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Network Working Group                                          S. WeilerRequest for Comments: 4470                                  SPARTA, Inc.Updates:4035,4034                                             J. IhrenCategory: Standards Track                                  Autonomica AB                                                              April 2006Minimally Covering NSEC Records and DNSSEC On-line SigningStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   This document describes how to construct DNSSEC NSEC resource records   that cover a smaller range of names than called for byRFC 4034.  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.Table of Contents1. Introduction ....................................................12. Applicability of This Technique .................................23. Minimally Covering NSEC Records .................................24. Better Epsilon Functions ........................................45. Security Considerations .........................................56. Acknowledgements ................................................67. Normative References ............................................61.  Introduction   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.Weiler & Ihren              Standards Track                     [Page 1]

RFC 4470                      NSEC Epsilon                    April 2006   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 inSection 5.1.2.  Keywords   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [4].2.  Applicability of This Technique   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.   As discussed inSection 5, on-line signing has several security   risks, including an increased likelihood of private keys being   disclosed and an increased risk of denial of service attack.  Anyone   contemplating use of this technique is strongly encouraged to review   the discussion of the risks of on-line signing inSection 5.   Furthermore, at the time this document was published, the DNSEXT   working group was actively working on a mechanism to prevent zone   walking that does not require on-line signing (tentatively called   NSEC3).  The new mechanism is likely to expose slightly more   information about the zone than this technique (e.g., the number of   instantiated names), but it may be preferable to this technique.3.  Minimally Covering NSEC Records   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.Weiler & Ihren              Standards Track                     [Page 2]

RFC 4470                      NSEC Epsilon                    April 2006   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 inRFC 4034 [2]Section 6.1.  This relaxes the requirement inSection4.1.1 of RFC 4034 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 generated NSEC record's type bitmap MUST have the RRSIG and NSEC   bits set and SHOULD NOT have any other bits set.  This relaxes the   requirement inSection 2.3 of RFC4035 that NSEC RRs not appear at   names that did not exist before the zone was signed.   The functions to generate the lexically following and proceeding   names need not be perfect or 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.  In this document, the functions that generate the   nearby names are called "epsilon" functions, a reference to the   mathematical convention of using the greek letter epsilon to   represent small deviations.   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 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 )Weiler & Ihren              Standards Track                     [Page 3]

RFC 4470                      NSEC Epsilon                    April 2006   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 epsilon 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 inRFC 4035 [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.4.  Better Epsilon FunctionsSection 6.1 of RFC 4034 defines a strict ordering of DNS names.   Working backward from that definition, it should be possible to   define epsilon 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 leftmost 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 the following:Weiler & Ihren              Standards Track                     [Page 4]

RFC 4470                      NSEC Epsilon                    April 2006     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     \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 do not 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 epsilon 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.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 digital signatures tends to be   computationally demanding, the requirement for on-demand signing   makes authoritative servers vulnerable to a denial of service attack.   Last, if the epsilon 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 is worth noting   that although 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 records to prevent zone   walking depends greatly on the quality of the epsilon functionsWeiler & Ihren              Standards Track                     [Page 5]

RFC 4470                      NSEC Epsilon                    April 2006   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 epsilon 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 inSection 4 above.   Another obvious but misguided concern is the danger from synthesized   NSEC records being replayed.  It is 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 [3].  The techniques described here actually decrease   the danger, since the span covered by any NSEC record is smaller than   before.  Choosing better epsilon functions will further reduce this   danger.6.  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.   In addition, the editors would like to thank Ed Lewis, Scott Rose,   and David Blacka for their careful review of the document.7.  Normative References   [1]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,        "DNS Security Introduction and Requirements",RFC 4033, March        2005.   [2]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,        "Resource Records for the DNS Security Extensions",RFC 4034,        March 2005.   [3]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,        "Protocol Modifications for the DNS Security Extensions",RFC4035, March 2005.   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.Weiler & Ihren              Standards Track                     [Page 6]

RFC 4470                      NSEC Epsilon                    April 2006Authors' Addresses   Samuel Weiler   SPARTA, Inc.   7075 Samuel Morse Drive   Columbia, Maryland  21046   US   EMail: weiler@tislabs.com   Johan Ihren   Autonomica AB   Bellmansgatan 30   Stockholm  SE-118 47   Sweden   EMail: johani@autonomica.seWeiler & Ihren              Standards Track                     [Page 7]

RFC 4470                      NSEC Epsilon                    April 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Weiler & Ihren              Standards Track                     [Page 8]