A Root Key Trust Anchor
Sentinel for DNSSECgih@apnic.nethttp://www.apnic.netjoao@apnic.nethttp://www.apnic.netwarren@kumari.net
Internet
DNSOPThe DNS Security Extensions (DNSSEC) were developed to provide origin
authentication and integrity protection for DNS data by using digital
signatures. These digital signatures can be verified by building a chain
of trust starting from a trust anchor and proceeding down to a
particular node in the DNS. This document specifies a mechanism that
will allow an end user and third parties to determine the trusted key
state for the root key of the resolvers that handle that user's DNS
queries. Note that this method is only applicable for determining which
keys are in the trust store for the root key.[ This document is being collaborated on in Github at:
https://github.com/APNIC-Labs/draft-kskroll-sentinel. The most recent
version of the document, open issues, etc should all be available here.
The authors (gratefully) accept pull requests. RFC Editor, please remove
text in square brackets before publication. ]The DNS Security Extensions (DNSSEC) , and 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" ), 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 security-aware DNS resolvers that perform
validation of their responses can respond to certain queries in a manner
that allows an agent performing the queries to deduce whether a
particular key for the root has been loaded into that resolver's trusted
key store. This document also describes a procedure where a collection
of resolvers can be tested to determine if at least one of these
resolvers has loaded a given key into its trusted key store. These tests
can be used to determine whether a certain root zone KSK is ready to be
used as a trusted key, within the context of a planned root zone KSK key
roll.There are two primary use cases for this mechanism:Users may wish to ascertain whether their DNS resolution
environment resolvers is ready for an upcoming root KSK
rollover.Researchers want to perform Internet-wide studies about the
proportion of users who will be negatively impacted an upcoming root
KSK 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 tests described in this document use a test
comprising of a set of DNS queries to domain names that have special
values for the left-most label. The test relies on recursive resolvers
supporting a mechanism that recognises this special name pattern in
queries, and under certain defined circumstances will return a DNS
SERVFAIL response code (RCODE 2), mimicking the response code that is
returned by security-aware resolvers when DNSSEC validation fails.If a browser or operating system is configured with multiple
resolvers, and those resolvers have different properties (for example,
one performs DNSSEC validation and one does not), the sentinel test
described in this document can still be used, 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 stub resolver to query the next recursive
resolver in the locally configured set upon receipt of a SERVFAIL
response code) then this test may produce indeterminate or inconsistent
results. In some cases where these assumptions do not hold, repeating
the same test query set may generate different results.Note that the sentinel mechanism described here measures a very
different (and likely more useful) metric than .
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.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.DNSSEC-Validating resolvers that implement this mechanism MUST
perform validation of responses in accordance with the DNSSEC response
validation specification .This sentinel mechanism makes use of two special labels:root-key-sentinel-is-ta-<key-tag>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 ,
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 which the validating DNS
resolver is currently trusting as a trust anchor?" Labels containing
"root-key-sentinel-not-ta-<key-tag>" is used to answer the
question "Is this the Key Tag of a key which the validating DNS resolver
is *not* currently trusting as a trust anchor?"All of the following conditions must be met to trigger special
processing inside resolver code: The DNS response is DNSSEC validated.The result of validation is "Secure".The Checking Disabled (CD) bit in the query is not set.The QTYPE is either A or AAAA (Query Type value 1 or 28).The OPCODE is QUERY.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.Responses which fulfil all of the preconditions in 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 that is
not in either the AddPend or Revoked state as described in ).Second, the resolver alters the response being sent to the original
query based on both the left-most label and the presence of a key with
given Key Tag in the trust anchor store. Two labels and two possible
states of the corresponding key generate four possible combinations
summarized in the table:Instruction "return SERVFAIL" means that the resolver MUST set
RCODE=SERVFAIL (value 2) and the ANSWER section of the DNS response
