idnits 2.17.1 draft-ietf-dnsext-delegation-signer-12.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** Looks like you're using RFC 2026 boilerplate. This must be updated to follow RFC 3978/3979, as updated by RFC 4748. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- ** The document seems to lack a 1id_guidelines paragraph about 6 months document validity -- however, there's a paragraph with a matching beginning. 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(30 more instances...) -- The abstract seems to indicate that this document updates RFC3090, but the header doesn't have an 'Updates:' line to match this. -- The abstract seems to indicate that this document updates RFC1035, but the header doesn't have an 'Updates:' line to match this. -- The abstract seems to indicate that this document updates RFC2535, but the header doesn't have an 'Updates:' line to match this. -- The abstract seems to indicate that this document updates RFC3008, but the header doesn't have an 'Updates:' line to match this. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the RFC 3978 Section 5.4 Copyright Line does not match the current year == The "Author's Address" (or "Authors' Addresses") section title is misspelled. == The expression 'MAY NOT', while looking like RFC 2119 requirements text, is not defined in RFC 2119, and should not be used. Consider using 'MUST NOT' instead (if that is what you mean). Found 'MAY NOT' in this paragraph: The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED", "RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be interpreted as described in RFC2119. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: A server authoritative for only the child zone at a delegation point that is also a caching server MAY (if the RD bit is set in the query) perform recursion to find the DS record at the delegation point, and may return the DS record from its cache. In this case, the AA bit MUST not be set in the response. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (December 2002) is 7795 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) -- Missing reference section? 'RFC1035' on line 602 looks like a reference -- Missing reference section? 'RFC2535' on line 608 looks like a reference -- Missing reference section? 'RFC3090' on line 614 looks like a reference -- Missing reference section? 'RFC3008' on line 611 looks like a reference -- Missing reference section? 'RFC3225' on line 617 looks like a reference -- Missing reference section? 'RFC3226' on line 620 looks like a reference -- Missing reference section? 'RFC 2181' on line 210 looks like a reference -- Missing reference section? 'RFC2181' on line 605 looks like a reference Summary: 3 errors (**), 0 flaws (~~), 7 warnings (==), 14 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DNSEXT Working Group Olafur Gudmundsson 3 INTERNET-DRAFT December 2002 4 6 Updates: RFC 1035, RFC 2535, RFC 3008, RFC 3090. 8 Delegation Signer Resource Record 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as ``work in progress.'' 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html 31 Comments should be sent to the authors or the DNSEXT WG mailing list 32 namedroppers@ops.ietf.org 34 This draft expires on June 4, 2003. 36 Copyright Notice 38 Copyright (C) The Internet Society (2002). All rights reserved. 40 Abstract 42 The delegation signer (DS) resource record is inserted at a zone cut 43 (i.e., a delegation point) to indicate that the delegated zone is 44 digitally signed and that the delegated zone recognizes the indicated 45 key as a valid zone key for the delegated zone. The DS RR is a 46 modification to the DNS Security Extensions definition, motivated by 47 operational considerations. The intent is to use this resource record 48 as an explicit statement about the delegation, rather than relying on 49 inference. 51 This document defines the DS RR, gives examples of how it is used and 52 the implications of this record on resolvers. This change is not 53 backwards compatible with RFC 2535. 54 This document updates RFC1035, RFC2535, RFC3008 and RFC3090. 56 1 Introduction 58 Familiarity with the DNS system [RFC1035], DNS security extensions 59 [RFC2535] and DNSSEC terminology [RFC3090] is important. 61 Experience shows that when the same data can reside in two 62 administratively different DNS zones, the data frequently gets out of 63 sync. The presence of an NS RRset in a zone anywhere other than at 64 the apex indicates a zone cut or delegation. The RDATA of the NS 65 RRset specifies the authoritative servers for the delegated or 66 "child" zone. Based on actual measurements, 10-30% of all delegations 67 on the Internet have differing NS RRsets at parent and child. There 68 are a number of reasons for this, including a lack of communication 69 between parent and child and bogus name servers being listed to meet 70 registry requirements. 72 DNSSEC [RFC2535,RFC3008,RFC3090] specifies that a child zone needs to 73 have its KEY RRset signed by its parent to create a verifiable chain 74 of KEYs. There has been some debate on where the signed KEY RRset 75 should reside, whether at the child [RFC2535] or at the parent. If 76 the KEY RRset resides at the child, maintaining the signed KEY RRset 77 in the child requires frequent two-way communication between the two 78 parties. First the child transmits the KEY RRset to the parent and 79 then the parent sends the signature(s) to the child. Storing the KEY 80 RRset at the parent was thought to simplify the communication. 82 DNSSEC [RFC2535] requires that the parent store a NULL KEY record for 83 an unsecure child zone to indicate that the child is unsecure. A NULL 84 KEY record is a waste: an entire signed RRset is used to communicate 85 effectively one bit of information--that the child is unsecure. 86 Chasing down NULL KEY RRsets complicates the resolution process in 87 many cases, because servers for both parent and child need to be 88 queried for the KEY RRset if the child server does not return it. 89 Storing the KEY RRset only in the parent zone simplifies this and 90 would allow the elimination of the NULL KEY RRsets entirely. For 91 large delegation zones the cost of NULL keys is a significant barrier 92 to deployment. 94 Another complication of the DNSSEC key model is that the KEY record 95 can be used to store public keys for other protocols in addition to 96 DNSSEC keys. There are number of potential problems with this, 97 including: 98 1. The KEY RRset can become quite large if many applications and 99 protocols store their keys at the zone apex. Possible protocols 100 are IPSEC, HTTP, SMTP, SSH and others that use public key 101 cryptography. 102 2. The KEY RRset may require frequent updates. 103 3. The probability of compromised or lost keys, which trigger 104 emergency key rollover procedures, increases. 105 4. The parent may refuse sign KEY RRsets with non-DNSSEC zone keys. 106 5. The parent may not meet the child's expectations in turnaround 107 time for resigning the KEY RRset. 109 Given these reasons, SIG@parent isn't any better than SIG/KEY@Child. 111 1.2 Reserved Words 113 The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED", 114 "RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be 115 interpreted as described in RFC2119. 117 2 Specification of the Delegation key Signer 119 This section defines the Delegation Signer (DS) RR type and the 120 changes to DNS to accommodate it. 122 2.1 Delegation Signer Record Model 124 This document presents a replacement for the DNSSEC KEY record chain 125 of trust [RFC2535] that uses a new RR that resides only at the 126 parent. This record identifies the key(s) that the child uses to 127 self-sign its own KEY RRset. 129 The chain of trust is now established by verifying the parent KEY 130 RRset, the DS RRset from the parent and the KEY RRset at the child. 131 This is cryptographically equivalent to using just KEY records. 133 Communication between the parent and child is greatly reduced, since 134 the child only needs to notify the parent about changes in keys that 135 sign its apex KEY RRset. The parent is ignorant of all other keys in 136 the child's apex KEY RRset. Furthermore, the child maintains full 137 control over the apex KEY RRset and its content. The child can 138 maintain any policies regarding its KEY usage for DNSSEC and other 139 applications and protocols with minimal impact on the parent. Thus if 140 the child wants to have frequent key rollover for its DNS zone keys, 141 the parent does not need to be aware of it: the child can use one key 142 to sign only its apex KEY RRset and other keys to sign the other 143 RRsets in the zone. 145 This model fits well with a slow roll out of DNSSEC and the islands 146 of security model. In this model, someone who trusts "good.example." 