1 Internet Engineering Task Force (IETF) S. Morris 2 Request for Comments: 7583 ISC 3 Category: Informational J. Ihren 4 ISSN: 2070-1721 Netnod 5 J. Dickinson 6 Sinodun 7 W. Mekking 8 Dyn 9 October 2015 10 11 12 DNSSEC Key Rollover Timing Considerations 13 14 Abstract 15 16 This document describes the issues surrounding the timing of events 17 in the rolling of a key in a DNSSEC-secured zone. It presents 18 timelines for the key rollover and explicitly identifies the 19 relationships between the various parameters affecting the process. 20 21 Status of This Memo 22 23 This document is not an Internet Standards Track specification; it is 24 published for informational purposes. 25 26 This document is a product of the Internet Engineering Task Force 27 (IETF). It represents the consensus of the IETF community. It has 28 received public review and has been approved for publication by the 29 Internet Engineering Steering Group (IESG). Not all documents 30 approved by the IESG are a candidate for any level of Internet 31 Standard; see Section 2 of RFC 5741. 32 33 Information about the current status of this document, any errata, 34 and how to provide feedback on it may be obtained at 35 http://www.rfc-editor.org/info/rfc7583. 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 Morris, et al. Informational [Page 1] 53 RFC 7583 Key Timing October 2015 54 55 56 Copyright Notice 57 58 Copyright (c) 2015 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 60 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. 70 71 Table of Contents 72 73 1. Introduction ....................................................3 74 1.1. Key Rolling Considerations .................................3 75 1.2. Types of Keys ..............................................4 76 1.3. Terminology ................................................4 77 1.4. Limitation of Scope ........................................5 78 2. Rollover Methods ................................................5 79 2.1. ZSK Rollovers ..............................................5 80 2.2. KSK Rollovers ..............................................7 81 3. Key Rollover Timelines ..........................................8 82 3.1. Key States .................................................8 83 3.2. ZSK Rollover Timelines ....................................10 84 3.2.1. Pre-Publication Method .............................10 85 3.2.2. Double-Signature Method ............................12 86 3.3. KSK Rollover Timelines ....................................14 87 3.3.1. Double-KSK Method ..................................14 88 3.3.2. Double-DS Method ...................................17 89 3.3.3. Double-RRset Method ................................20 90 3.3.4. Interaction with Configured Trust Anchors ..........22 91 3.3.5. Introduction of First Keys .........................24 92 4. Standby Keys ...................................................24 93 5. Algorithm Considerations .......................................25 94 6. Summary ........................................................26 95 7. Security Considerations ........................................26 96 8. Normative References ...........................................26 97 Appendix A. List of Symbols ......................................28 98 Acknowledgements ..................................................31 99 Authors' Addresses ................................................31 100 101 102 103 104 105 106 107 Morris, et al. Informational [Page 2] 108 RFC 7583 Key Timing October 2015 109 110 111 1. Introduction 112 113 1.1. Key Rolling Considerations 114 115 When a zone is secured with DNSSEC, the zone manager must be prepared 116 to replace ("roll") the keys used in the signing process. The 117 rolling of keys may be caused by compromise of one or more of the 118 existing keys, or it may be due to a management policy that demands 119 periodic key replacement for security or operational reasons. In 120 order to implement a key rollover, the keys need to be introduced 121 into and removed from the zone at the appropriate times. 122 Considerations that must be taken into account are: 123 124 o DNSKEY records and associated information (such as the DS records 125 or RRSIG records created with the key) are not only held at the 126 authoritative nameserver, they are also cached by resolvers. The 127 data on these systems can be interlinked, e.g., a validating 128 resolver may try to validate a signature retrieved from a cache 129 with a key obtained separately. 130 131 o Zone "bootstrapping" events, where a zone is signed for the first 132 time, can be common in configurations where a large number of 133 zones are being served. Procedures should be able to cope with 134 the introduction of keys into the zone for the first time as well 135 as "steady-state", where the records are being replaced as part of 136 normal zone maintenance. 137 138 o To allow for an emergency re-signing of the zone as soon as 139 possible after a key compromise has been detected, standby keys 140 (additional keys over and above those used to sign the zone) need 141 to be present. 142 143 o A query for the DNSKEY RRset returns all DNSKEY records in the 144 zone. As there is limited space in the UDP packet (even with 145 EDNS0 support), key records no longer needed must be periodically 146 removed. (For the same reason, the number of standby keys in the 147 zone should be restricted to the minimum required to support the 148 key management policy.) 149 150 Management policy, e.g., how long a key is used for, also needs to be 151 considered. However, the point of key management logic is not to 152 ensure that a rollover is completed at a certain time but rather to 153 ensure that no changes are made to the state of keys published in the 154 zone until it is "safe" to do so ("safe" in this context meaning that 155 at no time during the rollover process does any part of the zone ever 156 go bogus). In other words, although key management logic enforces 157 policy, it may not enforce it strictly. 158 159 160 161 162 Morris, et al. Informational [Page 3] 163 RFC 7583 Key Timing October 2015 164 165 166 A high-level overview of key rollover can be found in [RFC6781]. In 167 contrast, this document focuses on the low-level timing detail of two 168 classes of operations described there, the rollover of Zone Signing 169 Keys (ZSKs), and the rollover of Key Signing Keys (KSKs). 170 171 Note that the process for the introduction of keys into a zone is 172 different from that of rolling a key; see Section 3.3.5 for more 173 information. 174 175 1.2. Types of Keys 176 177 Although DNSSEC validation treats all keys equally, [RFC4033] 178 recognizes the broad classification of ZSKs and KSKs. A ZSK is used 179 to authenticate information within the zone; a KSK is used to 180 authenticate the zone's DNSKEY RRset. The main implication for this 181 distinction concerns the consistency of information during a 182 rollover. 183 184 During operation, a validating resolver must use separate pieces of 185 information to perform an authentication. At the time of 186 authentication, each piece of information may be in its cache or may 187 need to be retrieved from an authoritative server. The rollover 188 process needs to happen in such a way that the information is 189 consistent at all times during the rollover. With a ZSK, the 190 information is the RRSIG (plus associated RRset) and the DNSKEY. 191 These are both obtained from the same zone. In the case of the KSK, 192 the information is the DNSKEY and DS RRset with the latter being 193 obtained from a different zone. 194 195 Although there are similarities in the algorithms to roll ZSKs and 196 KSKs, there are a number of differences. For this reason, the two 197 types of rollovers are described separately. 