Internet Engineering Task Force                               D. Wessels
Internet-Draft                                                  Verisign Labs
Intended status: Standards Track                        December 9, 2015                               W. Kumari
Expires: June 11, September 10, 2016                                       Google
                                                           March 9, 2016

                        The EDNS Key Tag Option


   The DNS Security Extensions (DNSSEC) were developed to provide origin
   authentication and integrity protection for DNS data by using digital
   signatures.  These digital signatures can be verified by building a
   chain-of-trust starting from a trust anchor and proceeding down to a
   particular node in the DNS.  This document specifies a way for
   validating end-system resolvers to signal to a server which keys are
   referenced in their chain-of-trust.  The extensions allow zone
   administrators to monitor the progress of rollovers in a DNSSEC-
   signed zone.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on June 11, September 10, 2016.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Terminology . . . . . . . . . . . . . . . . . . . 3  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . . 3  4
   4.  Option Format  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Use By Queriers  . . . . . . . . . . . . . . . . . . . . . . . . 4  5
     5.1.  Stub Resolvers . . . . . . . . . . . . . . . . . . . . . .  5
       5.1.1.  Validating Stub Resolvers  . . . . . . . . . . . . . . . 5  6
       5.1.2.  Non-validating Stub Resolvers  . . . . . . . . . . . . .  6
     5.2.  Recursive Resolvers  . . . . . . . . . . . . . . . . . . . .  6
       5.2.1.  Validating Recursive Resolvers . . . . . . . . . . . .  6
       5.2.2.  Non-validating Recursive Resolvers . . . . . . . . . . 6  7
   6.  Use By Responders  . . . . . . . . . . . . . . . . . . . . . . . 6  7
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . .  7
   9.  Privacy Considerations . . . . . . . . . . . . . . . . . . . .  8
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  8
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8  9
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 8  9
     11.2. Informative References . . . . . . . . . . . . . . . . . .  9
   Author's Address
   Appendix A.  Changes / Author Notes. . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . 9 . . . . . . . . . . . . . . 10

1.  Introduction

   The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and
   [RFC4035] were developed to provide origin authentication and
   integrity protection for DNS data by using digital signatures.
   DNSSEC uses Key Tags to efficiently match signatures to the keys from
   which they are generated.  The Key Tag is a 16-bit value computed
   from the RDATA portion of a DNSKEY RR using a formula not unlike a
   ones-complement checksum.  RRSIG RRs contain a Key Tag field whose
   value is equal to the Key Tag of the DNSKEY RR that validates the

   Likewise, Delegation Signer (DS) RRs also contain a Key Tag field
   whose value is equal to the Key Tag of the DNSKEY RR to which it

   This draft sets out to specify a way for validating end-system
   resolvers to tell a server in a DNS query which DNSSEC key(s) they
   would use to validate the expected response.  This is done using the
   new EDNS option specified below in Section 4 for use in the OPT
   meta-RR [RFC6891].  This new EDNS option code is OPTIONAL to
   implement and use.

   This proposed EDNS option serves to measure the acceptance and use of
   new trust anchors and key signing keys (KSKs).  This signaling option
   can be used by zone administrators as a gauge to measure the
   successful deployment of new keys.  This is of particular interest
   for the DNS root zone in the event of key and/or algorithm rollovers
   which relies
   that rely on [RFC5011] to automatically update a validating end-
   system's trust anchor.

   [ FOR WG DISCUSSION: There is some reluctance within the working
   group to use EDNS0 to convey the key tags upstream.  In particular
   there is a concern that middleboxes might block messages with unknown
   option codes.  Also since EDNS0 is hop-by-hop, middleboxes and un-
   upgraded recursives won't necessarily know whether or not the edns-
   key-tag options should be forwarded.  RFC6891 says: "OPTION-CODE
   values not understood by a responder or requestor MUST be ignored."
   draft-wkumari-dnsop-trust-management proposed encoding this
   information in query names, but sufficient issues with this approach
   were discovered that the authors of the above decided to abandon this
   work.  The authors of draft-ietf-dnsop-edns-key-tag are willing to
   consider this alternative if so guided by the working group. ]

   This draft does not seek to introduce another process for rolling
   keys or updating trust anchors.  Rather, this document specifies a
   means by which a client query can signal the set of keys that a
   client uses for DNSSEC validation.