147 can preconfigure a key from "good.example." as a trusted key, and 148 from then on trusts any data signed by that key or that has a chain 149 of trust to that key. If "example." starts advertising DS records, 150 "good.example." does not have to change operations by suspending 151 self-signing. DS records can also be used to identify trusted keys 152 instead of KEY records. Another significant advantage is that the 153 amount of information stored in large delegation zones is reduced: 154 rather than the NULL KEY record at every unsecure delegation required 155 by RFC 2535, only secure delegations require additional information 156 in the form of a signed DS RRset. 158 The main disadvantage of this approach is that verifying a zone's KEY 159 RRset requires two signature verification operations instead of the 160 one required by RFC 2535. There is no impact on the number of 161 signatures verified for other types of RRsets. 163 Even though DS identifies two roles for KEY's, Key Signing Key (KSK) 164 and Zone Sigining Key (ZSK), there is no requirement that zone use 165 two different keys for these roles. It is expected that many small 166 zones will only use one key, while larger organizations will be more 167 likely to use multiple keys. 169 2.2 Protocol Change 171 All DNS servers and resolvers that support DS MUST support the OK bit 172 [RFC3225] and a larger message size [RFC3226]. In order for a 173 delegation to be considered secure the delegation MUST contain a DS 174 RRset. If a query contains the OK bit, a server returning a referral 175 for the delegation MUST include the following RRsets in the authority 176 section in this order: 177 If DS RRset is present: 178 parent NS RRset 179 DS and SIG(DS) 180 If no DS RRset is present: 181 parent NS RRset 182 parent NXT and SIG(NXT) 184 This increases the size of referral messages and may cause some or 185 all glue to be omitted. If the DS or NXT RRsets with signatures do 186 not fit in the DNS message, the TC bit MUST be set. Additional 187 section processing is not changed. 189 A DS RRset accompanying a NS RRset indicates that the child zone is 190 secure. If a NS RRset exists without a DS RRset, the child zone is 191 unsecure (from the parents point of view). DS RRsets MUST NOT appear 192 at non-delegation points or at a zone's apex. 194 Section 2.2.1 defines special considerations related to authoritative 195 servers responding to DS queries and replaces RFC2535 sections 2.3.4 196 and 3.4. Section 2.2.2 replaces RFC3008 section 2.7, and section 197 2.2.3 updates RFC3090. 199 2.2.1 RFC2535 2.3.4 and 3.4: Special Considerations at Delegation Points 201 DNS security views each zone as a unit of data completely under the 202 control of the zone owner with each entry (RRset) signed by a special 203 private key held by the zone manager. But the DNS protocol views the 204 leaf nodes in a zone that are also the apex nodes of a child zone 205 (i.e., delegation points) as "really" belonging to the child zone. 206 The corresponding domain names appear in two master files and might 207 have RRsets signed by both the parent and child zones' keys. A 208 retrieval could get a mixture of these RRsets and SIGs, especially 209 since one server could be serving both the zone above and below a 210 delegation point [RFC 2181]. 212 Each DS RRset stored in the parent zone MUST be signed by, at least, 213 one of the parent zone's private key. The parent zone MUST NOT 214 contain a KEY RRset at any delegation point. Delegations in the 215 parent MAY contain only the following RR types: NS, DS, NXT and SIG. 216 The NS RRset MUST NOT be signed. The NXT RRset is the exceptional 217 case: it will always appear differently and authoritatively in both 218 the parent and child zones if both are secure. 220 A secure zone MUST contain a self-signed KEY RRset at its apex. Upon 221 verifying the DS RRset from the parent, a resolver MAY trust any KEY 222 identified in the DS RRset as a valid signer of the child's apex KEY 223 RRset. Resolvers configured to trust one of the keys signing the KEY 224 RRset MAY now treat any data signed by the zone keys in the KEY RRset 225 as secure. In all other cases resolvers MUST consider the zone 226 unsecure. A DS RRset MUST NOT appear at a zone's apex. 228 An authoritative server queried for type DS MUST return the DS RRset 229 in the answer section. 