< draft-weiler-dnsext-dnssec-online-signing-00.txt		draft-weiler-dnsext-dnssec-online-signing-01.txt >
	Internet-Draft S. Weiler
	SPARTA, Inc.
	J. Ihren
	Autonomica
	30 October 2004
	Minimally Covering NSEC Records and DNSSEC On-line Signing		Network Working Group J. Ihren
	draft-weiler-dnsext-dnssec-online-signing-00.txt		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
	Status of this Memo		Status of this Memo
	This document is an Internet-Draft and is subject to all provisions		This document is an Internet-Draft and is subject to all provisions
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			Copyright Notice
			Copyright (C) The Internet Society (2005).
	Abstract		Abstract
	This document describes how to construct DNSSEC NSEC resource		This document describes how to construct DNSSEC NSEC resource records
	records that cover a smaller range of names than called for by		that cover a smaller range of names than called for by [-records].
	[-records]. By generating and signing these records on demand,		By generating and signing these records on demand, authoritative name
	authoritative name servers can effectively stop the disclosure of		servers can effectively stop the disclosure of zone contents
	zone contents otherwise made possible by walking the chain of NSEC		otherwise made possible by walking the chain of NSEC records in a
	records in a signed zone.		signed zone.
	Introduction and Terminology		Changes -00 to -01
	With DNSSEC [-records], an NSEC record lists the next instantiated		Clarified that this updates -records by relaxing requirements on the
	name in its zone, proving that no names exist in the "span" between		next name field.
	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		Added examples covering wildcard names.
	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		In the 'better functions' section, reiterated that perfect functions
	constructing NSEC records that cover fewer names. These records		aren't needed.
	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		Added a reference to 2119.
	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",		1. Introduction and Terminology
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
	this document are to be interpreted as described in RFC 2119.
	[RFC2119].
	Minimally Covering NSEC Records		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.
	This mechanism involves both changes to NSEC records for		Through repeated queries that return NSEC records, it is possible to
	instantiated names, which can still be generated and signed in		retrieve all of the names in the zone, a process commonly called
	advance, as well as the on-demand generation and signing of new		"walking" the zone. Some zone owners have policies forbidding zone
	NSEC records whenever a name must be proven not to exist.		transfers by arbitrary clients; this side-effect of the NSEC
			architecture subverts those policies.
	In the 'next name' field of instantiated names' NSEC records,		This document presents a way to prevent zone walking by constructing
	rather than list the next instantiated name in the zone, list any		NSEC records that cover fewer names. These records can make zone
	name that falls lexically after the NSEC's owner name and before		walking take approximately as many queries as simply asking for all
	the next instantiated name in the zone, according to the ordering		possible names in a zone, making zone walking impractical. Some of
	function in [-records] section 6.2. These NSEC records are		these records must be created and signed on demand, which requires
	returned whenever proving something specifically about the owner		on-line private keys. Anyone contemplating use of this technique is
	name (e.g. that no resource records of a given type appear at that		strongly encouraged to review the discussion of the risks of on-line
	name).		signing in Section 5.
	Whenever an NSEC record is needed to prove the non-existence of a		The technique presented here may be useful to a zone owner that wants
	name, a new NSEC record is produced and signed. The new NSEC		to use DNSSEC, is concerned about exposure of its zone contents via
	record has an owner name lexically before the QNAME but lexically		zone walking, and is willing to bear the costs of on-line signing.
	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		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	names need not be perfect nor consistent, but the generated NSEC		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	records must not cover any existing names. Furthermore, this		document are to be interpreted as described in RFC 2119 [4].
	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		2. Minimally Covering NSEC Records
	might produce the following NSEC record:
	exampld.com 3600 IN NSEC example-.com ( RRSIG NSEC )		This mechanism involves 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.
