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Ihren 5 Expires: July 9, 2006 Autonomica AB 6 January 5, 2006 8 Minimally Covering NSEC Records and DNSSEC On-line Signing 9 draft-ietf-dnsext-dnssec-online-signing-01 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on July 9, 2006. 36 Copyright Notice 38 Copyright (C) The Internet Society (2006). 40 Abstract 42 This document describes how to construct DNSSEC NSEC resource records 43 that cover a smaller range of names than called for by RFC4034. By 44 generating and signing these records on demand, authoritative name 45 servers can effectively stop the disclosure of zone contents 46 otherwise made possible by walking the chain of NSEC records in a 47 signed zone. 49 Changes from ietf-00 to ietf-01 51 Added an applicability statement, making reference to ongoing work on 52 NSEC3. 54 Added the phrase "epsilon functions", which has been commonly used to 55 describe the technique and already appeared in the header of each 56 page, in place of "increment and decrement functions". Also added an 57 explanatory sentence. 59 Corrected references from 4034 section 6.2 to section 6.1. 61 Fixed an out-of-date reference to [-bis] and other typos. 63 Replaced IANA Considerations text. 65 Escaped close parentheses in examples. 67 Added some more acknowledgements. 69 Changes from weiler-01 to ietf-00 71 Inserted RFC numbers for 4033, 4034, and 4035. 73 Specified contents of bitmap field in synthesized NSEC RR's, pointing 74 out that this relaxes a constraint in 4035. Added 4035 to the 75 Updates header. 77 Changes from weiler-00 to weiler-01 79 Clarified that this updates RFC4034 by relaxing requirements on the 80 next name field. 82 Added examples covering wildcard names. 84 In the 'better functions' section, reiterated that perfect functions 85 aren't needed. 87 Added a reference to RFC 2119. 89 Table of Contents 91 1. Introduction and Terminology . . . . . . . . . . . . . . . . . 4 92 2. Applicability of This Technique . . . . . . . . . . . . . . . 4 93 3. Minimally Covering NSEC Records . . . . . . . . . . . . . . . 5 94 4. Better Epsilon Functions . . . . . . . . . . . . . . . . . . . 6 95 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 96 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 97 7. Normative References . . . . . . . . . . . . . . . . . . . . . 8 98 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 9 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 100 Intellectual Property and Copyright Statements . . . . . . . . . . 11 102 1. Introduction and Terminology 104 With DNSSEC [1], an NSEC record lists the next instantiated name in 105 its zone, proving that no names exist in the "span" between the 106 NSEC's owner name and the name in the "next name" field. In this 107 document, an NSEC record is said to "cover" the names between its 108 owner name and next name. 110 Through repeated queries that return NSEC records, it is possible to 111 retrieve all of the names in the zone, a process commonly called 112 "walking" the zone. Some zone owners have policies forbidding zone 113 transfers by arbitrary clients; this side-effect of the NSEC 114 architecture subverts those policies. 116 This document presents a way to prevent zone walking by constructing 117 NSEC records that cover fewer names. These records can make zone 118 walking take approximately as many queries as simply asking for all 119 possible names in a zone, making zone walking impractical. Some of 120 these records must be created and signed on demand, which requires 121 on-line private keys. Anyone contemplating use of this technique is 122 strongly encouraged to review the discussion of the risks of on-line 123 signing in Section 6. 125 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 126 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 127 document are to be interpreted as described in RFC 2119 [4]. 129 2. Applicability of This Technique 131 The technique presented here may be useful to a zone owner that wants 132 to use DNSSEC, is concerned about exposure of its zone contents via 133 zone walking, and is willing to bear the costs of on-line signing. 135 As discussed in Section 6, on-line signing has several security 136 risks, including an increased likelihood of private keys being 137 disclosed and an increased risk of denial of service attack. Due to 138 these concerns, this technique should only be used when the need to 139 avoid exposure of zone contents is overwhelming. Anyone 140 contemplating use of this technique is strongly encouraged to review 141 the discussion of the risks of on-line signing in Section 6. 143 Furthermore, at the time this document was published, the DNSEXT 144 working group was actively working on a mechanism to prevent zone 145 walking that does not require on-line signing (tentatively called 146 NSEC3). This technique is likely to expose slightly more information 147 about the zone than this technique (e.g. the number of instantiated 148 names), but it will still likely be preferable to this technique. 150 Anyone contemplating use of this technique is strongly encouraged to 151 look for such future innovations. 