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rfc:rfc8509

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.

Huston, et al. Standards Track [Page 1] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

Copyright Notice

 Copyright (c) 2018 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (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

Huston, et al. Standards Track [Page 2] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

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.

Huston, et al. Standards Track [Page 3] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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?"

Huston, et al. Standards Track [Page 4] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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

Huston, et al. Standards Track [Page 5] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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.

Huston, et al. Standards Track [Page 6] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

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.

Huston, et al. Standards Track [Page 7] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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.

Huston, et al. Standards Track [Page 8] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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.

Huston, et al. Standards Track [Page 9] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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.

Huston, et al. Standards Track [Page 10] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

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

Huston, et al. Standards Track [Page 11] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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.

Huston, et al. Standards Track [Page 12] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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.

Huston, et al. Standards Track [Page 13] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

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>.

Huston, et al. Standards Track [Page 14] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 [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>.

Huston, et al. Standards Track [Page 15] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

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.

Huston, et al. Standards Track [Page 16] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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

Huston, et al. Standards Track [Page 17] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

 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.

Huston, et al. Standards Track [Page 18] RFC 8509 DNSSEC Trusted Key Sentinel December 2018

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

Huston, et al. Standards Track [Page 19]

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