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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, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                          J. Hodges, Ed.
Expires: September 9, 2010                                        PayPal
                                                           March 8, 2010


   Representation and Verification of Application Server Identity in
         Certificates Used with Transport Layer Security (TLS)
                draft-saintandre-tls-server-id-check-03

Abstract

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

Status of this Memo

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

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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



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   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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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.4.  Contributors . . . . . . . . . . . . . . . . . . . . . . .  4
     1.5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . .  4
   2.  Architectural Assumptions  . . . . . . . . . . . . . . . . . .  5
   3.  Representation of Server Identity  . . . . . . . . . . . . . .  6
   4.  Verification of Server Identity  . . . . . . . . . . . . . . .  7
     4.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Comparison Rules . . . . . . . . . . . . . . . . . . . . .  8
       4.2.1.  Traditional Domain Names . . . . . . . . . . . . . . .  9
       4.2.2.  Internationalized Domain Names . . . . . . . . . . . .  9
       4.2.3.  Wildcards  . . . . . . . . . . . . . . . . . . . . . .  9
       4.2.4.  Common Names . . . . . . . . . . . . . . . . . . . . . 10
       4.2.5.  Relative Distinguished Names . . . . . . . . . . . . . 10
     4.3.  Outcome  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Prior Art . . . . . . . . . . . . . . . . . . . . . . 15
     A.1.  IMAP, POP3, and ACAP (1999)  . . . . . . . . . . . . . . . 15
     A.2.  HTTP (2000)  . . . . . . . . . . . . . . . . . . . . . . . 16
     A.3.  LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 17
     A.4.  SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 20
     A.5.  XMPP (2004)  . . . . . . . . . . . . . . . . . . . . . . . 21
     A.6.  NNTP (2006)  . . . . . . . . . . . . . . . . . . . . . . . 22
     A.7.  NETCONF (2006/2009)  . . . . . . . . . . . . . . . . . . . 23
     A.8.  Syslog (2009)  . . . . . . . . . . . . . . . . . . . . . . 25
     A.9.  SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27






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1.  Introduction

1.1.  Motivation

   When a client wishes to establish a secure communication channel with
   an application server (e.g., "the HTTP server for example.com"), at a
   minimum the client needs to verify the identity of the server in
   order to prevent certain passive and active attacks against the
   connection.  To establish secure connections, many application
   technologies use Transport Layer Security [TLS] with certificates
   issued by certification authorities that are part of the Internet
   X.509 Public Key Infrastructure (PKI) as described in [X509]; such
   application technologies include but are not limited to the
   following:

   o  The Hypertext Transfer Protocol [HTTP], for which see also
      [HTTP-TLS]
   o  The Internet Message Access Protocol [IMAP] and the Post Office
      Protocol [POP3], for which see also [USINGTLS]
   o  The Lightweight Directory Access Protocol [LDAP], for which see
      also [LDAP-AUTH] and its predecessor [LDAP-TLS]
   o  The NETCONF Configuration Protocol [NETCONF], for which see also
      [NETCONF-SSH] and [NETCONF-TLS]
   o  The Network News Transfer Protocol [NNTP], for which see also
      [NNTP-TLS]
   o  The Session Initiation Protocol [SIP], for which see also
      [SIP-CERTS]
   o  The Simple Mail Transfer Protocol [SMTP], for which see also
      [SMTP-AUTH] and [SMTP-TLS]
   o  The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS]
   o  The Extensible Messaging and Presence Protocol [XMPP], for which
      see also [XMPPBIS]

   Different 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, protocol designers, and
   application developers.

   Futhermore, currently the vast majority of deployed application
   servers use domain names in their certificates (typically via a
   subjectAltName extension of dNSName or a subjectName component of
   Common Name).  Ideally, service operators would use application
   service identities in their certificates (such as an SRVName
   [SRVNAME], a URI, or an application-specific name form), since this
   would reduce the possibility of attacks against unrelated services at
   domain names that provide many different application services.




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   To codify secure authentication practices, this document specifies
   recommended procedures for representing and verifying server
   identities in certificates intended for use in applications that
   employ TLS.

1.2.  Scope

   This document applies only to server identities associated with
   domain names, only to TLS, and only to the PKI.  Similar
   considerations might apply to client identities (e.g., for mutual
   authentication), to identities other than domain names (e.g., IP
   addresses), to security protocols other than TLS (e.g., [IPSEC] and
   [DTLS]), and to keys or certificates created outside the context of
   the PKI (e.g., where a dependency on Certificate Revocation Lists or
   the Online Certificate Status Protocol might introduce unacceptable
   latency or denial of service attacks).  Although those scenarios
   might be able to re-use some aspects of the representation and
   verification rules provided here, they are outside the scope of this
   document and need to be addressed by separate specifications.

