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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Hoffman 3 Internet-Draft ICANN 4 Obsoletes: 7719 (if approved) A. Sullivan 5 Updates: 2308 (if approved) 6 Intended status: Best Current Practice K. Fujiwara 7 Expires: February 14, 2019 JPRS 8 August 13, 2018 10 DNS Terminology 11 draft-ietf-dnsop-terminology-bis-12 13 Abstract 15 The domain name system (DNS) is defined in literally dozens of 16 different RFCs. The terminology used by implementers and developers 17 of DNS protocols, and by operators of DNS systems, has sometimes 18 changed in the decades since the DNS was first defined. This 19 document gives current definitions for many of the terms used in the 20 DNS in a single document. 22 This document will be the successor to RFC 7719, and thus will 23 obsolete RFC 7719. It will also update RFC 2308. 25 Status of This Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at https://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on February 14, 2019. 42 Copyright Notice 44 Copyright (c) 2018 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (https://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 60 2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 61 3. DNS Response Codes . . . . . . . . . . . . . . . . . . . . . 9 62 4. DNS Transactions . . . . . . . . . . . . . . . . . . . . . . 10 63 5. Resource Records . . . . . . . . . . . . . . . . . . . . . . 12 64 6. DNS Servers and Clients . . . . . . . . . . . . . . . . . . . 15 65 7. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 66 8. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . 25 67 9. Registration Model . . . . . . . . . . . . . . . . . . . . . 26 68 10. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 28 69 11. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 33 70 12. Security Considerations . . . . . . . . . . . . . . . . . . . 34 71 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34 72 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 73 14.1. Normative References . . . . . . . . . . . . . . . . . . 34 74 14.2. Informative References . . . . . . . . . . . . . . . . . 37 75 Appendix A. Definitions Updated by this Document . . . . . . . . 41 76 Appendix B. Definitions First Defined in this Document . . . . . 41 77 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 78 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 47 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48 81 1. Introduction 83 The Domain Name System (DNS) is a simple query-response protocol 84 whose messages in both directions have the same format. (Section 2 85 gives a definition of "public DNS", which is often what people mean 86 when they say "the DNS".) The protocol and message format are 87 defined in [RFC1034] and [RFC1035]. These RFCs defined some terms, 88 but later documents defined others. Some of the terms from [RFC1034] 89 and [RFC1035] now have somewhat different meanings than they did in 90 1987. 92 This document collects a wide variety of DNS-related terms. Some of 93 them have been precisely defined in earlier RFCs, some have been 94 loosely defined in earlier RFCs, and some are not defined in any 95 earlier RFC at all. 97 Most of the definitions here are the consensus definition of the DNS 98 community -- both protocol developers and operators. Some of the 99 definitions differ from earlier RFCs, and those differences are 100 noted. In this document, where the consensus definition is the same 101 as the one in an RFC, that RFC is quoted. Where the consensus 102 definition has changed somewhat, the RFC is mentioned but the new 103 stand-alone definition is given. See Appendix A for a list of the 104 definitions that this document updates. 106 It is important to note that, during the development of this 107 document, it became clear that some DNS-related terms are interpreted 108 quite differently by different DNS experts. Further, some terms that 109 are defined in early DNS RFCs now have definitions that are generally 110 agreed to, but that are different from the original definitions. 111 Therefore, this document is a substantial revision to [RFC7719]. 113 The terms are organized loosely by topic. Some definitions are for 114 new terms for things that are commonly talked about in the DNS 115 community but that never had terms defined for them. 117 Other organizations sometimes define DNS-related terms their own way. 118 For example, the W3C defines "domain" at 119 . The Root Server System Advisory 120 Committee (RSSAC) has a good lexicon [RSSAC026]. 122 Note that there is no single consistent definition of "the DNS". It 123 can be considered to be some combination of the following: a commonly 124 used naming scheme for objects on the Internet; a distributed 125 database representing the names and certain properties of these 126 objects; an architecture providing distributed maintenance, 127 resilience, and loose coherency for this database; and a simple 128 query-response protocol (as mentioned below) implementing this 129 architecture. Section 2 defines "global DNS" and "private DNS" as a 130 way to deal with these differing definitions. 132 Capitalization in DNS terms is often inconsistent among RFCs and 133 various DNS practitioners. The capitalization used in this document 134 is a best guess at current practices, and is not meant to indicate 135 that other capitalization styles are wrong or archaic. In some 136 cases, multiple styles of capitalization are used for the same term 137 due to quoting from different RFCs. 139 2. Names 141 Naming system: A naming system associates names with data. Naming 142 systems have many significant facets that help differentiate them. 143 Some commonly-identified facets include: 145 * Composition of names 147 * Format of names 149 * Administration of names 151 * Types of data that can be associated with names 153 * Types of metadata for names 155 * Protocol for getting data from a name 157 * Context for resolving a name 159 Note that this list is a small subset of facets that people have 160 identified over time for naming systems, and the IETF has yet to 161 agree on a good set of facets that can be used to compare naming 162 systems. For example, other facets might include "protocol to 163 update data in a name", "privacy of names", and "privacy of data 164 associated with names", but those are not as well-defined as the 165 ones listed above. The list here is chosen because it helps 166 describe the DNS and naming systems similar to the DNS. 168 Domain name: An ordered list of one or more labels. 170 Note that this is a definition independent of the DNS RFCs, and 171 the definition here also applies to systems other than the DNS. 172 [RFC1034] defines the "domain name space" using mathematical trees 173 and their nodes in graph theory, and this definition has the same 174 practical result as the definition here. Using graph theory, a 175 domain name is a list of labels identifying a portion along one 176 edge of a directed acyclic graph. A domain name can be relative 177 to parts of the tree, or it can be fully qualified (in which case, 178 it begins at the common root of the graph). 180 Also note that different IETF and non-IETF documents have used the 181 term "domain name" in many different ways. It is common for 182 earlier documents to use "domain name" to mean "names that match 183 the syntax in [RFC1035]", but possibly with additional rules such 184 as "and are, or will be, resolvable in the global DNS" or "but 185 only using the presentation format". 187 Label: An ordered list of zero or more octets that makes up a 188 portion of a domain name. Using graph theory, a label identifies 189 one node in a portion of the graph of all possible domain names. 191 Global DNS: Using the short set of facets listed in "Naming system", 192 the global DNS can be defined as follows. Most of the rules here 193 come from [RFC1034] and [RFC1035], although the term "global DNS" 194 has not been defined before now. 196 Composition of names -- A name in the global DNS has one or more 197 labels. The length of each label is between 0 and 63 octets 198 inclusive. In a fully-qualified domain name, the first label in 199 the ordered list is 0 octets long; it is the only label whose 200 length may be 0 octets, and it is called the "root" or "root 201 label". A domain name in the global DNS has a maximum total 202 length of 255 octets in the wire format; the root represents one 203 octet for this calculation. (Multicast DNS [RFC6762] allows names 204 up to 255 bytes plus a terminating zero byte based on a different 205 interpretation of RFC 1035 and what is included in the 255 206 octets.) 208 Format of names -- Names in the global DNS are domain names. 209 There are three formats: wire format, presentation format, and 210 common display. 212 The basic wire format for names in the global DNS is a list of 213 labels ordered by decreasing distance from the root, with the root 214 label last. Each label is preceded by a length octet. [RFC1035] 215 also defines a compression scheme that modifies this format. 217 The presentation format for names in the global DNS is a list of 218 labels ordered by decreasing distance from the root, encoded as 219 ASCII, with a "." character between each label. In presentation 220 format, a fully-qualified domain name includes the root label and 221 the associated separator dot. For example, in presentation 222 format, a fully-qualified domain name with two non-root labels is 223 always shown as "example.tld." instead of "example.tld". 224 [RFC1035] defines a method for showing octets that do not display 225 in ASCII. 227 The common display format is used in applications and free text. 228 It is the same as the presentation format, but showing the root 229 label and the "." before it is optional and is rarely done. For 230 example, in common display format, a fully-qualified domain name 231 with two non-root labels is usually shown as "example.tld" instead 232 of "example.tld.". Names in the common display format are 233 normally written such that the directionality of the writing 234 system presents labels by decreasing distance from the root (so, 235 in both English and C the root or TLD label in the ordered list is 236 right-most; but in Arabic it may be left-most, depending on local 237 conventions). 239 Administration of names -- Administration is specified by 240 delegation (see the definition of "delegation" in Section 7). 242 Policies for administration of the root zone in the global DNS are 243 determined by the names operational community, which convenes 244 itself in the Internet Corporation for Assigned Names and Numbers 245 (ICANN). The names operational community selects the IANA 246 Functions Operator for the global DNS root zone. At the time this 247 document is published, that operator is Public Technical 248 Identifiers (PTI). (See for more 249 information about PTI operating the IANA Functions.) The name 250 servers that serve the root zone are provided by independent root 251 operators. Other zones in the global DNS have their own policies 252 for administration. 