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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 6125

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                     Cisco
Intended status: BCP                                           J. Hodges
Expires: December 13, 2010                                        PayPal
                                                           June 11, 2010


   Representation and Verification of Domain-Based Application Server
      Identity in Certificates Used with Transport Layer Security
                draft-saintandre-tls-server-id-check-06

Abstract

   Many application technologies enable a secure connection between two
   entities using certificates in the context of Transport Layer
   Security (TLS).  This document specifies best current practices for
   representing and verifying the identity of application servers in
   such interactions.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 13, 2010.

Copyright Notice

   Copyright (c) 2010 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
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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       1.2.1.  In Scope . . . . . . . . . . . . . . . . . . . . . . .  5
       1.2.2.  Out of Scope . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7
     1.4.  Contributors . . . . . . . . . . . . . . . . . . . . . . .  9
     1.5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . .  9
     1.6.  Discussion Venue . . . . . . . . . . . . . . . . . . . . .  9
   2.  Representation of Server Identity  . . . . . . . . . . . . . .  9
     2.1.  Subject Naming in PKIX Certificates  . . . . . . . . . . .  9
     2.2.  PKIX Certificate Name Rules  . . . . . . . . . . . . . . . 10
   3.  Verification of Server Identity  . . . . . . . . . . . . . . . 12
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.2.  Constructing an Ordered List of Reference Identifiers  . . 12
     3.3.  Seeking a Match  . . . . . . . . . . . . . . . . . . . . . 14
     3.4.  Verifying a Domain Name  . . . . . . . . . . . . . . . . . 15
       3.4.1.  Checking of Traditional Domain Names . . . . . . . . . 15
       3.4.2.  Checking of Internationalized Domain Names . . . . . . 16
       3.4.3.  Checking of Wildcard Labels  . . . . . . . . . . . . . 16
       3.4.4.  Checking of DNS Domain Names in Common Names . . . . . 17
     3.5.  Verifying an Application Type  . . . . . . . . . . . . . . 17
       3.5.1.  SRV-ID . . . . . . . . . . . . . . . . . . . . . . . . 17
       3.5.2.  URI-ID . . . . . . . . . . . . . . . . . . . . . . . . 18
     3.6.  Outcome  . . . . . . . . . . . . . . . . . . . . . . . . . 18
       3.6.1.  Case #1: Match Found . . . . . . . . . . . . . . . . . 18
       3.6.2.  Case #2: No Match Found, Cached Certificate  . . . . . 18
       3.6.3.  Case #3: No Match Found, Uncached Certificate  . . . . 18
         3.6.3.1.  Interactive User Agent . . . . . . . . . . . . . . 18
         3.6.3.2.  Automated Client . . . . . . . . . . . . . . . . . 19
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     4.1.  Service Delegation . . . . . . . . . . . . . . . . . . . . 19
     4.2.  Wildcard Certificates  . . . . . . . . . . . . . . . . . . 20
     4.3.  Internationalized Domain Names . . . . . . . . . . . . . . 20
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Prior Art . . . . . . . . . . . . . . . . . . . . . . 25
     A.1.  IMAP, POP3, and ACAP (1999)  . . . . . . . . . . . . . . . 25
     A.2.  HTTP (2000)  . . . . . . . . . . . . . . . . . . . . . . . 26
     A.3.  LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 27



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     A.4.  SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 30
     A.5.  XMPP (2004)  . . . . . . . . . . . . . . . . . . . . . . . 31
     A.6.  NNTP (2006)  . . . . . . . . . . . . . . . . . . . . . . . 32
     A.7.  NETCONF (2006/2009)  . . . . . . . . . . . . . . . . . . . 33
     A.8.  Syslog (2009)  . . . . . . . . . . . . . . . . . . . . . . 35
     A.9.  SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 36
     A.10. GIST (2010)  . . . . . . . . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38











































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1.  Introduction

1.1.  Motivation

   The visible face of the Internet consists of services that employ a
   client-server architecture in which an interactive or automated
   client connects to an application server in order to retrieve or
   upload information, communicate with other entities, or access a
   broader network of services.  When a client connects to an
   application server using Transport Layer Security [TLS] (or, less
   commonly, [DTLS]), it references some conception of the server's
   identity while attempting to establish a secure connection (e.g.,
   "the web site at example.com").  Likewise, during TLS negotiation the
   server presents its conception of the server's identity in the form
   of a public-key certificate that was issued by a certification
   authority 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 that it can be sure that the certificate
   can legitimately be used to authenticate the connection.

