A Root Key Trust Anchor Sentinel for DNSSEC
draft-ietf-dnsop-kskroll-sentinel-17

DNSOP                                                          G. Huston
Internet-Draft                                                  J. Damas
Intended status: Standards Track                                   APNIC
Expires: April 23, 2019                                        W. Kumari
                                                                  Google
                                                        October 20, 2018

              A Root Key Trust Anchor Sentinel for DNSSEC
                  draft-ietf-dnsop-kskroll-sentinel-17

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 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 determining
   which keys are in the trust store for the root key.

   [ 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.  RFC
   Editor, please remove text in square brackets before publication. ]

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 https://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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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 23, 2019.

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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
   (https://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 . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Sentinel Mechanism in Resolvers . . . . . . . . . . . . . . .   4
     2.1.  Preconditions . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Special Processing  . . . . . . . . . . . . . . . . . . .   5
   3.  Sentinel Tests for a Single DNS Resolver  . . . . . . . . . .   6
     3.1.  Forwarders  . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Sentinel Tests for Multiple Resolvers . . . . . . . . . . . .  10
     4.1.  Test Scenario and Objective . . . . . . . . . . . . . . .  10
     4.2.  Test Assumptions  . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Test Procedure  . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  13
   7.  Implementation Experience . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   10. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     11.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  Protocol Walkthrough Example . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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 of a DNSKEY RR as described in Appendix B of

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   [RFC4034].  RRSIG RRs contain a Key Tag field whose value is equal to
   the Key Tag of the DNSKEY RR that was used to generate the
   corresponding signature.

   This document specifies how security-aware DNS resolvers that perform
   validation of their responses can respond to certain queries in a
   manner that allows an agent performing the queries to deduce whether
   a particular key for the root has been loaded into that resolver's
   trusted key store.  This document also describes a procedure where a
   collection of resolvers can be tested to determine if at least one of
   these resolvers has loaded a given key into its trusted key store.
   These tests can be used to determine whether a certain root zone Key
   Signing Key (KSK) is ready to be used as a trusted key, within the
   context of a planned root zone KSK key roll.

   There are two primary use cases for this mechanism:

   o  Users may wish to ascertain whether their DNS resolution
      environment's resolver is ready for an upcoming root KSK rollover.

   o  Researchers want to perform Internet-wide studies about the
      proportion of users who will be negatively impacted by an upcoming
      root KSK rollover.

   The mechanism described in this document satisfy the requirements of
   both these use-cases.  This mechanism is OPTIONAL to implement and
   use.  If implemented, this mechanism SHOULD be enabled by default to
   facilitate Internet-wide measurement.  Configuration options MAY be
   provided to disable the mechanism for reasons of local policy.

   The KSK sentinel tests described in this document use a test
   comprising of a set of DNS queries to domain names that have special
   values for the left-most label.  The test relies on recursive
   resolvers supporting a mechanism that recognises this special name
   pattern in queries, and under certain defined circumstances will
   return a DNS SERVFAIL response code (RCODE 2), mimicking the response
   code that is returned by security-aware resolvers when DNSSEC
   validation fails.

   If a browser or operating system is configured with multiple
   resolvers, and those resolvers have different properties (for
   example, one performs DNSSEC validation and one does not), the
   sentinel test described in this document can still be used.  The
   sentinel test makes a number of assumptions about DNS resolution
   behaviour that may not necessarily hold in all environments; if these
   assumptions do not hold (such as, for example, requiring the stub
   resolver to query the next recursive resolver in the locally
   configured set upon receipt of a SERVFAIL response code) then this

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   test may produce indeterminate or inconsistent results.  In some
   cases where these assumptions do not hold, repeating the same test
   query set may generate different results.

   Note that the measurements facilitated by the mechanism described in
   this document are different from those of [RFC8145].  RFC 8145 relies
   on resolvers reporting towards the root servers a list of locally
   cached trust anchors for the root zone.  Those reports can be used to
   infer how many resolvers may be impacted by a KSK roll, but not what
   the user impact of the KSK roll will be.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document contains a number of terms related to the DNS.  The
   current definitions of these terms can be found in [RFC7719].