MUST be empty, ignoring all other documents which specify content of
the ANSWER section.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 applies instead to
a collection of DNS resolvers, as might be found in the DNS
configuration of an end-user environment.The critical aspect of the DNS names used in this mechanism is that
they contain the specified label for either the positive and negative
test as the left-most label in the query name.The sentinel detection procedure can test a DNS resolver using three
queries: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.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.A query name that is signed with a DNSSEC signature that cannot
be validated (described as a "bogus" RRset in Section 5 of , when, for example, an RRset is not signed with a
valid RRSIG record).The responses received from queries to resolve each of these 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 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, DNS resolvers are
classified by five distinct behavior types using the labels: "Vnew",
"Vold", "Vind", "nonV", and "other". These labels correspond to resolver
system behaviour types as follows:A DNS resolver that is configured to implement
this mechanism and has loaded the nominated key into their local
trusted key stores will respond with an A or AAAA RRset response for
the associated "root-key-sentinel-is-ta" queries, SERVFAIL for
"root-key-sentinel-not-ta" queries and SERVFAIL for the signed name
queries that return "bogus" validation status.A DNS resolver that is configured to implement
this mechanism and has not loaded the nominated key into their local
trusted key stores will respond with an SERVFAIL for the associated
"root-key-sentinel-is-ta" queries, an A or AAAA RRset response for
"root-key-sentinel-not-ta" queries and SERVFAIL for the signed name
queries that return "bogus" validation status.A DNS resolver that has is not configured to
implement this mechanism will respond with an A or AAAA RRset
response for "root-key-sentinel-is-ta", an A or AAAA RRset response
for "root-key-sentinel-not-ta" and SERVFAIL for the name that
returns "bogus" validation status. This set of responses does not
give any information about the trust anchors used by this
resolver.A 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 name that returns "bogus" validation
status.There is the potential to admit other
combinations of responses to these three queries. While this may
appear self-contradictory, there are cases where such an outcome is
possible. For example, in DNS resolver farms what appears to be a
single DNS resolver that responds to queries passed to a single IP
address is in fact constructed as a a collection of slave resolvers,
and the query is passed to one of these internal resolver engines.
If these individual slave resolvers in the farm do not behave
identically, then other sets of results can be expected from these
three queries. In such a case, no determination about the
capabilities of this DNS resolver farm can be made.Note that SERVFAIL might be cached according to Section 7 of for up to 5 minutes and a positive answer for up to
its TTL.If a client directs these three queries to a single resolver, the
responses should allow the client to determine the capability of the
resolver, and if it supports this sentinel mechanism, whether or not it
has a particular key in its trust anchor store, as in the following
table:The nominated key is trusted by the
resolver.The nominated key is not yet trusted by the
resolver.There is no information about the trust anchors
of the resolver.The resolver does not perform DNSSEC
validation.The properties of the resolver cannot be
analyzed by this protocol.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,
then the resolver will presumably mirror the capabilities of the
forwarder target resolver.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.A more complex case is where all of the following conditions hold:
Both the validating resolver and the forwarder target resolver
support this trusted key sentinel mechanismThe local resolver's queries do not have the CD bit setThe trusted key state differs between the forwarding resolver
and the forwarder target resolver In such a case, either the outcome is indeterminate
validating ("Vind"), or a case of mixed signals such as SERVFAIL in
all three responses, ("other") which is similarly an indeterminate
response with respect to the trusted key state.The description in 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 response from one resolver may cause the
local stub resolver to repeat the query against one of the other
configured resolvers and the results may be inconclusive.In describing a test procedure that can be used in this environment
of a set of DNS resolvers there are some necessary changes to the nature
of the question that this test can answer, the assumptions about the
behaviour of the DNS resolution environment, and some further
