wdiff draft-ietf-dnsop-kskroll-sentinel-12.txt draft-ietf-dnsop-kskroll-sentinel-13.txt

DNSOP G. Huston
Internet-Draft J. Damas
Intended status: Standards Track APNIC
Expires: November 3, December 6, 2018 W. Kumari
Google
May 2,
June 4, 2018

A Root Key Trust Anchor Sentinel for DNSSEC
draft-ietf-dnsop-kskroll-sentinel-12
draft-ietf-dnsop-kskroll-sentinel-13

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.

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

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

[ NOTE: This version uses the labels "root-key-sentinel-is-ta-", and
"root-key-sentinel-not-ta-".; older versions of this document used
"kskroll-sentinel-is-ta-<key-tag>", "kskroll-sentinel-not-ta-<key-
tag>", and before that, "_is-ta-<key-tag>", "_not-ta-<key-tag>".
Also note that the format of the tag-index is now zero-filled
decimal. Apologies to those who have begun implementing earlier
versions of this specification.]

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
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on November 3, December 6, 2018.

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
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Sentinel Mechanism in Resolvers . . . . . . . . . . . . . . . 4
2.1. Preconditions . . . . . . . . . . . . . . . . . . . . . . 4 5
2.2. Special Processing . . . . . . . . . . . . . . . . . . . 5
3. Processing Sentinel Results Tests for a Single DNS Resolver . . . . . . . . . . 6
3.1. Forwarders . . . . . . . 5 . . . . . . . . . . . . . . . . 8
4. Sentinel Tests for a Set of Resolvers . . . . . . . . . . . . 9
4.1. Test Result Considerations Scenario and Objective . . . . . . . . . . . . . 7 . . 9
4.2. Test Assumptions . . . . . . . . . . . . . . . . . . . . 10
4.3. Test Procedure . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 12
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9 12
7. Implementation Experience . . . . . . . . . . . . . . . . . . 9 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 13
10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 10 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 13 16
11.2. Informative References . . . . . . . . . . . . . . . . . 13 17
Appendix A. Protocol Walkthrough Example . . . . . . . . . . . . 14 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 20

1. Introduction

The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and
[RFC4035] were developed to provide origin authentication and
integrity protection for DNS data by using digital signatures.
DNSSEC uses Key Tags to efficiently match signatures to the keys from
which they are generated. The Key Tag is a 16-bit value computed
from the RDATA portion of a DNSKEY RR using a formula found in "Key
Tag Calculation" (Appendix B of "Resource Records for the DNS
Security Extensions" [RFC4034]), a formula similar to a ones-
complement checksum. RRSIG RRs contain a Key Tag field whose value
is equal to the Key Tag of the DNSKEY RR that validates the
signature.

This document specifies how validating security-aware DNS resolvers that perform
validation of their responses can respond to certain queries in a
manner that allows a querier an agent performing the queries to deduce whether
a particular key for the root has been loaded into that resolver's
trusted key store. In particular, this response mechanism 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 KSK
is ready to be used as a trusted key, within the context of a planned
root zone KSK key
roll, by this resolver. roll.

There are two primary use cases for this mechanism:

o Users want may wish to know ascertain whether the their DNS resolution
environment resolvers they use are is ready for an upcoming root KSK rollover rollover.

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

The mechanism described in this document meets both of these use
cases. This new mechanism is OPTIONAL to implement and use, although
for reasons of supporting broad-based measurement techniques, it is
strongly preferred that configurations of DNSSEC-validating resolvers
enabled this mechanism by default, allowing for local configuration
directives to disable this mechanism if desired.

The KSK sentinel test tests described in this document determines whether use a
user's browser or operating system looking up the special test
comprising of a set of DNS queries to domain names that
are used in this protocol would be able to validate using the root
KSK indicated by the have special names. The protocol uses
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) for this purpose because that is 2), mimicking the response
code that is returned by security-aware resolvers when DNSSEC
validation fails.

If a browser or operating system has is configured with multiple resolvers
configured,
resolvers, and those resolvers have different properties (for
example, one performs DNSSEC validation and one does not), the
sentinel mechanism might search among test described in this document can still be used, but it
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 different resolvers, or
might not, depending on how stub resolver to query
the browser next recursive resolver in the locally configured set upon
receipt of a SERVFAIL response code) then this test may produce
indeterminate or operating system is
configured. inconsistent results. In some cases where these
assumptions do not hold, repeating the same test query set may
generate different results.

