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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Downref: Normative reference to an Historic RFC: RFC 5074 ** Downref: Normative reference to an Informational RFC: RFC 7129 ** Obsolete normative reference: RFC 7719 (Obsoleted by RFC 8499) Summary: 3 errors (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group K. Fujiwara 3 Internet-Draft JPRS 4 Updates: 4035 (if approved) A. Kato 5 Intended status: Standards Track Keio/WIDE 6 Expires: July 16, 2017 W. Kumari 7 Google 8 January 12, 2017 10 Aggressive use of DNSSEC-validated Cache 11 draft-ietf-dnsop-nsec-aggressiveuse-08 13 Abstract 15 The DNS relies upon caching to scale; however, the cache lookup 16 generally requires an exact match. This document specifies the use 17 of NSEC/NSEC3 resource records to allow DNSSEC validating resolvers 18 to generate negative answers within a range, and positive answers 19 from wildcards. This increases performance / decreases latency, 20 decreases resource utilization on both authoritative and recursive 21 servers, and also increases privacy. It may also help increase 22 resilience to certain DoS attacks in some circumstances. 24 This document updates RFC4035 by allowing validating resolvers to 25 generate negative answers based upon NSEC/NSEC3 records (and positive 26 answers in the presence of wildcards). 28 [ Ed note: Text inside square brackets ([]) is additional background 29 information, answers to frequently asked questions, general musings, 30 etc. They will be removed before publication.This document is being 31 collaborated on in Github at: https://github.com/wkumari/draft-ietf- 32 dnsop-nsec-aggressiveuse. The most recent version of the document, 33 open issues, etc should all be available here. The authors 34 (gratefully) accept pull requests.] 36 Status of This Memo 38 This Internet-Draft is submitted in full conformance with the 39 provisions of BCP 78 and BCP 79. 41 Internet-Drafts are working documents of the Internet Engineering 42 Task Force (IETF). Note that other groups may also distribute 43 working documents as Internet-Drafts. The list of current Internet- 44 Drafts is at http://datatracker.ietf.org/drafts/current/. 46 Internet-Drafts are draft documents valid for a maximum of six months 47 and may be updated, replaced, or obsoleted by other documents at any 48 time. It is inappropriate to use Internet-Drafts as reference 49 material or to cite them other than as "work in progress." 51 This Internet-Draft will expire on July 16, 2017. 53 Copyright Notice 55 Copyright (c) 2017 IETF Trust and the persons identified as the 56 document authors. All rights reserved. 58 This document is subject to BCP 78 and the IETF Trust's Legal 59 Provisions Relating to IETF Documents 60 (http://trustee.ietf.org/license-info) in effect on the date of 61 publication of this document. Please review these documents 62 carefully, as they describe your rights and restrictions with respect 63 to this document. Code Components extracted from this document must 64 include Simplified BSD License text as described in Section 4.e of 65 the Trust Legal Provisions and are provided without warranty as 66 described in the Simplified BSD License. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 71 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 72 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 73 4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 74 5. Aggressive use of Cache . . . . . . . . . . . . . . . . . . . 6 75 5.1. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 6 76 5.2. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6 77 5.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 6 78 5.4. Consideration on TTL . . . . . . . . . . . . . . . . . . 7 79 6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7 80 7. Update to RFC 4035 . . . . . . . . . . . . . . . . . . . . . 8 81 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 82 9. Security Considerations . . . . . . . . . . . . . . . . . . . 8 83 10. Implementation Status . . . . . . . . . . . . . . . . . . . . 9 84 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 85 11.1. Change History . . . . . . . . . . . . . . . . . . . . . 10 86 11.1.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . 13 87 11.1.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . 13 88 11.1.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . 13 89 11.2. new section . . . . . . . . . . . . . . . . . . . . . . 13 90 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 91 12.1. Normative References . . . . . . . . . . . . . . . . . . 13 92 12.2. Informative References . . . . . . . . . . . . . . . . . 14 93 Appendix A. Detailed implementation notes . . . . . . . . . . . 14 94 Appendix B. Procedure for determining ENT vs NXDOMAN with NSEC . 15 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 97 1. Introduction 99 A DNS negative cache exists, and is used to cache the fact that an 100 RRset does not exist. This method of negative caching requires exact 101 matching; this leads to unnecessary additional lookups, increases 102 latency, leads to extra resource utilization on both authoritative 103 and recursive servers, and decreases privacy by leaking queries. 105 This document updates RFC 4035 to allow recursive resolvers to use 106 NSEC/NSEC3 resource records to synthesize negative answers from the 107 information they have in the cache. This allows validating resolvers 108 to respond with a negative answer immediately if the name in question 109 falls into a range expressed by a NSEC/NSEC3 resource record already 110 in the cache. It also allows the synthesis of positive answers in 111 the presence of wildcard records. 113 Aggressive Negative Caching was first proposed in Section 6 of DNSSEC 114 Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC 115 records efficiently. 117 [RFC8020], and [I-D.vixie-dnsext-resimprove] proposes first steps to 118 using NXDOMAIN information for more effective caching. This takes 119 this technique further. 121 2. Terminology 123 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 124 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 125 document are to be interpreted as described in RFC 2119 [RFC2119]. 127 Many of the specialized terms used in this document are defined in 128 DNS Terminology [RFC7719]. 130 The key words "Source of Synthesis" in this document are to be 131 interpreted as described in [RFC4592]. 133 3. Problem Statement 135 The DNS negative cache caches negative (non-existent) information, 136 and requires an exact match in most instances [RFC2308]. 138 Assume that the (DNSSEC signed) "example.com" zone contains: 140 albatross.example.com IN A 192.0.2.1 141 elephant.example.com IN A 192.0.2.2 142 zebra.example.com IN A 192.0.2.3 143 If a validating resolver receives a query for cat.example.com, it 144 contacts its resolver (which may be itself) to query the example.com 145 servers and will get back an NSEC record stating that there are no 146 records (alphabetically) between albatross and elephant, or an NSEC3 147 record stating there is nothing between two hashed names. The 148 resolver then knows that cat.example.com does not exist; however, it 149 does not use the fact that the proof covers a range (albatross to 150 elephant) to suppress queries for other labels that fall within this 151 range. This means that if the validating resolver gets a query for 152 ball.example.com (or dog.example.com) it will once again go off and 153 query the example.com servers for these names. 155 Apart from wasting bandwidth, this also wastes resources on the 156 recursive server (it needs to keep state for outstanding queries), 157 wastes resources on the authoritative server (it has to answer 158 additional questions), increases latency (the end user has to wait 159 longer than necessary to get back an NXDOMAIN answer), can be used by 160 attackers to cause a DoS (see additional resources), and also has 161 privacy implications (e.g: typos leak out further than necessary). 163 Another example: assume that the (DNSSEC signed) "example.org" zone 164 contains: 166 avocado.example.org IN A 192.0.2.1 167 *.example.org IN A 192.0.2.2 168 zucchini.example.org IN A 192.0.2.3 170 If a query is received for leek.example.org, it contacts its resolver 171 (which may be itself) to query the example.org servers and will get 172 back an NSEC record stating that there are no records 173 (alphabetically) between avocado and zucchini (or an NSEC3 record 174 stating there is nothing between two hashed names), as well as an 175 answer for leek.example.org, with the label count of the signature 176 set to two (see [RFC7129], section 5.3 for more details). 