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Sood 6 Expires: March 21, 2020 Google 7 September 18, 2019 9 Serving Stale Data to Improve DNS Resiliency 10 draft-ietf-dnsop-serve-stale-08 12 Abstract 14 This draft defines a method (serve-stale) for recursive resolvers to 15 use stale DNS data to avoid outages when authoritative nameservers 16 cannot be reached to refresh expired data. One of the motivations 17 for serve-stale is to make the DNS more resilient to DoS attacks, and 18 thereby make them less attractive as an attack vector. This document 19 updates the definitions of TTL from RFC 1034 and RFC 1035 so that 20 data can be kept in the cache beyond the TTL expiry, updates RFC 2181 21 by interpreting values with the high order bit set as being positive, 22 rather than 0, and suggests a cap of 7 days. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on March 21, 2020. 41 Copyright Notice 43 Copyright (c) 2019 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 61 4. Standards Action . . . . . . . . . . . . . . . . . . . . . . 4 62 5. Example Method . . . . . . . . . . . . . . . . . . . . . . . 4 63 6. Implementation Considerations . . . . . . . . . . . . . . . . 6 64 7. Implementation Caveats . . . . . . . . . . . . . . . . . . . 8 65 8. Implementation Status . . . . . . . . . . . . . . . . . . . . 9 66 9. EDNS Option . . . . . . . . . . . . . . . . . . . . . . . . . 10 67 10. Security Considerations . . . . . . . . . . . . . . . . . . . 10 68 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11 69 12. NAT Considerations . . . . . . . . . . . . . . . . . . . . . 11 70 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 71 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 72 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 73 15.1. Normative References . . . . . . . . . . . . . . . . . . 11 74 15.2. Informative References . . . . . . . . . . . . . . . . . 12 75 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 77 1. Introduction 79 Traditionally the Time To Live (TTL) of a DNS resource record has 80 been understood to represent the maximum number of seconds that a 81 record can be used before it must be discarded, based on its 82 description and usage in [RFC1035] and clarifications in [RFC2181]. 84 This document expands the definition of the TTL to explicitly allow 85 for expired data to be used in the exceptional circumstance that a 86 recursive resolver is unable to refresh the information. It is 87 predicated on the observation that authoritative answer 88 unavailability can cause outages even when the underlying data those 89 servers would return is typically unchanged. 91 We describe a method below for this use of stale data, balancing the 92 competing needs of resiliency and freshness. 94 This document updates the definitions of TTL from [RFC1034] and 95 [RFC1035] so that data can be kept in the cache beyond the TTL 96 expiry, and also updates [RFC2181] by interpreting values with the 97 high order bit set as being positive, rather than 0, and also 98 suggests a cap of 7 days. 100 2. Terminology 102 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 103 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 104 "OPTIONAL" in this document are to be interpreted as described in BCP 105 14 [RFC2119] [RFC8174] when, and only when, they appear in all 106 capitals, as shown here. 108 For a comprehensive treatment of DNS terms, please see [RFC8499]. 110 3. Background 112 There are a number of reasons why an authoritative server may become 113 unreachable, including Denial of Service (DoS) attacks, network 114 issues, and so on. If a recursive server is unable to contact the 115 authoritative servers for a query but still has relevant data that 116 has aged past its TTL, that information can still be useful for 117 generating an answer under the metaphorical assumption that "stale 118 bread is better than no bread." 120 [RFC1035] Section 3.2.1 says that the TTL "specifies the time 121 interval that the resource record may be cached before the source of 122 the information should again be consulted", and Section 4.1.3 further 123 says the TTL, "specifies the time interval (in seconds) that the 124 resource record may be cached before it should be discarded." 126 A natural English interpretation of these remarks would seem to be 127 clear enough that records past their TTL expiration must not be used. 128 However, [RFC1035] predates the more rigorous terminology of 129 [RFC2119] which softened the interpretation of "may" and "should". 131 [RFC2181] aimed to provide "the precise definition of the Time to 132 Live", but in Section 8 was mostly concerned with the numeric range 133 of values and the possibility that very large values should be 134 capped. (It also has the curious suggestion that a value in the 135 range 2147483648 to 4294967295 should be treated as zero.) It closes 136 that section by noting, "The TTL specifies a maximum time to live, 137 not a mandatory time to live." This wording again does not contain 138 BCP 14 [RFC2119] key words, but does convey the natural language 139 connotation that data becomes unusable past TTL expiry. 