Note that the sentinel mechanism described here measures a very
different (and likely more useful) metric than [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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.

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>

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).

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

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 Checking Disabled (CD) bit in the query is not set.

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

o The OPCODE is QUERY 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>" "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.

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.

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 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 key
with given Key Tag in the trust anchor store. Two labels and two
possible states of the corresponding Key key generate four possible
combinations summarized in the table:

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

3. Processing Sentinel Results Tests for a Single DNS Resolver

This proposed test that uses section describes the use of the sentinel detection mechanism
described
against a single DNS recursive resolver in order to determine whether
this document resolver is based on the use of three DNS names using a particular trust anchor to validate DNSSEC-
signed responses.

Note that have three distinct the test in this section applies to a single DNS resolution behaviours. resolver.
The test is
intended described in Section 4 applies instead to allow a user or a third party to determine the state collection of
their
DNS resolution system, and, resolvers, as might be found in particular, whether or not they
are using one or more validating DNS resolvers that use a particular
trust anchor for the root zone. 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 process uses a procedure can test with a DNS resolver using three query names:
queries:

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

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

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

The responses received from queries to resolve each of these names
would allow us
can be evaluated to infer a trust key state of the resolution
environment. The techniques describes in DNS resolver.

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

To describe this process of classification, we can classify DNS resolvers
into four are
classified by five distinct behavior types, for which we will use types using the labels: "Vnew",
"Vold", "Vleg", "Vind", "nonV", and "nonV". "other". These labels correspond to
resolver system behaviour types as follows:

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

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

Vleg:
queries that return "bogus" validation status.

Vind: A DNSSEC-Validating DNS resolver that does 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 invalid name. 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-DNSSEC-Validating non-security-aware DNS resolver will respond with an A or
AAAA record response for "root-key-sentinel-is-ta", an A record
response for "root-key-sentinel-not-ta" and an A or AAAA RRset
response for the invalid name.

Given name that returns "bogus" validation status.

other: There is the clear delineation amongst potential to admit other combinations of
responses to these three cases, if 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 simple single resolver, the variation in
response to the three queries
responses should allow the client to determine the category capability of the
resolver, and if it supports this sentinel mechanism, whether or not
it has a particular key in its trust anchor store. store, as in the
following table:

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

A "Vnew" type says that the

Vnew: The nominated key is trusted by the resolver
and has been loaded into its local trusted key stash. A "Vold" type
says that the resolver.

Vold: The nominated key is not yet trusted by the resolver in its
own right. A "Vleg" type does not give any resolver.

Vind: There is no information about the trust anchors, and a "nonV" type indicates that anchors of the
resolver.

nonV: The resolver does not perform DNSSEC validation.

4. Sentinel Test Result Considerations

other: The description in the previous section describes a simple situation
where the test queries were being passed to a single recursive
resolver that directly queried authoritative name servers, including
the root servers.

There is also the common case where the end client's browser or
operating system is configured to use multiple resolvers. In these
cases, a SERVFAIL response from one resolver may cause the end client
to repeat the query against one of the other configured resolvers.

If the client's browser or operating system does not try the
additional resolvers, the sentinel test will effectively only be for
the first resolver.

If any of the client's resolvers are non-validating resolvers, the
tests will result in the client reporting that it has a non-
validating DNS environment ("nonV"), which is effectively the case.

If all properties of the client resolvers are DNSSEC-validating resolvers, but
some do not support this trusted key mechanism, then the result will resolver cannot be indeterminate with respect to trusted key status ("Vleg").
Similarly, if all the client's resolvers support analyzed by this mechanism, but
some have loaded the key into the trusted key stash and some have
not, then the result is indeterminate ("Vleg").
protocol.

3.1. Forwarders

There is also the common case of a recursive resolver using a
forwarder.

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

If this non-validating resolver it
has multiple forwarders, then the above considerations will apply.

If the validating resolver has a forwarding configuration, and uses
the CD bit on all forwarded queries, then this resolver is acting in
a manner that is identical to a standalone resolver. The same
consideration applies if any one of the forwarder targets is a non-
validating resolver. Similarly, if all the forwarder targets do not
apply this trusted key mechanism, the same considerations apply.