   Many application technologies adhere to the pattern outlined here,
   including but not limited to the following:

   o  The Internet Message Access Protocol [IMAP] and the Post Office
      Protocol [POP3], for which see also [USINGTLS]

   o  The Hypertext Transfer Protocol [HTTP], for which see also
      [HTTP-TLS]

   o  The Lightweight Directory Access Protocol [LDAP], for which see
      also [LDAP-AUTH] and its predecessor [LDAP-TLS]

   o  The Simple Mail Transfer Protocol [SMTP], for which see also
      [SMTP-AUTH] and [SMTP-TLS]

   o  The Extensible Messaging and Presence Protocol [XMPP], for which
      see also [XMPPBIS]

   o  The Network News Transfer Protocol [NNTP], for which see also
      [NNTP-TLS]

   o  The NETCONF Configuration Protocol [NETCONF], for which see also
      [NETCONF-SSH] and [NETCONF-TLS]




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   o  The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS]

   o  The Session Initiation Protocol [SIP], for which see also
      [SIP-CERTS]

   o  The General Internet Signalling Transport [GIST]


   Application protocols have traditionally specified their own rules
   for representing and verifying server identities.  Unfortunately,
   this divergence of approaches has caused some confusion among
   certification authorities, application developers, and protocol
   designers.

   To codify best current practices regarding the implementation and
   deployment of secure PKIX-based authentication, this document
   specifies recommended procedures for representing and verifying
   server identities in certificates intended for use in applications
   employing TLS.

1.2.  Scope

1.2.1.  In Scope

   This document applies only to server identities associated with
   fully-qualified DNS domain names, only to TLS, 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).

1.2.2.  Out of Scope

   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 gives 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 (e.g., IP
      addresses).

      Some certification authorities issue server certificates based on
      IP addresses, but preliminary evidence indicates that such



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      certificates are a very small percentage of issued certificates
      (e.g., less than 1%).  Furthermore, IP addresses are not
      necessarily reliable identifiers for application servers 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.

   o  Security protocols other than [TLS] or [DTLS].

      Although other secure, lower-layer protocols exist and also at
      times employ PKIX certificates, e.g.  [IPSEC], their use cases can
      differ from those of TLS-based or DTLS-based application
      technologies.  Furtermore, 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], or use self-signed certificates,
      or are deployed on networks 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 syntax of
      OpenPGP keys differs significantly from X.509 certificates, 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 OSCP 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.


   Furthermore, this document also does not address various
   certification authority policies, such as:

   o  What classes and types of certificates to issue and whether to
      apply different policies for them (e.g., allow the wildcard
      character in Class 2 certificates but not in Class 1 or Extended
      Validation certificates).



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   o  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.

   o  Which identifiers to include (e.g., whether to include the SRVName
      and uniformResourceIdentifier extensions).

   o  How to certify or validate the information contained in a
      certificate.


   Finally, this specification is mostly silent about user interface
   issues, which in general are properly the responsibility of client
   software developers and standards development organizations dedicated
   to particular application technologies (see for example [WSC-UI].

1.3.  Terminology

   Because the concept of "identity" is too vague to be actionable in
   application protocols, we define a set of more concrete terms:

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

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

   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 also used for matching
      purposes; the types covered in this specification are:

      *  CN-ID = a Relative Distinguished Name (RDN) in the certificate
         subject that contains one and only one attribute value
         assertion (AVA) whose attribute type is Common Name (CN)

      *  DC-ID = a series of Domain Component (DC) attributes in the
         certificate subject, with one RDN per domain label and one DC
         in each RDN.

