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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 DNSIND Working Group Paul Vixie (Ed.) (ISC) 2 INTERNET-DRAFT Susan Thomson (Bellcore) 3 Yakov Rekhter (Cisco) 4 Jim Bound (DEC) 5 Amends: RFC 1035 March 1996 7 Dynamic Updates in the Domain Name System (DNS UPDATE) 9 Status of this Memo 11 This document is an Internet-Draft. Internet-Drafts are working 12 documents of the Internet Engineering Task Force (IETF), its areas, 13 and its working groups. Note that other groups may also distribute 14 working documents as Internet-Drafts. 16 Internet-Drafts are draft documents valid for a maximum of six months 17 and may be updated, replaced, or obsoleted by other documents at any 18 time. It is inappropriate to use Internet-Drafts as reference 19 material or to cite them other than as ``work in progress.'' 21 To learn the current status of any Internet-Draft, please check the 22 ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow 23 Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), 24 munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or 25 ftp.isi.edu (US West Coast). 27 Abstract 29 The Domain Name System was originally designed to support queries of 30 a statically configured database. While the data was expected to 31 change, the frequency of those changes was expected to be fairly low, 32 and all updates were made as external edits to a zone's Master File. 34 Using this specification of the UPDATE opcode, it is possible to add 35 or delete RRs or RRsets from a specified zone. Prerequisites are 36 specified separately from update operations, and can specify a 37 dependency upon either the previous existence or nonexistence of an 38 RRset, or the existence of a single RR. 40 UPDATE is atomic, i.e., all prerequisites must be satisfied or else 41 no update operations will take place. There are no data dependent 42 error conditions defined after the prerequisites have been met. 44 1 - Definitions 46 This document intentionally gives more definition to the roles of 47 ``Master,'' ``Slave,'' and ``Primary Master'' servers, and their 48 enumeration in NS RRs, and the SOA MNAME field. In that sense, the 49 following server type definitions can be considered an addendum to 50 [RFC1035], and are intended to be consistent with [NOTIFY]: 52 Slave an authoritative server that uses AXFR or IXFR to 53 retrieve the zone and is named in the zone's NS 54 RRset. 56 Master an authoritative server configured to be the source 57 of AXFR or IXFR data for one or more slave servers. 59 Primary Master master server at the root of the AXFR/IXFR dependency 60 graph. The primary master is named in the zone's SOA 61 MNAME field and optionally by an NS RR. There is by 62 definition only one primary master server per zone. 64 A domain name identifies a node within the domain name space tree 65 structure. Each node has a set (possibly empty) of Resource Records 66 (RRs). All RRs having the same NAME, CLASS and TYPE are called a 67 Resource Record Set (RRset). 69 The pseudocode used in this document is for example purposes only. If 70 it is found to disagree with the text, the text shall be considered 71 authoritative. If the text is found to be ambiguous, the pseudocode can 72 be used to help resolve the ambiguity. 74 1.1 - Comparison Rules 76 1.1.1. Two RRs are considered equal if their NAME, CLASS, TYPE, RDLENGTH 77 and RDATA fields are equal. Note that the time-to-live (TTL) field is 78 explicitly excluded from the comparison. 80 1.1.2. The rules for comparison of character strings in names are 81 specified in [RFC1035 2.3.3]. 83 1.1.3. Wildcarding is disabled. That is, a wildcard (``*'') in an 84 update only matches a wildcard (``*'') in the zone, and vice versa. 86 1.1.4. Aliasing is disabled: A CNAME in the zone matches a CNAME in the 87 update, and will not otherwise be followed. All UPDATE operations are 88 done on the basis of canonical names. 90 1.1.5. The following RR types cannot be appended to an RRset. If the 91 following comparison rules are met, then an attempt to add the new RR 92 will result in the replacement of the previous RR: 94 SOA compare only NAME, CLASS and TYPE -- it is not possible to 95 have more than one SOA per zone, even if any of the data 96 fields differ. 98 WKS compare only NAME, CLASS, TYPE, ADDRESS, and PROTOCOL -- only 99 one WKS RR is possible for this tuple, even if the services 100 masks differ. 102 CNAME compare only NAME, CLASS, and TYPE -- it is not possible to 103 have more than one CNAME RR, even if their data fields differ. 105 1.2 - Glue RRs 107 For the purpose of determining whether a domain name used in the UPDATE 108 protocol is contained within a specified zone, a domain name is ``in'' a 109 zone if it is owned by that zone's domain name. See section 7.19 for 110 details. 112 1.3 - New Assigned Numbers 114 CLASS = NONE (TBD: 254) 115 RCODE = YXDOMAIN (TBD: 6) 116 RCODE = YXRRSET (TBD: 7) 117 RCODE = NXRRSET (TBD: 8) 118 RCODE = NOTAUTH (TBD: 9) 119 RCODE = NOTZONE (TBD: 10?) 120 Opcode = UPDATE (5) 122 2 - Update Message Format 124 The DNS Message Format is defined by [RFC1035 4.1]. Some extensions are 125 necessary (for example, more error codes are possible under UPDATE than 126 under QUERY) and some fields must be overloaded (see description of 127 CLASS fields below). 129 The overall format of an UPDATE message is, following [ibid]: 131 +---------------------+ 132 | Header | 133 +---------------------+ 134 | Zone | specifies the zone to be updated 135 +---------------------+ 136 | Prerequisite | RRs or RRsets which must (not) preexist 137 +---------------------+ 138 | Update | RRs or RRsets to be added or deleted 139 +---------------------+ 140 | Additional Data | additional data 141 +---------------------+ 143 The Header Section specifies that this message is an UPDATE, and 144 describes the size of the other sections. The Zone Section names the 145 zone that is to be updated by this message. The Prerequisite Section 146 specifies the starting invariants (in terms of zone content) required 147 for this update. The Update Section contains the edits to be made, and 148 the Additional Data Section contains data which may be necessary to 149 complete, but is not part of, this update. 