A Sentinel for Detecting Trusted Keys in DNSSEC

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Document Type Active Internet-Draft (dnsop WG)
Authors Geoff Huston  , Joao Damas  , Warren Kumari 
Last updated 2018-03-05
Replaces draft-huston-kskroll-sentinel
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DNSOP                                                          G. Huston
Internet-Draft                                                  J. Damas
Intended status: Standards Track                                   APNIC
Expires: September 6, 2018                                     W. Kumari
                                                           March 5, 2018

            A Sentinel for Detecting Trusted Keys in DNSSEC


   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 and third parties 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-<key-
   tag>", "kskroll-sentinel-not-ta-<key-tag>"; older versions of this
   document used "_is-ta-<key-tag>", "_not-ta-<key-tag>".  Also note
   that the format of the tag-index is now zero-filled decimal.
   Apolgies to those who have began implmenting.]

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/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 September 6, 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
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   publication of this document.  Please review these documents
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   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.  Protocol Walkthrough Example  . . . . . . . . . . . . . . . .   3
   3.  Sentinel Mechanism in Resolvers . . . . . . . . . . . . . . .   6
     3.1.  Preconditions . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Special processing  . . . . . . . . . . . . . . . . . . .   7
   4.  Processing Sentinel Results . . . . . . . . . . . . . . . . .   8
   5.  Sentinel Test Result Considerations . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   10. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     11.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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

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   which they are generated.  The Key Tag is a 16-bit value computed
   from the RDATA portion of a DNSKEY RR using a formula similar to 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

   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.

   There are two primary use cases for this mechanism:

   o  Users want to know whether the resolvers they use are ready for an
      upcoming root KSK rollover

   o  Researchers want to perform Internet-wide studies about the
      percentage of users who will be ready for an upcoming root KSK

   The mechanism described in this document meets both of these use
   cases.  This new mechanism is OPTIONAL to implement and use, although
   for reasons of supporting broad-based measurement techniques, it is
   strongly preferred that 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",
   document are to be interpreted as described in RFC 2119.

2.  Protocol Walkthrough Example

   [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

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   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 Tag of 11112, the new KSK has a Key Tag of 02323.  Users want to
   verify that their resolver will not break after Alice rolls the root
   KSK key (that is, starts signing with just the KSK whose Key Tag is

   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.  Bob's ISP does not perform
   validation.  Charlie's ISP does validate, but the resolvers have not
   yet been upgraded to support this mechanism.  Dave and Ed's resolvers
   have been upgraded to support this mechanism; Dave's resolver has the
   new KSK, Ed's resolver hasn't managed to install the 02323 KSK in its
   trust store yet.

   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 three address records to

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

      kskroll-sentinel-is-ta-02323.example.com.  IN AAAA 2001:db8::1

      kskroll-sentinel-not-ta-02323.example.com.  IN AAAA 2001:db8::1

   Note that the use of "example.com" names and the addresses here are
   examples.  In a real deployment, the domain names need to be under
   control of the researcher, and the addresses much be real, reachable

   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,

   Geoff then asks Bob, Charlie, Dave and Ed to browse to
   www.example.com.  Using the methods described in this document, the

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   users can figure out what their fate will be when the 11112 KSK is

   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

   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 to the question of what root trust anchors
   this resolver is using.

   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-02323.example.com name normally (it contacts the example.com
   authoritative servers, etc); as it supports the sentinel mechanism,
   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 Tag of 02323 in its root trust store.  This
   means that that this part of the KSK Sentinel check passes (it is
   true that Key Tag 02323 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-02323.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 02323 in it's trust anchor store, the answer to "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-02323", but he cannot fetch "kskroll-sentinel-not-ta-
   02323".  From this, Dave knows that he is behind an upgraded,
   validating resolver, which has successfully installed the new, 02323

   Just like Charlie and Dave, Ed cannot fetch the "invalid" record.
   This tells him that his resolvers are validating.  When his

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   (upgraded) resolver performs the KSK Sentinel check for "kskroll-
   sentinel-is-ta-02323", it does *not* have the (new, 02323) KSK in
   it's trust anchor store.  This means check fails, and Ed's recursive
   resolver converts the (valid) answer into a SERVFAIL error response.
   It performs the same check for kskroll-sentinel-not-ta-
   02323.example.com; as it does not have the 02323 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-02323" resource, but he can fetch
   the "kskroll-sentinel-not-ta-02323" resource.  This tells Ed that his
   resolvers have not installed the new KSK.

   Geoff would like to do a large scale test and provide the information
   back to Alice.  He uses some mechanism such as causing users to go to
   a web page to cause a large number of users to attempt to resolve the
   three 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
   the tests, sends the results to Geoff, or both.  This sentinel
   mechanism does not rely on the web: it can equally be used by trying
   to resolve the names (for example, using the common "dig" command)
   and checking which result in a SERVFAIL.

   Note that the sentinel mechanism described here measures a very
   different (and likely more useful) metric than [RFC8145].  RFC 8145
   relies on resolvers reporting the list of keys that they have to root
   servers.  That reflects on how many resolvers will be impacted by a
   KSK roll, but not what the user impact of the KSK roll will be.

3.  Sentinel Mechanism in Resolvers

   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 two special labels:

   o  kskroll-sentinel-is-ta-<key-tag>

   o  kskroll-sentinel-not-ta-<key-tag>

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   Note that the <key-tag> is specified in the DNS label as unsigned
   decimal integer (as described in [RFC4034], section 5.3), but zero-
   padded to five digits (i.e: 42 would be represented as 00042).

