idnits 2.17.1 draft-ietf-dnsext-delegation-signer-13.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. Boilerplate error? == No 'Intended status' indicated for this document; assuming Proposed Standard Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. RFC 2119 keyword, line 171: '... that support DS MUST support the OK b...' RFC 2119 keyword, line 173: '...ecure the delegation MUST contain a DS...' RFC 2119 keyword, line 175: '...r the delegation MUST include the foll...' RFC 2119 keyword, line 186: '...sage, the TC bit MUST be set. Additio...' RFC 2119 keyword, line 191: '...t of view). DS RRsets MUST NOT appear...' (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 (February 2003) is 7713 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: 'RFC1035' is mentioned on line 682, but not defined == Missing Reference: 'RFC2535' is mentioned on line 685, but not defined ** Obsolete undefined reference: RFC 2535 (Obsoleted by RFC 4033, RFC 4034, RFC 4035) == Missing Reference: 'RFC3090' is mentioned on line 691, but not defined ** Obsolete undefined reference: RFC 3090 (Obsoleted by RFC 4033, RFC 4034, RFC 4035) == Missing Reference: 'RFC3008' is mentioned on line 688, but not defined ** Obsolete undefined reference: RFC 3008 (Obsoleted by RFC 4033, RFC 4034, RFC 4035) == Missing Reference: 'RFC 2181' is mentioned on line 210, but not defined == Missing Reference: 'RFC3445' is mentioned on line 694, but not defined ** Obsolete undefined reference: RFC 3445 (Obsoleted by RFC 4033, RFC 4034, RFC 4035) == Unused Reference: 'RFC2181' is defined on line 699, but no explicit reference was found in the text Summary: 7 errors (**), 0 flaws (~~), 12 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DNSEXT Working Group Olafur Gudmundsson 3 INTERNET-DRAFT February 2003 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 August 28, 2003. 36 Copyright Notice 38 Copyright (C) The Internet Society (2003). 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 Signing Key (ZSK), there is no requirement that zone use two 165 different keys for these roles. It is expected that many small zones 166 will only use one key, while larger organizations will be more likely 167 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 causing some or all 185 glue to be omitted. If the DS or NXT RRsets with signatures do not 186 fit in the DNS message, the TC bit MUST be set. Additional section 187 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 Special rules are needed to permit DS RR aware servers to gracefully 262 interact with older caches which otherwise might falsely label a 263 server as lame because of the new placement of the DS RR set. 265 Such a situation might arise when a server is authoritative for both 266 a zone and it's grandparent, but not the parent. This sounds like an 267 obscure example, but it is very real. The root zone is currently 268 served on 13 machines, and "root-servers.net." is served on 4 of the 269 same 13, but "net." is served elsewhere. 271 When a server receives a query for (, DS, IN), the response 272 MUST be determined from reading these rules in order: 274 1) If the server is authoritative for the zone that holds the DS RR 275 set (i.e., the zone that delegates away, aka the "parent" 276 zone), the response contains the DS RR set as an authoritative 277 answer. 279 2) If the server is offering recursive service and the RD bit is set 280 in the query, the server performs the query itself (according to the 281 rules for resolvers described below) and returns it's findings. 283 3) If the server is authoritative for the zone that holds the 284 's SOA RR set, the response is an authoritative negative 285 answer as described in 2.2.1.1. 287 4) If the server is authoritative for a zone or zones above the 288 QNAME, a referral to the most enclosing zone's servers is made. 290 5) If the server is not authoritative for any part of the QNAME, a 291 response indicating a lame server for QNAME is given. 293 Using these rules will require some special processing on the part of 294 a DS RR aware resolver. To illustrate this, an example is used. 296 Assuming a server is authoritative for roots.example.net. and for the 297 root zone but not the intervening two zones (or the intervening two 298 label deep zone). Assume that QNAME=roots.example.net., QTYPE=DS, 299 and QCLASS=IN. 301 The resolver will issue this request (assuming no cached data) 302 expecting a referral to a net. server. Instead, rule number 3 above 303 applies and a negative answer is returned by the server. The 304 reaction by the resolver is not to accept this answer as final as it 305 can determine from the SOA RR in the negative answer the context 306 within which the server has answered. 308 A solution to this is to instruct the resolver to hunt for the 309 authoritative zone of the data in a brute force manner. 311 This can be accomplished by taking the owner name of the returned SOA 312 RR and strip off enough left-hand labels until a successful NS 313 response is obtained. A successful response here means that the 314 answer has NS records in it. (Entertaining the possibility that a 315 cut point may be two labels down in a zone.) 317 Returning to the example, the response will include a negative answer 318 with either the SOA RR for "roots.example.net." or "example.net." 319 depending on whether roots.example.net is a delegated domain. In 320 either case, removing the least significant label of the SOA owner 321 name will lead to the location of the desired data. 323 2.2.1.3 Modification on KEY RR in the construction of Responses 325 This section updates RFC2535 section 3.5 by replacing it with the 326 following: 328 An query for KEY RR MUST NOT trigger any additional section 329 processing. Security aware resolver will include corresponding SIG 330 records in the answer section. 332 KEY records SHOULD NOT be added to additional records section in 333 response to any query. 335 RFC2535 included rules to in add KEY records to additional section 336 when SOA or NS records where included in an answer. The is was done 337 to reduce round trips (in the case of SOA) and to force out NULL 338 KEY's (in the NS case), as this document obsoletes NULL keys there is 339 no need for the second case, the first case causes redundant 340 transfers of KEY RRset as SOA is included in the authority section of 341 negative answers. 343 RFC2535 section 3.5 also included rule for adding KEY RRset to query 344 for A and AAAA, as Restrict KEY[RFC3445] eliminated use of KEY RR by 345 all applications therfore the rule is not needed anymore. 347 2.2.2 Signer's Name (replaces RFC3008 section 2.7) 349 The signer's name field of a SIG RR MUST contain the name of the zone 350 to which the data and signature belong. The combination of signer's 351 name, key tag, and algorithm MUST identify a zone key if the SIG is 352 to be considered material. This document defines a standard policy 353 for DNSSEC validation; local policy may override the standard policy. 355 There are no restrictions on the signer field of a SIG(0) record. 356 The combination of signer's name, key tag, and algorithm MUST 357 identify a key if this SIG(0) is to be processed. 359 2.2.3 Changes to RFC3090 361 A number of sections of RFC3090 need to be updated to reflect the DS 362 record. 364 2.2.3.1 RFC3090: Updates to section 1: Introduction 366 Most of the text is still relevant but the words ``NULL key'' are to 367 be replaced with ``missing DS RRset''. In section 1.3 the last three 368 paragraphs discuss the confusion in sections of RFC 2535 that are 369 replaced in section 2.2.1 above. Therefore, these paragraphs are now 370 obsolete. 372 2.2.3.2 RFC3090 section 2.1: Globally Secured 374 Rule 2.1.b is replaced by the following rule: 376 2.1.b. The KEY RRset at a zone's apex MUST be self-signed by a 377 private key whose public counterpart MUST appear in a zone signing 378 KEY RR (2.a) owned by the zone's apex and specifying a mandatory-to- 379 implement algorithm. This KEY RR MUST be identified by a DS RR in a 380 signed DS RRset in the parent zone. 382 If a zone cannot get its parent to advertise a DS record for it, the 383 child zone cannot be considered globally secured. The only exception 384 to this is the root zone, for which there is no parent zone. 386 2.2.3.3 RFC3090 section 3: Experimental Status. 388 The only difference between experimental status and globally secured 389 is the missing DS RRset in the parent zone. All locally secured zones 390 are experimental. 392 2.2.4 NULL KEY elimination 394 RFC3445 section 3 elminates the top two bits in the flags field of 395 KEY RR. These two bits where used to indicate NULL KEY or NO KEY. 396 RFC3090 defines that zone that defines that zone is either secure or 397 not, eliminates the possible need to put NULL keys in the zone apex 398 to indicate that the zone is not secured for a algorithm. Along with 399 this document these other two elminate all uses for the NULL KEY, 400 Thus this document obsoletes NULL KEY. 402 2.3 Comments on Protocol Changes 404 Over the years there have been various discussions surrounding the 405 DNS delegation model, declaring it to be broken because there is no 406 good way to assert if a delegation exists. In the RFC2535 version of 407 DNSSEC, the presence of the NS bit in the NXT bit map proves there is 408 a delegation at this name. Something more explicit is needed and the 409 DS record addresses this need for secure delegations. 