A Sentinel for Detecting Trusted Keys in DNSSEC
draft-ietf-dnsop-kskroll-sentinel-00

DNSOP                                                          G. Huston
Internet-Draft                                                  J. Damas
Intended status: Standards Track                                   APNIC
Expires: June 14, 2018                                         W. Kumari
                                                                  Google
                                                       December 11, 2017

            A Sentinel for Detecting Trusted Keys in DNSSEC
                draft-ietf-dnsop-kskroll-sentinel-00.txt

Abstract

   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 mechanism that
   will allow an end user to determine the trusted key state of the
   resolvers that handle the user's DNS queries.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on June 14, 2018.

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   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Sentinel Mechanism  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Sentinel Processing . . . . . . . . . . . . . . . . . . . . .   4
   4.  Sentinel Test Result Considerations . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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
   signature.

   This document specifies how validating resolvers can respond to
   certain queries in a manner that allows a querier to deduce whether a
   particular key has been loaded into that resolver's trusted key
   store.  In particular, this response mechanism can be used to
   determine whether a certain Root Zone KSK is ready to be used as a
   trusted key within the context of a key roll by this resolver.

   This new mechanism is OPTIONAL to implement and use, although for
   reasons of supporting broad-based measurement techniques, it is
   strongly preferred if configurations of DNSSEC-validating resolvers
   enabled this mechanism by default, allowing for local configuration
   directives to disable this mechanism if desired.

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1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

2.  Sentinel Mechanism

   DNSSEC-Validating resolvers that implement this mechanism MUST be
   performing validation of responses in accordance with the DNSSEC
   response validation specification [RFC4035].

   This sentinel mechanism makes use of 2 special labels, "_is-ta-<tag-
   index>." (intended to be used in a query where the response can
   answer the question: Is this the key tag a trust anchor which the
   validating DNS resolver is currently trusting?) and "_not-ta-<tag-
   index>." (intended to be used in a query where the response can
   answer the question: Is this the key tag of a key that is NOT in the
   resolver's current trust store?).  The use of the positive question
   and its inverse allows for queries to detect whether resolvers
   support this sentinel mechanism.

   If the outcome of the DNSSEC validation process on the response RRset
   indicates that the response RRset is authentic, and if the left-most
   label of the original query name matches the template "_is-ta-<tag-
   index>.", then the following rule should be applied to the response:
   If the resolver has placed a Root Zone Key Signing Key with tag index
   value matching the value specified in the query into the local
   resolver's store of trusted keys, then the resolver should return a
   response indicating that the response contains authenticated data
   according to section 5.8 of [RFC6840].  Otherwise, the resolver MUST
   return RCODE 2 (server failure).  Note that the <tag-index> is
   specified in the DNS label using hexadecimal notation.

   If the outcome of the DNSSEC validation process aplied to the
   response RRset indicates that the response RRset is authentic, and if
   the left-most label of the original query name matches the template
   "_not-ta-<tag-index>.", then the following rule should be applied to
   the response: If the resolver has not placed a Root Zone Key Signing
   Key with tag index value matching the value specified in the query
   into the local resolver's store of trusted keys, then the resolver
   should return a response indicating that the response contains
   authenticated data according to section 5.8 of [RFC6840].  Otherwise,
   the resolver MUST return RCODE 2 (server failure).  Note that the
   <tag-index> is specified in the DNS label using hexadecimal notation.

   In all other cases the resolver MUST NOT alter the outcome of the DNS
   response validation process.

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   This mechanism is to be applied only by resolvers that are performing
   DNSSEC validation, and applies only to RRset responses to an A or
   AAAA query (Query Type value 1 or 28) where the resolver has
   authenticated the response RRset according to the DNSSEC validation
   process and where the query name contains either of the labels
   described in this section as its left-most label.  In this case, the
   resolver is to perform an additional test following the conventional
   validation function, as described in this section.  The result of
   this additional test determines whether the resolver will alter its
   response that would've indicated that the RRset is authentic to a
   response that indicates DNSSEC validation failure via the use of
   RCODE 2.