178 If the validating resolver gets a query for banana.example.org it 179 will once again go off and query the example.org servers for 180 banana.example.org (even though it already has proof that there is a 181 wildcard record) - just like above, this has privacy implications, 182 wastes resources, can be used to contribute to a DoS, etc. 184 4. Background 186 DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of 187 existence"; this is a cryptographic proof that the queried for name 188 does not exist or type does not exist. Proof that a name does not 189 exist is accomplished by providing a (DNSSEC secured) record 190 containing the names which appear alphabetically before and after the 191 queried for name. In the first example above, if the (DNSSEC 192 validating) recursive server were to query for dog.example.com it 193 would receive a (signed) NSEC record stating that there are no labels 194 between "albatross" and "elephant" (or, for NSEC3, a similar pair of 195 hashed names). This is a signed, cryptographic proof that these 196 names are the ones before and after the queried for label. As 197 dog.example.com falls within this range, the recursive server knows 198 that dog.example.com really does not exist. Proof that a type does 199 not exist is accomplished by providing a (DNSSEC secured) record 200 containing the queried for name, and a type bitmap which does not 201 include the requested type. 203 This document specifies that this NSEC/NSEC3 record should be used to 204 generate negative answers for any queries that the validating server 205 receives that fall within the range covered by the record (for the 206 TTL for the record). This document also specifies that a positive 207 answer should be generated for any queries that the validating server 208 receives that are proven to be covered by a wildcard record. 210 Section 4.5 of [RFC4035] says: 212 "In theory, a resolver could use wildcards or NSEC RRs to generate 213 positive and negative responses (respectively) until the TTL or 214 signatures on the records in question expire. However, it seems 215 prudent for resolvers to avoid blocking new authoritative data or 216 synthesizing new data on their own. Resolvers that follow this 217 recommendation will have a more consistent view of the namespace." 218 and "The reason for these recommendations is that, between the 219 initial query and the expiration of the data from the cache, the 220 authoritative data might have been changed (for example, via dynamic 221 update).". In other words, if a resolver generates negative answers 222 from an NSEC record, it will not send any queries for names within 223 that NSEC range (for the TTL). If a new name is added to the zone 224 during this interval the resolver will not know this. Similarly, if 225 the resolver is generating responses from a wildcard record, it will 226 continue to do so (for the TTL). 228 We believe this recommendation can be relaxed because, in the absence 229 of this technique, a lookup for the exact name could have come in 230 during this interval, and so a negative answer could already be 231 cached (see [RFC2308] for more background). This means that zone 232 operators should have no expectation that an added name would work 233 immediately. With DNSSEC and Aggressive NSEC, the TTL of the NSEC/ 234 NSEC3 record and the SOA.MINIMUM field are the authoritative 235 statement of how quickly a name can start working within a zone. 237 5. Aggressive use of Cache 239 This document relaxes the restriction given in Section 4.5 of 240 [RFC4035], see Section 7 for more detail. 242 If the negative cache of the validating resolver has sufficient 243 information to validate the query, the resolver SHOULD use NSEC, 244 NSEC3 and wildcard records aggressively. Otherwise, it MUST fall 245 back to send the query to the authoritative DNS servers. 247 5.1. NSEC 249 The validating resolver needs to check the existence of an NSEC RR 250 matching/covering the source of synthesis and an NSEC RR covering the 251 query name. 253 If denial of existence can be determined according to the rules set 254 out in Section 5.4 of [RFC4035], using NSEC records in the cache, 255 then the resolver can immediately return an NXDOMAIN or NODATA (as 256 appropriate) response. 258 5.2. NSEC3 260 NSEC3 aggressive negative caching is more difficult than NSEC 261 aggressive caching. If the zone is signed with NSEC3, the validating 262 resolver needs to check the existence of non-terminals and wildcards 263 which derive from query names. 