411 The DS record is a major change to DNS: it is the first resource 412 record that can appear only on the upper side of a delegation. Adding 413 it will cause interoperabilty problems and requires a flag day for 414 DNSSEC. Many old servers and resolvers MUST be upgraded to take 415 advantage of DS. Some old servers will be able to be authoritative 416 for zones with DS records but will not add the NXT or DS records to 417 the authority section. The same is true for caching servers; in 418 fact, some may even refuse to pass on the DS or NXT records. 420 2.4 Wire Format of the DS record 422 The DS (type=TDB) record contains these fields: key tag, algorithm, 423 digest type, and the digest of a public key KEY record that is 424 allowed and/or used to sign the child's apex KEY RRset. Other keys 425 MAY sign the child's apex KEY RRset. 427 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 428 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 1 429 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 430 | key tag | algorithm | Digest type | 431 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 432 | digest (length depends on type) | 433 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 434 | (SHA-1 digest is 20 bytes) | 435 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 436 | | 437 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 438 | | 439 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 440 | | 441 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 443 The key tag is calculated as specified in RFC2535. Algorithm MUST be 444 an algorithm number assigned in the range 1..251 and the algorithm 445 MUST be allowed to sign DNS data. The digest type is an identifier 446 for the digest algorithm used. The digest is calculated over the 447 canonical name of the delegated domain name followed by the whole 448 RDATA of the KEY record (all four fields). 450 digest = hash( canonical FQDN on KEY RR | KEY_RR_rdata) 452 KEY_RR_rdata = Flags | Protocol | Algorithm | Public Key 454 Digest type value 0 is reserved, value 1 is SHA-1, and reserving 455 other types requires IETF standards action. For interoperabilty 456 reasons, as few digest algorithms as possible should be reserved. The 457 only reason to reserve additional digest types is to increase 458 security. 460 DS records MUST point to zone KEY records that are allowed to 461 authenticate DNS data. The indicated KEY record's protocol field 462 MUST be set to 3; flag field bit 7 MUST be set to 1. The value of 463 other flag bits is not significant for the purposes of this document. 465 The size of the DS RDATA for type 1 (SHA-1) is 24 bytes, regardless 466 of key size, new digest types probably will have larger digests. 468 2.4.1 Justifications for Fields 470 The algorithm and key tag fields are present to allow resolvers to 471 quickly identify the candidate KEY records to examine. SHA-1 is a 472 strong cryptographic checksum: it is computationally infeasible for 473 an attacker to generate a KEY record that has the same SHA-1 digest. 474 Combining the name of the key and the key rdata as input to the 475 digest provides stronger assurance of the binding. Having the key 476 tag in the DS record adds greater assurance than the SHA-1 digest 477 alone, as there are now two different mapping functions that a KEY RR 478 must match. 480 This format allows concise representation of the keys that the child 481 will use, thus keeping down the size of the answer for the 482 delegation, reducing the probability of DNS message overflow. The 483 SHA-1 hash is strong enough to uniquely identify the key and is 484 similar to the PGP key footprint. The digest type field is present 485 for possible future expansion. 487 The DS record is well suited to listing trusted keys for islands of 488 security in configuration files. 490 2.5 Presentation Format of the DS Record 492 The presentation format of the DS record consists of three numbers 493 (key tag, algorithm and digest type) followed by the digest itself 494 presented in hex: 495 example. DS 12345 3 1 123456789abcdef67890123456789abcdef67890 497 2.6 Transition Issues for Installed Base 499 No backwards compatibility with RFC2535 is provided. 501 RFC2535-compliant resolvers will assume that all DS-secured 502 delegations are locally secure. This is bad, but the DNSEXT Working 503 Group has determined that rather than dealing with both 504 RFC2535-secured zones and DS-secured zones, a rapid adoption of DS is 505 preferable. Thus the only option for early adopters is to upgrade to 506 DS as soon as possible. 508 2.6.1 Backwards compatibility with RFC2535 and RFC1035 510 This section documents how a resolver determines the type of 511 delegation. 