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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) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DNSOP G. Huston 3 Internet-Draft J. Damas 4 Intended status: Standards Track APNIC 5 Expires: September 1, 2018 W. Kumari 6 Google 7 February 28, 2018 9 A Sentinel for Detecting Trusted Keys in DNSSEC 10 draft-ietf-dnsop-kskroll-sentinel-03 12 Abstract 14 The DNS Security Extensions (DNSSEC) were developed to provide origin 15 authentication and integrity protection for DNS data by using digital 16 signatures. These digital signatures can be verified by building a 17 chain of trust starting from a trust anchor and proceeding down to a 18 particular node in the DNS. This document specifies a mechanism that 19 will allow an end user to determine the trusted key state for the 20 root key of the resolvers that handle that user's DNS queries. Note 21 that this method is only applicable for determing which keys are in 22 the trust store for the root key. 24 There is an example / toy implementation of this at http://www.ksk- 25 test.net . 27 [ This document is being collaborated on in Github at: 28 https://github.com/APNIC-Labs/draft-kskroll-sentinel. The most 29 recent version of the document, open issues, etc should all be 30 available here. The authors (gratefully) accept pull requests. Text 31 in square brackets will be removed before publication. ] 33 [ NOTE: This version uses the labels "kskroll-sentinel-is-ta-", "kskroll-sentinel-not-ta-"; older versions of 35 this document used "_is-ta-", "_not-ta-". Also 36 note that the format of the tag-index is now decimal. Apolgies to 37 those who have began implmenting.] 39 Status of This Memo 41 This Internet-Draft is submitted in full conformance with the 42 provisions of BCP 78 and BCP 79. 44 Internet-Drafts are working documents of the Internet Engineering 45 Task Force (IETF). Note that other groups may also distribute 46 working documents as Internet-Drafts. The list of current Internet- 47 Drafts is at http://datatracker.ietf.org/drafts/current/. 49 Internet-Drafts are draft documents valid for a maximum of six months 50 and may be updated, replaced, or obsoleted by other documents at any 51 time. It is inappropriate to use Internet-Drafts as reference 52 material or to cite them other than as "work in progress." 54 This Internet-Draft will expire on September 1, 2018. 56 Copyright Notice 58 Copyright (c) 2018 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 61 This document is subject to BCP 78 and the IETF Trust's Legal 62 Provisions Relating to IETF Documents 63 (http://trustee.ietf.org/license-info) in effect on the date of 64 publication of this document. Please review these documents 65 carefully, as they describe your rights and restrictions with respect 66 to this document. Code Components extracted from this document must 67 include Simplified BSD License text as described in Section 4.e of 68 the Trust Legal Provisions and are provided without warranty as 69 described in the Simplified BSD License. 71 Table of Contents 73 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 74 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 75 2. Use Case . . . . . . . . . . . . . . . . . . . . . . . . . . 3 76 3. Sentinel Mechanism in Resolvers . . . . . . . . . . . . . . . 6 77 4. Processing Sentinel Results . . . . . . . . . . . . . . . . . 7 78 5. Sentinel Test Result Considerations . . . . . . . . . . . . . 9 79 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 80 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 81 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 82 9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 11 83 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 84 10.1. Normative References . . . . . . . . . . . . . . . . . . 12 85 10.2. Informative References . . . . . . . . . . . . . . . . . 13 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 88 1. Introduction 90 The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and 91 [RFC4035] were developed to provide origin authentication and 92 integrity protection for DNS data by using digital signatures. 93 DNSSEC uses Key Tags to efficiently match signatures to the keys from 94 which they are generated. The Key Tag is a 16-bit value computed 95 from the RDATA portion of a DNSKEY RR using a formula similar to a 96 ones-complement checksum. RRSIG RRs contain a Key Tag field whose 97 value is equal to the Key Tag of the DNSKEY RR that validates the 98 signature. 100 This document specifies how validating resolvers can respond to 101 certain queries in a manner that allows a querier to deduce whether a 102 particular key for the root has been loaded into that resolver's 103 trusted key store. In particular, this response mechanism can be 104 used to determine whether a certain root zone KSK is ready to be used 105 as a trusted key within the context of a key roll by this resolver. 