Internet-Draft S. Weiler
SPARTA, Inc.
J. Ihren
Autonomica
30 October 2004
Minimally Covering NSEC Records and DNSSEC On-line Signing
draft-weiler-dnsext-dnssec-online-signing-00.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on 30 April 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 signed zone.
Introduction and Terminology
With DNSSEC [-records], 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 zones 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 [Security
Considerations].
The technique presented here may be useful to a zone 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.
[RFC2119].
Minimally Covering NSEC Records
This mechanism involves both changes to NSEC records for
instantiated 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] section 6.2. 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 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 better when the generated NSEC records cover very
little of the zone's namespace.
For example, a query for the non-instantiated name example.com
might produce the following NSEC record:
exampld.com 3600 IN NSEC example-.com ( RRSIG NSEC )
Before answering a query with this record, 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),
a better increment or decrement function may be used or the covered
name closest to the QNAME could be used as the 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] 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.
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 preceeding 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 MUST ensure than no
generated NSEC covers any existing name.
To increment a name, add a leading label with a single null 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 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 an NSEC record 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 )
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.
IANA Considerations
This document has no IANA Actions.
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. If any "mandatory to implement" algorithm is
found to be particularly vulnerable to chosen plaintext attack, a
zone may which to switch to another algorithm or use less
predictable increment and decrement function.
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
instantiated name is likewise a poor choice. A good choice of
increment and decrement functions are the ones that produce the
immediately following and preceeding names, respectively, though
zone administrators may wish to use less perfect functions that
return more human-friendly names than the functions described in
section X 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.
Normative References
(out of date versions)
[I-D.ietf-dnsext-dnssec-intro]
Arends, R., Austein, R., Larson, M., Massey, D. and S.
Rose, "DNS Security Introduction and Requirements",
draft-ietf-dnsext-dnssec-intro-10 (work in progress),
May 2004.
[I-D.ietf-dnsext-dnssec-records]
Arends, R., Austein, R., Larson, M., Massey, D. and S.
Rose, "Resource Records for DNS Security Extensions",
draft-ietf-dnsext-dnssec-records-08 (work in progress),
May 2004.
[I-D.ietf-dnsext-dnssec-protocol]
Arends, R., Austein, R., Larson, M., Massey, D. and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", draft-ietf-dnsext-dnssec-protocol-06 (work
in progress), May 2004.
Acknowledgements
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, Jakob Schlyter, Bill Manning, and Joao Damas.
Authors' Addresses
Samuel Weiler
SPARTA, Inc.
7075 Samuel Morse Drive
Columbia, MD 21046
USA
EMail: weiler@tislabs.com
Johan Ihren
Autonomica
Bellmansgatan 30
SE-118 47 Stockholm, Sweden
Mail: johani@autonomica.se
Copyright Notice
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.