265 If denial of existence can be determined according to the rules set 266 out in [RFC5155] Sections 8.4, 8.5, 8.6, 8.7, using NSEC3 records in 267 the cache, then the resolver can immediately return an NXDOMAIN or 268 NODATA response (as appropriate). 270 If a covering NSEC3 RR has Opt-Out flag, the covering NSEC3 RR does 271 not prove the non-existence of the domain name and the aggressive 272 negative caching is not possible for the domain name. 274 5.3. Wildcards 276 The last paragraph of [RFC4035] Section 4.5 also discusses the use of 277 wildcards and NSEC RRs to generate positive responses and recommends 278 that it not be relied upon. Just like the case for the aggressive 279 use of NSEC/NSEC3 for negative answers, we revise this 280 recommendation. 282 As long as the validating resolver can determine that a name would 283 not exist without the wildcard match, determined according to the 284 rules set out in Section 5.3.4 of [RFC4035] (NSEC), or in Section 8.8 285 of [RFC5155], it SHOULD synthesize an answer (or NODATA response) for 286 that name using the cached deduced wildcard. If the corresponding 287 wildcard record is not in the cache, it MUST fall back to send the 288 query to the authoritative DNS servers. 290 5.4. Consideration on TTL 292 The TTL value of negative information is especially important, 293 because newly added domain names cannot be used while the negative 294 information is effective. 296 Section 5 of [RFC2308] states that the maximum number of negative 297 cache TTL value is 3 hours (10800). It is RECOMMENDED that 298 validating resolvers limit the maximum effective TTL value of 299 negative responses (NSEC/NSEC3 RRs) to this same value. 301 Section 5 of [RFC2308] also states that a negative cache entry TTL is 302 taken from the minimum of the SOA.MINIMUM field and SOA's TTL. This 303 can be less than the TTL of an NSEC or NSEC3 record, since their TTL 304 is equal to the SOA.MINIMUM field (see [RFC4035]section 2.3 and 305 [RFC5155] section 3.) 307 A resolver that supports aggressive use of NSEC and NSEC3 SHOULD 308 reduce the TTL of NSEC and NSEC3 records to match the SOA.MINIMUM 309 field in the authority section of a negative response, if SOA.MINIMUM 310 is smaller. 312 6. Benefits 314 The techniques described in this document provide a number of 315 benefits, including (in no specific order): 317 Reduced latency: By answering directly from cache, validating 318 resolvers can immediately inform clients that the name they are 319 looking for does not exist, improving the user experience. 321 Decreased recursive server load: By answering queries from the cache 322 by synthesizing answers, validating servers avoid having to send a 323 query and wait for a response. In addition to decreasing the 324 bandwidth used, it also means that the server does not need to 325 allocate and maintain state, thereby decreasing memory and CPU 326 load. 328 Decreased authoritative server load: Because recursive servers can 329 answer queries without asking the authoritative server, the 330 authoritative servers receive fewer queries. This decreases the 331 authoritative server bandwidth, queries per second and CPU 332 utilization. 334 The scale of the benefit depends upon multiple factors, including the 335 query distribution. For example, at the time of this writing, around 336 65% of queries to Root Name servers result in NXDOMAIN responses (see 337 statistics from [root-servers.org]); this technique will eliminate a 338 sizable quantity of these. 340 The technique described in this document may also mitigate so-called 341 "random QNAME attacks", in which attackers send many queries for 342 random sub-domains to resolvers. As the resolver will not have the 343 answers cached, it has to ask external servers for each random query, 344 leading to a DoS on the authoritative servers (and often resolvers). 345 Aggressive NSEC may help mitigate these attacks by allowing the 346 resolver to answer directly from cache for any random queries which 347 fall within already requested ranges. It will not always work as an 348 effective defense, not least because not many zones are DNSSEC signed 349 at all -- but it will still provide an additional layer of defense. 351 As these benefits are only accrued by those using DNSSEC, it is hoped 352 that these techniques will lead to more DNSSEC deployment. 