151 2.1 - Transport Issues 153 An update transaction may be carried in a UDP datagram, if the request 154 fits, or in a TCP connection (at the discretion of the requestor). When 155 TCP is used, the message is in the format described in [RFC1035 4.2.2]. 157 2.2 - Message Header 159 The header of the DNS Message Format is defined by [RFC 1035 4.1]. Not 160 all opcodes define the same set of flag bits, though as a practical 161 matter most of the bits defined for QUERY (in [ibid]) are identically 162 defined by the other opcodes. UPDATE uses only one flag bit (QR). 164 The DNS Message Format specifies record counts for its four sections 165 (Question, Answer, Authority, and Additional). UPDATE uses the same 166 fields, and the same section formats, but the naming and use of these 167 sections differs as shown in the following modified header, after 168 [RFC1035 4.1.1]: 170 1 1 1 1 1 1 171 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 172 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 173 | ID | 174 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 175 |QR| Opcode | Z | RCODE | 176 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 177 | ZOCOUNT | 178 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 179 | PRCOUNT | 180 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 181 | UPCOUNT | 182 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 183 | ADCOUNT | 184 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 186 These fields are used as follows: 188 ID A 16-bit identifier assigned by the entity that generates any 189 kind of request. This identifier is copied in the 190 corresponding reply and can be used by the requestor to match 191 replies to outstanding requests, or by the server to detect 192 duplicated requests from some requestor. 194 QR A one bit field that specifies whether this message is a 195 request (0), or a response (1). 197 Opcode A four bit field that specifies the kind of request in this 198 message. This value is set by the originator of a request 199 and copied into the response. The Opcode value that 200 identifies an UPDATE message is five (5). 202 Z Reserved for future use. Should be zero (0) in all requests 203 and responses. A non-zero Z field should be ignored by 204 implementations of this specification. 206 RCODE Response code - this four bit field is undefined in requests 207 and set in responses. The values and meanings of this field 208 within responses are as follows: 210 Mneumonic Value Description 211 ------------------------------------------------------------ 212 NOERROR 0 No error condition. 213 FORMERR 1 The name server was unable to interpret 214 the request due to a format error. 215 SERVFAIL 2 The name server encountered an internal 216 failure while processing this request, 217 for example an operating system error 218 or a forwarding timeout. 219 NXDOMAIN 3 Some name that ought to exist, 220 does not exist. 221 NOTIMP 4 The name server does not support 222 the specified Opcode. 223 REFUSED 5 The name server refuses to perform the 224 specified operation for policy or 225 security reasons. 226 YXDOMAIN 6? Some name that ought not to exist, 227 does exist. 228 YXRRSET 7? Some RRset that ought not to exist, 229 does exist. 230 NXRRSET 8? Some RRset that ought to exist, 231 does not exist. 232 NOTAUTH 9? The server is not authoritative for 233 the zone named in the Zone Section. 234 NOTZONE 10? A name used in the Prerequisite or 235 Update Section is not within the 236 zone denoted by the Zone Section. 238 ZOCOUNT The number of RRs in the Zone Section. 240 PRCOUNT The number of RRs in the Prerequisite Section. 242 UPCOUNT The number of RRs in the Update Section. 244 ADCOUNT The number of RRs in the Additional Data Section. 246 2.3 - Zone Section 248 The Zone Section has the same format as that specified in [RFC1035 249 4.1.2], with the fields redefined as follows: 251 1 1 1 1 1 1 252 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 253 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 254 | | 255 / ZNAME / 256 / / 257 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 258 | ZTYPE | 259 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 260 | ZCLASS | 261 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 263 UPDATE uses this section to denote the zone of the records being 264 updated. All records to be updated must be in the same zone, and 265 therefore the Zone Section is allowed to contain exactly one record. 266 The ZNAME is the zone name, the ZTYPE must be SOA, and the ZCLASS is the 267 zone's class. 269 2.4 - Prerequisite Section 271 This section contains a set of RRset prerequisites which must be 272 satisfied at the time the UPDATE packet is received by the primary 273 master server. The format of this section is as specified by [RFC1035 274 4.1.3]. There are five possible sets of semantics that can be expressed 275 here, summarized as follows and then explained below. 277 (1) RRset exists (value independent). At least one RR with a 278 specified NAME and TYPE (in the zone and class specified by the 279 Zone Section) must exist. 281 (2) RRset exists (value dependent). A set of RRs with a specified 282 NAME and TYPE exists and has the same members with the same 283 RDATAs as the RRset specified here in this Section. 285 (3) RRset does not exist. No RRs with a specified NAME and TYPE (in 286 the zone and class denoted by the Zone Section) can exist. 288 (4) Name is in use. At least one RR with a specified NAME (in the 289 zone and class specified by the Zone Section) must exist. Note 290 that this prerequisite is NOT satisfied by empty nonterminals. 292 (5) Name is not in use. No RR of any type is owned by a specified 293 NAME. Note that this prerequisite IS satisfied by empty 294 nonterminals. 296 The syntax of these is as follows: 298 2.4.1 - RRset Exists (Value Independent) 300 At least one RR with a specified NAME and TYPE (in the zone and class 301 specified in the Zone Section) must exist. 