Commit c6b8cd34 authored by Mark Andrews's avatar Mark Andrews
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new draft

parent 11468384
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Network Working Group S. Weiler
Internet-Draft SPARTA, Inc.
Updates: 4033, 4034, 4035, 5155 D. Blacka
(if approved) VeriSign, Inc.
Intended status: Standards Track January 14, 2012
Expires: July 17, 2012
(if approved) Verisign, Inc.
Intended status: Standards Track April 30, 2012
Expires: November 1, 2012
Clarifications and Implementation Notes for DNSSECbis
draft-ietf-dnsext-dnssec-bis-updates-16
draft-ietf-dnsext-dnssec-bis-updates-18
Abstract
......@@ -37,7 +37,7 @@ Status of this Memo
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 July 17, 2012.
This Internet-Draft will expire on November 1, 2012.
Copyright Notice
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Internet-Draft DNSSECbis Implementation Notes January 2012
Internet-Draft DNSSECbis Implementation Notes April 2012
to this document. Code Components extracted from this document must
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Internet-Draft DNSSECbis Implementation Notes January 2012
Internet-Draft DNSSECbis Implementation Notes April 2012
Table of Contents
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4.2. Validating Responses to an ANY Query . . . . . . . . . . . 6
4.3. Check for CNAME . . . . . . . . . . . . . . . . . . . . . 6
4.4. Insecure Delegation Proofs . . . . . . . . . . . . . . . . 6
5. Interoperability Concerns . . . . . . . . . . . . . . . . . . 6
5. Interoperability Concerns . . . . . . . . . . . . . . . . . . 7
5.1. Errors in Canonical Form Type Code List . . . . . . . . . 7
5.2. Unknown DS Message Digest Algorithms . . . . . . . . . . . 7
5.3. Private Algorithms . . . . . . . . . . . . . . . . . . . . 8
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5.6. Setting the DO Bit on Replies . . . . . . . . . . . . . . 9
5.7. Setting the AD Bit on Queries . . . . . . . . . . . . . . 9
5.8. Setting the AD Bit on Replies . . . . . . . . . . . . . . 9
5.9. Always set the CD bit on Queries . . . . . . . . . . . . . 9
5.9. Always set the CD bit on Queries . . . . . . . . . . . . . 10
5.10. Nested Trust Anchors . . . . . . . . . . . . . . . . . . . 10
5.10.1. Closest Encloser . . . . . . . . . . . . . . . . . . 10
5.10.2. Accept Any Success . . . . . . . . . . . . . . . . . 11
5.10.3. Preference Based on Source . . . . . . . . . . . . . 11
5.11. Mandatory Algorithm Rules . . . . . . . . . . . . . . . . 12
5.12. Expect Extra Signatures From Strange Keys . . . . . . . . 12
6. Minor Corrections and Clarifications . . . . . . . . . . . . . 13
6.1. Finding Zone Cuts . . . . . . . . . . . . . . . . . . . . 13
6.2. Clarifications on DNSKEY Usage . . . . . . . . . . . . . . 13
6.3. Errors in Examples . . . . . . . . . . . . . . . . . . . . 13
6.4. Errors in RFC 5155 . . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 16
Appendix B. Discussion of Setting the CD Bit . . . . . . . . . . 17
5.11. Mandatory Algorithm Rules . . . . . . . . . . . . . . . . 11
5.12. Ignore Extra Signatures From Unknown Keys . . . . . . . . 11
6. Minor Corrections and Clarifications . . . . . . . . . . . . . 12
6.1. Finding Zone Cuts . . . . . . . . . . . . . . . . . . . . 12
6.2. Clarifications on DNSKEY Usage . . . . . . . . . . . . . . 12
6.3. Errors in Examples . . . . . . . . . . . . . . . . . . . . 12
6.4. Errors in RFC 5155 . . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 15
Appendix B. Discussion of Setting the CD Bit . . . . . . . . . . 15
Appendix C. Discussion of Trust Anchor Preference Options . . . . 18
C.1. Closest Encloser . . . . . . . . . . . . . . . . . . . . . 18
C.2. Accept Any Success . . . . . . . . . . . . . . . . . . . . 19
C.3. Preference Based on Source . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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Internet-Draft DNSSECbis Implementation Notes January 2012
Internet-Draft DNSSECbis Implementation Notes April 2012
1. Introduction and Terminology
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1.1. Structure of this Document
The clarifications to DNSSECbis are sorted according to their
importance, starting with ones which could, if ignored, lead to
The clarifications and changes to DNSSECbis are sorted according to
their importance, starting with ones which could, if ignored, lead to
security problems and progressing down to clarifications that are
expected to have little operational impact.