	Before answering a query with this record, an authoritative server		In the 'next name' field of instantiated names' NSEC records, rather
	must test for the existence of names between these endpoints. If		than list the next instantiated name in the zone, list any name that
	the generated NSEC would cover existing names (e.g. exampldd.com),		falls lexically after the NSEC's owner name and before the next
	a better increment or decrement function may be used or the covered		instantiated name in the zone, according to the ordering function in
	name closest to the QNAME could be used as the NSEC owner name or		[-records] [2] section 6.2. This relaxes the requirement in section
	next name, as appropriate. If an existing name is used as the NSEC		4.1.1 of [-records] that the 'next name' field contains the next
	owner name, that name's real NSEC record MUST be returned. Using		owner name in the zone. This change is expected to be fully
	the same example, assuming an exampldd.com delegation exists, this		compatible with all existing DNSSEC validators. These NSEC records
	record might be returned from the parent:		are returned whenever proving something specifically about the owner
			name (e.g. that no resource records of a given type appear at that
			name).
	exampldd.com 3600 IN NSEC example-.com ( NS DS RRSIG NSEC )		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.
	Like every authoritative record in the zone, each generated NSEC		The functions to generate the lexically following and proceeding
	record MUST have corresponding RRSIGs generated using each		names need not be perfect nor consistent, but the generated NSEC
	algorithm (but not necessarily each DNSKEY) in the zone's DNSKEY		records must not cover any existing names. Furthermore, this
	RRset, as described in [-protocol] section 2.2. To minimize the		technique works best when the generated NSEC records cover as few
	number of signatures that must be generated, a zone may wish to		names as possible.
	limit the number of algorithms in its DNSKEY RRset.
	Better Increment & Decrement Functions		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.
	Section 6.2 of [-records] defines a strict ordering of DNS names.		For example, a query for the non-instantiated name example.com might
	Working backwards from that definition, it should be possible to		produce the following NSEC record: For example, a query for an A
	define increment and decrement functions that generate the		record at the non-instantiated name example.com might produce the
	immediately following and preceeding names, respectively. This		following two NSEC records, the first denying the existence of the
	document does not define such functions. Instead, this section		name example.com and the second denying the existence of a wildcard:
	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.		exampld.com 3600 IN NSEC example-.com ( RRSIG NSEC )
			).com 3600 IN NSEC +.com ( RRSIG NSEC )
	To decrement a name, decrement the last character of the leftmost		Before answering a query with these records, an authoritative server
	label, then fill that label to a length of 63 octets with octets of		must test for the existence of names between these endpoints. If the
	value 255. To decrement a null octet, remove the octet -- if an		generated NSEC would cover existing names (e.g. exampldd.com or
	empty label is left, remove the label. Defining this function		*bizarre.example.com), a better increment or decrement function may
	numerically: fill the left-most label to its maximum length with		be used or the covered name closest to the QNAME could be used as the
	zeros (numeric, not ASCII zeros) and subtract one.		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:
	In response to a query for the non-existent name foo.example.com,		exampldd.com 3600 IN NSEC example-.com ( NS DS RRSIG NSEC )
	these functions produce an NSEC record of:
			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		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\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.example.com 3600 IN NSEC \000.foo.example.com ( NSEC RRSIG )
	Both of these functions are imperfect: they don't take into account		)\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
	constraints on number of labels in a name nor total length of a		\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
	name.		\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.*.example.com ( NSEC RRSIG )
	IANA Considerations		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.
	This document has no IANA Actions.		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.
	Security Considerations		4. IANA Considerations
	This approach requires on demand generation of RRSIG records. This		This document specifies no IANA Actions.
	creates several new vulnerabilities.
	First, on demand signing requires that a zone's authoritative		5. Security Considerations
	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		This approach requires on-demand generation of RRSIG records. This
	computationally demanding, the requirement for on demand signing		creates several new vulnerabilities.
	makes authoritative servers vulnerable to a denial of service
	attack.
	Lastly, if the increment and decrement functions are predictable,		First, on-demand signing requires that a zone's authoritative servers
	on-demand signing may enable a chosen-plaintext attack on a zone's		have access to its private keys. Storing private keys on well-known
	private keys. Zones using this approach should attempt to use		internet-accessible servers may make them more vulnerable to
	cryptographic algorithms that are resistant to chosen-plaintext		unintended disclosure.
	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		Second, since generation of public key signatures tends to be
	walking depends greatly on the quality of the increment and		computationally demanding, the requirement for on-demand signing
	decrement functions chosen. An increment function that chooses a		makes authoritative servers vulnerable to a denial of service attack.
