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Versions: 00 01 02 03 04 05 RFC 4985

INTERNET-DRAFT                                  S. Santesson (Microsoft)
Intended Category: Standards Track
Expires April 2007                                          October 2006

                Internet X.509 Public Key Infrastructure
        Subject Alternative Name for expression of service name
                    <draft-ietf-pkix-srvsan-03.txt>


Status of this Memo

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Abstract

   This document defines a new name form for inclusion in the otherName
   field of an X.509 Subject Alternative Name extension which allows a
   certificate subject to be associated with the service name and domain
   name components of a DNS Service Resource Record.













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Table of Contents

   1  Introduction ................................................    2
   2  Name Definitions ............................................    3
   3  Name Constraints Matching Rules .............................    5
   4  Security Considerations .....................................    6
   5  IANA Considerations .........................................    6
   6  References ..................................................    6
   Appendix A. ASN.1 Syntax .......................................    7
      Appendix A.1.  1988 ASN.1 Module ............................    7
      Appendix A.2.  1993 ASN.1 Module ............................    8
   Authors' Addresses .............................................   10
   Full Copyright Statement .......................................   10
   Intellectual Property ..........................................   10

1.  Introduction

   RFC 2782 [N3] Defines a DNS RR (Resource Record) for specifying the
   location of services (SRV RR) which allows clients to ask for a
   specific service/protocol for a specific domain and get back the
   names of any available servers.

   Server discovery through a DNS query based on service/protocol
   relative to a domain is from an authentication perspective
   fundamentally different from when a client has prior trusted
   knowledge about the name and address of the server it attempts to
   connect. While authentication of the name and address of a server
   makes sense when the name and address of the server is prior
   knowledge, it typically has very little value if the name and address
   of the server is obtained from an untrusted source.

   Subsequent authentication of a server discovered through DNS RR
   lookup based on service name typically requires the client to
   authenticate that the connected server is authorized to provide the
   requested service rather than authenticating the servers host name.

   While DNS servers may have the capacity to provide trusted
   information, they may in many other situations not be trusted enough
   to do that, in which case the server may be required to provide
   verifiable credentials to support its due authorization to provide a
   requested service.

   One example where expression of such authorization can be very useful
   is when locating and authenticating a legitimate Kerberos KDC server.

   To support these scenarios, this standard defines a new name form for
   expression of service name relative to a domain in X.509
   certificates.



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   Current dNSName GeneralName Subject Alternative name form only
   provide for DNS host names to be expressed in "preferred name
   syntax," as specified by RFC 1034 [N4]. This definition is therefore
   not broad enough to allow expression of a service related to that
   domain.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [N1].

2.  Name Definitions

   This section defines the SRVName name as a form of otherName from the
   GeneralName structure in SubjectAltName defined in RFC 3280 [N2].

      id-on-dnsSRV OBJECT IDENTIFIER ::= { id-on 7 }

      SRVName ::= UTF8String    (SIZE (1..MAX))

   The SRVName, if present, MUST contain a service name and a domain
   name in the following form:

      _Service.Name

   The content of the components of this name form MUST be consistent
   with the corresponding definition of these components in an SRV RR
   according to RFC 2782 [N3].

   The content of these components are:

      Service
         The symbolic name of the desired service, as defined in
         Assigned Numbers [N5] or locally.  An underscore (_) is
         prepended to the service identifier to avoid collisions with
         DNS labels that occur in nature.  Some widely used services,
         notably POP, don't have a single universal name.  If Assigned
         Numbers names the service indicated, that name is the only name
         which is allowed in the service component of this name form.
         The Service is case insensitive.

         All characters in the service name MUST be in the ASCII range
         (0..7F)

      Name
         The DNS domain name of the domain where the specified service
         is located.



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         If the domain name is an Internationalized domain name (IDN)
         then all DNS labels in SRVName MUST have been processed with
         NAMEPREP (RFC3491) [N6].


   Example: _mail.example.com

   Example: The "mail" service at na<LATIN SMALL LETTER I WITH
   DIAERESIS>ve.net (an IDN, which becomes xn--nave-6pa.net when encoded
   as an IDNA) would use the following 15-character SRVName value:

       _mail.na<LATIN SMALL LETTER I WITH DIAERESIS>ve.net

   Its 16-byte UTF-8 encoding is (in hex):

       5F 6D 61 69 6C 2E 6E 61 C3 AF 76 65 2E 6E 65 74


   Even though this name form is based on the service resource record
   (SRV RR) definition in RFC 2782 [N3] and may be used to enhance
   subsequent authentication of DNS based service discovery, this
   standard does not define any new conditions or requirements regarding
   use of SRV RR for service discovery or where and when such use is
   appropriate.



























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3  Name Constraints Matching Rules

   Name constraining, as specified in RFC 3280, MAY be applied to the
   SRVName by adding name restriction in the name constraints extension
   in the form of an SRVName.

   SRVName restrictions are expressed as a complete SRVName
   (_mail.example.com), just a service name (_mail) or just as a DNS
   name (example.com). The name restriction of the service name part and
   the DNS name part of SRVName are handled separately.

   If a service name is included in the restriction then that
   restriction can only be satisfied by an SRVName which includes a
   corresponding service name. If the restriction has an absent service
   name, then that restriction is satisfied by any SRVName that match
   the domain part of the restriction.

   DNS name restrictions are expressed as host.example.com.  Any DNS
   name that can be constructed by simply adding subdomains to the left
   hand side of the name satisfies the DNS name part of the name
   constraint.  For example, www.host.example.com would satisfy the
   constraint (host.example.com) but 1host.example.com would not.