231 2.2.1.1 Special processing for DS queries 233 When a server is authoritative for the parent zone at a delegation 234 point and receives a query for the DS record at that name, it will 235 return the DS from the parent zone. This is true whether or not it 236 is also authoritative for the child zone. 238 When the server is authoritative for the child zone at a delegation 239 point but not the parent zone, there is no natural response, since 240 the child zone is not authoritative for the DS record at the zone's 241 apex. As these queries are only expected to originate from recursive 242 servers which are not DS-aware, the authoritative server MUST answer 243 with: 244 RCODE: NOERROR 245 AA bit: set 246 Answer Section: Empty 247 Authority Section: SOA [+ SIG(SOA) + NXT + SIG(NXT)] 249 That is, it answers as if it is authoritative and the DS record does 250 not exist. DS-aware recursive servers will query the parent zone at 251 delegation points, so will not be affected by this. 253 A server authoritative for only the child zone at a delegation point 254 that is also a caching server MAY (if the RD bit is set in the query) 255 perform recursion to find the DS record at the delegation point, and 256 may return the DS record from its cache. In this case, the AA bit 257 MUST not be set in the response. 259 2.2.1.2 Special processing when child and an ancestor share server 261 When a child zone and a ancestor other than parent share an 262 authorative server, a DS aware server MUST answer with information 263 from child zone, as specified in section 2.2.1.1. This is to prevent 264 the server to be marked as lame for child. 266 This answer can cause problem for a DS aware resolver that is 267 traversing this branch of the DNS tree for the first time. The 268 resolver is expecting to get back either DS record or a delegation 269 information. The SOA with same name as QNAME informs the resolver 270 that the answer orignated from the zone below the one where the DS 271 resides. At this point the resolver has no information on how to get 272 from the ancestor to the parent. In this case the resolver SHOULD 273 attempt to fetch the delegation information by issuing a query with a 274 QNAME one label shorter and type NS. This will yield the NS set for 275 the parent, allowing the resolver to query for the DS record. 277 2.2.2 Signer's Name (replaces RFC3008 section 2.7) 279 The signer's name field of a SIG RR MUST contain the name of the zone 280 to which the data and signature belong. The combination of signer's 281 name, key tag, and algorithm MUST identify a zone key if the SIG is 282 to be considered material. This document defines a standard policy 283 for DNSSEC validation; local policy may override the standard policy. 285 There are no restrictions on the signer field of a SIG(0) record. 286 The combination of signer's name, key tag, and algorithm MUST 287 identify a key if this SIG(0) is to be processed. 289 2.2.3 Changes to RFC3090 291 A number of sections of RFC3090 need to be updated to reflect the DS 292 record. 294 2.2.3.1 RFC3090: Updates to section 1: Introduction 296 Most of the text is still relevant but the words ``NULL key'' are to 297 be replaced with ``missing DS RRset''. In section 1.3 the last three 298 paragraphs discuss the confusion in sections of RFC 2535 that are 299 replaced in section 2.2.1 above. Therefore, these paragraphs are now 300 obsolete. 302 2.2.3.2 RFC3090 section 2.1: Globally Secured 304 Rule 2.1.b is replaced by the following rule: 306 2.1.b. The KEY RRset at a zone's apex MUST be self-signed by a 307 private key whose public counterpart MUST appear in a zone signing 308 KEY RR (2.a) owned by the zone's apex and specifying a mandatory-to- 309 implement algorithm. This KEY RR MUST be identified by a DS RR in a 310 signed DS RRset in the parent zone. 312 If a zone cannot get its parent to advertise a DS record for it, the 313 child zone cannot be considered globally secured. The only exception 314 to this is the root zone, for which there is no parent zone. 316 2.2.3.3 RFC3090 section 3: Experimental Status. 318 The only difference between experimental status and globally secured 319 is the missing DS RRset in the parent zone. All locally secured zones 320 are experimental. 