, Peter Sylvester, Martin Thomson, Paul
 Tiemann, Sean Turner, Nicolas Williams, Dan Wing, Dan Winship, and
 Stefan Winter.
 Thanks also to Barry Leiba and Ben Campbell for their reviews on
 behalf of the Security Directorate and the General Area Review Team,
 respectively.
 The responsible Area Director was Alexey Melnikov.

Saint-Andre & Hodges Standards Track [Page 33] RFC 6125 Service Identity March 2011

10. References

10.1. Normative References

 [DNS-CONCEPTS]   Mockapetris, P., "Domain names - concepts and
                  facilities", STD 13, RFC 1034, November 1987.
 [DNS-SRV]        Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR
                  for specifying the location of services (DNS SRV)",
                  RFC 2782, February 2000.
 [IDNA-DEFS]      Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Definitions and Document
                  Framework", RFC 5890, August 2010.
 [IDNA-PROTO]     Klensin, J., "Internationalized Domain Names in
                  Applications (IDNA): Protocol", RFC 5891,
                  August 2010.
 [KEYWORDS]       Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.
 [LDAP-DN]        Zeilenga, K., Ed., "Lightweight Directory Access
                  Protocol (LDAP): String Representation of
                  Distinguished Names", RFC 4514, June 2006.
 [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, May 2008.
 [SRVNAME]        Santesson, S., "Internet X.509 Public Key
                  Infrastructure Subject Alternative Name for
                  Expression of Service Name", RFC 4985, August 2007.
 [URI]            Berners-Lee, T., Fielding, R., and L. Masinter,
                  "Uniform Resource Identifier (URI): Generic Syntax",
                  STD 66, RFC 3986, January 2005.

10.2. Informative References

 [ABNF]           Crocker, D., Ed. and P. Overell, "Augmented BNF for
                  Syntax Specifications: ABNF", STD 68, RFC 5234,
                  January 2008.
 [DNS-CASE]       Eastlake 3rd, D., "Domain Name System (DNS) Case
                  Insensitivity Clarification", RFC 4343,
                  January 2006.

Saint-Andre & Hodges Standards Track [Page 34] RFC 6125 Service Identity March 2011

 [DNSSEC]         Arends, R., Austein, R., Larson, M., Massey, D., and
                  S. Rose, "DNS Security Introduction and
                  Requirements", RFC 4033, March 2005.
 [DTLS]           Rescorla, E. and N. Modadugu, "Datagram Transport
                  Layer Security", RFC 4347, April 2006.
 [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>.
 [EMAIL-SRV]      Daboo, C., "Use of SRV Records for Locating Email
                  Submission/Access Services", RFC 6186, March 2011.
 [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>.
 [GIST]           Schulzrinne, H. and R. Hancock, "GIST: General
                  Internet Signalling Transport", RFC 5971,
                  October 2010.
 [HTTP]           Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                  Masinter, L., Leach, P., and T. Berners-Lee,
                  "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                  June 1999.
 [HTTP-TLS]       Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
 [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>.
 [IDNA2003]       Faltstrom, P., Hoffman, P., and A. Costello,
                  "Internationalizing Domain Names in Applications
                  (IDNA)", RFC 3490, March 2003.
 [IMAP]           Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
                  VERSION 4rev1", RFC 3501, March 2003.
 [IP]             Postel, J., "Internet Protocol", STD 5, RFC 791,
                  September 1981.

Saint-Andre & Hodges Standards Track [Page 35] RFC 6125 Service Identity March 2011

 [IPSEC]          Kent, S. and K. Seo, "Security Architecture for the
                  Internet Protocol", RFC 4301, December 2005.
 [IPv6]           Deering, S. and R. Hinden, "Internet Protocol,
                  Version 6 (IPv6) Specification", RFC 2460,
                  December 1998.
 [LDAP]           Sermersheim, J., "Lightweight Directory Access
                  Protocol (LDAP): The Protocol", RFC 4511, June 2006.
 [LDAP-AUTH]      Harrison, R., "Lightweight Directory Access Protocol
                  (LDAP): Authentication Methods and Security
                  Mechanisms", RFC 4513, June 2006.
 [LDAP-SCHEMA]    Sciberras, A., Ed., "Lightweight Directory Access
                  Protocol (LDAP): Schema for User Applications",
                  RFC 4519, June 2006.
 [LDAP-TLS]       Hodges, J., Morgan, R., and M. Wahl, "Lightweight
                  Directory Access Protocol (v3): Extension for
                  Transport Layer Security", RFC 2830, May 2000.
 [NAPTR]          Mealling, M., "Dynamic Delegation Discovery System
                  (DDDS) Part Three: The Domain Name System (DNS)
                  Database", RFC 3403, October 2002.
 [NETCONF]        Enns, R., Ed., "NETCONF Configuration Protocol",
                  RFC 4741, December 2006.
 [NETCONF-SSH]    Wasserman, M. and T. Goddard, "Using the NETCONF
                  Configuration Protocol over Secure SHell (SSH)",
                  RFC 4742, December 2006.
 [NETCONF-TLS]    Badra, M., "NETCONF over Transport Layer Security
                  (TLS)", RFC 5539, May 2009.
 [NNTP]           Feather, C., "Network News Transfer Protocol
                  (NNTP)", RFC 3977, October 2006.
 [NNTP-TLS]       Murchison, K., Vinocur, J., and C. Newman, "Using
                  Transport Layer Security (TLS) with Network News
                  Transfer Protocol (NNTP)", RFC 4642, October 2006.
 [OCSP]           Myers, M., Ankney, R., Malpani, A., Galperin, S.,
                  and C. Adams, "X.509 Internet Public Key
                  Infrastructure Online Certificate Status Protocol -
                  OCSP", RFC 2560, June 1999.