3.  Sentinel Processing

   This proposed test that uses the sentinel detection mechanism
   described in this document is based on the use of three DNS names
   that have three distinct DNS resolution behaviours.  The test is
   intended to allow a user to determine the state of their DNS
   resolution system, and, in particular, whether or not they are using
   validating DNS resolvers that have picked up an incoming trust anchor
   as a trusted key in a root zone KSK roll scenario.

   The name format can be defined in a number of ways, and no name form
   is intrinsically better than any other in terms of the test itself.
   The critical aspect of the DNS names used in any such test is that
   they contain the specified label for either the positive and negative
   test as the left-most label in the query name.

   The sentinel detection process is envisaged to use a test with three
   query names:

   a.  a query name containing the left-most label "_is-ta-<tag-
       index>.".  This corresponds to a a validly-signed RRset in the
       zone, so that responses associated with queried names in this
       zone can be authenticated by a DNSSEC-validating resolver.  Any
       validly-signed DNS zone can be used for this test.

   b.  a query name containing the left-most label "_not-ta-<tag-
       index>.".  This is also a validly-signed name.  Any validly-
       signed DNS zone can be used for this test.

   c.  a third query name that is signed with a DNSSEC signature that
       cannot be validated (i.e. the corresponding RRset is not signed
       with a valid RRSIG record).

   The responses received from queries to resolve each of these names
   would allow us to infer a trust key state of the resolution

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   environment.  To describe this process of classification, we can
   classify resolvers into four distinct behavior types, for which we
   will use the labels: "Vnew", "Vold", "Vleg", and "nonV".  These
   labels correspond to resolver behaviour types as follows:

   o  Vnew: A DNSSEC-Validating resolver that is configured to implement
      this mechanism has loaded the nominated key into its local trusted
      key store will respond with an A or AAAA RRset response for "_is-
      ta" queries, SERVFAIL for "_not-ta" queries and SERVFAIL for the
      invalidly signed name queries.

   o  Vold: A DNSSEC-Validating resolver that is configured to implement
      this mechanism that has not loaded the nominated key into its
      local trusted key store will respond with an SERVFAIL for "_is-ta"
      queries, an A or AAAA RRset response for "_not-ta" queries and
      SERVFAIL for the invalidly signed name queries.

   o  Vleg: A DNSSEC-Validating resolver that does not implement this
      mechanism will respond with an A or AAAA RRSET response for "_is-
      ta", an A record response for "_not-ta" and SERVFAIL for the
      invalid name.

   o  nonV: A non-DNSSEC-Validating resolver will respond with an A
      record response for "_is-ta", an A record response for "_not-ta"
      and an A record response for the invalid name.

   Given the clear delineation amongst these three cases, if a client
   directs these three queries to a simple resolver, the variation in
   response to the three queries should allow the client to determine
   the category of the resolver, and if it supports this mechanism,
   whether or not it has loaded a particular key into its local trusted
   key stash.

      +-------------+----------+-----------+------------+
      | Type\Query  |  _is-ta  |  _not-ta  |  invalid   |
      +-------------+----------+-----------+------------+
      | Vnew        |    A     |  SERVFAIL |  SERVFAIL  |
      | Vold        | SERVFAIL |      A    |  SERVFAIL  |
      | Vleg        |    A     |      A    |  SERVFAIL  |
      | nonV        |    A     |      A    |     A      |
      +-------------+----------+-----------+------------+

   A "Vnew" response pattern says that the nominated key is trusted by
   the resolver and has been loaded into its local trusted key stash.  A
   "Vold" response pattern says that the nominated key is not yet
   trusted by the resolver in its own right.  A "Vleg" response pattern

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   is indeterminate, and a "nonV" response pattern indicates that the
   resolver does not perform DNSSEC validation.

