452 lines
17 KiB
Plaintext
452 lines
17 KiB
Plaintext
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Network Working Group S. Weiler
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Request for Comments: 4470 SPARTA, Inc.
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Updates: 4035, 4034 J. Ihren
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Category: Standards Track Autonomica AB
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April 2006
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Minimally Covering NSEC Records and DNSSEC On-line Signing
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Status of This Memo
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice
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Copyright (C) The Internet Society (2006).
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Abstract
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This document describes how to construct DNSSEC NSEC resource records
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that cover a smaller range of names than called for by RFC 4034. By
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generating and signing these records on demand, authoritative name
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servers can effectively stop the disclosure of zone contents
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otherwise made possible by walking the chain of NSEC records in a
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signed zone.
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Table of Contents
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1. Introduction ....................................................1
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2. Applicability of This Technique .................................2
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3. Minimally Covering NSEC Records .................................2
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4. Better Epsilon Functions ........................................4
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5. Security Considerations .........................................5
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6. Acknowledgements ................................................6
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7. Normative References ............................................6
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1. Introduction
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With DNSSEC [1], an NSEC record lists the next instantiated name in
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its zone, proving that no names exist in the "span" between the
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NSEC's owner name and the name in the "next name" field. In this
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document, an NSEC record is said to "cover" the names between its
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owner name and next name.
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Weiler & Ihren Standards Track [Page 1]
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RFC 4470 NSEC Epsilon April 2006
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Through repeated queries that return NSEC records, it is possible to
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retrieve all of the names in the zone, a process commonly called
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"walking" the zone. Some zone owners have policies forbidding zone
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transfers by arbitrary clients; this side effect of the NSEC
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architecture subverts those policies.
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This document presents a way to prevent zone walking by constructing
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NSEC records that cover fewer names. These records can make zone
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walking take approximately as many queries as simply asking for all
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possible names in a zone, making zone walking impractical. Some of
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these records must be created and signed on demand, which requires
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on-line private keys. Anyone contemplating use of this technique is
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strongly encouraged to review the discussion of the risks of on-line
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signing in Section 5.
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1.2. Keywords
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The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in RFC 2119 [4].
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2. Applicability of This Technique
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The technique presented here may be useful to a zone owner that wants
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to use DNSSEC, is concerned about exposure of its zone contents via
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zone walking, and is willing to bear the costs of on-line signing.
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As discussed in Section 5, on-line signing has several security
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risks, including an increased likelihood of private keys being
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disclosed and an increased risk of denial of service attack. Anyone
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contemplating use of this technique is strongly encouraged to review
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the discussion of the risks of on-line signing in Section 5.
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Furthermore, at the time this document was published, the DNSEXT
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working group was actively working on a mechanism to prevent zone
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walking that does not require on-line signing (tentatively called
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NSEC3). The new mechanism is likely to expose slightly more
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information about the zone than this technique (e.g., the number of
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instantiated names), but it may be preferable to this technique.
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3. Minimally Covering NSEC Records
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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
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the on-demand generation and signing of new NSEC records whenever a
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name must be proven not to exist.
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Weiler & Ihren Standards Track [Page 2]
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RFC 4470 NSEC Epsilon April 2006
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In the "next name" field of instantiated names' NSEC records, rather
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than list the next instantiated name in the zone, list any name that
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falls lexically after the NSEC's owner name and before the next
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instantiated name in the zone, according to the ordering function in
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RFC 4034 [2] Section 6.1. This relaxes the requirement in Section
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4.1.1 of RFC 4034 that the "next name" field contains the next owner
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name in the zone. This change is expected to be fully compatible
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with all existing DNSSEC validators. These NSEC records are returned
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whenever proving something specifically about the owner name (e.g.,
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that no resource records of a given type appear at that name).
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Whenever an NSEC record is needed to prove the non-existence of a
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name, a new NSEC record is dynamically produced and signed. The new
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NSEC record has an owner name lexically before the QNAME but
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lexically following any existing name and a "next name" lexically
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following the QNAME but before any existing name.
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The generated NSEC record's type bitmap MUST have the RRSIG and NSEC
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bits set and SHOULD NOT have any other bits set. This relaxes the
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requirement in Section 2.3 of RFC4035 that NSEC RRs not appear at
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names that did not exist before the zone was signed.