141 Several recursive resolver operators, including Akamai, currently use 142 stale data for answers in some way. A number of recursive resolver 143 packages (including BIND, Knot, OpenDNS, Unbound) provide options to 144 use stale data. Apple MacOS can also use stale data as part of the 145 Happy Eyeballs algorithms in mDNSResponder. The collective 146 operational experience is that using stale data can provide 147 significant benefit with minimal downside. 149 4. Standards Action 151 The definition of TTL in [RFC1035] Sections 3.2.1 and 4.1.3 is 152 amended to read: 154 TTL a 32-bit unsigned integer number of seconds that specifies the 155 duration that the resource record MAY be cached before the source 156 of the information MUST again be consulted. Zero values are 157 interpreted to mean that the RR can only be used for the 158 transaction in progress, and should not be cached. Values SHOULD 159 be capped on the orders of days to weeks, with a recommended cap 160 of 604,800 seconds (seven days). If the data is unable to be 161 authoritatively refreshed when the TTL expires, the record MAY be 162 used as though it is unexpired. See the Section 5 and Section 6 163 sections for details. 165 Interpreting values which have the high order bit set as being 166 positive, rather than 0, is a change from [RFC2181]. Suggesting a 167 cap of seven days, rather than the 68 years allowed by [RFC2181], 168 reflects the current practice of major modern DNS resolvers. 170 When returning a response containing stale records, a recursive 171 resolver MUST set the TTL of each expired record in the message to a 172 value greater than 0, with a RECOMMENDED value of 30 seconds. See 173 Section 6 for explanation. 175 Answers from authoritative servers that have a DNS Response Code of 176 either 0 (NoError) or 3 (NXDomain) and the Authoritative Answers (AA) 177 bit set MUST be considered to have refreshed the data at the 178 resolver. Answers from authoritative servers that have any other 179 response code SHOULD be considered a failure to refresh the data and 180 therefor leave any previous state intact. See Section 6 for a 181 discussion. 183 5. Example Method 185 There is more than one way a recursive resolver could responsibly 186 implement this resiliency feature while still respecting the intent 187 of the TTL as a signal for when data is to be refreshed. 189 In this example method four notable timers drive considerations for 190 the use of stale data: 192 o A client response timer, which is the maximum amount of time a 193 recursive resolver should allow between the receipt of a 194 resolution request and sending its response. 196 o A query resolution timer, which caps the total amount of time a 197 recursive resolver spends processing the query. 199 o A failure recheck timer, which limits the frequency at which a 200 failed lookup will be attempted again. 202 o A maximum stale timer, which caps the amount of time that records 203 will be kept past their expiration. 205 Most recursive resolvers already have the query resolution timer, and 206 effectively some kind of failure recheck timer. The client response 207 timer and maximum stale timer are new concepts for this mechanism. 209 When a request is received by a recursive resolver, it should start 210 the client response timer. This timer is used to avoid client 211 timeouts. It should be configurable, with a recommended value of 1.8 212 seconds as being just under a common timeout value of 2 seconds while 213 still giving the resolver a fair shot at resolving the name. 215 The resolver then checks its cache for any unexpired records that 216 satisfy the request and returns them if available. If it finds no 217 relevant unexpired data and the Recursion Desired flag is not set in 218 the request, it should immediately return the response without 219 consulting the cache for expired records. Typically this response 220 would be a referral to authoritative nameservers covering the zone, 221 but the specifics are implementation-dependent. 223 If iterative lookups will be done, then the failure recheck timer is 224 consulted. Attempts to refresh from non-responsive or otherwise 225 failing authoritative nameservers are recommended to be done no more 226 frequently than every 30 seconds. If this request was received 227 within this period, the cache may be immediately consulted for stale 228 data to satisfy the request. 230 Outside the period of the failure recheck timer, the resolver should 231 start the query resolution timer and begin the iterative resolution 232 process. This timer bounds the work done by the resolver when 233 contacting external authorities, and is commonly around 10 to 30 234 seconds. If this timer expires on an attempted lookup that is still 235 being processed, the resolution effort is abandoned. 