198 199 1.3. Terminology 200 201 The terminology used in this document is as defined in [RFC4033] and 202 [RFC5011]. 203 204 A number of symbols are used to identify times, intervals, etc. All 205 are listed in Appendix A. 206 207 208 209 210 211 212 213 214 215 216 217 Morris, et al. Informational [Page 4] 218 RFC 7583 Key Timing October 2015 219 220 221 1.4. Limitation of Scope 222 223 This document represents current thinking at the time of publication. 224 However, the subject matter is evolving and it is not possible for 225 the document to be comprehensive. In particular, it does not cover: 226 227 o Rolling a key that is used as both a ZSK and KSK. 228 229 o Algorithm rollovers. Only the rolling of keys of the same 230 algorithm is described here: not transitions between algorithms. 231 232 o Changing TTLs. 233 234 Algorithm rollover is excluded from the document owing to the need 235 for there to be an RRSIG for at least one DNSKEY of each algorithm in 236 the DNSKEY RRset [RFC4035]. This introduces additional constraints 237 on rollovers that are not considered here. Such considerations do 238 not apply where other properties of the key, such as key length, are 239 changed during the rollover: the DNSSEC protocol does not impose any 240 restrictions in these cases. 241 242 Also excluded from consideration is the effect of changing the Time 243 to Live (TTL) of records in a zone. TTLs can be changed at any time, 244 but doing so around the time of a key rollover may have an impact on 245 event timings. In the timelines below, it is assumed that TTLs are 246 constant. 247 248 2. Rollover Methods 249 250 2.1. ZSK Rollovers 251 252 For ZSKs, the issue for the zone operator/signer is to ensure that 253 any caching validator that has access to a particular signature also 254 has access to the corresponding valid ZSK. 255 256 A ZSK can be rolled in one of three ways: 257 258 o Pre-Publication: described in [RFC6781], the new key is introduced 259 into the DNSKEY RRset, which is then re-signed. This state of 260 affairs remains in place for long enough to ensure that any cached 261 DNSKEY RRsets contain both keys. At that point, signatures 262 created with the old key can be replaced by those created with the 263 new key. During the re-signing process (which may or may not be 264 atomic depending on how the zone is managed), it doesn't matter 265 with which key an RRSIG record retrieved by a resolver was 266 created; cached copies of the DNSKEY RRset will contain both the 267 old and new keys. 268 269 270 271 272 Morris, et al. Informational [Page 5] 273 RFC 7583 Key Timing October 2015 274 275 276 Once the zone contains only signatures created with the new key, 277 there is an interval during which RRSIG records created with the 278 old key expire from caches. After this, there will be no 279 signatures anywhere that were created using the old key, and it 280 can be removed from the DNSKEY RRset. 281 282 o Double-Signature: also mentioned in [RFC6781], this involves 283 introducing the new key into the zone and using it to create 284 additional RRSIG records; the old key and existing RRSIG records 285 are retained. During the period in which the zone is being signed 286 (again, the signing process may not be atomic), validating 287 resolvers are always able to validate RRSIGs: any combination of 288 old and new DNSKEY RRset and RRSIGs allows at least one signature 289 to be validated. 290 291 Once the signing process is complete and enough time has elapsed 292 to make sure that all validators that have the DNSKEY and 293 signatures in cache have both the old and new information, the old 294 key and signatures can be removed from the zone. As before, 295 during this period any combination of DNSKEY RRset and RRSIGs will 296 allow validation of at least one signature. 297 298 o Double-RRSIG: strictly speaking, the use of the term "Double- 299 Signature" above is a misnomer as the method is not only double 300 signature, it is also double key as well. A true Double-Signature 301 method (here called the Double-RRSIG method) involves introducing 302 new signatures in the zone (while still retaining the old ones) 303 but not introducing the new key. 304 305 Once the signing process is complete and enough time has elapsed 306 to ensure that all caches that may contain an RR and associated 307 RRSIG have a copy of both signatures, the key is changed. After a 308 further interval during which the old DNSKEY RRset expires from 309 caches, the old signatures are removed from the zone. 310 311 Of the three methods, Double-Signature is conceptually the simplest: 312 introduce the new key and new signatures, then approximately one TTL 313 later remove the old key and old signatures. It is also the fastest, 314 but suffers from increasing the size of the zone and the size of 315 responses. 316 317 Pre-Publication is more complex: introduce the new key, approximately 318 one TTL later sign the records, and approximately one TTL after that 319 remove the old key. It does however keep the zone and response sizes 320 to a minimum. 321 322 323 324 325 326 327 Morris, et al. Informational [Page 6] 328 RFC 7583 Key Timing October 2015 329 330 331 Double-RRSIG is essentially the reverse of Pre-Publication: introduce 332 the new signatures, approximately one TTL later change the key, and 333 approximately one TTL after that remove the old signatures. However, 334 it has the disadvantage of the Pre-Publication method in terms of 335 time taken to perform the rollover, the disadvantage of the Double- 336 Signature rollover in terms of zone and response sizes, and none of 337 the advantages of either. For these reasons, it is unlikely to be 338 used in any real-world situations and so will not be considered 339 further in this document. 340 341 2.2. KSK Rollovers 342 343 In the KSK case, there should be no problem with a caching validator 344 not having access to a signature created with a valid KSK. The KSK 345 is only used for one signature (that over the DNSKEY RRset) and both 346 the key and the signature travel together. Instead, the issue is to 347 ensure that the KSK is trusted. 348 349 Trust in the KSK is due to either the existence of a signed and 350 validated DS record in the parent zone or an explicitly configured 351 trust anchor. If the former, the rollover algorithm will need to 352 involve the parent zone in the addition and removal of DS records, so 353 timings are not wholly under the control of the zone manager. If the 354 latter, [RFC5011] timings will be needed to roll the keys. (Even in 355 the case where authentication is via a DS record, the zone manager 356 may elect to include [RFC5011] timings in the key rolling process so 357 as to cope with the possibility that the key has also been explicitly 358 configured as a trust anchor.) 359 360 It is important to note that the need to interact with the parent 361 does not preclude the development of key rollover logic; in 362 accordance with the goal of the rollover logic, being able to 363 determine when a state change is "safe", the only effect of being 364 dependent on the parent is that there may be a period of waiting for 365 the parent to respond in addition to any delay the key rollover logic 366 requires. Although this introduces additional delays, even with a 367 parent that is less than ideally responsive, the only effect will be 368 a slowdown in the rollover state transitions. This may cause a 369 policy violation, but will not cause any operational problems. 370 371 Like the ZSK case, there are three methods for rolling a KSK: 372 373 o Double-KSK: the new KSK is added to the DNSKEY RRset, which is 374 then signed with both the old and new key. After waiting for the 375 old RRset to expire from caches, the DS record in the parent zone 376 is changed. After waiting a further interval for this change to 377 be reflected in caches, the old key is removed from the RRset. 