107 This new mechanism is OPTIONAL to implement and use, although for 108 reasons of supporting broad-based measurement techniques, it is 109 strongly preferred that configurations of DNSSEC-validating resolvers 110 enabled this mechanism by default, allowing for local configuration 111 directives to disable this mechanism if desired. 113 1.1. Terminology 115 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 116 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 117 document are to be interpreted as described in RFC 2119. 119 Note that example.com, AAAA records and the IPv6 documentation prefix 120 (2001:db8::/32) are only examples - A records (or CNAMES), other IPs, 121 other domains work just as well. 123 2. Use Case 125 [Ed note: This is currently towards the front of the document; we 126 will make it an appendix at publication time, but until then it is 127 worth having up front, as it makes the rest of the document much 128 easier to understand ] 130 This section provides a non-normative example of how the sentinel 131 mechanism could be used, and what each participant does. It is 132 provided in a conversational tone to be easier to follow. 134 Alice is in charge of the DNS root KSK (Key Signing Key), and would 135 like to roll / replace the key with a new one. She publishes the new 136 KSK, but would like to be able to predict / measure what the impact 137 will be before removing/revoking the old key. The current KSK has a 138 key ID of 1111, the new KSK has a key ID of 2222. Users want to 139 verify that their resolver will not break after Alice rolls the root 140 KSK key (that is, starts signing with just the KSK whose key ID is 141 2222). 143 Bob, Charlie, Dave, Ed are all users. They use the DNS recursive 144 resolvers supplied by their ISPs. They would like to confirm that 145 their ISPs have picked up the new KSK. Bob's ISP does not perform 146 validation. Charlie's ISP does validate, but the resolvers have not 147 yet been upgraded to support this mechanism. Dave and Ed's resolvers 148 have been upgraded to support this mechanism; Dave's resolver has the 149 new KSK, Ed's resolver hasn't managed to install the 2222 KSK in its 150 trust store yet. 152 Geoff is a researcher, and would like to both provide a means for 153 Bob, Charlie, Dave and Ed to be able to perform tests, and also would 154 like to be able to perform Internet-wide measurements of what the 155 impact will be (and report this back to Alice). 157 Geoff sets an authoritative DNS server for example.com, and also a 158 webserver (www.example.com). He adds 3 AAAA records to example.com: 160 invalid.example.com. IN AAAA 2001:db8::1 162 kskroll-sentinel-is-ta-2222.example.com. IN AAAA 2001:db8::1 164 kskroll-sentinel-not-ta-2222.example.com. IN AAAA 2001:db8::1 166 Geoff then DNSSEC signs the example.com zone, and intentionally makes 167 the invalid.example.com record invalid/bogus (for example, by editing 168 the signed zone and entering garbage for the signature). Geoff also 169 configures his webserver to listen on 2001:db8::1 and serve a 170 resource (for example, a 1x1 GIF, 1x1.gif) for all of these names. 171 The webserver also serves a webpage (www.example.com) which contains 172 links to these 3 resources (http://invalid.example.com/1x1.gif, 173 http://kskroll-sentinel-is-ta-2222.example.com/1x1.gif, 174 http://kskroll-sentinel-not-ta-2222.example.com/1x1.gif). 176 Geoff then asks Bob, Charlie, Dave and Ed to browse to 177 www.example.com. Using the methods described in this document, the 178 users can figure out what their fate will be when the 1111 KSK is 179 removed. 181 Bob is not using a validating resolver. This means that he will be 182 able to resolve invalid.example.com (and fetch the 1x1 GIF) - this 183 tells him that the KSK roll does not affect him, and so he will be 184 OK. 186 Charlie's resolvers are validating, but they have not been upgraded 187 to support the KSK sentinel mechanism. Charlie will not be able to 188 fetch the http://invalid.example.com/1x1.gif resource (the 189 invalid.example.com record is bogus, and none of his resolvers will 190 resolve it). He is able to fetch both of the other resources - from 191 this he knows (see the logic below) that he is using legacy, 192 validating resolvers. The KSK sentinel method cannot provided him 193 with a definitive answer to the question of what root trust anchors 194 this resolver is using. 