237 If the answer has not been completely determined by the time the 238 client response timer has elapsed, the resolver should then check its 239 cache to see whether there is expired data that would satisfy the 240 request. If so, it adds that data to the response message with a TTL 241 greater than 0 (as specified in Section 4). The response is then 242 sent to the client while the resolver continues its attempt to 243 refresh the data. 245 When no authorities are able to be reached during a resolution 246 attempt, the resolver should attempt to refresh the delegation and 247 restart the iterative lookup process with the remaining time on the 248 query resolution timer. This resumption should be done only once 249 during one resolution effort. 251 Outside the resolution process, the maximum stale timer is used for 252 cache management and is independent of the query resolution process. 253 This timer is conceptually different from the maximum cache TTL that 254 exists in many resolvers, the latter being a clamp on the value of 255 TTLs as received from authoritative servers and recommended to be 256 seven days in the TTL definition in Section 4. The maximum stale 257 timer should be configurable, and defines the length of time after a 258 record expires that it should be retained in the cache. The 259 suggested value is between 1 and 3 days. 261 6. Implementation Considerations 263 This document mainly describes the issues behind serving stale data 264 and intentionally does not provide a formal algorithm. The concept 265 is not overly complex, and the details are best left to resolver 266 authors to implement in their codebases. The processing of serve- 267 stale is a local operation, and consistent variables between 268 deployments are not needed for interoperability. However, we would 269 like to highlight the impact of various implementation choices, 270 starting with the timers involved. 272 The most obvious of these is the maximum stale timer. If this 273 variable is too large it could cause excessive cache memory usage, 274 but if it is too small, the serve-stale technique becomes less 275 effective, as the record may not be in the cache to be used if 276 needed. Shorter values, even less than a day, can effectively handle 277 the vast majority of outages. Longer values, as much as a week, give 278 time for monitoring systems to notice a resolution problem and for 279 human intervention to fix it; operational experience has been that 280 sometimes the right people can be hard to track down and 281 unfortunately slow to remedy the situation. 283 Increased memory consumption could be mitigated by prioritizing 284 removal of stale records over non-expired records during cache 285 exhaustion. Implementations may also wish to consider whether to 286 track the names in requests for their last time of use or their 287 popularity, using that as an additional factor when considering cache 288 eviction. A feature to manually flush only stale records could also 289 be useful. 291 The client response timer is another variable which deserves 292 consideration. If this value is too short, there exists the risk 293 that stale answers may be used even when the authoritative server is 294 actually reachable but slow; this may result in sub-optimal answers 295 being returned. Conversely, waiting too long will negatively impact 296 user experience. 298 The balance for the failure recheck timer is responsiveness in 299 detecting the renewed availability of authorities versus the extra 300 resource use for resolution. If this variable is set too large, 301 stale answers may continue to be returned even after the 302 authoritative server is reachable; per [RFC2308], Section 7, this 303 should be no more than five minutes. If this variable is too small, 304 authoritative servers may be rapidly hit with a significant amount of 305 traffic when they become reachable again. 307 Regarding the TTL to set on stale records in the response, 308 historically TTLs of zero seconds have been problematic for some 309 implementations, and negative values can't effectively be 310 communicated to existing software. Other very short TTLs could lead 311 to congestive collapse as TTL-respecting clients rapidly try to 312 refresh. The recommended value of 30 seconds not only sidesteps 313 those potential problems with no practical negative consequences, it 314 also rate limits further queries from any client that honors the TTL, 315 such as a forwarding resolver. 317 Another implementation consideration is the use of stale nameserver 318 addresses for lookups. This is mentioned explicitly because, in some 319 resolvers, getting the addresses for nameservers is a separate path 320 from a normal cache lookup. If authoritative server addresses are 321 not able to be refreshed, resolution can possibly still be successful 322 if the authoritative servers themselves are up. For instance, 323 consider an attack on a top-level domain that takes its nameservers 324 offline; serve-stale resolvers that had expired glue addresses for 325 subdomains within that TLD would still be able to resolve names 326 within those subdomains, even those it had not previously looked up. 328 The directive in Section 4 that only NoError and NXDomain responses 329 should invalidate any previously associated answer stems from the 330 fact that no other RCODEs that a resolver normally encounters make 331 any assertions regarding the name in the question or any data 332 associated with it. This comports with existing resolver behavior 333 where a failed lookup (say, during pre-fetching) doesn't impact the 334 existing cache state. Some authoritative server operators have said 335 that they would prefer stale answers to be used in the event that 336 their servers are responding with errors like ServFail instead of 337 giving true authoritative answers. Implementers MAY decide to return 338 stale answers in this situation. 