378 379 380 381 382 Morris, et al. Informational [Page 7] 383 RFC 7583 Key Timing October 2015 384 385 386 o Double-DS: the new DS record is published. After waiting for this 387 change to propagate into caches, the KSK is changed. After a 388 further interval during which the old DNSKEY RRset expires from 389 caches, the old DS record is removed. 390 391 o Double-RRset: the new KSK is added to the DNSKEY RRset, which is 392 then signed with both the old and new key, and the new DS record 393 is added to the parent zone. After waiting a suitable interval 394 for the old DS and DNSKEY RRsets to expire from caches, the old 395 DNSKEY and DS records are removed. 396 397 In essence, Double-KSK means that the new KSK is introduced first and 398 used to sign the DNSKEY RRset. The DS record is changed, and finally 399 the old KSK is removed. It limits interactions with the parent to a 400 minimum but, for the duration of the rollover, the size of the DNSKEY 401 RRset is increased. 402 403 With Double-DS, the order of operations is the other way around: 404 introduce the new DS, change the DNSKEY, then remove the old DS. The 405 size of the DNSKEY RRset is kept to a minimum, but two interactions 406 are required with the parent. 407 408 Finally, Double-RRset is the fastest way to roll the KSK, but has the 409 drawbacks of both of the other methods: a larger DNSKEY RRset and two 410 interactions with the parent. 411 412 3. Key Rollover Timelines 413 414 3.1. Key States 415 416 DNSSEC validation requires both the DNSKEY and information created 417 from it (referred to as "associated data" in this section). In the 418 case of validation of an RR, the data associated with the key is the 419 corresponding RRSIG. Where there is a need to validate a chain of 420 trust, the associated data is the DS record. 421 422 During the rolling process, keys move through different states. The 423 defined states are: 424 425 Generated Although keys may be created immediately prior to first 426 use, some implementations may find it convenient to 427 create a pool of keys in one operation and draw from it 428 as required. (Note: such a pre-generated pool must be 429 secured against surreptitious use.) In the timelines 430 below, before the first event, the keys are considered to 431 be created but not yet used: they are said to be in the 432 "Generated" state. 433 434 435 436 437 Morris, et al. Informational [Page 8] 438 RFC 7583 Key Timing October 2015 439 440 441 Published A key enters the published state when either it or its 442 associated data first appears in the appropriate zone. 443 444 Ready The DNSKEY or its associated data have been published for 445 long enough to guarantee that copies of the key(s) it is 446 replacing (or associated data related to that key) have 447 expired from caches. 448 449 Active The data is starting to be used for validation. In the 450 case of a ZSK, it means that the key is now being used to 451 sign RRsets and that both it and the created RRSIGs 452 appear in the zone. In the case of a KSK, it means that 453 it is possible to use it to validate a DNSKEY RRset as 454 both the DNSKEY and DS records are present in their 455 respective zones. Note that when this state is entered, 456 it may not be possible for validating resolvers to use 457 the data for validation in all cases: the zone signing 458 may not have finished or the data might not have reached 459 the resolver because of propagation delays and/or caching 460 issues. If this is the case, the resolver will have to 461 rely on the predecessor data instead. 462 463 Retired The data has ceased to be used for validation. In the 464 case of a ZSK, it means that the key is no longer used to 465 sign RRsets. In the case of a KSK, it means that the 466 successor DNSKEY and DS records are in place. In both 467 cases, the key (and its associated data) can be removed 468 as soon as it is safe to do so, i.e., when all validating 469 resolvers are able to use the new key and associated data 470 to validate the zone. However, until this happens, the 471 current key and associated data must remain in their 472 respective zones. 473 474 Dead The key and its associated data are present in their 475 respective zones, but there is no longer information 476 anywhere that requires their presence for use in 477 validation. Hence, they can be removed at any time. 478 479 Removed Both the DNSKEY and its associated data have been removed 480 from their respective zones. 481 482 Revoked The DNSKEY is published for a period with the "revoke" 483 bit set as a way of notifying validating resolvers that 484 have configured it as a trust anchor, as used in 485 [RFC5011], that it is about to be removed from the zone. 486 This state is used when [RFC5011] considerations are in 487 effect (see Section 3.3.4). 488 489 490 491 492 Morris, et al. Informational [Page 9] 493 RFC 7583 Key Timing October 2015 494 495 496 3.2. ZSK Rollover Timelines 497 498 The following sections describe the rolling of a ZSK. They show the 499 events in the lifetime of a key (referred to as "key N") and cover 500 its replacement by its successor (key N+1). 501 502 3.2.1. Pre-Publication Method 503 504 In this method, the new key is introduced into the DNSKEY RRset. 505 After enough time to ensure that any cached DNSKEY RRsets contain 506 both keys, the zone is signed using the new key and the old 507 signatures are removed. Finally, when all signatures created with 508 the old key have expired from caches, the old key is removed. 509 510 The following diagram shows the timeline of a Pre-Publication 511 rollover. Time increases along the horizontal scale from left to 512 right and the vertical lines indicate events in the process. 513 Significant times and time intervals are marked. 514 515 |1| |2| |3| |4| |5| |6| |7| |8| 516 | | | | | | | | 517 Key N |<-Ipub->|<--->|<-------Lzsk------>|<-Iret->|<--->| 518 | | | | | | | | 519 Key N+1 | | | |<-Ipub->|<-->|<---Lzsk---- - - 520 | | | | | | | | 521 Key N Tpub Trdy Tact Tret Tdea Trem 522 Key N+1 Tpub Trdy Tact 523 524 ---- Time ----> 525 526 Figure 1: Timeline for a Pre-Publication ZSK Rollover 527 528 Event 1: Key N's DNSKEY record is put into the zone, i.e., it is 529 added to the DNSKEY RRset, which is then re-signed with the currently 530 active KSKs. The time at which this occurs is the publication time 531 (Tpub), and the key is now said to be published. Note that the key 532 is not yet used to sign records. 533 534 Event 2: Before it can be used, the key must be published for long 535 enough to guarantee that any cached version of the zone's DNSKEY 536 RRset includes this key. 537 538 This interval is the publication interval (Ipub) and, for the second 539 or subsequent keys in the zone, is given by: 540 541 Ipub = Dprp + TTLkey 542 543 544 545 546 547 Morris, et al. Informational [Page 10] 548 RFC 7583 Key Timing October 2015 549 550 551 Here, Dprp is the propagation delay -- the time taken for a change 552 introduced at the master to replicate to all nameservers. TTLkey is 553 the TTL for the DNSKEY records in the zone. The sum is therefore the 554 maximum time taken for existing DNSKEY records to expire from caches, 555 regardless of the nameserver from which they were retrieved. 556 557 (The case of introducing the first ZSK into the zone is discussed in 558 Section 3.3.5.) 559 560 After a delay of Ipub, the key is said to be ready and could be used 561 to sign records. The time at which this event occurs is key N's 562 ready time (Trdy), which is given by: 563 564 Trdy(N) = Tpub(N) + Ipub 565 566 Event 3: At some later time, the key starts being used to sign 567 RRsets. This point is the activation time (Tact) and after this, key 568 N is said to be active. 