      *  DNS-ID = a subjectAltName identifier of type dNSName





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      *  SRV-ID = the SRVName form of otherName from the GeneralName
         structure in SubjectAltName

      *  URI-ID = a subjectAltName identifier of type
         uniformResourceIdentifier


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

   PKIX certificate:  An X.509v3 certificate generated and employed in
      the context of the Internet Public Key Infrastructure using X.509
      [PKIX].

   presented identifier:  An identifier that is presented by a server to
      a client within the server's PKIX certificate when the client
      attempts to establish a secure connection with the server; the
      certificate can include one or more presented identifiers of
      different types.

   reference identifier:  An identifier that is used by the client for
      matching purposes when checking the presented identifiers; the
      client can attempt to match multiple reference identifiers of
      different types.

   service type:  A formal identifier for the application protocol used
      to provide a particular kind of service at a domain; the service
      type typically takes the form of a URI scheme or a DNS SRV
      Service.

   source domain:  The fully-qualified DNS domain name that a client
      expects an application server to present in the certificate.

   target 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) or that a natural person directly controlling
      an interactive client has explicitly configured for connecting to
      the source domain.


   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", "certificate", "credential", "identity",
   "self-signed certificate", "trust", "trust anchor", "trust chain",
   "validate", and "verify".



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   The following capitalized keywords are to be interpreted as described
   in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT";
   "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY",
   "OPTIONAL".

1.4.  Contributors

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

1.5.  Acknowledgements

   The editors and contributors wish to thank the following individuals
   for their feedback and suggestions: Nelson Bolyard, Kaspar Brand,
   Scott Cantor, Dave Crocker, Cyrus Daboo, Charles Gardiner, Philip
   Guenther, Bruno Harbulot, David Harrington, Paul Hoffman, Harry Hotz,
   Geoff Keating, Scott Lawrence, Matt McCutchen, Alexey Melnikov, Eddy
   Nigg, Ludwig Nussel, Joe Orton, Tom Petch, Yngven Pettersen, Tim
   Polk, Eric Rescorla, Pete Resnick, Martin Rex, Joe Salowey, Rob
   Stradling, Peter Sylvester, Dan Wing, Dan Winship, and Kurt Zeilenga.

1.6.  Discussion Venue

   [[ RFC Editor: please remove this section. ]]

   The editors are actively seeking input from certification
   authorities, application developers, and protocol designers regarding
   the recommendations in this document.  Please send feedback to the
   editors directly or post to the <certid@ietf.org> mailing list, for
   which archives and subscription information are available at
   <https://www.ietf.org/mailman/listinfo/certid>.


2.  Representation of Server Identity

   This section enumerates the rules for representing application server
   identity in PKIX certificates.  First we provide a brief tutorial
   about subject naming, then we provide the rules.

2.1.  Subject Naming in PKIX Certificates

   The application server is the subject of a server certificate.  As
   [PKIX] says, "[the] subject field identifies the entity associated
   with the public key stored in the subject public key field."

   The application server is identified by a name or names carried in
   the subject field and/or the subjectAltName extension of the
   certificate.  See sections 4.1.2.6 and 4.2.1.6 of [PKIX] for details.



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   This section only briefly introduces distinguished names and their
   components, as well as subjectAltNames and the particular
   subjectAltName extension types explicitly mentioned in this
   specification.

   The subject field of a PKIX certificate is defined as an X.501 type
   Name and known as a Distinguished Name (DN) -- see [X.501] and
   [PKIX].  A DN is an ordered sequence of Relative Distinguished Names
   (RDNs), where each RDN is a set (i.e., an unordered group) of type-
   and-value pairs or "attribute value assertions" (AVAs) [LDAP-DN],
   each of which asserts some attribute about the subject of the
   certificate.  In the DER encoding of a DN, the RDNs are always in
   order from most significant to least significant (i.e., the first RDN
   is most significant and the last RDN is least significant); however,
   in the string representation of a DN as used in various protocols and
   data formats, the RDNs might be ordered from most significant to
   least significant (e.g., this is true of LDAP) or from least
   significant to most significant.

   Note that certificates are binary objects -- they are encoded using
   distinguished encoding rules (DER).  Thus, displayable (a.k.a.
   printable) renderings of certificate subject (and issuer) names means
   that the DER-encoded sequences are decoded and converted into a
   "string representation" of a DN before being rendered.  Often such a
   DN string representation is ordered from left-to-right, most specific
   to most general.  See [LDAP-DN] for details.