2.  Sentinel Mechanism in Resolvers

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

   This sentinel mechanism makes use of two special labels:

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

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

   These labels trigger special processing in the validating DNS
   resolver when responses from authoritative servers are received.
   Labels containing "root-key-sentinel-is-ta-<key-tag>" is used to
   answer the question "Is this the Key Tag of a key which the
   validating DNS resolver is currently trusting as a trust anchor?"
   Labels containing "root-key-sentinel-not-ta-<key-tag>" is used to
   answer the question "Is this the Key Tag of a key which the
   validating DNS resolver is *not* currently trusting as a trust
   anchor?".

   The special labels defined here came after extensive IETF evaluation
   of alternative patterns and approaches in light of the desired
   behaviour (sections 2.1, 2.2) within the resolver and the applied
   testing methodology (section 4.3).  As one example, underscore

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   prefixed names were rejected because some browsers and operating
   systems would not fetch them because they domain names but not valid
   hostnames (see [RFC7719] for these definitions).  Attention was paid
   to the consideration of local collisions and the reservation of
   leftmost labels of a domain name, and the impact upon zone operators
   who might desire to use a similarly constructed hostname for a
   purpose other than as documented here.  Therefore, it is important to
   note that the reservation of the labels in this manner is definitely
   not considered "best practice".

2.1.  Preconditions

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

   o  The DNS response is DNSSEC validated.

   o  The result of validation is "Secure".

   o  The EDNS(0) Checking Disabled (CD) bit in the query is not set.

   o  The QTYPE is either A or AAAA (Query Type value 1 or 28).

   o  The OPCODE is QUERY.

   o  The leftmost label of the original QNAME (the name sent in the
      Question Section in the original query) is either "root-key-
      sentinel-is-ta-<key-tag>" or "root-key-sentinel-not-ta-<key-tag>".

   If any one of the preconditions is not met, the resolver MUST NOT
   alter the DNS response based on the mechanism in this document.

   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 (for example, a Key Tag value of 42 would be
   represented in the label as 00042).  The precise specification of the
   special labels above should be followed exactly.  For example, a
   label that does not include a Key Tag zero-padded to five digits does
   not match this specification, and should not be processed as if they
   did -- in other words, such queries should be handled as any other
   label and not according to Section 2.2.

2.2.  Special Processing

   Responses which fulfil all of the preconditions in Section 2.1
   require special processing, depending on leftmost label in the QNAME.

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   First, the resolver determines if the numerical value of <key-tag> is
   equal to any of the Key Tag values of an active root zone KSK which
   is currently trusted by the local resolver and is stored in its store
   of trusted keys.  An active root zone KSK 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]).

   Second, the resolver alters the response being sent to the original
   query based on both the left-most label and the presence of a key
   with given Key Tag in the trust anchor store.  Two labels and two
   possible states of the corresponding key generate four possible
   combinations summarized in the table:

    Label      | Key is trusted          | Key is not trusted
    ------------------------------------------------------------------
    is-ta      | return original answer  | return SERVFAIL
    not-ta     | return SERVFAIL         | return original answer

   Instruction "return SERVFAIL" means that the resolver MUST set
   RCODE=SERVFAIL (value 2) and the ANSWER section of the DNS response
   MUST be empty, ignoring all other documents which specify content of
   the ANSWER section.

   Instruction "return original answer" means that the resolver MUST
   process the query without any further special processing; that is,
   exactly as if the mechanism described in this document was not
   implemented or was disabled.  The answer for the A or AAAA query is
   sent on to the client.

3.  Sentinel Tests for a Single DNS Resolver

   This section describes the use of the sentinel detection mechanism
   against a single DNS recursive resolver in order to determine whether
   this resolver is using a particular trust anchor to validate DNSSEC-
   signed responses.

   Note that the test in this section applies to a single DNS resolver.
   The test described in Section 4 applies instead to a collection of
   DNS resolvers, as might be found in the DNS configuration of an end-
   user environment.

   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 procedure can test a DNS resolver using three
   queries:

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   o  A query name containing the left-most label "root-key-sentinel-is-
      ta-<key-tag>".  This corresponds to a validly-signed name in the
      parent zone, so that responses associated with this query name can
      be authenticated by a DNSSEC-validating resolver.  Any validly-
      signed DNS zone can be used as the parent zone for this test.

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

   o  A query name that is signed with a DNSSEC signature that cannot be
      validated (described as a "bogus" RRset in Section 5 of [RFC4033],
      when, for example, an RRset associated with a zone that is not
      signed with a valid RRSIG record).