   Examples:

      Name Constraints
      SRVName restriction   Matching SRVName      non-matching SRVName
      ===================   ================      ====================
      example.com           _mail.example.com     _mail.1example.com
                            _ntp.example.com
                            _mail.1.example.com

      _mail                 _mail.example.com     _ntp.example.com
                            _mail.1example.com

      _mail.example.com     _mail.example.com     _mail.1example.com
                            _mail.1.example.com   _ntp.example.com














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4  Security Considerations

   Assignment of services to hosts may be subject to change.
   Implementers should be aware of the need to revoke old certificates
   that no longer reflect the current assignment of services and thus
   make sure that all issued certificates are up to date.

   When X.509 certificates enhanced with the name form specified in this
   standard is used to enhance authentication of service discovery based
   on a SRV RR query to a DNS server, all security considerations of RFC
   2782 applies.

5  IANA Considerations

   This document has no actions for IANA.

6 References

   Normative references:

   [N1]        S. Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [N2]        R. Housley, W. Polk, W. Ford, and D. Solo, "Internet
               X.509 Public Key Infrastructure: Certificate and
               Certificate Revocation List (CRL) Profile", RFC 3280,
               April 2002.

   [N3]        A. Gulbrandsen and P. Vixie, "A DNS RR for specifying the
               location of services (DNS SRV)", RFC 2782, February 2000.

   [N4]        P. Mockapetris, "DOMAIN NAMES - CONCEPTS AND FACILITIES",
               RFC 1034, November 1987

   [N5]        J. Reynolds, "Assigned Numbers: RFC 1700 is Replaced by
               an On-line Database", RFC 3232, January 2002.

   [N6]        Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
               Profile for Internationalized Domain Names (IDN)", RFC
               3491, March 2003.

   [N7]        ITU-T Recommendation X.501 (2001): Information
               technology - Open Systems Interconnection - The
               Directory: Models







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Appendix A.  ASN.1 Syntax

   As in RFC 2459, ASN.1 modules are supplied in two different variants
   of the ASN.1 syntax.

   This section describes data objects used by conforming PKI components
   in an "ASN.1-like" syntax.  This syntax is a hybrid of the 1988 and
   1993 ASN.1 syntaxes.  The 1988 ASN.1 syntax is augmented with the
   1993 UNIVERSAL Type UTF8String.

   The ASN.1 syntax does not permit the inclusion of type statements in
   the ASN.1 module, and the 1993 ASN.1 standard does not permit use of
   the new UNIVERSAL types in modules using the 1988 syntax.  As a
   result, this module does not conform to either version of the ASN.1
   standard.

   Appendix A.1 may be parsed by an 1988 ASN.1-parser by replacing the
   definitions for the UNIVERSAL Types with the 1988 catch-all "ANY".

   Appendix A.2 may be parsed "as is" by an 1997-compliant ASN.1 parser.

   In case of discrepancies between these modules, the 1988 module is
   the normative one.


Appendix A.1.  1988 ASN.1 Module


   PKIXServiceNameSAN88 {iso(1) identified-organization(3) dod(6)
         internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-dns-srv-name-88(39) }

   DEFINITIONS EXPLICIT TAGS ::=

      BEGIN

      -- EXPORTS ALL --

      IMPORTS

   -- UTF8String, / move hyphens before slash if UTF8String does not
   -- resolve with your compiler

        id-pkix
              FROM PKIX1Explicit88 { iso(1) identified-organization(3)
              dod(6) internet(1) security(5) mechanisms(5) pkix(7)
              id-mod(0) id-pkix1-explicit(18) } ;
              -- from RFC3280 [N2]



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     -- Service Name Object Identifier and Syntax
     -- id-pkix OBJECT IDENTIFIER ::= {1 3 6 1 5 5 7}

     id-on   OBJECT IDENTIFIER ::= { id-pkix 8 }

     id-on-dnsSRV OBJECT IDENTIFIER ::= { id-on 7 }

     SRVName ::= UTF8String    (SIZE (1..MAX))


   END



Appendix A.2.  1993 ASN.1 Module


   PKIXServiceNameSAN93 {iso(1) identified-organization(3) dod(6)
       internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-dns-srv-name-93(40) }

   DEFINITIONS EXPLICIT TAGS ::=

   BEGIN

   -- EXPORTS ALL --

   IMPORTS

        id-pkix
              FROM PKIX1Explicit88 { iso(1) identified-organization(3)
              dod(6) internet(1) security(5) mechanisms(5) pkix(7)
              id-mod(0) id-pkix1-explicit(18) }
               -- from RFC 3280 [N2]

        ATTRIBUTE
              FROM InformationFramework {joint-iso-itu-t ds(5) module(1)
              informationFramework(1) 4};
               -- from X.501 [N7]

   -- Service Name Object Identifier

   id-on   OBJECT IDENTIFIER ::= { id-pkix 8 }

   id-on-dnsSRV OBJECT IDENTIFIER ::= { id-on 7 }






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   -- Service Name

   srvName ATTRIBUTE ::= {
          WITH SYNTAX     SRVName
          ID              id-on-dnsSRV }

   SRVName ::= UTF8String    (SIZE (1..MAX))


   END









































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Authors' Addresses


   Stefan Santesson
   Microsoft
   Tuborg Boulevard 12
   2900 Hellerup
   Denmark

   EMail: stefans@microsoft.com


Full Copyright Statement

   Copyright (C) The Internet Society (2006).

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   copyrights, patents or patent applications, or other proprietary



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   rights that may cover technology that may be required to implement
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Expires April 2007













































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