observations about potential variability in the test outcomes.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.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.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.When the user's stub resolver passes a query to a resolver in
the configured resolver set, it will get a consistent answer over
the timeframe of the queries. This assumption implies that if the
same query is asked by the same stub resolver multiple times in
succession to the same recursive resolver, the recursive
resolver's response will be the same for each of these
queries.All DNSSEC-validating resolvers have KSK-current in their local
trust anchor cache.There is no current published measurement data that indicates to
what extent the first two assumptions listed here are valid, and how
many end users may be impacted by these assumptions. In particular,
the first assumption, that a consistent SERFAIL response will cause
the local stub DNS resolution environment to query all of its
configured recursive resolvers before concluding that the name cannot
be resolved, is a very critical assumption for this test.The sentinel detection process tests a DNS resolution environment
with three query names:A query name that is signed with a DNSSEC signature that cannot
be validated (described as a "bogus" RRset in Section 5 of , when, for example, an RRset is not signed with
a valid RRSIG record).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.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: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.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.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.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.This document describes a mechanism to allow users to determine the
trust anchor state of root zone key signing keys in the DNS resolution
system that they use. If the user executes third party code, then this
information may also be available to the third party.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.The mechanism in this document enables third parties (with either
good or bad intentions) to learn something about the security
configuration of recursive DNS resolvers. That is, someone who can cause
an Internet user to make specific DNS queries (e.g. via web-based
advertisements or javascript in web pages), can, under certain specific
circumstances that includes additional knowledge of the resolvers that
are invoked by the user, determine which trust anchors are configured in
these resolvers. Without this additional knowledge, the third party can
infer the aggregate capabilities of the user's DNS resolution
environment, but cannot necessarily infer the trust configuration of any
recursive name server.[ RFC Editor: Please remove before publication. As this section will
be removed, it is more conversational than would appear in a published
doc. ] List of known resolver implementations (alphabetical):Ondrej Sury of ISC reported to the DNSOP Working
Group in April 2018 that this technique was peer-reviewed and merged
into BIND master branch with the intent to backport the feature into
older release branches. The merge request:
https://gitlab.isc.org/isc-projects/bind9/merge_requests/123
Information on configuring this can be found in the BIND 9.13.0
Administrator Reference Manual (ARM), available at
https://ftp.isc.org/isc/bind9/9.13.0/doc/arm/Bv9ARM.pdfPetr Spacek implemented early versions
of this technique into the Knot resolver, identified a number of
places where it wasn't clear, and provided very helpful text to
address these issues and make the document mode clear. Petr also
identified an embarrassingly large number of typos (and similar) in
the ksk-test setup. More information is at
http://knot-resolver.readthedocs.io/en/stable/modules.html#sentinel-for-detecting-trusted-keys
Benno Overeinder of NLnet Labs reported to the
DNSOP Working Group in April 2018 an intention to support this
technique in Unbound in the near future. This is now implemented in
Unbound version 1.7.1, available from
http://unbound.nlnetlabs.nl/download.html . Configuration
information is at
http://unbound.nlnetlabs.nl/documentation/unbound.conf.htmlA (partial) list of "client" / user side implementations (the author
was keeping a more complete list of implementations, but has misplaced
it - apologies, I'm happy to re-add them if you send me a note.):An Javascript implementation
of the client side of this protocol is available at:
http://www.ksk-test.netHugo
Salgado-Hernández has created an implementation at
http://test.kskroll.dnssec.lab.nic.cl/The code for this
implementation is published at
https://github.com/paulehoffman/sentinel-testbed [Note to IANA, to be removed prior to publication: there are no IANA
considerations stated in this version of the document.]This document has borrowed extensively from
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-Hernández, 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.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 -13 to -14:Addressed nits from Bob Harold -