   The responses received from queries to resolve each of these query
   names can be evaluated to infer a trust key state of the DNS
   resolver.

   An essential assumption here is that this technique relies on
   security-aware (DNSSEC validating) resolvers responding with a
   SERVFAIL response code to queries where DNSSEC checking is requested
   and the response cannot be validated.  Note that other issues can
   also cause a resolver to return SERVFAIL responses, and so the
   sentinel processing may sometimes result in incorrect or
   indeterminate conclusions.

   To describe this process of classification, DNS resolvers are
   classified by five distinct behavior types using the labels: "Vnew",
   "Vold", "Vind", "nonV", and "other".  These labels correspond to
   resolver system behaviour types as follows:

   Vnew:  A DNS resolver that is configured to implement this mechanism
      and has loaded the nominated key into their local trusted key
      stores will respond with an A or AAAA RRset response for the
      associated "root-key-sentinel-is-ta" queries, SERVFAIL for "root-
      key-sentinel-not-ta" queries and SERVFAIL for the signed name
      queries that return "bogus" validation status.

   Vold:  A DNS resolver that is configured to implement this mechanism
      and has not loaded the nominated key into their local trusted key
      stores will respond with an SERVFAIL for the associated "root-key-
      sentinel-is-ta" queries, an A or AAAA RRset response for "root-
      key-sentinel-not-ta" queries and SERVFAIL for the signed name
      queries that return "bogus" validation status.

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   Vind:  A DNS resolver that has is not configured to implement this
      mechanism will respond with an A or AAAA RRset response for "root-
      key-sentinel-is-ta", an A or AAAA RRset response for "root-key-
      sentinel-not-ta" and SERVFAIL for the name that returns "bogus"
      validation status.  This set of responses does not give any
      information about the trust anchors used by this resolver.

   nonV:  A non-security-aware DNS resolver will respond with an A or
      AAAA RRset response for "root-key-sentinel-is-ta", an A or AAAA
      RRset response for "root-key-sentinel-not-ta" and an A or AAAA
      RRset response for the name that returns "bogus" validation
      status.

   other:  There is the potential to admit other combinations of
      responses to these three queries.  While this may appear self-
      contradictory, there are cases where such an outcome is possible.
      For example, in DNS resolver farms what appears to be a single DNS
      resolver that responds to queries passed to a single IP address is
      in fact constructed as a a collection of slave resolvers, and the
      query is passed to one of these internal resolver engines.  If
      these individual slave resolvers in the farm do not behave
      identically, then other sets of results can be expected from these
      three queries.  In such a case, no determination about the
      capabilities of this DNS resolver farm can be made.

   Note that SERVFAIL might be cached according to Section 7 of
   [RFC2308] for up to 5 minutes and a positive answer for up to its
   TTL.

   If a client directs these three queries to a single resolver, the
   responses should allow the client to determine the capability of the
   resolver, and if it supports this sentinel mechanism, whether or not
   it has a particular key in its trust anchor store, as in the
   following table:

                                    Query
                      +----------+-----------+------------+
                      |  is-ta   |  not-ta   |   bogus    |
              +-------+----------+-----------+------------+
              | Vnew  |    Y     |  SERVFAIL |  SERVFAIL  |
              | Vold  | SERVFAIL |      Y    |  SERVFAIL  |
        Type  | Vind  |    Y     |      Y    |  SERVFAIL  |
              | nonV  |    Y     |      Y    |     Y      |
              | other |    *     |      *    |     *      |
              +-------+----------+-----------+------------+

   In this table, the "Y" response denotes an A or AAAA RRset response
   (depending on the query type of A or AAAA records), "SERVFAIL"

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   denotes a DNS SERVFAIL response code (RCODE 2), and "*" denotes
   either response.

   Vnew:  The nominated key is trusted by the resolver.

   Vold:  The nominated key is not yet trusted by the resolver.

   Vind:  There is no information about the trust anchors of the
      resolver.

   nonV:  The resolver does not perform DNSSEC validation.

   other:  The properties of the resolver cannot be analyzed by this
      protocol.

3.1.  Forwarders

   Some resolvers are configured not to answer queries using the
   recursive algorithm first described in [RFC1034] section 4.3.2, but
   instead relay queries to one or more other resolvers.  Resolvers
   configured in this manner are referred to in this document as
   "forwarders".

