Network Working Group J. Ihren
Internet-Draft Autonomica AB
Updates: [-records] (if approved) S. Weiler
Expires: August 24, 2005 SPARTA, Inc
February 20, 2005
Minimally Covering NSEC Records and DNSSEC On-line Signing
draft-weiler-dnsext-dnssec-online-signing-01
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes how to construct DNSSEC NSEC resource records
that cover a smaller range of names than called for by [-records].
By generating and signing these records on demand, authoritative name
servers can effectively stop the disclosure of zone contents
otherwise made possible by walking the chain of NSEC records in a
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signed zone.
Changes -00 to -01
Clarified that this updates -records by relaxing requirements on the
next name field.
Added examples covering wildcard names.
In the 'better functions' section, reiterated that perfect functions
aren't needed.
Added a reference to 2119.
1. Introduction and Terminology
With DNSSEC [1], an NSEC record lists the next instantiated name in
its zone, proving that no names exist in the "span" between the
NSEC's owner name and the name in the "next name" field. In this
document, an NSEC record is said to "cover" the names between its
owner name and next name.
Through repeated queries that return NSEC records, it is possible to
retrieve all of the names in the zone, a process commonly called
"walking" the zone. Some zone owners have policies forbidding zone
transfers by arbitrary clients; this side-effect of the NSEC
architecture subverts those policies.
This document presents a way to prevent zone walking by constructing
NSEC records that cover fewer names. These records can make zone
walking take approximately as many queries as simply asking for all
possible names in a zone, making zone walking impractical. Some of
these records must be created and signed on demand, which requires
on-line private keys. Anyone contemplating use of this technique is
strongly encouraged to review the discussion of the risks of on-line
signing in Section 5.
The technique presented here may be useful to a zone owner that wants
to use DNSSEC, is concerned about exposure of its zone contents via
zone walking, and is willing to bear the costs of on-line signing.
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 [4].
2. Minimally Covering NSEC Records
This mechanism involves changes to NSEC records for instantiated
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names, which can still be generated and signed in advance, as well as
the on-demand generation and signing of new NSEC records whenever a
name must be proven not to exist.
In the 'next name' field of instantiated names' NSEC records, rather
than list the next instantiated name in the zone, list any name that
falls lexically after the NSEC's owner name and before the next
instantiated name in the zone, according to the ordering function in
[-records] [2] section 6.2. This relaxes the requirement in section
4.1.1 of [-records] that the 'next name' field contains the next
owner name in the zone. This change is expected to be fully
compatible with all existing DNSSEC validators. These NSEC records
are returned whenever proving something specifically about the owner
name (e.g. that no resource records of a given type appear at that
name).
Whenever an NSEC record is needed to prove the non-existence of a
name, a new NSEC record is dynamically produced and signed. The new
NSEC record has an owner name lexically before the QNAME but
lexically following any existing name and a 'next name' lexically
following the QNAME but before any existing name.
The functions to generate the lexically following and proceeding
names need not be perfect nor consistent, but the generated NSEC
records must not cover any existing names. Furthermore, this
technique works best when the generated NSEC records cover as few
names as possible.
An NSEC record denying the existence of a wildcard may be generated
in the same way. Since the NSEC record covering a non-existent
wildcard is likely to be used in response to many queries,
authoritative name servers using the techniques described here may
want to pregenerate or cache that record and its corresponding RRSIG.
For example, a query for the non-instantiated name example.com might
produce the following NSEC record: For example, a query for an A
record at the non-instantiated name example.com might produce the
following two NSEC records, the first denying the existence of the
name example.com and the second denying the existence of a wildcard:
exampld.com 3600 IN NSEC example-.com ( RRSIG NSEC )
).com 3600 IN NSEC +.com ( RRSIG NSEC )
Before answering a query with these records, an authoritative server
must test for the existence of names between these endpoints. If the
generated NSEC would cover existing names (e.g. exampldd.com or
*bizarre.example.com), a better increment or decrement function may
be used or the covered name closest to the QNAME could be used as the
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NSEC owner name or next name, as appropriate. If an existing name is
used as the NSEC owner name, that name's real NSEC record MUST be
returned. Using the same example, assuming an exampldd.com
delegation exists, this record might be returned from the parent:
exampldd.com 3600 IN NSEC example-.com ( NS DS RRSIG NSEC )
Like every authoritative record in the zone, each generated NSEC
record MUST have corresponding RRSIGs generated using each algorithm
(but not necessarily each DNSKEY) in the zone's DNSKEY RRset, as
described in [-protocol] [3] section 2.2. To minimize the number of
signatures that must be generated, a zone may wish to limit the
number of algorithms in its DNSKEY RRset.
3. Better Increment & Decrement Functions
Section 6.2 of [-records] defines a strict ordering of DNS names.
