Internet Engineering Task Force S. Huque
Internet-Draft Verisign Labs
Updates: 6698 (if approved) D. James
Intended status: Standards Track Verisign, Inc.
Expires: December 29, 2015 V. Dukhovni
Two Sigma
June 27, 2015

Client Certificates in DANE TLSA Records


The current DNS TLSA record format [RFC6698] describes how to specify TLS server certificates or their public keys in the DNS. This document makes a narrowly focused update to RFC 6698. It describes how to additionally use the TLSA record to specify TLS client certificates and the rules and considerations for using them.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on December 29, 2015.

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Table of Contents

1. Introduction and Motivation

The Transport Layer Security (TLS) protocol [RFC5246] optionally supports the authentication of clients using X.509 certificates [RFC5280]. TLS Applications currently employing DANE authentication of servers using TLSA records may also desire to authenticate clients using the same mechanism, especially if the client identity is in the form of or can be represented by a DNS domain name. Some design patterns from the Internet of Things (IoT) make use of this form of authentication, where large networks of physical objects identified by DNS names may authenticate themselves using TLS to centralized device management and control platforms.

In this document, the term TLS is used generically to describe both the TLS and DTLS (Datagram Transport Layer Security) [RFC6347] protocols.

2. Associating Client Identities in TLSA Records

When specifying client identities (i.e. client domain names) in TLSA records, the owner name of the TLSA record is proposed to have one of the two formats described below:

Format 1: The first label identifies the application service, and the second identifies the transport protocol. The remaining labels are composed of the client domain name. So the form looks like _service._transport.[client-domain-name].

Format 2: The first label is the constant string "_client" and the second label identifies the application service. The remaining labels are composed of the client domain name. So the form looks like _client._service.[client-domain-name]. This form is transport protocol agnostic, and has a constant string that can be used to easily identify TLSA records for client certificates.

Encoding the application service name into the owner name allows the same client domain name to have different authentication credentials for different application services.

The _service label could be a custom string for an application, but more commonly is expected to be service name registered in the IANA Service Name Registry [SRVREG].

Note: The final version of this document will specify only one format for the owner name, based on IETF working group consensus.

The RDATA or data field portion of the TLSA record is formed exactly as specified in RFC 6698, and carries the same meaning.

3. Authentication Model

The authentication model assumed in this document is the following: The client is assigned an identity corresponding to a DNS domain name (which does not necessarily have any relation to its network layer addresses). The client generates (or has generated for it) a private and public key pair, and a certificate binding the name to its public key. This certificate has a corresponding TLSA record published in the DNS, which allows it to be authenticated directly via the DNS (using the DANE-TA or DANE-EE usage modes) or via a PKIX public CA system constraint (using the PKIX-TA or PKIX-EE usage modes).

4. Client Identifiers in X.509 certificates

The client certificate MUST have have the client's DNS name specified in the Subject Alternative Name extension's dNSName type. Or, if an application specific identity is preferred or needed, the SRV-ID (PKIX OtherName SRVName) MUST be used to specify the application service and the client's name, e.g. "". See [RFC6125] and [RFC4985] for a discussion of application specific identifiers in X.509 certificates.

The initial revision of this document talks mainly about dNSName identifiers, because SRV-ID has not seen much adoption in the Internet to date. However, with TLSA usage modes except for DANE-EE, there may be a problem separating different client credentials with the same underlying domain name, but distinct services unless SRV-ID is employed.

5. Signaling the Client's DANE Identity in TLS

The protocol described in the initial version of this document assumes either that client authentication is mandatory, or that where it is optional, clients can handle a Client Certificate Request message from the server without issues if they are not equipped with client certificates. Technically, the TLS protocol specification states that the client may respond with a Client Certificate message with no certificate, and that the server may at its discretion continue the handshake without client authentication. However in practice, problems may arise. There are deployed client software implementations that do not react gracefully when encountering a certificate request that they did not expect.

Furthermore, a server may want an explicit indication from the client that it has a DANE record, so as to avoid unnecessary DNS queries in-band with the TLS handshake for clients that don't support this.

Hence, to address this issue generally, a client identity signaling solution will need to be devised, whereby the client indicates its DANE identity (i.e. its domain name identity and the fact that this identity has an associated TLSA record) to the server. Application specific protocol enhancements are one way to achieve this, e.g. a new SMTP command. A more general way would be to develop a new TLS extension to convey this information.

[Another internet draft is currently being written to define such a TLS extension to convey DANE client identity.]

6. Example TLSA records for clients

The following examples are provided in the textual presentation format of the TLSA record.

An example TLSA record for the client "" and the application "smtp-client" on transport "tcp". This record record specifies the use of TLS, and specifies the SHA-256 hash of a PKIX CA certificate to authenticate the client's certificate.