   If the resolver is non-validating, and it has a single forwarder,
   then the resolver will presumably mirror the capabilities of the
   forwarder's target resolver.

   If the validating resolver has a forwarding configuration, and it
   sets the EDNS(0) Checking Disabled (CD) bit as described in
   Section 3.2.2 of [RFC4035] on all forwarded queries, then this
   resolver is acting in a manner that is identical to a standalone
   resolver.

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

   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 EDNS(0) CD bit set

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

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

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4.  Sentinel Tests for Multiple Resolvers

   The description in Section 3 describes a trust anchor test that can
   be used in the simple situation where the test queries were being
   passed to a single recursive resolver that directly queried
   authoritative name servers.

   However, the common end-user scenario is where a user's local DNS
   resolution environment is configured to use more than one recursive
   resolver.  The single resolver test technique will not function
   reliably in such cases, as a a SERVFAIL response from one resolver
   may cause the local stub resolver to repeat the query against one of
   the other configured resolvers and the results may be inconclusive.

   In describing a test procedure that can be used in this environment
   of a set of DNS resolvers there are some necessary changes to the
   nature of the question that this test can answer, the assumptions
   about the behaviour of the DNS resolution environment, and some
   further observations about potential variability in the test
   outcomes.

4.1.  Test Scenario and Objective

   This test is not intended to expose which trust anchors are used by
   any single DNS resolver.

   The test scenario is explicitly restricted to that of the KSK
   environment where a current active KSK (called "KSK-current") is to
   be replaced with a new KSK (called "KSK-new").  The test is designed
   to be run between when KSK-new is introduced into the root zone and
   when the root zone is signed with KSK-new.

   The objective of the test is to determine if the user will be
   negatively impacted by the KSK roll.  A "negative impact" for the
   user is defined such that all the configured resolvers are security-
   aware resolvers that perform validation of DNSSEC-signed responses,
   and none of these resolvers have loaded KSK-new into their local
   trust anchor set.  In this situation, it is anticipated that once the
   KSK is rolled the entire set of the user's resolvers will not be able
   to validate the contents of the root zone and the user is likely to
   lose DNS service as a result of this inability to perform successful
   DNSSEC validation.

4.2.  Test Assumptions

   There are a number of assumptions about the DNS environment used in
   this test.  Where these assumptions do not hold, the results of the
   test will be indeterminate.

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   o  When a recursive resolver returns SERVFAIL to the user's stub
      resolver, the stub resolver will send the same query to the next
      resolver in the locally configured resolver set.  It will continue
      to do this until it gets a non-SERVFAIL response or until it runs
      out of resolvers to try.

   o  When the user's stub resolver passes a query to a resolver in the
      configured resolver set, it will get a consistent answer over the
      timeframe of the queries.  This assumption implies that if the
      same query is asked by the same stub resolver multiple times in
      succession to the same recursive resolver, the recursive
      resolver's response will be the same for each of these queries.

   o  All DNSSEC-validating resolvers have KSK-current in their local
      trust anchor cache.

   There is no current published measurement data that indicates to what
   extent the first two assumptions listed here are valid, and how many
   end users may be impacted by these assumptions.  In particular, the
   first assumption, that a consistent SERFAIL response will cause the
   local stub DNS resolution environment to query all of its configured
   recursive resolvers before concluding that the name cannot be
   resolved, is a very critical assumption for this test.

   Note that additional precision / determinism may be achievable by
   bypassing the normal OS behavior and explicitly testing using each
   configured recursive resolver (e.g using 'dig').

4.3.  Test Procedure

   The sentinel detection process tests a DNS resolution environment
   with three query names.  Note that these same general categories of
   query as in Section 3 but the key tag used is different for some
   queries:

   o  A query name that is signed with a DNSSEC signature that cannot be
      validated (described as a "bogus" RRset in Section 5 of [RFC4033],
      when, for example, an RRset is not signed with a valid RRSIG
      record).

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

   o  A query name containing the left-most label "root-key-sentinel-is-
      ta-<key-tag-of-KSK-new>".  This name MUST be a validly-signed
      name.  Any validly-signed DNS zone can be used for this test.