322 2.3 Comments on Protocol Changes 324 Over the years there have been various discussions surrounding the 325 DNS delegation model, declaring it to be broken because there is no 326 good way to assert if a delegation exists. In the RFC2535 version of 327 DNSSEC, the presence of the NS bit in the NXT bit map proves there is 328 a delegation at this name. Something more explicit is needed and the 329 DS record addresses this need for secure delegations. 331 The DS record is a major change to DNS: it is the first resource 332 record that can appear only on the upper side of a delegation. Adding 333 it will cause interoperabilty problems and requires a flag day for 334 DNSSEC. Many old servers and resolvers MUST be upgraded to take 335 advantage of DS. Some old servers will be able to be authoritative 336 for zones with DS records but will not add the NXT or DS records to 337 the authority section. The same is true for caching servers; in 338 fact, some may even refuse to pass on the DS or NXT records. 340 2.4 Wire Format of the DS record 342 The DS (type=TDB) record contains these fields: key tag, algorithm, 343 digest type, and the digest of a public key KEY record that is 344 allowed and/or used to sign the child's apex KEY RRset. Other keys 345 MAY sign the child's apex KEY RRset. 346 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 347 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 348 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 349 | key tag | algorithm | Digest type | 350 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 351 | digest (length depends on type) | 352 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 353 | (SHA-1 digest is 20 bytes) | 354 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 355 | | 356 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 357 | | 358 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 359 | | 360 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 362 The key tag is calculated as specified in RFC2535. Algorithm MUST be 363 an algorithm number assigned in the range 1..251 and the algorithm 364 MUST be allowed to sign DNS data. The digest type is an identifier 365 for the digest algorithm used. The digest is calculated over the 366 canonical name of the delegated domain name followed by the whole 367 RDATA of the KEY record (all four fields). 369 digest = hash( canonical FQDN on KEY RR | KEY_RR_rdata) 371 KEY_RR_rdata = Flags | Protocol | Algorithm | Public Key 373 Digest type value 0 is reserved, value 1 is SHA-1, and reserving 374 other types requires IETF standards action. For interoperabilty 375 reasons, as few digest algorithms as possible should be reserved. The 376 only reason to reserve additional digest types is to increase 377 security. 379 DS records MUST point to zone KEY records that are allowed to 380 authenticate DNS data. The indicated KEY record's protocol field 381 MUST be set to 3; flag field bits 0 and 6 MUST be set to 0; bit 7 382 MUST be set to 1. The value of other bits is not significant for the 383 purposes of this document. 385 The size of the DS RDATA for type 1 (SHA-1) is 24 bytes, regardless 386 of key size, new digest types probably will have larger digests. 388 2.4.1 Justifications for Fields 390 The algorithm and key tag fields are present to allow resolvers to 391 quickly identify the candidate KEY records to examine. SHA-1 is a 392 strong cryptographic checksum: it is computationally infeasible for 393 an attacker to generate a KEY record that has the same SHA-1 digest. 394 Combining the name of the key and the key rdata as input to the 395 digest provides stronger assurance of the binding. Having the key 396 tag in the DS record adds greater assurance than the SHA-1 digest 397 alone, as there are now two different mapping functions that a KEY RR 398 must match. 400 This format allows concise representation of the keys that the child 401 will use, thus keeping down the size of the answer for the 402 delegation, reducing the probability of DNS message overflow. The 403 SHA-1 hash is strong enough to uniquely identify the key and is 404 similar to the PGP key footprint. The digest type field is present 405 for possible future expansion. 407 The DS record is well suited to listing trusted keys for islands of 408 security in configuration files. 