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rfc:rfc4985

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

Santesson Standards Track [Page 1] RFC 4985 DNS SRV RR otherName August 2007

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))

Santesson Standards Track [Page 2] RFC 4985 DNS SRV RR otherName August 2007

 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.

Santesson Standards Track [Page 3] RFC 4985 DNS SRV RR otherName August 2007

  • 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";

Santesson Standards Track [Page 4] RFC 4985 DNS SRV RR otherName August 2007

  • 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

Santesson Standards Track [Page 5] RFC 4985 DNS SRV RR otherName August 2007

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.

Santesson Standards Track [Page 6] RFC 4985 DNS SRV RR otherName August 2007

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
  1. - EXPORTS ALL –
    IMPORTS
  1. - UTF8String, / move hyphens before slash if UTF8String does not
  2. - 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]

Santesson Standards Track [Page 7] RFC 4985 DNS SRV RR otherName August 2007

  1. - Service Name Object Identifier and Syntax
  2. - 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
  1. - 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]
  1. - In the GeneralName definition using the 1993 ASN.1 syntax
  2. - includes:
 OTHER-NAME ::= TYPE-IDENTIFIER
  1. - Service Name Object Identifier
 id-on   OBJECT IDENTIFIER ::= { id-pkix 8 }
 id-on-dnsSRV OBJECT IDENTIFIER ::= { id-on 7 }

Santesson Standards Track [Page 8] RFC 4985 DNS SRV RR otherName August 2007

  1. - 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

Santesson Standards Track [Page 9] RFC 4985 DNS SRV RR otherName August 2007

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 Copyright (C) The IETF Trust (2007).
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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Santesson Standards Track [Page 10]

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