569 570 Tact(N) >= Trdy(N) 571 572 Event 4: At some point thought must be given to its successor (key 573 N+1). As with the introduction of the currently active key into the 574 zone, the successor key will need to be published at least Ipub 575 before it is activated. The publication time of key N+1 depends on 576 the activation time of key N: 577 578 Tpub(N+1) <= Tact(N) + Lzsk - Ipub 579 580 Here, Lzsk is the length of time for which a ZSK will be used (the 581 ZSK lifetime). It should be noted that in the diagrams, the actual 582 key lifetime is represented; this may differ slightly from the 583 intended lifetime set by key management policy. 584 585 Event 5: While key N is still active, its successor becomes ready. 586 From this time onwards, key N+1 could be used to sign the zone. 587 588 Event 6: When key N has been in use for an interval equal to the ZSK 589 lifetime, it is retired (i.e., it will never again be used to 590 generate new signatures) and key N+1 activated and used to sign the 591 zone. This is the retire time of key N (Tret), and is given by: 592 593 Tret(N) = Tact(N) + Lzsk 594 595 It is also the activation time of the successor key N+1. Note that 596 operational considerations may cause key N to remain in use for a 597 longer (or shorter) time than the lifetime set by the key management 598 policy. 599 600 601 602 Morris, et al. Informational [Page 11] 603 RFC 7583 Key Timing October 2015 604 605 606 Event 7: The retired key needs to be retained in the zone whilst any 607 RRSIG records created using this key are still published in the zone 608 or held in caches. (It is possible that a validating resolver could 609 have an old RRSIG record in the cache, but the old DNSKEY RRset has 610 expired when it is asked to provide both to a client. In this case 611 the DNSKEY RRset would need to be looked up again.) This means that 612 once the key is no longer used to sign records, it should be retained 613 in the zone for at least the retire interval (Iret) given by: 614 615 Iret = Dsgn + Dprp + TTLsig 616 617 Dsgn is the delay needed to ensure that all existing RRsets have been 618 re-signed with the new key. Dprp is the propagation delay, required 619 to guarantee that the updated zone information has reached all slave 620 servers, and TTLsig is the maximum TTL of all the RRSIG records in 621 the zone created with the retiring key. 622 623 The time at which all RRSIG records created with this key have 624 expired from resolver caches is the dead time (Tdea), given by: 625 626 Tdea(N) = Tret(N) + Iret 627 628 ... at which point the key is said to be dead. 629 630 Event 8: At any time after the key becomes dead, it can be removed 631 from the zone's DNSKEY RRset, which must then be re-signed with the 632 current KSK. This time is the removal time (Trem), given by: 633 634 Trem(N) >= Tdea(N) 635 636 ... at which time the key is said to be removed. 637 638 3.2.2. Double-Signature Method 639 640 In this rollover, a new key is introduced and used to sign the zone; 641 the old key and signatures are retained. Once all cached DNSKEY and/ 642 or RRSIG information contains copies of the new DNSKEY and RRSIGs 643 created with it, the old DNSKEY and RRSIGs can be removed from the 644 zone. 645 646 The timeline for a Double-Signature rollover is shown below. The 647 diagram follows the convention described in Section 3.2.1. 648 649 650 651 652 653 654 655 656 657 Morris, et al. Informational [Page 12] 658 RFC 7583 Key Timing October 2015 659 660 661 |1| |2| |3| |4| 662 | | | | 663 Key N |<-------Lzsk----------->|<--->| 664 | | | | 665 | |<--Iret-->| | 666 | | | | 667 Key N+1 | |<----Lzsk------- - - 668 | | | | 669 Key N Tact Tdea Trem 670 Key N+1 Tact 671 672 ---- Time ----> 673 674 Figure 2: Timeline for a Double-Signature ZSK Rollover 675 676 Event 1: Key N is added to the DNSKEY RRset and is then used to sign 677 the zone; existing signatures in the zone are not removed. The key 678 is published and active: this is key N's activation time (Tact), 679 after which the key is said to be active. 680 681 Event 2: As the current key (key N) approaches the end of its actual 682 lifetime (Lzsk), the successor key (key N+1) is introduced into the 683 zone and starts being used to sign RRsets: neither the current key 684 nor the signatures created with it are removed. The successor key is 685 now also active. 686 687 Tact(N+1) = Tact(N) + Lzsk - Iret 688 689 Event 3: Before key N can be withdrawn from the zone, all RRsets that 690 need to be signed must have been signed by the successor key (key 691 N+1) and any old RRsets that do not include the new key or new RRSIGs 692 must have expired from caches. Note that the signatures are not 693 replaced: each RRset is signed by both the old and new key. 694 695 This takes Iret, the retire interval, given by the expression: 696 697 Iret = Dsgn + Dprp + max(TTLkey, TTLsig) 698 699 As before, Dsgn is the delay needed to ensure that all existing 700 RRsets have been signed with the new key and Dprp is the propagation 701 delay, required to guarantee that the updated zone information has 702 reached all slave servers. The final term (the maximum of TTLkey and 703 TTLsig) is the period to wait for key and signature data associated 704 with key N to expire from caches. (TTLkey is the TTL of the DNSKEY 705 RRset and TTLsig is the maximum TTL of all the RRSIG records in the 706 zone created with the ZSK. The two may be different: although the 707 708 709 710 711 712 Morris, et al. Informational [Page 13] 713 RFC 7583 Key Timing October 2015 714 715 716 TTL of an RRSIG is equal to the TTL of the RRs in the associated 717 RRset [RFC4034], the DNSKEY RRset only needs to be signed with the 718 KSK.) 719 720 At the end of this interval, key N is said to be dead. This occurs 721 at the dead time (Tdea) so: 722 723 Tdea(N) = Tact(N+1) + Iret 724 725 Event 4: At some later time, key N and the signatures generated with 726 it can be removed from the zone. This is the removal time (Trem), 727 given by: 728 729 Trem(N) >= Tdea(N) 730 731 3.3. KSK Rollover Timelines 732 733 The following sections describe the rolling of a KSK. They show the 734 events in the lifetime of a key (referred to as "key N") and cover it 735 replacement by its successor (key N+1). (The case of introducing the 736 first KSK into the zone is discussed in Section 3.3.5.) 737 738 3.3.1. Double-KSK Method 739 740 In this rollover, the new DNSKEY is added to the zone. After an 741 interval long enough to guarantee that any cached DNSKEY RRsets 742 contain the new DNSKEY, the DS record in the parent zone is changed. 743 After a further interval to allow the old DS record to expire from 744 caches, the old DNSKEY is removed from the zone. 745 746 The timeline for a Double-KSK rollover is shown below. The diagram 747 follows the convention described in Section 3.2.1. 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 Morris, et al. Informational [Page 14] 768 RFC 7583 Key Timing October 2015 769 770 771 |1| |2| |3| |4| 772 | | | | 773 Key N |<-IpubC->|<--->|<-Dreg->|<-----Lksk--- - - 774 | | | | 775 Key N+1 | | | | 776 | | | | 777 Key N Tpub Trdy Tsbm Tact 778 Key N+1 779 780 ---- Time ----> 781 782 (continued ...) 783 784 |5| |6| |7| |8| |9| |10| 785 | | | | | | 786 Key N - - --------------Lksk------->|<-Iret->|<----->| 787 | | | | | | 788 Key N+1 |<-IpubC->|<--->|<-Dreg->|<--------Lksk----- - - 789 | | | | | | 790 Key N Tret Tdea Trem 791 Key N+1 Tpub Trdy Tsbm Tact 792 793 ---- Time (cont.) ----> 794 795 Figure 3: Timeline for a Double-KSK Rollover 796 797 Event 1: Key N is introduced into the zone; it is added to the DNSKEY 798 RRset, which is then signed by all currently active KSKs. (So at 799 this point, the DNSKEY RRset is signed by both key N and its 800 predecessor KSK. If other KSKs were active, it is signed by these as 801 well.) This is the publication time of key N (Tpub); after this, the 802 key is said to be published. 