Saint-Andre & Hodges Standards Track [Page 36] RFC 6125 Service Identity March 2011

 [OPENPGP]        Callas, J., Donnerhacke, L., Finney, H., Shaw, D.,
                  and R. Thayer, "OpenPGP Message Format", RFC 4880,
                  November 2007.
 [PKIX-OLD]       Housley, R., Ford, W., Polk, T., and D. Solo,
                  "Internet X.509 Public Key Infrastructure
                  Certificate and CRL Profile", RFC 2459,
                  January 1999.
 [POP3]           Myers, J. and M. Rose, "Post Office Protocol -
                  Version 3", STD 53, RFC 1939, May 1996.
 [PRIVATE]        Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
                  G., and E. Lear, "Address Allocation for Private
                  Internets", BCP 5, RFC 1918, February 1996.
 [S-NAPTR]        Daigle, L. and A. Newton, "Domain-Based Application
                  Service Location Using SRV RRs and the Dynamic
                  Delegation Discovery Service (DDDS)", RFC 3958,
                  January 2005.
 [SECTERMS]       Shirey, R., "Internet Security Glossary, Version 2",
                  RFC 4949, August 2007.
 [SIP]            Rosenberg, J., Schulzrinne, H., Camarillo, G.,
                  Johnston, A., Peterson, J., Sparks, R., Handley, M.,
                  and E. Schooler, "SIP: Session Initiation Protocol",
                  RFC 3261, June 2002.
 [SIP-CERTS]      Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
                  Certificates in the Session Initiation Protocol
                  (SIP)", RFC 5922, June 2010.
 [SIP-SIPS]       Audet, F., "The Use of the SIPS URI Scheme in the
                  Session Initiation Protocol (SIP)", RFC 5630,
                  October 2009.
 [SMTP]           Klensin, J., "Simple Mail Transfer Protocol",
                  RFC 5321, October 2008.
 [SMTP-AUTH]      Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP
                  Service Extension for Authentication", RFC 4954,
                  July 2007.
 [SMTP-TLS]       Hoffman, P., "SMTP Service Extension for Secure SMTP
                  over Transport Layer Security", RFC 3207,
                  February 2002.

Saint-Andre & Hodges Standards Track [Page 37] RFC 6125 Service Identity March 2011

 [SNMP]           Harrington, D., Presuhn, R., and B. Wijnen, "An
                  Architecture for Describing Simple Network
                  Management Protocol (SNMP) Management Frameworks",
                  STD 62, RFC 3411, December 2002.
 [SNMP-TLS]       Hardaker, W., "Transport Layer Security (TLS)
                  Transport Model for the Simple Network Management
                  Protocol (SNMP)", RFC 5953, August 2010.
 [SYSLOG]         Gerhards, R., "The Syslog Protocol", RFC 5424,
                  March 2009.
 [SYSLOG-DTLS]    Salowey, J., Petch, T., Gerhards, R., and H. Feng,
                  "Datagram Transport Layer Security (DTLS) Transport
                  Mapping for Syslog", RFC 6012, October 2010.
 [SYSLOG-TLS]     Miao, F., Ed., Ma, Y., Ed., and J. Salowey, Ed.,
                  "Transport Layer Security (TLS) Transport Mapping
                  for Syslog", RFC 5425, March 2009.
 [TLS]            Dierks, T. and E. Rescorla, "The Transport Layer
                  Security (TLS) Protocol Version 1.2", RFC 5246,
                  August 2008.
 [TLS-EXT]        Eastlake 3rd, D., "Transport Layer Security (TLS)
                  Extensions: Extension Definitions", RFC 6066,
                  January 2011.
 [US-ASCII]       American National Standards Institute, "Coded
                  Character Set - 7-bit American Standard Code for
                  Information Interchange", ANSI X3.4, 1986.
 [USINGTLS]       Newman, C., "Using TLS with IMAP, POP3 and ACAP",
                  RFC 2595, June 1999.
 [WSC-UI]         Saldhana, A. and T. Roessler, "Web Security Context:
                  User Interface Guidelines", World Wide Web
                  Consortium LastCall WD-wsc-ui-20100309, March 2010,
                  <http://www.w3.org/TR/2010/WD-wsc-ui-20100309>.
 [X.500]          International Telecommunications Union, "Information
                  Technology - Open Systems Interconnection - The
                  Directory: Overview of concepts, models and
                  services", ITU-T Recommendation X.500, ISO Standard
                  9594-1, August 2005.

