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>
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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
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Expires April 2007
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