TLS                                                             M. Shore
Internet-Draft                                      No Mountain Software
Intended status: Standards Track                               R. Barnes
Expires: December 6, 2016 January 8, 2017                                         Mozilla
                                                                S. Huque
                                                           Verisign Labs
                                                               W. Toorop
                                                              NLNet Labs
                                                            June 4,
                                                            July 7, 2016

    A DANE Record and DNSSEC Authentication Chain Extension for TLS


   This draft describes a new TLS extension for transport of a DNS
   record set serialized with the DNSSEC signatures needed to
   authenticate that record set.  The intent of this proposal is to
   allow TLS clients to perform DANE authentication of a TLS server
   certificate without needing to perform additional DNS record lookups.
   It will typically not be used for general DNSSEC validation of TLS
   endpoint names.

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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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 6, 2016. January 8, 2017.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.

Table of Contents

   1.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  DNSSEC Authentication Chain Extension . . . . . . . . . . . .   3
     3.1.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  DNSSEC Authentication Chain Data  . . . . . . . . . . . .   4
   4.  Construction of Serialized Authentication Chains  . . . . . .   7
   5.  Caching and Regeneration of the Authentication Chain  . . . .   7
   6.  Verification  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Trust Anchor Maintenance  . . . . . . . . . . . . . . . . . .   8
   8.  Mandating use of this extension . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Pseudocode example  Updates from -00 . . . . . . . . . . . . . . . . . .  12
   Appendix B.  Test vector  . . . . . . . . . . . . . . . . . . . .  12

1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Introduction

   This draft describes a new TLS [RFC5246] extension for transport of a
   DNS record set serialized with the DNSSEC signatures [RFC4034] needed
   to authenticate that record set.  The intent of this proposal is to
   allow TLS clients to perform DANE authentication [RFC6698] of a TLS
   server certificate without performing perform additional DNS record lookups
   and incurring the associated latency penalty.  It also provides the
   ability to avoid potential problems with TLS clients being unable to
   look up DANE records because of an interfering or broken middlebox on
   the path between the endpoint client and a DNS server.  And lastly, it allows
   a TLS client to validate DANE records itself without necessarily
   needing access to a validating DNS resolver to which it has a secure
   connection.  It will typically not be used for general DNSSEC
   validation of endpoint names, but is more appropriate for validation
   of DANE TLSA records.

   This mechanism is useful for TLS applications that need to address
   the problems described above, typically web browsers or VoIP and XMPP
   applications.  It may not be relevant for many other applications.
   For example, SMTP MTAs are usually located in data centers, may
   tolerate extra DNS lookup latency, are on servers where it is easier
   to provision a validating resolver, or are less likely to experience
   traffic interference from misconfigured middleboxes.  Furthermore,
   SMTP MTAs usually employ Opportunistic Security [RFC7435], in which
   the presence of the DNS TLSA records is used to determine whether to
   enforce an authenticated TLS connection.  Hence DANE authentication
   of SMTP MTAs [RFC7672] will typically not use this mechanism.

   The extension described here allows a TLS client to request in the
   client hello message that the DNS authentication chain be returned in
   the (extended) server hello message.  If the server is configured for
   DANE authentication, then it performs the appropriate DNS queries,
   builds the authentication chain, and returns it to the client.  The
   server will usually use a previously cached authentication chain, but
   it will need to rebuild it periodically as described in Section 5.
   The client then authenticates the chain using a pre-configured trust

   This specification is based on Adam Langley's original proposal for
   serializing DNSSEC authentication chains and delivering them in an
   X.509 certificate extension [AGL]. [I-D.agl-dane-serializechain].  It
   modifies the approach by using wire format DNS records in the
   serialized data (assuming that the data will be prepared and consumed
   by a DNS-specific library), and by using a TLS extension to deliver
   the data.

3.  DNSSEC Authentication Chain Extension

3.1.  Protocol

   A client MAY include an extension of type "dnssec_chain" in the
   (extended) ClientHello.  The "extension_data" field of this extension
   MUST be empty.