Saint-Andre & Hodges Standards Track [Page 38] RFC 6125 Service Identity March 2011

 [X.501]          International Telecommunications Union, "Information
                  Technology - Open Systems Interconnection - The
                  Directory: Models", ITU-T Recommendation X.501,
                  ISO Standard 9594-2, August 2005.
 [X.509]          International Telecommunications Union, "Information
                  Technology - Open Systems Interconnection - The
                  Directory: Public-key and attribute certificate
                  frameworks", ITU-T Recommendation X.509,
                  ISO Standard 9594-8, August 2005.
 [X.520]          International Telecommunications Union, "Information
                  Technology - Open Systems Interconnection - The
                  Directory: Selected attribute types", ITU-
                  T Recommendation X.509, ISO Standard 9594-6,
                  August 2005.
 [X.690]          International Telecommunications Union, "Information
                  Technology - ASN.1 encoding rules: Specification of
                  Basic Encoding Rules (BER), Canonical Encoding Rules
                  (CER) and Distinguished Encoding Rules (DER)", ITU-
                  T Recommendation X.690, ISO Standard 8825-1,
                  August 2008.
 [XMPP]           Saint-Andre, P., "Extensible Messaging and Presence
                  Protocol (XMPP): Core", RFC 6120, March 2011.
 [XMPP-OLD]       Saint-Andre, P., Ed., "Extensible Messaging and
                  Presence Protocol (XMPP): Core", RFC 3920,
                  October 2004.

Saint-Andre & Hodges Standards Track [Page 39] RFC 6125 Service Identity March 2011

Appendix A. Sample Text

 At the time of this writing, two application technologies reuse the
 recommendations in this specification: email [EMAIL-SRV] and XMPP
 [XMPP].  Here we include the text from [XMPP] to illustrate the
 thought process that might be followed by protocol designers for
 other application technologies.  Specifically, because XMPP uses DNS
 SRV records for resolution of the DNS domain names for application
 services, the XMPP specification recommends the use of SRV-IDs.
 The text regarding certificate issuance is as follows:
 ######
 In a PKIX certificate to be presented by an XMPP server (i.e., a
 "server certificate"), the certificate MUST include one or more XMPP
 addresses (i.e., domainparts) associated with XMPP services hosted at
 the server.  The rules and guidelines defined in [this specification]
 apply to XMPP server certificates, with the following XMPP-specific
 considerations:
 o  Support for the DNS-ID identifier type [PKIX] is REQUIRED in XMPP
    client and server software implementations.  Certification
    authorities that issue XMPP-specific certificates MUST support the
    DNS-ID identifier type.  XMPP service providers SHOULD include the
    DNS-ID identifier type in certificate requests.
 o  Support for the SRV-ID identifier type [SRVNAME] is REQUIRED for
    XMPP client and server software implementations (for verification
    purposes XMPP client implementations need to support only the
    "_xmpp-client" application service type, whereas XMPP server
    implementations need to support both the "_xmpp-client" and
    "_xmpp-server" application service types).  Certification
    authorities that issue XMPP-specific certificates SHOULD support
    the SRV-ID identifier type.  XMPP service providers SHOULD include
    the SRV-ID identifier type in certificate requests.
 o  Support for the XmppAddr identifier type is encouraged in XMPP
    client and server software implementations for the sake of
    backward-compatibility, but is no longer encouraged in
    certificates issued by certification authorities or requested by
    XMPP service providers.
 o  DNS domain names in server certificates MAY contain the wildcard
    character '*' as the complete left-most label within the
    identifier.
 ######

Saint-Andre & Hodges Standards Track [Page 40] RFC 6125 Service Identity March 2011

 The text regarding certificate verification is as follows:
 ######
 For server certificates, the rules and guidelines defined in [this
 specification] apply, with the proviso that the XmppAddr identifier
 is allowed as a reference identifier.
 The identities to be checked are set as follows:
 o  The initiating entity sets its reference identifier to the 'to'
    address it communicates in the initial stream header; i.e., this
    is the identity it expects the receiving entity to provide in a
    PKIX certificate.
 o  The receiving entity sets its reference identifier to the 'from'
    address communicated by the initiating entity in the initial
    stream header; i.e., this is the identity that the initiating
    entity is trying to assert.
 ######

Saint-Andre & Hodges Standards Track [Page 41] RFC 6125 Service Identity March 2011

Appendix B. Prior Art

 (This section is non-normative.)
 The recommendations in this document are an abstraction from
 recommendations in specifications for a wide range of application
 protocols.  For the purpose of comparison and to delineate the
 history of thinking about application service identity verification
 within the IETF, this informative section gathers together prior art
 by including the exact text from various RFCs (the only modifications
 are changes to the names of several references to maintain coherence
 with the main body of this document, and the elision of irrelevant
 text as marked by the characters "[...]").