254 Types of data that can be associated with names -- A name can have 255 zero or more resource records associated with it. There are 256 numerous types of resource records with unique data structures 257 defined in many different RFCs and in the IANA registry at 258 [IANA_Resource_Registry]. 260 Types of metadata for names -- Any name that is published in the 261 DNS appears as a set of resource records (see the definition of 262 "RRset" in Section 5). Some names do not themselves have data 263 associated with them in the DNS, but "appear" in the DNS anyway 264 because they form part of a longer name that does have data 265 associated with it (see the definition of "empty non-terminals" in 266 Section 7). 268 Protocol for getting data from a name -- The protocol described in 269 [RFC1035]. 271 Context for resolving a name -- The global DNS root zone 272 distributed by PTI. 274 Private DNS: Names that use the protocol described in [RFC1035] but 275 that do not rely on the global DNS root zone, or names that are 276 otherwise not generally available on the Internet but are using 277 the protocol described in [RFC1035]. A system can use both the 278 global DNS and one or more private DNS systems; for example, see 279 "Split DNS" in Section 6. 281 Note that domain names that do not appear in the DNS, and that are 282 intended never to be looked up using the DNS protocol, are not 283 part of the global DNS or a private DNS even though they are 284 domain names. 286 Multicast DNS: "Multicast DNS (mDNS) provides the ability to perform 287 DNS-like operations on the local link in the absence of any 288 conventional Unicast DNS server. In addition, Multicast DNS 289 designates a portion of the DNS namespace to be free for local 290 use, without the need to pay any annual fee, and without the need 291 to set up delegations or otherwise configure a conventional DNS 292 server to answer for those names." (Quoted from [RFC6762], 293 Abstract) Although it uses a compatible wire format, mDNS is 294 strictly speaking a different protocol than DNS. Also, where the 295 above quote says "a portion of the DNS namespace", it would be 296 clearer to say "a portion of the domain name space" The names in 297 mDNS are not intended to be looked up in the DNS. 299 Locally served DNS zone: A locally served DNS zone is a special case 300 of private DNS. Names are resolved using the DNS protocol in a 301 local context. [RFC6303] defines subdomains of IN-ADDR.ARPA that 302 are locally served zones. Resolution of names through locally 303 served zones may result in ambiguous results. For example, the 304 same name may resolve to different results in different locally 305 served DNS zone contexts. The context through which a locally 306 served zone may be explicit, for example, as defined in [RFC6303], 307 or implicit, as defined by local DNS administration and not known 308 to the resolution client. 310 Fully qualified domain name (FQDN): This is often just a clear way 311 of saying the same thing as "domain name of a node", as outlined 312 above. However, the term is ambiguous. Strictly speaking, a 313 fully qualified domain name would include every label, including 314 the zero-length label of the root: such a name would be written 315 "www.example.net." (note the terminating dot). But because every 316 name eventually shares the common root, names are often written 317 relative to the root (such as "www.example.net") and are still 318 called "fully qualified". This term first appeared in [RFC0819]. 319 In this document, names are often written relative to the root. 321 The need for the term "fully qualified domain name" comes from the 322 existence of partially qualified domain names, which are names 323 where one or more of the earliest labels in the ordered list are 324 omitted (for example, a name of "www" derived from 325 "www.example.net"). Such relative names are understood only by 326 context. 328 Host name: This term and its equivalent, "hostname", have been 329 widely used but are not defined in [RFC1034], [RFC1035], 330 [RFC1123], or [RFC2181]. The DNS was originally deployed into the 331 Host Tables environment as outlined in [RFC0952], and it is likely 332 that the term followed informally from the definition there. Over 333 time, the definition seems to have shifted. "Host name" is often 334 meant to be a domain name that follows the rules in Section 3.5 of 335 [RFC1034], the "preferred name syntax" (that is, every character 336 in each label is a letter, a digit, or a hyphen). Note that any 337 label in a domain name can contain any octet value; hostnames are 338 generally considered to be domain names where every label follows 339 the rules in the "preferred name syntax", with the amendment that 340 labels can start with ASCII digits (this amendment comes from 341 Section 2.1 of [RFC1123]). 343 People also sometimes use the term hostname to refer to just the 344 first label of an FQDN, such as "printer" in 345 "printer.admin.example.com". (Sometimes this is formalized in 346 configuration in operating systems.) In addition, people 347 sometimes use this term to describe any name that refers to a 348 machine, and those might include labels that do not conform to the 349 "preferred name syntax". 351 TLD: A Top-Level Domain, meaning a zone that is one layer below the 352 root, such as "com" or "jp". There is nothing special, from the 353 point of view of the DNS, about TLDs. Most of them are also 354 delegation-centric zones (defined in Section 7, and there are 355 significant policy issues around their operation. TLDs are often 356 divided into sub-groups such as Country Code Top-Level Domains 357 (ccTLDs), Generic Top-Level Domains (gTLDs), and others; the 358 division is a matter of policy, and beyond the scope of this 359 document. 361 IDN: The common abbreviation for "Internationalized Domain Name". 362 The IDNA protocol is the standard mechanism for handling domain 363 names with non-ASCII characters in applications in the DNS. The 364 current standard, normally called "IDNA2008", is defined in 365 [RFC5890], [RFC5891], [RFC5892], [RFC5893], and [RFC5894]. These 366 documents define many IDN-specific terms such as "LDH label", 367 "A-label", and "U-label". [RFC6365] defines more terms that 368 relate to internationalization (some of which relate to IDNs), and 369 [RFC6055] has a much more extensive discussion of IDNs, including 370 some new terminology. 372 Subdomain: "A domain is a subdomain of another domain if it is 373 contained within that domain. This relationship can be tested by 374 seeing if the subdomain's name ends with the containing domain's 375 name." (Quoted from [RFC1034], Section 3.1). For example, in the 376 host name "nnn.mmm.example.com", both "mmm.example.com" and 377 "nnn.mmm.example.com" are subdomains of "example.com". 379 Alias: The owner of a CNAME resource record, or a subdomain of the 380 owner of a DNAME resource record (DNAME records are defined in 381 [RFC6672]). See also "canonical name". 383 Canonical name: A CNAME resource record "identifies its owner name 384 as an alias, and specifies the corresponding canonical name in the 385 RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2) 386 This usage of the word "canonical" is related to the mathematical 387 concept of "canonical form". 389 CNAME: "It is traditional to refer to the owner of a CNAME record as 390 'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of 391 'canonical name', and the owner of a CNAME record is an alias, not 392 a canonical name." (Quoted from [RFC2181], Section 10.1.1) 394 3. DNS Response Codes 396 Some of response codes that are defined in [RFC1035] have acquired 397 their own shorthand names. All of the RCODEs are listed at 398 [IANA_Resource_Registry], although that site uses mixed-case 399 capitalization, while most documents use all-caps. Some of the 400 common names are described here, but the official list is in the IANA 401 registry. 403 NOERROR: "No error condition" (Quoted from [RFC1035], 404 Section 4.1.1.) 406 FORMERR: "Format error - The name server was unable to interpret the 407 query." (Quoted from [RFC1035], Section 4.1.1.) 409 SERVFAIL: "Server failure - The name server was unable to process 410 this query due to a problem with the name server." (Quoted from 411 [RFC1035], Section 4.1.1.) 413 NXDOMAIN: "Name Error - This code signifies that the domain name 414 referenced in the query does not exist." (Quoted from [RFC1035], 415 Section 4.1.1.) [RFC2308] established NXDOMAIN as a synonym for 416 Name Error. 418 NOTIMP: "Not Implemented - The name server does not support the 419 requested kind of query." (Quoted from [RFC1035], Section 4.1.1.) 421 REFUSED: "Refused - The name server refuses to perform the specified 422 operation for policy reasons. For example, a name server may not 423 wish to provide the information to the particular requester, or a 424 name server may not wish to perform a particular operation (e.g., 425 zone transfer) for particular data." (Quoted from [RFC1035], 426 Section 4.1.1.) 428 NODATA: "A pseudo RCODE which indicates that the name is valid for 429 the given class, but there are no records of the given type. A 430 NODATA response has to be inferred from the answer." (Quoted from 431 [RFC2308], Section 1.) "NODATA is indicated by an answer with the 432 RCODE set to NOERROR and no relevant answers in the answer 433 section. The authority section will contain an SOA record, or 434 there will be no NS records there." (Quoted from [RFC2308], 435 Section 2.2.) Note that referrals have a similar format to NODATA 436 replies; [RFC2308] explains how to distinguish them. 438 The term "NXRRSET" is sometimes used as a synonym for NODATA. 439 However, this is a mistake, given that NXRRSET is a specific error 440 code defined in [RFC2136]. 442 Negative response: A response that indicates that a particular RRset 443 does not exist, or whose RCODE indicates the nameserver cannot 444 answer. Sections 2 and 7 of [RFC2308] describe the types of 445 negative responses in detail. 447 4. DNS Transactions 449 The header of a DNS message is its first 12 octets. Many of the 450 fields and flags in the header diagram in Sections 4.1.1 through 451 4.1.3 of [RFC1035] are referred to by their names in that diagram. 452 For example, the response codes are called "RCODEs", the data for a 453 record is called the "RDATA", and the authoritative answer bit is 454 often called "the AA flag" or "the AA bit". 