196 Dave's resolvers implement the sentinel method, and have picked up 197 the new KSK. For the same reason as Charlie, he cannot fetch the 198 "invalid" resource. His resolver resolves the kskroll-sentinel-is- 199 ta-2222.example.com name normally (it contacts the example.com 200 authoritative servers, etc); as it supports the sentinel mechanism, 201 just before Dave's recursive server send the reply to Dave's stub, it 202 performs the KSK Sentinel check (see below). The QNAME starts with 203 "kskroll-sentinel-is-ta-", and the recursive resolver does indeed 204 have a key with the Key ID of 2222 in its root trust store. This 205 means that that this part of the KSK Sentinel check passes (it is 206 true that 2222 is in the trust anchor store), and the recursive 207 resolver replies normally (with the answer provided by the 208 authoritative server). Dave's recursive resolver then resolves the 209 kskroll-sentinel-not-ta-2222.example.com name. Once again, it 210 performs the normal resolution process, but because it implements KSK 211 Sentinel (and the QNAME starts with "kskroll-sentinel-not-ta-"), just 212 before sending the reply, it performs the KSK Sentinel check. As it 213 has 2222 in it's trust anchor store, the answer to "is this *not* a 214 trust anchor" is false, and so the recursive resolver does not reply 215 with the answer from the authoritative server - instead, it replies 216 with a SERVFAIL (note that replying with SERVFAIL instead of the 217 original answer is the only mechanism that KSK Sentinel uses). This 218 means that Dave cannot fetch "invalid", he can fetch "kskroll- 219 sentinel-is-ta-2222", but he cannot fetch "kskroll-sentinel-not-ta- 220 2222". From this, Dave knows that he is behind an upgraded, 221 validating resolver, which has successfully installed the new, 2222 222 KSK. 224 Just like Charlie and Dave, Ed cannot fetch the "invalid" record. 225 This tells him that his resolvers are validating. When his 226 (upgraded) resolver performs the KSK Sentinel check for "kskroll- 227 sentinel-is-ta-2222", it does *not* have the (new, 2222) KSK in it's 228 trust anchor store. This means check fails, and Ed's recursive 229 resolver converts the (valid) answer into a SERVFAIL error response. 230 It performs the same check for kskroll-sentinel-not-ta- 231 2222.example.com; as it does not have the 2222 KSK, it is true that 232 this is not a trust anchor for it, and so it replies normally. This 233 means that Ed cannot fetch the "invalid" resource, he also cannot 234 fetch the "kskroll-sentinel-is-ta-2222" resource, but he can fetch 235 the "kskroll-sentinel-not-ta-2222" resource. This tells Ed that his 236 resolvers have not installed the new KSK. 238 Geoff would like to do a large scale test and provide the information 239 back to Alice. He uses some mechanism such as causing users to go to 240 a web page to cause a large number of users to attempt to resolve the 241 three resources, and then analyzes the results of the tests to 242 determine what percentage of users will be affected by the KSK 243 rollover event. 245 The above description is a simplified example - it is not anticipated 246 that Bob, Charlie, Dave and Ed will actually look for the absence or 247 presence of web resources; instead, the webpage that they load would 248 likely contain JavaScript (or similar) which displays the result of 249 the tests, sends the results to Geoff, or both. This sentinel 250 mechanism does not rely on the web: it can equally be used by trying 251 to resolve the names (for example, using the common "dig" command) 252 and checking which result in a SERVFAIL. 254 Note that the sentinel mechanism described here measures a very 255 different (and likely more useful) metric than [RFC8145]. RFC 8145 256 relies on resolvers reporting the list of keys that they have to root 257 servers. That reflects on how many resolvers will be impacted by a 258 KSK roll, but not what the user impact of the KSK roll will be. 260 3. Sentinel Mechanism in Resolvers 262 DNSSEC-Validating resolvers that implement this mechanism MUST be 263 performing validation of responses in accordance with the DNSSEC 264 response validation specification [RFC4035]. 266 This sentinel mechanism makes use of two special labels. The 267 "kskroll-sentinel-is-ta-" label is used in a query where 268 the response can answer whether this is the key tag of a trust anchor 269 which the validating DNS resolver is currently trusting. The 270 "kskroll-sentinel-not-ta-" label is used in a query where 271 the response can answer whether this is the key tag of a trust anchor 272 which the validating DNS resolver is NOT currently trusting. 