   Servers receiving a "dnssec_chain" extension in the client hello, and
   which are capable of being authenticated via DANE, SHOULD MAY return a
   serialized authentication chain in the extended ServerHello message,
   using the format described below.  If a server is unable to return a an
   authentication chain, or does not wish to return a an authentication
   chain, it does not include a dnssec_chain extension.  As with all TLS
   extensions, if the server does not support this extension it will not
   return any authentication chain.

   A client must not be able to force a server to perform lookups on
   arbitrary domain names using this mechanism.  Therefore, a server
   MUST NOT construct chains for domain names other than its own.

3.2.  DNSSEC Authentication Chain Data

   The "extension_data" field of the "dnssec_chain" extension MUST
   contain a DNSSEC Authentication Chain encoded in the following form:

             opaque AuthenticationChain<0..2^16-1>; AuthenticationChain<0..2^16-1>

   The AuthenticationChain structure is composed of a sequence of
   uncompressed wire format DNS resource record sets (RRset) and
   corresponding signatures (RRsig) records.  The record sets and
   signatures are presented in validation order, starting at the target
   DANE record, followed by the DNSKEY and DS record sets for each
   intervening DNS zone up to a trust anchor chosen by the server,
   typically the DNS root.

   This sequence of native DNS wire format records enables easier
   generation of the data structure on the server and easier
   verification of the data on client by means of existing DNS library
   functions.  However this document describes the data structure in
   sufficient detail that implementers if they desire can write their
   own code to do this.

   [TODO: mention that to reduce the size of the chain, the server can
   deliver exactly one RRsig per RRset, namely the one used to validate
   the chain as it is built.]

   Each RRset in the chain is composed of a sequence of wire format DNS
   resource records.  The format of the resource record is described in
   RFC 1035 [RFC1035], Section 3.2.1.  The resource records SHOULD be
   presented in the canonical form and ordering as described in RFC 4034

             RR(i) = owner | type | class | TTL | RDATA length | RDATA

   RRs within the RRset are ordered canonically, by treating the RDATA
   portion of each RR as a left-justified unsigned octet sequence in
   which the absence of an octet sorts before a zero octet.

   The RRsig record is in DNS wire format as described in RFC 4034
   [RFC4034], Section 3.1.  The signature portion of the RDATA, as
   described in the same section, is the following:

             signature = sign(RRSIG_RDATA | RR(1) | RR(2)... )

   where, RRSIG_RDATA is the wire format of the RRSIG RDATA fields with
   the Signer's Name field in canonical form and the signature field

   The first RRset in the chain MUST contain the DANE records being
   presented.  The subsequent RRsets MUST be a sequence of DNSKEY and DS
   RRsets, starting with a DNSKEY RRset.  Each RRset MUST authenticate
   the preceding RRset:

   o  A DNSKEY RRset must include the DNSKEY RR containing the public
      key used to verify the previous RRset.

   o  For a DS RRset, the set of key hashes MUST overlap with the
      preceding set of DNSKEY records.

   In addition, a DNSKEY RRset followed by a DS RRset MUST be self-
   signed, in the sense that its RRSIG MUST verify under one of the keys
   in the DNSKEY RRSET.

   The final DNSKEY RRset in the authentication chain, containing the
   trust anchor may be omitted.  If omitted, the client MUST verify that
   the key tag and owner name in the final RRSIG record correspond to a
   trust anchor.  There may however be reason to include the trust
   anchor RRset and signature if clients are expected to use RFC5011
   compliant key rollover functions inband via the chain data.  In that
   case, they will need to periodically inspect flags (revocation and
   secure entry point flags) on the trust anchor DNSKEY RRset.

   For example, for an HTTPS server at, where there are
   zone cuts at "com." and "", the AuthenticationChain
   structure would comprise the following RRsets and signatures (the
   data field of the records are omitted here for brevity):

             RRSIG( TLSA)
             RRSIG( DNSKEY)
             RRSIG( DS)
             com. DNSKEY
             RRSIG(com. DNSKEY)
             com. DS
             RRSIG(com. DS)
             . DNSKEY
             RRSIG(. DNSKEY)

   Names that are aliased via CNAME and/or DNAME records may involve
   multiple branches of the DNS tree.  In this case the authentication
   chain structure will be composed of a sequence of these multiple
   intersecting branches.  DNAME chains should omit unsigned CNAME
   records that may have been synthesized in the response from a DNS
   resolver.  Wildcard DANE records will need to include the wildcard
   name as well as a
   name, and negative proof (i.e.  NSEC or NSEC3 records) that no closer
   name exists. exists MUST be included.