   Certificate subjects may also have "alternative" names.  These are
   represented within certificates using the SubjectAltName field.  This
   field is a sequence of typed fields, where each type is a distinct
   type of identifier.  For example, the subjectAltName identifier types
   noted in this specification are:

   o  dNSName -- a (fully-qualified) DNS domain name [PKIX]
   o  SRVName -- a DNS SRV service name [DNS-SRV] [SRVNAME]
   o  uniformResourceIdentifier -- a Uniform Resource Identifier [URI]
      [PKIX]

2.2.  PKIX Certificate Name Rules

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

   1.  The certificate MUST include a "DNS-ID" (i.e., a subjectAltName
       identifier of type dNSName).




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   2.  If the service using the certificate deploys a technology in
       which a server is discovered by means of DNS SRV records
       [DNS-SRV] (e.g., this is true of [XMPP]), then the certificate
       SHOULD include an "SRV-ID" (i.e., an instance of the SRVName form
       of otherName from the GeneralName structure in the subjectAltName
       as specified in [SRVNAME]).

   3.  If the service using the certificate deploys a technology in
       which a server is typically associated with a URI (e.g., this is
       true of [SIP]), then the certificate SHOULD include an URI-ID
       (i.e., a subjectAltName identifier of type
       uniformResourceIdentifier); the scheme SHALL be that of the
       protocol associated with the application type and the authority
       component SHALL be the fully-qualified DNS domain name of the
       service.

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

   5.  The certificate SHOULD NOT represent the server's fully-qualified
       DNS domain name in a Relative Distinguished Name (RDN) of type
       Common Name (CN) (see [LDAP-SCHEMA]), even though we recognize
       that many deployed clients still check for this legacy identifier
       configuration within certificate subjectName.  However, if this
       legacy identifer configuration is employed, then the server's
       fully-qualified DNS domain name MUST be placed in the last (most
       specific) RDN within the RDN sequence making up the certificate's
       subjectName, as the order of RDNs is determined by the DER-
       encoded Name within the server's PKIX certificate.  Furthermore,
       the certificate's subject Distinguished Name SHOULD NOT contain
       more than one Common Name attribute, and MUST NOT contain RDNs
       which consist of multiple Common Name attributes.

   6.  The certificate SHOULD NOT represent the server's fully-qualified
       DNS domain name by means of a DC-ID, i.e., a series of Domain
       Component (DC) attributes in the certificate subject, with one
       RDN per domain label and one DC in each RDN.  Although (for
       example) <dc=www,dc=example,dc=com> could be used to represent
       the DNS domain name "www.example.com", given the fact that the
       DNS-ID can be used instead, the DC-ID is NOT RECOMMENDED.

   7.  The fully-qualified DNS domain name portion of the DN-ID or CN-ID
       MAY contain one instance of the wildcard character '*', but only
       as the left-most label of the domain name component of the



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       identifier (following the definition of "label" from [DNS]).
       Specifications that profile the rules defined in this document
       MUST specify whether the wildcard character is or is not allowed
       in certificates issued under that profile; by default wildcard
       certificates SHOULD NOT be allowed.



3.  Verification of Server Identity

3.1.  Overview

   At a high level, the client verifies the server's identity by
   performing the following actions:

   1.  Before connecting to the server, the client constructs an ordered
       list of reference identifiers against which to check the
       presented identifiers.

   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 (a) MUST match the source domain (or, in
       some cases, target domain) of the identifiers and (b) MAY also
       match the service type of the identifiers.


      Implementation Note: 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 server
      identity presented in a certificate, and therefore methods for
      doing so (e.g., consulting local policy information) are out of
      scope for this document.

3.2.  Constructing an Ordered List of Reference Identifiers

   Before connecting to the server, the client MUST construct an ordered
   list of acceptable reference identifiers.

   The inputs here might be a URI that a user has typed into an
   interface (e.g., an HTTP URL for a web site), configured account
   information (e.g., the username of an IMAP or POP3 account for
   retrieving email), or some other combination of information that can



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   yield a source domain and an application type.