......@@ -207,9 +207,9 @@ Internet-Draft DNSSECbis Implementation Notes January 2012
[RFC5155] describes the use and behavior of the NSEC3 and NSEC3PARAM
records for hashed denial of existence. Validator implementations
are strongly encouraged to include support for NSEC3 because a number
of highly visible zones are expected to use it. Validators that do
not support validation of responses using NSEC3 will likely be
hampered in validating large portions of the DNS space.
of highly visible zones use it. Validators that do not support
validation of responses using NSEC3 will be hampered in validating
large portions of the DNS space.
[RFC5155] should be considered part of the DNS Security Document
Family as described by [RFC4033], Section 10.
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MAY indeed be using either and validators supporting these algorithms
MUST support both NSEC3 and NSEC responses.
MAY be using either and validators supporting these algorithms MUST
support both NSEC3 and NSEC responses.
2.2. SHA-2 Support
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Section 4.7 of RFC4035 permits security-aware resolvers to implement
a BAD cache. Because of scaling concerns not discussed in this
document, that guidance has changed: security-aware resolvers SHOULD
implement a BAD cache, as described in RFC4035.
implement a BAD cache as described in RFC4035.
4. Security Concerns
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Similarly, the algorithm would also allow an NSEC RR at the same
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on (or that should be based on) CNAMEs. When validating a NOERROR/
NODATA response, validators MUST check the CNAME bit in the matching
NSEC or NSEC3 RR's type bitmap in addition to the bit for the query
type. Without this check, an attacker could successfully transform a
positive CNAME response into a NOERROR/NODATA response.
type.
Without this check, an attacker could successfully transform a
positive CNAME response into a NOERROR/NODATA response by (e.g.)
simply stripping the CNAME RRset from the response. A naive
validator would then note that the QTYPE was not present in the
matching NSEC/NSEC3 RR, but fail to notice that the CNAME bit was
set, and thus the response should have been a positive CNAME
response.
4.4. Insecure Delegation Proofs
[RFC4035] Section 5.2 specifies that a validator, when proving a
delegation is not secure, needs to check for the absence of the DS
and SOA bits in the NSEC (or NSEC3) type bitmap. The validator also
needs to check for the presence of the NS bit in the matching NSEC
(or NSEC3) RR (proving that there is, indeed, a delegation), or
MUST check for the presence of the NS bit in the matching NSEC (or
NSEC3) RR (proving that there is, indeed, a delegation), or
alternately make sure that the delegation is covered by an NSEC3 RR
with the Opt-Out flag set. If this is not checked, spoofed unsigned
delegations might be used to claim that an existing signed record is
not signed.
with the Opt-Out flag set.
5. Interoperability Concerns
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Without this check, an attacker could reuse an NSEC or NSEC3 RR
matching a non-delegation name to spoof an unsigned delegation at
that name. This would claim that an existing signed RRset (or set of
signed RRsets) is below an unsigned delegation, thus not signed and
vulnerable to further attack.
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5. Interoperability Concerns
5.1. Errors in Canonical Form Type Code List
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authenticated NSEC RRset proving that no DS RRset exists, as
described above.
To paraphrase the above, when determining the security status of a
zone, a validator disregards any DS records listing unknown or
unsupported algorithms. If none are left, the zone is treated as if
it were unsigned.
Modified to consider DS message digest algorithms, a validator also
disregards any DS records using unknown or unsupported message digest
algorithms.
In other words, when determining the security status of a zone, a
validator disregards any authenticated DS records that specify
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unknown or unsupported DNSKEY algorithms. If none are left, the zone
is treated as if it were unsigned.
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This document modifies the above text to additionally disregard
authenticated DS records using unknown or unsupported message digest
algorithms.
5.3. Private Algorithms
As discussed above, section 5.2 of [RFC4035] requires that validators
As discussed above, Section 5.2 of [RFC4035] requires that validators
make decisions about the security status of zones based on the public
key algorithms shown in the DS records for those zones. In the case
of private algorithms, as described in [RFC4034] Appendix A.1.1, the
eight-bit algorithm field in the DS RR is not conclusive about what
algorithm(s) is actually in use.