803 804 Event 2: Before it can be used, the key must be published for long 805 enough to guarantee that any validating resolver that has a copy of 806 the DNSKEY RRset in its cache will have a copy of the RRset that 807 includes this key: in other words, that any prior cached information 808 about the DNSKEY RRset has expired. 809 810 The interval is the publication interval in the child zone (IpubC) 811 and is given by: 812 813 IpubC = DprpC + TTLkey 814 815 816 817 818 819 820 821 822 Morris, et al. Informational [Page 15] 823 RFC 7583 Key Timing October 2015 824 825 826 ... where DprpC is the propagation delay for the child zone (the zone 827 containing the KSK being rolled) and TTLkey the TTL for the DNSKEY 828 RRset. The time at which this occurs is the key N's ready time, 829 Trdy, given by: 830 831 Trdy(N) = Tpub(N) + IpubC 832 833 Event 3: At some later time, the DS record corresponding to the new 834 KSK is submitted to the parent zone for publication. This time is 835 the submission time, Tsbm: 836 837 Tsbm(N) >= Trdy(N) 838 839 Event 4: The DS record is published in the parent zone. As this is 840 the point at which all information for authentication -- both DNSKEY 841 and DS record -- is available in the two zones, in analogy with other 842 rollover methods, this is called the activation time of key N (Tact): 843 844 Tact(N) = Tsbm(N) + Dreg 845 846 ... where Dreg is the registration delay, the time taken after the DS 847 record has been submitted to the parent zone manager for it to be 848 placed in the zone. (Parent zones are often managed by different 849 entities, and this term accounts for the organizational overhead of 850 transferring a record. In practice, Dreg will not be a fixed time: 851 instead, the end of Dreg will be signaled by the appearance of the DS 852 record in the parent zone.) 853 854 Event 5: While key N is active, thought needs to be given to its 855 successor (key N+1). At some time before the scheduled end of the 856 KSK lifetime, the successor KSK is published in the zone. (As 857 before, this means that the DNSKEY RRset is signed by all KSKs.) 858 This time is the publication time of the successor key N+1, given by: 859 860 Tpub(N+1) <= Tact(N) + Lksk - Dreg - IpubC 861 862 ... where Lksk is the actual lifetime of the KSK, and Dreg the 863 registration delay. 864 865 Event 6: After an interval IpubC, key N+1 becomes ready (in that all 866 caches that have a copy of the DNSKEY RRset have a copy of this key). 867 This time is the ready time of the successor key N+1 (Trdy). 868 869 Event 7: At the submission time of the successor key N+1, Tsbm(N+1), 870 the DS record corresponding to key N+1 is submitted to the parent 871 zone. 872 873 874 875 876 877 Morris, et al. Informational [Page 16] 878 RFC 7583 Key Timing October 2015 879 880 881 Event 8: The successor DS record is published in the parent zone and 882 the current DS record withdrawn. Key N is said to be retired and the 883 time at which this occurs is Tret(N), given by: 884 885 Tret(N) = Tsbm(N+1) + Dreg 886 887 Event 9: Key N must remain in the zone until any caches that contain 888 a copy of the DS RRset have a copy containing the new DS record. 889 This interval is the retire interval, given by: 890 891 Iret = DprpP + TTLds 892 893 ... where DprpP is the propagation delay in the parent zone and TTLds 894 the TTL of a DS record in the parent zone. 895 896 As the key is no longer used for anything, it is said to be dead. 897 This point is the dead time (Tdea), given by: 898 899 Tdea(N) = Tret(N) + Iret 900 901 Event 10: At some later time, key N is removed from the zone's DNSKEY 902 RRset (at the remove time Trem); the key is now said to be removed. 903 904 Trem(N) >= Tdea(N) 905 906 3.3.2. Double-DS Method 907 908 In this rollover, the new DS record is published in the parent zone. 909 When any caches that contain the DS RRset contain a copy of the new 910 record, the KSK in the zone is changed. After a further interval for 911 the old DNSKEY RRset to expire from caches, the old DS record is 912 removed from the parent. 913 914 The timeline for a Double-DS rollover is shown below. The diagram 915 follows the convention described in Section 3.2.1. 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 Morris, et al. Informational [Page 17] 933 RFC 7583 Key Timing October 2015 934 935 936 |1| |2| |3| |4| |5| 937 | | | | | 938 Key N |<-Dreg->|<-IpubP->|<-->|<-------Lksk---- - - 939 | | | | | 940 Key N+1 | | | | |<--Dreg-- - - 941 | | | | | 942 Key N Tsbm Tpub Trdy Tact 943 Key N+1 Tsbm 944 ---- Time ----> 945 946 (continued ...) 947 948 |6| |7| |8| |9| |10| 949 | | | | | 950 Key N - ----------Lksk--------->|<-Iret->|<---->| 951 | | | | | 952 Key N+1 - --Dreg-->|<-IpubP->|<-->|<---Lksk------- - - 953 | | | | | 954 Key N Tret Tdea Trem 955 Key N+1 Tpub Trdy Tact 956 957 ---- Time ----> 958 959 Figure 4: Timeline for a Double-DS KSK Rollover 960 961 Event 1: The DS RR is submitted to the parent zone for publication. 962 This time is the submission time, Tsbm. 963 964 Event 2: After the registration delay, Dreg, the DS record is 965 published in the parent zone. This is the publication time (Tpub) of 966 key N, given by: 967 968 Tpub(N) = Tsbm(N) + Dreg 969 970 As before, in practice, Dreg will not be a fixed time. Instead, the 971 end of Dreg will be signaled by the appearance of the DS record in 972 the parent zone. 973 974 Event 3: At some later time, any cache that has a copy of the DS 975 RRset will have a copy of the DS record for key N. At this point, 976 key N, if introduced into the DNSKEY RRset, could be used to validate 977 the zone. For this reason, this time is known as the ready time, 978 Trdy, and is given by: 979 980 Trdy(N) = Tpub(N) + IpubP 981 982 983 984 985 986 987 Morris, et al. Informational [Page 18] 988 RFC 7583 Key Timing October 2015 989 990 991 IpubP is the publication interval of the DS record (in the parent 992 zone) and is given by the expression: 993 994 IpubP = DprpP + TTLds 995 996 ... where DprpP is the propagation delay for the parent zone and 997 TTLds the TTL assigned to DS records in that zone. 998 999 Event 4: At some later time, the key rollover takes place and the new 1000 key (key N) is introduced into the DNSKEY RRset and used to sign it. 1001 This time is key N's activation time (Tact) and at this point key N 1002 is said to be active: 1003 1004 Tact(N) >= Trdy(N) 1005 1006 Event 5: At some point, thought must be given to key replacement. 1007 The DS record for the successor key must be submitted to the parent 1008 zone at a time such that when the current key is withdrawn, any cache 1009 that contains the zone's DS records has data about the DS record of 1010 the successor key. The time at which this occurs is the submission 1011 time of the successor key N+1, given by: 1012 1013 Tsbm(N+1) <= Tact(N) + Lksk - IpubP - Dreg 1014 1015 ... where Lksk is the actual lifetime of key N (which may differ 1016 slightly from the lifetime set in the key management policy) and Dreg 1017 is the registration delay. 1018 1019 Event 6. After an interval Dreg, the successor DS record is 1020 published in the zone. 1021 1022 Event 7: The successor key (key N+1) enters the ready state, i.e., 1023 its DS record is now in caches that contain the parent DS RRset. 1024 1025 Event 8: When key N has been active for its lifetime (Lksk), it is 1026 replaced in the DNSKEY RRset by key N+1; the RRset is then signed 1027 with the new key. At this point, as both the old and new DS records 1028 have been in the parent zone long enough to ensure that they are in 1029 caches that contain the DS RRset, the zone can be authenticated 1030 throughout the rollover. A validating resolver can authenticate 1031 either the old or new KSK. 1032 1033 This time is the retire time (Tret) of key N, given by: 1034 1035 Tret(N) = Tact(N) + Lksk 1036 1037 This is also the activation time of the successor key N+1. 