303 For this prerequisite, a requestor adds to the section a single RR whose 304 NAME and TYPE are equal to that of the zone RRset whose existence is 305 required. RDLENGTH is zero and RDATA is therefore empty. CLASS must be 306 specified as ANY to differentiate this condition from that of an actual 307 RR whose RDLENGTH is naturally zero (0) (e.g., NULL). TTL is specified 308 as zero (0). 310 2.4.2 - RRset Exists (Value Dependent) 312 A set of RRs with a specified NAME and TYPE exists and has the same 313 members with the same RDATAs as the RRset specified here in this 314 section. While RRset ordering is undefined and therefore not 315 significant to this comparison, the sets be identical in their extent. 317 For this prerequisite, a requestor adds to the section an entire RRset 318 whose preexistence is required. NAME and TYPE are that of the RRset 319 being denoted. CLASS is that of the zone. TTL must be specified as 320 zero (0) and is ignored when comparing RRsets for identity. 322 2.4.3 - RRset Does Not Exist 324 No RRs with a specified NAME and TYPE (in the zone and class denoted by 325 the Zone Section) can exist. 327 For this prerequisite, a requestor adds to the section a single RR whose 328 NAME and TYPE are equal to that of the RRset whose nonexistence is 329 required. The RDLENGTH of this record is zero (0), and RDATA field is 330 therefore empty. CLASS must be specified as NONE in order to 331 distinguish this condition from a valid RR whose RDLENGTH is naturally 332 zero (0) (for example, the NULL RR). TTL must be specified as zero (0). 334 2.4.4 - Name Is In Use 336 Name is in use. At least one RR with a specified NAME (in the zone and 337 class specified by the Zone Section) must exist. Note that this 338 prerequisite is NOT satisfied by empty nonterminals. 340 For this prerequisite, a requestor adds to the section a single RR whose 341 NAME is equal to that of the name whose ownership of an RR is required. 342 RDLENGTH is zero and RDATA is therefore empty. CLASS must be specified 343 as ANY to differentiate this condition from that of an actual RR whose 344 RDLENGTH is naturally zero (0) (e.g., NULL). TYPE must be specified as 345 ANY to differentiate this case from that of an RRset existence test. 346 TTL is specified as zero (0). 348 2.4.5 - Name Is Not In Use 350 Name is not in use. No RR of any type is owned by a specified NAME. 351 Note that this prerequisite IS satisfied by empty nonterminals. 353 For this prerequisite, a requestor adds to the section a single RR whose 354 NAME is equal to that of the name whose nonownership of any RRs is 355 required. RDLENGTH is zero and RDATA is therefore empty. CLASS must be 356 specified as NONE. TYPE must be specified as ANY. TTL must be 357 specified as zero (0). 359 2.5 - Update Section 361 This section contains RRs to be added to or deleted from the zone. The 362 format of this section is as specified by [RFC1035 4.1.3]. There are 363 four possible sets of semantics, summarized below and with details to 364 follow. 366 (1) Add RRs to an RRset. 367 (2) Delete an RRset. 368 (3) Delete all RRsets from a name. 369 (4) Delete an RR from an RRset. 371 The syntax of these is as follows: 373 2.5.1 - Add To An RRset 375 RRs are added to the Update Section whose NAME, TYPE, TTL, RDLENGTH and 376 RDATA are those being added, and CLASS is the same as the zone class. 377 Any duplicate RRs will be silently ignored by the primary master. 379 2.5.2 - Delete An RRset 381 One RR is added to the Update Section whose NAME and TYPE are those of 382 the RRset to be deleted. TTL must be specified as zero (0) and is 383 otherwise not used by the primary master. CLASS must be specified as 384 ANY. RDLENGTH must be zero (0) and RDATA must therefore be empty. If 385 no such RRset exists, then this Update RR will be silently ignored by 386 the primary master. 388 2.5.3 - Delete All RRsets From A Name 390 One RR is added to the Update Section whose NAME is that of the name to 391 be cleansed of RRsets. TYPE must be specified as ANY. TTL must be 392 specified as zero (0) and is otherwise not used by the primary master. 393 CLASS must be specified as ANY. RDLENGTH must be zero (0) and RDATA 394 must therefore be empty. If no such RRsets exist, then this Update RR 395 will be silently ignored by the primary master. 397 2.5.4 - Delete An RR From An RRset 399 RRs to be deleted are added to the Update Section. The NAME, TYPE, 400 RDLENGTH and RDATA must match the RR being deleted. TTL must be 401 specified as zero (0) and will otherwise be ignored by the primary 402 master. CLASS must be specified as NONE to distinguish this from an RR 403 addition. If no such RRs exist, then this Update RR will be silently 404 ignored by the primary master. 406 2.6 - Additional Data Section 408 This section contains RRs which are related to the update itself, or to 409 new RRs being added by the update. For example, out of zone glue (A RRs 410 referred to by new NS RRs) should be presented here. The server can use 411 or ignore out of zone glue, at the discretion of the server implementor. 412 The format of this section is as specified by [RFC1035 4.1.3]. 414 3 - Server Behavior 416 A server, upon receiving an UPDATE request, will signal NOTIMP to the 417 requestor if the UPDATE opcode is not recognized or if it is recognized 418 but has not been implemented. Otherwise, processing continues as 419 follows. 421 3.1 - Process Zone Section 423 3.1.1. The Zone Section is checked to see that there is exactly one RR 424 therein and that the RR's ZTYPE is SOA, else signal FORMERR to the 425 requestor. Next, the ZNAME and ZCLASS are checked to see if the zone so 426 named is one of this server's authority zones, else signal NOTAUTH to 427 the requestor. If the server is a zone slave, the request will be 428 forwarded toward the primary master. 430 3.1.2 - Pseudocode For Zone Section Processing 432 if (zcount != 1 || ztype != SOA) 433 return (FORMERR) 434 if (zone_type(zname, zclass) == SLAVE) 435 return forward() 436 if (zone_type(zname, zclass) == MASTER) 437 return update() 438 return (NOTAUTH) 440 Sections 3.2 through 3.8 describe the primary master's behaviour, 441 whereas Section 6 describes a forwarder's behaviour. 