354 7. Update to RFC 4035 356 Section 4.5 of [RFC4035] shows that "In theory, a resolver could use 357 wildcards or NSEC RRs to generate positive and negative responses 358 (respectively) until the TTL or signatures on the records in question 359 expire. However, it seems prudent for resolvers to avoid blocking 360 new authoritative data or synthesizing new data on their own. 361 Resolvers that follow this recommendation will have a more consistent 362 view of the namespace". 364 The paragraph is updated as follows: 366 +-----------------------------------------------------------------+ 367 | Once the records are validated, DNSSEC enabled validating | 368 | resolvers SHOULD use wildcards and NSEC/NSEC3 resource records | 369 | to generate positive and negative responses until the | 370 | effective TTLs or signatures for those records expire. | 371 +-----------------------------------------------------------------+ 373 8. IANA Considerations 375 This document has no IANA actions. 377 9. Security Considerations 379 Use of NSEC / NSEC3 resource records without DNSSEC validation may 380 create serious security issues, and so this technique requires DNSSEC 381 validation. 383 Newly registered resource records may not be used immediately. 384 However, choosing suitable TTL value and negative cache TTL value 385 (SOA MINIMUM field) will mitigate the delay concern, and it is not a 386 security problem. 388 It is also suggested to limit the maximum TTL value of NSEC / NSEC3 389 resource records in the negative cache to, for example, 10800 seconds 390 (3hrs), to mitigate this issue. 392 Although the TTL of NSEC/NSEC3 records is typically fairly short 393 (minutes or hours), their RRSIG expiration time can be much further 394 in the future (weeks). An attacker who is able to successfully spoof 395 responses might poison a cache with old NSEC/NSEC3 records. If the 396 resolver is NOT making aggressive use of NSEC/NSEC3, the attacker has 397 to repeat the attack for every query. If the resolver IS making 398 aggressive use of NSEC/NSEC3, one successful attack would be able to 399 suppress many queries for new names, up to the negative TTL. 401 10. Implementation Status 403 [ Editor note: RFC Editor, please remove this entire section. 404 RFC6982 says: "Since this information is necessarily time dependent, 405 it is inappropriate for inclusion in a published RFC." ] 407 Unbound currently implements aggressive negative caching, as does 408 Google Public DNS. 410 11. Acknowledgments 412 The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler 413 and the Unbound developers. 415 Thanks to Mark Andrews for providing the helpful notes for 416 implementors provided in Appendix B. 418 The authors would like to specifically thank Stephane Bortzmeyer (for 419 standing next to and helping edit), Ralph Dolmans, Tony Finch, Tatuya 420 JINMEI for extensive review and comments, and also Mark Andrews, 421 Casey Deccio, Alexander Dupuy, Olafur Gudmundsson, Bob Harold, Shumon 422 Huque, John Levine, Pieter Lexis, Matthijs Mekking (who even sent 423 pull requests!) and Ondrej Sury. Mark Andrews also provided the 424 helpful notes for implementors (https://www.ietf.org/mail- 425 archive/web/dnsop/current/msg18332.html) which we made into 426 Appendix B. 428 11.1. Change History 430 RFC Editor: Please remove this section prior to publication. 432 -05 to -06: 434 o Moved some dangling text around - when the examples were added 435 some text added in the wrong place. 437 o There were some bits which mentioned "negative" in the title. 439 o We had the cut-and-paste of what changed in 4035 twice. 441 o Clarified that this also allows NODATA responses to be 442 synthesized. 444 -04 to -05: 446 o Bob pointed out that I did a stupid - when I added the wildcard to 447 'example.com' I made the example wrong / confusing. I have 448 attempted to fix this by adding a second example zone 449 (example.org) with the wildcard instead. 451 o More helpful changes (in a pull request, thanks!) from Matthijs 453 o Included Mark Andrew's useful explanation of how to tell ENT from 454 NXD as an Appendix. 456 -03 to -04: 458 o Working group does want the "positive" answers, not just negative 459 ones. This requires reading what used to be Section 7, and a 460 bunch of cleanup, including: 462 * Additional text in the Problem Statement 464 * Added a wildcard record to the zone. 466 * Added "or positive answers from wildcards" type text (where 467 appropriate) to explain that this isn't just for negative 468 answers. 