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   The responses received from queries to resolve each of these names
   can be evaluated to infer a trust key state of the user's DNS
   resolution environment.

   The responses to these queries are described using a simplified
   notation.  Each query will either result in a SERFVAIL response
   (denoted as "S"), indicating that all of the resolvers in the
   recursive resolver set returned the SERVFAIL response code, or result
   in a response with the desire RRset value (denoted as "A").  The
   queries are ordered by the "invalid" name, the "root-key-sentinel-
   not-ta" label, then the "root-key-sentinel-is-ta" label, and a
   triplet notation denotes a particular response.  For example, the
   triplet "(S S A)" denotes a SERVFAIL response to the invalid query, a
   SERVFAIL response to the "root-key-sentinel-not-ta" query and a RRset
   response to the "root-key-sentinel-is-ta" query.

   The set of all possible responses to these three queries are:

   (A * *):  If any resolver returns an "A" response for the query for
      the invalid name, then the resolver set contains at least one non-
      validating DNS resolver, and the user will not be impacted by the
      KSK roll.

   (S A *):  If any of the resolvers returns an "A" response the the
      "root-key-sentinel-not-ta" query, then at least one of the
      resolvers does not recognise the sentinel mechanism, and the
      behaviour of the collection of resolvers during the KSK roll
      cannot be reliably determined.

   (S S A):  This case implies that all of the resolvers in the set
      perform DNSSEC-validation, all of the resolvers are aware of the
      sentinel mechanism, and at least one resolver has loaded KSK-new
      as a local trust anchor.  The user will not be impacted by the KSK
      roll.

   (S S S):  This case implies that all of the resolvers in the set
      perform DNSSEC-validation, all of the resolvers are aware of the
      sentinel mechanism, and none of the resolvers has loaded KSK-new
      as a local trust anchor.  The user will be negatively impacted by
      the KSK roll.

5.  Security Considerations

   This document describes a mechanism to allow users to determine the
   trust anchor state of root zone key signing keys in the DNS
   resolution system that they use.  If the user executes third party
   code, then this information may also be available to the third party.

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   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 normal DNSSEC validation processing load.

6.  Privacy Considerations

   The mechanism in this document enables third parties (with either
   good or bad intentions) to learn something about the security
   configuration of recursive DNS resolvers.  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, under certain
   specific circumstances that includes additional knowledge of the
   resolvers that are invoked by the user, determine which trust anchors
   are configured in these resolvers.  Without this additional
   knowledge, the third party can infer the aggregate capabilities of
   the user's DNS resolution environment, but cannot necessarily infer
   the trust configuration of any recursive name server.

7.  Implementation Experience

   [ RFC Editor: Please remove before publication.  As this section will
   be removed, it is more conversational than would appear in a
   published doc. ]

   List of known resolver implementations (alphabetical):

   BIND   Ondrej Sury of ISC reported to the DNSOP Working Group in
      April 2018 that this technique was peer-reviewed and merged into
      BIND master branch with the intent to backport the feature into
      older release branches.  The merge request:
      https://gitlab.isc.org/isc-projects/bind9/merge_requests/123
      Information on configuring this can be found in the BIND 9.13.0
      Administrator Reference Manual (ARM), available at
      https://ftp.isc.org/isc/bind9/9.13.0/doc/arm/Bv9ARM.pdf

   Knot resolver  Petr Spacek implemented 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
      these issues and make the document mode clear.  Petr also
      identified an embarrassingly large number of typos (and similar)
      in the ksk-test setup.  More information is at http://knot-

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      resolver.readthedocs.io/en/stable/modules.html#sentinel-for-
      detecting-trusted-keys

   Unbound  Benno Overeinder of NLnet Labs reported to the DNSOP Working
      Group in April 2018 an intention to support this technique in
      Unbound in the near future.  This is now implemented in Unbound
      version 1.7.1, available from http://unbound.nlnetlabs.nl/
      download.html . Configuration information is at
      http://unbound.nlnetlabs.nl/documentation/unbound.conf.html

   A (partial) list of "client" / user side implementations (the author
   was keeping a more complete list of implementations, but has
   misplaced it - apologies, I'm happy to re-add them if you send me a
   note.):

   http://www.ksk-test.net  An Javascript implementation of the client
      side of this protocol is available at: http://www.ksk-test.net

   http://test.kskroll.dnssec.lab.nic.cl/  Hugo Salgado-Hernandez has
      created an implementation at
      http://test.kskroll.dnssec.lab.nic.cl/

   http://sentinel.research.icann.org/  The code for this implementation
      is published at https://github.com/paulehoffman/sentinel-testbed

   http://www.bellis.me.uk/sentinel/  Ray Bellis client implementation -
      http://www.bellis.me.uk/sentinel/

8.  IANA Considerations

   This document has no IANA actions.