443 3.2 - Process Prerequisite Section 445 Next, the Prerequisite Section is checked to see that all prerequisites 446 are satisfied by the current state of the zone. Using the definitions 447 expressed in Section 1.2, if any RR's NAME is not within the zone 448 specified in the Zone Section, signal NOTZONE to the requestor. 450 3.2.1. For RRs in this section whose CLASS is ANY, test to see that TTL 451 and RDLENGTH are both zero (0), else signal FORMERR to the requestor. 452 If TYPE is ANY, test to see that there is at least one RR in the zone 453 whose NAME is the same as that of the Prerequisite RR, else signal 454 NXDOMAIN to the requestor. If TYPE is not ANY, test to see that there 455 is at least one RR in the zone whose NAME and TYPE are the same as that 456 of the Prerequisite RR, else signal NXRRSET to the requestor. 458 3.2.2. For RRs in this section whose CLASS is NONE, test to see that the 459 TTL and RDLENGTH are both zero (0), else signal FORMERR to the 460 requestor. If the TYPE is ANY, test to see that there are no RRs in the 461 zone whose NAME is the same as that of the Prerequisite RR, else signal 462 YXDOMAIN to the requestor. If the TYPE is not ANY, test to see that 463 there are no RRs in the zone whose NAME and TYPE are the same as that of 464 the Prerequisite RR, else signal YXRRSET to the requestor. 466 3.2.3. For RRs in this section whose CLASS is the same as the ZCLASS, 467 test to see that the TTL is zero (0), else signal FORMERR to the 468 requestor. Then, build an RRset for each unique and compare 469 each resulting RRset for set equality (same members, no more, no less) 470 with RRsets in the zone. If any Prerequisite RRset is not entirely and 471 exactly matched by a zone RRset, signal NXRRSET to the requestor. If 472 any RR in this section has a CLASS other than ZCLASS or NONE or ANY, 473 signal FORMERR to the requestor. 475 3.2.4 - Table Of Metavalues Used In Prerequisite Section 477 CLASS TYPE RDATA Meaning 478 ------------------------------------------------------------ 479 ANY ANY empty Name is in use 480 ANY rrset empty RRset exists (value independent) 481 NONE ANY empty Name is not in use 482 NONE rrset empty RRset does not exist 483 zone rrset rr RRset exists (value dependent) 484 3.2.5 - Pseudocode for Prerequisite Section Processing 486 for rr in prerequisites 487 if (rr.ttl != 0) 488 return (FORMERR) 489 if (zone_of(rr.name) != ZNAME) 490 return (NOTZONE); 491 if (rr.class == ANY) 492 if (rr.rdlength != 0) 493 return (FORMERR) 494 if (rr.type == ANY) 495 if (!zone_name) 496 return (NXDOMAIN) 497 else 498 if (!zone_rrset) 499 return (NXRRSET) 500 if (rr.class == NONE) 501 if (rr.rdlength != 0) 502 return (FORMERR) 503 if (rr.type == ANY) 504 if (zone_name) 505 return (YXDOMAIN) 506 else 507 if (zone_rrset) 508 return (YXRRSET) 509 if (rr.class == zclass) 510 temp += rr 511 else 512 return (FORMERR) 514 for rrset in temp 515 if (zone_rrset != rrset) 516 return (NXDOMAIN) 518 3.3 - Check Requestor's Permissions 520 3.3.1. Next, the requestor's permission to update the RRs named in the 521 Update Section may be tested in an implementation dependent fashion or 522 using mechanisms specified in a subsequent Secure DNS Update protocol. 523 If the requestor does not have permission to perform these updates, the 524 server may write a warning message in its operations log, and may either 525 signal REFUSED to the requestor, or ignore the permission problem and 526 proceed with the update. 528 3.3.2. While the exact processing is implementation defined, if these 529 verification activities are to be performed, this is the point in the 530 server's processing where such performance should take place, since if a 531 REFUSED condition is encountered after an update has been partially 532 applied, it will be necessary to undo the partial update and restore the 533 zone to its original state before answering the requestor. 535 3.3.3 - Pseudocode for Permission Checking 537 if (security policy exists) 538 if (this update is not permitted) 539 if (local option) 540 log a message about permission problem 541 if (local option) 542 return (REFUSED) 544 3.4 - Process Update Section 546 Next, the Update Section is processed as follows. 548 3.4.1 - Prescan 550 The Update Section is parsed into RRs and each RR's CLASS is checked to 551 see if it is ANY, NONE, or the same as the Zone Class, else signal a 552 FORMERR to the requestor. Using the definitions in Section 1.2, each 553 RR's NAME must be in the zone specified by the Zone Section, else signal 554 NOTZONE to the requestor. 556 3.4.1.2. For RRs whose CLASS is not ANY, check the TYPE and if it is 557 ANY, AXFR, MAILA, MAILB, or any other QUERY metatype, or any 558 unrecognized type, then signal FORMERR to the requestor. For RRs whose 559 CLASS is ANY or NONE, check the TTL to see that it is zero (0), else 560 signal a FORMERR to the requestor. For any RR whose CLASS is ANY, check 561 the RDLENGTH to make sure that it is zero (0) (that is, the RDATA field 562 is empty), and that the TYPE is not AXFR, MAILA, MAILB, or any other 563 QUERY metatype besides ANY, or any unrecognized type, else signal 564 FORMERR to the requestor. 566 3.4.1.3 - Pseudocode For Update Section Prescan 568 [rr] for rr in updates 569 if (zone_of(rr.name) != ZNAME) 570 return (NOTZONE); 571 if (rr.class == zclass) 572 if (rr.type & ANY|AXFR|MAILA|MAILB) 573 return (FORMERR) 574 elsif (rr.class == ANY) 575 if (rr.ttl != 0 || rr.rdlength != 0 576 || rr.type & AXFR|MAILA|MAILB) 577 return (FORMERR) 578 elsif (rr.class == NONE) 579 if (rr.ttl != 0 || rr.type & ANY|AXFR|MAILA|MAILB) 580 return (FORMERR) 581 else 582 return (FORMERR) 584 3.4.2 - Update 586 The Update Section is parsed into RRs and these RRs are processed in 587 order. 589 3.4.2.1. If any system failure (such as an out of memory condition, or a 590 hardware error in persistent storage) occurs during the processing of 591 this section, signal SERVFAIL to the requestor and undo all updates 592 applied to the zone during this transaction. 