470 * Reworded much of the Wildcard text. 472 o Incorporated pull request from Tony Finch (thanks!): 473 https://github.com/wkumari/draft-ietf-dnsop-nsec-aggressiveuse/ 474 pull/1 476 o More fixups from Tony (including text): https://www.ietf.org/mail- 477 archive/web/dnsop/current/msg18271.html. This included much 478 clearer text on TTL, references to the NSEC / NSEC3 RFCs (instead 479 of my clumsy summary), good text on replays, etc. 481 o Converted the "zone file" to a figure to make it more readable. 483 o Text from Tim W: "If a validating resolver receives a query for 484 cat.example.com, it contacts its resolver (which may be itself) to 485 query..." - which satisfies Jinmei's concern (which I was too 486 dense to grock). 488 o Fixup of the "validation required" in security considerations. 490 -02 to -03: 492 o Integrated a bunch of comments from Matthijs Mekking - details in: 493 https://github.com/wkumari/draft-ietf-dnsop-nsec-aggressiveuse/ 494 pull/1. I decided to keep "Aggressive Negative Caching" instead 495 of "Aggressive USE OF Negative Caching" for readability. 497 o Attempted to address Bob Harold's comment on the readability 498 issues with "But, it will be more effective when both are 499 enabled..." in Section 5.4 - https://www.ietf.org/mail- 500 archive/web/dnsop/current/msg17997.html 502 o MAYs and SHOULD drifted in the text block. Fixed - thanks to 503 https://mailarchive.ietf.org/arch/msg/ 504 dnsop/2ljmmzxtIMCFMLOZmWcSbTYVOy4 506 o A number of good edits from Stephane in: https://www.ietf.org/ 507 mail-archive/web/dnsop/current/msg18109.html 509 o A bunch more edits from Jinmei, as in: https://www.ietf.org/mail- 510 archive/web/dnsop/current/msg18206.html 512 -01 to -02: 514 o Added Section 6 - Benefits (as suggested by Jinmei). 516 o Removed Appendix B (Jinmei) 518 o Replaced "full-service" with "validating" (where applicable) 520 o Integrated other comments from Jinmei from https://www.ietf.org/ 521 mail-archive/web/dnsop/current/msg17875.html 523 o Integrated comment from co-authors, including re-adding parts of 524 Appendix B, terminology, typos. 526 o Tried to explain under what conditions this may actually mitigate 527 attacks. 529 -00 to -01: 531 o Comments from DNSOP meeting in Berlin. 533 o Changed intended status to Standards Track (updates RFC 4035) 535 o Added a section "Updates to RFC 4035" 537 o Some language clarification / typo / cleanup 539 o Cleaned up the TTL section a bit. 541 o Removed Effects section, Additional proposal section, and pseudo 542 code. 544 o Moved "mitigation of random subdomain attacks" to Appendix. 546 From draft-fujiwara-dnsop-nsec-aggressiveuse-03 -> draft-ietf-dnsop- 547 nsec-aggressiveuse 549 o Document adopted by DNSOP WG. 551 o Adoption comments 553 o Changed main purpose to performance 555 o Use NSEC3/Wildcard keywords 557 o Improved wordings (from good comments) 559 o Simplified pseudo code for NSEC3 561 o Added Warren as co-author. 563 o Reworded much of the problem statement 565 o Reworked examples to better explain the problem / solution. 567 11.1.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 569 o Added reference to DLV [RFC5074] and imported some sentences. 571 o Added Aggressive Negative Caching Flag idea. 573 o Added detailed algorithms. 575 11.1.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 577 o Added reference to [I-D.vixie-dnsext-resimprove] 579 o Added considerations for the CD bit 581 o Updated detailed algorithms. 583 o Moved Aggressive Negative Caching Flag idea into Additional 584 Proposals 586 11.1.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 588 o Added "Partial implementation" 590 o Section 4,5,6 reorganized for better representation 592 o Added NODATA answer in Section 4 594 o Trivial updates 596 o Updated pseudo code 598 11.2. new section 600 12. References 602 12.1. Normative References 604 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 605 Requirement Levels", BCP 14, RFC 2119, 606 DOI 10.17487/RFC2119, March 1997, 607 . 609 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 610 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 611 . 613 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 614 Rose, "Protocol Modifications for the DNS Security 615 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 616 . 618 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name 619 System", RFC 4592, DOI 10.17487/RFC4592, July 2006, 620 . 