PKIX Repository Locator: Internet X.509 Public Key Infrastructure Repository Locator Service
How a certificate-using program can find a domain's PKI repository by asking DNS for an SRV record.
Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key Infrastructure Repository Locator Service", RFC 4386, DOI 10.17487/RFC4386, February 2006, <https://www.rfc-editor.org/info/rfc4386>.
@misc{rfc4386,
series = {Request for Comments},
number = 4386,
howpublished = {RFC 4386},
publisher = {RFC Editor},
doi = {10.17487/RFC4386},
url = {https://www.rfc-editor.org/info/rfc4386},
author = {S. Boeyen and P. Hallam-Baker},
title = {{Internet X.509 Public Key Infrastructure Repository Locator Service}},
pagetotal = 6,
year = 2006,
month = feb,
} RFC text
Contents
Network Working Group S. Boeyen
Request for Comments: 4386 Entrust Inc.
Category: Experimental P. Hallam-Baker
VeriSign Inc.
February 2006
Internet X.509 Public Key Infrastructure
Repository Locator Service
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines a Public Key Infrastructure (PKI) repository
locator service. The service makes use of DNS SRV records defined in
accordance with RFC 2782. The service enables certificate-using
systems to locate PKI repositories.
Table of Contents
1. Overview ........................................................2
1.1. Conventions Used in This Document ..........................2
2. SRV RR Definition ...............................................2
2.1. Assignment of New Protocol Prefixes ........................3
2.2. Use of Multiple Repositories ...............................3
2.3. SRV RR Example .............................................3
3. Security Considerations .........................................4
4. IANA Considerations .............................................4
5. Informative References ..........................................4
1. Overview
A number of RFCs (including [RFC2559], [RFC2560], and [RFC2585]) have
specified operational protocols for retrieval of PKI data, including
public-key certificates and revocation information, from PKI
repositories. These RFCs assume that a certificate-using system has
the information necessary to identify, locate, and connect to the PKI
repository with a specific protocol. Although some tools are
available in protocol-specific environments for this purpose, such as
knowledge references in directory systems, these are restricted for
use with a single protocol and do not share a common means of
publication. This document provides a solution to this problem
through the use of Service Record (SRV) Resource Records (RRs) in
DNS. This solution is expected to be particularly useful in
environments where only a domain name is available. In other
situations (e.g., where a certificate is available that contains the
required information), such a DNS lookup is not needed.
[RFC2782] defines a DNS RR for specifying the location of services
(SRV). This document defines SRV records for a PKI repository
locator service to enable PKI clients to obtain the necessary
information to connect to a domain's PKI repository, including
information about each protocol that is supported by that domain for
access to its repository. This document includes the definition of
an SRV RR format for this service and an example of its potential use
in an email environment.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document (in uppercase,
as shown) are to be interpreted as described in [RFC2119].
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively.
2. SRV RR Definition
The format of the SRV RR, whose DNS type code is 33, is:
_Service._Proto.Name TTL Class SRV Priority Weight Port Target
For the PKI repository locator service, this document uses the
symbolic name "PKIXREP". Note that when used in an SRV RR, this name
MUST be prepended with an "_" character.
The protocols that can be included in PKIXREP SRV RRs are:
Protocol SRV Prefix
LDAP _LDAP
HTTP _HTTP
OCSP _OCSP
2.1. Assignment of New Protocol Prefixes
Protocol prefix assignments for new PKIX repository protocols SHOULD
be defined in the document that specifies the protocol.
2.2. Use of Multiple Repositories
The existence of multiple repositories MAY be determined by making
separate DNS queries for each of the protocols supported by the
client.
If this approach is found to be unacceptably inefficient due to a
proliferation of repository protocols at a future date, the service
discovery protocol could be extended to allow the repository to
advertise the protocols supported.
2.3. SRV RR Example
This example uses the fictional domain "example.com" as an aid in
understanding the use of SRV records by a certificate-using system.
Assume that Alice is an email client that needs a certificate for a
recipient. Alice's client system supports LDAP for certificate
retrieval. Assume the message recipient is Bob and that Bob's email
address is bob@example.com. Assume that example.test maintains a
"border directory" PKI repository and that Bob's certificate is
available from that directory, "border.example.com", via LDAP.
Alice's client system retrieves, via DNS, the SRV record for
_PKIXREP._LDAP.example.com.
- The QNAME of the DNS query is _PKIXREP._LDAP.example.com.
- The QCLASS of the DNS query is IN.
- The QTYPE of the DNS query is SRV.
The result SHOULD include the host address for example.com's border
directory system.
Note that if example.com operated its service on a number of hosts,
more than one SRV RR would be returned. In this case, RFC 2782
defines the procedure to be followed in determining which of these
should be accessed first.
3. Security Considerations
Security issues regarding PKI repositories themselves are outside the
scope of this document. For LDAP repositories, for example, specific
security considerations are addressed in RFC 2559.
Security issues with respect to the use of SRV records in general are
addressed in RFC 2782, and these issues apply to the use of SRV
records in the context of the PKIXREP service defined here.
4. IANA Considerations
This document reserves the use of "_PKIXREP" service label. Since
this relates to a service that may pass messages over a number of
different message transports, each message must be associated with a
specific transport.
In order to ensure that the association between "_PKIXREP" and their
respective underlying services is deterministic, the IANA has created
a new registry: PKIX SRV Protocol Labels.