4.  Sentinel Test Result Considerations

   The description in the previous section describes a simple situation
   where the test queries were being passed to a single recursive
   resolver that directly queried authoritative name servers, including
   the root servers.

   There is also the common case where the end client is configured to
   use multiple resolvers.  In these cases the SERVFAIL responses from
   one resolver will prompt the end client to repeat the query against
   one of the other configured resolvers.

   If any of the client's resolvers are non-validating resolvers, the
   tests will result in the client reporting that it has a non-
   validating DNS environment ("nonV"), which is effectively the case.

   If all of the client resolvers are DNSSEC-validating resolvers, but
   some do not support this trusted key mechanism, then the result will
   be indeterminate with respect to trusted key status ("Vleg").
   Simlarly, if all the client's resolvers support this mechanism, but
   some have loaded the key into the trusted key stash and some have
   not, then the result is indeterminate ("Vleg").

   There is also the common case of a recursive resolver using a
   forwarder.

   If the resolver is non-validating, and it has a single forwarder
   clause, then the resolver will presumably mirror the capabilities of
   the forwarder target resolver.  If this non-validating resolver it
   has multiple forwarders, then the above considerations will apply.

   If the validating resolver has a forwarding configuration, and uses
   the CD flag on all forwarded queries, then this resolver is acting in
   a manner that is identical to a standalone resolver.  The same
   consideration applies if any one one of the forwarder targets is a
   non-validating resolver.  Similarly, if all the forwarder targets do
   not apply this trusted key mechanism, the same considerations apply.

   A more complex case is where the following conditions all hold:

      both the validating resolver and the forwarder target resolver
      support this trusted key sentinel mechanism, and

      the local resolver's queries do not have the CD bit set, and

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      the trusted key state differs between the forwarding resolver and
      the forwarder target resolver

   then either the outcome is indeterminate validating ("Vleg"), or a
   case of mixed signals (SERVFAIL in all three responses), which is
   similarly an indeterminate response with respect to the trusted key
   state.

5.  Security Considerations

   This document describes a mechanism to allow users to determine the
   trust state of root zone key signing keys in the DNS resolution
   system that they use.

   The mechanism does not require resolvers to set otherwise
   unauthenticated responses to be marked as authenticated, and does not
   alter the security properties of DNSSEC with respect to the
   interpretation of the authenticity of responses that are so marked.

   The mechanism does not require any further significant processing of
   DNS responses, and queries of the form described in this document do
   not impose any additional load that could be exploited in an attack
   over the the normal DNSSEC validation processing load.

6.  IANA Considerations

   [Note to IANA, to be removed prior to publication: there are no IANA
   considerations stated in this version of the document.]

7.  Acknowledgements

   This document has borrowed extensively from [RFC8145] for the
   introductory text, and the authors would like to acknowledge and
   thank the authors of that document both for some text excerpts and
   for the more general stimulation of thoughts about monitoring the
   progress of a roll of the Key Signing Key of the Root Zone of the
   DNS.

8.  References

8.1.  Normative References

   [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,
              <https://www.rfc-editor.org/info/rfc4033>.

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   [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,
              <https://www.rfc-editor.org/info/rfc4034>.

   [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,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC6840]  Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
              Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
              DOI 10.17487/RFC6840, February 2013,
              <https://www.rfc-editor.org/info/rfc6840>.

8.2.  Informative References

   [RFC8145]  Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
              Anchor Knowledge in DNS Security Extensions (DNSSEC)",
              RFC 8145, DOI 10.17487/RFC8145, April 2017,
              <https://www.rfc-editor.org/info/rfc8145>.

Authors' Addresses

   Geoff Huston

   Email: gih@apnic.net
   URI:   http://www.apnic.net

   Joao Silva Damas

   Email: joao@apnic.net
   URI:   http://www.apnic.net

   Warren Kumari

   Email: warren@kumari.net

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