Format 1: IN TLSA (
   0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
         7983a1d16e8a410e4561cb106618e971 )

Format 2: IN TLSA (
   0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
         7983a1d16e8a410e4561cb106618e971 )

An example TLSA record for the client "" and the application "localsvc" on transport "udp". This record specifies the use of DTLS, and specifies the SHA-512 hash of the subject public key component of the client's certificate. The usage mode for this record is 3 (DANE-EE) and hence no PKIX validation for this certificate should be performed.

Format 1: IN TLSA (
   3 1 2 0f8b48ff5fd94117f21b6550aaee89c8
         b408534451fd1b9702f8de0532ecd03c )

Format 2: IN TLSA (
   3 1 2 0f8b48ff5fd94117f21b6550aaee89c8
         b408534451fd1b9702f8de0532ecd03c )

7. Changes to Client and Server behavior

[Note: As the client identity signaling solution is developed, this section will undergo enhancements to use it. A future revision of this document will also explicitly address the use case of raw public keys instead of X.509 certificates.]

A TLS Client conforming to this specification MUST have a signed DNS TLSA record published corresponding to its DNS name and X.509 certificate. The client presents this certificate in the TLS handshake with the server. The presented client certificate MUST have have the client's DNS name specified either in the Subject Alternative Name extension's dNSName type, or the SRVName type.

A TLS Server implementing this specification performs the following steps:

Request a client certificate in the TLS handshake (the "Client Certificate Request" message).
Extract the client identity from the Subject Alternative Name extension's dNSName or SRVName type in the client certificate. (If no client certificate is provided, then the server may terminate the connection, or at its discretion may continue the handshake without client authentication.)
Construct the DNS query name for the corresponding TLSA record, by prepending the application service, transport protocol, and "_client" labels according to which format is being used. See Section 2 for the proposed owner name formats.
Look up the TLSA record in the DNS. The response MUST be cryptographically validated using DNSSEC. The server could perform the DNSSEC validation itself. It could also be configured to trust responses obtained via a validating resolver to which it has a secure connection.
Extract the RDATA of the TLSA record and match it to the presented client certificate according to the rules specified in the DANE TLS protocol [RFC6698]. If successfully matched, the client is authenticated and the TLS session proceeds. If not, the session is terminated with a "bad_certificate" alert message.
If there are multiple records in the TLSA record set, then the client is authenticated as long as at least one of the TLSA records matches.

Specific applications may be designed to require more detailed validation steps. For example, a server might want to verify the client's IP address is associated with the certificate in some manner, e.g. by confirming that a secure reverse DNS lookup of that address ties it back to the same domain name, or by requiring an iPAddress component to be included in the certificate. Such details are outside the scope of this document, and should be outlined in other application specific documents.

Servers may have their own whitelisting and authorization rules for which certificates they accept. For example a TLS server may be configured to only allow TLS sessions from clients with certificate identities within a specific domain or set of domains.

If the presented client certificate has multiple distinct reference identifier types (e.g. a dNSName, and an rfc822Name) then TLS servers configured to perform DANE authentication according to this specification should only examine and authenticate the dNSName or SRVName identity. See [RFC6125] for a description of reference identifiers and matching rules.

If the presented client certificate has multiple dNSName or SRVName identities, then the client MUST use an identity signalling mechanism to indicate the intended name to the server.

8. Raw Public Keys

This specification can also support the use of raw public keys in TLS [RFC7250]. This use case employs only usage mode 3 (DANE-EE) and a selector value of 1 (SPKI) in the DANE TLSA record, as described in [DANERAW]. It requires the use of the new client identity signaling solution discussed previously.

9. Open Issues

Should this document also consider client identities in the form of e-mail addresses? The use case might be an SMTP client talking to an SMTP submission server. In that case, the email address of a user would most likely be conveyed in the certificate in a subject alt name rfc822Name type. The corresponding TLSA record would have to then have an owner name format similar to the OPENPGPKEY or SMIMEA records. This use case might be best left to the SMIMEA specification to consider.

10. Acknowledgements

This document benefited from discussions with the following people: Duane Wessels, Allison Mankin, Casey Deccio, and Warren Kumari.

11. IANA Considerations

This document includes no request to IANA.

12. Security Considerations

This document makes a narrow update to RFC 6698 by defining the usage of the TLSA record for client TLS certificates. There are no security considerations for this document beyond those described in RFC 6698 and in the specifications for TLS and DTLS [RFC5246], [RFC6347].

13. References

13.1. Normative References

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005.
[RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name", RFC 4985, August 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, March 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, August 2012.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S. and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, June 2014.

13.2. Informative References

[DANERAW] Gilmore, J., "Authenticating Raw Public Keys with DANE TLSA"
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, July 2003.
[SRVREG] IANA, , "Service Name and Transport Protocol Port Number Registry"

Authors' Addresses

Shumon Huque Verisign Labs 12061 Bluemont Way Reston, VA 20190 US EMail:
Dan James Verisign, Inc. 12061 Bluemont Way Reston, VA 20190 US EMail:
Viktor Dukhovni Two Sigma EMail: