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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (March 09, 2019) is 1874 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Obsolete informational reference (is this intentional?): RFC 7719 (Obsoleted by RFC 8499) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 5 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DNSOP Working Group D. Lawrence 3 Internet-Draft Oracle 4 Updates: 1034, 1035 (if approved) W. Kumari 5 Intended status: Standards Track P. Sood 6 Expires: September 10, 2019 Google 7 March 09, 2019 9 Serving Stale Data to Improve DNS Resiliency 10 draft-ietf-dnsop-serve-stale-04 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. It updates the definition 17 of TTL from [RFC1034], [RFC1035], and [RFC2181] to make it clear that 18 data can be kept in the cache beyond the TTL expiry and used for 19 responses when a refreshed answer is not readily available. One of 20 the motivations for serve-stale is to make the DNS more resilient to 21 DoS attacks, and thereby make them less attractive as an attack 22 vector. 24 Ed note 26 Text inside square brackets ([]) is additional background 27 information, answers to frequently asked questions, general musings, 28 etc. They will be removed before publication. This document is 29 being collaborated on in GitHub at . The most recent version of the document, open issues, etc 31 should all be available here. The authors gratefully accept pull 32 requests. 34 Status of This Memo 36 This Internet-Draft is submitted in full conformance with the 37 provisions of BCP 78 and BCP 79. 39 Internet-Drafts are working documents of the Internet Engineering 40 Task Force (IETF). Note that other groups may also distribute 41 working documents as Internet-Drafts. The list of current Internet- 42 Drafts is at https://datatracker.ietf.org/drafts/current/. 44 Internet-Drafts are draft documents valid for a maximum of six months 45 and may be updated, replaced, or obsoleted by other documents at any 46 time. It is inappropriate to use Internet-Drafts as reference 47 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on September 10, 2019. 50 Copyright Notice 52 Copyright (c) 2019 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (https://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 68 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 69 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 70 4. Standards Action . . . . . . . . . . . . . . . . . . . . . . 4 71 5. Example Method . . . . . . . . . . . . . . . . . . . . . . . 4 72 6. Implementation Considerations . . . . . . . . . . . . . . . . 6 73 7. Implementation Caveats . . . . . . . . . . . . . . . . . . . 8 74 8. Implementation Status . . . . . . . . . . . . . . . . . . . . 9 75 9. EDNS Option . . . . . . . . . . . . . . . . . . . . . . . . . 9 76 10. Security Considerations . . . . . . . . . . . . . . . . . . . 10 77 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10 78 12. NAT Considerations . . . . . . . . . . . . . . . . . . . . . 10 79 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 80 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 81 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 82 15.1. Normative References . . . . . . . . . . . . . . . . . . 11 83 15.2. Informative References . . . . . . . . . . . . . . . . . 11 84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 86 1. Introduction 88 Traditionally the Time To Live (TTL) of a DNS resource record has 89 been understood to represent the maximum number of seconds that a 90 record can be used before it must be discarded, based on its 91 description and usage in [RFC1035] and clarifications in [RFC2181]. 93 This document proposes that the definition of the TTL be explicitly 94 expanded to allow for expired data to be used in the exceptional 95 circumstance that a recursive resolver is unable to refresh the 96 information. It is predicated on the observation that authoritative 97 answer unavailability can cause outages even when the underlying data 98 those servers would return is typically unchanged. 100 We describe a method below for this use of stale data, balancing the 101 competing needs of resiliency and freshness. 103 2. Terminology 105 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 106 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 107 "OPTIONAL" in this document are to be interpreted as described in BCP 108 14 [RFC2119] [RFC8174] when, and only when, they appear in all 109 capitals, as shown here. 111 For a comprehensive treatment of DNS terms, please see [RFC7719]. 113 3. Background 115 There are a number of reasons why an authoritative server may become 116 unreachable, including Denial of Service (DoS) attacks, network 117 issues, and so on. If the recursive server is unable to contact the 118 authoritative servers for a query but still has relevant data that 119 has aged past its TTL, that information can still be useful for 120 generating an answer under the metaphorical assumption that "stale 121 bread is better than no bread." 123 [RFC1035] Section 3.2.1 says that the TTL "specifies the time 124 interval that the resource record may be cached before the source of 125 the information should again be consulted", and Section 4.1.3 further 126 says the TTL, "specifies the time interval (in seconds) that the 127 resource record may be cached before it should be discarded." 129 A natural English interpretation of these remarks would seem to be 130 clear enough that records past their TTL expiration must not be used. 131 However, [RFC1035] predates the more rigorous terminology of 132 [RFC2119] which softened the interpretation of "may" and "should". 134 [RFC2181] aimed to provide "the precise definition of the Time to 135 Live", but in Section 8 was mostly concerned with the numeric range 136 of values and the possibility that very large values should be 137 capped. (It also has the curious suggestion that a value in the 138 range 2147483648 to 4294967295 should be treated as zero.) It closes 139 that section by noting, "The TTL specifies a maximum time to live, 140 not a mandatory time to live." This is again not [RFC2119]-normative 141 language, but does convey the natural language connotation that data 142 becomes unusable past TTL expiry. 144 Several major recursive resolver operators currently use stale data 145 for answers in some way, including Akamai (in three different 146 resolver implementations), BIND, Knot, OpenDNS, and Unbound. Apple 147 MacOS can also use stale data as part of the Happy Eyeballs 148 algorithms in mDNSResponder. The collective operational experience 149 is that it provides significant benefit with minimal downside. 151 4. Standards Action 153 The definition of TTL in [RFC1035] Sections 3.2.1 and 4.1.3 is 154 amended to read: 156 TTL a 32-bit unsigned integer number of seconds that specifies the 157 duration that the resource record MAY be cached before the source 158 of the information MUST again be consulted. Zero values are 159 interpreted to mean that the RR can only be used for the 160 transaction in progress, and should not be cached. Values SHOULD 161 be capped on the orders of days to weeks, with a recommended cap 162 of 604,800 seconds. If the data is unable to be authoritatively 163 refreshed when the TTL expires, the record MAY be used as though 164 it is unexpired. 166 Interpreting values which have the high order bit set as being 167 positive, rather than 0, is a change from [RFC2181]. Suggesting a 168 cap of seven days, rather than the 68 years allowed by [RFC2181], 169 reflects the current practice of major modern DNS resolvers. 171 When returning a response containing stale records, the recursive 172 resolver MUST set the TTL of each expired record in the message to a 173 value greater than 0, with 30 seconds RECOMMENDED. 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. 182 5. Example Method 184 There is conceivably more than one way a recursive resolver could 185 responsibly implement this resiliency feature while still respecting 186 the intent of the TTL as a signal for when data is to be refreshed. 188 In this example method four notable timers drive considerations for 189 the use of stale data, as follows: 191 o A client response timer, which is the maximum amount of time a 192 recursive resolver should allow between the receipt of a 193 resolution request and sending its response. 195 o A query resolution timer, which caps the total amount of time a 196 recursive resolver spends processing the query. 198 o A failure recheck timer, which limits the frequency at which a 199 failed lookup will be attempted again. 201 o A maximum stale timer, which caps the amount of time that records 202 will be kept past their expiration. 204 Most recursive resolvers already have the query resolution timer, and 205 effectively some kind of failure recheck timer. The client response 206 timer and maximum stale timer are new concepts for this mechanism. 208 When a request is received by the recursive resolver, it SHOULD start 209 the client response timer. This timer is used to avoid client 210 timeouts. It SHOULD be configurable, with a recommended value of 1.8 211 seconds as being just under a common timeout value of 2 seconds while 212 still giving the resolver a fair shot at resolving the name. 214 The resolver then checks its cache for any unexpired data that 215 satisfies the request and of course returns them if available. If it 216 finds no relevant unexpired data and the Recursion Desired flag is 217 not set in the request, it SHOULD immediately return the response 218 without consulting the cache for expired records. Typically this 219 response would be a referral to authoritative nameservers covering 220 the zone, but the specifics are implementation dependent. 222 If iterative lookups will be done, then the failure recheck timer is 223 consulted. Attempts to refresh from non-responsive or otherwise 224 failing authoritative nameservers are recommended to be done no more 225 frequently than every 30 seconds. If this request was received 226 within this period, the cache may be immediately consulted for stale 227 data to satisfy the request. 