1.3.  Terminology

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

   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: Dave Crocker, Cyrus Daboo, Philip
   Guenther, David Harrington, Paul Hoffman, Scott Lawrence, Alexey
   Melnikov, Tom Petch, Pete Resnick, Joe Salowey, and Dan Wing.







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2.  Architectural Assumptions

   Internet applications often use a client-server architecture in which
   a client connects to a server in order to retrieve or upload data,
   access a broader network of services, or communicate with other
   entities on the network.  From the security perspective, a client
   might be a user agent controlled by a human, an automated process
   such as a bot, or a peer server.  We assume that an application
   server hosts information, enables a provisioned account to perform
   authorized services, or provides network access on behalf of a
   particular organization or service that is canonically identified by
   a domain name (not, e.g., an IP address).  The specific application
   provided (e.g., a web site or an instant messaging system) might
   further restrict the identity type that is represented or verified in
   a TLS interaction; such an identity type is often informal (e.g.,
   most people expect a service that is provided on port 443 to be a web
   server) but can be specified more formally through the use of DNS SRV
   records [DNS-SRV], a Uniform Resource Identifier [URI] represented by
   a subjectAltName of uniformResourceIdentifier, the SRVName
   certificate extension defined in [SRVNAME], or other certificate
   extensions that have been defined for particular identity types
   (e.g., the XmppAddr extension for use in [XMPP]).  The certificate
   presented by an application server might contain one identity or it
   might contain multiple identities of different types (e.g., one
   identity might be a simple dNSName and another might be an SRVName
   that restricts the certificate to use in the context of the specified
   service).

   When a client connects to an application server, it has some
   conception of either the server's identity (e.g., "an XMPP server for
   example.com") or of the server's location (e.g., "the SMTP server at
   mail.example.com").  The client expects at least one of the server's
   presented identities to match this reference identity.  It is
   important to note that the reference identity is provided by a human
   user or configured into the client application (e.g., in the domain
   part of an "Application Unique String" as described in [SIP-LOC]),
   and is not an identity that is derived from the reference identity in
   an automated fashion (e.g., an IP address discovered through DNS
   resolution of the reference identity).  Only a match between the
   reference identity and a presented identity enables the client to be
   sure that the certificate can legitimately be used to secure the
   connection.  However, a user-oriented client MAY provide a
   configuration setting that enables a human user to explicitly specify
   a particular hostname to be checked for connection purposes, thus
   explicitly overriding matching rules.

   To summarize, we define the following terms to assist in
   understanding the process of representing and verifying server



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   identity:

   identity set:  The set of identities that are presented by a server
      to a client (in the form of the server's X.509 certificate) when
      the client attempts to establish a secure connection with the
      server.
   identity type:  The "natural kind" of identity to which a presented
      identity or reference identity belongs.  For example, the
      reference identity might be a domain name, an IPv4 or IPv6
      address, or a particular service in the sense of [DNS-SRV] or
      [SRVNAME].  This specification does not provide a complete
      taxonomy of identity types but assumes that an application server
      is identified by a domain name that could be represented in the
      form of several identity types (e.g., a dNSName and an SRVName).
   presented identity:  A single member of the identity set.
   reference identity:  The client's conception of the server's identity
      before it attempts to establish a secure connection with the
      server, i.e. the identity that the client expects the server to
      present and to which the client makes reference when attempting to
      verify the server's identity.  The reference identity MUST be
      provided by the human user controlling the client (if any), 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.  The reference identity MAY be reflected in the
      TLS "server_name" extension as specified in [TLS].


3.  Representation of Server Identity

   The following rules apply to the representation of application server
   identities in certificates issued by certification authorities, i.e.,
   certificates that are to used for securing connections to application
   servers such as web sites, email services, and instant messaging
   services.

   1.  The certification authority MUST issue the certificate based on
       the domain name at which the server will provide the relevant
       service (not an IP address or host name for a specific machine).
   2.  An identity MAY contain one instance of the wildcard character
       '*' but only as the left-most label.  An application protocol
       that re-uses the rules specified in this document MUST specify
       whether the wildcard character is or is not allowed in
       certificates issued for use with that protocol.
   3.  If the service at which the certificate will be used deploys a
       technology that is discovered by means of DNS SRV records
       [DNS-SRV], then the certificate SHOULD include an identity of
       type SRVName [SRVNAME].