1038 1039 1040 1041 1042 Morris, et al. Informational [Page 19] 1043 RFC 7583 Key Timing October 2015 1044 1045 1046 Event 9: At some later time, all copies of the old DNSKEY RRset have 1047 expired from caches and the old DS record is no longer needed. In 1048 analogy with other rollover methods, this is called the dead time, 1049 Tdea, and is given by: 1050 1051 Tdea(N) = Tret(N) + Iret 1052 1053 ... where Iret is the retire interval of the key, given by: 1054 1055 Iret = DprpC + TTLkey 1056 1057 As before, this term includes DprpC, the time taken to propagate the 1058 RRset change through the master-slave hierarchy of the child zone and 1059 TTLkey, the time taken for the DNSKEY RRset to expire from caches. 1060 1061 Event 10: At some later time, the DS record is removed from the 1062 parent zone. In analogy with other rollover methods, this is the 1063 removal time (Trem), given by: 1064 1065 Trem(N) >= Tdea(N) 1066 1067 3.3.3. Double-RRset Method 1068 1069 In the Double-RRset rollover, the new DNSKEY and DS records are 1070 published simultaneously in the appropriate zones. Once enough time 1071 has elapsed for the old DNSKEY and DS RRsets to expire from caches, 1072 the old DNSKEY and DS records are removed from their respective 1073 zones. 1074 1075 The timeline for this rollover is shown below. The diagram follows 1076 the convention described in Section 3.2.1. 1077 1078 |1| |2| |3| |4| |5| 1079 | | | | | 1080 Key N |<-----------Lksk---------->|<---->| 1081 | | | | | 1082 | |<------Ipub----->| | 1083 | | | | | 1084 | |<-Dreg->|<-Iret->| | 1085 | | | | | 1086 Key N+1 | | |<----Lksk-------- - - 1087 | | | | | 1088 Key N Tact Tret Tdea Trem 1089 Key N+1 Tpub Tact 1090 1091 ---- Time ----> 1092 1093 Figure 5: Timeline for a Double-RRset KSK Rollover 1094 1095 1096 1097 Morris, et al. Informational [Page 20] 1098 RFC 7583 Key Timing October 2015 1099 1100 1101 Event 1: The DS and DNSKEY records have appeared in their respective 1102 zones and the latter has been used to sign the DNSKEY RRset. The key 1103 is published and active: this is key N's activation time (Tact). 1104 1105 Event 2: As the current key (key N) approaches the end of its actual 1106 lifetime (Lksk), the successor key (key N+1) is introduced into the 1107 zone and is used to sign the DNSKEY RRset. At the same time, the 1108 successor DS record is submitted to the parent zone. This is the 1109 publication time of the successor key (Tpub): 1110 1111 Tpub(N+1) <= Tact(N) + Lksk - Ipub 1112 1113 ... where Ipub is defined below. 1114 1115 Event 3: After the registration delay (Dreg), the DS record appears 1116 in the parent zone. The DNSKEY record is already in the child zone, 1117 so with both the new key and its associated data now visible, this is 1118 the key's activation time (Tact) and the key is now said to be 1119 active. 1120 1121 Tact(N+1) = Tpub(N+1) + Dreg 1122 1123 Event 4: Before key N and its associated data can be withdrawn, all 1124 RRsets in the caches of validating resolvers must contain the new DS 1125 and/or DNSKEY. The time at which this occurs is the dead time of key 1126 N (Tdea), given by: 1127 1128 Tdea(N) = Tpub(N+1) + Ipub 1129 1130 Ipub is the time it takes to guarantee that any prior cached 1131 information about the DNSKEY and the DS RRsets have expired. For the 1132 DNSKEY, this is the publication interval of the child (IpubC). For 1133 the DS, the publication interval (IpubP) starts once the record 1134 appears in the parent zone, which is Dreg after it has been 1135 submitted. Hence: 1136 1137 Ipub = max(Dreg + IpubP, IpubC) 1138 1139 The parent zone's publication interval is given by: 1140 1141 IpubP = DprpP + TTLds 1142 1143 where DprpP is the parent zone's propagation delay and TTLds is the 1144 TTL of the DS record in that zone. 1145 1146 1147 1148 1149 1150 1151 1152 Morris, et al. Informational [Page 21] 1153 RFC 7583 Key Timing October 2015 1154 1155 1156 The child zone's publication interval is given by a similar equation: 1157 1158 IpubC = DprpC + TTLkey 1159 1160 where DprpC is the propagation delay in the child zone and TTLkey the 1161 TTL of a DNSKEY record. 1162 1163 In analogy with other rollovers, we can also define a retire interval 1164 -- the interval between a key becoming active and the time at which 1165 its predecessor is considered dead. In this case, Iret is given by: 1166 1167 Iret = Ipub - Dreg 1168 1169 In other words, the retire interval of the predecessor key is the 1170 greater of the publication interval of the parent, or the publication 1171 interval of the child minus the registration delay. 1172 1173 Event 5: At some later time, the key N's DS and DNSKEY records are 1174 removed from their respective zones. In analogy with other rollover 1175 methods, this is the removal time (Trem), given by: 1176 1177 Trem(N) >= Tdea(N) 1178 1179 3.3.4. Interaction with Configured Trust Anchors 1180 1181 Although the preceding sections have been concerned with rolling 1182 KSKs, where the trust anchor is a DS record in the parent zone, zone 1183 managers may want to take account of the possibility that some 1184 validating resolvers may have configured trust anchors directly. 1185 1186 Rolling a configured trust anchor is dealt with in [RFC5011]. It 1187 requires introducing the KSK to be used as the trust anchor into the 1188 zone for a period of time before use and retaining it (with the 1189 "revoke" bit set) for some time after use. 1190
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1191 126.96.36.199. Addition of KSK 1192 1193 When the new key is introduced, the expression for the publication 1194 interval of the DNSKEY (IpubC) in the Double-KSK and Double-RRset 1195 methods is modified to: 1196 1197 IpubC >= DprpC + max(Itrp, TTLkey) 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 Morris, et al. Informational [Page 22] 1208 RFC 7583 Key Timing October 2015 1209 1210 1211 ... where the right-hand side of the expression now includes the 1212 "trust point" interval. This term is the interval required to 1213 guarantee that a resolver configured for the automatic update of keys 1214 according to [RFC5011] will accept the new key as a new trust point. 1215 That interval is given by: 1216 1217 Itrp >= queryInterval + AddHoldDownTime + queryInterval 1218 1219 ... where queryInterval is as defined in Section 2.3 of [RFC5011] and 1220 AddHoldDownTime is the Add Hold-Down Time defined in Section 2.4.1 of 1221 the same document. 1222 1223 The first term of the expression (queryInterval) represents the time 1224 after which all validating resolvers can be guaranteed to have 1225 obtained a copy of the DNSKEY RRset containing the new key. Once 1226 retrieved, a validating resolver needs to wait for AddHoldDownTime. 1227 Providing it does not see a validly signed DNSKEY RRset without the 1228 new key in that period, it will treat it as a trust anchor the next 1229 time it retrieves the RRset, a process that can take up to another 1230 queryInterval (the third term). 