If no private algorithms appear in the DS set or if any supported
algorithm appears in the DS set, no special processing will be
needed. In the remaining cases, the security status of the zone
depends on whether or not the resolver supports any of the private
algorithms in use (provided that these DS records use supported hash
functions, as discussed in Section 5.2). In these cases, the
resolver MUST retrieve the corresponding DNSKEY for each private
algorithm DS record and examine the public key field to determine the
algorithm in use. The security-aware resolver MUST ensure that the
hash of the DNSKEY RR's owner name and RDATA matches the digest in
the DS RR. If they do not match, and no other DS establishes that
the zone is secure, the referral should be considered Bogus data, as
discussed in [RFC4035].
If no private algorithms appear in the DS RRset, or if any supported
algorithm appears in the DS RRset, no special processing is needed.
Furthermore, if the validator implementation does not support any
private algorithms, or only supports private algorithms using an
algorithm number not present in the DS RRset, no special processing
is needed.
In the remaining cases, the security status of the zone depends on
whether or not the resolver supports any of the private algorithms in
use (provided that these DS records use supported hash functions, as
discussed in Section 5.2). In these cases, the resolver MUST
retrieve the corresponding DNSKEY for each private algorithm DS
record and examine the public key field to determine the algorithm in
use. The security-aware resolver MUST ensure that the hash of the
DNSKEY RR's owner name and RDATA matches the digest in the DS RR as
described in Section 5.2 of [RFC4035], authenticating the DNSKEY. If
all of the retrieved and authenticated DNSKEY RRs use unknown or
unsupported private algorithms, then the zone is treated as if it
were unsigned.
Note that if none of the private algorithm DS RRs can be securely
matched to DNSKEY RRs and no other DS establishes that the zone is
secure, the referral should be considered Bogus data as discussed in
[RFC4035].
This clarification facilitates the broader use of private algorithms,
as suggested by [RFC4955].
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When multiple RRSIGs cover a given RRset, [RFC4035] Section 5.3.3
suggests that "the local resolver security policy determines whether
the resolver also has to test these RRSIG RRs and how to resolve
conflicts if these RRSIG RRs lead to differing results." In most
cases, a resolver would be well advised to accept any valid RRSIG as
sufficient. If the first RRSIG tested fails validation, a resolver
would be well advised to try others, giving a successful validation
result if any can be validated and giving a failure only if all
RRSIGs fail validation.
If a resolver adopts a more restrictive policy, there's a danger that
properly-signed data might unnecessarily fail validation, perhaps
because of cache timing issues. Furthermore, certain zone management
techniques, like the Double Signature Zone-signing Key Rollover
method described in section 4.2.1.2 of [RFC4641] might not work
reliably.
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the resolver also has to test these RRSIG RRs and how to resolve
conflicts if these RRSIG RRs lead to differing results."
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This document specifies that a resolver SHOULD accept any valid RRSIG
as sufficient, and only determine that an RRset is Bogus if all
RRSIGs fail validation.
If a resolver adopts a more restrictive policy, there's a danger that
properly-signed data might unnecessarily fail validation due to cache
timing issues. Furthermore, certain zone management techniques, like
the Double Signature Zone-signing Key Rollover method described in
section 4.2.1.2 of [RFC4641], will not work reliably. Such a
resolver is also vulnerable to malicious insertion of gibberish
signatures.
5.5. Key Tag Calculation
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5.6. Setting the DO Bit on Replies
As stated in [RFC3225], the DO bit of the query MUST be copied in the
response. At least one implementation has done something different,
so it may be wise for resolvers to be liberal in what they accept.
response. However, in order to interoperate with implementations
that ignore this rule on sending, resolvers MUST ignore the DO bit in
responses.
5.7. Setting the AD Bit on Queries
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Section 3.2.3 of [RFC4035] describes under which conditions a
validating resolver should set or clear the AD bit in a response. In
order to protect legacy stub resolvers and middleboxes, validating
resolvers SHOULD only set the AD bit when a response both meets the
conditions listed in RFC 4035, section 3.2.3, and the request
contained either a set DO bit or a set AD bit.
5.9. Always set the CD bit on Queries
When processing a request with the CD bit set, a resolver SHOULD
attempt to return all responsive data, even data that has failed
DNSSEC validation. RFC4035 section 3.2.2 requires a resolver
processing a request with the CD bit set to set the CD bit on its
upstream queries.
order to interoperate with legacy stub resolvers and middleboxes that
neither understand nor ignore the AD bit, validating resolvers SHOULD
only set the AD bit when a response both meets the conditions listed
in RFC 4035, section 3.2.3, and the request contained either a set DO
bit or a set AD bit.