622 [RFC5074] Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074, 623 DOI 10.17487/RFC5074, November 2007, 624 . 626 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 627 Security (DNSSEC) Hashed Authenticated Denial of 628 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008, 629 . 631 [RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of 632 Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, 633 February 2014, . 635 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 636 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 637 2015, . 639 12.2. Informative References 641 [I-D.vixie-dnsext-resimprove] 642 Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS 643 Resolvers for Resiliency, Robustness, and Responsiveness", 644 draft-vixie-dnsext-resimprove-00 (work in progress), June 645 2010. 647 [RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is 648 Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020, 649 November 2016, . 651 [root-servers.org] 652 IANA, "Root Server Technical Operations Assn", 653 . 655 Appendix A. Detailed implementation notes 657 o Previously, cached negative responses were indexed by QNAME, 658 QCLASS, QTYPE, and the setting of the CD bit (see RFC 4035, 659 Section 4.7), and only queries matching the index key would be 660 answered from the cache. With aggressive negative caching, the 661 validator, in addition to checking to see if the answer is in its 662 cache before sending a query, checks to see whether any cached and 663 validated NSEC record denies the existence of the sought 664 record(s). Using aggressive negative caching, a validator will 665 not make queries for any name covered by a cached and validated 666 NSEC record. Furthermore, a validator answering queries from 667 clients will synthesize a negative answer (or NODATA response) 668 whenever it has an applicable validated NSEC in its cache unless 669 the CD bit was set on the incoming query. (Imported from 670 Section 6 of [RFC5074]). 672 o Implementing aggressive negative caching suggests that a validator 673 will need to build an ordered data structure of NSEC and NSEC3 674 records for each signer domain name of NSEC / NSEC3 records in 675 order to efficiently find covering NSEC / NSEC3 records. Call the 676 table as NSEC_TABLE. (Imported from Section 6.1 of [RFC5074] and 677 expanded.) 679 o The aggressive negative caching may be inserted at the cache 680 lookup part of the recursive resolvers. 682 o If errors happen in aggressive negative caching algorithm, 683 resolvers MUST fall back to resolve the query as usual. "Resolve 684 the query as usual" means that the resolver must process the query 685 as though it does not implement aggressive negative caching. 687 Appendix B. Procedure for determining ENT vs NXDOMAN with NSEC 689 This procedure outlines how to determine if a given name does not 690 exist, or is an ENT (Empty Non-Terminal, see [RFC5155] Section 1.3) 691 with NSEC. 693 If the NSEC record has not been verified as secure discard it. 695 If the given name sorts before or matches the NSEC owner name discard 696 it as it does not prove the NXDOMAIN or ENT. 698 If the given name is a subdomain of the NSEC owner name and the NS 699 bit is present and the SOA bit is absent then discard the NSEC as it 700 is from a parent zone. 702 If the next domain name sorts after the NSEC owner name and the given 703 name sorts after or matches next domain name then discard the NSEC 704 record as it does not prove the NXDOMAIN or ENT. 706 If the next domain name sorts before or matches the NSEC owner name 707 and the given name is not a subdomain of the next domain name then 708 discard the NSEC as it does not prove the NXDOMAIN or ENT. 710 You now have a NSEC record that proves the NXDOMAIN or ENT. 712 If the next domain name is a subdomain of the given name you have a 713 ENT otherwise you have a NXDOMAIN. 715 Authors' Addresses 717 Kazunori Fujiwara 718 Japan Registry Services Co., Ltd. 719 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda 720 Chiyoda-ku, Tokyo 101-0065 721 Japan 723 Phone: +81 3 5215 8451 724 Email: fujiwara@jprs.co.jp 726 Akira Kato 727 Keio University/WIDE Project 728 Graduate School of Media Design, 4-1-1 Hiyoshi 729 Kohoku, Yokohama 223-8526 730 Japan 732 Phone: +81 45 564 2490 733 Email: kato@wide.ad.jp 735 Warren Kumari 736 Google 737 1600 Amphitheatre Parkway 738 Mountain View, CA 94043 739 US 741 Email: warren@kumari.net