   The client might need to derive the source domain and application
   type from the input(s) it has received.  The derived data MUST
   include only information that can be securely parsed out of the
   inputs (e.g., extracting the fully-qualified DNS domain name from the
   authority component of a URI or extracting the application type from
   the scheme of a URI) or information for which the derivation is
   performed in a secure manner (e.g., using [DNSSEC]).

   In some cases the inputs might include more than one fully-qualified
   DNS domain name, because a user might have explicitly configured the
   client to associate a target domain with the source domain.  Such
   delegation can occur by means of user-approved DNS SRV records (e.g.,
   _xmpp-server._tcp.im.example.com might yield a hostname of
   hosting.example.net) or a user-configured lookup table for host-to-
   address or address-to-host translations (e.g., the Unix "hosts"
   file).  See under Section 4 for further discussion of service
   delegation.

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

   o  The list MUST 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
      target domain).

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

   o  If a server for the application type is typically associated with
      a URI, then the list SHOULD include a URI-ID

   o  The list SHOULD NOT include a CN-ID; however, the CN-ID (if
      included) MUST be constructed only from the source domain and
      never from a target domain.

   o  The list SHOULD NOT include a DC-ID; however, the DC-ID (if
      included) MUST be constructed only from the source domain and
      never from a target domain.







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      Implementation Note: The client does not need to actually
      construct the foregoing identifiers in the formats found in a
      certificate (e.g., as ASN.1 object identifiers), only the
      functional equivalent of such identifiers for matching purposes.

      Security Note: A client MUST NOT construct a reference identifier
      corresponding to Relative Distinguished Names (RDNs) other than
      the Common Name or Domain Components (DCs) and MUST NOT check for
      such RDNs in the presented identifiers.

   The client then orders the list in accordance with the following
   rules:

   o  Reference identifiers that include the source domain MUST be
      preferred over reference identifiers that include a target domain
      (if any).

   o  Reference identifiers that include both a fully-qualified DNS
      domain name and an application type MUST be preferred over
      reference identifiers that include only a fully-qualified DNS
      domain name.  Therefore an SRV-ID or URI-ID is to be preferred
      over a DNS-ID.

   o  Notwithstanding any of the foregoing rules, reference identifiers
      of type CN-ID (if included) MUST always be the lowest-priority
      reference identifiers in the list.


   To illustrate the ordering rules, consider the case where the inputs
   are a source domain of "im.example.com" and an application type of
   "XMPP" (for which application servers are discovered via DNS SRV
   records) and the user of the client has also explicitly configured a
   target domain of "hosting.example.net".  In this case, the ordered
   list would be:

   1.  SRV-ID of "_xmpp.im.example.com"
   2.  DNS-ID of "im.example.com"
   3.  SRV-ID of "_xmpp.hosting.example.net"
   4.  DNS-ID of "hosting.example.net"
   5.  CN-ID of "im.example.com"

3.3.  Seeking a Match

   Once the client has constructed its order 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.  It does so by seeking a match in preference order



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   and aborting the search if any presented identifier matches one of
   its reference identifiers.  The search fails if the client exhausts
   its list of reference identifiers without finding a match.  Detailed
   comparison rules for finding a match are provided in the following
   sections.

      Security Note: A client MUST NOT seek a match for a reference
      identifier of CN-ID if the presented identifiers include an
      SRV-ID, URI-ID, DNS-ID, or any application-specific subjectAltName
      extensions supported by the client.  Furthermore, a client SHALL
      check only against the last Common Name RDN in the sequence of
      RDNs making up the Distinguished Name within the certificate's
      subjectName (where the term "last" refers to the DER order, which
      is often not the string order presented to a user; the order that
      is applied here MUST be the DER order).

3.4.  Verifying a Domain Name

   This document assumes that each reference identifier contains a
   fully-qualified DNS domain name that is a "traditional domain name"
   or an "internationalized domain name".  The client MUST match the
   source domain of a reference identifier according to the following
   rules.