1231 1232 However, the expression for queryInterval given in [RFC5011] contains 1233 the DNSKEY's RRSIG expiration interval, a parameter that only the 1234 validating resolver can really calculate. In practice, a modified 1235 query interval that depends only on TTLkey can be used: 1236 1237 modifiedQueryInterval = MAX(1hr, MIN(15 days, TTLkey / 2)) 1238 1239 (This is obtained by taking the expression for queryInterval in 1240 [RFC5011] and assuming a worst case for RRsigExpirationInterval. It 1241 is greater than or equal to queryInterval for all values of the 1242 expiration time.) The expression above then becomes (after 1243 collecting terms): 1244 1245 Itrp >= AddHoldDownTime + 2 * modifiedQueryInterval 1246 1247 In the Double-DS method, instead of swapping the KSK RRs in a single 1248 step, there must now be a period of overlap. In other words, the new 1249 KSK must be introduced into the zone at least: 1250 1251 DprpC + max(Itrp, TTLkey) 1252 1253 ... before the switch is made. 1254 1255 1256 1257 1258 1259 1260 1261 1262 Morris, et al. Informational [Page 23] 1263 RFC 7583 Key Timing October 2015 1264 1265 1266 188.8.131.52. Removal of KSK 1267 1268 The timeline for the removal of the key in all methods is modified by 1269 introducing a new state, "revoked". When the key reaches its dead 1270 time, instead of being declared "dead", it is revoked; the "revoke" 1271 bit is set in the published DNSKEY RR, and the DNSKEY RRset re-signed 1272 with the current and revoked keys. The key is maintained in this 1273 state for the revoke interval, Irev, given by: 1274 1275 Irev >= DprpC + modifiedQueryInterval 1276 1277 As before, DprpC is the time taken for the revoked DNSKEY to 1278 propagate to all slave zones, and modifiedQueryInterval is the time 1279 after which it can be guaranteed that all validating resolvers that 1280 adhere to RFC 5011 have retrieved a copy of the DNSKEY RRset 1281 containing the revoked key. 1282 1283 After this time, the key is dead and can be removed from the zone. 1284 1285 3.3.5. Introduction of First Keys 1286 1287 There are no timing considerations associated with the introduction 1288 of the first keys into a zone other that they must be introduced and 1289 the zone validly signed before a chain of trust to the zone is 1290 created. 1291 1292 In the case of a secure parent, it means ensuring that the DS record 1293 is not published in the parent zone until there is no possibility 1294 that a validating resolver can obtain the record yet is not able to 1295 obtain the corresponding DNSKEY. In the case of an insecure parent, 1296 i.e., the initial creation of a chain of trust or "security apex", it 1297 is not possible to guarantee this. It is up to the operator of the 1298 validating resolver to wait for the new KSK to appear at all servers 1299 for the zone before configuring the trust anchor. 1300 1301 4. Standby Keys 1302 1303 Although keys will usually be rolled according to some regular 1304 schedule, there may be occasions when an emergency rollover is 1305 required, e.g., if the active key is suspected of being compromised. 1306 The aim of the emergency rollover is to allow the zone to be 1307 re-signed with a new key as soon as possible. As a key must be in 1308 the ready state to sign the zone, having at least one additional key 1309 (a standby key) in this state at all times will minimize delay. 1310 1311 In the case of a ZSK, a standby key only really makes sense with the 1312 Pre-Publication method. A permanent standby DNSKEY RR should be 1313 included in the zone or successor keys could be introduced as soon as 1314 1315 1316 1317 Morris, et al. Informational [Page 24] 1318 RFC 7583 Key Timing October 2015 1319 1320 1321 possible after a key becomes active. Either way results in one or 1322 more additional ZSKs in the DNSKEY RRset that can immediately be used 1323 to sign the zone if the current key is compromised. 1324 1325 (Although, in theory, the mechanism could be used with both the 1326 Double-Signature and Double-RRSIG methods, it would require 1327 pre-publication of the signatures. Essentially, the standby key 1328 would be permanently active, as it would have to be periodically used 1329 to renew signatures. Zones would also permanently require two sets 1330 of signatures.) 1331 1332 It is also possible to have a standby KSK. The Double-KSK method 1333 requires that the standby KSK be included in the DNSKEY RRset; 1334 rolling the key then requires just the introduction of the DS record 1335 in the parent. Note that the standby KSK should also be used to sign 1336 the DNSKEY RRset. As the RRset and its signatures travel together, 1337 merely adding the KSK without using it to sign the DNSKEY RRset does 1338 not provide the desired time saving: for a KSK to be used in a 1339 rollover, the DNSKEY RRset must be signed with it, and this would 1340 introduce a delay while the old RRset (not signed with the new key) 1341 expires from caches. 1342 1343 The idea of a standby KSK in the Double-RRset rollover method 1344 effectively means having two active keys (as the standby KSK and 1345 associated DS record would both be published at the same time in 1346 their respective zones). 1347 1348 Finally, in the Double-DS method of rolling a KSK, it is not a 1349 standby key that is present, it is a standby DS record in the parent 1350 zone. 1351 1352 Whatever algorithm is used, the standby item of data can be included 1353 in the zone on a permanent basis, or be a successor introduced as 1354 early as possible. 1355 1356 5. Algorithm Considerations 1357 1358 The preceding sections have implicitly assumed that all keys and 1359 signatures are created using a single algorithm. However, 1360 Section 2.2 of [RFC4035] requires that there be an RRSIG for each 1361 RRset using at least one DNSKEY of each algorithm in the zone apex 1362 DNSKEY RRset. 1363 1364 Except in the case of an algorithm rollover -- where the algorithms 1365 used to create the signatures are being changed -- there is no 1366 relationship between the keys of different algorithms. This means 1367 that they can be rolled independently of one another. In other 1368 1369 1370 1371 1372 Morris, et al. Informational [Page 25] 1373 RFC 7583 Key Timing October 2015 1374 1375 1376 words, the key-rollover logic described above should be run 1377 separately for each algorithm; the union of the results is included 1378 in the zone, which is signed using the active key for each algorithm. 1379 1380 6. Summary 1381 1382 For ZSKs, the Pre-Publication method is generally considered to be 1383 the preferred way of rolling keys. As shown in this document, the 1384 time taken to roll is wholly dependent on parameters under the 1385 control of the zone manager. 1386 1387 In contrast, the Double-RRset method is the most efficient for KSK 1388 rollover due to the ability to have new DS records and DNSKEY RRsets 1389 propagate in parallel. The time taken to roll KSKs may depend on 1390 factors related to the parent zone if the parent is signed. For 1391 zones that intend to comply with the recommendations of [RFC5011], in 1392 many cases, the rollover time will be determined by the times defined 1393 by RFC 5011. It should be emphasized that this delay is a policy 1394 choice and not a function of timing values and that it also requires 1395 changes to the rollover process due to the need to manage revocation 1396 of trust anchors. 1397 1398 Finally, the treatment of emergency key rollover is significantly 1399 simplified by the introduction of standby keys as standard practice 1400 during all types of rollovers. 1401 1402 7. Security Considerations 1403 1404 This document does not introduce any new security issues beyond those 1405 already discussed in [RFC4033], [RFC4034], [RFC4035], and [RFC5011]. 1406 1407 8. Normative References 1408 1409 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1410 Rose, "DNS Security Introduction and Requirements", 1411 RFC 4033, DOI 10.17487/RFC4033, March 2005, 1412 <http://www.rfc-editor.org/info/rfc4033>. 1413 1414 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1415 Rose, "Resource Records for the DNS Security Extensions", 1416 RFC 4034, DOI 10.17487/RFC4034, March 2005, 1417 <http://www.rfc-editor.org/info/rfc4034>. 1418 1419 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1420 Rose, "Protocol Modifications for the DNS Security 1421 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 1422 <http://www.rfc-editor.org/info/rfc4035>. 1423 1424 1425 1426 1427 Morris, et al. Informational [Page 26] 1428 RFC 7583 Key Timing October 2015 1429 1430 1431 [RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC) 1432 Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011, 1433 September 2007, <http://www.rfc-editor.org/info/rfc5011>. 