456 QNAME: The most commonly-used rough definition is that the QNAME is 457 a field in the Question section of a query. "A standard query 458 specifies a target domain name (QNAME), query type (QTYPE), and 459 query class (QCLASS) and asks for RRs which match." (Quoted from 460 [RFC1034], Section 3.7.1.). Strictly speaking, the definition 461 comes from [RFC1035], Section 4.1.2, where the QNAME is defined in 462 respect of the Question Section. This definition appears to be 463 applied consistently: the discussion of inverse queries in section 464 6.4 refers to the "owner name of the query RR and its TTL", 465 because inverse queries populate the Answer Section and leave the 466 Question Section empty. (Inverse queries are deprecated in 467 [RFC3425], and so relevant definitions do not appear in this 468 document.) 470 [RFC2308], however, has an alternate definition that puts the 471 QNAME in the answer (or series of answers) instead of the query. 472 It defines QNAME as: "...the name in the query section of an 473 answer, or where this resolves to a CNAME, or CNAME chain, the 474 data field of the last CNAME. The last CNAME in this sense is 475 that which contains a value which does not resolve to another 476 CNAME." This definition has a certain internal logic, because of 477 the way CNAME substitution works and the definition of CNAME. If 478 a name server does not find an RRset that matches a query, but it 479 finds the same name in the same class with a CNAME record, then 480 the name server "includes the CNAME record in the response and 481 restarts the query at the domain name specified in the data field 482 of the CNAME record." ([RFC1034] Section 3.6.2). This is made 483 explicit in the resolution algorithm outlined in Section 4.3.2 of 484 [RFC1034], which says to "change QNAME to the canonical name in 485 the CNAME RR, and go back to step 1" in the case of a CNAME RR. 486 Since a CNAME record explicitly declares that the owner name is 487 canonically named what is in the RDATA, then there is a way to 488 view the new name (i.e. the name that was in the RDATA of the 489 CNAME RR) as also being the QNAME. 491 This creates a kind of confusion, however, because the response to 492 a query that results in CNAME processing contains in the echoed 493 Question Section one QNAME (the name in the original query), and a 494 second QNAME that is in the data field of the last CNAME. The 495 confusion comes from the iterative/recursive mode of resolution, 496 which finally returns an answer that need not actually have the 497 same owner name as the QNAME contained in the original query. 499 To address this potential confusion, it is helpful to distinguish 500 between three meanings: 502 * QNAME (original): The name actually sent in the Question 503 Section in the original query, which is always echoed in the 504 (final) reply in the Question Section when the QR bit is set to 505 1. 507 * QNAME (effective): A name actually resolved, which is either 508 the name originally queried, or a name received in a CNAME 509 chain response. 511 * QNAME (final): The name actually resolved, which is either the 512 name actually queried or else the last name in a CNAME chain 513 response. 515 Note that, because the definition in [RFC2308] is actually for a 516 different concept than what was in [RFC1034], it would have be 517 better if [RFC2308] had used a different name for that concept. 518 In general use today, QNAME almost always means what is defined 519 above as "QNAME (original)". 521 Referrals: A type of response in which a server, signaling that it 522 is not (completely) authoritative for an answer, provides the 523 querying resolver with an alternative place to send its query. 524 Referrals can be partial. 526 A referral arises when a server is not performing recursive 527 service while answering a query. It appears in step 3(b) of the 528 algorithm in [RFC1034], Section 4.3.2. 530 There are two types of referral response. The first is a downward 531 referral (sometimes described as "delegation response"), where the 532 server is authoritative for some portion of the QNAME. The 533 authority section RRset's RDATA contains the name servers 534 specified at the referred-to zone cut. In normal DNS operation, 535 this kind of response is required in order to find names beneath a 536 delegation. The bare use of "referral" means this kind of 537 referral, and many people believe that this is the only legitimate 538 kind of referral in the DNS. 540 The second is an upward referral (sometimes described as "root 541 referral"), where the server is not authoritative for any portion 542 of the QNAME. When this happens, the referred-to zone in the 543 authority section is usually the root zone (.). In normal DNS 544 operation, this kind of response is not required for resolution or 545 for correctly answering any query. There is no requirement that 546 any server send upward referrals. Some people regard upward 547 referrals as a sign of a misconfiguration or error. Upward 548 referrals always need some sort of qualifier (such as "upward" or 549 "root"), and are never identified by the bare word "referral". 551 A response that has only a referral contains an empty answer 552 section. It contains the NS RRset for the referred-to zone in the 553 authority section. It may contain RRs that provide addresses in 554 the additional section. The AA bit is clear. 556 In the case where the query matches an alias, and the server is 557 not authoritative for the target of the alias but it is 558 authoritative for some name above the target of the alias, the 559 resolution algorithm will produce a response that contains both 560 the authoritative answer for the alias, and also a referral. Such 561 a partial answer and referral response has data in the answer 562 section. It has the NS RRset for the referred-to zone in the 563 authority section. It may contain RRs that provide addresses in 564 the additional section. The AA bit is set, because the first name 565 in the answer section matches the QNAME and the server is 566 authoritative for that answer (see [RFC1035], Section 4.1.1). 568 5. Resource Records 570 RR: An acronym for resource record. ([RFC1034], Section 3.6.) 572 RRset: A set of resource records with the same label, class and 573 type, but with different data. (Definition from [RFC2181]) Also 574 spelled RRSet in some documents. As a clarification, "same label" 575 in this definition means "same owner name". In addition, 576 [RFC2181] states that "the TTLs of all RRs in an RRSet must be the 577 same". 579 Note that RRSIG resource records do not match this definition. 580 [RFC4035] says: "An RRset MAY have multiple RRSIG RRs associated 581 with it. Note that as RRSIG RRs are closely tied to the RRsets 582 whose signatures they contain, RRSIG RRs, unlike all other DNS RR 583 types, do not form RRsets. In particular, the TTL values among 584 RRSIG RRs with a common owner name do not follow the RRset rules 585 described in [RFC2181]." 587 Master file: "Master files are text files that contain RRs in text 588 form. Since the contents of a zone can be expressed in the form 589 of a list of RRs a master file is most often used to define a 590 zone, though it can be used to list a cache's contents." 591 ([RFC1035], Section 5.) Master files are sometimes called "zone 592 files". 594 Presentation format: The text format used in master files. This 595 format is shown but not formally defined in [RFC1034] and 596 [RFC1035]. The term "presentation format" first appears in 597 [RFC4034]. 599 EDNS: The extension mechanisms for DNS, defined in [RFC6891]. 600 Sometimes called "EDNS0" or "EDNS(0)" to indicate the version 601 number. EDNS allows DNS clients and servers to specify message 602 sizes larger than the original 512 octet limit, to expand the 603 response code space, and potentially to carry additional options 604 that affect the handling of a DNS query. 606 OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to 607 contain control information pertaining to the question-and-answer 608 sequence of a specific transaction. (Definition from [RFC6891], 609 Section 6.1.1) It is used by EDNS. 611 Owner: The domain name where a RR is found ([RFC1034], Section 3.6). 612 Often appears in the term "owner name". 614 SOA field names: DNS documents, including the definitions here, 615 often refer to the fields in the RDATA of an SOA resource record 616 by field name. "SOA" stands for "start of a zone of authority". 617 Those fields are defined in Section 3.3.13 of [RFC1035]. The 618 names (in the order they appear in the SOA RDATA) are MNAME, 619 RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. Note that the 620 meaning of MINIMUM field is updated in Section 4 of [RFC2308]; the 621 new definition is that the MINIMUM field is only "the TTL to be 622 used for negative responses". This document tends to use field 623 names instead of terms that describe the fields. 625 TTL: The maximum "time to live" of a resource record. "A TTL value 626 is an unsigned number, with a minimum value of 0, and a maximum 627 value of 2147483647. That is, a maximum of 2^31 - 1. When 628 transmitted, the TTL is encoded in the less significant 31 bits of 629 the 32 bit TTL field, with the most significant, or sign, bit set 630 to zero." (Quoted from [RFC2181], Section 8) (Note that [RFC1035] 631 erroneously stated that this is a signed integer; that was fixed 632 by [RFC2181].) 634 The TTL "specifies the time interval that the resource record may 635 be cached before the source of the information should again be 636 consulted". (Quoted from [RFC1035], Section 3.2.1) Also: "the 637 time interval (in seconds) that the resource record may be cached 638 before it should be discarded". (Quoted from [RFC1035], 639 Section 4.1.3). Despite being defined for a resource record, the 640 TTL of every resource record in an RRset is required to be the 641 same ([RFC2181], Section 5.2). 643 The reason that the TTL is the maximum time to live is that a 644 cache operator might decide to shorten the time to live for 645 operational purposes, such as if there is a policy to disallow TTL 646 values over a certain number. Some servers are known to ignore 647 the TTL on some RRsets (such as when the authoritative data has a 648 very short TTL) even though this is against the advice in RFC 649 1035. An RRset can be flushed from the cache before the end of 650 the TTL interval, at which point the value of the TTL becomes 651 unknown because the RRset with which it was associated no longer 652 exists. 654 There is also the concept of a "default TTL" for a zone, which can 655 be a configuration parameter in the server software. This is 656 often expressed by a default for the entire server, and a default 657 for a zone using the $TTL directive in a zone file. The $TTL 658 directive was added to the master file format by [RFC2308]. 660 Class independent: A resource record type whose syntax and semantics 661 are the same for every DNS class. A resource record type that is 662 not class independent has different meanings depending on the DNS 663 class of the record, or the meaning is undefined for some class. 664 Most resource record types are defined for class 1 (IN, the 665 Internet), but many are undefined for other classes. 667 Address records: Records whose type is A or AAAA. [RFC2181] 668 informally defines these as "(A, AAAA, etc)". Note that new types 669 of address records could be defined in the future. 671 6. DNS Servers and Clients 673 This section defines the terms used for the systems that act as DNS 674 clients, DNS servers, or both. In the RFCs, DNS servers are 675 sometimes called "name servers", "nameservers", or just "servers". 676 There is no formal definition of DNS server, but the RFCs generally 677 assume that it is an Internet server that listens for queries and 678 sends responses using the DNS protocol defined in [RFC1035] and its 679 successors. 681 It is important to note that the terms "DNS server" and "name server" 682 require context in order to understand the services being provided. 683 Both authoritative servers and recursive resolvers are often called 684 "DNS servers" and "name servers" even though they serve different 685 roles (but may be part of the same software package). 687 For terminology specific to the public DNS root server system, see 688 [RSSAC026]. That document defines terms such as "root server", "root 689 server operator", and terms that are specific to the way that the 690 root zone of the public DNS is served. 692 Resolver: A program "that extract[s] information from name servers 693 in response to client requests." (Quoted from [RFC1034], 694 Section 2.4) A resolver performs queries for a name, type, and 695 class, and receives responses. The logical function is called 696 "resolution". In practice, the term is usually referring to some 697 specific type of resolver (some of which are defined below), and 698 understanding the use of the term depends on understanding the 699 context. 701 A related term is "resolve", which is not formally defined in 702 [RFC1034] or [RFC1035]. An imputed definition might be "asking a 703 question that consists of a domain name, class, and type, and 704 receiving some sort of response". Similarly, an imputed 705 definition of "resolution" might be "the response received from 706 resolving". 708 Stub resolver: A resolver that cannot perform all resolution itself. 709 Stub resolvers generally depend on a recursive resolver to 710 undertake the actual resolution function. Stub resolvers are 711 discussed but never fully defined in Section 5.3.1 of [RFC1034]. 712 They are fully defined in Section 6.1.3.1 of [RFC1123]. 714 Iterative mode: A resolution mode of a server that receives DNS 715 queries and responds with a referral to another server. 716 Section 2.3 of [RFC1034] describes this as "The server refers the 717 client to another server and lets the client pursue the query". A 718 resolver that works in iterative mode is sometimes called an 719 "iterative resolver". See also "iterative resolution" later in 720 this section. 722 Recursive mode: A resolution mode of a server that receives DNS 723 queries and either responds to those queries from a local cache or 724 sends queries to other servers in order to get the final answers 725 to the original queries. Section 2.3 of [RFC1034] describes this 726 as "The first server pursues the query for the client at another 727 server". Section 4.3.1 of of [RFC1034] says "in [recursive] mode 728 the name server acts in the role of a resolver and returns either 729 an error or the answer, but never referrals." That same section 730 also says "The recursive mode occurs when a query with RD set 731 arrives at a server which is willing to provide recursive service; 732 the client can verify that recursive mode was used by checking 733 that both RA and RD are set in the reply." 735 A server operating in recursive mode may be thought of as having a 736 name server side (which is what answers the query) and a resolver 737 side (which performs the resolution function). Systems operating 738 in this mode are commonly called "recursive servers". Sometimes 739 they are called "recursive resolvers". While strictly the 740 difference between these is that one of them sends queries to 741 another recursive server and the other does not, in practice it is 742 not possible to know in advance whether the server that one is 743 querying will also perform recursion; both terms can be observed 744 in use interchangeably. 746 Recursive resolver: A resolver that acts in recursive mode. In 747 general, a recursive resolver is expected to cache the answers it 748 receives (which would make it a full-service resolver), but some 749 recursive resolvers might not cache. 751 [RFC4697] tried to differentiate between a recursive resolver and 752 an iterative resolver. 754 Recursive query: A query with the Recursion Desired (RD) bit set to 755 1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive 756 service is available and is requested by the RD bit in the query, 757 the server uses its resolver to answer the query. (See 758 Section 4.3.2 of [RFC1035].) 760 Non-recursive query: A query with the Recursion Desired (RD) bit set 761 to 0 in the header. A server can answer non-recursive queries 762 using only local information: the response contains either an 763 error, the answer, or a referral to some other server "closer" to 764 the answer. (See Section 4.3.1 of [RFC1035].) 766 Iterative resolution: A name server may be presented with a query 767 that can only be answered by some other server. The two general 768 approaches to dealing with this problem are "recursive", in which 769 the first server pursues the query on behalf of the client at 770 another server, and "iterative", in which the server refers the 771 client to another server and lets the client pursue the query 772 there. (See Section 2.3 of [RFC1034].) 774 In iterative resolution, the client repeatedly makes non-recursive 775 queries and follows referrals and/or aliases. The iterative 776 resolution algorithm is described in Section 5.3.3 of [RFC1034]. 778 Full resolver: This term is used in [RFC1035], but it is not defined 779 there. RFC 1123 defines a "full-service resolver" that may or may 780 not be what was intended by "full resolver" in [RFC1035]. This 781 term is not properly defined in any RFC. 783 Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this 784 term to mean a resolver that acts in recursive mode with a cache 785 (and meets other requirements). 787 Priming: "The act of finding the list of root servers from a 788 configuration that lists some or all of the purported IP addresses 789 of some or all of those root servers." (Quoted from [RFC8109], 790 Section 2.) In order to operate in recursive mode, a resolver 791 needs to know the address of at least one root server. Priming is 792 most often done from a configuration setting that contains a list 793 of authoritative servers for the root zone. 795 Root hints: "Operators who manage a DNS recursive resolver typically 796 need to configure a 'root hints file'. This file contains the 797 names and IP addresses of the authoritative name servers for the 798 root zone, so the software can bootstrap the DNS resolution 799 process. For many pieces of software, this list comes built into 800 the software." (Quoted from [IANA_RootFiles]) This file is often 801 used in priming. 803 Negative caching: "The storage of knowledge that something does not 804 exist, cannot give an answer, or does not give an answer." 805 (Quoted from [RFC2308], Section 1) 807 Authoritative server: "A server that knows the content of a DNS zone 808 from local knowledge, and thus can answer queries about that zone 809 without needing to query other servers." (Quoted from [RFC2182], 810 Section 2.) An authoritative server is named in the NS ("name 811 server") record in a zone. It is a system that responds to DNS 812 queries with information about zones for which it has been 813 configured to answer with the AA flag in the response header set 814 to 1. It is a server that has authority over one or more DNS 815 zones. Note that it is possible for an authoritative server to 816 respond to a query without the parent zone delegating authority to 817 that server. Authoritative servers also provide "referrals", 818 usually to child zones delegated from them; these referrals have 819 the AA bit set to 0 and come with referral data in the Authority 820 and (if needed) the Additional sections. 822 Authoritative-only server: A name server that only serves 823 authoritative data and ignores requests for recursion. It will 824 "not normally generate any queries of its own. Instead, it 825 answers non-recursive queries from iterative resolvers looking for 826 information in zones it serves." (Quoted from [RFC4697], 827 Section 2.4) In this case, "ignores requests for recursion" means 828 "responds to requests for recursion with responses indicating that 829 recursion was not performed". 831 Zone transfer: The act of a client requesting a copy of a zone and 832 an authoritative server sending the needed information. (See 833 Section 7 for a description of zones.) There are two common 834 standard ways to do zone transfers: the AXFR ("Authoritative 835 Transfer") mechanism to copy the full zone (described in 836 [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy 837 only parts of the zone that have changed (described in [RFC1995]). 838 Many systems use non-standard methods for zone transfer outside 839 the DNS protocol. 841 Slave server: See "Secondary server". 843 Secondary server: "An authoritative server which uses zone transfer 844 to retrieve the zone" (Quoted from [RFC1996], Section 2.1). 845 Secondary servers are also discussed in [RFC1034]. [RFC2182] 846 describes secondary servers in more detail. Although early DNS 847 RFCs such as [RFC1996] referred to this as a "slave", the current 848 common usage has shifted to calling it a "secondary". 850 Master server: See "Primary server". 852 Primary server: "Any authoritative server configured to be the 853 source of zone transfer for one or more [secondary] servers" 854 (Quoted from [RFC1996], Section 2.1) or, more specifically, "an 855 authoritative server configured to be the source of AXFR or IXFR 856 data for one or more [secondary] servers" (Quoted from [RFC2136]). 857 Primary servers are also discussed in [RFC1034]. Although early 858 DNS RFCs such as [RFC1996] referred to this as a "master", the 859 current common usage has shifted to "primary". 