153 3. Minimally Covering NSEC Records 155 This mechanism involves changes to NSEC records for instantiated 156 names, which can still be generated and signed in advance, as well as 157 the on-demand generation and signing of new NSEC records whenever a 158 name must be proven not to exist. 160 In the 'next name' field of instantiated names' NSEC records, rather 161 than list the next instantiated name in the zone, list any name that 162 falls lexically after the NSEC's owner name and before the next 163 instantiated name in the zone, according to the ordering function in 164 RFC4034 [2] section 6.1. This relaxes the requirement in section 165 4.1.1 of RFC4034 that the 'next name' field contains the next owner 166 name in the zone. This change is expected to be fully compatible 167 with all existing DNSSEC validators. These NSEC records are returned 168 whenever proving something specifically about the owner name (e.g. 169 that no resource records of a given type appear at that name). 171 Whenever an NSEC record is needed to prove the non-existence of a 172 name, a new NSEC record is dynamically produced and signed. The new 173 NSEC record has an owner name lexically before the QNAME but 174 lexically following any existing name and a 'next name' lexically 175 following the QNAME but before any existing name. 177 The generated NSEC record's type bitmap SHOULD have the RRSIG and 178 NSEC bits set and SHOULD NOT have any other bits set. This relaxes 179 the requirement in Section 2.3 of RFC4035 that NSEC RRs not appear at 180 names that did not exist before the zone was signed. 182 The functions to generate the lexically following and proceeding 183 names need not be perfect nor consistent, but the generated NSEC 184 records must not cover any existing names. Furthermore, this 185 technique works best when the generated NSEC records cover as few 186 names as possible. In this document, the functions that generate the 187 nearby names are called 'epsilon' functions, a reference to the 188 mathematical convention of using the greek letter epsilon to 189 represent small deviations. 191 An NSEC record denying the existence of a wildcard may be generated 192 in the same way. Since the NSEC record covering a non-existent 193 wildcard is likely to be used in response to many queries, 194 authoritative name servers using the techniques described here may 195 want to pregenerate or cache that record and its corresponding RRSIG. 197 For example, a query for an A record at the non-instantiated name 198 example.com might produce the following two NSEC records, the first 199 denying the existence of the name example.com and the second denying 200 the existence of a wildcard: 202 exampld.com 3600 IN NSEC example-.com ( RRSIG NSEC ) 204 \).com 3600 IN NSEC +.com ( RRSIG NSEC ) 206 Before answering a query with these records, an authoritative server 207 must test for the existence of names between these endpoints. If the 208 generated NSEC would cover existing names (e.g. exampldd.com or 209 *bizarre.example.com), a better epsilon function may be used or the 210 covered name closest to the QNAME could be used as the NSEC owner 211 name or next name, as appropriate. If an existing name is used as 212 the NSEC owner name, that name's real NSEC record MUST be returned. 213 Using the same example, assuming an exampldd.com delegation exists, 214 this record might be returned from the parent: 216 exampldd.com 3600 IN NSEC example-.com ( NS DS RRSIG NSEC ) 218 Like every authoritative record in the zone, each generated NSEC 219 record MUST have corresponding RRSIGs generated using each algorithm 220 (but not necessarily each DNSKEY) in the zone's DNSKEY RRset, as 221 described in RFC4035 [3] section 2.2. To minimize the number of 222 signatures that must be generated, a zone may wish to limit the 223 number of algorithms in its DNSKEY RRset. 225 4. Better Epsilon Functions 227 Section 6.1 of RFC4034 defines a strict ordering of DNS names. 228 Working backwards from that definition, it should be possible to 229 define epsilon functions that generate the immediately following and 230 preceding names, respectively. This document does not define such 231 functions. Instead, this section presents functions that come 232 reasonably close to the perfect ones. As described above, an 233 authoritative server should still ensure than no generated NSEC 234 covers any existing name. 236 To increment a name, add a leading label with a single null (zero- 237 value) octet. 239 To decrement a name, decrement the last character of the leftmost 240 label, then fill that label to a length of 63 octets with octets of 241 value 255. To decrement a null (zero-value) octet, remove the octet 242 -- if an empty label is left, remove the label. Defining this 243 function numerically: fill the left-most label to its maximum length 244 with zeros (numeric, not ASCII zeros) and subtract one. 246 In response to a query for the non-existent name foo.example.com, 247 these functions produce NSEC records of: 249 fon\255\255\255\255\255\255\255\255\255\255\255\255\255\255 250 \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255 251 \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255 252 \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255 253 \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\255 256 \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255 257 \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255 258 \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255 259 \255\255.example.com 3600 IN NSEC \000.