Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name
RFC 4985

Network Working Group                                       S. Santesson
Request for Comments: 4985                                     Microsoft
Category: Standards Track                                    August 2007

                Internet X.509 Public Key Infrastructure
        Subject Alternative Name for Expression of Service Name

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

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

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................2
   2. Name Definitions ................................................2
   3. Internationalized Domain Names ..................................4
   4. Name Constraints Matching Rules .................................5
   5. Security Considerations .........................................6
   6. Normative References ............................................6
   Appendix A. ASN.1 Syntax ...........................................7
      Appendix A.1. 1988 ASN.1 Module .................................7
      Appendix A.2. 1993 ASN.1 Module .................................8

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1.  Introduction

   This document specifies a name form for inclusion in X.509
   certificates that may be used by a certificate relying party to
   verify that a particular host is authorized to provide a specific
   service within a domain.

   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.

   Existing name forms in X.509 certificates support authentication of a
   host name.  This is useful when the name of the host is known by the
   client prior to authentication.

   When a server host name is discovered through DNS RR lookup query
   based on service name, the client may need to authenticate the
   server's authorization to provide the requested service in addition
   to the server's host name.

   While DNS servers may have the capacity to provide trusted
   information, there may be many other situations where the binding
   between the name of the host and the provided service needs to be
   supported by additional credentials.

   Current dNSName GeneralName Subject Alternative name form only
   provides 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 ::= IA5String (SIZE (1..MAX))

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   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
         that is allowed in the service component of this name form.
         The Service is case insensitive.

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

         If the domain name is an Internationalized Domain Name (IDN),
         then encoding in ASCII form SHALL be done as defined in section
         3.

   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.

   The format of a DNS RR, according to RFC 2782, also includes a
   protocol component (_Service._Proto.Name).  This protocol component
   is not included in the SRVName specified in this document.  The
   purpose of the SRVName is limited to authorization of service
   provision within a domain.  It is outside the scope of the SRVName to
   provide any means to verify that the host is using any intended
   protocol.  By omitting the protocol component from the SRVName two
   important advantages have been achieved:

   * One certificate with a single SRVName can be issued to a host that
     offers multiple protocol alternatives.

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   * Name constraints processing rules (specified in section 4)are
     significantly less complex to define without the protocol
     component.

   A present SRVName in a certificate MUST NOT be used to identify a
   host unless one of the following conditions applies:

   *  Use of this name form is specified by the security protocol being
      used and the identified service has a defined service name
      according to RFC 2782, or;

   *  Use of this name form is configured by local policy.

3.  Internationalized Domain Names

   IA5String is limited to the set of ASCII characters.  To accommodate
   internationalized domain names in the current structure, conforming
   implementations MUST convert internationalized domain names to the
   ASCII Compatible Encoding (ACE) format as specified in section 4 of
   RFC 3490 [N6] before storage in the Name part of SRVName.
   Specifically, conforming implementations MUST perform the conversion
   operation specified in section 4 of RFC 3490 [N6], with the following
   clarifications:

      *  in step 1, the domain name SHALL be considered a "stored
         string".  That is, the AllowUnassigned flag SHALL NOT be set;

      *  in step 3, set the flag called "UseSTD3ASCIIRules";

      *  in step 4, process each label with the "ToASCII" operation; and

      *  in step 5, change all label separators to U+002E (full stop).

   When comparing DNS names for equality, conforming implementations
   MUST perform a case-insensitive exact match on the entire domain
   name.  When evaluating name constraints, conforming implementations
   MUST perform a case-insensitive exact match on a label-by-label
   basis.

   Implementations SHOULD convert IDNs to Unicode before display.
   Specifically, conforming implementations SHOULD perform the
   conversion operation specified in section 4 of RFC 3490 [N6], with
   the following clarifications:

      *  in step 1, the domain name SHALL be considered a "stored
         string".  That is, the AllowUnassigned flag SHALL NOT be set;

      *  in step 3, set the flag called "UseSTD3ASCIIRules";

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      *  in step 4, process each label with the "ToUnicode" operation;
         and

      *  skip step 5.

   Note:  Implementations MUST allow for increased space requirements
   for IDNs.  An IDN ACE label will begin with the four additional
   characters "xn--" and may require as many as five ASCII characters to
   specify a single international character.

4.  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 that includes a
   corresponding service name.  If the restriction has an absent service
   name, then that restriction is satisfied by any SRVName that matches
   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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5.  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 an SRV RR query to a DNS server, all security considerations of
   RFC 2782 applies.

6.  Normative References

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

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

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

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

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

   [N6]   Faltstrom, P., Hoffman, P., and A. Costello,
          "Internationalizing Domain Names in Applications (IDNA)", RFC
          3490, March 2003.

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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 Public Key
   Infrastructure (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 a 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 ::= IA5String    (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]

   -- In the GeneralName definition using the 1993 ASN.1 syntax
   -- includes:

   OTHER-NAME ::= TYPE-IDENTIFIER

   -- 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 OTHER-NAME ::= { SRVName IDENTIFIED BY { id-on-dnsSRV }}

   SRVName ::= IA5String (SIZE (1..MAX))

   END

Author's Address

   Stefan Santesson
   Microsoft
   Tuborg Boulevard 12
   2900 Hellerup
   Denmark

   EMail: stefans@microsoft.com

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