3.4.1.  Checking of Traditional Domain Names

   The term "traditional domain name" is a contraction of this more
   formal and accurate name: "traditional US-ASCII letter-digit-hyphen
   DNS domain name".  Traditional domain names are defined in
   [DNS-CONCEPTS] and [DNS] in conjunction with [HOSTS] as further
   explained in [IDNA2003].  In essence, a traditional domain name
   consists of a set of one or more labels (e.g., "www", "example", and
   "com"), with the labels usually shown separated by dots (e.g.,
   "www.example.com").  Labels nominally consist of only [US-ASCII]
   uppercase and lowercase letters, digits, and hyphen.  There are
   additional qualifications (please refer to the above-referenced
   specifications for details) but they are not germane to this
   specification.

   If the source domain 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
   components 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.





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3.4.2.  Checking of Internationalized Domain Names

   [[ Editorial Note: This section needs to be updated to reflect
   [IDNA2008]. ]]

   The term "internationalized domain name" refers to a DNS domain name
   that conforms to the overall form of a domain name (dot-separated
   labels) but that can include Unicode code points outside the
   traditional US-ASCII range, as explained by [IDNA2003] and
   [IDNA2008].

   If the source domain of a reference identifier is an
   internationalized domain name, then an implementation MUST convert
   the domain name to the ASCII Compatible Encoding (ACE) format as
   specified in Section 4 of [IDNA2003] before comparison; specifically,
   the conversion operation specified in Section 4 of [IDNA2003] MUST be
   performed 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.
   o  In step 5, change all label separators to U+002E (full stop).

   After performing the "to-ASCII" conversion with regard to an
   internationalized domain name, the DNS labels and names MUST be
   compared for equality according to the rules specified in Section 3
   of [IDNA2003], i.e. once all label separators are replaced with
   U+002E (dot) they are compared in a case-insensitive manner.

3.4.3.  Checking of Wildcard Labels

   Unless forbidden by a specification that profiles the best practices
   defined herein, a client employing this specification's rules MAY
   match the reference identifier against a presented identifier
   containing one instance of the wildcard character '*', but only as
   the left-most label of the domain name, e.g. *.example.com (following
   the definition of "label" from [DNS]).

   If such a wildcard identifier is presented, the wildcard MUST be used
   to match only the one position-wise corresponding label (thus
   *.example.com matches foo.example.com but not bar.foo.example.com or
   example.com).  The client MUST fail to match a presented identifier
   in which the wildcard character is contained within a label fragment
   (e.g., baz*.example.net is not allowed and MUST NOT be taken to match
   baz1.example.net and baz2.example.net), or in which the wildcard
   character does not comprise the left-most label in the presented
   identifier (e.g., neither bar.*.example.net nor bar.f*o.example.net
   are allowed).



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3.4.4.  Checking of DNS Domain Names in Common Names

   As noted, a client MUST NOT seek a match for a reference identifier
   of CN-ID if the presented identifiers include an SRV-ID, URI-ID,
   DNS-ID, or any application-specific subjectAltName extensions
   supported by the client.

   Therefore, if and only if the identity set does not include
   subjectAltName extensions of type dNSName, SRVName, or
   uniformResourceIdentifier (or any application-specific subjectAltName
   extensions supported by the client), the client MAY as a fallback
   check for a fully-qualified DNS domain name in the last Common Name
   RDN in the sequence of RDNs making up the Distinguished Name within
   the certificate's subjectName (where the term "last" refers to the
   DER order, which is often not the string order presented to a user;
   the order that is applied here MUST be the DER order).

   In existing certificates, the CN is often used for representing a
   fully-qualified DNS domain name; for example, consider the following
   subjectName, where the last RDN is a Common Name that is intended to
   represent a fully-qualified DNS domain name:

   cn=www.example.com,c=GB,ou=Web Services

   Here the Common Name is "www.example.com" (a string whose form
   matches that of a fully-qualified DNS domain name) and the client
   could choose to compare a reference identifier of type CN-ID against
   that string.  When doing so, the client MUST follow the comparison
   rules for the source domain of an identifier of type DNS-ID, SRV-ID,
   or URI-ID, as described under Section 3.4.