594 3.4.2.2. Any Update RR whose CLASS is the same as ZCLASS is added to the 595 zone. In case of duplicate RDATAs (which for SOA RRs is always the 596 case, and for WKS RRs is the case if the ADDRESS and PROTOCOL fields 597 both match), the Zone RR is replaced by Update RR. If the TYPE is SOA 598 and there is no Zone SOA RR, or the new SOA.SERIAL is lower (according 599 to [KRE1996]) than the current Zone SOA RR's SOA.SERIAL, the Update RR 600 is ignored. In the case of a CNAME Update RR and a non-CNAME Zone RRset 601 or vice versa, ignore the CNAME Update RR, otherwise replace the CNAME 602 Zone RR with the CNAME Update RR. 604 3.4.2.3. For any Update RR whose CLASS is ANY and whose TYPE is ANY, all 605 Zone RRs with the same NAME are deleted, unless the NAME is the same as 606 ZNAME in which case only those RRs whose TYPE is other than SOA or NS 607 are deleted. For any Update RR whose CLASS is ANY and whose TYPE is not 608 ANY all Zone RRs with the same NAME and TYPE are deleted, unless the 609 NAME is the same as ZNAME in which case neither SOA or NS RRs will be 610 deleted. 612 3.4.2.4. For any Update RR whose class is NONE, any Zone RR whose NAME, 613 TYPE, RDATA and RDLENGTH are equal to the Update RR is deleted, unless 614 the NAME is the same as ZNAME and either the TYPE is SOA or the TYPE is 615 NS and the matching Zone RR is the only NS remaining in the RRset, in 616 which case this Update RR is ignored. 618 3.4.2.5. Signal NOERROR to the requestor. 620 3.4.2.6 - Table Of Metavalues Used In Update Section 622 CLASS TYPE RDATA Meaning 623 --------------------------------------------------------- 624 ANY ANY empty Delete all RRsets from a name 625 ANY rrset empty Delete an RRset 626 NONE rrset rr Delete an RR from an RRset 627 zone rrset rr Add to an RRset 628 3.4.2.7 - Pseudocode For Update Section Processing 630 [rr] for rr in updates 631 if (rr.class == zclass) 632 if (rr.type == CNAME) 633 if (zone_rrset) 634 next [rr] 635 elsif (zone_rrset) 636 next [rr] 637 if (rr.type == SOA) 638 if (!zone_rrset || 639 zone_rr.serial > rr.soa.serial) 640 next [rr] 641 for zrr in zone_rrset 642 if (rr.type == CNAME || rr.type == SOA || 643 (rr.type == WKS && rr.proto == zrr.proto && 644 rr.address == zrr.address) || 645 rr.rdata == zrr.rdata) 646 zrr = rr 647 next [rr] 648 zone_rrset += rr 649 elsif (rr.class == ANY) 650 if (rr.type == ANY) 651 if (rr.name == zname) 652 zone_rrset = Nil 653 else 654 zone_rrset = Nil 655 elsif (rr.name == zname && 656 (rr.type == SOA || rr.type == NS)) 657 next [rr] 658 else 659 zone_rrset = Nil 660 elsif (rr.class == NONE) 661 if (rr.type == SOA) 662 next [rr] 663 if (rr.type == NS && zone_rrset == rr) 664 next [rr] 665 zone_rr = Nil 666 return (NOERROR) 668 3.5 - Stability 670 When a zone is modified by an UPDATE operation, the server must commit 671 the change to nonvolatile storage before sending a response to the 672 requestor or answering any queries or transfers for the modified zone. 673 It is reasonable for a server to store only the update records as long 674 as a system reboot or power failure will cause these update records to 675 be incorporated into the zone the next time the server is started. It 676 is also reasonable for the server to copy the entire modified zone to 677 nonvolatile storage after each update operation, though this would have 678 suboptimal performance for large zones. 680 3.6 - Zone Identity 682 If the zone's SOA SERIAL is changed by an update operation, that change 683 must be in a positive direction (using modulo 2**32 arithmetic as 684 specified by [KRE1996]). Attempts to replace an SOA with one whose 685 SERIAL is less than the current one will be silently ignored by the 686 primary master server. 688 If the zone's SOA's SERIAL is not changed as a result of an update 689 operation, then the server shall increment it automatically before the 690 SOA or any changed name or RR or RRset is included in any response or 691 transfer. The primary master server's implementor might choose to 692 autoincrement the SOA SERIAL if any of the following events occurs: 694 (1) Each update operation. 696 (2) A name, RR or RRset in the zone has changed and has subsequently 697 been visible to a DNS client since the unincremented SOA was 698 visible to a DNS client, and the SOA is about to become visible to 699 a DNS client. 701 (3) A configurable period of time has elapsed since the last update 702 operation. This period shall be less than or equal to one third of 703 the zone refresh time, and the default shall be the lesser of that 704 maximum and 300 seconds. 706 (4) A configurable number of updates has been applied since the last 707 SOA change. The default value for this configuration parameter 708 shall be one hundred (100). 710 It is imperative that the zone's contents and the SOA's SERIAL be 711 tightly synchronized. If the zone appears to change, the SOA must 712 appear to change as well. 714 3.7 - Atomicity 716 During the processing of an UPDATE transaction, the server must ensure 717 atomicity with respect to other (concurrent) UPDATE or QUERY 718 transactions. No two transactions can be processed concurrently if 719 either depends on the final results of the other; in particular, a QUERY 720 should not be able to retrieve RRsets which have been partially modified 721 by a concurrent UPDATE, and an UPDATE should not be able to start from 722 prerequisites that might not still hold at the completion of some other 723 concurrent UPDATE. Finally, if two UPDATE transactions would modify the 724 same names, RRs or RRsets, then such UPDATE transactions must be 725 serialized. 727 3.8 - Response 729 At the end of UPDATE processing, a response code will be known. A 730 response message is generated by copying the ID and Opcode fields from 731 the request, and either copying the ZOCOUNT, PRCOUNT, UPCOUNT, and 732 ADCOUNT fields and associated sections, or placing zeros (0) in the 733 these ``count'' fields and not including any part of the original 734 update. The QR bit is set to one (1), and the response is sent back to 735 the requestor. If the requestor used UDP, then the response will be 736 sent to the requestor's source UDP port. If the requestor used TCP, 737 then the response will be sent back on the requestor's open TCP 738 connection. 740 4 - Requestor Behaviour 742 4.1. From a requestor's point of view, any authoritative server for the 743 zone can appear to be able to process update requests, even though only 744 the primary master server is actually able to modify the zone's master 745 file. Requestors are expected to know the name of the zone they intend 746 to update and to know or be able to determine the name servers for that 747 zone. 749 4.2. If update ordering is desired, the requestor will need to know the 750 value of the existing SOA RR. Requestors who update the SOA RR must 751 update the SOA SERIAL field in a positive direction (as defined by 752 [KRE1996]) and to preserve the other SOA fields unless the requestor's 753 explicit intent is to change them. The SOA SERIAL field must never be 754 set to zero (0). 756 4.3. If the requestor has reasonable cause to believe that all of a 757 zone's servers will be equally reachable, then it should arrange to try 758 the primary master server (as given by the SOA MNAME field if matched by 759 some NS NSDNAME) first to avoid unnecessary forwarding inside the slave 760 servers. (Note that the primary master will in some cases not be 761 reachable by all requestors, due to firewalls or network partitioning.) 763 4.4. Once the zone's name servers been found and possibly sorted so that 764 the ones more likely to be reachable and/or support the UPDATE opcode 765 are listed first, the requestor composes an UPDATE message of the 766 following form and sends it to the first name server on its list: 768 ID: (new) 769 Opcode: UPDATE 770 Zone zcount: 1 771 Zone zname: (zone name) 772 Zone zclass: (zone class) 773 Zone ztype: T_SOA 774 Prerequisite Section: (see previous text) 775 Update Section: (see previous text) 776 Additional Data Section: (empty) 778 4.5. If the requestor receives a response, and the response has an RCODE 779 other than SERVFAIL or NOTIMP, then the requestor returns an appropriate 780 response to its caller. 782 4.6. If a response is received whose RCODE is SERVFAIL or NOTIMP, or if 783 no response is received within an implementation dependent timeout 784 period, or if an ICMP error is received indicating that the server's 785 port is unreachable, then the requestor will delete the unusable server 786 from its internal name server list and try the next one, repeating until 787 the name server list is empty. If the requestor runs out of servers to 788 try, an appropriate error will be returned to the requestor's caller. 790 5 - Duplicate Detection, Ordering and Mutual Exclusion 792 5.1. For correct operation, mechanisms may be needed to ensure 793 idempotence, order UPDATE requests and provide mutual exclusion. An 794 UPDATE message or response might be delivered zero times, one time, or 795 multiple times. Datagram duplication is of particular interest since it 796 covers the case of the so-called ``replay attack'' where a correct 797 request is duplicated maliciously by an intruder. 799 5.2. Multiple UPDATE requests or responses in transit might be delivered 800 in any order, due to network topology changes or load balancing, or to 801 multipath forwarding graphs wherein several slave servers all forward to 802 the primary master. In some cases, it might be required that the 803 earlier update not be applied after the later update, where ``earlier'' 804 and ``later'' are defined by an external time base visible to some set 805 of requestors, rather than by the order of request receipt at the 806 primary master. 808 5.3. A requestor can ensure transaction idempotence by explicitly 809 deleting some ``marker RR'' (rather than deleting the RRset of which it 810 is a part) and then adding a new ``marker RR'' with a different RDATA 811 field. The Prerequisite Section should specify that the original 812 ``marker RR'' must be present in order for this UPDATE message to be 813 accepted by the server. 815 5.4. If the request is duplicated by a network error, all duplicate 816 requests will fail since only the first will find the original ``marker 817 RR'' present and having its known previous value. The decisions of 818 whether to use such a ``marker RR'' and what RR to use are left up to 819 the application programmer, though one obvious choice is the zone's SOA 820 RR as described below. 822 5.5. Requestors can ensure update ordering by externally synchronizing 823 their use of successive values of the ``marker RR.'' Mutual exclusion 824 can be addressed as a degenerate case, in that a single succession of 825 the ``marker RR'' is all that is needed. 827 5.6. A special case where update ordering and datagram duplication 828 intersect is when an RR validly changes to some new value and then back 829 to its previous value. Without a ``marker RR'' as described above, this 830 sequence of updates can leave the zone in an undefined state if 831 datagrams are duplicated. 833 5.7. To achieve an atomic multitransaction ``read-modify-write'' cycle, 834 a requestor could first retrieve the SOA RR, and build an UPDATE message 835 one of whose prerequisites was the old SOA RR. It would then specify 836 updates that would delete this SOA RR and add a new one with an 837 incremented SOA SERIAL, along with whatever actual prerequisites and 838 updates were the object of the transaction. If the transaction 839 succeeds, the requestor knows that the RRs being changed were not 840 otherwise altered by any other requestor. 842 6 - Forwarding 844 When a zone slave forwards an UPDATE message upward toward the zone's 845 primary master server, it must allocate a new ID and prepare to enter 846 the role of ``forwarding server,'' which is a requestor with respect to 847 the forward server. 849 6.1. The set of forward servers will be same as the set of servers this 850 zone slave would use as the source of AXFR or IXFR data. So, while the 851 original requestor might have used the zone's NS RRset to locate its 852 update server, a forwarder always forwards toward its designated zone 853 master servers. 855 6.2. If the original requestor used TCP, then the TCP connection from 856 the requestor is still open and the forwarder must use TCP to forward 857 the message. If the original requestor used UDP, the forwarder may use 858 either UDP or TCP to forward the message, at the whim of the 859 implementor. 