    Internet Engineering Task Force (IETF)                         G. Huston
    Request for Comments: 8509                                      J. Damas
    Category: Standards Track                                          APNIC
    ISSN: 2070-1721                                                W. Kumari
                                                                      Google
                                                               December 2018

                  A Root Key Trust Anchor Sentinel for DNSSEC

    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.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8509.

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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
       to this document.  Code Components extracted from this document must
       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  . . . . . . . . . . . . . . . . . . . . . . . .   3
         1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
       2.  Sentinel Mechanism in Resolvers . . . . . . . . . . . . . . .   4
         2.1.  Preconditions . . . . . . . . . . . . . . . . . . . . . .   5
         2.2.  Special Processing  . . . . . . . . . . . . . . . . . . .   6
       3.  Sentinel Tests for a Single DNS Resolver  . . . . . . . . . .   7
         3.1.  Forwarders  . . . . . . . . . . . . . . . . . . . . . . .   9
       4.  Sentinel Tests for Multiple Resolvers . . . . . . . . . . . .  10
         4.1.  Test Scenario and Objective . . . . . . . . . . . . . . .  11
         4.2.  Test Assumptions  . . . . . . . . . . . . . . . . . . . .  11
         4.3.  Test Procedure  . . . . . . . . . . . . . . . . . . . . .  12
       5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
       6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  14
       7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
       8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
         8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
         8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
       Appendix A.  Protocol Walk-Through Example  . . . . . . . . . . .  16
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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    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 Resource Record (RR) as described in
       Appendix B of [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 whether 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 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 satisfies 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 a set of DNS queries to domain names that have special
       values for the leftmost label.  The test relies on recursive
       resolvers supporting a mechanism that recognizes this special name
       pattern in queries; under certain defined circumstances, it will
       return a DNS SERVFAIL response code (RCODE 2), mimicking the response
       code that is returned by security-aware resolvers when DNSSEC
       validation fails.

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       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
       behavior that may not necessarily hold in all environments; if these
       assumptions do not hold, then this test may produce indeterminate or
       inconsistent results.  This might occur, for example, if the stub
       resolver is required to query the next recursive resolver in the
       locally configured set upon receipt of a SERVFAIL response code.  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>" are used to
       answer the question, "Is this the Key Tag of a key that the
       validating DNS resolver is currently trusting as a trust anchor?"

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       Labels containing "root-key-sentinel-not-ta-<key-tag>" are used to
       answer the question, "Is this the Key Tag of a key that the
       validating DNS resolver is *not* currently trusting as a trust
       anchor?"

       The special labels defined here were chosen after extensive IETF
       evaluation of alternative patterns and approaches in light of the
       desired behavior (Sections 2.1 and 2.2) within the resolver and the
       applied testing methodology (Section 4.3).  As one example,
       underscore-prefixed names were rejected because some browsers and
       operating systems would not fetch them because they are domain names
       but not valid hostnames (see [RFC7719] for these definitions).
       Consideration was given to local collisions and the reservation of
       leftmost labels of a domain name, as well as the impact upon zone
       operators who might desire to use a similarly constructed hostname
       for a purpose other than those 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 Extension Mechanisms for DNS (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 an unsigned
       decimal integer (as described in [RFC4034], Section 5.3) but is 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

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       not match this specification and should not be processed as if it 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 that fulfill all of the preconditions in Section 2.1
       require special processing, depending on the leftmost label in the
       QNAME.

       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 that is
       currently trusted by the local resolver and stored in its store of
       trusted keys.  An active root zone KSK is one that 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 leftmost 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

       The 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 that specify the content
       of the Answer Section.

       The 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.

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    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 or negative
       test as the leftmost label in the query name.

       The sentinel detection procedure can test a DNS resolver using three
       queries:

       o  A query name containing the leftmost 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 leftmost 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 is 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.

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       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 behavior types as follows:

       Vnew:  A DNS resolver that is configured to implement this mechanism
          and has loaded the nominated key into its 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 its local trusted-key
          stores will respond with a 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.

       Vind:  A DNS resolver that 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 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.

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       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"
       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 has a single forwarder, then it
       will presumably mirror the capabilities of the forwarder's target
       resolver.

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       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 there are mixed signals such as SERVFAIL in all three
       responses ("other"), which is similarly an indeterminate response
       with respect to the trusted key state.

    4.  Sentinel Tests for Multiple Resolvers

       Section 3 describes a trust-anchor test that can be used in the
       simple situation where the test queries are being passed to a single
       recursive resolver that directly queries 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 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 for 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
       behavior of the DNS resolution environment, and some further
       observations about potential variability in the test outcomes.

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    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.

       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 either gets a non-SERVFAIL response or 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
          time frame 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 or how many
       end users may be impacted by these assumptions.  In particular, the
       first assumption, that a consistent SERVFAIL response will cause the

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       local stub DNS resolution environment to query all of its configured
       recursive resolvers before concluding that the name cannot be
       resolved, is a 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 are the 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 leftmost 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 leftmost 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.

       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 result in either a SERVFAIL response
       (denoted "S"), indicating that all of the resolvers in the recursive
       resolver set returned the SERVFAIL response code, or a response with
       the desired RRset value (denoted "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 an RRset response to the "root-
       key-sentinel-is-ta" query.

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       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 for the
          "root-key-sentinel-not-ta" query, then at least one of the
          resolvers does not recognize the sentinel mechanism, and the
          behavior 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 for allowing 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.

       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.

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    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 include 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.  IANA Considerations

       This document has no IANA actions.

    8.  References

    8.1.  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>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [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>.

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       [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>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

    8.2.  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>.

       [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>.

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    Appendix A.  Protocol Walk-Through 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 (that is, starts signing with just the KSK whose Key Tag is
       02323).

       Bob, Charlie, Dave, and 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, but Ed's resolver hasn't managed to install the 02323 KSK in
       its trust store yet.

       Geoff is a researcher.  He would like to both provide a means for
       Bob, Charlie, Dave, and Ed to perform tests and himself 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 web
       server (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.
       In a real deployment, the domain names need to be under the control
       of the researcher, and the addresses must be real, reachable
       addresses.

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       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 web server to listen on
       2001:db8::1 and serve a resource (for example, a 1x1 GIF, 1x1.gif)
       for all of these names.  The web server also serves a web page
       (www.example.com) that contains links to these three resources
       (http://bogus.example.com/1x1.gif, http://root-key-sentinel-is-ta-
       02323.example.com/1x1.gif, and 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 that 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
       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-
       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

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       key with key-tag 11112 in its 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 a
       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 perform 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 the 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 recognize 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 impacted 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 he 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 web page that they load would
       likely contain JavaScript (or similar) that 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 names result in a SERVFAIL.

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    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.

       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!

    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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