DNSEXT R. Arends
Internet-Draft Telematica Instituut
Expires: August 28, 2003 M. Kosters
D. Blacka
Verisign, Inc.
February 27, 2003
DNSSEC Opt-In
draft-ietf-dnsext-dnssec-opt-in-05
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on August 28, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
In RFC 2535, delegations to unsigned subzones are cryptographically
secured. Maintaining this cryptography is not practical or
necessary. This document describes an "Opt-In" model that allows
administrators to omit this cryptography and manage the cost of
adopting DNSSEC with large zones.
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Table of Contents
1. Definitions and Terminology . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Additions . . . . . . . . . . . . . . . . . . . . . 5
3.1 Server Considerations . . . . . . . . . . . . . . . . . . . 6
3.1.1 Delegations Only . . . . . . . . . . . . . . . . . . . . . . 6
3.1.2 Insecure Delegation Responses . . . . . . . . . . . . . . . 6
3.1.3 Wildcards and Opt-In . . . . . . . . . . . . . . . . . . . . 6
3.1.4 Dynamic Update . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Client Considerations . . . . . . . . . . . . . . . . . . . 7
3.2.1 Delegations Only . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2 Validation Process Changes . . . . . . . . . . . . . . . . . 7
3.2.3 NXT Record Caching . . . . . . . . . . . . . . . . . . . . . 8
3.2.4 Use of the AD bit . . . . . . . . . . . . . . . . . . . . . 8
4. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Transition Issues . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 17
Normative References . . . . . . . . . . . . . . . . . . . . 18
Informative References . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19
A. Implementing Opt-In using "Views" . . . . . . . . . . . . . 21
B. Changes from Prior Versions . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . 25
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1. Definitions and Terminology
Throughout this document, familiarity with the DNS system, RFC 1035
[1], DNS security extensions, RFC 2535 [2], and DNSSEC terminology
RFC 3090 [8] is assumed.
The following abbreviations and terms are used in this document:
RR: is used to refer to a DNS resource record.
RRset: refers to a Resource Record Set, as defined by [6]. In this
document, the RRset is also defined to include the covering SIG
records, if any exist.
signed name: refers to a DNS name that has, at minimum, a (signed)
NXT record.
unsigned name: refers to a DNS name that does not (at least) have a
NXT record.
covering NXT record/RRset: is the NXT record used to prove
(non)existence of a particular name or RRset. This means that for
a RRset or name 'N', the covering NXT record has the name 'N', or
has an owner name less than 'N' and "next" name greater than 'N'.
delegation: refers to a NS RRset with a name different from the
current zone apex (non-zone-apex), signifying a delegation to a
subzone.
secure delegation: refers to a signed name containing a delegation
(NS RRset), and a signed DS RRset, signifying a delegation to a
signed subzone.
2535/DS insecure delegation: refers to a signed name containing a
delegation (NS RRset), but lacking a DS RRset, signifying a
delegation to an unsigned subzone.
Opt-In insecure delegation: refers to an unsigned name containing
only a delegation NS RRset. The covering NXT record uses the
Opt-In methodology described in this document.
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 [5].
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2. Overview
The cost to cryptographically secure delegations to unsigned zones is
high for large delegation-centric zones and zones where insecure
delegations will be updated rapidly. For these zones, the costs of
maintaining the NXT record chain may be extremely high relative to
the gain of cryptographically authenticating existence of unsecured
zones.
This document describes a method of eliminating the superfluous
cryptography present in secure delegations to insecure zones. Using
"Opt-In", a zone administrator can choose to remove insecure
delegations from the NXT chain. This is accomplished by extending
the semantics of the NXT record by using a redundant bit in the type
map.
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3. Protocol Additions
In RFC 2535, delegation NS RRsets are not signed, but are instead
accompanied by a NXT RRset of the same name, and possibly a
("no-key") KEY RR [2] or DS record [3]. The security status of the
subzone is determined by the presence of the KEY or DS records,
cryptographically proven by the NXT record. Opt-In expands this
definition by allowing insecure delegations to exist within an
otherwise signed zone without the corresponding NXT record at the
delegation's owner name. These insecure delegations are proven
insecure by using a covering NXT record.