861 6.3. It is reasonable for forward servers to be forwarders themselves, 862 if the AXFR dependency graph being followed is a deep one involving 863 firewalls and multiple connectivity realms. In most cases the AXFR 864 dependency graph will be shallow and the forward server will be the 865 primary master server. 867 6.4. The forwarder will not respond to its requestor until it receives a 868 response from its forward server. UPDATE transactions involving 869 forwarders are therefore time synchronized with respect to the original 870 requestor and the primary master server. 872 6.5. When there are multiple possible sources of AXFR data and therefore 873 multiple possible forward servers, a forwarder will use the same 874 fallback strategy with respect to connectivity or timeout errors that it 875 would use when performing an AXFR. This is implementation dependent. 877 6.6. When a forwarder receives a response from a forward server, it 878 copies this response into a new response message, assigns its 879 requestor's ID to that message, and sends the response back to the 880 requestor. 882 7 - Design, Implementation, Operation, and Protocol Notes 884 Some of the principles which guided the design of this UPDATE 885 specification are as follows. Note that these are not part of the 886 formal specification and any disagreement between this section and any 887 other section of this document should be resolved in favour of the other 888 section. 890 7.1. Using metavalues for CLASS is possible only because all RRs in the 891 packet are assumed to be in the same zone, and CLASS is an attribute of 892 a zone rather than of an RRset. (It is for this reason that the Zone 893 Section is not optional.) 895 7.2. Since there are no data-present or data-absent errors possible from 896 processing the Update Section, it is necessary to state data-present and 897 data-absent dependencies in the Prerequisite Section. 899 7.3. The Additional Data Section can be used to supply a server with out 900 of zone glue that will be needed in referrals. For example, if adding a 901 new NS RR to HOME.VIX.COM specifying a nameserver called NS.AU.OZ, the A 902 RR for NS.AU.OZ can be included in the Additional Data Section. Servers 903 can use this information or ignore it, at the discretion of the 904 implementor. 906 7.4. The Additional Data Section might be used if some of the RRs later 907 needed for Secure DNS Update are not actually zone updates, but rather 908 ancillary keys or signatures not intended to be stored in the zone (as 909 an update would be), yet necessary for validating the update operation. 911 7.5. It is expected that in the absence of Secure DNS Update, a server 912 will only accept updates if they come from a source address that has 913 been statically configured in the server's description of a primary 914 master zone. DHCP servers would be likely candidates for inclusion in 915 this statically configured list. 917 7.6. It is not possible to create a zone using this protocol, since 918 there is no provision for a slave server to be told who its master 919 servers are. It is expected that this protocol will be extended in the 920 future to cover this case. Therefore, at this time, the addition of SOA 921 RRs is unsupported. For similar reasons, deletion of SOA RRs is also 922 unsupported. 924 7.7. The prerequisite for specifying that a name own at least one RR 925 differs semantically from QUERY, in that QUERY would return 926 rather than NXDOMAIN if queried for an RRset at this 927 name, while UPDATE's prerequisite condition [Section 2.4.4] would NOT be 928 satisfied. 930 7.8. It is possible for a UDP response to be lost in transit and for a 931 request to be retried due to a timeout condition. In this case an 932 UPDATE that was successful the first time it was received by the primary 933 master might ultimately appear to have failed when the response to a 934 duplicate request is finally received by the requestor. (This is 935 because the original prerequisites may no longer be satisfied after the 936 update has been applied.) For this reason, requestors who require an 937 accurate response code must use TCP. 939 7.9. Because a requestor who requires an accurate response code will 940 initiate their UPDATE transaction using TCP, a forwarder who receives a 941 request via TCP must forward it using TCP. 943 7.10. Deferral of SOA SERIAL autoincrements is made possible so that 944 serial numbers can be conserved and wraparound at 2**32 can be made an 945 infrequent occurance. Visible (to DNS clients) SOA SERIALs need to 946 differ if the zone differs. Note that the Authority Section SOA in a 947 QUERY response is a form of visibility, for the purposes of this 948 prerequisite. 950 7.11. A zone's SOA SERIAL should never be set to zero (0) due to 951 interoperability problems with some older but widely installed 952 implementations of DNS. When incrementing an SOA SERIAL, if the result 953 of the increment is zero (0) (as will be true when wrapping around 954 2**32), it is necessary to increment it again or set it to one (1). See 955 [KRE1996] for more detail on this subject. 957 7.12. Due to the TTL minimalization necessary when caching an RRset, it 958 is recommended that all TTLs in an RRset be set to the same value. 959 While the DNS Message Format permits variant TTLs to exist in the same 960 RRset, and this variance can exist inside a zone, such variance will 961 have counterintuitive results and its use is discouraged. 963 7.13. Zone cut management presents some obscure corner cases to the add 964 and delete operations in the Update Section. It is possible to delete 965 an NS RR as long as it's not the last RR in the RRset. If deleting all 966 RRs from a name, SOA and NS RRs at the top of a zone are unaffected. If 967 deleting RRsets, it is not possible to delete either SOA or NS RRsets at 968 the top of a zone. An attempt to add an SOA will be treated as a 969 replace operation. 