	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		Lastly, if the increment and decrement functions are predictable,
	synthesized NSEC records being replayed. It's possible for an		on-demand signing may enable a chosen-plaintext attack on a zone's
	attacker to replay an old but still validly signed NSEC record		private keys. Zones using this approach should attempt to use
	after a new name has been added in the span covered by that NSEC,		cryptographic algorithms that are resistant to chosen-plaintext
	incorrectly proving that there is no record at that name. This		attacks. It's worth noting that while DNSSEC has a "mandatory to
	danger exists with DNSSEC as defined in [-bis]. The techniques		implement" algorithm, that is a requirement on resolvers and
	described here actually decrease the danger, since the span covered		validators -- there is no requirement that a zone be signed with any
	by any NSEC record is smaller than before. Choosing better		given algorithm.
	increment and decrement functions will further reduce this danger.
	Normative References		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. 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.
	(out of date versions)		Another obvious but misguided concern is the danger from synthesized
	[I-D.ietf-dnsext-dnssec-intro]		NSEC records being replayed. It's possible for an attacker to replay
	Arends, R., Austein, R., Larson, M., Massey, D. and S.		an old but still validly signed NSEC record after a new name has been
	Rose, "DNS Security Introduction and Requirements",		added in the span covered by that NSEC, incorrectly proving that
	draft-ietf-dnsext-dnssec-intro-10 (work in progress),		there is no record at that name. This danger exists with DNSSEC as
	May 2004.		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.
	[I-D.ietf-dnsext-dnssec-records]		6. Normative References
	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]		[1] Arends, R., Austein, R., Massey, D., Larson, M. and S. Rose,
	Arends, R., Austein, R., Larson, M., Massey, D. and S.		"DNS Security Introduction and Requirements",
	Rose, "Protocol Modifications for the DNS Security		Internet-Draft draft-ietf-dnsext-dnssec-intro-13, October 2004.
	Extensions", draft-ietf-dnsext-dnssec-protocol-06 (work
	in progress), May 2004.
	Acknowledgements		[2] Arends, R., "Resource Records for the DNS Security Extensions",
			Internet-Draft draft-ietf-dnsext-dnssec-records-11, October
			2004.
	Many individuals contributed to this design. They include, in		[3] Arends, R., "Protocol Modifications for the DNS Security
	addition to the authors of this document, Olaf Kolkman, Ed Lewis,		Extensions",
	Peter Koch, Matt Larson, David Blacka, Suzanne Woolf, Jaap		Internet-Draft draft-ietf-dnsext-dnssec-protocol-09, October
	Akkerhuis, Jakob Schlyter, Bill Manning, and Joao Damas.		2004.
			[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
			Levels", BCP 14, RFC 2119, March 1997.
	Authors' Addresses		Authors' Addresses
	Samuel Weiler		Johan Ihren
	SPARTA, Inc.		Autonomica AB
	7075 Samuel Morse Drive		Bellmansgatan 30
	Columbia, MD 21046		Stockholm SE-118 47
	USA		Sweden
	EMail: weiler@tislabs.com		Email: johani@autonomica.se
			Samuel Weiler
			SPARTA, Inc
			7075 Samuel Morse Drive
			Columbia, Maryland 21046
			US
	Johan Ihren		Email: weiler@tislabs.com
	Autonomica
	Bellmansgatan 30
	SE-118 47 Stockholm, Sweden
	Mail: johani@autonomica.se
	Copyright Notice		Appendix A. Acknowledgments
	Copyright (C) The Internet Society (2004). This document is subject		Many individuals contributed to this design. They include, in
	to the rights, licenses and restrictions contained in BCP 78, and		addition to the authors of this document, Olaf Kolkman, Ed Lewis,
	except as set forth therein, the authors retain all their rights.		Peter Koch, Matt Larson, David Blacka, Suzanne Woolf, Jaap Akkerhuis,
			Jacob Schlyter, Bill Manning, and Joao Damas.
	This document and the information contained herein are provided on		The key innovation of this document, namely that perfect increment
	an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE		and decrement functions are not necessary, arose during a discussion
	REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND		among the above-listed people at the RIPE49 meeting in September
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