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. 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