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The functions to generate the lexically following and proceeding
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names need not be perfect or consistent, but the generated NSEC
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records must not cover any existing names. Furthermore, this
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technique works best when the generated NSEC records cover as few
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names as possible. In this document, the functions that generate the
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nearby names are called "epsilon" functions, a reference to the
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mathematical convention of using the greek letter epsilon to
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represent small deviations.
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An NSEC record denying the existence of a wildcard may be generated
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in the same way. Since the NSEC record covering a non-existent
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wildcard is likely to be used in response to many queries,
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authoritative name servers using the techniques described here may
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want to pregenerate or cache that record and its corresponding RRSIG.
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For example, a query for an A record at the non-instantiated name
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example.com might produce the following two NSEC records, the first
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denying the existence of the name example.com and the second denying
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the existence of a wildcard:
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exampld.com 3600 IN NSEC example-.com ( RRSIG NSEC )
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\).com 3600 IN NSEC +.com ( RRSIG NSEC )
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Weiler & Ihren Standards Track [Page 3]
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RFC 4470 NSEC Epsilon April 2006
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Before answering a query with these records, an authoritative server
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must test for the existence of names between these endpoints. If the
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generated NSEC would cover existing names (e.g., exampldd.com or
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*bizarre.example.com), a better epsilon function may be used or the
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covered name closest to the QNAME could be used as the NSEC owner
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name or next name, as appropriate. If an existing name is used as
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the NSEC owner name, that name's real NSEC record MUST be returned.
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Using the same example, assuming an exampldd.com delegation exists,
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this record might be returned from the parent:
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exampldd.com 3600 IN NSEC example-.com ( NS DS RRSIG NSEC )
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Like every authoritative record in the zone, each generated NSEC
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record MUST have corresponding RRSIGs generated using each algorithm
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(but not necessarily each DNSKEY) in the zone's DNSKEY RRset, as
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described in RFC 4035 [3] Section 2.2. To minimize the number of
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signatures that must be generated, a zone may wish to limit the
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number of algorithms in its DNSKEY RRset.
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4. Better Epsilon Functions
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Section 6.1 of RFC 4034 defines a strict ordering of DNS names.
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Working backward from that definition, it should be possible to
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define epsilon functions that generate the immediately following and
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preceding names, respectively. This document does not define such
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functions. Instead, this section presents functions that come
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reasonably close to the perfect ones. As described above, an
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authoritative server should still ensure than no generated NSEC
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covers any existing name.
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To increment a name, add a leading label with a single null (zero-
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value) octet.
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To decrement a name, decrement the last character of the leftmost
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label, then fill that label to a length of 63 octets with octets of
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value 255. To decrement a null (zero-value) octet, remove the octet
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-- if an empty label is left, remove the label. Defining this
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function numerically: fill the leftmost label to its maximum length
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with zeros (numeric, not ASCII zeros) and subtract one.
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In response to a query for the non-existent name foo.example.com,
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these functions produce NSEC records of the following:
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Weiler & Ihren Standards Track [Page 4]
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RFC 4470 NSEC Epsilon April 2006
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fon\255\255\255\255\255\255\255\255\255\255\255\255\255\255
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\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
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\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
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\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
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\255.example.com 3600 IN NSEC \000.foo.example.com ( NSEC RRSIG )
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\)\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
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\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
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\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
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\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 )
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The first of these NSEC RRs proves that no exact match for
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foo.example.com exists, and the second proves that there is no
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wildcard in example.com.
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Both of these functions are imperfect: they do not take into account
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constraints on number of labels in a name nor total length of a name.
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As noted in the previous section, though, this technique does not
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depend on the use of perfect epsilon functions: it is sufficient to
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test whether any instantiated names fall into the span covered by the
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generated NSEC and, if so, substitute those instantiated owner names
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for the NSEC owner name or next name, as appropriate.
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5. Security Considerations
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This approach requires on-demand generation of RRSIG records. This
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creates several new vulnerabilities.
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First, on-demand signing requires that a zone's authoritative servers
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have access to its private keys. Storing private keys on well-known
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Internet-accessible servers may make them more vulnerable to
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unintended disclosure.
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Second, since generation of digital signatures tends to be
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computationally demanding, the requirement for on-demand signing
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makes authoritative servers vulnerable to a denial of service attack.