861 Primary master: "The primary master is named in the zone's SOA MNAME 862 field and optionally by an NS RR". (Quoted from [RFC1996], 863 Section 2.1). [RFC2136] defines "primary master" as "Master 864 server at the root of the AXFR/IXFR dependency graph. The primary 865 master is named in the zone's SOA MNAME field and optionally by an 866 NS RR. There is by definition only one primary master server per 867 zone." The idea of a primary master is only used by [RFC2136], 868 and is considered archaic in other parts of the DNS. 870 The idea of a primary master is only used in [RFC1996] and 871 [RFC2136]. A modern interpretation of the term "primary master" 872 is a server that is both authoritative for a zone and that gets 873 its updates to the zone from configuration (such as a master file) 874 or from UPDATE transactions. 876 Stealth server: This is "like a slave server except not listed in an 877 NS RR for the zone." (Quoted from [RFC1996], Section 2.1) 879 Hidden master: A stealth server that is a primary server for zone 880 transfers. "In this arrangement, the master name server that 881 processes the updates is unavailable to general hosts on the 882 Internet; it is not listed in the NS RRset." (Quoted from 883 [RFC6781], Section 3.4.3). An earlier RFC, [RFC4641], said that 884 the hidden master's name "appears in the SOA RRs MNAME field", 885 although in some setups, the name does not appear at all in the 886 public DNS. A hidden master can also be a secondary server for 887 the zone itself. 889 Forwarding: The process of one server sending a DNS query with the 890 RD bit set to 1 to another server to resolve that query. 891 Forwarding is a function of a DNS resolver; it is different than 892 simply blindly relaying queries. 894 [RFC5625] does not give a specific definition for forwarding, but 895 describes in detail what features a system that forwards needs to 896 support. Systems that forward are sometimes called "DNS proxies", 897 but that term has not yet been defined (even in [RFC5625]). 899 Forwarder: Section 1 of [RFC2308] describes a forwarder as "a 900 nameserver used to resolve queries instead of directly using the 901 authoritative nameserver chain". [RFC2308] further says "The 902 forwarder typically either has better access to the internet, or 903 maintains a bigger cache which may be shared amongst many 904 resolvers." That definition appears to suggest that forwarders 905 normally only query authoritative servers. In current use, 906 however, forwarders often stand between stub resolvers and 907 recursive servers. [RFC2308] is silent on whether a forwarder is 908 iterative-only or can be a full-service resolver. 910 Policy-implementing resolver: A resolver acting in recursive mode 911 that changes some of the answers that it returns based on policy 912 criteria, such as to prevent access to malware sites or 913 objectionable content. In general, a stub resolver has no idea 914 whether upstream resolvers implement such policy or, if they do, 915 the exact policy about what changes will be made. In some cases, 916 the user of the stub resolver has selected the policy-implementing 917 resolver with the explicit intention of using it to implement the 918 policies. In other cases, policies are imposed without the user 919 of the stub resolver being informed. 921 Open resolver: A full-service resolver that accepts and processes 922 queries from any (or nearly any) client. This is sometimes also 923 called a "public resolver", although the term "public resolver" is 924 used more with open resolvers that are meant to be open, as 925 compared to the vast majority of open resolvers that are probably 926 misconfigured to be open. Open resolvers are discussed in 927 [RFC5358] 929 Split DNS: The terms "split DNS" and "split-horizon DNS" have long 930 been used in the DNS community without formal definition. In 931 general, they refer to situations in which DNS servers that are 932 authoritative for a particular set of domains provide partly or 933 completely different answers in those domains depending on the 934 source of the query. The effect of this is that a domain name 935 that is notionally globally unique nevertheless has different 936 meanings for different network users. This can sometimes be the 937 result of a "view" configuration, described below. 939 [RFC2775], Section 3.8 gives a related definition that is too 940 specific to be generally useful. 942 View: A configuration for a DNS server that allows it to provide 943 different responses depending on attributes of the query, such as 944 for "split DNS". Typically, views differ by the source IP address 945 of a query, but can also be based on the destination IP address, 946 the type of query (such as AXFR), whether it is recursive, and so 947 on. Views are often used to provide more names or different 948 addresses to queries from "inside" a protected network than to 949 those "outside" that network. Views are not a standardized part 950 of the DNS, but they are widely implemented in server software. 952 Passive DNS: A mechanism to collect DNS data by storing DNS 953 responses from name servers. Some of these systems also collect 954 the DNS queries associated with the responses, although doing so 955 raises some privacy concerns. Passive DNS databases can be used 956 to answer historical questions about DNS zones such as which 957 values were present at a given time in the past, or when a name 958 was spotted first. Passive DNS databases allow searching of the 959 stored records on keys other than just the name and type, such as 960 "find all names which have A records of a particular value". 962 Anycast: "The practice of making a particular service address 963 available in multiple, discrete, autonomous locations, such that 964 datagrams sent are routed to one of several available locations." 965 (Quoted from [RFC4786], Section 2) See [RFC4786] for more detail 966 on Anycast and other terms that are specific to its use. 968 Instance: "When anycast routing is used to allow more than one 969 server to have the same IP address, each one of those servers is 970 commonly referred to as an 'instance'." "An instance of a server, 971 such as a root server, is often referred to as an 'Anycast 972 instance'." (Quoted from [RSSAC026]) 974 Privacy-enabling DNS server: "A DNS server that implements DNS over 975 TLS [RFC7858] and may optionally implement DNS over DTLS 976 [RFC8094]." (Quoted from [RFC8310], Section 2) 978 7. Zones 980 This section defines terms that are used when discussing zones that 981 are being served or retrieved. 983 Zone: "Authoritative information is organized into units called 984 'zones', and these zones can be automatically distributed to the 985 name servers which provide redundant service for the data in a 986 zone." (Quoted from [RFC1034], Section 2.4) 988 Child: "The entity on record that has the delegation of the domain 989 from the Parent." (Quoted from [RFC7344], Section 1.1) 991 Parent: "The domain in which the Child is registered." (Quoted from 992 [RFC7344], Section 1.1) Earlier, "parent name server" was defined 993 in [RFC0882] as "the name server that has authority over the place 994 in the domain name space that will hold the new domain". (Note 995 that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].) 996 [RFC0819] also has some description of the relationship between 997 parents and children. 999 Origin: 1001 (a) "The domain name that appears at the top of a zone (just below 1002 the cut that separates the zone from its parent). The name of the 1003 zone is the same as the name of the domain at the zone's origin." 1004 (Quoted from [RFC2181], Section 6.) These days, this sense of 1005 "origin" and "apex" (defined below) are often used 1006 interchangeably. 1008 (b) The domain name within which a given relative domain name 1009 appears in zone files. Generally seen in the context of 1010 "$ORIGIN", which is a control entry defined in [RFC1035], 1011 Section 5.1, as part of the master file format. For example, if 1012 the $ORIGIN is set to "example.org.", then a master file line for 1013 "www" is in fact an entry for "www.example.org.". 1015 Apex: The point in the tree at an owner of an SOA and corresponding 1016 authoritative NS RRset. This is also called the "zone apex". 1017 [RFC4033] defines it as "the name at the child's side of a zone 1018 cut". The "apex" can usefully be thought of as a data-theoretic 1019 description of a tree structure, and "origin" is the name of the 1020 same concept when it is implemented in zone files. The 1021 distinction is not always maintained in use, however, and one can 1022 find uses that conflict subtly with this definition. [RFC1034] 1023 uses the term "top node of the zone" as a synonym of "apex", but 1024 that term is not widely used. These days, the first sense of 1025 "origin" (above) and "apex" are often used interchangeably. 1027 Zone cut: The delimitation point between two zones where the origin 1028 of one of the zones is the child of the other zone. 1030 "Zones are delimited by 'zone cuts'. Each zone cut separates a 1031 'child' zone (below the cut) from a 'parent' zone (above the 1032 cut)." (Quoted from [RFC2181], Section 6; note that this is 1033 barely an ostensive definition.) Section 4.2 of [RFC1034] uses 1034 "cuts" instead of "zone cut". 1036 Delegation: The process by which a separate zone is created in the 1037 name space beneath the apex of a given domain. Delegation happens 1038 when an NS RRset is added in the parent zone for the child origin. 1039 Delegation inherently happens at a zone cut. The term is also 1040 commonly a noun: the new zone that is created by the act of 1041 delegating. 1043 Authoritative data: "All of the RRs attached to all of the nodes 1044 from the top node of the zone down to leaf nodes or nodes above 1045 cuts around the bottom edge of the zone." (Quoted from [RFC1034], 1046 Section 4.2.1) Note that this definition might inadvertently also 1047 cause any NS records that appear in the zone to be included, even 1048 those that might not truly be authoritative because there are 1049 identical NS RRs below the zone cut. This reveals the ambiguity 1050 in the notion of authoritative data, because the parent-side NS 1051 records authoritatively indicate the delegation, even though they 1052 are not themselves authoritative data. 1054 [RFC4033], Section 2, defines "Authoritative RRset" which is 1055 related to authoritative data but has a more precise definition. 