         A CNAME example:
   IN   CNAME              IN   TLSA     2 0 1 ...

         Here the authentication chain structure is composed of two
         consecutive chains, one for
         and one for The second chain can omit
         the record sets at the end that overlap with the first.

         TLS DNSSEC chain components:
         RRSIG( DNSKEY) DS
         RRSIG( DS)
         com. DNSKEY
         RRSIG(com. DNSKEY)
         com. DS
         RRSIG(com. DS)
         . DNSKEY
         RRSIG(. DNSKEY)
         RRSIG( DNSKEY) DS
         RRSIG( DS)
         net. DNSKEY
         RRSIG(net. DNSKEY)
         net. DS
         RRSIG(net. DS)

   Note as well that if a user has a specific TLSA record for port 443,
   and a different wildcard covering other ports, attackers MUST NOT be
   able to substitute the wildcard TLSA RRset for the more specific one
   for port 443.  DNSSEC wildcards must not be confused with the X.509

4.  Construction of Serialized Authentication Chains

   This section describes a possible procedure for the server to use to
   build the serialized DNSSEC chain.

   When the goal is to perform DANE authentication [RFC6698] of the
   server's X.509 certificate, the DNS record set to be serialized is a
   TLSA record set corresponding to the server's domain name.

   The domain name of the server MUST be that included in the TLS Server
   Name Indication extension [RFC6066] when present.  If the Server Name
   Indication extension is not present, or if the server does not
   recognize the provided name and wishes to proceed with the handshake
   rather than to abort the connection, the server uses the domain name
   associated with the server IP address to which the connection has
   been established.

   The TLSA record to be queried is constructed by prepending the _port
   and _transport labels to the domain name as described in [RFC6698],
   where "port" is the port number associated with the TLS server.  The
   transport is "tcp" for TLS servers, and "udp" for DTLS servers.  The
   port number label is the left-most label, followed by the transport,
   followed by the base domain name.

   The components of the authentication chain are built by starting at
   the target record set and its corresponding RRSIG.  Then traversing
   the DNS tree upwards towards the trust anchor zone (normally the DNS
   root), for each zone cut, the DNSKEY and DS RRsets and their
   signatures are added.  If DNS responses messages contain any domain
   names utilizing name compression [RFC1035], then they must be

   In the future, proposed DNS protocol enhancements, such as the EDNS
   Chain Query extension [CHAINQUERY] [I-D.ietf-dnsop-edns-client-subnet] may offer
   easy ways to obtain all of the chain data in one transaction with an
   upstream DNSSEC aware recursive server.

5.  Caching and Regeneration of the Authentication Chain

   DNS records have Time To Live (TTL) parameters, and DNSSEC signatures
   have validity periods (specifically signature expiration times).
   After the TLS server constructs the serialized authentication chain,
   it SHOULD cache and reuse it in multiple TLS connection handshakes.
   However, it MUST refresh and rebuild the chain as TTLs and signature
   validity periods dictate.  A server implementation could carefully
   track these parameters and requery component records in the chain
   correspondingly.  Alternatively, it could be configured to rebuild
   the entire chain at some predefined periodic interval that does not
   exceed the DNS TTLs or signature validity periods of the component
   records in the chain.

6.  Verification

   A TLS client making use of this specification, and which receives a
   DNSSEC authentication chain extension from a server, SHOULD use this
   information to perform DANE authentication of the server certificate.
   In order to do this, it uses the mechanism specified by the DNSSEC
   protocol [RFC4035].  This mechanism is sometimes implemented in a
   DNSSEC validation engine or library.

   If the authentication chain is correctly verified, the client then
   performs DANE authentication of the server according to the DANE TLS
   protocol [RFC6698], and the additional protocol requirements outlined
   in [RFC7671].