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, Hugo Salgado-
   Hernandez, Andreas Schulze, Mukund Sivaraman, Petr Spacek, Job
   Snijders, Andrew Sullivan, Ondrej Sury, Paul Vixie, Duane Wessels and
   Paul Wouters for their helpful feedback.

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   The authors would like to especially call out Paul Hoffman and Duane
   Wessels for providing comments in the form of pull requests.  Joe
   Abley also helpfully provided extensive review and OLD / NEW text.

   Petr Spacek wrote some very early implementations, and provided
   significant feedback (including pointing out when the test bed didn't
   match the document!)

10.  Change Log

   RFC Editor: Please remove this section!

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

   From -16 to -17:

   o  Thank to Paul Hoffman for "Lots of editorial fixes for post-IESG
      draft" ( https://github.com/APNIC-Labs/draft-kskroll-sentinel/
      pull/28 )

   o  Repeat after me: Do not drive git while on cold meds...

   From -15 to -16:

   o  Addressed IESG comments

   o  Benjamin Kaduk's Discuss on draft-ietf-dnsop-kskroll-sentinel

   o  Also added Terry's "This a bad design pattern, but we decided the
      benefits outweigh the costs this time." text.

   o  Suggestion from Adam to clarify that bypassing e.g gethostbyname()
      can provide better testing.

   o  Nit: Forgot 'name' in 'This name MUST be a validly-signed name.'

   o  Clarified that 'bogus.example.com' is intentionally DNSSEC bogus /
      invalid.

   From -14 to -15:

   o  Addressed Joe Abley's thorough review, at:
      https://mailarchive.ietf.org/arch/msg/dnsop/8ZnN1xj55Yimet2cg-
      LrdoJafEA

   From -13 to -14:

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   o  Addressed nits from Bob Harold -
      https://mailarchive.ietf.org/arch/msg/dnsop/
      j4Serw0z24o470AnlD8ISo8o9k4

   o  Formatting changes (and a bit more text) in the implementation
      section.

   o  Closes PR #21: Clarify indeterminate and resolution systems,

   o  Closes PR #22: Updates to -13 describing the test procedure for a
      set of resolvers

   o  Closes PR #23: Fix sundry typos,

   o  Closes PR #24: Editorial and clarifications to the new text

   o  Closes PR #25: Clarified when the test can be run

   From -12 to -13:

   o  Merged Paul Hoffmans PR#19, PR#20.

   o  Moved toy ksk-test.net to implementation section.

   o  Split the test procedures between the test of a single DNS
      resolvers and the test of a collection of DNS resolvers as would
      be found in an end user environment.

   From -11 to -12:

   o  Moved the Walkthrough Example to the end of the document as an
      appendix.

   o  Incorporated changes as proposed by Ondrej Sury, relating to a
      consistent use of Key Tag and a reference to the definition of a
      Bogus RRset.

   o  Corrected minor typos.

   o  Revised the Privacy Considerations.

   o  In response to a request from DNSOP Working Group chairs, a
      section on reported Implementation Experience has been added,
      based on postings to the DNSOP Working Group mailing list.

   From -10 to -11:

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   o  Clarified the preconditions for this mechanism as per Working
      Group mailing list discussion.

   o  Corrected minor typo.

   From -09 to -10:

   o  Clarified the precondition list to specify that the resolver had
      performed DNSSEC-validation by setting the AD bit in the response

   o  Clarified the language referring to the operation of RFC8145
      signalling.

   From -08 to -09:

   o  Incorporated Paul Hoffman's PR # 15 (Two issues from the
      Hackathon) - https://github.com/APNIC-Labs/draft-kskroll-sentinel/
      pull/15

   o  Clarifies that the match is on the *original* QNAME.