1057 Lame delegation: "A lame delegations exists when a nameserver is 1058 delegated responsibility for providing nameservice for a zone (via 1059 NS records) but is not performing nameservice for that zone 1060 (usually because it is not set up as a primary or secondary for 1061 the zone)." (Quoted from [RFC1912], Section 2.8) 1063 Another definition is that a lame delegation "happens when a name 1064 server is listed in the NS records for some domain and in fact it 1065 is not a server for that domain. Queries are thus sent to the 1066 wrong servers, who don't know nothing (at least not as expected) 1067 about the queried domain. Furthermore, sometimes these hosts (if 1068 they exist!) don't even run name servers." (Quoted from 1069 [RFC1713], Section 2.3) 1071 Glue records: "[Resource records] which are not part of the 1072 authoritative data [of the zone], and are address resource records 1073 for the [name servers in subzones]. These RRs are only necessary 1074 if the name server's name is 'below' the cut, and are only used as 1075 part of a referral response." Without glue "we could be faced 1076 with the situation where the NS RRs tell us that in order to learn 1077 a name server's address, we should contact the server using the 1078 address we wish to learn." (Definition from [RFC1034], 1079 Section 4.2.1) 1081 A later definition is that glue "includes any record in a zone 1082 file that is not properly part of that zone, including nameserver 1083 records of delegated sub-zones (NS records), address records that 1084 accompany those NS records (A, AAAA, etc), and any other stray 1085 data that might appear" ([RFC2181], Section 5.4.1). Although glue 1086 is sometimes used today with this wider definition in mind, the 1087 context surrounding the [RFC2181] definition suggests it is 1088 intended to apply to the use of glue within the document itself 1089 and not necessarily beyond. 1091 Bailiwick: "In-bailiwick" is an adjective to describe a name server 1092 whose name is either a subdomain of or (rarely) the same as the 1093 origin of the zone that contains the delegation to the name 1094 server. In-bailiwick name servers may have glue records in their 1095 parent zone (using the first of the definitions of "glue records" 1096 in the definition above). (The term "bailiwick" means the 1097 district or territory where a bailiff or policeman has 1098 jurisdiction.) 1100 "In-bailiwick" names are divided into two type of name server 1101 names: "in-domain" names and "sibling domain" names. 1103 * In-domain: an adjective to describe a name server whose name is 1104 either subordinate to or (rarely) the same as the owner name of 1105 the NS resource records. An in-domain name server name MUST 1106 have glue records or name resolution fails. For example, a 1107 delegation for "child.example.com" may have "in-domain" name 1108 server name "ns.child.example.com". 1110 * Sibling domain: a name server's name that is either subordinate 1111 to or (rarely) the same as the zone origin and not subordinate 1112 to or the same as the owner name of the NS resource records. 1113 Glue records for sibling domains are allowed, but not 1114 necessary. For example, a delegation for "child.example.com" 1115 in "example.com" zone may have "sibling" name server name 1116 "ns.another.example.com". 1118 "Out-of-bailiwick" is the antonym of in-bailiwick. An adjective 1119 to describe a name server whose name is not subordinate to or the 1120 same as the zone origin. Glue records for out-of-bailiwick name 1121 servers are useless. Following table shows examples of delegation 1122 types. 1124 Delegation |Parent|Name Server Name | Type 1125 -----------+------+------------------+----------------------------- 1126 com | . |a.gtld-servers.net|in-bailiwick / sibling domain 1127 net | . |a.gtld-servers.net|in-bailiwick / in-domain 1128 example.org| org |ns.example.org |in-bailiwick / in-domain 1129 example.org| org |ns.ietf.org |in-bailiwick / sibling domain 1130 example.org| org |ns.example.com |out-of-bailiwick 1131 example.jp | jp |ns.example.jp |in-bailiwick / in-domain 1132 example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain 1133 example.jp | jp |ns.example.com |out-of-bailiwick 1135 Root zone: The zone of a DNS-based tree whose apex is the zero- 1136 length label. Also sometimes called "the DNS root". 1138 Empty non-terminals (ENT): "Domain names that own no resource 1139 records but have subdomains that do." (Quoted from [RFC4592], 1140 Section 2.2.2.) A typical example is in SRV records: in the name 1141 "_sip._tcp.example.com", it is likely that "_tcp.example.com" has 1142 no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV 1143 RRset. 1145 Delegation-centric zone: A zone that consists mostly of delegations 1146 to child zones. This term is used in contrast to a zone that 1147 might have some delegations to child zones, but also has many data 1148 resource records for the zone itself and/or for child zones. The 1149 term is used in [RFC4956] and [RFC5155], but is not defined there. 1151 Occluded name: "The addition of a delegation point via dynamic 1152 update will render all subordinate domain names to be in a limbo, 1153 still part of the zone, but not available to the lookup process. 1154 The addition of a DNAME resource record has the same impact. The 1155 subordinate names are said to be 'occluded'." (Quoted from 1156 [RFC5936], Section 3.5) 1158 Fast flux DNS: This "occurs when a domain is found in DNS using A 1159 records to multiple IP addresses, each of which has a very short 1160 Time-to-Live (TTL) value associated with it. This means that the 1161 domain resolves to varying IP addresses over a short period of 1162 time." (Quoted from [RFC6561], Section 1.1.5, with typo 1163 corrected) In addition to having legitimate uses, fast flux DNS 1164 can used to deliver malware. Because the addresses change so 1165 rapidly, it is difficult to ascertain all the hosts. It should be 1166 noted that the technique also works with AAAA records, but such 1167 use is not frequently observed on the Internet as of this writing. 1169 Reverse DNS, reverse lookup: "The process of mapping an address to a 1170 name is generally known as a 'reverse lookup', and the IN- 1171 ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse 1172 DNS'." (Quoted from [RFC5855], Section 1) 1174 Forward lookup: "Hostname-to-address translation". (Quoted from 1175 [RFC2133], Section 6) 1177 arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain 1178 was originally established as part of the initial deployment of 1179 the DNS, to provide a transition mechanism from the Host Tables 1180 that were common in the ARPANET, as well as a home for the IPv4 1181 reverse mapping domain. During 2000, the abbreviation was 1182 redesignated to 'Address and Routing Parameter Area' in the hope 1183 of reducing confusion with the earlier network name." (Quoted 1184 from [RFC3172], Section 2.) .arpa is an "infrastructure domain", a 1185 domain whose "role is to support the operating infrastructure of 1186 the Internet". (Quoted from [RFC3172], Section 2.) See [RFC3172] 1187 for more history of this name. 1189 Service name: "Service names are the unique key in the Service Name 1190 and Transport Protocol Port Number registry. This unique symbolic 1191 name for a service may also be used for other purposes, such as in 1192 DNS SRV records." (Quoted from [RFC6335], Section 5.) 1194 8. Wildcards 1196 Wildcard: [RFC1034] defined "wildcard", but in a way that turned out 1197 to be confusing to implementers. For an extended discussion of 1198 wildcards, including clearer definitions, see [RFC4592]. Special 1199 treatment is given to RRs with owner names starting with the label 1200 "*". "Such RRs are called 'wildcards'. Wildcard RRs can be 1201 thought of as instructions for synthesizing RRs." (Quoted from 1202 [RFC1034], Section 4.3.3) 1204 Asterisk label: "The first octet is the normal label type and length 1205 for a 1-octet-long label, and the second octet is the ASCII 1206 representation for the '*' character. A descriptive name of a 1207 label equaling that value is an 'asterisk label'." (Quoted from 1208 [RFC4592], Section 2.1.1) 1210 Wildcard domain name: "A 'wildcard domain name' is defined by having 1211 its initial (i.e., leftmost or least significant) label be 1212 asterisk label." (Quoted from [RFC4592], Section 2.1.1) 1214 Closest encloser: "The longest existing ancestor of a name." 1215 (Quoted from [RFC5155], Section 1.3) An earlier definition is "The 1216 node in the zone's tree of existing domain names that has the most 1217 labels matching the query name (consecutively, counting from the 1218 root label downward). Each match is a 'label match' and the order 1219 of the labels is the same." (Quoted from [RFC4592], 1220 Section 3.3.1) 1222 Closest provable encloser: "The longest ancestor of a name that can 1223 be proven to exist. Note that this is only different from the 1224 closest encloser in an Opt-Out zone." (Quoted from [RFC5155], 1225 Section 1.3) 1227 Next closer name: "The name one label longer than the closest 1228 provable encloser of a name." (Quoted from [RFC5155], 1229 Section 1.3) 1231 Source of Synthesis: "The source of synthesis is defined in the 1232 context of a query process as that wildcard domain name 1233 immediately descending from the closest encloser, provided that 1234 this wildcard domain name exists. 'Immediately descending' means 1235 that the source of synthesis has a name of the form: .." (Quoted from [RFC4592], 1237 Section 3.3.1) 1239 9. Registration Model 1241 Registry: The administrative operation of a zone that allows 1242 registration of names within that zone. People often use this 1243 term to refer only to those organizations that perform 1244 registration in large delegation-centric zones (such as TLDs); but 1245 formally, whoever decides what data goes into a zone is the 1246 registry for that zone. This definition of "registry" is from a 1247 DNS point of view; for some zones, the policies that determine 1248 what can go in the zone are decided by superior zones and not the 1249 registry operator. 1251 Registrant: An individual or organization on whose behalf a name in 1252 a zone is registered by the registry. In many zones, the registry 1253 and the registrant may be the same entity, but in TLDs they often 1254 are not. 1256 Registrar: A service provider that acts as a go-between for 1257 registrants and registries. Not all registrations require a 1258 registrar, though it is common to have registrars involved in 1259 registrations in TLDs. 1261 EPP: The Extensible Provisioning Protocol (EPP), which is commonly 1262 used for communication of registration information between 1263 registries and registrars. EPP is defined in [RFC5730]. 1265 WHOIS: A protocol specified in [RFC3912], often used for querying 1266 registry databases. WHOIS data is frequently used to associate 1267 registration data (such as zone management contacts) with domain 1268 names. The term "WHOIS data" is often used as a synonym for the 1269 registry database, even though that database may be served by 1270 different protocols, particularly RDAP. The WHOIS protocol is 1271 also used with IP address registry data. 1273 RDAP: The Registration Data Access Protocol, defined in [RFC7480], 1274 [RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The 1275 RDAP protocol and data format are meant as a replacement for 1276 WHOIS. 