Network Working Group R. Arends
Internet-Draft Nominum, Inc.
Expires: December 26, 2002 M. Kosters
D. Blacka
Verisign, Inc.
June 27, 2002
DNSSEC Opt-In
draft-ietf-dnsext-dnssec-opt-in-02
Status of this Memo
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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.
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Table of Contents
1. Definitions and Terminology . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Additions . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Server Considerations . . . . . . . . . . . . . . . . . . . . 5
3.2 Client Considerations . . . . . . . . . . . . . . . . . . . . 6
4. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
A. Implementing Opt-In using "Views" . . . . . . . . . . . . . . 16
B. Changes from Prior Versions . . . . . . . . . . . . . . . . . 18
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 19
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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].
covering NXT record/RRset: is the NXT record used to prove
(non)existance 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 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 or absence 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 the 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.
3.1 Server Considerations
This protocol change dictates a number of changes to the operation of
an authoritative server:
o The server MUST enforce the protocol requirement that ONLY
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insecure delegation nodes may exist between the secure nodes of
the zone.
o The server must be able to retrieve the proper NXT records along
with referrals to insecure subzone.
In the delegation signer proposal, NXT records already must be
returned along with referrals to insecure delegations. The primary
difference that this proposal introduces is that the appropriate NXT
record will have a different owner name.
3.2 Client Considerations
Opt-In imposes some new requirements on the DNS resolver (caching or
otherwise):
o Resolvers MUST be able to use Opt-In style NXT records to
cryptographically prove the validity and security status (as
insecure) of a referral:
* In RFC 2535, this is proven by existence of a verified "no-key"
KEY RRset.
* Using Delegation Signer, this is proven by the existence of a
verified NXT record. This NXT record has same name as the
delegation RRset and does not have the DS bit set in the type
map.
* Using Opt-In, this is proven by the existence of a verified
Opt-In NXT record. This NXT record does not have the NXT bit
set in the type map (that is, it is an Opt-In style NXT record)
and the name of the delegation RRset is lexicographically
between the owner and next names of the NXT record.
Note that 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.
o Resolvers MUST reject as invalid non-NS RRsets that fall within an
Opt-In tagged NXT record's span.
o Caching resolvers must be able to retrieve the appropriate
covering Opt-In NXT record when returning referrals that need
them. This is only a difference when you consider that the
covering NXT record will not have the same name as the delegation
RRset itself.
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o The AD bit (as defined by [8]) MUST NOT be set in a response
containing an Opt-In tagged NXT record in the authority section.
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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 only in the fact that the NXT record will be tagged as
Opt-In.
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.UNSECURE.EXAMPLE. A
RCODE=NOERROR
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. Security Considerations
Opt-In allows for unsigned delegations. All unsigned names are
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 a
secured 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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7. IANA Considerations
None.
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8. 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-07 (work in progress), March 2002.
[8] Gudmundsson, O. and B. Wellington, "Redefinition of DNS AD bit",
draft-ietf-dnsext-ad-is-secure-05 (work in progress), March
2002.
Authors' Addresses
Roy Arends
Nominum, Inc.
950 Charter Street
Redwood City, CA 94063
US
Phone: +1 650 381 6000
EMail: Roy.Arends@nominum.com
URI: http://www.nominum.com
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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
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 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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