229 Outside the period of the failure recheck timer, the resolver SHOULD 230 start the query resolution timer and begin the iterative resolution 231 process. This timer bounds the work done by the resolver when 232 contacting external authorities, and is commonly around 10 to 30 233 seconds. If this timer expires on an attempted lookup that is still 234 being processed, the resolution effort is abandoned. 236 If the answer has not been completely determined by the time the 237 client response timer has elapsed, the resolver SHOULD then check its 238 cache to see whether there is expired data that would satisfy the 239 request. If so, it adds that data to the response message with a TTL 240 greater than 0 per Section 4. The response is then sent to the 241 client while the resolver continues its attempt to refresh the data. 243 When no authorities are able to be reached during a resolution 244 attempt, the resolver SHOULD attempt to refresh the delegation and 245 restart the iterative lookup process with the remaining time on the 246 query resolution timer. This resumption should be done only once 247 during one resolution effort. 249 Outside the resolution process, the maximum stale timer is used for 250 cache management and is independent of the query resolution process. 251 This timer is conceptually different from the maximum cache TTL that 252 exists in many resolvers, the latter being a clamp on the value of 253 TTLs as received from authoritative servers and recommended to be 7 254 days in the TTL definition above. The maximum stale timer SHOULD be 255 configurable, and defines the length of time after a record expires 256 that it SHOULD be retained in the cache. The suggested value is 7 257 days, which gives time for monitoring to notice the resolution 258 problem and for human intervention to fix it. 260 6. Implementation Considerations 262 This document mainly describes the issues behind serving stale data 263 and intentionally does not provide a formal algorithm. The concept 264 is not overly complex, and the details are best left to resolver 265 authors to implement in their codebases. The processing of serve- 266 stale is a local operation, and consistent variables between 267 deployments are not needed for interoperability. However, we would 268 like to highlight the impact of various implementation choices, 269 starting with the timers involved. 271 The most obvious of these is the maximum stale timer. If this 272 variable is too large it could cause excessive cache memory usage, 273 but if it is too small, the serve-stale technique becomes less 274 effective, as the record may not be in the cache to be used if 275 needed. Increased memory consumption could be mitigated by 276 prioritizing removal of stale records over non-expired records during 277 cache exhaustion. Implementations may also wish to consider whether 278 to track the names in requests for their last time of use or their 279 popularity, using that as an additional factor when considering cache 280 eviction. A feature to manually flush only stale records could also 281 be useful. 283 The client response timer is another variable which deserves 284 consideration. If this value is too short, there exists the risk 285 that stale answers may be used even when the authoritative server is 286 actually reachable but slow; this may result in sub-optimal answers 287 being returned. Conversely, waiting too long will negatively impact 288 user experience. 290 The balance for the failure recheck timer is responsiveness in 291 detecting the renewed availability of authorities versus the extra 292 resource use for resolution. If this variable is set too large, 293 stale answers may continue to be returned even after the 294 authoritative server is reachable; per [RFC2308], Section 7, this 295 should be no more than five minutes. If this variable is too small, 296 authoritative servers may be rapidly hit with a significant amount of 297 traffic when they become reachable again. 299 Regarding the TTL to set on stale records in the response, 300 historically TTLs of zero seconds have been problematic for some 301 implementations, and negative values can't effectively be 302 communicated to existing software. Other very short TTLs could lead 303 to congestive collapse as TTL-respecting clients rapidly try to 304 refresh. The recommended value of 30 seconds not only sidesteps 305 those potential problems with no practical negative consequences, it 306 also rate limits further queries from any client that honors the TTL, 307 such as a forwarding resolver. 