   These labels trigger special processing in the resolver as specified
   bellow.  The labels containing "is-ta" and "not-ta" are used to
   answer questions "Is (or is not, respectivaly) this the key tag a
   trust anchor which the validating DNS resolver is currently
   trusting?"  Processing of both labels has the very same preconditions
   for both labels and differs only in last step.

   The use of the positive question and its inverse allows for queries
   to detect whether resolvers support this sentinel mechanism.

3.1.  Preconditions

   All of the following conditions must be met to trigger special
   processing inside resolver code:

   a.  DNS response for particular query is DNSSEC validated and result
       of validation is SECURE.

   b.  QTYPE is A or AAAA (Query Type value 1 or 28)

   c.  The OPCODE is QUERY

   d.  The leftmost label of the QNAME is either "kskroll-sentinel-is-
       ta-<tag-index>" or "kskroll-sentinel-not-ta-<tag-index>"

   If all preconditions are not met, the resolver MUST NOT alter the DNS

3.2.  Special processing

   Responses which fullfill all preconditions in section 3.1 are subject
   of special processing, depending on leftmost label of the QNAME.

   First, the resolver determines if the numerical value of <key-tag> is
   equal to any of the key tags of an active Root Zone Key Signing Key
   which is currently trusted by the local resolver and is stored in its
   store of trusted keys.  An active key is one which could currently be
   used for validation (that is, a key that is not in either the AddPend
   or Revoked state as described in [RFC5011]).

   As second step, the resolver alters response depending on meaning of
   the label and presence of key with given keytag among trusted keys.
   Two labels and two possible states of the keytag generate four
   possible combinations summarized in the table:

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                              Keytag trusted
   label type |       yes               |       no
   is-ta      | return original answer  | return SERVFAIL
   not-ta     | return SERVFAIL         | return original answer

4.  Processing Sentinel Results

   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 or a third party to determine the state of
   their DNS resolution system, and, in particular, whether or not they
   are using validating DNS resolvers that use a particular trust anchor
   for the root zone.

   The critical aspect of the DNS names used in this mechanism 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 uses a test with three query names:

   o  A query name containing the left-most label "kskroll-sentinel-is-
      ta-<key-tag>".  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.

   o  A query name containing the left-most label "kskroll-sentinel-not-
      ta-<key-tag>".  This is also a validly-signed name.  Any validly-
      signed DNS zone can be used for this test.

   o  A query name that is signed with a DNSSEC signature that cannot be
      validated (such as if 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, and so the sentinel processing may sometimes
   result in incorrect conclusions.

   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:

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   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

   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.

   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 or AAAA RRset response for
      "kskroll-sentinel-not-ta" and SERVFAIL for the invalid name.

   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 or AAAA RRset
      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 a particular key in its trust anchor store.

                      |  is-ta   |  not-ta   |  invalid   |
              | Vnew  |    A     |  SERVFAIL |  SERVFAIL  |
              | Vold  | SERVFAIL |      A    |  SERVFAIL  |
        Type  | Vleg  |    A     |      A    |  SERVFAIL  |
              | nonV  |    A     |      A    |     A      |

   A "Vnew" type says that the nominated key is trusted by the resolver
   and has been loaded into its local trusted key stash.  A "Vold" type
   says that the nominated key is not yet trusted by the resolver in its
   own right.  A "Vleg" type does not give any information about the
   trust anchors, and a "nonV" type indicates that the resolver does not
   perform DNSSEC validation.

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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

   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 all of the following conditions all

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

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

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

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   In such a case, 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 and third parties
   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.  Privacy Considerations

   The mechansim in this document enables third parties (with either
   good or bad intentions) to learn something about the security
   configuration of recursive name servers.  That is, someone who can
   cause an Internet user to make specific DNS queries (e.g. via web-
   based advertisements or javascript in web pages), can then determine
   which trust anchors are configured in the user's resolver.

8.  IANA Considerations

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

9.  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 KSK of the root zone of the DNS.

   The authors would like to 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

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   Schulze, Mukund Sivaraman, Petr Spacek, Andrew Sullivan, Paul Vixie,
   Duane Wessels and Paul Wouters for their helpful feedback.

   The authors would like to especially call out Paul Hoffman and Duane
   Wessels for providing comments in the form of a pull request.  Petr
   Specek implmented early versions of this technique into the Knot
   resolver, identified a number of places where it wasn't clear, and
   provided very helpful text to address this.

10.  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 -04 to -05:

   o  Incorporated Duane's #10

   o  Integrated Petr Spacek's Issue - https://github.com/APNIC-Labs/
      draft-kskroll-sentinel/issues/9 (note that commit-log incorrectly
      referred to Duane's PR as number 9, it is actually 10).

   From -03 to -04:

   o  Addressed GitHub pull requests #4, #5, #6, #7 #8.

   o  Added Duane's privacy concerns

   o  Makes the use cases clearer

   o  Fixed some A/AAAA stuff

   o  Changed the example numbers

   o  Made it clear that names and addresses must be real

   From -02 to -03:

   o  Integrated / published comments from Paul in GitHub PR #2 -

   o  Made the keytag be decimal, not hex (thread / consensus in
      Kg7AtDhFRNw31He8n0_bMr9hBuE )

   From -01 to 02:

   o  Removed Address Record definition.

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   o  Clarified that many things can cause SERVFAIL.

   o  Made examples FQDN.

   o  Fixed a number of typos.

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

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

   o  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.

   o  Changed the label template from _is-ta-<key-tag> to kskroll-
      sentinel-is-ta-<key-tag>.  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.

11.  References

11.1.  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-

   [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,

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   [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-

11.2.  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-

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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