274 The use of the positive question and its inverse allows for queries 275 to detect whether resolvers support this sentinel mechanism. Note 276 that the test is "Is there an active key with this KeyID in the 277 resolver's current trust store for the DNS root?", not "Is there any 278 key with this KeyID in the trust store", nor "Was a key with this 279 KeyID used to validate this query?". An active key is one which 280 could currently be used for validation (that is, a key that is not in 281 either the AddPend or Revoked state as described in [RFC5011]). 283 If the outcome of the DNSSEC validation process on the response 284 indicates that the response is authentic, and if the left-most label 285 of the original query name matches the template "kskroll-sentinel-is- 286 ta-.", then the following rule should be applied to the 287 response: If the resolver has placed a root zone KSK with tag index 288 value matching the value specified in the query into the local 289 resolver's store of trusted keys, then the resolver should return a 290 response indicating that the response contains authenticated data 291 according to section 5.8 of [RFC6840]. Otherwise, the resolver MUST 292 return RCODE 2 (server failure). Note that the is 293 specified in the DNS label using decimal notation (as described in 294 [RFC4034], section 5.3), zero padded to 5 digits. 296 If the outcome of the DNSSEC validation process applied to the 297 response indicates that the response is authentic, and if the left- 298 most label of the original query name matches the template "kskroll- 299 sentinel-not-ta-.", then the following rule should be 300 applied to the response: If the resolver has not placed a root zone 301 KSK with tag index value matching the value specified in the query 302 into the local resolver's store of trusted keys, then the resolver 303 should return a response indicating that the response contains 304 authenticated data according to section 5.8 of [RFC6840]. Otherwise, 305 the resolver MUST return RCODE 2 (server failure). Note that the 306 is specified in the DNS label using decimal notation. 308 In all other cases the resolver MUST NOT alter the outcome of the DNS 309 response validation process. 311 This mechanism is to be applied only by resolvers that are performing 312 DNSSEC validation, and applies only to responses to an A or AAAA 313 query (Query Type value 1 or 28) where the resolver has authenticated 314 the response according to the DNSSEC validation process and where the 315 query name contains either of the labels described in this section as 316 its left-most label. In this case, the resolver is to perform an 317 additional test following the conventional validation function, as 318 described in this section. The result of this additional test 319 determines whether the resolver will alter its response that would 320 have indicated that the RRset is authentic to a response that 321 indicates DNSSEC validation failure via the use of RCODE 2. 323 4. Processing Sentinel Results 325 This proposed test that uses the sentinel detection mechanism 326 described in this document is based on the use of three DNS names 327 that have three distinct DNS resolution behaviours. The test is 328 intended to allow a user to determine the state of their DNS 329 resolution system, and, in particular, whether or not they are using 330 validating DNS resolvers that use a particular trust anchor for the 331 root zone. 333 The critical aspect of the DNS names used in this mechanism is that 334 they contain the specified label for either the positive and negative 335 test as the left-most label in the query name. 337 The sentinel detection process uses a test with three query names: 339 o A query name containing the left-most label "kskroll-sentinel-is- 340 ta-.". This corresponds to a a validly-signed RRset in 341 the zone, so that responses associated with queried names in this 342 zone can be authenticated by a DNSSEC-validating resolver. Any 343 validly-signed DNS zone can be used for this test. 345 o A query name containing the left-most label "kskroll-sentinel-not- 346 ta-.". This is also a validly-signed name. Any 347 validly-signed DNS zone can be used for this test. 349 o A query name that is signed with a DNSSEC signature that cannot be 350 validated (such as if the corresponding RRset is not signed with a 351 valid RRSIG record). 353 The responses received from queries to resolve each of these names 354 would allow us to infer a trust key state of the resolution 355 environment. The techniques describes in this document rely on 356 (DNSSEC validating) resolvers responding with SERVFAIL (RCODE 2) to 357 valid answers. Note that a slew of other issues can also cause 358 SERVFAIL responses, and so the sentinel processing may sometimes 359 result in incorrect conclusions. 