1278 DNS operator: An entity responsible for running DNS servers. For a 1279 zone's authoritative servers, the registrant may act as their own 1280 DNS operator, or their registrar may do it on their behalf, or 1281 they may use a third-party operator. For some zones, the registry 1282 function is performed by the DNS operator plus other entities who 1283 decide about the allowed contents of the zone. 1285 Public suffix: "A domain that is controlled by a public registry." 1286 (Quoted from [RFC6265], Section 5.3) A common definition for this 1287 term is a domain under which subdomains can be registered by third 1288 parties, and on which HTTP cookies (which are described in detail 1289 in [RFC6265]) should not be set. There is no indication in a 1290 domain name whether it is a public suffix; that can only be 1291 determined by outside means. In fact, both a domain and a 1292 subdomain of that domain can be public suffixes. 1294 There is nothing inherent in a domain name to indicate whether it 1295 is a public suffix. One resource for identifying public suffixes 1296 is the Public Suffix List (PSL) maintained by Mozilla 1297 (http://publicsuffix.org/). 1299 For example, at the time this document is published, the "com.au" 1300 domain is listed as a public suffix in the PSL. (Note that this 1301 example might change in the future.) 1303 Note that the term "public suffix" is controversial in the DNS 1304 community for many reasons, and may be significantly changed in 1305 the future. One example of the difficulty of calling a domain a 1306 public suffix is that designation can change over time as the 1307 registration policy for the zone changes, such as was the case 1308 with the "uk" TLD in 2014. 1310 Subordinate and Superordinate: These terms are introduced in 1311 [RFC3731] for use in the registration model, but not defined 1312 there. Instead, they are given in examples. "For example, domain 1313 name 'example.com' has a superordinate relationship to host name 1314 ns1.example.com'." "For example, host ns1.example1.com is a 1315 subordinate host of domain example1.com, but it is a not a 1316 subordinate host of domain example2.com." (Quoted from [RFC3731], 1317 Section 1.1.) These terms are strictly ways of referring to the 1318 relationship standing of two domains where one is a subdomain of 1319 the other. 1321 10. General DNSSEC 1323 Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and 1324 [RFC5155]. The terms that have caused confusion in the DNS community 1325 are highlighted here. 1327 DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in 1328 some RFCs, have not been formally defined. However, Section 2 of 1329 [RFC4033] defines many types of resolvers and validators, 1330 including "non-validating security-aware stub resolver", "non- 1331 validating stub resolver", "security-aware name server", 1332 "security-aware recursive name server", "security-aware resolver", 1333 "security-aware stub resolver", and "security-oblivious 1334 'anything'". (Note that the term "validating resolver", which is 1335 used in some places in DNSSEC-related documents, is also not 1336 defined in those RFCs, but is defined below.) 1338 Signed zone: "A zone whose RRsets are signed and that contains 1339 properly constructed DNSKEY, Resource Record Signature (RRSIG), 1340 Next Secure (NSEC), and (optionally) DS records." (Quoted from 1341 [RFC4033], Section 2.) It has been noted in other contexts that 1342 the zone itself is not really signed, but all the relevant RRsets 1343 in the zone are signed. Nevertheless, if a zone that should be 1344 signed contains any RRsets that are not signed (or opted out), 1345 those RRsets will be treated as bogus, so the whole zone needs to 1346 be handled in some way. 1348 It should also be noted that, since the publication of [RFC6840], 1349 NSEC records are no longer required for signed zones: a signed 1350 zone might include NSEC3 records instead. [RFC7129] provides 1351 additional background commentary and some context for the NSEC and 1352 NSEC3 mechanisms used by DNSSEC to provide authenticated denial- 1353 of-existence responses. NSEC and NSEC3 are described below. 1355 Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that 1356 is not signed". Section 2 of [RFC4035] defines this as "A zone 1357 that does not include these records [properly constructed DNSKEY, 1358 Resource Record Signature (RRSIG), Next Secure (NSEC), and 1359 (optionally) DS records] according to the rules in this section". 1360 There is an important note at the end of Section 5.2 of [RFC4035] 1361 that defines an additional situation in which a zone is considered 1362 unsigned: "If the resolver does not support any of the algorithms 1363 listed in an authenticated DS RRset, then the resolver will not be 1364 able to verify the authentication path to the child zone. In this 1365 case, the resolver SHOULD treat the child zone as if it were 1366 unsigned." 1368 NSEC: "The NSEC record allows a security-aware resolver to 1369 authenticate a negative reply for either name or type non- 1370 existence with the same mechanisms used to authenticate other DNS 1371 replies." (Quoted from [RFC4033], Section 3.2.) In short, an 1372 NSEC record provides authenticated denial of existence. 1374 "The NSEC resource record lists two separate things: the next 1375 owner name (in the canonical ordering of the zone) that contains 1376 authoritative data or a delegation point NS RRset, and the set of 1377 RR types present at the NSEC RR's owner name." (Quoted from 1378 Section 4 of RFC 4034) 1380 NSEC3: Like the NSEC record, the NSEC3 record also provides 1381 authenticated denial of existence; however, NSEC3 records mitigate 1382 against zone enumeration and support Opt-Out. NSEC3 resource 1383 records require associated NSEC3PARAM resource records. NSEC3 and 1384 NSEC3PARAM resource records are defined in [RFC5155]. 1386 Note that [RFC6840] says that [RFC5155] "is now considered part of 1387 the DNS Security Document Family as described by Section 10 of 1388 [RFC4033]". This means that some of the definitions from earlier 1389 RFCs that only talk about NSEC records should probably be 1390 considered to be talking about both NSEC and NSEC3. 1392 Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover 1393 unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1.) 1394 Opt-out tackles the high costs of securing a delegation to an 1395 insecure zone. When using Opt-Out, names that are an insecure 1396 delegation (and empty non-terminals that are only derived from 1397 insecure delegations) don't require an NSEC3 record or its 1398 corresponding RRSIG records. Opt-Out NSEC3 records are not able 1399 to prove or deny the existence of the insecure delegations. 1400 (Adapted from [RFC7129], Section 5.1) 1402 Insecure delegation: "A signed name containing a delegation (NS 1403 RRset), but lacking a DS RRset, signifying a delegation to an 1404 unsigned subzone." (Quoted from [RFC4956], Section 2.) 1406 Zone enumeration: "The practice of discovering the full content of a 1407 zone via successive queries." (Quoted from [RFC5155], 1408 Section 1.3.) This is also sometimes called "zone walking". Zone 1409 enumeration is different from zone content guessing where the 1410 guesser uses a large dictionary of possible labels and sends 1411 successive queries for them, or matches the contents of NSEC3 1412 records against such a dictionary. 1414 Validation: Validation, in the context of DNSSEC, refers to one of 1415 the following: 1417 * Checking the validity of DNSSEC signatures 1419 * Checking the validity of DNS responses, such as those including 1420 authenticated denial of existence 1422 * Building an authentication chain from a trust anchor to a DNS 1423 response or individual DNS RRsets in a response 1425 The first two definitions above consider only the validity of 1426 individual DNSSEC components such as the RRSIG validity or NSEC 1427 proof validity. The third definition considers the components of 1428 the entire DNSSEC authentication chain, and thus requires 1429 "configured knowledge of at least one authenticated DNSKEY or DS 1430 RR" (as described in [RFC4035], Section 5). 1432 [RFC4033], Section 2, says that a "Validating Security-Aware Stub 1433 Resolver... performs signature validation" and uses a trust anchor 1434 "as a starting point for building the authentication chain to a 1435 signed DNS response", and thus uses the first and third 1436 definitions above. The process of validating an RRSIG resource 1437 record is described in [RFC4035], Section 5.3. 1439 [RFC5155] refers to validating responses throughout the document, 1440 in the context of hashed authenticated denial of existence; this 1441 uses the second definition above. 1443 The term "authentication" is used interchangeably with 1444 "validation", in the sense of the third definition above. 1445 [RFC4033], Section 2, describes the chain linking trust anchor to 1446 DNS data as the "authentication chain". A response is considered 1447 to be authentic if "all RRsets in the Answer and Authority 1448 sections of the response [are considered] to be authentic" 1449 ([RFC4035]). DNS data or responses deemed to be authentic or 1450 validated have a security status of "secure" ([RFC4035], 1451 Section 4.3; [RFC4033], Section 5). "Authenticating both DNS keys 1452 and data is a matter of local policy, which may extend or even 1453 override the [DNSSEC] protocol extensions" ([RFC4033], 1454 Section 3.1). 1456 The term "verification", when used, is usually synonym for 1457 "validation". 1459 Validating resolver: A security-aware recursive name server, 1460 security-aware resolver, or security-aware stub resolver that is 1461 applying at least one of the definitions of validation (above), as 1462 appropriate to the resolution context. For the same reason that 1463 the generic term "resolver" is sometimes ambiguous and needs to be 1464 evaluated in context (see Section 6), "validating resolver" is a 1465 context-sensitive term. 1467 Key signing key (KSK): DNSSEC keys that "only sign the apex DNSKEY 1468 RRset in a zone."(Quoted from [RFC6781], Section 3.1) 1470 Zone signing key (ZSK): "DNSSEC keys that can be used to sign all 1471 the RRsets in a zone that require signatures, other than the apex 1472 DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Also note 1473 that a ZSK is sometimes used to sign the apex DNSKEY RRset. 