Since this represents a change of the interpretation of NXT records,
resolvers must be able to distinguish between RFC 2535 NXT records
and Opt-In NXT records. This is accomplished by "tagging" the NXT
records that cover (or potentially cover) insecure delegation nodes.
This tag is indicated by the absence of the NXT bit in the type map.
Since the NXT bit in the type map merely indicates the existence of
the record itself, this bit is redundant and safe for use as a tag.
An Opt-In tagged NXT record does not assert the (non)existence of the
delegations that it covers (except for a delegation with the same
name). This allows for the addition or removal of these delegations
without recalculating or resigning the NXT chain. However, Opt-In
tagged NXT records do assert the (non)existence of other RRsets.
An Opt-In NXT record MAY have the same name as an insecure
delegation. In this case, the delegation is proven insecure by the
lack of a DS bit in type map, or the presence of a "no-key" KEY
RRset, and the NXT record does assert the existence of the
delegation.
Zones using Opt-In MAY contain a mixture of Opt-In tagged NXT records
and RFC 2535 NXT records. If a NXT record is not Opt-In, there MUST
NOT be any insecure delegations (or any other records) between it and
the RRsets indicated by the 'next domain name' in the NXT RDATA. If
it is Opt-In, there MUST only be insecure delegations between it and
the next node indicated by the 'next domain name' in the NXT RDATA.
In summary,
o An Opt-In NXT type is identified by a zero-valued (or
not-specified) NXT bit in the type bit map of the NXT record.
o A RFC2535 NXT type is identified by a one-valued NXT bit in the
type bit map of the NXT record.
and,
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o An Opt-In NXT record does not assert the non-existence of a name
between its owner name and "next" name, although it does assert
that any name in this span MUST be an insecure delegation.
o An Opt-In NXT record does assert the (non)existence of RRsets with
the same owner name.
3.1 Server Considerations
Opt-In imposes some new requirements on authoritative DNS servers.
3.1.1 Delegations Only
This specification dictates that only insecure delegations may exist
between the owner and "next" names of an Opt-In tagged NXT record.
Signing tools SHOULD NOT generate signed zones that violate this
restriction. Servers SHOULD refuse to load and/or serve zones that
violate this restriction.
3.1.2 Insecure Delegation Responses
When returning an Opt-In insecure delegation, the server MUST return
the covering NXT RRset in the Authority section.
This presents a change from RFC 2535, where the "no-key" KEY RRset
would be returned instead. However, in the delegation signer
proposal, NXT records already must be returned along with the
insecure delegation. The primary difference that this proposal
introduces is that the Opt-In tagged NXT record will have a different
owner name from the delegation RRset. This may require
implementations to do a NXT search on cached responses.
3.1.3 Wildcards and Opt-In
RFC 2535, in section 5.3, describes the practice of returning NXT
records to prove the non-existence of an applicable wildcard in
non-existent name responses. This NXT record can be described as a
"negative wildcard proof". The use of Opt-In NXT records changes the
necessity for this practice. For non-existent name responses when the
query name (qname) is covered by an Opt-In tagged NXT record, servers
MAY choose to omit the wildcard proof record, and clients MUST NOT
treat the absence of this NXT record as a validation error.
The intent of the RFC 2535 negative wildcard proof requirement is to
prevent malicious users from undetectably removing valid wildcard
responses. In order for this cryptographic proof to work, the
resolver must be able to prove:
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1. The exact qname does not exist. This is done by the "normal" NXT
record.
2. No applicable wildcard exists. This is done by returning a NXT
record proving that the wildcard does not exist (negative
wildcard proof).
However, if the NXT record covering the exact qname is an Opt-In NXT
record, the resolver will not be able to prove the first part of this
equation, as the qname might exist as an insecure delegation. Thus,
since the total proof cannot be completed, the negative wildcard
proof record is not useful.
The negative wildcard proof is also not useful when returned as part
of an Opt-In insecure delegation response for a similar reason: the
resolver cannot prove that the qname does or does not exist, and
therefore cannot prove that a wildcard expansion is valid.
The presence of an Opt-In tagged NXT record does not change the
practice of returning a NXT along with a wildcard expansion. Even
though the Opt-In NXT will not be able to prove that the wildcard
expansion is valid, it will prove that the wildcard expansion is not
masking any signed records.
3.1.4 Dynamic Update
Opt-In changes the semantics of Secure DNS Dynamic Update [7]. In
particular, it introduces the need for rules that describe when to
add or remove a delegation name from the NXT chain. This document
does not attempt to define these rules. Until these rules are
defined, servers MUST NOT process DNS Dynamic Update requests against
zones that use Opt-In NXT records.
3.2 Client Considerations
Opt-In imposes some new requirements on DNS resolvers (caching or
otherwise).
3.2.1 Delegations Only
As stated in the "Server Considerations" section above, this
specification restricts the namespace covered by Opt-In tagged NXT
records to insecure delegations only. Thus, resolvers MUST reject as
invalid any records that fall within an Opt-In NXT record's span that
are not NS records or corresponding glue records.
3.2.2 Validation Process Changes
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This specification does not change the resolver's resolution
algorithm. However, it does change the DNSSEC validation process.
Resolvers MUST be able to use Opt-In tagged NXT records to
cryptographically prove the validity and security status (as
insecure) of a referral. Resolvers determine the security status of
the referred-to zone as follows:
o In RFC 2535, the security status is proven by existence of a
verified "no-key" KEY RRset. The absence of the "no-key" KEY
RRset indicates that the referred-to zone is secure.
o Using Delegation Signer, the security status is proven by the
existence or absence of a DS RRset at the same name as the
delegation. The existence of the DS RRset indicates that the
referred-to zone is secure. The absence of the DS RRset is proven
using a verified NXT record of the same name that does not have
the DS bit set in the type map. This NXT record MAY also be
tagged as Opt-In.
o Using Opt-In, the security status is proven by the existence of a
DS record (for secure) or the presence of a verified Opt-In tagged
NXT record that covers the delegation name. That is, the NXT
record does not have the NXT bit set in the type map, and the
delegation name falls between the NXT's owner and "next" name.
Using Opt-In does not substantially change the nature of following
referrals within DNSSEC. At every delegation point, the resolver
will have cryptographic proof that the subzone is secure or insecure.
When receiving either an Opt-In insecure delegation response or a
non-existent name response where that name is covered by an Opt-In
tagged NXT record, the resolver MUST NOT require proof (in the form
of a NXT record) that a wildcard did not exist.
3.2.3 NXT Record Caching
Caching resolvers MUST be able to retrieve the appropriate covering
Opt-In NXT record when returning referrals that need them. This
requirement differs from Delegation Signer in that the covering NXT
will not have the same owner name as the delegation. Some
implementations may have to use new methods for finding these NXT
records.
3.2.4 Use of the AD bit
The AD bit, as defined by [4], MUST NOT be set when:
o sending a non-existent name (NXDOMAIN) response where the covering
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NXT is tagged as Opt-In.
o sending an Opt-In insecure delegation response, unless the
covering (Opt-In) NXT record's owner name equals the delegation
name.
This rule is based on what the Opt-In NXT record actually proves.
For names that exist between the Opt-In NXT record's owner and "next"
names, the Opt-In NXT record cannot prove the non-existence or
existence of the name. As such, not all data in the response has
been cryptographically verified, so the AD bit cannot be set.
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4. Benefits
Using Opt-In allows administrators of large and/or changing
delegation-centric zones to minimize the overhead involved in
maintaining the security of the zone.
Opt-In accomplishes this by eliminating the need for both "no-key"
KEY (in [2]) and NXT records for insecure delegations. This, in a
zone with a large number of delegations to unsigned subzones, can
lead to substantial space savings (both in memory and on disk).
Additionally, Opt-In allows for the addition or removal of insecure
delegations without modifying the NXT record chain. Zones that are
frequently updating insecure delegations (e.g., TLDs) can avoid the
substantial overhead of modifying and resigning the affected NXT
records.
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5. Example
Consider the zone EXAMPLE, shown below. This is a zone where all of
the NXT records are tagged as Opt-In.
Example A: Fully DS/Opt-In Zone.
EXAMPLE. SOA ...
EXAMPLE. SIG SOA ...
EXAMPLE. NS FIRST-SECURE.EXAMPLE.
EXAMPLE. SIG NS ...
EXAMPLE. KEY ...
EXAMPLE. SIG KEY ...
EXAMPLE. NXT FIRST-SECURE.EXAMPLE. SOA NS SIG KEY
EXAMPLE. SIG NXT ...
FIRST-SECURE.EXAMPLE. A ...
FIRST-SECURE.EXAMPLE. SIG A ...
FIRST-SECURE.EXAMPLE. NXT NOT-SECURE-2.EXAMPLE. A SIG
FIRST-SECURE.EXAMPLE. SIG NXT ...
NOT-SECURE.EXAMPLE. NS NS.NOT-SECURE.EXAMPLE.
NS.NOT-SECURE.EXAMPLE. A ...
NOT-SECURE-2.EXAMPLE. NS NS.NOT-SECURE.EXAMPLE.
NOT-SECURE-2.EXAMPLE NXT SECOND-SECURE.EXAMPLE NS SIG
NOT-SECURE-2.EXAMPLE SIG NXT ...
SECOND-SECURE.EXAMPLE. NS NS.ELSEWHERE.
SECOND-SECURE.EXAMPLE. DS ...
SECOND-SECURE.EXAMPLE. SIG DS ...
SECOND-SECURE.EXAMPLE. NXT EXAMPLE. NS SIG KEY
SECOND-SECURE.EXAMPLE. SIG NXT ...
UNSIGNED.EXAMPLE. NS NS.UNSIGNED.EXAMPLE.
NS.UNSIGNED.EXAMPLE. A ...
In this example, a query for a signed RRset (e.g.,
"FIRST-SECURE.EXAMPLE A"), or a secure delegation
("WWW.SECOND-SECURE.EXAMPLE A") will result in a standard RFC 2535
response.
A query for a nonexistent RRset will result in a response that
differs from RFC 2535 by: the NXT record will be tagged as Opt-In,
there may be no NXT record proving the non-existence of a matching
wildcard record, and the AD bit will not be set.
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A query for an insecure delegation RRset (or a referral) will return
both the answer (in the Authority section) and the corresponding
Opt-In NXT record to prove that it is not secure.
Example A.1: Response to query for WWW.UNSIGNED.EXAMPLE. A
RCODE=NOERROR, AD=0
Answer Section:
Authority Section:
UNSIGNED.EXAMPLE. NS NS.UNSIGNED.EXAMPLE
SECOND-SECURE.EXAMPLE. NXT EXAMPLE. NS SIG DS
SECOND-SECURE.EXAMPLE. SIG NXT ...
Additional Section:
NS.UNSIGNED.EXAMPLE. A ...
In the Example A.1 zone, the EXAMPLE. node MAY use either style of
NXT record, because there are no insecure delegations that occur
between it and the next node, FIRST-SECURE.EXAMPLE. In other words,
Example A would still be a valid zone if the NXT record for EXAMPLE.
was changed to the following RR:
EXAMPLE. NXT FIRST-SECURE.EXAMPLE. SOA NS SIG KEY NXT
However, the other NXT records (FIRST-SECURE.EXAMPLE. and
SECOND-SECURE.EXAMPLE.) MUST be tagged as Opt-In because there are
insecure delegations in the range they define. (NOT-SECURE.EXAMPLE.
and UNSIGNED.EXAMPLE., respectively).