361 To describe this process of classification, we can classify resolvers 362 into four distinct behavior types, for which we will use the labels: 363 "Vnew", "Vold", "Vleg", and "nonV". These labels correspond to 364 resolver behaviour types as follows: 366 Vnew: A DNSSEC-Validating resolver that is configured to implement 367 this mechanism has loaded the nominated key into its local trusted 368 key store will respond with an A or AAAA RRset response for 369 "kskroll-sentinel-is-ta" queries, SERVFAIL for "kskroll-sentinel- 370 not-ta" queries and SERVFAIL for the invalidly signed name 371 queries. 373 Vold: A DNSSEC-Validating resolver that is configured to implement 374 this mechanism that has not loaded the nominated key into its 375 local trusted key store will respond with an SERVFAIL for 376 "kskroll-sentinel-is-ta" queries, an A or AAAA RRset response for 377 "kskroll-sentinel-not-ta" queries and SERVFAIL for the invalidly 378 signed name queries. 380 Vleg: A DNSSEC-Validating resolver that does not implement this 381 mechanism will respond with an A or AAAA RRset response for 382 "kskroll-sentinel-is-ta", an A record response for "kskroll- 383 sentinel-not-ta" and SERVFAIL for the invalid name. 385 nonV: A non-DNSSEC-Validating resolver will respond with an A or 386 AAAA record response for "kskroll-sentinel-is-ta", an A record 387 response for "kskroll-sentinel-not-ta" and an A record response 388 for the invalid name. 390 Given the clear delineation amongst these three cases, if a client 391 directs these three queries to a simple resolver, the variation in 392 response to the three queries should allow the client to determine 393 the category of the resolver, and if it supports this mechanism, 394 whether or not it has a particular key in its trust anchor store. 396 Query 397 +----------+-----------+------------+ 398 | is-ta | not-ta | invalid | 399 +-------+----------+-----------+------------+ 400 | Vnew | A | SERVFAIL | SERVFAIL | 401 | Vold | SERVFAIL | A | SERVFAIL | 402 Type | Vleg | A | A | SERVFAIL | 403 | nonV | A | A | A | 404 +-------+----------+-----------+------------+ 406 A "Vnew" type says that the nominated key is trusted by the resolver 407 and has been loaded into its local trusted key stash. A "Vold" type 408 says that the nominated key is not yet trusted by the resolver in its 409 own right. A "Vleg" type does not give the user any information 410 about the trust anchors, and a "nonV" type indicates that the 411 resolver does not perform DNSSEC validation. 413 5. Sentinel Test Result Considerations 415 The description in the previous section describes a simple situation 416 where the test queries were being passed to a single recursive 417 resolver that directly queried authoritative name servers, including 418 the root servers. 420 There is also the common case where the end client is configured to 421 use multiple resolvers. In these cases the SERVFAIL responses from 422 one resolver will prompt the end client to repeat the query against 423 one of the other configured resolvers. 425 If any of the client's resolvers are non-validating resolvers, the 426 tests will result in the client reporting that it has a non- 427 validating DNS environment ("nonV"), which is effectively the case. 429 If all of the client resolvers are DNSSEC-validating resolvers, but 430 some do not support this trusted key mechanism, then the result will 431 be indeterminate with respect to trusted key status ("Vleg"). 432 Simlarly, if all the client's resolvers support this mechanism, but 433 some have loaded the key into the trusted key stash and some have 434 not, then the result is indeterminate ("Vleg"). 436 There is also the common case of a recursive resolver using a 437 forwarder. 439 If the resolver is non-validating, and it has a single forwarder 440 clause, then the resolver will presumably mirror the capabilities of 441 the forwarder target resolver. If this non-validating resolver it 442 has multiple forwarders, then the above considerations will apply. 444 If the validating resolver has a forwarding configuration, and uses 445 the CD flag on all forwarded queries, then this resolver is acting in 446 a manner that is identical to a standalone resolver. The same 447 consideration applies if any one one of the forwarder targets is a 448 non-validating resolver. Similarly, if all the forwarder targets do 449 not apply this trusted key mechanism, the same considerations apply. 451 A more complex case is where all of the following conditions all 452 hold: 454 o Both the validating resolver and the forwarder target resolver 455 support this trusted key sentinel mechanism 457 o The local resolver's queries do not have the CD bit set 459 o The trusted key state differs between the forwarding resolver and 460 the forwarder target resolver 462 In such a case, either the outcome is indeterminate validating 463 ("Vleg"), or a case of mixed signals (SERVFAIL in all three 464 responses), which is similarly an indeterminate response with respect 465 to the trusted key state. 