512 RFC1035 delegation (in parent) has: 514 RFC1035 NS 516 RFC2535 adds the following two cases: 518 Secure RFC2535: NS + NXT + SIG(NXT) 519 NXT bit map contains: NS SIG NXT 520 Unsecure RFC2535: NS + KEY + SIG(KEY) + NXT + SIG(NXT) 521 NXT bit map contains: NS SIG KEY NXT 522 KEY must be a NULL key. 524 DNSSEC with DS has the following two states: 526 Secure DS: NS + DS + SIG(DS) 527 NXT bit map contains: NS SIG NXT DS 528 Unsecure DS: NS + NXT + SIG(NXT) 529 NXT bit map contains: NS SIG NXT 531 It is difficult for a resolver to determine if a delegation is secure 532 RFC 2535 or unsecure DS. This could be overcome by adding a flag to 533 the NXT bit map, but only upgraded resolvers would understand this 534 flag, anyway. Having both parent and child signatures for a KEY RRset 535 might allow old resolvers to accept a zone as secure, but the cost of 536 doing this for a long time is much higher than just prohibiting RFC 537 2535-style signatures at child zone apexes and forcing rapid 538 deployment of DS-enabled servers and resolvers. 540 RFC 2535 and DS can in theory be deployed in parallel, but this would 541 require resolvers to deal with RFC 2535 configurations forever. This 542 document obsoletes the NULL KEY in parent zones, which is a difficult 543 enough change that a flag day is required. 545 2.7 KEY and corresponding DS record example 547 This is an example of a KEY record and the corresponding DS record. 549 dskey.example. KEY 256 3 1 ( 550 AQPwHb4UL1U9RHaU8qP+Ts5bVOU1s7fYbj2b3CCbzNdj 551 4+/ECd18yKiyUQqKqQFWW5T3iVc8SJOKnueJHt/Jb/wt 552 ) ; key id = 28668 553 DS 28668 1 1 49FD46E6C4B45C55D4AC69CBD3CD34AC1AFE51DE 555 3 Resolver 557 3.1 DS Example 559 To create a chain of trust, a resolver goes from trusted KEY to DS to 560 KEY. 562 Assume the key for domain "example." is trusted. Zone "example." 563 contains at least the following records: 564 example. SOA 565 example. NS ns.example. 566 example. KEY 567 example. NXT NS SOA KEY SIG NXT secure.example. 568 example. SIG(SOA) 569 example. SIG(NS) 570 example. SIG(NXT) 571 example. SIG(KEY) 572 secure.example. NS ns1.secure.example. 573 secure.example. DS tag=12345 alg=3 digest_type=1 574 secure.example. NXT NS SIG NXT DS unsecure.example. 575 secure.example. SIG(NXT) 576 secure.example. SIG(DS) 577 unsecure.example NS ns1.unsecure.example. 578 unsecure.example. NXT NS SIG NXT example. 579 unsecure.example. SIG(NXT) 581 In zone "secure.example." following records exist: 582 secure.example. SOA 583 secure.example. NS ns1.secure.example. 584 secure.example. KEY 585 secure.example. KEY 586 secure.example. NXT 587 secure.example. SIG(KEY) 588 secure.example. SIG(SOA) 589 secure.example. SIG(NS) 590 secure.example. SIG(NXT) 592 In this example the private key for "example." signs the DS record 593 for "secure.example.", making that a secure delegation. The DS record 594 states which key is expected to sign the KEY RRset at 595 "secure.example.". Here "secure.example." signs its KEY RRset with 596 the KEY identified in the DS RRset, thus the KEY RRset is validated 597 and trusted. 599 This example has only one DS record for the child, but parents MUST 600 allow multiple DS records to facilitate key rollover and multiple KEY 601 algorithms. 603 The resolver determines the security status of "unsecure.example." by 604 examining the parent zone's NXT record for this name. The absence of 605 the DS bit indicates an unsecure delegation. Note the NXT record 606 SHOULD only be examined after verifying the corresponding signature. 608 3.1 Resolver Cost Estimates for DS Records 610 From a RFC2535 resolver point of view, for each delegation followed 611 to chase down an answer, one KEY RRset has to be verified. 612 Additional RRsets might also need to be verified based on local 613 policy (e.g., the contents of the NS RRset). Once the resolver gets 614 to the appropriate delegation, validating the answer might require 615 verifying one or more signatures. A simple A record lookup requires 616 at least N delegations to be verified and one RRset. For a DS-enabled 617 resolver, the cost is 2N+1. For an MX record, where the target of 618 the MX record is in the same zone as the MX record, the costs are N+2 619 and 2N+2, for RFC 2535 and DS, respectively. In the case of negatives 620 answer the same ratios hold true. 622 The resolver may require an extra query to get the DS record and this 623 may add to the overall cost of the query, but this is never worse 624 than chasing down NULL KEY records from the parent in RFC2535 DNSSEC. 