Prevailing wisdom suggests that a validating resolver SHOULD set the
CD bit on every upstream query regardless of whether the CD bit was
set on the incoming query or whether it has a trust anchor at or
above the QNAME. In other words, a validating resolver should
attempt to retrieve all possible data -- even that which it can not
validate itself -- on the grounds that a later query might come with
the CD bit set.
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RFC4035 is ambiguous about what to do when a cached response was
obtained with the CD bit not set, a case that only arises when the
5.9. Always set the CD bit on Queries
When processing a request with the CD bit set, a resolver SHOULD
attempt to return all response data, even data that has failed DNSSEC
validation. RFC4035 section 3.2.2 requires a resolver processing a
request with the CD bit set to set the CD bit on its upstream
queries.
This document further specifies that validating resolvers SHOULD set
the CD bit on every upstream query. This is regardless of whether
the CD bit was set on the incoming query or whether it has a trust
anchor at or above the QNAME.
[RFC4035] is ambiguous about what to do when a cached response was
obtained with the CD bit unset, a case that only arises when the
resolver chooses not to set the CD bit on all upstream queries, as
suggested above. In the typical case, no new query is required, nor
specified above. In the typical case, no new query is required, nor
does the cache need to track the state of the CD bit used to make a
given query. The problem arises when the cached response is a server
failure (RCODE 2), which may indicate that the requested data failed
......@@ -530,103 +543,39 @@ Internet-Draft DNSSECbis Implementation Notes January 2012
When presented with this situation, DNSSEC validators have a choice
of which trust anchor(s) to use. Which to use is a matter of
implementation choice. It is possible and perhaps advisable to
expose the choice of policy as a configuration option. The rest of
this section discusses some possible policies. As a default, we
suggest that validators implement the "Accept Any Success" policy
described below in Section 5.10.2 while exposing other policies as
configuration options.
5.10.1. Closest Encloser
One policy is to choose the trust anchor closest to the QNAME of the
response. In our example, that would be the "zone.example." trust
anchor.
implementation choice. Appendix C discusses several possible
algorithms.
This policy has the advantage of allowing the operator to trivially
override a parent zone's trust anchor with one that the operator can
validate in a stronger way, perhaps because the resolver operator is
affiliated with the zone in question. This policy also minimizes the
number of public key operations needed, which may be of benefit in
resource-constrained environments.
It is possible and advisable to expose the choice of policy as a
configuration option. As a default, it is suggested that validators
implement the "Accept Any Success" policy described in Appendix C.2
while exposing other policies as configuration options.
This policy has the disadvantage of possibly giving the user some
unexpected and unnecessary validation failures when sub-zone trust
anchors are neglected. As a concrete example, consider a validator
The "Accept Any Success" policy is to try all applicable trust
anchors until one gives a validation result of Secure, in which case
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that configured a trust anchor for "zone.example." in 2009 and one
for "example." in 2011. In 2012, "zone.example." rolls its KSK and
updates its DS records, but the validator operator doesn't update its
trust anchor. With the "closest encloser" policy, the validator gets
validation failures.
5.10.2. Accept Any Success
Another policy is to try all applicable trust anchors until one gives
a validation result of Secure, in which case the final validation
result is Secure. If and only if all applicable trust anchors give a
result of Insecure, the final validation result is Insecure. If one
or more trust anchors lead to a Bogus result and there is no Secure
result, then the final validation result is Bogus.
This has the advantage of causing the fewer validation failures,
which may deliver a better user experience. If one trust anchor is
out of date (as in our above example), the user may still be able to
get a Secure validation result (and see DNS responses).
This policy has the disadvantage of making the validator subject to
compromise of the weakest of these trust anchors while making its
relatively painless to keep old trust anchors configured in
perpetuity.
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5.10.3. Preference Based on Source
When the trust anchors have come from different sources (e.g.
automated updates ([RFC5011]), one or more DLV registries
([RFC5074]), and manually configured), a validator may wish to choose
between them based on the perceived reliability of those sources.
The order of precedence might be exposed as a configuration option.
For example, a validator might choose to prefer trust anchors found
in a DLV registry over those manually configured on the theory that
the manually configured ones will not be as aggressively maintained.
Conversely, a validator might choose to prefer manually configured
trust anchors over those obtained from a DLV registry on the theory
that the manually configured ones have been more carefully
authenticated.