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   4.  If the service is associated with a particular URI, then the
       certificate MAY include an identity of type
       uniformResourceIdentifier (i.e., a subjectAltName extension that
       includes a uniformResourceIdentifier name form whose authority
       component is the domain name of the service).
   5.  Although the certificate MAY include other application-specific
       identities for types that were defined before specification of
       the SRVName extension (e.g., XmppAddr for [XMPP]) or for which
       service names do not exist, the SRVName extension is preferred.
   6.  If the certificate does not include an SRVName,
       uniformResourceIdentifier, or other application-specific identity
       type, then the certificate MUST include an identity of dNSName.
   7.  The certificate SHOULD NOT represent the server's domain name in
       an identity of Common Name (CN) (see [LDAP-SCHEMA]) in the leaf
       Relative Distinguished Name (RDN) of the subjectName, even though
       it is recognized that many deployed clients still check this
       legacy identity.  For example, here the CN is the leaf RDN, which
       is acceptable:

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

   8.  The certificate MUST NOT represent the server's domain name in an
       identity of Common Name (CN) where the CN is in something other
       than the "leaf" (left-most) position within the Relative
       Distinguished Names (RDNs) of the subjectName.  For example, here
       the CN is not the leaf RDN, which is unacceptable:

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

   9.  The certificate MUST NOT represent the server's domain name by
       means of a series of Domain Component (DC) attributes (because
       the order of Domain Components is not guaranteed, certain attacks
       are possible if DC attributes are included).


4.  Verification of Server Identity

4.1.  Overview

   At a high level, the client verifies the server's identity in
   accordance with the following rules:

   1.  Before connecting to the server, the client determines the
       possible identity type(s) of the reference identity.
   2.  During the process of attempting to establish a secure
       connection, the server MUST present its identity set to the
       client in the form of an X.509 certificate [X509].




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   3.  Upon being presented with the server's identity set, the client
       MUST check the reference identity against the presented
       identities for the purpose of finding a match.  To do so, the
       client iterates through all of the subjectAltName extensions it
       recognizes in the server's certificate and compares the value of
       each extension to the reference identity until it has either
       produced a match or exhausted the identities in the identity set
       (comparison rules for matching particular identity types are
       provided under Section 4.2, including fallbacks to several
       subjectName fields).  Even if the application technology does not
       define an application-specific preference order for checking of
       identities, the client SHOULD check them in order from most
       specific (e.g., SRVName) to least specific (e.g., dNSName).
   4.  Before attempting to find a match in relation to a particular
       presented identity, the client MAY map the reference identity to
       a different identity type.  Such a mapping MAY be performed for
       any available subjectAltName type to which the reference identity
       can be mapped; however, the reference identity SHOULD be mapped
       only to types for which the mapping is inherently secure (e.g.,
       extracting the domain name from a URI to match against a
       subjectAltName of type dNSName or SRVName).
   5.  The client MUST NOT attempt to match against an an identity of
       type SRVName [SRVNAME] unless the service to which the client has
       connected deploys a technology that is discovered by means of DNS
       SRV records [DNS-SRV].
   6.  If the identity set has more than one member, a match with any of
       the presented identities is acceptable.

      Note: A hostname that is resolved via the Domain Name System MUST
      NOT be used as the reference identity unless an administrator or
      human user has explicitly configured an application to associate a
      particular hostname (and potentially port) with the hostname to
      which the application connects (e.g., to "hardcode" an association
      between an original hostname of example.net and a configured
      hostname of connector.example.com:443).

   In addition to the identity check described in this section, a client
   might complete further verification to ensure that the server is
   authorized to provide the service it is requested to provide.
   Methods for doing so (which might include consulting local policy
   information) are out of scope for this document.

4.2.  Comparison Rules

   This document assumes that the reference identity is a domain name as
   defined by [RFC1034] and [RFC1035] for "traditional" domain names or
   by [IDNA2003] or [IDNA2008] for internationalized domain names.  The
   client MUST match the reference identity against subjectAltName



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   extensions of type dNSName and SRVName [SRVNAME] according to the
   following rules.

4.2.1.  Traditional Domain Names

   If the reference identity is a "traditional" domain name, then
   matching of reference identity against the presented identity is
   performed by comparing the set of domain components using a case-
   insensitive ASCII comparison (as clarified by [DNS-CASE]).

