Minimally Covering NSEC Records and DNSSEC On-line Signing
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Copyright © The Internet Society (2006).
This document describes how to construct DNSSEC NSEC resource records that cover a smaller range of names than called for by RFC 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 Contents
1. Introduction ....................................................1 2. Applicability of This Technique .................................2 3. Minimally Covering NSEC Records .................................2 4. Better Epsilon Functions ........................................4 5. Security Considerations .........................................5 6. Acknowledgements ................................................6 7. Normative References ............................................6
With DNSSEC , 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 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 in Section 5.
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 in RFC 2119 .
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 in Section 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 in Section 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.
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 RFC 4034  Section 6.1. This relaxes the requirement in Section 4.1.1 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 in Section 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 )
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 in RFC 4035  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 Functions
Section 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:
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 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 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 in Section 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 . 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.
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
 Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005.  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005.  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
Samuel Weiler SPARTA, Inc. 7075 Samuel Morse Drive Columbia, Maryland 21046 US
email@example.com Johan Ihren Autonomica AB Bellmansgatan 30 Stockholm SE-118 47 Sweden
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