2.  Requirements Terminology

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

3.  Terminology

   Trust Anchor:  A configured DNSKEY RR or DS RR hash of a DNSKEY RR.
      A validating security-aware resolver uses this public key or hash
      as a starting point for building the authentication chain to a
      signed DNS response.  In general, a validating resolver will have
      to obtain the initial values of its trust anchors via some secure
      or trusted means outside the DNS protocol.  Presence of a trust
      anchor also implies that the resolver should expect the zone to
      which the trust anchor points to be signed. (quoted from [RFC4033]
      Section 2)

   Key Tag:  A 16-bit integer that identifies and enables efficient
      selection of DNSSEC public keys.  A Key Tag value can be computed
      over the RDATA of a DNSKEY RR.  The Key Tag field in the RRSIG and
      DS records can be used to help select the corresponding DNSKEY RR
      efficiently when more than one candidate DNSKEY RR is available.
      For most algorithms the Key Tag is a simple 16-bit modular sum of
      the DNSKEY RDATA.  See [RFC4034] Appendix B.

4.  Option Format

   The edns-key-tag option is encoded as follows:

   0                       8                      16
   |                  OPTION-CODE                  |
   |                 OPTION-LENGTH                 |
   |                    KEY-TAG                    |
   |                      ...                      /


   OPTION-CODE:   The EDNS0 option code assigned to edns-key-tag, [TBD].

   OPTION-LENGTH:   The value 2 x number of key-tag values present.

   KEY-TAG:   One or more 16-bit Key Tag values ([RFC4034], Appendix B).

5.  Use By Queriers

   A validating end-system resolver sets the edns-key-tag option in the
   OPT meta-RR when sending a DNSKEY query.  The validating end-system
   resolver SHOULD also set the DNSSEC OK bit [RFC4034] to indicate that
   it wishes to receive DNSSEC RRs in the response.

   A DNS client MUST NOT include the edns-key-tag option for non-DNSKEY

   The KEY-TAG value(s) included in the edns-key-tag option represent
   the Key Tag of the Trust Anchor or DNSKEY RR that will be used to
   validate the expected response.  When the client sends a DNSKEY
   query, the edns-key-tag option represents the Key Tag(s) of the
   KSK(s) of the zone for which the server is authoritative.  A
   validating end-system resolver learns the Key Tag(s) of the KSK(s)
   from the zone's DS record(s) (found in the parent), or from a
   configured trust anchor.

   A DNS client SHOULD include the edns-key-tag option when issuing a
   DNSKEY query for a zone corresponding to a configured Trust Anchor.

   A DNS client MAY include the edns-key-tag option when issuing a
   DNSKEY query for a non-Trust Anchor zone (i.e., Key Tags learned via
   DS records).  Since some DNSSEC validators implement bottom-up
   validation, non-Trust Anchor Key Tags zone might not be known at the
   time of the query.  Such a validator can include the edns-key-tag
   option based on previously cached data.

   A DNS client MUST NOT include Key Tag(s) for keys which are not
   learned via either configured Trust Anchor or DS records.

   Since the edns-key-tag option is only set in the query, if a client
   sees these options in the response, no action needs to be taken and
   the client MUST ignore the option values.

5.1.  Stub Resolvers

   Typically, stub resolvers rely on an upstream recursive server (or
   cache) to provide a response.  Optimal setting of the edns-key-tag
   option depends on whether the stub resolver elects to perform its own

5.1.1.  Validating Stub Resolvers

   A validating stub resolver sets the DNSSEC OK (DO) bit [RFC4034] to
   indicate that it wishes to receive additional DNSSEC RRs (i.e., RRSIG
   RRs) in the response.  Such validating resolvers SHOULD include the
   edns-key-tag option in the OPT RR when sending a DNSKEY query.

5.1.2.  Non-validating Stub Resolvers

   The edns-key-tag option MUST NOT be included by non-validating stub

5.2.  Recursive Resolvers

5.2.1.  Validating Recursive Resolvers

   A validating recursive resolver sets is, by definition, configured with at
   least one trust anchor.  Thus, a recursive resolver SHOULD include
   the edns-key-tag option when
   performing recursion based on relevant keys it knows and any edns-
   key-tag values in its DNSKEY queries as described above.

   In addition, the stub client query. clients of a validating recursive resolver might be
   configured to do their own validation, with their own trust
   anchor(s).  When the a validating recursive server resolver receives a query with the option set, the recursive server SHOULD set
   that includes the edns-key-tag option with a Key Tag list for any outgoing iterative queries for that
   resolution chain to a union of
   differs from its own, it SHOULD forward both the stub client's Key Tag(s) and Tag
   list as well as its own.  When doing so, the
   validating recursive resolver's resolver
   SHOULD transmit the two Key Tag(s). Tag lists using separate instances of the
   edns-key-tag option code in the OPT meta-RR.  For example, if the
   recursive resolver's Key Tag list is (19036, 12345) and the stub's stub/
   client's list is (19036, 34567), the final edns-key-tag list recursive would be include the
   edns-key-tag option twice: Once with values (19036, 12345) and once
   with values (19036,
   12345, 34567).