4.2.2.  Internationalized Domain Names

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

   If the reference identity is an internationalized domain name, then
   an implementation MUST convert the reference identity to the ASCII
   Compatible Encoding (ACE) format as specified in Section 4 of
   [IDNA2003] before comparison with subjectAltName values of type
   dNSName; 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.

4.2.3.  Wildcards

   Unless otherwise specified by an application protocol that re-uses
   the rules specified in this document, the client MAY match against an
   identity that contains one instance of the wildcard character '*' as
   the left-most label of the domain name.  If it does so, the client
   MUST match the reference identity against the entire left-most label
   only (thus *.example.com matches foo.example.com but not
   bar.foo.example.com or example.com itself).  The client MUST ignore a
   presented identity 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).







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4.2.4.  Common Names

   If and only if the identity set does not include subjectAltName
   extensions of type dNSName, SRVName, uniformResourceIdentifier (or
   other application-specific subjectAltName extensions), the client MAY
   as a fallback check the value of the Common Name (CN) if presented as
   the leaf (left-most) Relative Distinguished Name (RND) in the
   subjectName component of the server's X.509 certificate.  In existing
   certificates, the CN is often used for representing a domain name;
   for example, consider the following subjectName with a leaf RDN of
   Common Name:

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

   Here the Common Name is "www.example.com" and the client could choose
   to compare the reference identity against that CN.  When doing so,
   the client MUST follow the comparison rules described above for
   subjectAltName extensions of type dNSName and SRVName.

   A client MUST NOT check the Common Name if it is other than the leaf
   (left-most) Relative Distinguished Name (RDN) in the subjectName.

4.2.5.  Relative Distinguished Names

   A client MUST NOT check Relative Distinguished Names (RDNs) other
   than the Common Name; in particular, this means that a series of
   Domain Component (DC) attributes MUST NOT be checked (because the
   order of Domain Components is not guaranteed, certain attacks are
   possible if DC attributes are checked).

4.3.  Outcome

   The outcome of the checking procedure is one of the following:

   Case #1:  The client finds at least one presented identity that
      matches the reference identity; the entity MUST use this as the
      validated identity of the server.
   Case #2:  The client finds no subjectAltName or subjectName that
      matches the reference identity but a human user has permanently
      accepted the certificate during a previous connection attempt; the
      client MUST verify that the cached certificate was presented and
      MUST notify the user if the certificate has changed since the last
      time that a secure connection was successfully negotiated.
   Case #3:  The client finds no subjectAltName or subjectName that
      matches the reference identity and a human user has not
      permanently accepted the certificate during a previous connection
      attempt.  The client MUST NOT use the presented identity (if any).
      Instead, if the client is a user-oriented application, then it



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      MUST either (1) automatically terminate the connection with a bad
      certificate error or (2) show the certificate (including the
      entire certificate chain) to the user and give the user the choice
      of terminating the connecting or accepting the certificate
      temporarily (i.e., for this connection attempt only) or
      permanently (i.e., for all future connection attempts) and then
      continuing with the connection; if a user permanently accepts a
      certificate in this way, the client MUST cache the certificate (or
      some non-forgeable representation such as a hash value) and in
      future connection MUST attempt behave as in Case #2.  (It is the
      resposibility of the human user to verify the hash value or
      fingerprint of the certificate with the server over a trusted
      communication layer.)  If the client is an automated application,
      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.


5.  Security Considerations

   This entire document discusses security.


6.  IANA Considerations

   This document has no actions for the IANA.


7.  References

7.1.  Normative References

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

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

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and



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              specification", STD 13, RFC 1035, November 1987.

   [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.

   [X509]     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.

7.2.  Informative References

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

   [DNS-SRV]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [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.

   [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.

   [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



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              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.

   [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.

   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC2459]  Housley, R., Ford, W., Polk, T., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and CRL
              Profile", RFC 2459, January 1999.

   [SECTERMS]



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              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)",
              draft-ietf-sip-domain-certs-04 (work in progress),
              May 2009.

   [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.

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

   [USINGTLS]
              Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

   [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-04 (work
              in progress), November 2009.


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
   [RFC2459].  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 leaf
   Relative Distinguished Name (RDN) of the subjectName field of the
   server's certificate.  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";





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   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 [X509]).
   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 [X509].
       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 [X509] 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 [X509] 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 [X509].
   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 [X509].
   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 [X509], 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 [X509].
      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".  [X509], 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.

   ######


Authors' Addresses

   Peter Saint-Andre (editor)
   Cisco

   Email: psaintan@cisco.com


   Jeff Hodges (editor)
   PayPal

   Email: Jeff.Hodges@PayPal.com






















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