B.1. IMAP, POP3, and ACAP (1999)

 In 1999, [USINGTLS] specified the following text regarding
 application service identity verification in IMAP, POP3, and ACAP:
 ######
 2.4.  Server Identity Check
 During the TLS negotiation, the client MUST check its understanding
 of the server hostname against the server's identity as presented in
 the server Certificate message, in order to prevent man-in-the-middle
 attacks.  Matching is performed according to these rules:
 o  The client MUST use the server hostname it used to open the
    connection as the value to compare against the server name as
    expressed in the server certificate.  The client MUST NOT use any
    form of the server hostname derived from an insecure remote source
    (e.g., insecure DNS lookup).  CNAME canonicalization is not done.
 o  If a subjectAltName extension of type dNSName is present in the
    certificate, it SHOULD be used as the source of the server's
    identity.
 o  Matching is case-insensitive.
 o  A "*" wildcard character MAY be used as the left-most name
    component in the certificate.  For example, *.example.com would
    match a.example.com, foo.example.com, etc. but would not match
    example.com.
 o  If the certificate contains multiple names (e.g. more than one
    dNSName field), then a match with any one of the fields is
    considered acceptable.

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 If the match fails, the client SHOULD either ask for explicit user
 confirmation, or terminate the connection and indicate the server's
 identity is suspect.
 ######

B.2. HTTP (2000)

 In 2000, [HTTP-TLS] specified the following text regarding
 application service identity verification in HTTP:
 ######
 3.1.  Server Identity
 In general, HTTP/TLS requests are generated by dereferencing a URI.
 As a consequence, the hostname for the server is known to the client.
 If the hostname is available, the client MUST check it against the
 server's identity as presented in the server's Certificate message,
 in order to prevent man-in-the-middle attacks.
 If the client has external information as to the expected identity of
 the server, the hostname check MAY be omitted.  (For instance, a
 client may be connecting to a machine whose address and hostname are
 dynamic but the client knows the certificate that the server will
 present.)  In such cases, it is important to narrow the scope of
 acceptable certificates as much as possible in order to prevent man
 in the middle attacks.  In special cases, it may be appropriate for
 the client to simply ignore the server's identity, but it must be
 understood that this leaves the connection open to active attack.
 If a subjectAltName extension of type dNSName is present, that MUST
 be used as the identity.  Otherwise, the (most specific) Common Name
 field in the Subject field of the certificate MUST be used.  Although
 the use of the Common Name is existing practice, it is deprecated and
 Certification Authorities are encouraged to use the dNSName instead.
 Matching is performed using the matching rules specified by
 [PKIX-OLD].  If more than one identity of a given type is present in
 the certificate (e.g., more than one dNSName name, a match in any one
 of the set is considered acceptable.)  Names may contain the wildcard
 character * which is considered to match any single domain name
 component or component fragment.  E.g., *.a.com matches foo.a.com but
 not bar.foo.a.com. f*.com matches foo.com but not bar.com.
 In some cases, the URI is specified as an IP address rather than a
 hostname.  In this case, the iPAddress subjectAltName must be present
 in the certificate and must exactly match the IP in the URI.

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 If the hostname does not match the identity in the certificate, user
 oriented clients MUST either notify the user (clients MAY give the
 user the opportunity to continue with the connection in any case) or
 terminate the connection with a bad certificate error.  Automated
 clients MUST log the error to an appropriate audit log (if available)
 and SHOULD terminate the connection (with a bad certificate error).
 Automated clients MAY provide a configuration setting that disables
 this check, but MUST provide a setting which enables it.
 Note that in many cases the URI itself comes from an untrusted
 source.  The above-described check provides no protection against
 attacks where this source is compromised.  For example, if the URI
 was obtained by clicking on an HTML page which was itself obtained
 without using HTTP/TLS, a man in the middle could have replaced the
 URI.  In order to prevent this form of attack, users should carefully
 examine the certificate presented by the server to determine if it
 meets their expectations.
 ######

B.3. LDAP (2000/2006)

 In 2000, [LDAP-TLS] specified the following text regarding
 application service identity verification in LDAP:
 ######
 3.6.  Server Identity Check
 The client MUST check its understanding of the server's hostname
 against the server's identity as presented in the server's
 Certificate message, in order to prevent man-in-the-middle attacks.
 Matching is performed according to these rules:
 o  The client MUST use the server hostname it used to open the LDAP
    connection as the value to compare against the server name as
    expressed in the server's certificate.  The client MUST NOT use
    the server's canonical DNS name or any other derived form of name.
 o  If a subjectAltName extension of type dNSName is present in the
    certificate, it SHOULD be used as the source of the server's
    identity.
 o  Matching is case-insensitive.
 o  The "*" wildcard character is allowed.  If present, it applies
    only to the left-most name component.