   A validating recursive resolver MAY combine stub client stub/client Key Tag
   values from multiple incoming queries into a single outgoing query.
   It is RECOMMENDED that implementations place reasonable limits on the
   number of Key Tags to include in the outgoing edns-key-tag option.

   If the client included the DO and Checking Disabled (CD) bits, but
   did not include the edns-key-tag option in the query, the validating
   recursive resolver MAY include the option with its own Key Tag values
   in full.

   Validating recursive resolvers MUST NOT set the edns-key-tag option
   in the final response to the stub client.

5.2.2.  Non-validating Recursive Resolvers

   Recursive resolvers that do not validate responses SHOULD copy the
   edns-key-tag option seen in received queries, as they represent the
   wishes of the validating downstream resolver that issued the original

6.  Use By Responders

   An authoritative name server receiving queries with the edns-key-tag
   option MAY log or otherwise collect the Key Tag values to provide
   information to the zone operator.

   A responder MUST NOT include the edns-key-tag option in any DNS

7.  IANA Considerations

   The IANA is directed to assign an EDNS0 option code for the edns-key-
   tag option from the DNS EDNS0 Option Codes (OPT) registry as follows:

           | Value | Name         | Status   | Reference       |
           | [TBA] | edns-key-tag | Optional | [This document] |

8.  Security Considerations

   This document specifies a way for a client to signal its trust anchor
   knowledge to a cache or server.  The signals are optional codes
   contained in the OPT meta-RR used with EDNS.  The goal of these
   options is to signal new trust anchor uptake in clients to allow zone
   administrators to know when it is possible to complete a key rollover
   in a DNSSEC-signed zone.

   There is a possibility that an eavesdropper or server could infer the
   validator in use by a client by the Key Tag list seen in queries.
   This may allow an attacker to find validators using old, possibly
   broken, keys.  It could also be used to identify the validator or
   narrow down the possible validator implementations in use by a
   client, which could have a known vulnerability that could be
   exploited by the attacker.

   Consumers of data collected from the edns-key-tag option are advised
   that provided Key Tag values might be "made up" by some DNS clients
   with malicious or at least mischievous intentions.  For example, an
   attacker with sufficient resources might try to generate large
   numbers of queries including only old Key Tag values, with the
   intention of delaying the completion of a key rollover.

   DNSSEC does not require keys in a zone to have unique Key Tags.
   During a rollover there is a small possibility that an old key and a
   new key will have identical Key Tag values.  Zone operators relying
   on the edns-key-tag mechanism SHOULD take care to ensure that new
   keys have unique Key Tag values.

9.  Privacy Considerations

   This proposal adds additional additional, optional "signaling" to DNS queries in
   the form of Key Tag values.  While Key Tag values themselves are not
   considered private information, it may be possible for an
   eavesdropper to use Key Tag values as a fingerprinting technique to
   identify particular DNS validating clients.  This may be especially
   true if the validator is configured with trust anchor for zones in
   addition to the root zone.

   A validating end-system resolver need not transmit the edns-key-tag
   option in every applicable query.  Due to privacy concerns, such a
   resolver MAY choose to transmit the edns-key-tag option for a subset
   of queries (e.g., every 25th time), or by random chance with a
   certain probability (e.g., 5%).

   Implementations of this specification MAY be administratively
   configured to only transmit the edns-key-tag option for certain
   zones.  For example, the software's configuration file may specify a
   list of zones for which use of the option is allowed or denied.
   Since the primary motivation for this specification is to provide
   operational measurement data for root zone key rollovers, it is
   RECOMMENDED that implementations at least include the edns-key-tag
   option for root zone DNSKEY queries.

10.  Acknowledgments

   This document was inspired by and borrows heavily from [RFC6975] by
   Scott Rose and Steve Crocker.  The author authors would like to thank to Casey Deccio and
   Deccio, Burt Kalisky Kalisky, Bob Harold, Tim Wicinski, Suzanne Woolf, and
   other members of the dnsop working group for early feedback. their input.

11.  References
11.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [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,

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [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,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/
              RFC6891, April 2013,

11.2.  Informative References

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

   [RFC6975]  Crocker, S. and S. Rose, "Signaling Cryptographic
              Algorithm Understanding in DNS Security Extensions
              (DNSSEC)", RFC 6975, DOI 10.17487/RFC6975, July 2013,

Author's Address

Appendix A.  Changes / Author Notes.

   [RFC Editor: Please remove this section before publication]

   From -00 to -01:

   o  Changed how a recursive should combine a stub's key tag values
      with its own.  Previously it was to be a union of key tag values.
      Now it is a separate instance of the option code for recursive and

   o  Added Warren as coauthor.

Authors' Addresses

   Duane Wessels
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190
   United States

   Phone: +1 703 948-3200

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   United States