410 2.5 Presentation Format of the DS Record 412 The presentation format of the DS record consists of three numbers 413 (key tag, algorithm and digest type) followed by the digest itself 414 presented in hex: 415 example. DS 12345 3 1 123456789abcdef67890123456789abcdef67890 417 2.6 Transition Issues for Installed Base 419 No backwards compatibility with RFC2535 is provided. 421 RFC2535-compliant resolvers will assume that all DS-secured 422 delegations are locally secure. This is bad, but the DNSEXT Working 423 Group has determined that rather than dealing with both 424 RFC2535-secured zones and DS-secured zones, a rapid adoption of DS is 425 preferable. Thus the only option for early adopters is to upgrade to 426 DS as soon as possible. 428 2.6.1 Backwards compatibility with RFC2535 and RFC1035 430 This section documents how a resolver determines the type of 431 delegation. 432 RFC1035 delegation (in parent) has: 434 RFC1035 NS 436 RFC2535 adds the following two cases: 438 Secure RFC2535: NS + NXT + SIG(NXT) 439 NXT bit map contains: NS SIG NXT 440 Unsecure RFC2535: NS + KEY + SIG(KEY) + NXT + SIG(NXT) 441 NXT bit map contains: NS SIG KEY NXT 442 KEY must be a NULL key. 444 DNSSEC with DS has the following two states: 446 Secure DS: NS + DS + SIG(DS) 447 NXT bit map contains: NS SIG NXT DS 448 Unsecure DS: NS + NXT + SIG(NXT) 449 NXT bit map contains: NS SIG NXT 451 It is difficult for a resolver to determine if a delegation is secure 452 RFC 2535 or unsecure DS. This could be overcome by adding a flag to 453 the NXT bit map, but only upgraded resolvers would understand this 454 flag, anyway. Having both parent and child signatures for a KEY RRset 455 might allow old resolvers to accept a zone as secure, but the cost of 456 doing this for a long time is much higher than just prohibiting RFC 457 2535-style signatures at child zone apexes and forcing rapid 458 deployment of DS-enabled servers and resolvers. 460 RFC 2535 and DS can in theory be deployed in parallel, but this would 461 require resolvers to deal with RFC 2535 configurations forever. This 462 document obsoletes the NULL KEY in parent zones, which is a difficult 463 enough change that a flag day is required. 465 2.7 KEY and corresponding DS record example 467 This is a example of a KEY record and corresponding DS record. 469 dskey.example. KEY 256 3 1 ( 470 AQPwHb4UL1U9RHaU8qP+Ts5bVOU1s7fYbj2b3CCbzNdj 471 4+/ECd18yKiyUQqKqQFWW5T3iVc8SJOKnueJHt/Jb/wt 472 ) ; key id = 28668 473 DS 28668 1 1 49FD46E6C4B45C55D4AC69CBD3CD34AC1AFE51DE 475 3 Resolver 477 3.1 DS Example 479 To create a chain of trust, a resolver goes from trusted KEY to DS to 480 KEY. 482 Assume the key for domain "example." is trusted. Zone "example." 483 contains at least the following records: 484 example. SOA 485 example. NS ns.example. 486 example. KEY 487 example. NXT NS SOA KEY SIG NXT secure.example. 488 example. SIG(SOA) 489 example. SIG(NS) 490 example. SIG(NXT) 491 example. SIG(KEY) 492 secure.example. NS ns1.secure.example. 493 secure.example. DS tag=12345 alg=3 digest_type=1 494 secure.example. NXT NS SIG NXT DS unsecure.example. 495 secure.example. SIG(NXT) 496 secure.example. SIG(DS) 497 unsecure.example NS ns1.unsecure.example. 498 unsecure.example. NXT NS SIG NXT example. 499 unsecure.example. SIG(NXT) 501 In zone "secure.example." following records exist: 502 secure.example. SOA 503 secure.example. NS ns1.secure.example. 504 secure.example. KEY 505 secure.example. KEY 506 secure.example. NXT 507 secure.example. SIG(KEY) 508 secure.example. SIG(SOA) 509 secure.example. SIG(NS) 510 secure.example. SIG(NXT) 512 In this example the private key for "example." signs the DS record 513 for "secure.example.", making that a secure delegation. The DS record 514 states which key is expected to sign the KEY RRset at 515 "secure.example.". Here "secure.example." signs its KEY RRset with 516 the KEY identified in the DS RRset, thus the KEY RRset is validated 517 and trusted. 519 This example has only one DS record for the child, but parents MUST 520 allow multiple DS records to facilitate key rollover and multiple KEY 521 algorithms. 523 The resolver determines the security status of "unsecure.example." by 524 examining the parent zone's NXT record for this name. The absence of 525 the DS bit indicates an unsecure delegation. Note the NXT record 526 SHOULD only be examined after verifying the corresponding signature. 528 3.1 Resolver Cost Estimates for DS Records 530 From a RFC2535 resolver point of view, for each delegation followed 531 to chase down an answer, one KEY RRset has to be verified. 