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 E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not
 bar.com.  If more than one identity of a given type is present in the
 certificate (e.g. more than one dNSName name), a match in any one of
 the set is considered acceptable.
 If the hostname does not match the dNSName-based identity in the
 certificate per the above check, user-oriented clients SHOULD either
 notify the user (clients MAY give the user the opportunity to
 continue with the connection in any case) or terminate the connection
 and indicate that the server's identity is suspect.  Automated
 clients SHOULD close the connection, returning and/or logging an
 error indicating that the server's identity is suspect.
 Beyond the server identity checks described in this section, clients
 SHOULD be prepared to do further checking to ensure that the server
 is authorized to provide the service it is observed to provide.  The
 client MAY need to make use of local policy information.
 ######
 In 2006, [LDAP-AUTH] specified the following text regarding
 application service identity verification in LDAP:
 ######
 3.1.3.  Server Identity Check
 In order to prevent man-in-the-middle attacks, the client MUST verify
 the server's identity (as presented in the server's Certificate
 message).  In this section, the client's understanding of the
 server's identity (typically the identity used to establish the
 transport connection) is called the "reference identity".
 The client determines the type (e.g., DNS name or IP address) of the
 reference identity and performs a comparison between the reference
 identity and each subjectAltName value of the corresponding type
 until a match is produced.  Once a match is produced, the server's
 identity has been verified, and the server identity check is
 complete.  Different subjectAltName types are matched in different
 ways.  Sections 3.1.3.1 - 3.1.3.3 explain how to compare values of
 various subjectAltName types.
 The client may map the reference identity to a different type prior
 to performing a comparison.  Mappings may be performed for all
 available subjectAltName types to which the reference identity can be
 mapped; however, the reference identity should only be mapped to
 types for which the mapping is either inherently secure (e.g.,
 extracting the DNS name from a URI to compare with a subjectAltName

Saint-Andre & Hodges Standards Track [Page 45] RFC 6125 Service Identity March 2011

 of type dNSName) or for which the mapping is performed in a secure
 manner (e.g., using [DNSSEC], or using user- or admin-configured
 host-to-address/address-to-host lookup tables).
 The server's identity may also be verified by comparing the reference
 identity to the Common Name (CN) [LDAP-SCHEMA] value in the last
 Relative Distinguished Name (RDN) of the subject field of the
 server's certificate (where "last" refers to the DER-encoded order,
 not the order of presentation in a string representation of DER-
 encoded data).  This comparison is performed using the rules for
 comparison of DNS names in Section 3.1.3.1, below, with the exception
 that no wildcard matching is allowed.  Although the use of the Common
 Name value is existing practice, it is deprecated, and Certification
 Authorities are encouraged to provide subjectAltName values instead.
 Note that the TLS implementation may represent DNs in certificates
 according to X.500 or other conventions.  For example, some X.500
 implementations order the RDNs in a DN using a left-to-right (most
 significant to least significant) convention instead of LDAP's right-
 to-left convention.
 If the server identity check fails, user-oriented clients SHOULD
 either notify the user (clients may give the user the opportunity to
 continue with the LDAP session in this case) or close the transport
 connection and indicate that the server's identity is suspect.
 Automated clients SHOULD close the transport connection and then
 return or log an error indicating that the server's identity is
 suspect or both.
 Beyond the server identity check described in this section, clients
 should be prepared to do further checking to ensure that the server
 is authorized to provide the service it is requested to provide.  The
 client may need to make use of local policy information in making
 this determination.
 3.1.3.1.  Comparison of DNS Names
 If the reference identity is an internationalized domain name,
 conforming implementations MUST convert it to the ASCII Compatible
 Encoding (ACE) format as specified in Section 4 of RFC 3490
 [IDNA2003] before comparison with subjectAltName values of type
 dNSName.  Specifically, conforming implementations MUST perform the
 conversion operation specified in Section 4 of RFC 3490 as follows:
 o  in step 1, the domain name SHALL be considered a "stored string";
 o  in step 3, set the flag called "UseSTD3ASCIIRules";
 o  in step 4, process each label with the "ToASCII" operation; and

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 o  in step 5, change all label separators to U+002E (full stop).
 After performing the "to-ASCII" conversion, the DNS labels and names
 MUST be compared for equality according to the rules specified in
 Section 3 of RFC3490.
 The '*' (ASCII 42) wildcard character is allowed in subjectAltName
 values of type dNSName, and then only as the left-most (least
 significant) DNS label in that value.  This wildcard matches any
 left-most DNS label in the server name.  That is, the subject
 *.example.com matches the server names a.example.com and
 b.example.com, but does not match example.com or a.b.example.com.
 3.1.3.2.  Comparison of IP Addresses
 When the reference identity is an IP address, the identity MUST be
 converted to the "network byte order" octet string representation
 [IP] [IPv6].  For IP Version 4, as specified in RFC 791, the octet
 string will contain exactly four octets.  For IP Version 6, as
 specified in RFC 2460, the octet string will contain exactly sixteen
 octets.  This octet string is then compared against subjectAltName
 values of type iPAddress.  A match occurs if the reference identity
 octet string and value octet strings are identical.
 3.1.3.3.  Comparison of Other subjectName Types
 Client implementations MAY support matching against subjectAltName
 values of other types as described in other documents.
 ######

B.4. SMTP (2002/2007)

 In 2002, [SMTP-TLS] specified the following text regarding
 application service identity verification in SMTP:
 ######
 4.1 Processing After the STARTTLS Command
 [...]
 The decision of whether or not to believe the authenticity of the
 other party in a TLS negotiation is a local matter.  However, some
 general rules for the decisions are:

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 o  A SMTP client would probably only want to authenticate an SMTP
    server whose server certificate has a domain name that is the
    domain name that the client thought it was connecting to.
 [...]
 ######
 In 2006, [SMTP-AUTH] specified the following text regarding
 application service identity verification in SMTP:
 ######
 14.  Additional Requirements When Using SASL PLAIN over TLS
 [...]
 After a successful [TLS] negotiation, the client MUST check its
 understanding of the server hostname against the server's identity as
 presented in the server Certificate message, in order to prevent man-
 in-the-middle attacks.  If the match fails, the client MUST NOT
 attempt to authenticate using the SASL PLAIN mechanism.  Matching is
 performed according to the following rules:
    The client MUST use the server hostname it used to open the
    connection as the value to compare against the server name as
    expressed in the server certificate.  The client MUST NOT use any
    form of the server hostname derived from an insecure remote source
    (e.g., insecure DNS lookup).  CNAME canonicalization is not done.
    If a subjectAltName extension of type dNSName is present in the
    certificate, it SHOULD be used as the source of the server's
    identity.
    Matching is case-insensitive.
    A "*" wildcard character MAY be used as the leftmost name
    component in the certificate.  For example, *.example.com would
    match a.example.com, foo.example.com, etc., but would not match
    example.com.
    If the certificate contains multiple names (e.g., more than one
    dNSName field), then a match with any one of the fields is
    considered acceptable.
 ######

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B.5. XMPP (2004)

 In 2004, [XMPP-OLD] specified the following text regarding
 application service identity verification in XMPP:
 ######
 14.2.  Certificate Validation
 When an XMPP peer communicates with another peer securely, it MUST
 validate the peer's certificate.  There are three possible cases:
 Case #1:  The peer contains an End Entity certificate which appears
    to be certified by a certification path terminating in a trust
    anchor (as described in Section 6.1 of [PKIX]).
 Case #2:  The peer certificate is certified by a Certificate
    Authority not known to the validating peer.
 Case #3:  The peer certificate is self-signed.
 In Case #1, the validating peer MUST do one of two things:
 1.  Verify the peer certificate according to the rules of [PKIX].
     The certificate SHOULD then be checked against the expected
     identity of the peer following the rules described in [HTTP-TLS],
     except that a subjectAltName extension of type "xmpp" MUST be
     used as the identity if present.  If one of these checks fails,
     user-oriented clients MUST either notify the user (clients MAY
     give the user the opportunity to continue with the connection in
     any case) or terminate the connection with a bad certificate
     error.  Automated clients SHOULD terminate the connection (with a
     bad certificate error) and log the error to an appropriate audit
     log.  Automated clients MAY provide a configuration setting that
     disables this check, but MUST provide a setting that enables it.
 2.  The peer SHOULD show the certificate to a user for approval,
     including the entire certification path.  The peer MUST cache the
     certificate (or some non-forgeable representation such as a
     hash).  In future connections, the peer MUST verify that the same
     certificate was presented and MUST notify the user if it has
     changed.
 In Case #2 and Case #3, implementations SHOULD act as in (2) above.
 ######

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 Although [XMPP-OLD] defined its own rules, [XMPP] reuses the rules in
 this document regarding application service identity verification in
 XMPP.

B.6. NNTP (2006)

 In 2006, [NNTP-TLS] specified the following text regarding
 application service identity verification in NNTP:
 ######
 5.  Security Considerations
 [...]
 During the TLS negotiation, the client MUST check its understanding
 of the server hostname against the server's identity as presented in
 the server Certificate message, in order to prevent man-in-the-middle
 attacks.  Matching is performed according to these rules:
 o  The client MUST use the server hostname it used to open the
    connection (or the hostname specified in TLS "server_name"
    extension [TLS]) as the value to compare against the server name
    as expressed in the server certificate.  The client MUST NOT use
    any form of the server hostname derived from an insecure remote
    source (e.g., insecure DNS lookup).  CNAME canonicalization is not
    done.
 o  If a subjectAltName extension of type dNSName is present in the
    certificate, it SHOULD be used as the source of the server's
    identity.
 o  Matching is case-insensitive.
 o  A "*" wildcard character MAY be used as the left-most name
    component in the certificate.  For example, *.example.com would
    match a.example.com, foo.example.com, etc., but would not match
    example.com.
 o  If the certificate contains multiple names (e.g., more than one
    dNSName field), then a match with any one of the fields is
    considered acceptable.
 If the match fails, the client SHOULD either ask for explicit user
 confirmation or terminate the connection with a QUIT command and
 indicate the server's identity is suspect.

Saint-Andre & Hodges Standards Track [Page 50] RFC 6125 Service Identity March 2011

 Additionally, clients MUST verify the binding between the identity of
 the servers to which they connect and the public keys presented by
 those servers.  Clients SHOULD implement the algorithm in Section 6
 of [PKIX] for general certificate validation, but MAY supplement that
 algorithm with other validation methods that achieve equivalent
 levels of verification (such as comparing the server certificate
 against a local store of already-verified certificates and identity
 bindings).
 ######

B.7. NETCONF (2006/2009)

 In 2006, [NETCONF-SSH] specified the following text regarding
 application service identity verification in NETCONF:
 ######
 6.  Security Considerations
 The identity of the server MUST be verified and authenticated by the
 client according to local policy before password-based authentication
 data or any configuration or state data is sent to or received from
 the server.  The identity of the client MUST also be verified and
 authenticated by the server according to local policy to ensure that
 the incoming client request is legitimate before any configuration or
 state data is sent to or received from the client.  Neither side
 should establish a NETCONF over SSH connection with an unknown,
 unexpected, or incorrect identity on the opposite side.
 ######
 In 2009, [NETCONF-TLS] specified the following text regarding
 application service identity verification in NETCONF:
 ######
 3.1.  Server Identity
 During the TLS negotiation, the client MUST carefully examine the
 certificate presented by the server to determine if it meets the
 client's expectations.  Particularly, the client MUST check its
 understanding of the server hostname against the server's identity as
 presented in the server Certificate message, in order to prevent man-
 in-the-middle attacks.

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 Matching is performed according to the rules below (following the
 example of [NNTP-TLS]):
 o  The client MUST use the server hostname it used to open the
    connection (or the hostname specified in the TLS "server_name"
    extension [TLS]) as the value to compare against the server name
    as expressed in the server certificate.  The client MUST NOT use
    any form of the server hostname derived from an insecure remote
    source (e.g., insecure DNS lookup).  CNAME canonicalization is not
    done.
 o  If a subjectAltName extension of type dNSName is present in the
    certificate, it MUST be used as the source of the server's
    identity.
 o  Matching is case-insensitive.
 o  A "*" wildcard character MAY be used as the leftmost name
    component in the certificate.  For example, *.example.com would
    match a.example.com, foo.example.com, etc., but would not match
    example.com.
 o  If the certificate contains multiple names (e.g., more than one
    dNSName field), then a match with any one of the fields is
    considered acceptable.
 If the match fails, the client MUST either ask for explicit user
 confirmation or terminate the connection and indicate the server's
 identity is suspect.
 Additionally, clients MUST verify the binding between the identity of
 the servers to which they connect and the public keys presented by
 those servers.  Clients SHOULD implement the algorithm in Section 6
 of [PKIX] for general certificate validation, but MAY supplement that
 algorithm with other validation methods that achieve equivalent
 levels of verification (such as comparing the server certificate
 against a local store of already-verified certificates and identity
 bindings).
 If the client has external information as to the expected identity of
 the server, the hostname check MAY be omitted.
 ######

B.8. Syslog (2009)

 In 2009, [SYSLOG-TLS] specified the following text regarding
 application service identity verification in Syslog:

Saint-Andre & Hodges Standards Track [Page 52] RFC 6125 Service Identity March 2011

 ######
 5.2.  Subject Name Authorization
 Implementations MUST support certification path validation [PKIX].
 In addition, they MUST support specifying the authorized peers using
 locally configured host names and matching the name against the
 certificate as follows.
 o  Implementations MUST support matching the locally configured host
    name against a dNSName in the subjectAltName extension field and
    SHOULD support checking the name against the common name portion
    of the subject distinguished name.
 o  The '*' (ASCII 42) wildcard character is allowed in the dNSName of
    the subjectAltName extension (and in common name, if used to store
    the host name), but only as the left-most (least significant) DNS
    label in that value.  This wildcard matches any left-most DNS
    label in the server name.  That is, the subject *.example.com
    matches the server names a.example.com and b.example.com, but does
    not match example.com or a.b.example.com.  Implementations MUST
    support wildcards in certificates as specified above, but MAY
    provide a configuration option to disable them.
 o  Locally configured names MAY contain the wildcard character to
    match a range of values.  The types of wildcards supported MAY be
    more flexible than those allowed in subject names, making it
    possible to support various policies for different environments.
    For example, a policy could allow for a trust-root-based
    authorization where all credentials issued by a particular CA
    trust root are authorized.
 o  If the locally configured name is an internationalized domain
    name, conforming implementations MUST convert it to the ASCII
    Compatible Encoding (ACE) format for performing comparisons, as
    specified in Section 7 of [PKIX].
 o  Implementations MAY support matching a locally configured IP
    address against an iPAddress stored in the subjectAltName
    extension.  In this case, the locally configured IP address is
    converted to an octet string as specified in [PKIX], Section
    4.2.1.6.  A match occurs if this octet string is equal to the
    value of iPAddress in the subjectAltName extension.
 ######

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B.9. SIP (2010)