Working backwards from that definition, it should be possible to
define increment and decrement functions that generate the
immediately following and preceding names, respectively. This
document does not define such functions. Instead, this section
presents functions that come reasonably close to the perfect ones.
As described above, an authoritative server should still ensure than
no generated NSEC covers any existing name.
To increment a name, add a leading label with a single null
(zero-value) octet.
To decrement a name, decrement the last character of the leftmost
label, then fill that label to a length of 63 octets with octets of
value 255. To decrement a null (zero-value) octet, remove the octet
-- if an empty label is left, remove the label. Defining this
function numerically: fill the left-most label to its maximum length
with zeros (numeric, not ASCII zeros) and subtract one.
In response to a query for the non-existent name foo.example.com,
these functions produce NSEC records of:
fon\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255.example.com 3600 IN NSEC \000.foo.example.com ( NSEC RRSIG )
)\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
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\255\255.example.com 3600 IN NSEC \000.*.example.com ( NSEC RRSIG )
The first of these NSEC RRs proves that no exact match for
foo.example.com exists, and the second proves that there is no
wildcard in example.com.
Both of these functions are imperfect: they don't take into account
constraints on number of labels in a name nor total length of a name.
As noted in the previous section, though, this technique does not
depend on the use of perfect increment or decrement functions: it is
sufficient to test whether any instantiated names fall into the span
covered by the generated NSEC and, if so, substitute those
instantiated owner names for the NSEC owner name or next name, as
appropriate.
4. IANA Considerations
This document specifies no IANA Actions.
5. Security Considerations
This approach requires on-demand generation of RRSIG records. This
creates several new vulnerabilities.
First, on-demand signing requires that a zone's authoritative servers
have access to its private keys. Storing private keys on well-known
internet-accessible servers may make them more vulnerable to
unintended disclosure.
Second, since generation of public key signatures tends to be
computationally demanding, the requirement for on-demand signing
makes authoritative servers vulnerable to a denial of service attack.
Lastly, if the increment and decrement functions are predictable,
on-demand signing may enable a chosen-plaintext attack on a zone's
private keys. Zones using this approach should attempt to use
cryptographic algorithms that are resistant to chosen-plaintext
attacks. It's worth noting that while DNSSEC has a "mandatory to
implement" algorithm, that is a requirement on resolvers and
validators -- there is no requirement that a zone be signed with any
given algorithm.
The success of using minimally covering NSEC record to prevent zone
walking depends greatly on the quality of the increment and decrement
functions chosen. An increment function that chooses a name
obviously derived from the next instantiated name may be easily
reverse engineered, destroying the value of this technique. An
increment function that always returns a name close to the next
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instantiated name is likewise a poor choice. Good choices of
increment and decrement functions are the ones that produce the
immediately following and preceding names, respectively, though zone
administrators may wish to use less perfect functions that return
more human-friendly names than the functions described in Section 3
above.
Another obvious but misguided concern is the danger from synthesized
NSEC records being replayed. It's possible for an attacker to replay
an old but still validly signed NSEC record after a new name has been
added in the span covered by that NSEC, incorrectly proving that
there is no record at that name. This danger exists with DNSSEC as
defined in [-bis]. The techniques described here actually decrease
the danger, since the span covered by any NSEC record is smaller than
before. Choosing better increment and decrement functions will
further reduce this danger.
6. Normative References
[1] Arends, R., Austein, R., Massey, D., Larson, M. and S. Rose,
"DNS Security Introduction and Requirements",
Internet-Draft draft-ietf-dnsext-dnssec-intro-13, October 2004.
[2] Arends, R., "Resource Records for the DNS Security Extensions",
Internet-Draft draft-ietf-dnsext-dnssec-records-11, October
2004.
[3] Arends, R., "Protocol Modifications for the DNS Security
Extensions",
Internet-Draft draft-ietf-dnsext-dnssec-protocol-09, October
2004.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Authors' Addresses
Johan Ihren
Autonomica AB
Bellmansgatan 30
Stockholm SE-118 47
Sweden
Email: johani@autonomica.se
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Samuel Weiler
SPARTA, Inc
7075 Samuel Morse Drive
Columbia, Maryland 21046
US
Email: weiler@tislabs.com
Appendix A. Acknowledgments
Many individuals contributed to this design. They include, in
addition to the authors of this document, Olaf Kolkman, Ed Lewis,
Peter Koch, Matt Larson, David Blacka, Suzanne Woolf, Jaap Akkerhuis,
Jacob Schlyter, Bill Manning, and Joao Damas.
The key innovation of this document, namely that perfect increment
and decrement functions are not necessary, arose during a discussion
among the above-listed people at the RIPE49 meeting in September
2004.
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