340 Since the goal of serve-stale is to provide resiliency for all 341 obvious errors to refresh data, these other RCODEs are treated as 342 though they are equivalent to not getting an authoritative response. 343 Although NXDomain for a previously existing name might well be an 344 error, it is not handled that way because there is no effective way 345 to distinguish operator intent for legitimate cases versus error 346 cases. 348 During discussion in the IETF, it was suggested that, if all 349 authorities return responses with RCODE of Refused, it may be an 350 explicit signal to take down the zone from servers that still have 351 the zone's delegation pointed to them. Refused, however, is also 352 overloaded to mean multiple possible failures which could represent 353 transient configuration failures. Operational experience has shown 354 that purposely returning Refused is a poor way to achieve an explicit 355 takedown of a zone compared to either updating the delegation or 356 returning NXDomain with a suitable SOA for extended negative caching. 357 Implementers MAY nonetheless consider whether to treat all 358 authorities returning Refused as preempting the use of stale data. 360 7. Implementation Caveats 362 Stale data is used only when refreshing has failed in order to adhere 363 to the original intent of the design of the DNS and the behaviour 364 expected by operators. If stale data were to always be used 365 immediately and then a cache refresh attempted after the client 366 response has been sent, the resolver would frequently be sending data 367 that it would have had no trouble refreshing. Because modern 368 resolvers use techniques like pre-fetching and request coalescing for 369 efficiency, it is not necessary that every client request needs to 370 trigger a new lookup flow in the presence of stale data, but rather 371 that a good-faith effort has been recently made to refresh the stale 372 data before it is delivered to any client. 374 It is important to continue the resolution attempt after the stale 375 response has been sent, until the query resolution timeout, because 376 some pathological resolutions can take many seconds to succeed as 377 they cope with unavailable servers, bad networks, and other problems. 378 Stopping the resolution attempt when the response with expired data 379 has been sent would mean that answers in these pathological cases 380 would never be refreshed. 382 The continuing prohibition against using data with a 0 second TTL 383 beyond the current transaction explicitly extends to it being 384 unusable even for stale fallback, as it is not to be cached at all. 386 Be aware that Canonical Name (CNAME) and DNAME [RFC6672] records 387 mingled in the expired cache with other records at the same owner 388 name can cause surprising results. This was observed with an initial 389 implementation in BIND when a hostname changed from having an IPv4 390 Address (A) record to a CNAME. The version of BIND being used did 391 not evict other types in the cache when a CNAME was received, which 392 in normal operations is not a significant issue. However, after both 393 records expired and the authorities became unavailable, the fallback 394 to stale answers returned the older A instead of the newer CNAME. 396 8. Implementation Status 398 [RFC Editor: per RFC 6982 this section should be removed prior to 399 publication.] 401 The algorithm described in Section 5 was originally implemented as a 402 patch to BIND 9.7.0. It has been in production on Akamai's 403 production network since 2011, and effectively smoothed over 404 transient failures and longer outages that would have resulted in 405 major incidents. The patch was contributed to Internet Systems 406 Consortium and the functionality is now available in BIND 9.12 via 407 the options stale-answer-enable, stale-answer-ttl, and max-stale-ttl. 409 Unbound has a similar feature for serving stale answers, but will 410 respond with stale data immediately if it has recently tried and 411 failed to refresh the answer by pre-fetching. 413 Knot Resolver has a demo module here: https://knot- 414 resolver.readthedocs.io/en/stable/modules.html#serve-stale 416 Details of Apple's implementation are not currently known. 418 In the research paper "When the Dike Breaks: Dissecting DNS Defenses 419 During DDoS" [DikeBreaks], the authors detected some use of stale 420 answers by resolvers when authorities came under attack. Their 421 research results suggest that more widespread adoption of the 422 technique would significantly improve resiliency for the large number 423 of requests that fail or experience abnormally long resolution times 424 during an attack. 