A Sentinel for Detecting Trusted Keys in DNSSEC
draft-ietf-dnsop-kskroll-sentinel-02

DNSOP                                                          G. Huston
Internet-Draft                                                  J. Damas
Intended status: Standards Track                                   APNIC
Expires: August 25, 2018                                       W. Kumari
                                                                  Google
                                                       February 21, 2018

            A Sentinel for Detecting Trusted Keys in DNSSEC
                  draft-ietf-dnsop-kskroll-sentinel-02

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 for the
   root key of the resolvers that handle that user's DNS queries.  Note
   that this method is only applicable for determing which keys are in
   the trust store for the root key.

   There is an example / toy implementation of this at http://www.ksk-
   test.net .

   [ This document is being collaborated on in Github at:
   https://github.com/APNIC-Labs/draft-kskroll-sentinel.  The most
   recent version of the document, open issues, etc should all be
   available here.  The authors (gratefully) accept pull requests.  Text
   in square brackets will be removed before publication. ]

   [ NOTE: This version uses the labels "kskroll-sentinel-is-ta-<tag-
   index>", "kskroll-sentinel-not-ta-<tag-index>"; older versions of
   this document used "_is-ta-<tag-index>", "_not-ta-<tag-index>".  ]

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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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 August 25, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Use Case  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Sentinel Mechanism  . . . . . . . . . . . . . . . . . . . . .   6
   4.  Sentinel Processing . . . . . . . . . . . . . . . . . . . . .   7
   5.  Sentinel Test Result Considerations . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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.

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   This document specifies how validating resolvers can respond to
   certain queries in a manner that allows a querier to deduce whether a
   particular key for the root 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.

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.

   Note that example.com, AAAA records and the IPv6 documentation prefix
   (2001:db8::/32) are only examples - A records (or CNAMES), other IPs,
   other domains work just as well.

2.  Use Case

   [Ed note: This is currently towards the front of the document; we
   will make it an appendix at publication time, but until then it is
   worth having up front, as it makes the rest of the document much
   easier to understand ]

   This section provides a non-normative example of how the sentinel
   mechanism could be used, and what each participant does.  It is
   provided in a conversational tone to be easier to follow.

   Alice is in charge of the DNS root KSK (Key Signing Key), and would
   like to roll / replace the key with a new one.  She publishes the new
   KSK, but would like to be able to predict / measure what the impact
   will be before removing/revoking the old key.  The current KSK has a
   key ID of 1111, the new KSK has a key ID of 2222

   Bob, Charlie, Dave, Ed are all users.  They use the DNS recursive
   resolvers supplied by their ISPs.  They would like to confirm that
   their ISPs have picked up the new KSK and will not break.  Bob's ISP
   does not perform validation.  Charlie's ISP does validate, but the
   resolvers have not yet been upgraded to support sentinel.  Dave and
   Ed's resolvers have been upgraded to support sentinel; Dave's
   resolver has the new KSK, Ed's resolver hasn't managed to install the
   2222 KSK in its trust store yet.

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   Geoff is a researcher, and would like to both provide a means for
   Bob, Charlie, Dave and Ed to be able to perform tests, and also would
   like to be able to perform Internet wide measurements of what the
   impact will be (and report this back to Alice).

   Geoff sets an authoritative DNS server for example.com, and also a
   webserver (www.example.com).  He adds 3 AAAA records to example.com:

      invalid.example.com.  IN AAAA 2001:db8::1

      kskroll-sentinel-is-ta-2222.example.com.  IN AAAA 2001:db8::1

      kskroll-sentinel-not-ta-2222.example.com.  IN AAAA 2001:db8::1

   Geoff then DNSSEC signs the example.com zone, and intentionally makes
   the invalid.example.com record invalid/bogus (for example, by editing
   the signed zone and entering garbage for the signature).  Geoff also
   configures his webserver to listen on 2001:db8::1 and serve a
   resource (for example, a 1x1 GIF, 1x1.gif) for all of these names.
   The webserver also serves a webpage (www.example.com) which contains
   links to these 3 resources (http://invalid.example.com/1x1.gif,
   http://kskroll-sentinel-is-ta-2222.example.com/1x1.gif,
   http://kskroll-sentinel-not-ta-2222.example.com/1x1.gif).

   Geoff then asks Bob, Charlie, Dave and Ed to browse to
   www.example.com.  Using the methods described in this document, the
   users can figure out what their fate will be when the 1111 KSK is
   removed.