   From -08 to -07:

   o  Changed title from "A Sentinel for Detecting Trusted Keys in
      DNSSEC" to "A Root Key Trust Anchor Sentinel for DNSSEC".

   o  Changed magic string from "kskroll-sentinel-" to "root-key-
      sentinel-" -- this time for sure, Rocky!

   From -07 to -06:

   o  Addressed GitHub PR #14: Clarifications regarding caching and
      SERVFAIL responses

   o  Addressed GitHub PR #12, #13: Clarify situation with multiple
      resolvers, Fix editorial nits.

   From -05 to -06:

   o  Paul improved my merging of Petr's text to make it more readable.
      Minor change, but this is just before the cut-off, so I wanted it
      maximally readable.

   From -04 to -05:

   o  Incorporated Duane's #10

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   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 -
      https://github.com/APNIC-Labs/draft-kskroll-sentinel/pull/2

   o  Made the Key Tag be decimal, not hex (thread / consensus in
      https://mailarchive.ietf.org/arch/msg/dnsop/
      Kg7AtDhFRNw31He8n0_bMr9hBuE )

   From -01 to 02:

   o  Removed Address Record definition.

   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 Key Tags 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:

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   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

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
              <https://www.rfc-editor.org/info/rfc2308>.

   [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>.

11.2.  Informative References

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <https://www.rfc-editor.org/info/rfc7719>.

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   [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>.

Appendix A.  Protocol Walkthrough Example

   This Appendix 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.  The
   examples here all assume that each person has just one resolver, or a
   system of resolvers that have the same properties.

   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 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
   02323).

   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
   example.com:

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

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

      root-key-sentinel-not-ta-11112.example.com.  IN AAAA 2001:db8::1

   Note that the use of "example.com" names and the addresses here are
   examples, and "bogus" intentionally has invalid DNSSEC signatures.

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   In a real deployment, the domain names need to be under control of
   the researcher, and the addresses must be real, reachable addresses.

   Geoff then DNSSEC signs the example.com zone, and intentionally makes
   the bogus.example.com record have bogus validation status (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://bogus.example.com/1x1.gif, http://root-key-sentinel-is-ta-
   02323.example.com/1x1.gif, http://root-key-sentinel-not-ta-
   11112.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 11112 KSK is
   removed.

   Bob is not using a validating resolver.  This means that he will be
   able to resolve bogus.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://bogus.example.com/1x1.gif resource (the
   bogus.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 in the body of this document) that he is
   using validating resolvers, but at least one of these resolvers is
   not configured to perform sentinel processing.  The KSK sentinel
   method cannot provide him with a definitive answer to the question of
   whether he will be impacted by the KSK roll.

   Dave's resolvers implement the sentinel method, and have picked up
   the new KSK.  For the same reason as Charlie, he cannot fetch the
   "bogus" resource.  His resolver resolves the root-key-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 resolver sends the reply to Dave's stub,
   it performs the KSK Sentinel check.  The QNAME starts with "root-key-
   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 root-key-

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   sentinel-not-ta-11112.example.com name.  Once again, it performs the
   normal resolution process, but because it implements KSK Sentinel
   (and the QNAME starts with "root-key-sentinel-not-ta-"), just before
   sending the reply, it performs the KSK Sentinel check.  As it has the
   key with key-tag 11112 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 "bogus", he can
   fetch "root-key-sentinel-is-ta-02323", but he cannot fetch "root-key-
   sentinel-not-ta-11112".  From this, Dave knows that he is behind an
   collection of resolvers that all validate, all have the key with key
   tag 11112 loaded and at least one of these resolvers has loaded the
   key with key-tag 02323 into its local trust anchor cache, Dave will
   not be impacted by the KSK roll.

   Just like Charlie and Dave, Ed cannot fetch the "bogus" record.  This
   tells him that his resolvers are validating.  When his (sentinel-
   aware) resolvers performs the KSK Sentinel check for "root-key-
   sentinel-is-ta-02323", none of them have loaded the new key with key-
   tag 02323 in their local 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 root-key-
   sentinel-not-ta-11112.example.com, and as all of Ed's resolvers both
   perform DNSSEC validation and recognise the sentinel label Ed will be
   unable to fetch the "root-key-sentinel-not-ta-11112" resource.  This
   tells Ed that his resolvers have not installed the new KSK and he
   will be negatively implacted by the KSK roll..

   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.

   This 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.

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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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