426 9. EDNS Option 428 During the discussion of serve-stale in the IETF, it was suggested 429 that an EDNS option should be available to either explicitly opt-in 430 to getting data that is possibly stale, or at least as a debugging 431 tool to indicate when stale data has been used for a response. 433 The opt-in use case was rejected as the technique was meant to be 434 immediately useful in improving DNS resiliency for all clients. 436 The reporting case was ultimately also rejected because even the 437 simpler version of a proposed option was still too much bother to 438 implement for too little perceived value. 440 10. Security Considerations 442 The most obvious security issue is the increased likelihood of DNSSEC 443 validation failures when using stale data because signatures could be 444 returned outside their validity period. Stale negative records can 445 increase the time window where newly published TLSA or DS RRs may not 446 be used due to cached NSEC or NSEC3 records. These scenarios would 447 only be an issue if the authoritative servers are unreachable, the 448 only time the techniques in this document are used, and thus does not 449 introduce a new failure in place of what would have otherwise been 450 success. 452 Additionally, bad actors have been known to use DNS caches to keep 453 records alive even after their authorities have gone away. The serve 454 stale feature potentially makes the attack easier, although without 455 introducing a new risk. In addition, attackers could combine this 456 with a DDoS attack on authoritative servers with the explicit intent 457 of having stale information cached for longer. But if attackers have 458 this capacity, they probably could do much worse than prolonging the 459 life of old data. 461 In [CloudStrife], it was demonstrated how stale DNS data, namely 462 hostnames pointing to addresses that are no longer in use by the 463 owner of the name, can be used to co-opt security such as to get 464 domain-validated certificates fraudulently issued to an attacker. 465 While this document does not create a new vulnerability in this area, 466 it does potentially enlarge the window in which such an attack could 467 be made. A proposed mitigation is that certificate authorities 468 should fully look up each name starting at the DNS root for every 469 name lookup. Alternatively, CAs should use a resolver that is not 470 serving stale data. 472 11. Privacy Considerations 474 This document does not add any practical new privacy issues. 476 12. NAT Considerations 478 The method described here is not affected by the use of NAT devices. 480 13. IANA Considerations 482 There are no IANA considerations. 484 14. Acknowledgements 486 The authors wish to thank Robert Edmonds, Tony Finch, Bob Harold, 487 Tatuya Jinmei, Matti Klock, Jason Moreau, Giovane Moura, Jean Roy, 488 Mukund Sivaraman, Davey Song, Paul Vixie, Ralf Weber and Paul Wouters 489 for their review and feedback. 491 Paul Hoffman deserves special thanks for submitting a number of Pull 492 Requests. 494 15. References 496 15.1. Normative References 498 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 499 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 500 . 502 [RFC1035] Mockapetris, P., "Domain names - implementation and 503 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 504 November 1987, . 506 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 507 Requirement Levels", BCP 14, RFC 2119, 508 DOI 10.17487/RFC2119, March 1997, . 511 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 512 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 513 . 515 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 516 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 517 . 519 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 520 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 521 May 2017, . 523 15.2. Informative References 525 [CloudStrife] 526 Borgolte, K., Fiebig, T., Hao, S., Kruegel, C., and G. 527 Vigna, "Cloud Strife: Mitigating the Security Risks of 528 Domain-Validated Certificates", ACM 2018 Applied 529 Networking Research Workshop, DOI 10.1145/3232755.3232859, 530 July 2018, . 534 [DikeBreaks] 535 Moura, G., Heidemann, J., Mueller, M., Schmidt, R., and M. 536 Davids, "When the Dike Breaks: Dissecting DNS Defenses 537 During DDos", ACM 2018 Internet Measurement Conference, 538 DOI 10.1145/3278532.3278534, October 2018, 539 . 541 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 542 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, 543 . 545 [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 546 Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, 547 January 2019, . 549 Authors' Addresses 551 David C Lawrence 552 Oracle 554 Email: tale@dd.org 556 Warren "Ace" Kumari 557 Google 558 1600 Amphitheatre Parkway 559 Mountain View CA 94043 560 USA 562 Email: warren@kumari.net 563 Puneet Sood 564 Google 566 Email: puneets@google.com