626 DS adds processing overhead on resolvers and increases the size of 627 delegation answers, but much less than storing signatures in the 628 parent zone. 630 4 Security Considerations: 632 This document proposes a change to the validation chain of KEY 633 records in DNSSEC. The change is not believed to reduce security in 634 the overall system. In RFC2535 DNSSEC, the child zone has to 635 communicate keys to its parent and prudent parents will require some 636 authentication with that transaction. The modified protocol will 637 require the same authentication, but allows the child to exert more 638 local control over its own KEY RRset. 640 There is a remote possibility that an attacker could generate a valid 641 KEY that matches all the DS fields, of a specific DS set, and thus 642 forge data from the child. This possibility is considered 643 impractical, as on average more than 644 2 ^ (160 - ) 645 keys would have to be generated before a match would be found. 647 An attacker that wants to match any DS record will have to generate 648 on average at least 2^80 keys. 650 The DS record represents a change to the DNSSEC protocol and there is 651 an installed base of implementations, as well as textbooks on how to 652 set up secure delegations. Implementations that do not understand the 653 DS record will not be able to follow the KEY to DS to KEY chain and 654 will consider all zones secured that way as unsecure. 656 5 IANA Considerations: 658 IANA needs to allocate an RR type code for DS from the standard RR 659 type space (type 43 requested). 661 IANA needs to open a new registry for the DS RR type for digest 662 algorithms. Defined types are: 663 0 is Reserved, 664 1 is SHA-1. 665 Adding new reservations requires IETF standards action. 667 6 Acknowledgments 669 Over the last few years a number of people have contributed ideas 670 that are captured in this document. The core idea of using one key to 671 sign only the KEY RRset comes from discussions with Bill Manning and 672 Perry Metzger on how to put in a single root key in all resolvers. 673 Alexis Yushin, Brian Wellington, Paul Vixie, Jakob Schlyter, Scott 674 Rose, Edward Lewis, Lars-Johan Liman, Matt Larson, Mark Kosters, Dan 675 Massey, Olaf Kolman, Phillip Hallam-Baker, Miek Gieben, Havard 676 Eidnes, Donald Eastlake 3rd., Randy Bush, David Blacka, Steve 677 Bellovin, Rob Austein, Derek Atkins, Roy Arends, Mark Andrews, Harald 678 Alvestrand, and others have provided useful comments. 680 Normative References: 682 [RFC1035] P. Mockapetris, ``Domain Names - Implementation and 683 Specification'', STD 13, RFC 1035, November 1987. 685 [RFC2535] D. Eastlake, ``Domain Name System Security Extensions'', RFC 686 2535, March 1999. 688 [RFC3008] B. Wellington, ``Domain Name System Security (DNSSEC) Signing 689 Authority'', RFC 3008, November 2000. 691 [RFC3090] E. Lewis `` DNS Security Extension Clarification on Zone 692 Status'', RFC 3090, March 2001. 694 [RFC3445] D. Massey, S. Rose ``Limiting the scope of the KEY Resource 695 Record (RR)``, RFC 3445, December 2002. 697 Informational References 699 [RFC2181] R. Elz, R. Bush, ``Clarifications to the DNS Specification'', 700 RFC 2181, July 1997. 702 [RFC3225] D. Conrad, ``Indicating Resolver Support of DNSSEC'', RFC 703 3225, December 2001. 705 [RFC3226] O. Gudmundsson, ``DNSSEC and IPv6 A6 aware server/resolver 706 message size requirements'', RFC 3226, December 2001. 708 Author Address 710 Olafur Gudmundsson 711 3821 Village Park Drive 712 Chevy Chase, MD, 20815 713 USA 714 716 Full Copyright Statement 718 Copyright (C) The Internet Society (2003). All Rights Reserved. 720 This document and translations of it may be copied and furnished to 721 others, and derivative works that comment on or otherwise explain it 722 or assist in its implementation may be prepared, copied, published 723 and distributed, in whole or in part, without restriction of any 724 kind, provided that the above copyright notice and this paragraph are 725 included on all such copies and derivative works. However, this 726 document itself may not be modified in any way, such as by removing 727 the copyright notice or references to the Internet Society or other 728 Internet organizations, except as needed for the purpose of 729 developing Internet standards in which case the procedures for 730 copyrights defined in the Internet Standards process must be 731 followed, or as required to translate it into languages other than 732 English. 734 The limited permissions granted above are perpetual and will not be 735 revoked by the Internet Society or its successors or assigns. 737 This document and the information contained herein is provided on an 738 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 739 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 740 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 741 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 742 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."