path: root/doc/rfc/rfc6125.txt

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.




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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



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   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.



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   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.






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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.







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   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]



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   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.




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      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]).



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      *  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.






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   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]




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      *  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,




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      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.






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   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.




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   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:




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   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



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   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



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   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.



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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





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   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.



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   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.




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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):




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   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



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   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.





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   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



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   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




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      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



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   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).



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   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



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      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.





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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



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   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.



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   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



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   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.






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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.



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   [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.




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   [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.




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   [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.




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   [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.






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   [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.





















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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.

   ######



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   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.

   ######






























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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



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   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.





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   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:



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   ######

   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.

   ######









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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:






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   ######

   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.

   ######











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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
































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