971 7.14. No semantic checking is required in the primary master server when 972 adding new RRs. Therefore a requestor can cause CNAME or NS or any 973 other kind of RR to be added even if their target name does not exist or 974 does not have the proper RRsets to make the original RR useful. Primary 975 master servers which implement this kind of checking should take great 976 care to avoid out-of-zone dependencies (whose veracity cannot be 977 authoritatively checked) or signals to the requestor during processing 978 of the Update Section after the prescan. 980 7.15. Nonterminal or wildcard CNAMEs are not well specified by RFC 1035 981 and their use will probably lead to unpredictable results. Their use is 982 discouraged. 984 7.16. Before adding a delegation to a zone, all RRsets at or below the 985 new zone cut should be removed, except for ``glue'' which are A RRs 986 below the zone cut which are targets of NS RRs at the zone cut. 988 7.17. A primary server implementation may choose to perform part of its 989 permission checking during the Update Section processing. This may be 990 needed if the permissions won't be known until the final form of an 991 RRset is known. In this case, a primary server can signal REFUSED to 992 the requestor as long as it also undoes all partial updates and restores 993 the zone to its original state. 995 7.18. In a deep AXFR dependency graph, it has not historically been an 996 error for slaves to depend mutually upon each other. This configuration 997 has been used to enable a zone to flow from the primary master to all 998 slaves even though not all slaves have continuous connectivity to the 999 primary master. UPDATE's use of the AXFR dependency graph for 1000 forwarding prohibits this kind of dependency loop, since UPDATE 1001 forwarding has no loop detection analagous to the SOA SERIAL pretest 1002 used by AXFR. 1004 7.19. For UPDATE's purposes, a zone is said to own all names at or below 1005 the zone's root. This allows an UPDATE message to add or delete names 1006 below a zone cut so as to create and maintain ``glue'' records needed 1007 for delegation when a name server is within the zone being delegated. 1009 7.20. Previously existing names which are occluded by a new zone cut are 1010 still considered part of the parent zone, for the purposes of zone 1011 transfers, even though queries for such names will be referred to the 1012 new subzone's servers. If a zone cut is removed, all parent zone names 1013 that were occluded by it will again become visible to queries. (This is 1014 a clarification of RFC 1034.) 1016 7.21. If a node contains any name server delegations (NS RRs), this node 1017 is said to be owned by the child zone, and the parent will answer only 1018 with a nonauthoritative referral to the child zone's servers if queried 1019 for a name at or below the child zone's root, except in the case of an 1020 QTYPE=NS query at the zone cut itself, for which the parent zone's 1021 servers would answer authoritatively. (This is a clarification of RFC 1022 1034.) 1024 7.22. If a server is authoritative for both a zone and its child, then 1025 queries for names at the zone cut between them will be answered 1026 authoritatively using only data from the child zone. (This is a 1027 clarification of RFC 1034.) 1029 7.23. Update ordering using the SOA RR is problematic since there is no 1030 way to know which of a zone's NS RRs represents the primary master, and 1031 the zone slaves can be out of date if their SOA.REFRESH timers have not 1032 elapsed since the last time the zone was changed on the primary master. 1033 We recommend that a zone needing ordered updates use only servers which 1034 implement NOTIFY (see [NOTIFY]) and IXFR (see [IXFR]), and that a client 1035 receiving a prerequisite error while attempting an ordered update simply 1036 retry after a random delay period to allow the zone to settle. 1038 8 - Security Considerations 1040 In the absence of DNS Security, the protocol described by this document 1041 makes it possible for anyone who can reach an authoritative name server 1042 to alter the contents of a zone. This strongly indicates a need for out 1043 of band access control such as static access control lists enforced by 1044 the server combined with the strongest possible firewall techniques. 1046 At the time of this writing, work is progressing (see [DNSSEC]) on the 1047 general problem of DNS Security, and for Secure DNS Updates (see 1048 [SECUPD]). No updates should be accepted from hosts outside an 1049 enterprise network's security perimeter until and unless Secure DNS 1050 Updates have been implemented. For the purpose of this recommendation, 1051 a slave server acting as a forwarder, or the primary master itself, is 1052 outside the security perimeter if it is allowed to exchange DNS messages 1053 with hosts outside that perimeter. 1055 Acknowledgements 1057 We would like to thank the IETF DNSIND working group for their input and 1058 assistance, in particular, Rob Austein, Randy Bush, Donald Eastlake, 1059 Masataka Ohta, Mark Andrews, and Robert Elz. Special thanks to Bill 1060 Simpson and Ken Wallich for reviewing this document. 1062 References 1064 [RFC1035] 1065 P. Mockapetris, ``Domain Names - Implementation and Specification,'' 1066 RFC 1035, USC/Information Sciences Institute, November 1987. 1068 [IXFR] 1069 M. Ohta, ``Incremental Zone Transfer,'' Internet Draft, February 1070 1996, . 1072 [NOTIFY] 1073 P. Vixie, ``A Mechanism for Prompt Notification of Zone Changes (DNS 1074 NOTIFY),'' Internet Draft, March 1996, . 1077 [KRE1996] 1078 R. Elz, ``Serial Number Arithmetic,'' Internet Draft, February 1996, 1079 . 1081 [DNSSEC] 1082 Donald E. Eastlake and Charles W. Kaufman, ``Domain Name System 1083 Protocol Security Extensions,'' Internet Draft, January 1996, . 1086 [SECUPD] 1087 Donald E. Eastlake, ``Secure Domain Name System Dynamic Update,'' 1088 Internet Draft, February 1996, 1090 Authors' Addresses 1092 Yakov Rekhter Susan Thomson 1093 Cisco Systems Bellcore 1094 170 West Tasman Drive 445 South Street 1095 San Jose, CA 95134-1706 Morristown, NJ 07960 1096 +1 914 528 0090 +1 201 829 4514 1097 1099 Jim Bound Paul Vixie 1100 Digital Equipment Corp. Internet Software Consortium 1101 110 Spitbrook Rd ZK3-3/U14 Star Route Box 159A 1102 Nashua, NH 03062-2698 Woodside, CA 94062 1103 +1 603 881 0400 +1 415 747 0204 1104