For this registry, an entry shall consist of a label name and a
pointer to a specification describing how the protocol named in the
label uses SRV. Specifications should conform to the requirements
listed in [RFC2434] for "specification required".
5. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2559] Boeyen, S., Howes, T., and P. Richard, "Internet X.509
Public Key Infrastructure Operational Protocols - LDAPv2",
RFC 2559, April 1999.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC
2585, May 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
Authors' Addresses
Sharon Boeyen
Entrust
1000 Innovation Drive
Ottawa, Ontario
Canada K2K 3E7
EMail: sharon.boeyen@entrust.com
Phillip M. Hallam-Baker
VeriSign Inc.
401 Edgewater Place, Suite 280
Wakefield MA 01880
EMail: pbaker@VeriSign.com
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Copyright (C) The Internet Society (2006).
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ISO Plain Language
On this page
§1Why a Repository Locator Service
A number of RFCs, among them the operational protocols for LDAP, OCSP and for FTP and HTTP, have specified operational protocols for retrieving PKI data, including public-key certificates and revocation information, from PKI repositories. Those documents assume that a certificate-using system already has the information necessary to identify, locate and connect to the PKI repository with a specific protocol.
Some tools are available in protocol-specific environments for this purpose, such as knowledge references in directory systems, but they are restricted to use with a single protocol and do not share a common means of publication. This document provides a solution through the use of Service Record resource records in DNS. The solution is expected to be particularly useful in environments where only a domain name is available; in other situations, for example where a certificate is available that already contains the required information, such a DNS lookup is not needed.
RFC 2782 defines the DNS record for specifying the location of services. This document defines SRV records for a PKI repository locator service, so that PKI clients can obtain the information they need to connect to a domain's PKI repository, including which protocols that domain supports for access to its repository.
The protocols that fetch certificates are already written. What they leave out is how a program finds the server to fetch from. The memo puts that answer in DNS, where a domain name is usually all a client has to go on.
- Retrieval protocols existRFCs already define how to fetch certificates and revocation data over LDAP, HTTP or OCSP.
- The gap they leaveThose documents assume the client already knows which repository to use and how to reach it.
- The fixPublish the repository in DNS as an SRV record under a fixed service label.
§2The SRV Record Definition
The format of the SRV record, whose DNS type code is 33, is _Service._Proto.Name TTL Class SRV Priority Weight Port Target. For the PKI repository locator service the symbolic name is PKIXREP, and when it is used in an SRV record this name must be prepended with an underscore character.
The protocols that can be included in PKIXREP SRV records are LDAP with the prefix _LDAP, HTTP with the prefix _HTTP, and OCSP with the prefix _OCSP. Assignments of protocol prefixes for new PKI repository protocols should be defined in the document that specifies the protocol.
The existence of multiple repositories may be determined by making separate DNS queries for each of the protocols supported by the client. If that turns out to be unacceptably inefficient because of a proliferation of repository protocols at some future date, the memo suggests the service discovery protocol could be extended to let a repository advertise the protocols it supports.
The name of the record says two things: that it points at a certificate repository, and which protocol to speak to it. A client that speaks LDAP looks for one name, and a client that speaks HTTP looks for another.
_Service._Proto.Name TTL Class SRV Priority Weight Port Target
Protocol SRV Prefix
LDAP _LDAP
HTTP _HTTP
OCSP _OCSP§2.3A Worked Example
The example uses the fictional domain example.com to show how a certificate-using system uses the records. Alice is an email client that needs a certificate for a recipient, and her client system supports LDAP for certificate retrieval. The recipient is Bob, whose email address is bob@example.com, and example.test maintains a border directory PKI repository from which Bob's certificate is available over LDAP at border.example.com.
Alice's client system retrieves, via DNS, the SRV record for _PKIXREP._LDAP.example.com. The QNAME of the DNS query is _PKIXREP._LDAP.example.com, the QCLASS is IN, and the QTYPE is SRV. The result should include the host address for example.com's border directory system.
If example.com operated its service on a number of hosts, more than one SRV record would be returned, and RFC 2782 defines the procedure for deciding which of them should be accessed first.
The client's mail program needs a certificate for a recipient and knows how to fetch certificates over LDAP but not where they are kept. It asks DNS for the name that combines the two, and DNS answers with the machine that holds them.
- DNS QUERYFor _PKIXREP._LDAP.example.com, with QCLASS=IN and QTYPE=SRV.
- SRV RECORDThe host address of the domain's border directory system.
- LDAP RETRIEVALThe certificate is fetched over the protocol named in the record.
§3Security Considerations
Security issues regarding the PKI repositories themselves are outside the scope of this document; for LDAP repositories, for example, the specific security considerations are addressed in RFC 2559. Security issues with respect to the use of SRV records in general are addressed in RFC 2782, and those issues apply to the use of SRV records in the context of the PKIXREP service defined here.
The memo does not add security of its own. It points at the documents that cover the repository and the DNS record, and says their warnings apply here too.
§4IANA Considerations
The document reserves the use of the _PKIXREP service label. Since the service may pass messages over a number of different transports, each message must be associated with a specific transport, and to keep the association between _PKIXREP and the underlying services deterministic, IANA created a new registry, PKIX SRV Protocol Labels. An entry in that registry consists of a label name and a pointer to a specification describing how the protocol named in the label uses SRV. Specifications should conform to the requirements listed in RFC 2434 for specification required.
The label _PKIXREP is reserved, and the protocols that may appear under it are listed in a registry so that each one always means the same thing.