309 Another implementation consideration is the use of stale nameserver 310 addresses for lookups. This is mentioned explicitly because, in some 311 resolvers, getting the addresses for nameservers is a separate path 312 from a normal cache lookup. If authoritative server addresses are 313 not able to be refreshed, resolution can possibly still be successful 314 if the authoritative servers themselves are up. For instance, 315 consider an attack on a top-level domain that takes its nameservers 316 offline; serve-stale resolvers that had expired glue addresses for 317 subdomains within that TLD would still be able to resolve names 318 within those subdomains, even those it had not previously looked up. 320 The directive in Section 4 that only NoError and NXDomain responses 321 should invalidate any previously associated answer stems from the 322 fact that no other RCODEs which a resolver normally encounters makes 323 any assertions regarding the name in the question or any data 324 associated with it. This comports with existing resolver behavior 325 where a failed lookup (say, during pre-fetching) doesn't impact the 326 existing cache state. Some authoritative servers operators have said 327 that they would prefer stale answers to be used in the event that 328 their servers are responding with errors like ServFail instead of 329 giving true authoritative answers. Implementers MAY decide to return 330 stale answers in this situation. 332 Since the goal of serve-stale is to provide resiliency for all 333 obvious errors to refresh data, these other RCODEs are treated as 334 though they are equivalent to not getting an authoritative response. 336 Although NXDomain for a previously existing name might well be an 337 error, it is not handled that way because there is no effective way 338 to distinguish operator intent for legitimate cases versus error 339 cases. 341 During discussion in dnsop it was suggested that Refused from all 342 authorities should be treated, from a serve-stale perspective, as 343 though it were equivalent to NXDomain because it represents an 344 explicit signal to take down the zone from servers that still have 345 the zone's delegation pointed to them. Refused, however, is also 346 overloaded to mean multiple possible failures which could represent 347 transient configuration failures. Operational experience has shown 348 that purposely returning Refused is a poor way to achieve an explicit 349 takedown of a zone compared to either updating the delegation or 350 returning NXDomain with a suitable SOA for extended negative caching. 351 Implementers MAY nonetheless consider whether to treat all 352 authorities returning Refused as preempting the use of stale data. 354 7. Implementation Caveats 356 Stale data is used only when refreshing has failed in order to adhere 357 to the original intent of the design of the DNS and the behaviour 358 expected by operators. If stale data were to always be used 359 immediately and then a cache refresh attempted after the client 360 response has been sent, the resolver would frequently be sending data 361 that it would have had no trouble refreshing. As modern resolvers 362 use techniques like pre-fetching and request coalescing for 363 efficiency, it is not necessary that every client request needs to 364 trigger a new lookup flow in the presence of stale data, but rather 365 that a good-faith effort has been recently made to refresh the stale 366 data before it is delivered to any client. 368 It is important to continue the resolution attempt after the stale 369 response has been sent, until the query resolution timeout, because 370 some pathological resolutions can take many seconds to succeed as 371 they cope with unavailable servers, bad networks, and other problems. 372 Stopping the resolution attempt when the response with expired data 373 has been sent would mean that answers in these pathological cases 374 would never be refreshed. 376 The continuing prohibition against using data with a 0 second TTL 377 beyond the current transaction explicitly extends to it being 378 unusable even for stale fallback, as it is not to be cached at all. 380 Be aware that Canonical Name (CNAME) records mingled in the expired 381 cache with other records at the same owner name can cause surprising 382 results. This was observed with an initial implementation in BIND 383 when a hostname changed from having an IPv4 Address (A) record to a 384 CNAME. The version of BIND being used did not evict other types in 385 the cache when a CNAME was received, which in normal operations is 386 not a significant issue. However, after both records expired and the 387 authorities became unavailable, the fallback to stale answers 388 returned the older A instead of the newer CNAME. 390 8. Implementation Status 392 [RFC Editor: per RFC 6982 this section should be removed prior to 393 publication.] 395 The algorithm described in Section 5 was originally implemented as a 396 patch to BIND 9.7.0. It has been in production on Akamai's 397 production network since 2011, and effectively smoothed over 398 transient failures and longer outages that would have resulted in 399 major incidents. The patch was contributed to Internet Systems 400 Consortium and the functionality is now available in BIND 9.12 via 401 the options stale-answer-enable, stale-answer-ttl, and max-stale-ttl. 403 Unbound has a similar feature for serving stale answers, but will 404 respond with stale data immediately if it has recently tried and 405 failed to refresh the answer by pre-fetching. 