Or the validator might do something more complicated: prefer a sub-
set of manually configured trust anchors (based on a configuration
option), then trust anchors that have been updated using the RFC5011
mechanism, then trust anchors from one DLV registry, then trust
anchors from a different DLV registry, then the rest of the manually
configured trust anchors.
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the final validation result is Secure. If and only if all applicable
trust anchors give a result of Insecure, the final validation result
is Insecure. If one or more trust anchors lead to a Bogus result and
there is no Secure result, then the final validation result is Bogus.
5.11. Mandatory Algorithm Rules
The last paragraph of RFC4035 Section 2.2 includes rules for which
algorithms must be used to sign a zone. Since these rules have been
confusing, we restate them in different language here:
The last paragraph of RFC4035 Section 2.2 includes rules describing
which algorithms must be used to sign a zone. Since these rules have
been confusing, they are restated using different language here:
The DS RRset and DNSKEY RRset are used to signal which algorithms
are used to sign a zone. The pressence of an algorithm in a
zone's DS or DNSKEY RRset set signals that that algorithm is used
to sign the entire zone.
are used to sign a zone. The presence of an algorithm in either a
zone's DS or DNSKEY RRset signals that that algorithm is used to
sign the entire zone.
A signed zone MUST include a DNSKEY for each algorithm present in
the zone's DS RRset and expected trust anchors for the zone. The
......@@ -643,19 +592,17 @@ Internet-Draft DNSSECbis Implementation Notes January 2012
Lastly, note that this a requirement at the server side, not the
client side. Validators SHOULD accept any single valid path. They
SHOULD NOT insist that all algorithms signalled in the DS RRset work,
and they MUST NOT insist that all algorithms signalled in the DNSKEY
SHOULD NOT insist that all algorithms signaled in the DS RRset work,
and they MUST NOT insist that all algorithms signaled in the DNSKEY
RRset work. A validator MAY have a configuration option to perform a
signature completeness test to support troubleshooting.
5.12. Expect Extra Signatures From Strange Keys
5.12. Ignore Extra Signatures From Unknown Keys
Validating resolvers should not be surprised to find RRSIGs in a zone
that do not (currently) have a corresponding DNSKEY in the zone.
Likewise, a validating resolver should not be surprised to find
RRSIGs with algorithm types that don't exist in the DNSKEY RRset or
DNSKEYs with algorithm types that don't appear in the zone's DS
RRset.
Validating resolvers MUST disregard RRSIGs in a zone that do not
(currently) have a corresponding DNSKEY in the zone. Similarly, a
validating resolver MUST disregard RRSIGs with algorithm types that
don't exist in the DNSKEY RRset.
Good key rollover and algorithm rollover practices, as discussed in
RFC4641 and its successor documents and as suggested by the rules in
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6. Minor Corrections and Clarifications
......@@ -690,18 +634,17 @@ Internet-Draft DNSSECbis Implementation Notes January 2012
6.2. Clarifications on DNSKEY Usage
Questions of the form "can I use a different DNSKEY for signing this
RRset" have occasionally arisen.
The short answer is "yes, absolutely". You can even use a different
DNSKEY for each RRset in a zone, subject only to practical limits on
the size of the DNSKEY RRset. However, be aware that there is no way
to tell resolvers what a particularly DNSKEY is supposed to be used
for -- any DNSKEY in the zone's signed DNSKEY RRset may be used to
authenticate any RRset in the zone. For example, if a weaker or less
trusted DNSKEY is being used to authenticate NSEC RRsets or all
dynamically updated records, that same DNSKEY can also be used to
sign any other RRsets from the zone.
It is possible to use different DNSKEYs to sign different subsets of
a zone, constrained only by the rules in Section 5.11. It is even
possible to use a different DNSKEY for each RRset in a zone, subject
only to practical limits on the size of the DNSKEY RRset and the
above rules. However, be aware that there is no way to tell
resolvers what a particular DNSKEY is supposed to be used for -- any
DNSKEY in the zone's signed DNSKEY RRset may be used to authenticate
any RRset in the zone. For example, if a weaker or less trusted
DNSKEY is being used to authenticate NSEC RRsets or all dynamically
updated records, that same DNSKEY can also be used to sign any other
RRsets from the zone.
Furthermore, note that the SEP bit setting has no effect on how a
DNSKEY may be used -- the validation process is specifically
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wildcard expansion. This is true for "x.w.example" but not for
"x.w.example.com", which of course has a label count of 4
(antithetically, a label count of 3 would imply the answer was the
result of a wildcard expansion).