Internet Engineering Task Force                                 S. Huque
Internet-Draft                                             Verisign Labs
Updates: 6698 (if approved)                                     D. James
Intended status: Standards Track                          Verisign, Inc.
Expires: January 06, 2016                                    V. Dukhovni
                                                               Two Sigma
                                                           July 05, 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 client
   certificates, and also the rules and considerations for using them
   with the TLS protocol.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 06, 2016.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction and Motivation . . . . . . . . . . . . . . . . .   2
   2.  Associating Client Identities in TLSA Records . . . . . . . .   2
   3.  Authentication Model  . . . . . . . . . . . . . . . . . . . .   3
   4.  Client Identifiers in X.509 certificates  . . . . . . . . . .   3
   5.  Signaling the Client's DANE Identity in TLS . . . . . . . . .   4
   6.  Example TLSA records for clients  . . . . . . . . . . . . . .   4
   7.  Changes to Client and Server behavior . . . . . . . . . . . .   5
   8.  Raw Public Keys . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   7
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   12. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     13.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     13.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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]

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 has the following format:

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   The first label identifies the application service name.  The
   remaining labels are composed of the client domain name.

   Encoding the application service name into the owner name allows the
   same client domain name to have different authentication credentials
   for different application services.  There is no need to encode the
   transport label - the same name form is usable with both TLS and

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

   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.  This domain name doesn't necessarily have any relation to its
   network layer addresses.  Clients often have dynamic or unpredictable
   addresses, and may move around the network, so tying their identity
   to network addresses is not feasible or wise in the general case.

   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.  "_smtp-".  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

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   Internet to date.  However, with TLSA usage modes except for DANE-EE,
   if there is a need to isolate multiple application specific
   credentials from each other on the same client (i.e. with the same
   underlying base domain name), then SRV-ID would need to be 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.

   More importantly, 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

   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".  This record specifies the SHA-256 hash of
   a PKIX CA certificate to authenticate the client's certificate.

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Internet-Draft          TLSA client certificates               July 2015 IN TLSA (
      0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
            7983a1d16e8a410e4561cb106618e971 )

   An example TLSA record for the client "" and the
   application "localsvc".  This record 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. 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 explicitly address the additional 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

   S1  Request a client certificate in the TLS handshake (the "Client
       Certificate Request" message).

   S2  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.)

   S3  Construct the DNS query name for the corresponding TLSA record.
       For dNSName, the underscored application service label is
       prepended to the domain name, corresponding to the application in

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       use.  For SRVName, the DNS query name is identical to the content
       of the SRVName identifier.  See Section 2 for the proposed owner
       name format.

   S4  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.

   S5  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.

   S6  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.

   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.  If the certificate contains both dNSName and
   SRVName identities, SRVName should be preferred.  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.

   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 documents specific to the applications that require this

   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.

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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
   [DANEOPS].  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],

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.

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   [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.

   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
              Conversations about DNS-Based Authentication of Named
              Entities (DANE)", RFC 7218, April 2014.

   [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

   [DANEOPS]  Dukhovni, V., "Updates to and Operational Guidance for the
              DANE Protocol", , <

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552, July

   [SRVREG]   IANA, ., "Service Name and Transport Protocol Port Number
              Registry", , <

Authors' Addresses

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   Shumon Huque
   Verisign Labs


   Dan James
   Verisign, Inc.


   Viktor Dukhovni
   Two Sigma


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