 In 2010, [SIP-CERTS] specified the following text regarding
 application service identity verification in SIP:
 ######
 7.2.  Comparing SIP Identities
 When an implementation (either client or server) compares two values
 as SIP domain identities:
    Implementations MUST compare only the DNS name component of each
    SIP domain identifier; an implementation MUST NOT use any scheme
    or parameters in the comparison.
    Implementations MUST compare the values as DNS names, which means
    that the comparison is case insensitive as specified by
    [DNS-CASE].  Implementations MUST handle Internationalized Domain
    Names (IDNs) in accordance with Section 7.2 of [PKIX].
    Implementations MUST match the values in their entirety:
       Implementations MUST NOT match suffixes.  For example,
       "foo.example.com" does not match "example.com".
       Implementations MUST NOT match any form of wildcard, such as a
       leading "." or "*." with any other DNS label or sequence of
       labels.  For example, "*.example.com" matches only
       "*.example.com" but not "foo.example.com".  Similarly,
       ".example.com" matches only ".example.com", and does not match
       "foo.example.com."
          [HTTP-TLS] allows the dNSName component to contain a
          wildcard; e.g., "DNS:*.example.com".  [PKIX], while not
          disallowing this explicitly, leaves the interpretation of
          wildcards to the individual specification.  [SIP] does not
          provide any guidelines on the presence of wildcards in
          certificates.  Through the rule above, this document
          prohibits such wildcards in certificates for SIP domains.
 ######

Saint-Andre & Hodges Standards Track [Page 54] RFC 6125 Service Identity March 2011

B.10. SNMP (2010)

 In 2010, [SNMP-TLS] specified the following text regarding
 application service identity verification in SNMP:
 ######
 If the server's presented certificate has passed certification path
 validation [PKIX] to a configured trust anchor, and an active row
 exists with a zero-length snmpTlstmAddrServerFingerprint value, then
 the snmpTlstmAddrServerIdentity column contains the expected host
 name.  This expected host name is then compared against the server's
 certificate as follows:
 o  Implementations MUST support matching the expected host name
    against a dNSName in the subjectAltName extension field and MAY
    support checking the name against the CommonName portion of the
    subject distinguished name.
 o  The '*' (ASCII 0x2a) wildcard character is allowed in the dNSName
    of the subjectAltName extension (and in common name, if used to
    store the host name), but only as the left-most (least
    significant) DNS label in that value.  This wildcard matches any
    left-most DNS label in the server name.  That is, the subject
    *.example.com matches the server names a.example.com and
    b.example.com, but does not match example.com or a.b.example.com.
    Implementations MUST support wildcards in certificates as
    specified above, but MAY provide a configuration option to disable
    them.
 o  If the locally configured name is an internationalized domain
    name, conforming implementations MUST convert it to the ASCII
    Compatible Encoding (ACE) format for performing comparisons, as
    specified in Section 7 of [PKIX].
 If the expected host name fails these conditions then the connection
 MUST be closed.
 ######

B.11. GIST (2010)

 In 2010, [GIST] specified the following text regarding application
 service identity verification in the General Internet Signalling
 Transport:

Saint-Andre & Hodges Standards Track [Page 55] RFC 6125 Service Identity March 2011

 ######
 5.7.3.1.  Identity Checking in TLS
 After TLS authentication, a node MUST check the identity presented by
 the peer in order to avoid man-in-the-middle attacks, and verify that
 the peer is authorised to take part in signalling at the GIST layer.
 The authorisation check is carried out by comparing the presented
 identity with each Authorised Peer Database (APD) entry in turn, as
 discussed in Section 4.4.2.  This section defines the identity
 comparison algorithm for a single APD entry.
 For TLS authentication with X.509 certificates, an identity from the
 DNS namespace MUST be checked against each subjectAltName extension
 of type dNSName present in the certificate.  If no such extension is
 present, then the identity MUST be compared to the (most specific)
 Common Name in the Subject field of the certificate.  When matching
 DNS names against dNSName or Common Name fields, matching is case-
 insensitive.  Also, a "*" wildcard character MAY be used as the left-
 most name component in the certificate or identity in the APD.  For
 example, *.example.com in the APD would match certificates for
 a.example.com, foo.example.com, *.example.com, etc., but would not
 match example.com.  Similarly, a certificate for *.example.com would
 be valid for APD identities of a.example.com, foo.example.com,
 *.example.com, etc., but not example.com.
 Additionally, a node MUST verify the binding between the identity of
 the peer to which it connects and the public key presented by that
 peer.  Nodes SHOULD implement the algorithm in Section 6 of [PKIX]
 for general certificate validation, but MAY supplement that algorithm
 with other validation methods that achieve equivalent levels of
 verification (such as comparing the server certificate against a
 local store of already-verified certificates and identity bindings).
 For TLS authentication with pre-shared keys, the identity in the
 psk_identity_hint (for the server identity, i.e. the Responding node)
 or psk_identity (for the client identity, i.e. the Querying node)
 MUST be compared to the identities in the APD.
 ######

Saint-Andre & Hodges Standards Track [Page 56] RFC 6125 Service Identity March 2011

Authors' Addresses

 Peter Saint-Andre
 Cisco
 1899 Wyknoop Street, Suite 600
 Denver, CO  80202
 USA
 Phone: +1-303-308-3282
 EMail: psaintan@cisco.com
 Jeff Hodges
 PayPal
 2211 North First Street
 San Jose, California  95131
 US
 EMail: Jeff.Hodges@PayPal.com

Saint-Andre & Hodges Standards Track [Page 57]

/data/webs/external/dokuwiki/data/pages/rfc/rfc6125.txt · Last modified: 2011/03/30 22:04 (external edit)