   Bob is not using a validating resolver.  This means that he will be
   able to resolve invalid.example.com (and fetch the 1x1 GIF) - this
   tells him that the KSK roll does not affect him, and so he will be
   OK.

   Charlie's resolvers are validating, but they have not been upgraded
   to support the KSK sentinel mechanism.  Charlie will not be able to
   fetch the http://invalid.example.com/1x1.gif resource (the
   invalid.example.com record is bogus, and none of his resolvers will
   resolve it).  He is able to fetch both of the other resources - from
   this he knows (see the logic below) that he is using legacy,
   validating resolvers.  The KSK sentinel method cannot provided him
   with a definitive answer.

   Dave's resolvers implement the sentinel method, and have picked up
   the new KSK.  For the same reason as Charlie, he cannot fetch the
   "invalid" resource.  His resolver resolves the kskroll-sentinel-is-
   ta-2222.example.com name normally (it contacts the example.com
   authoritative servers, etc); as it supports the sentinel mechanism,

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   just before Dave's recursive server send the reply to Dave's stub, it
   performs the KSK Sentinel check (see below).  The QNAME starts with
   "kskroll-sentinel-is-ta-", and the recursive resolver does indeed
   have a key with the Key ID of 2222 in its root trust store.  This
   means that that this part of the KSK Sentinel check passes (it is
   true that 2222 is in the Trust Anchor store), and the recursive
   resolver replies normally (with the answer provided by the
   authoritative server).  Dave's recursive resolver then resolves the
   kskroll-sentinel-not-ta-2222.example.com name.  Once again, it
   performs the normal resolution process, but because it implements KSK
   Sentinel (and the QNAME starts with "kskroll-sentinel-not-ta-"), just
   before sending the reply, it performs the KSK Sentinel check.  As it
   has 2222 in it's trust anchor store, the "Is this *not* a trust
   anchor" is false, and so the recursive resolver does not reply with
   the answer from the authoritative server - instead, it replies with a
   SERVFAIL (note that replying with SERVFAIL instead of the original
   answer is the only mechanism that KSK Sentinel uses).  This means
   that Dave cannot fetch "invalid", he can fetch "kskroll-sentinel-is-
   ta-2222", but he cannot fetch "kskroll-sentinel-not-ta-2222".  From
   this, Dave knows that he is behind an upgraded, validating resolver,
   which has successfully installed the new, 2222 KSK.  Dave has nothing
   to worry about - he will be fine with the old (1111) KSK is removed.

   Now for Ed.  Just like Charlie and Dave, Ed cannot fetch the
   "invalid" record.  This tells him that his resolvers are validating.
   When his (upgraded) resolver performs the KSK Sentinel check for
   "kskroll-sentinel-is-ta-2222", it does *not* have the (new, 2222) KSK
   in it's trust anchor store.  This means check fails, and Ed's
   recursive resolver converts the (valid) 2001:db8::1 answer into a
   SERVFAIL error response.  It performs the same check for kskroll-
   sentinel-not-ta-2222.example.com; as it does not have the 2222 KSK,
   it is true that this is not a trust anchor for it, and so it replies
   normally.  This means that Ed cannot fetch the "invalid" resource, he
   also cannot fetch the "kskroll-sentinel-is-ta-2222" resource, but he
   can fetch the "kskroll-sentinel-not-ta-2222" resource.  This tells Ed
   that his resolvers have not installed the new KSK, and, when the old
   KSK is removed, his DNS will break.

   Geoff would like to do a large scale test and provide the information
   back to Alice.  He uses some mechanism (such as an advertising
   network) to cause a large number of users to attempt to resolve the 3
   resources, and then analyzes the results of the tests to determine
   what percentage of users will be affected by the KSK rollover event.

   The above description is a simplified example - it is not anticipated
   that Bob, Charlie, Dave and Ed will actually look for the absence or
   presence of web resources; instead, the webpage that they load would
   likely contain JavaScript (or similar) which displays the result of

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   the tests.  An example of this is at http://www.ksk-test.net.  This
   KSK mechanism does not rely on the web - this method can equally be
   used by trying to resolve the names (for example, using 'dig') and
   checking which result in a SERVFAIL.