7.  Trust Anchor Maintenance

   The trust anchor may change periodically, e.g. when the operator of
   the trust anchor zone performs a DNSSEC key rollover.  Managed key
   rollovers typically use a process that can be tracked by verifiers
   allowing them to automatically update their trust anchors, as
   described in [RFC5011].  TLS clients using this specification are
   also expected to use such a mechanism to keep their trust anchors
   updated.  Some operating systems may have a system-wide service to
   maintain and keep the root trust anchor up to date.  In such cases,
   the TLS client application could simply reference that as its trust
   anchor, periodically checking whether it has changed.

8.  Mandating use of this extension

   A TLS server certificate MAY mandate the use of this extension by
   means of the X.509 TLS Feature Extension described in [RFC7633].
   This X.509 certificate extension, when populated with the
   dnssec_chain TLS extension identifier, indicates to the client that
   the server must deliver the authentication chain when asked to do so.
   (The X.509 TLS Feature Extension is the same mechanism used to
   deliver other mandatory signals, such as OCSP "must staple"

9.  Security Considerations

   The security considerations of the normatively referenced RFCs (1035,
   4034, 4035, 5246, 6066, 6698, 7633, 7671) all pertain to this
   extension.  Since the server is delivering a chain of DNS records and
   signatures to the client, it MUST rebuild the chain in accordance
   with TTL and signature expiration of the chain components as
   described in Section 5.  TLS clients need roughly accurate time in
   order to properly authenticate these signatures.  This could be
   achieved by running a time synchronization protocol like NTP
   [RFC5905] or SNTP [RFC5905], which are already widely used today.
   TLS clients MUST support a mechanism to track and rollover the trust
   anchor key, or be able to avail themselves of a service that does
   this, as described in Section 7.

10.  IANA Considerations

   This extension requires the registration of a new value in the TLS
   ExtensionsType registry.  The value requested from IANA is 53.  If
   the draft is adopted by the WG, the authors expect to make an early
   allocation request as specified in [RFC7120].

11.  Acknowledgments

   Many thanks to Adam Langley for laying the groundwork for this
   extension.  The original idea is his but our acknowledgment in no way
   implies his endorsement.  This document also benefited from
   discussions with and review from the following people: Viktor
   Dukhovni, Daniel Kahn Gillmor, Jeff Hodges, Allison Mankin, Patrick
   McManus, Rick van Rein, Gowri Visweswaran, Duane Wessels, Nico
   Williams, and Paul Wouters.

12.  References

12.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066, DOI
              10.17487/RFC6066, January 2011,

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <>.

   [RFC7633]  Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
              Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
              October 2015, <>.

   [RFC7671]  Dukhovni, V. and W. Hardaker, "The DNS-Based
              Authentication of Named Entities (DANE) Protocol: Updates
              and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
              October 2015, <>.

12.2.  Informative References

   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
              September 2007, <>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <>.

   [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", RFC 7672, DOI
              10.17487/RFC7672, October 2015,


              Langley, A., "Serializing DNS Records with DNSSEC
              Authentication", draft-agl-dane-serializechain-01 (work in
              progress), July 2011, <

              Wouters, P., "Chain Query Requests 2011.

              Contavalli, C., Gaast, W., tale, t., and W. Kumari,
              "Client Subnet in DNS",
              query>. DNS Queries", draft-ietf-dnsop-edns-
              client-subnet-07 (work in progress), March 2016.

Appendix A.  Pseudocode example

   [code goes here]  Updates from -00

   o  Edits based on comments from Rick van Rein

   o  Warning about not overloading X.509 wildcards on DNSSEC wildcards
      (from V.  Dukhovny)

   o  Added MUST include negative proof on wildcards (from V.  Dukhovny)

   o  Removed "TODO" on allowing the server to deliver only one
      signature per RRset

   o  Added additional minor edits suggested by Viktor Dukhovny

Appendix B.  Test vector

   [data go here]

Authors' Addresses

   Melinda Shore
   No Mountain Software


   Richard Barnes


   Shumon Huque
   Verisign Labs


   Willem Toorop
   NLNet Labs