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Last, if the epsilon functions are predictable, on-demand signing may
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enable a chosen-plaintext attack on a zone's private keys. Zones
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using this approach should attempt to use cryptographic algorithms
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that are resistant to chosen-plaintext attacks. It is worth noting
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that although DNSSEC has a "mandatory to implement" algorithm, that
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is a requirement on resolvers and validators -- there is no
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requirement that a zone be signed with any given algorithm.
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The success of using minimally covering NSEC records to prevent zone
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walking depends greatly on the quality of the epsilon functions
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Weiler & Ihren Standards Track [Page 5]
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RFC 4470 NSEC Epsilon April 2006
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chosen. An increment function that chooses a name obviously derived
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from the next instantiated name may be easily reverse engineered,
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destroying the value of this technique. An increment function that
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always returns a name close to the next instantiated name is likewise
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a poor choice. Good choices of epsilon functions are the ones that
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produce the immediately following and preceding names, respectively,
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though zone administrators may wish to use less perfect functions
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that return more human-friendly names than the functions described in
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Section 4 above.
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Another obvious but misguided concern is the danger from synthesized
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NSEC records being replayed. It is possible for an attacker to
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replay an old but still validly signed NSEC record after a new name
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has been added in the span covered by that NSEC, incorrectly proving
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that there is no record at that name. This danger exists with DNSSEC
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as defined in [3]. The techniques described here actually decrease
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the danger, since the span covered by any NSEC record is smaller than
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before. Choosing better epsilon functions will further reduce this
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danger.
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6. Acknowledgements
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Many individuals contributed to this design. They include, in
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addition to the authors of this document, Olaf Kolkman, Ed Lewis,
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Peter Koch, Matt Larson, David Blacka, Suzanne Woolf, Jaap Akkerhuis,
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Jakob Schlyter, Bill Manning, and Joao Damas.
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In addition, the editors would like to thank Ed Lewis, Scott Rose,
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and David Blacka for their careful review of the document.
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7. Normative References
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[1] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
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"DNS Security Introduction and Requirements", RFC 4033, March
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2005.
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[2] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
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"Resource Records for the DNS Security Extensions", RFC 4034,
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March 2005.
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[3] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
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"Protocol Modifications for the DNS Security Extensions", RFC
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4035, March 2005.
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[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
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Levels", BCP 14, RFC 2119, March 1997.
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Weiler & Ihren Standards Track [Page 6]
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RFC 4470 NSEC Epsilon April 2006
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Authors' Addresses
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Samuel Weiler
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SPARTA, Inc.
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7075 Samuel Morse Drive
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Columbia, Maryland 21046
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US
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EMail: weiler@tislabs.com
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Johan Ihren
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Autonomica AB
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Bellmansgatan 30
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Stockholm SE-118 47
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Sweden
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EMail: johani@autonomica.se
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Weiler & Ihren Standards Track [Page 7]
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RFC 4470 NSEC Epsilon April 2006
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Full Copyright Statement
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Copyright (C) The Internet Society (2006).
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This document is subject to the rights, licenses and restrictions
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contained in BCP 78, and except as set forth therein, the authors
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retain all their rights.
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This document and the information contained herein are provided on an
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"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Intellectual Property
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The IETF takes no position regarding the validity or scope of any
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Intellectual Property Rights or other rights that might be claimed to
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pertain to the implementation or use of the technology described in
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this document or the extent to which any license under such rights
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might or might not be available; nor does it represent that it has
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made any independent effort to identify any such rights. Information
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on the procedures with respect to rights in RFC documents can be
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found in BCP 78 and BCP 79.
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Copies of IPR disclosures made to the IETF Secretariat and any
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assurances of licenses to be made available, or the result of an
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attempt made to obtain a general license or permission for the use of
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such proprietary rights by implementers or users of this
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specification can be obtained from the IETF on-line IPR repository at
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http://www.ietf.org/ipr.
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The IETF invites any interested party to bring to its attention any
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copyrights, patents or patent applications, or other proprietary
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rights that may cover technology that may be required to implement
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||
this standard. Please address the information to the IETF at
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ietf-ipr@ietf.org.
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|
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Acknowledgement
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|
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Funding for the RFC Editor function is provided by the IETF
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Administrative Support Activity (IASA).
|
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Weiler & Ihren Standards Track [Page 8]
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