https://mailarchive.ietf.org/arch/msg/dnsop/j4Serw0z24o470AnlD8ISo8o9k4Formatting changes (and a bit more text) in the implementation
section.Closes PR #21: Clarify indeterminate and resolution systems,Closes PR #22: Updates to -13 describing the test procedure for a
set of resolversCloses PR #23: Fix sundry typos,Closes PR #24: Editorial and clarifications to the new textCloses PR #25: Clarified when the test can be runFrom -12 to -13:Merged Paul Hoffmans PR#19, PR#20.Moved toy ksk-test.net to implementation section.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:Moved the Walkthrough Example to the end of the document as an
appendix.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.Corrected minor typos.Revised the Privacy Considerations.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:Clarified the preconditions for this mechanism as per Working
Group mailing list discussion.Corrected minor typo.From -09 to -10:Clarified the precondition list to specify that the resolver had
performed DNSSEC-validation by setting the AD bit in the
responseClarified the language referring to the operation of RFC8145
signalling.From -08 to -09:Incorporated Paul Hoffman's PR # 15 (Two issues from the
Hackathon) -
https://github.com/APNIC-Labs/draft-kskroll-sentinel/pull/15Clarifies that the match is on the *original* QNAME.From -08 to -07:Changed title from "A Sentinel for Detecting Trusted Keys in
DNSSEC" to "A Root Key Trust Anchor Sentinel for DNSSEC".Changed magic string from "kskroll-sentinel-" to
"root-key-sentinel-" -- this time for sure, Rocky!From -07 to -06:Addressed GitHub PR #14: Clarifications regarding caching and
SERVFAIL responsesAddressed GitHub PR #12, #13: Clarify situation with multiple
resolvers, Fix editorial nits.From -05 to -06: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: Incorporated Duane's #10Integrated 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:Addressed GitHub pull requests #4, #5, #6, #7 #8.Added Duane's privacy concernsMakes the use cases clearerFixed some A/AAAA stuffChanged the example numbersMade it clear that names and addresses must be realFrom -02 to -03:Integrated / published comments from Paul in GitHub PR #2 -
https://github.com/APNIC-Labs/draft-kskroll-sentinel/pull/2Made the Key Tag be decimal, not hex (thread / consensus in
https://mailarchive.ietf.org/arch/msg/dnsop/Kg7AtDhFRNw31He8n0_bMr9hBuE
)From -01 to 02:Removed Address Record definition.Clarified that many things can cause SERVFAIL.Made examples FQDN.Fixed a number of typos.Had accidentally said that Charlie was using a non-validating
resolver in example.[ TODO(WK): Doc says Key Tags are hex, is this really what the WG
wants? ]And active key is one that can be used *now* (not e.g
AddPend)From -00 to 01:Added a conversational description of how the system is intended
to work.Clarification that this is for the root.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.This Appendix provides a non-normative example of how the sentinel
mechanism could be used, and what each participant does. It is provided
in a conversational tone to be easier to follow. The examples here all
assume that each person has just one resolver, or a system of resolvers
that have the same properties.Alice is in charge of the DNS root KSK (Key Signing Key), and would
like to roll / replace the key with a new one. She publishes the new
KSK, but would like to be able to predict / measure what the impact will
be before removing/revoking the old key. The current KSK has a Key Tag
of 11112, the new KSK has a Key Tag of 02323. Users want to verify that
their resolver will not break after Alice rolls the root KSK key (that
is, starts signing with just the KSK whose Key Tag is 02323).Bob, Charlie, Dave, Ed are all users. They use the DNS recursive
resolvers supplied by their ISPs. They would like to confirm that their
ISPs have picked up the new KSK. Bob's ISP does not perform validation.
Charlie's ISP does validate, but the resolvers have not yet been
upgraded to support this mechanism. Dave and Ed's resolvers have been
upgraded to support this mechanism; Dave's resolver has the new KSK,
Ed's resolver hasn't managed to install the 02323 KSK in its trust store
yet.Geoff is a researcher, and would like to both provide a means for
Bob, Charlie, Dave and Ed to be able to perform tests, and also would
like to be able to perform Internet-wide measurements of what the impact
will be (and report this back to Alice).Geoff sets an authoritative DNS server for example.com, and also a
webserver (www.example.com). He adds three address records to
example.com: bogus.example.com. IN AAAA 2001:db8::1root-key-sentinel-is-ta-02323.example.com. IN AAAA
2001:db8::1root-key-sentinel-not-ta-11112.example.com. IN AAAA
2001:db8::1Note 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 bogus.example.com record have bogus validation status (for example,
by editing the signed zone and entering garbage for the signature).