3.5.  Verifying an Application Type

   As specified under the ordering rules for reference identifiers, a
   client SHOULD check not only the domain name but also the application
   type of the service to which it connects.  This is a best practice
   because typically a client is not designed to connect to all kinds of
   services using all possible application protocols, but instead is
   designed to connect to a specific kind of service, such as a web
   site, an email service, or an instant messaging service.

   The application type is verified by means of either an SRV-ID or
   URI-ID.

3.5.1.  SRV-ID

   The service name portion of an SRV-ID (e.g., "xmpp") MUST be matched
   in a case-insensitive manner, in accordance with [DNS-SRV].  Note



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   that the "_" character is prepended to the service identifier in DNS
   SRV records.

3.5.2.  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 therefore does not need to be included in any
   comparison.

3.6.  Outcome

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

3.6.1.  Case #1: Match Found

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

3.6.2.  Case #2: No Match Found, Cached Certificate

   If the client finds no presented identifier that matches any of the
   reference identifiers but a natural person has permanently accepted
   the certificate during a previous connection attempt or via
   configured preferences, the certificate is cached.  In this case, the
   client MUST verify that the presented certificate matches the cached
   certificate and (if it is an interactive client) MUST notify the user
   if the certificate has changed since the last time a secure
   connection was successfully negotiated.

3.6.3.  Case #3: No Match Found, Uncached Certificate

   If the client finds no presented identifier that matches any of the
   reference identifiers and a human user has not permanently accepted
   the certificate for this application server during a previous
   connection attempt, the client MUST NOT consider the certificate to
   include a validated identity for the application server.

   Instead, the client MUST proceed as follows.

3.6.3.1.  Interactive User Agent

   If the client is an interactive client that is directly controlled by
   a natural person, then it MUST either do one of the following:




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   1.  Automatically terminate the connection with a bad certificate
       error; or

   2.  Actively warn the user that the certificate is untrusted and
       encourage the user to terminate the connection, but give advanced
       users the option to (a) view the entire certificate chain, (b)
       accept the certificate for this application server either
       temporarily (i.e., for this connection attempt only) or
       permanently (i.e., for all future connection attempts) despite
       the identity mismatch, and then (c) continue with the connection
       attempt.

   If a user permanently accepts a certificate for this application
   server despite an identity mismatch (an action referred to in
   [WSC-UI] as "pinning"), the client MUST cache the certificate (or
   some non-forgeable representation such as a hash value) and in future
   connection attempts MUST behave as in "Case #2: No Match Found,
   Cached Certificate" Section 3.6.2.

      Security Note: It is the responsibility of the human user to
      verify the hash value or fingerprint of the certificate with the
      server over a trusted communication layer.

      Informational Note: The guidelines provided here are roughly
      consistent with those provided for web browsers and other HTTP-
      aware interactive clients in [WSC-UI].

3.6.3.2.  Automated Client

   If the client is an automated application that is not directly
   controlled by a natural person, then it SHOULD terminate the
   connection with a bad certificate error and log the error to an
   appropriate audit log.  An automated application MAY provide a
   configuration setting that disables this check, but MUST enable the
   check by default.


4.  Security Considerations

4.1.  Service Delegation

   When the connecting application is an interactive client, the source
   domain name and application type MUST be provided by a human user
   (e.g. when specifying the server portion of the user's account name
   on the server or when explicitly configuring the client to connect to
   a particular host or URI as in [SIP-LOC]) and MUST NOT be derived
   from the user inputs in an automated fashion (e.g., a hostname
   address discovered through DNS resolution of the source domain).



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   This rule is important because only a match between the user inputs
   (in the form of a reference identifier) and a presented identifier
   enables the client to be sure that the certificate can legitimately
   be used to secure the connection.

   However, an interactive client MAY provide a configuration setting
   that enables a human user to explicitly specify a particular hostname
   (called a "target domain") to be checked for connection purposes.

4.2.  Wildcard Certificates

   Allowing the wildcard character in certificates has led to homograph
   attacks involving non-ASCII characters that look similar to
   characters commonly included in HTTP URLs, such as "/" and "?"; for
   discussion, see for example [Defeating-SSL].

4.3.  Internationalized Domain Names

   In addition to the wildcard certificate attacks previously mentioned,
   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].


5.  IANA Considerations

   This document has no actions for the IANA.


6.  References

6.1.  Normative References

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [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.

   [IDNA2003]
              Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.



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   [IDNA2008]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol",
              draft-ietf-idnabis-protocol-18 (work in progress),
              January 2010.

   [LDAP-DN]  Zeilenga, K., "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.

   [TERMS]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

6.2.  Informative References

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

   [DNS-CASE]
              Eastlake, D., "Domain Name System (DNS) Case Insensitivity
              Clarification", RFC 4343, January 2006.

   [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.

   [GIST]     Schulzrinne, H. and M. Stiemerling, "GIST: General
              Internet Signalling Transport", draft-ietf-nsis-ntlp-20
              (work in progress), June 2009.



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   [HOSTS]    Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [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.

   [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [IDNA-DEFS]
              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              draft-ietf-idnabis-defs-13 (work in progress),
              January 2010.

   [IP]       Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [IPv6]     Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [IPSEC]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [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., "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.

   [NETCONF]  Enns, R., "NETCONF Configuration Protocol", RFC 4741,
              December 2006.



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   [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.

   [OPENPGP]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.

   [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.

   [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.

   [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 B. Laboratories, "Domain
              Certificates in the Session Initiation Protocol (SIP)",



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              draft-ietf-sip-domain-certs-06 (work in progress),
              March 2010.

   [SIP-LOC]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [SMTP-AUTH]
              Siemborski, R. and A. Melnikov, "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.

   [SYSLOG]   Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.

   [SYSLOG-TLS]
              Miao, F., Ma, Y., and J. Salowey, "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.

   [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.501]    International Telecommunications Union, "Information
              Technology - Open Systems Interconnection - The Directory:
              Models", ITU-T Recommendation X.501, ISO Standard 9594-2,
              February 2001.

   [XMPP]     Saint-Andre, P., Ed., "Extensible Messaging and Presence



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              Protocol (XMPP): Core", RFC 3920, October 2004.

   [XMPPBIS]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-07 (work
              in progress), April 2010.


Appendix A.  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 server 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 "[...]").

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

   In 1999, [USINGTLS] specified the following text regarding server
   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.



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   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 and indicate the server's
   identity is suspect.

   ######

A.2.  HTTP (2000)

   In 2000, [HTTP-TLS] specified the following text regarding server
   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.



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   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.

   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.

   ######

A.3.  LDAP (2000/2006)

   In 2000, [LDAP-TLS] specified the following text regarding server
   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.





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   o  Matching is case-insensitive.
   o  The "*" wildcard character is allowed.  If present, it applies
      only to the left-most name component.

   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 server
   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



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   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
   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 subjectName 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:





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   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
   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.

   ######

A.4.  SMTP (2002/2007)

   In 2002, [SMTP-TLS] specified the following text regarding server
   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 server
   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.

   ######

A.5.  XMPP (2004)

   In 2004, [XMPP] specified the following text regarding server
   identity verification in XMPP:




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   ######

   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 chain of certificates 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 certificate chain.  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.

   ######

   At the time of this writing, [XMPPBIS] refers to this document for
   rules regarding server identity verification in XMPP.

A.6.  NNTP (2006)

   In 2006, [NNTP-TLS] specified the following text regarding server
   identity verification in NNTP:

   ######




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   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.

   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).

   ######

A.7.  NETCONF (2006/2009)

   In 2006, [NETCONF-SSH] specified the following text regarding server
   identity verification in NETCONF:




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   ######

   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 server
   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.

   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.



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   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.

   ######

A.8.  Syslog (2009)

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

   ######

   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



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      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.

   ######

A.9.  SIP (2010)

   At the time of this writing, [SIP-CERTS] specified text regarding
   server identity verification in the Session Initiation Protocol
   (SIP).  However, that specification has not yet been approved by the
   IESG and text cannot be considered final.

   The relevant text follows.

   ######

   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".





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         Implemenations 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.

   ######

A.10.  GIST (2010)

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

   ######

   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



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   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.

   ######


Authors' Addresses

   Peter Saint-Andre
   Cisco

   Email: psaintan@cisco.com


   Jeff Hodges
   PayPal

   Email: Jeff.Hodges@PayPal.com
























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