NOT-SECURE-2.EXAMPLE. is an example of an insecure delegation that is
part of the NXT chain and also covered by an Opt-In tagged NXT
record. Because NOT-SECURE-2.EXAMPLE. is a signed name, it cannot be
removed from the zone without modifying and resigning the prior NXT
record. Delegations with names that fall between
NOT-SECURE-2.EXAMPLE. and SECOND-SECURE.EXAMPLE. may be added or
removed without resigning any NXT records.
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6. Transition Issues
Opt-In is not backwards compatible with RFC 2535. RFC 2535 compliant
DNSSEC implementations will not recognize Opt-In tagged NXT records
as different from RFC 2535 NXT records. Because of this, RFC 2535
implementations will reject all Opt-In insecure delegations within a
zone as invalid.
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7. Security Considerations
Opt-In allows for unsigned names, in the form of delegations to
unsigned subzones, to exist within an otherwise signed zone. All
unsigned names are, by definition, insecure, and their validity or
existence cannot by cryptographically proven.
In general:
o Records with unsigned names (whether existing or not) suffer from
the same vulnerabilities as records in an unsigned zone. These
vulnerabilites are described in more detail in [10] (note in
particular sections 2.3, "Name Games" and 2.6, "Authenticated
Denial").
o Records with signed names have the same security whether or not
Opt-In is used.
Note that with or without Opt-In, an insecure delegation may have its
contents undetectably altered by an attacker. Because of this, the
primary difference in security that Opt-In introduces is the loss of
the ability to prove the existence or nonexistence of an insecure
delegation within the span of an Opt-In NXT record.
In particular, this means that a malicious entity may be able to
insert or delete records with unsigned names. These records are
normally NS records, but this also includes signed wildcard
expansions (while the wildcard record itself is signed, its expanded
name is an unsigned name).
For example, if a resolver received the following response from the
example zone above:
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Example S.1: Response to query for WWW.DOES-NOT-EXIST.EXAMPLE. A
RCODE=NOERROR
Answer Section:
Authority Section:
DOES-NOT-EXIST.EXAMPLE. NS NS.FORGED.
EXAMPLE. NXT FIRST-SECURE.EXAMPLE. SOA NS SIG KEY
EXAMPLE. SIG NXT ...
Additional Section:
The resolver would have no choice but to believe that the referral to
NS.FORGED. is valid. If a wildcard existed that would have been
expanded to cover "WWW.DOES-NOT-EXIST.EXAMPLE.", an attacker could
have undetectably removed it and replaced it with the forged
delegation.
Note that being able to add a delegation is functionally equivalent
to being able to add any record type: an attacker merely has to forge
a delegation to nameserver under his/her control and place whatever
records needed at the subzone apex.
While in particular cases, this issue may not present a significant
security problem, in general it should not be lightly dismissed.
Therefore, it is strongly RECOMMENDED that Opt-In be used sparingly.
In particular, zone signing tools SHOULD NOT default to Opt-In, and
MAY choose to not support Opt-In at all.
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8. IANA Considerations
None.
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9. Acknowledgments
The contributions, suggestions and remarks of the following persons
(in alphabetic order) to this draft are acknowledged:
Mats Dufberg, Miek Gieben, Olafur Gudmundsson, Bob Halley, Olaf
Kolkman, Edward Lewis, Ted Lindgreen, Rip Loomis, Bill Manning,
Dan Massey, Scott Rose, Mike Schiraldi, Jakob Schlyter, Brian
Wellington.
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Normative References
[1] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[2] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[3] Gudmundsson, O., "Delegation Signer Resource Record",
draft-ietf-dnsext-delegation-signer-12 (work in progress),
December 2002.
[4] Gudmundsson, O. and B. Wellington, "Redefinition of DNS AD bit",
draft-ietf-dnsext-ad-is-secure-06 (work in progress), June 2002.
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Informative References
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
[7] Eastlake, D., "Secure Domain Name System Dynamic Update", RFC
2137, April 1997.