A more complex case is where all of the following conditions all 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 CD bit set
o The trusted key state differs between the forwarding resolver and
the forwarder target resolver

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

Please note

4. Sentinel Tests for a Set of 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 queries
authoritative name servers.

However, the common end user scenario is where a user's local DNS
resolution environment is configured to use a set of recursive
resolvers. The single resolver test technique will not function
reliably in such cases, as a a SERVFAIL might 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 cached according 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 [RFC2308]
section 7 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 up 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 5 minutes validate the contents of the root zone and the user is likely to
loose DNS service as a positive 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 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 up 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
TTL. configured
recursive resolvers before concluding that the name cannot be
resolved, is a very critical assumption for this test.

4.3. Test Procedure

The sentinel detection process test a DNS resolution environment with
three query names:

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. 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.
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 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 "not-ta" label, then
the "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 "not-ta"
query and a RRset response to the "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
"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 and third parties 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 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 name servers. 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

Petr Spacek implemented early versions of this technique into the
Knot resolver, and identified a number of places where it wasn't
clear, and provided very helpful text to address this.

Ondrej Sury of ISC has 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.

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.

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

8. IANA Considerations

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

9. Acknowledgements

This document has borrowed extensively from [RFC8145] for the
introductory text, and the authors would like to acknowledge and
thank the authors of that document both for some text excerpts and
for the more general stimulation of thoughts about monitoring the
progress of a roll of the KSK of the root zone of the DNS.

The authors would like to thank Joe Abley, Mehmet Akcin, Mark
Andrews, Richard Barnes, Ray Bellis, Stephane Bortzmeyer, David
Conrad, Ralph Dolmans, John Dickinson, Steinar Haug, Bob Harold, Wes
Hardaker, Paul Hoffman, Matt Larson, Jinmei Tatuya, Edward Lewis,
George Michaelson, Benno Overeinder, Matthew Pounsett, Andreas
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 a pull request.

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 -12 to -13:

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

o Moved toy ksk-test.net to implmentation 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:

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

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:

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

[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

[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:

invalid.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-02323.example.com.

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. 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 invalid.example.com bogus.example.com record invalid/bogus 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://invalid.example.com/1x1.gif,
http://root-key-sentinel-is-ta-02323.example.com/1x1.gif,
http://root-key-sentinel-not-ta-02323.example.com/1x1.gif).
(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 invalid.example.com 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://invalid.example.com/1x1.gif http://bogus.example.com/1x1.gif resource (the
invalid.example.com
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 legacy, validating resolvers. 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 what root
trust anchors this resolver is using.
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
"invalid"
"bogus" resource. His resolver resolves the root-key-sentinel-is-
ta-02323.example.com 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-
sentinel-not-ta-02323.example.com
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
02323 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 "invalid", "bogus", he can
fetch "root-key-
sentinel-is-ta-02323", "root-key-sentinel-is-ta-02323", but he cannot fetch "root-key-sentinel-not-ta-
02323". "root-key-
sentinel-not-ta-11112". From this, Dave knows that he is behind an upgraded,
validating resolver, which
collection of resolvers that all validate, all have the key with key
tag 11112 loaded and at least one of these resolvers has successfully installed loaded the new,
key with key-tag 02323
KSK. into its local trust anchor cache, Dave will
not be impacted by the KSK roll.

Just like Charlie and Dave, Ed cannot fetch the "invalid" "bogus" record. This
tells him that his resolvers are validating. When his
(upgraded) resolver (sentinel-
aware) resolvers performs the KSK Sentinel check for "root-key-
sentinel-is-ta-02323", it does *not* none of them have loaded the (new, 02323) KSK new key with key-
tag 02323 in
it's 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-
02323.example.com; root-key-
sentinel-not-ta-11112.example.com, and as it does not have the 02323 KSK, it is true that
this is not a trust anchor for it, all of Ed's resolvers both
perform DNSSEC validation and so it replies normally. This
means that Ed cannot fetch recognise the "invalid" resource, he also cannot
fetch the "root-key-sentinel-is-ta-02323" resource, but he can sentinel label Ed will be
unable to fetch the "root-key-sentinel-not-ta-02323" "root-key-sentinel-not-ta-11112" resource. This
tells Ed that his resolvers have not installed the new KSK. 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.

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