Network Working Group R. Arends
Internet-Draft
Expires: April 14, 2003 M. Kosters
D. Blacka
Verisign, Inc.
October 14, 2002
DNSSEC Opt-In
draft-ietf-dnsext-dnssec-opt-in-03
Status of this Memo
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This Internet-Draft will expire on April 14, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). 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
3.2 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Transition Issues . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
A. Implementing Opt-In using "Views" . . . . . . . . . . . . . 18
B. Changes from Prior Versions . . . . . . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . 21
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1. Definitions and Terminology
Throughout this document, familiarity with the DNS system, RFC 1035
[1], DNS security extensions, RFC 2535 [4], and DNSSEC terminology
RFC 3090 [5] 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 [3]. In this
document, the RRset is also defined to include the covering SIG
records, if any exist.
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. A delegation returned in a DNS response is also called a
"referral".
secure delegation: refers to the NS, DS, NXT and SIG RRsets for a
non-zone-apex owner name, signifying a delegation to a DNSSEC
signed subzone.
2535/DS insecure delegation: refers to the NS, NXT, and SIG RRsets
for a non-zone-apex owner name, signifying a delegation to an
unsigned subzone. This differs from the secured delegation by the
absence of a DS RRset, marked by the zero value for the DS type
code in the NXT type map.
Opt-In insecure delegation: refers to the NS RRset for a non-zone-
apex owner name where 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 [2].
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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 not be relative to the gain of
cryptographically securing delegations to unsigned 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 instead are
accompanied by a NXT RRset of the same name, and possibly a ("no-
key") KEY RR [4] or DS record [7]. 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.
Using Opt-In, the existence or non-existence of insecure delegations
is not asserted by the tagged NXT records. This allows for the
addition or removal of delegations to unsigned zones without
recalculating and resigning the NXT chain. However, Opt-In NXT
records still assert the (non)existence of signed RRsets.
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 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,
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
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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.
Servers and signing tools MUST enforce 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 both non-existent name (NXDOMAIN)
responses and Opt-In insecure delegation responses, servers MUST NOT
return negative wildcard proof records when the query name (qname) is
covered by an Opt-In tagged NXT record.
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:
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 one or
more NXT records proving that the wildcards do not exist
(negative wildcard proofs).
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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 records are not useful.
The negative wildcard proofs are 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.
3.2 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
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 record at the same name as the
delegation. The absence 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.
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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 [8], MUST NOT be set when:
o sending a non-existent name (NXDOMAIN) response where the covering
NXT is tagged as Opt-In, unless the NXT record's owner name equals
the qname.
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 [4]) 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 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 SECOND-SECURE.EXAMPLE. A SIG
FIRST-SECURE.EXAMPLE. SIG NXT ...
NOT-SECURE.EXAMPLE. NS NS.NOT-SECURE.EXAMPLE.
NS.NOT-SECURE.EXAMPLE. A ...
SECOND-SECURE.EXAMPLE. NS NS.ELSEWHERE.
SECOND-SECURE.EXAMPLE. KEY ...
SECOND-SECURE.EXAMPLE. SIG KEY ...
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 will be no NXT record proving the non-existence of a matching
wildcard record, and the AD bit will not be set.
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.
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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 KEY
SECOND-SECURE.EXAMPLE. SIG NXT ...
Additional Section:
NS.UNSIGNED.EXAMPLE A ...
EXAMPLE. KEY ...
EXAMPLE. SIG KEY ...
In the Example A 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).
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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. All unsigned names are, by
definition, insecure, and their validity (or existence) can not be
cryptographically proven. With Opt-In, a malicious entity is able
to: insert, modify, or delete insecure delegation RRsets within the
Opt-In spans of a otherwise secured zone. In addition, a malicious
entity is able to replay or delete wildcard expansions (if there is
an existing applicable wildcard) in the Opt-In spans of the zone.
For example, if a resolver received the following response from the
example zone above:
Example S.1: Response to query for WWW.DOES-NOT-EXIST.EXAMPLE. A
RCODE=NOERROR
Authority Section:
DOES-NOT-EXIST.EXAMPLE. NS NS.FORGED.
EXAMPLE. NXT FIRST-SECURE.EXAMPLE. SOA NS SIG KEY
EXAMPLE. SIG NXT ...
Additional Section:
EXAMPLE. KEY ...
EXAMPLE. SIG KEY ...
The resolver would have no choice but to believe that the referral to
NS.FORGED. is valid.
While in particular cases, this issue may not present a significant
security problem, in general it should not be lightly dismissed. 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, Ted Lindgreen, Bill Manning, Dan Massey, Scott Rose, Mike
Schiraldi, Jakob Schlyter, Brian Wellington.
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References
[1] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
[4] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[5] Lewis, E., "DNS Security Extension Clarification on Zone
Status", RFC 3090, March 2001.
[6] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 3225,
December 2001.
[7] Gudmundsson, O., "Delegation Signer Resource Record", draft-
ietf-dnsext-delegation-signer-09 (work in progress), September
2002.
[8] Gudmundsson, O. and B. Wellington, "Redefinition of DNS AD bit",
draft-ietf-dnsext-ad-is-secure-06 (work in progress), June 2002.
Authors' Addresses
Roy Arends
Bankastraat 41-E
1094 EB Amsterdam
NL
Phone: +31206931681
EMail: roy@logmess.com
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 [4].
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 [6] 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 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
records
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Full Copyright Statement
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