532 Additional RRsets might also need to be verified based on local 533 policy (e.g., the contents of the NS RRset). Once the resolver gets 534 to the appropriate delegation, validating the answer might require 535 verifying one or more signatures. A simple A record lookup requires 536 at least N delegations to be verified and one RRset. For a DS-enabled 537 resolver, the cost is 2N+1. For an MX record, where the target of 538 the MX record is in the same zone as the MX record, the costs are N+2 539 and 2N+2, for RFC 2535 and DS, respectively. In the case of negatives 540 answer the same ratios hold true. 542 The resolver may require an extra query to get the DS record and this 543 may add to the overall cost of the query, but this is never worse 544 than chasing down NULL KEY records from the parent in RFC2535 DNSSEC. 546 DS adds processing overhead on resolvers and increases the size of 547 delegation answers, but much less than storing signatures in the 548 parent zone. 550 4 Security Considerations: 552 This document proposes a change to the validation chain of KEY 553 records in DNSSEC. The change is not believed to reduce security in 554 the overall system. In RFC2535 DNSSEC, the child zone has to 555 communicate keys to its parent and prudent parents will require some 556 authentication with that transaction. The modified protocol will 557 require the same authentication, but allows the child to exert more 558 local control over its own KEY RRset. 560 There is a remote possibility that an attacker could generate a valid 561 KEY that matches all the DS fields, of a specific DS set, and thus 562 forge data from the child. This possibility is considered 563 impractical, as on average more than 564 2 ^ (160 - ) 565 keys would have to be generated before a match would be found. 567 An attacker that wants to match any DS record will have to generate 568 on average at least 2^80 keys. 570 The DS record represents a change to the DNSSEC protocol and there is 571 an installed base of implementations, as well as textbooks on how to 572 set up secure delegations. Implementations that do not understand the 573 DS record will not be able to follow the KEY to DS to KEY chain and 574 will consider all zones secured that way as unsecure. 576 5 IANA Considerations: 578 IANA needs to allocate an RR type code for DS from the standard RR 579 type space (type 43 requested). 581 IANA needs to open a new registry for the DS RR type for digest 582 algorithms. Defined types are: 583 0 is Reserved, 584 1 is SHA-1. 585 Adding new reservations requires IETF standards action. 587 6 Acknowledgments 589 Over the last few years a number of people have contributed ideas 590 that are captured in this document. The core idea of using one key to 591 sign only the KEY RRset comes from discussions with Bill Manning and 592 Perry Metzger on how to put in a single root key in all resolvers. 593 Alexis Yushin, Brian Wellington, Paul Vixie, Jakob Schlyter, Scott 594 Rose, Edward Lewis, Lars-Johan Liman, Matt Larson, Mark Kosters, Dan 595 Massey, Olaf Kolman, Phillip Hallam-Baker, Miek Gieben, Havard 596 Eidnes, Donald Eastlake 3rd., Randy Bush, David Blacka, Steve 597 Bellovin, Rob Austein, Derek Atkins, Roy Arends, Mark Andrews, Harald 598 Alvestrand, and others have provided useful comments. 600 Normative References: 602 [RFC1035] P. Mockapetris, ``Domain Names - Implementation and 603 Specification'', STD 13, RFC 1035, November 1987. 605 [RFC2181] R. Elz, R. Bush, ``Clarifications to the DNS Specification'', 606 RFC 2181, July 1997. 608 [RFC2535] D. Eastlake, ``Domain Name System Security Extensions'', RFC 609 2535, March 1999. 611 [RFC3008] B. Wellington, ``Domain Name System Security (DNSSEC) Signing 612 Authority'', RFC 3008, November 2000. 614 [RFC3090] E. Lewis `` DNS Security Extension Clarification on Zone 615 Status'', RFC 3090, March 2001. 617 [RFC3225] D. Conrad, ``Indicating Resolver Support of DNSSEC'', RFC 618 3225, December 2001. 620 [RFC3226] O. Gudmundsson, ``DNSSEC and IPv6 A6 aware server/resolver 621 message size requirements'', RFC 3226, December 2001. 623 Author Address 625 Olafur Gudmundsson 626 PO Box 6306 627 Washington, DC, 20015 628 USA 629 631 Full Copyright Statement 633 Copyright (C) The Internet Society (2002). 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