1434 1435 [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC 1436 Operational Practices, Version 2", RFC 6781, 1437 DOI 10.17487/RFC6781, December 2012, 1438 <http://www.rfc-editor.org/info/rfc6781>. 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 Morris, et al. Informational [Page 27] 1483 RFC 7583 Key Timing October 2015 1484 1485 1486 Appendix A. List of Symbols 1487 1488 The document defines a number of symbols, all of which are listed 1489 here. All are of the form: 1490 1491 <TYPE><id><ZONE> 1492 1493 where: 1494 1495 <TYPE> is an uppercase character indicating what type the symbol is. 1496 Defined types are: 1497 1498 D delay: interval that is a feature of the process 1499 1500 I interval between two events 1501 1502 L lifetime: interval set by the zone manager 1503 1504 T a point in time 1505 1506 TTL TTL of a record 1507 1508 I, T, and TTL are self-explanatory. Like I, both D and L are time 1509 periods, but whereas I values are intervals between two events, a "D" 1510 interval (delay) is a feature of the process, probably outside 1511 control of the zone manager, and an "L" interval (lifetime) is chosen 1512 by the zone manager and is a feature of policy. 1513 1514 <id> is lowercase and defines what object or event the variable is 1515 related to, e.g., 1516 1517 act activation 1518 1519 pub publication 1520 1521 ret retire 1522 1523 <ZONE> is an optional uppercase letter that distinguishes between the 1524 same variable applied to different zones and is one of: 1525 1526 C child 1527 1528 P parent 1529 1530 Within the rollover descriptions, times may have a number in 1531 parentheses affixed to their end indicating the instance of the key 1532 to which they apply, e.g., Tact(N) is the activation time of key N, 1533 Tpub(N+1) the publication time of key N+1 etc. 1534 1535 1536 1537 Morris, et al. Informational [Page 28] 1538 RFC 7583 Key Timing October 2015 1539 1540 1541 The list of variables used in the text given below. 1542 1543 Dprp Propagation delay. The amount of time for a change made at 1544 a master nameserver to propagate to all the slave 1545 nameservers. 1546 1547 DprpC Propagation delay in the child zone. 1548 1549 DprpP Propagation delay in the parent zone. 1550 1551 Dreg Registration delay: the time taken for a DS record 1552 submitted to a parent zone to appear in it. As a parent 1553 zone is often managed by a different organization than that 1554 managing the child zone, the delays associated with passing 1555 data between organizations is captured by this term. 1556 1557 Dsgn Signing delay. After the introduction of a new ZSK, the 1558 amount of time taken for all the RRs in the zone to be 1559 signed with it. 1560 1561 Ipub Publication interval. The amount of time that must elapse 1562 after the publication of a DNSKEY and/or its associated 1563 data before it can be assumed that any resolvers that have 1564 the relevant RRset cached have a copy of the new 1565 information. 1566 1567 IpubC Publication interval in the child zone. 1568 1569 IpubP Publication interval in the parent zone. 1570 1571 Iret Retire interval. The amount of time that must elapse after 1572 a DNSKEY or associated data enters the retire state for any 1573 dependent information (e.g., RRSIG for a ZSK) to be purged 1574 from validating resolver caches. 1575 1576 Irev Revoke interval. The amount of time that a KSK must remain 1577 published with the "revoke" bit set to satisfy 1578 considerations of [RFC5011]. 1579 1580 Itrp Trust-point interval. The amount of time that a trust 1581 anchor must be published for in order to guarantee that a 1582 resolver configured for an automatic update of keys will 1583 see the new key at least twice. 1584 1585 1586 1587 1588 1589 1590 1591 1592 Morris, et al. Informational [Page 29] 1593 RFC 7583 Key Timing October 2015 1594 1595 1596 Lksk Lifetime of a KSK. This is the actual amount of time for 1597 which this particular KSK is regarded as the active KSK. 1598 Depending on when the key is rolled over, the actual 1599 lifetime may be longer or shorter than the intended key 1600 lifetime indicated by management policy. 1601 1602 Lzsk Lifetime of a ZSK. This is the actual amount of time for 1603 which the ZSK is used to sign the zone. Depending on when 1604 the key is rolled over, the actual lifetime may be longer 1605 or shorter than the intended key lifetime indicated by 1606 management policy. 1607 1608 Tact Activation time. The time at which the key is regarded as 1609 the principal key for the zone. 1610 1611 Tdea Dead time. The time at which any information held in 1612 validating resolver caches is guaranteed to contain 1613 information related to the successor key. At this point, 1614 the current key and its associated information are not 1615 longed required for validation purposes. 1616 1617 Tpub Publication time. The time that the key or associated data 1618 appears in the zone for the first time. 1619 1620 Trem Removal time. The time at which the key and its associated 1621 information starts being removed from their respective 1622 zones. 1623 1624 Tret Retire time. The time at which successor information 1625 starts being used. 1626 1627 Trdy Ready time. The time at which it can be guaranteed that 1628 validating resolvers that have information about the key 1629 and/or associated data cached have a copy of the new 1630 information. 1631 1632 Tsbm Submission time. The time at which the DS record of a KSK 1633 is submitted to the parent zone. 1634 1635 TTLds Time to live of a DS record. 1636 1637 TTLkey Time to live of a DNSKEY record. (By implication, this is 1638 also the time to live of the signatures on the DNSKEY 1639 RRset.) 1640 1641 TTLsig The maximum time to live of all the RRSIG records in the 1642 zone that were created with the ZSK. 1643 1644 1645 1646 1647 Morris, et al. Informational [Page 30] 1648 RFC 7583 Key Timing October 2015 1649 1650 1651 Acknowledgements 1652 1653 The authors gratefully acknowledge help and contributions from Roy 1654 Arends, Tim Wicinski, and Wouter Wijngaards. 1655 1656 Authors' Addresses 1657 1658 Stephen Morris 1659 Internet Systems Consortium 1660 950 Charter Street 1661 Redwood City, CA 94063 1662 United States 1663 1664 Email: email@example.com 1665 URI: http://www.isc.org 1666 1667 1668 Johan Ihren 1669 Netnod 1670 Franzengatan 5 1671 Stockholm SE-112 51 1672 Sweden 1673 1674 Email: firstname.lastname@example.org 1675 URI: http://www.netnod.se 1676 1677 1678 John Dickinson 1679 Sinodun Internet Technologies Ltd 1680 Magdalen Centre 1681 Oxford Science Park 1682 Robert Robertson Avenue 1683 Oxford, Oxfordshire OX4 4GA 1684 United Kingdom 1685 1686 Email: email@example.com 1687 URI: http://www.sinodun.com 1688 1689 1690 W. (Matthijs) Mekking 1691 Dyn, Inc. 1692 150 Dow St 1693 Manchester NH 03101 1694 United States 1695 1696 Email: firstname.lastname@example.org 1697 URI: https://www.dyn.com 1698 1699 1700 1701 1702 Morris, et al. Informational [Page 31] 1703
After a careful timing analysis of the steps defined in RFC5011 and the timing described in RFC7583, issues were found that could lead to denial of service attacks. draft-rfc5011-security-considerations discusses the issues found and the suggested fixes to the timing considerations and adding additional terms to the RFC7583 Itrp equation. Specifically, this never-published draft states:
Note: The equation for Itrp in RFC7583 is insecure as it does not include the sigExpirationTime listed above [which is defined as "The amount of time between the DNSKEY RRSIG's Signature Inception field and the Signature Expiration field."]. The Itrp equation in RFC7583 also does not include the 2*TTL safety margin, though that is an operational consideration.