[8] Lewis, E., "DNS Security Extension Clarification on Zone
Status", RFC 3090, March 2001.
[9] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 3225,
December 2001.
[10] Atkins, D. and R. Austein, "Threat Analysis Of The Domain Name
System", draft-ietf-dnsext-dns-threats-02 (work in progress),
November 2002.
Authors' Addresses
Roy Arends
Telematica Instituut
Drienerlolaan 5
7522 NB Enschede
NL
EMail: roy.arends@telin.nl
Mark Kosters
Verisign, Inc.
21355 Ridgetop Circle
Dulles, VA 20166
US
Phone: +1 703 948 3200
EMail: markk@verisign.com
URI: http://www.verisignlabs.com
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David Blacka
Verisign, Inc.
21355 Ridgetop Circle
Dulles, VA 20166
US
Phone: +1 703 948 3200
EMail: davidb@verisign.com
URI: http://www.verisignlabs.com
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Appendix A. Implementing Opt-In using "Views"
In many cases, it may be convenient to implement an Opt-In zone by
combining two separately maintained "views" of a zone at request
time. In this context, "view" refers to a particular version of a
zone, not to any specific DNS implementation feature.
In this scenario, one view is the secure view, the other is the
insecure (or legacy) view. The secure view consists of an entirely
signed zone using Opt-In tagged NXT records. The insecure view
contains no DNSSEC information. It is helpful, although not
necessary, for the secure view to be a subset (minus DNSSEC records)
of the insecure view.
In addition, the only RRsets that may solely exist in the insecure
view are non-zone-apex NS RRsets. That is, all non-NS RRsets (and the
zone apex NS RRset) MUST be signed and in the secure view.
These two views may be combined at request time to provide a virtual,
single Opt-In zone. The following algorithm is used when responding
to each query:
V_A is the secure view as described above.
V_B is the insecure view as described above.
R_A is a response generated from V_A, following RFC 2535 [2].
R_B is a response generated from V_B, following DNS resolution as
per RFC 1035 [1].
R_C is the response generated by combining R_A with R_B, as
described below.
A query is DNSSEC-aware if it either has the DO bit [9] turned on,
or is for a DNSSEC-specific record type.
1. If V_A is a subset of V_B and the query is not DNSSEC-aware,
generate and return R_B, otherwise
2. Generate R_A.
3. If R_A's RCODE != NXDOMAIN, return R_A, otherwise
4. Generate R_B and combine it with R_A to form R_C:
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For each section (ANSWER, AUTHORITY, ADDITIONAL), copy the
records from R_A into R_B, EXCEPT the AUTHORITY section SOA
record, if R_B's RCODE = NOERROR.
5. Return R_C.
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Appendix B. Changes from Prior Versions
Changes from version 04:
Added definitions for "signed name" and "unsigned name".
Added text to make it clear that insecure delegations may have
Opt-In NXT records of the same name. Updated the example to have
one of these.
Changed Server-side requirements from MUST NOT to SHOULD NOT and
added some basic description of what action to take in the face of
violating the delegation-only restriction.
Relaxed requirement that servers drop negative wildcard proof from
MUST to MAY, reiterated the client requirement.
Added section on Dynamic Update declaring it to be undefined wrt
Opt-In.
Essentially rewrote the "Security Considerations" section. It does
not actually say anything different, but hopefully it says it in a
clearer fashion.
Split references into Normative and Informative.
Fixed the example zone and responses to match Delegation Signer.
Changes from version 03:
Editorial changes for clarification only.
Changes from version 02:
Added text on changes to validation process, use of the AD bit,
and interactions with wildcards. Added wildcard caveats to the
"Security Considerations" section. Added "Transition Issues"
section.
Changes from version 01:
Changed to "delegation only". Strengthened "Security
Considerations" section. Added "Server Considerations" and "Client
Considerations" sections. Added AD bit requirement.
Changes from version 00:
Complete rewrite, altering approach from "views" to tagged NXT
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records
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