407 Knot Resolver has a demo module here: https://knot- 408 resolver.readthedocs.io/en/stable/modules.html#serve-stale 410 Details of Apple's implementation are not currently known. 412 In the research paper "When the Dike Breaks: Dissecting DNS Defenses 413 During DDoS" [DikeBreaks], the authors detected some use of stale 414 answers by resolvers when authorities came under attack. Their 415 research results suggest that more widespread adoption of the 416 technique would significantly improve resiliency for the large number 417 of requests that fail or experience abnormally long resolution times 418 during an attack. 420 9. EDNS Option 422 During the discussion of serve-stale in the IETF dnsop working group, 423 it was suggested that an EDNS option should be available to either 424 explicitly opt-in to getting data that is possibly stale, or at least 425 as a debugging tool to indicate when stale data has been used for a 426 response. 428 The opt-in use case was rejected as the technique was meant to be 429 immediately useful in improving DNS resiliency for all clients. 431 The reporting case was ultimately also rejected as working group 432 participants determined that even the simpler version of a proposed 433 option was still too much bother to implement for too little 434 perceived value. 436 10. Security Considerations 438 The most obvious security issue is the increased likelihood of DNSSEC 439 validation failures when using stale data because signatures could be 440 returned outside their validity period. This would only be an issue 441 if the authoritative servers are unreachable, the only time the 442 techniques in this document are used, and thus does not introduce a 443 new failure in place of what would have otherwise been success. 445 Additionally, bad actors have been known to use DNS caches to keep 446 records alive even after their authorities have gone away. This 447 potentially makes that easier, although without introducing a new 448 risk. 450 In [CloudStrife] it was demonstrated how stale DNS data, namely 451 hostnames pointing to addresses that are no longer in use by the 452 owner of the name, can be used to co-opt security such as to get 453 domain-validated certificates fraudulently issued to an attacker. 454 While this RFC does not create a new vulnerability in this area, it 455 does potentially enlarge the window in which such an attack could be 456 made. An obvious mitigation is that not only should a certificate 457 authority not use a resolver that has this feature enabled, it should 458 probably not use a caching resolver at all and instead fully look up 459 each name freshly from the root. 461 11. Privacy Considerations 463 This document does not add any practical new privacy issues. 465 12. NAT Considerations 467 The method described here is not affected by the use of NAT devices. 469 13. IANA Considerations 471 There are no IANA considerations. 473 14. Acknowledgements 475 The authors wish to thank Robert Edmonds, Tony Finch, Bob Harold, 476 Tatuya Jinmei, Matti Klock, Jason Moreau, Giovane Moura, Jean Roy, 477 Mukund Sivaraman, Davey Song, Paul Vixie, Ralf Weber and Paul Wouters 478 for their review and feedback. 480 15. References 482 15.1. Normative References 484 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 485 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 486 . 488 [RFC1035] Mockapetris, P., "Domain names - implementation and 489 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 490 November 1987, . 492 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 493 Requirement Levels", BCP 14, RFC 2119, 494 DOI 10.17487/RFC2119, March 1997, 495 . 497 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 498 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 499 . 501 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 502 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 503 . 505 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 506 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 507 May 2017, . 509 15.2. Informative References 511 [CloudStrife] 512 Borgolte, K., Fiebig, T., Hao, S., Kruegel, C., and G. 513 Vigna, "Cloud Strife: Mitigating the Security Risks of 514 Domain-Validated Certificates", ACM 2018 Applied 515 Networking Research Workshop, DOI 10.1145/3232755.3232859, 516 July 2018, . 520 [DikeBreaks] 521 Moura, G., Heidemann, J., Mueller, M., Schmidt, R., and M. 522 Davids, "When the Dike Breaks: Dissecting DNS Defenses 523 During DDos", ACM 2018 Internet Measurement Conference, 524 DOI 10.1145/3278532.3278534, October 2018, 525 . 527 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 528 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 529 2015, . 531 Authors' Addresses 533 David C Lawrence 534 Oracle 536 Email: tale@dd.org 538 Warren "Ace" Kumari 539 Google 540 1600 Amphitheatre Parkway 541 Mountain View CA 94043 542 USA 544 Email: warren@kumari.net 546 Puneet Sood 547 Google 549 Email: puneets@google.com