467 6. Security Considerations 469 This document describes a mechanism to allow users to determine the 470 trust state of root zone key signing keys in the DNS resolution 471 system that they use. 473 The mechanism does not require resolvers to set otherwise 474 unauthenticated responses to be marked as authenticated, and does not 475 alter the security properties of DNSSEC with respect to the 476 interpretation of the authenticity of responses that are so marked. 478 The mechanism does not require any further significant processing of 479 DNS responses, and queries of the form described in this document do 480 not impose any additional load that could be exploited in an attack 481 over the the normal DNSSEC validation processing load. 483 7. IANA Considerations 485 [Note to IANA, to be removed prior to publication: there are no IANA 486 considerations stated in this version of the document.] 488 8. Acknowledgements 490 This document has borrowed extensively from [RFC8145] for the 491 introductory text, and the authors would like to acknowledge and 492 thank the authors of that document both for some text excerpts and 493 for the more general stimulation of thoughts about monitoring the 494 progress of a roll of the KSK of the root zone of the DNS. 496 The authors would like to thank Joe Abley, Mehmet Akcin, Mark 497 Andrews, Richard Barnes, Ray Bellis, Stephane Bortzmeyer, David 498 Conrad, Ralph Dolmans, John Dickinson, Steinar Haug, Bob Harold, Wes 499 Hardaker, Paul Hoffman, Matt Larson, Jinmei Tatuya, Edward Lewis, 500 George Michaelson, Benno Overeinder, Matthew Pounsett, Andreas 501 Schulze, Mukund Sivaraman, Petr Spacek, Andrew Sullivan, Paul Vixie, 502 Duane Wessels and Paul Wouters for their helpful feedback. 504 The authors would like to especially call out Paul Hoffman for 505 providing comments in the form of a pull request. 507 9. Change Log 509 Note that this document is being worked on in GitHub - see Abstract. 510 The below is mainly large changes, and is not authoritative. 512 From -02 to -03: 514 o Integrated / published comments from Paul in GitHub PR #2 - 515 https://github.com/APNIC-Labs/draft-kskroll-sentinel/pull/2 517 o Made the keytab be decimal, not hex (thread / consensus in 518 https://mailarchive.ietf.org/arch/msg/dnsop/ 519 Kg7AtDhFRNw31He8n0_bMr9hBuE ) 521 From -01 to 02: 523 o Removed Address Record definition. 525 o Clarified that many things can cause SERVFAIL. 527 o Made examples FQDN. 529 o Fixed a number of typos. 531 o Had accidentally said that Charlie was using a non-validating 532 resolver in example. 534 o [ TODO(WK): Doc says keytags are hex, is this really what the WG 535 wants? ] 537 o And active key is one that can be used *now* (not e.g AddPend) 539 From -00 to 01: 541 o Added a conversational description of how the system is intended 542 to work. 544 o Clarification that this is for the root. 546 o Changed the label template from _is-ta- to kskroll-sentinel- 547 is-ta-. This is because BIND (at least) will not allow 548 records which start with an underscore to have address records 549 (CNAMEs, yes, A/AAAA no). Some browsers / operating systems also 550 will not fetch resources from names which start with an 551 underscore. 553 10. References 555 10.1. Normative References 557 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 558 Rose, "DNS Security Introduction and Requirements", RFC 559 4033, DOI 10.17487/RFC4033, March 2005, . 562 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 563 Rose, "Resource Records for the DNS Security Extensions", 564 RFC 4034, DOI 10.17487/RFC4034, March 2005, 565 . 567 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 568 Rose, "Protocol Modifications for the DNS Security 569 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 570 . 572 [RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC) 573 Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011, 574 September 2007, . 576 [RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and 577 Implementation Notes for DNS Security (DNSSEC)", RFC 6840, 578 DOI 10.17487/RFC6840, February 2013, . 581 10.2. Informative References 583 [RFC8145] Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust 584 Anchor Knowledge in DNS Security Extensions (DNSSEC)", RFC 585 8145, DOI 10.17487/RFC8145, April 2017, . 588 Authors' Addresses 590 Geoff Huston 592 Email: gih@apnic.net 593 URI: http://www.apnic.net 595 Joao Silva Damas 597 Email: joao@apnic.net 598 URI: http://www.apnic.net 600 Warren Kumari 602 Email: warren@kumari.net