Geoff also configures his webserver to listen on 2001:db8::1 and serve a
resource (for example, a 1x1 GIF, 1x1.gif) for all of these names. The
webserver also serves a webpage (www.example.com) which contains links
to these 3 resources (http://bogus.example.com/1x1.gif,
http://root-key-sentinel-is-ta-02323.example.com/1x1.gif,
http://root-key-sentinel-not-ta-11112.example.com/1x1.gif).Geoff then asks Bob, Charlie, Dave and Ed to browse to
www.example.com. Using the methods described in this document, the users
can figure out what their fate will be when the 11112 KSK is
removed.Bob is not using a validating resolver. This means that he will be
able to resolve bogus.example.com (and fetch the 1x1 GIF) - this tells
him that the KSK roll does not affect him, and so he will be OK.Charlie's resolvers are validating, but they have not been upgraded
to support the KSK sentinel mechanism. Charlie will not be able to fetch
the http://bogus.example.com/1x1.gif resource (the bogus.example.com
record is bogus, and none of his resolvers will resolve it). He is able
to fetch both of the other resources - from this he knows (see the logic
in the body of this document) that he is using validating resolvers, but
at least one of these resolvers is not configured to perform sentinel
processing. The KSK sentinel method cannot provide him with a definitive
answer to the question of whether he will be impacted by the KSK
roll.Dave's resolvers implement the sentinel method, and have picked up
the new KSK. For the same reason as Charlie, he cannot fetch the "bogus"
resource. His resolver resolves the
root-key-sentinel-is-ta-02323.example.com name normally (it contacts the
example.com authoritative servers, etc); as it supports the sentinel
mechanism, just before Dave's recursive resolver sends the reply to
Dave's stub, it performs the KSK Sentinel check. The QNAME starts with
"root-key-sentinel-is-ta-", and the recursive resolver does indeed have
a key with the Key Tag of 02323 in its root trust store. This means that
that this part of the KSK Sentinel check passes (it is true that Key Tag
02323 is in the trust anchor store), and the recursive resolver replies
normally (with the answer provided by the authoritative server). Dave's
recursive resolver then resolves the
root-key-sentinel-not-ta-11112.example.com name. Once again, it performs
the normal resolution process, but because it implements KSK Sentinel
(and the QNAME starts with "root-key-sentinel-not-ta-"), just before
sending the reply, it performs the KSK Sentinel check. As it has the key
with key-tag 11112 in it's trust anchor store, the answer to "is this
*not* a trust anchor" is false, and so the recursive resolver does not
reply with the answer from the authoritative server - instead, it
replies with a SERVFAIL (note that replying with SERVFAIL instead of the
original answer is the only mechanism that KSK Sentinel uses). This
means that Dave cannot fetch "bogus", he can fetch
"root-key-sentinel-is-ta-02323", but he cannot fetch
"root-key-sentinel-not-ta-11112". From this, Dave knows that he is
behind an collection of resolvers that all validate, all have the key
with key tag 11112 loaded and at least one of these resolvers has loaded
the key with key-tag 02323 into its local trust anchor cache, Dave will
not be impacted by the KSK roll.Just like Charlie and Dave, Ed cannot fetch the "bogus" record. This
tells him that his resolvers are validating. When his (sentinel-aware)
resolvers performs the KSK Sentinel check for
"root-key-sentinel-is-ta-02323", none of them have loaded the new key
with key-tag 02323 in their local trust anchor store. This means check
fails, and Ed's recursive resolver converts the (valid) answer into a
SERVFAIL error response. It performs the same check for
root-key-sentinel-not-ta-11112.example.com, and as all of Ed's resolvers
both perform DNSSEC validation and recognise the sentinel label Ed will
be unable to fetch the "root-key-sentinel-not-ta-11112" resource. This
tells Ed that his resolvers have not installed the new KSK and he will
be negatively implacted by the KSK roll..Geoff would like to do a large scale test and provide the information
back to Alice. He uses some mechanism such as causing users to go to a
web page to cause a large number of users to attempt to resolve the
three resources, and then analyzes the results of the tests to determine
what percentage of users will be affected by the KSK rollover event.This description is a simplified example - it is not anticipated that
Bob, Charlie, Dave and Ed will actually look for the absence or presence
of web resources; instead, the webpage that they load would likely
contain JavaScript (or similar) which displays the result of the tests,
sends the results to Geoff, or both. This sentinel mechanism does not
rely on the web: it can equally be used by trying to resolve the names
(for example, using the common "dig" command) and checking which result
in a SERVFAIL.