   [ Note that the KSK Sentinel mechanism measures a very different
   (and, in our opinion, much more useful!) metric than RFC8145 --
   RFC8145 relied on resolvers reporting the list of keys that they have
   -- this doesn't reflect what the *user* impact of the KSK roll will
   be.  As we cannot get perfect visibility into all resolvers, we will
   have to aim for "do the least harm", not "do no harm" ]

3.  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, "kskroll-
   sentinel-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
   "kskroll-sentinel-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.  Note that
   the test is "Is there an active key with this KeyID in the resolver's
   current trust store for the DNS root?", not "Is there any key with
   this KeyID in the trust store", nor "Was a key with this KeyID used
   to validate this query?".  An active key is one which could currently
   be used for validation (ie not in AddPend or Revoked state
   ([RFC5011])).

   If the outcome of the DNSSEC validation process on the response
   indicates that the response is authentic, and if the left-most label
   of the original query name matches the template "kskroll-sentinel-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 applied to the
   response indicates that the response is authentic, and if the left-

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   most label of the original query name matches the template "kskroll-
   sentinel-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.

   This mechanism is to be applied only by resolvers that are performing
   DNSSEC validation, and applies only to responses to an A or AAAA
   query (Query Type value 1 or 28) where the resolver has authenticated
   the response 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
   have indicated that the RRset is authentic to a response that
   indicates DNSSEC validation failure via the use of RCODE 2.

4.  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 "kskroll-sentinel-is-
       ta-<tag-index>.".  This corresponds to a a validly-signed RRset
       in the zone, so that responses associated with queried names in

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       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 "kskroll-sentinel-
       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
   environment.  The techniques describes in this document rely on
   (DNSSEC validating) resolvers responding with SERVFAIL (RCODE 2) to
   valid answers.  Note that a slew of other issues can also cause
   SERVFAIL responses, so false positive or negative results may
   sometimes occur.  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
      "kskroll-sentinel-is-ta" queries, SERVFAIL for "kskroll-sentinel-
      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
      "kskroll-sentinel-is-ta" queries, an A or AAAA RRset response for
      "kskroll-sentinel-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
      "kskroll-sentinel-is-ta", an A record response for "kskroll-
      sentinel-not-ta" and SERVFAIL for the invalid name.

   o  nonV: A non-DNSSEC-Validating resolver will respond with an A or
      AAAA record response for "kskroll-sentinel-is-ta", an A record
      response for "kskroll-sentinel-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

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

5.  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.

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   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:

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

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

   o  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.

6.  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.

7.  IANA Considerations

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

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8.  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.

   The authors would like the especially thank Joe Abley, Mehmet Akcin,
   Mark Andrews, Richard Barnes, Ray Bellis, Stephane Bortzmeyer, David
   Conrad, Ralph Dolmans, John Dickinson, Steinar Haug, Bob Harold, Wes
   Hardaker, Paul Hoffman, Matt Larson, Jinmei Tatuya, Edward Lewis,
   George Michaelson, Benno Overeinder, Matthew Pounsett, Andreas
   Schulze, Mukund Sivaraman, Petr Spacek.  Andrew Sullivan, Paul Vixie,
   Duane Wessels and Paul Wouters for their helpful feedback.

   [TODO: Add people who have contributed!]

9.  Change Log

   Note that this document is being worked on in GitHub - see Abstract.
   The below is mainly large changes, and is not authoritative.

   From -01 to 02:

      Removed Address Record definition.

      Clarified that many things can cause SERVFAIL.

      Made examples FQDN.

      Fixed a number of typos.

      Had accidentally said that Charlie was using a non-validating
      resolver in example.

      [ TODO(WK): Doc says keytags are hex, is this really what the WG
      wants? ]

      And active key is one that can be used *now* (not e.g AddPend)

   From -00 to 01:

   o  Added a conversational description of how the system is intended
      to work.

   o  Clarification that this is for the root.

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   o  Changed the label template from _is-ta-<tag> to kskroll-sentinel-
      is-ta-<tag-index>.  This is because BIND (at least) will not allow
      records which start with an underscore to have address records
      (CNAMEs, yes, A/AAAA no).  Some browsers / operating systems also
      will not fetch resources from names which start with an
      underscore.

10.  References

10.1.  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>.

   [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>.

   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
              September 2007, <https://www.rfc-editor.org/info/rfc5011>.

   [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>.

10.2.  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

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   Joao Silva Damas

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

   Warren Kumari

   Email: warren@kumari.net

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