*.example.com ( NSEC RRSIG ) 261 The first of these NSEC RRs proves that no exact match for 262 foo.example.com exists, and the second proves that there is no 263 wildcard in example.com. 265 Both of these functions are imperfect: they don't take into account 266 constraints on number of labels in a name nor total length of a name. 267 As noted in the previous section, though, this technique does not 268 depend on the use of perfect epsilon functions: it is sufficient to 269 test whether any instantiated names fall into the span covered by the 270 generated NSEC and, if so, substitute those instantiated owner names 271 for the NSEC owner name or next name, as appropriate. 273 5. IANA Considerations 275 This document specifies no IANA Actions. 277 6. Security Considerations 279 This approach requires on-demand generation of RRSIG records. This 280 creates several new vulnerabilities. 282 First, on-demand signing requires that a zone's authoritative servers 283 have access to its private keys. Storing private keys on well-known 284 internet-accessible servers may make them more vulnerable to 285 unintended disclosure. 287 Second, since generation of digital signatures tends to be 288 computationally demanding, the requirement for on-demand signing 289 makes authoritative servers vulnerable to a denial of service attack. 291 Lastly, if the epsilon functions are predictable, on-demand signing 292 may enable a chosen-plaintext attack on a zone's private keys. Zones 293 using this approach should attempt to use cryptographic algorithms 294 that are resistant to chosen-plaintext attacks. It's worth noting 295 that while DNSSEC has a "mandatory to implement" algorithm, that is a 296 requirement on resolvers and validators -- there is no requirement 297 that a zone be signed with any given algorithm. 299 The success of using minimally covering NSEC record to prevent zone 300 walking depends greatly on the quality of the epsilon functions 301 chosen. An increment function that chooses a name obviously derived 302 from the next instantiated name may be easily reverse engineered, 303 destroying the value of this technique. An increment function that 304 always returns a name close to the next instantiated name is likewise 305 a poor choice. Good choices of epsilon functions are the ones that 306 produce the immediately following and preceding names, respectively, 307 though zone administrators may wish to use less perfect functions 308 that return more human-friendly names than the functions described in 309 Section 4 above. 311 Another obvious but misguided concern is the danger from synthesized 312 NSEC records being replayed. It's possible for an attacker to replay 313 an old but still validly signed NSEC record after a new name has been 314 added in the span covered by that NSEC, incorrectly proving that 315 there is no record at that name. This danger exists with DNSSEC as 316 defined in [3]. The techniques described here actually decrease the 317 danger, since the span covered by any NSEC record is smaller than 318 before. Choosing better epsilon functions will further reduce this 319 danger. 321 7. Normative References 323 [1] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, 324 "DNS Security Introduction and Requirements", RFC 4033, 325 March 2005. 327 [2] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, 328 "Resource Records for the DNS Security Extensions", RFC 4034, 329 March 2005. 331 [3] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, 332 "Protocol Modifications for the DNS Security Extensions", 333 RFC 4035, March 2005. 335 [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement 336 Levels", BCP 14, RFC 2119, March 1997. 338 Appendix A. Acknowledgments 340 Many individuals contributed to this design. They include, in 341 addition to the authors of this document, Olaf Kolkman, Ed Lewis, 342 Peter Koch, Matt Larson, David Blacka, Suzanne Woolf, Jaap Akkerhuis, 343 Jakob Schlyter, Bill Manning, and Joao Damas. 345 In addition, the editors would like to thank Ed Lewis for his careful 346 review of the document. 348 Authors' Addresses 350 Samuel Weiler 351 SPARTA, Inc 352 7075 Samuel Morse Drive 353 Columbia, Maryland 21046 354 US 356 Email: weiler@tislabs.com 358 Johan Ihren 359 Autonomica AB 360 Bellmansgatan 30 361 Stockholm SE-118 47 362 Sweden 364 Email: johani@autonomica.se 366 Intellectual Property Statement 368 The IETF takes no position regarding the validity or scope of any 369 Intellectual Property Rights or other rights that might be claimed to 370 pertain to the implementation or use of the technology described in 371 this document or the extent to which any license under such rights 372 might or might not be available; nor does it represent that it has 373 made any independent effort to identify any such rights. 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