1475 Combined signing key (CSK): "In cases where the differentiation 1476 between the KSK and ZSK is not made, i.e., where keys have the 1477 role of both KSK and ZSK, we talk about a Single-Type Signing 1478 Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes 1479 called a "combined signing key" or CSK. It is operational 1480 practice, not protocol, that determines whether a particular key 1481 is a ZSK, a KSK, or a CSK. 1483 Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be 1484 used to distinguish between keys that are intended to be used as 1485 the secure entry point into the zone when building chains of 1486 trust, i.e., they are (to be) pointed to by parental DS RRs or 1487 configured as a trust anchor. Therefore, it is suggested that the 1488 SEP flag be set on keys that are used as KSKs and not on keys that 1489 are used as ZSKs, while in those cases where a distinction between 1490 a KSK and ZSK is not made (i.e., for a Single-Type Signing 1491 Scheme), it is suggested that the SEP flag be set on all keys." 1492 (Quoted from [RFC6781], Section 3.2.3.) Note that the SEP flag is 1493 only a hint, and its presence or absence may not be used to 1494 disqualify a given DNSKEY RR from use as a KSK or ZSK during 1495 validation. 1497 The original definition of SEPs was in [RFC3757]. That definition 1498 clearly indicated that the SEP was a key, not just a bit in the 1499 key. The abstract of [RFC3757] says: "With the Delegation Signer 1500 (DS) resource record (RR), the concept of a public key acting as a 1501 secure entry point (SEP) has been introduced. During exchanges of 1502 public keys with the parent there is a need to differentiate SEP 1503 keys from other public keys in the Domain Name System KEY (DNSKEY) 1504 resource record set. A flag bit in the DNSKEY RR is defined to 1505 indicate that DNSKEY is to be used as a SEP." That definition of 1506 the SEP as a key was made obsolete by [RFC4034], and the 1507 definition from [RFC6781] is consistent with [RFC4034]. 1509 Trust anchor: "A configured DNSKEY RR or DS RR hash of a DNSKEY RR. 1510 A validating security-aware resolver uses this public key or hash 1511 as a starting point for building the authentication chain to a 1512 signed DNS response. In general, a validating resolver will have 1513 to obtain the initial values of its trust anchors via some secure 1514 or trusted means outside the DNS protocol." (Quoted from 1515 [RFC4033], Section 2) 1517 DNSSEC Policy (DP): A statement that "sets forth the security 1518 requirements and standards to be implemented for a DNSSEC-signed 1519 zone." (Quoted from [RFC6841], Section 2) 1521 DNSSEC Practice Statement (DPS): "A practices disclosure document 1522 that may support and be a supplemental document to the DNSSEC 1523 Policy (if such exists), and it states how the management of a 1524 given zone implements procedures and controls at a high level." 1525 (Quoted from [RFC6841], Section 2) 1527 Hardware security module (HSM): A specialized piece of hardware that 1528 is used to create keys for signatures and to sign messages. In 1529 DNSSEC, HSMs are often used to hold the private keys for KSKs and 1530 ZSKs and to create the signatures used in RRSIG records at 1531 periodic intervals. 1533 Signing software: Authoritative DNS servers that support DNSSEC 1534 often contain software that facilitates the creation and 1535 maintenance of DNSSEC signatures in zones. There is also stand- 1536 alone software that can be used to sign a zone regardless of 1537 whether the authoritative server itself supports signing. 1538 Sometimes signing software can support particular HSMs as part of 1539 the signing process. 1541 11. DNSSEC States 1543 A validating resolver can determine that a response is in one of four 1544 states: secure, insecure, bogus, or indeterminate. These states are 1545 defined in [RFC4033] and [RFC4035], although the two definitions 1546 differ a bit. This document makes no effort to reconcile the two 1547 definitions, and takes no position as to whether they need to be 1548 reconciled. 1550 Section 5 of [RFC4033] says: 1552 A validating resolver can determine the following 4 states: 1554 Secure: The validating resolver has a trust anchor, has a chain 1555 of trust, and is able to verify all the signatures in the 1556 response. 1558 Insecure: The validating resolver has a trust anchor, a chain 1559 of trust, and, at some delegation point, signed proof of the 1560 non-existence of a DS record. This indicates that subsequent 1561 branches in the tree are provably insecure. A validating 1562 resolver may have a local policy to mark parts of the domain 1563 space as insecure. 1565 Bogus: The validating resolver has a trust anchor and a secure 1566 delegation indicating that subsidiary data is signed, but 1567 the response fails to validate for some reason: missing 1568 signatures, expired signatures, signatures with unsupported 1569 algorithms, data missing that the relevant NSEC RR says 1570 should be present, and so forth. 1572 Indeterminate: There is no trust anchor that would indicate that a 1573 specific portion of the tree is secure. This is the default 1574 operation mode. 1576 Section 4.3 of [RFC4035] says: 1578 A security-aware resolver must be able to distinguish between four 1579 cases: 1581 Secure: An RRset for which the resolver is able to build a chain 1582 of signed DNSKEY and DS RRs from a trusted security anchor to 1583 the RRset. In this case, the RRset should be signed and is 1584 subject to signature validation, as described above. 1586 Insecure: An RRset for which the resolver knows that it has no 1587 chain of signed DNSKEY and DS RRs from any trusted starting 1588 point to the RRset. This can occur when the target RRset lies 1589 in an unsigned zone or in a descendent [sic] of an unsigned 1590 zone. In this case, the RRset may or may not be signed, but 1591 the resolver will not be able to verify the signature. 1593 Bogus: An RRset for which the resolver believes that it ought to 1594 be able to establish a chain of trust but for which it is 1595 unable to do so, either due to signatures that for some reason 1596 fail to validate or due to missing data that the relevant 1597 DNSSEC RRs indicate should be present. This case may indicate 1598 an attack but may also indicate a configuration error or some 1599 form of data corruption. 1601 Indeterminate: An RRset for which the resolver is not able to 1602 determine whether the RRset should be signed, as the resolver 1603 is not able to obtain the necessary DNSSEC RRs. This can occur 1604 when the security-aware resolver is not able to contact 1605 security-aware name servers for the relevant zones. 1607 12. Security Considerations 1609 These definitions do not change any security considerations for the 1610 DNS. 1612 13. IANA Considerations 1614 None. 1616 14. References 1618 14.1. Normative References 1620 [IANA_RootFiles] 1621 Internet Assigned Numbers Authority, "IANA Root Files", 1622 2016, . 1624 [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities", 1625 RFC 882, DOI 10.17487/RFC0882, November 1983, 1626 . 1628 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1629 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1630 . 1632 [RFC1035] Mockapetris, P., "Domain names - implementation and 1633 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1634 November 1987, . 1636 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1637 Application and Support", STD 3, RFC 1123, 1638 DOI 10.17487/RFC1123, October 1989, 1639 . 1641 [RFC1912] Barr, D., "Common DNS Operational and Configuration 1642 Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996, 1643 . 1645 [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone 1646 Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996, 1647 August 1996, . 1649 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 1650 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1651 RFC 2136, DOI 10.17487/RFC2136, April 1997, 1652 . 1654 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1655 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1656 . 1658 [RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection 1659 and Operation of Secondary DNS Servers", BCP 16, RFC 2182, 1660 DOI 10.17487/RFC2182, July 1997, 1661 . 1663 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 1664 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 1665 . 1667 [RFC3731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP) 1668 Domain Name Mapping", RFC 3731, DOI 10.17487/RFC3731, 1669 March 2004, . 1671 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1672 Rose, "DNS Security Introduction and Requirements", 1673 RFC 4033, DOI 10.17487/RFC4033, March 2005, 1674 . 1676 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1677 Rose, "Resource Records for the DNS Security Extensions", 1678 RFC 4034, DOI 10.17487/RFC4034, March 2005, 1679 . 1681 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1682 Rose, "Protocol Modifications for the DNS Security 1683 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 1684 . 1686 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name 1687 System", RFC 4592, DOI 10.17487/RFC4592, July 2006, 1688 . 1690 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 1691 Security (DNSSEC) Hashed Authenticated Denial of 1692 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008, 1693 . 1695 [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive 1696 Nameservers in Reflector Attacks", BCP 140, RFC 5358, 1697 DOI 10.17487/RFC5358, October 2008, 1698 . 1700 [RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)", 1701 STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009, 1702 . 1704 [RFC5855] Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6 1705 Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855, 1706 May 2010, . 1708 [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol 1709 (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010, 1710 . 1712 [RFC6561] Livingood, J., Mody, N., and M. O'Reirdan, 1713 "Recommendations for the Remediation of Bots in ISP 1714 Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012, 1715 . 1717 [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC 1718 Operational Practices, Version 2", RFC 6781, 1719 DOI 10.17487/RFC6781, December 2012, 1720 . 1722 [RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and 1723 Implementation Notes for DNS Security (DNSSEC)", RFC 6840, 1724 DOI 10.17487/RFC6840, February 2013, 1725 . 1727 [RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A 1728 Framework for DNSSEC Policies and DNSSEC Practice 1729 Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013, 1730 . 1732 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 1733 for DNS (EDNS(0))", STD 75, RFC 6891, 1734 DOI 10.17487/RFC6891, April 2013, 1735 . 1737 [RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating 1738 DNSSEC Delegation Trust Maintenance", RFC 7344, 1739 DOI 10.17487/RFC7344, September 2014, 1740 . 1742 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1743 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 1744 2015, . 1746 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles 1747 for DNS over TLS and DNS over DTLS", RFC 8310, 1748 DOI 10.17487/RFC8310, March 2018, 1749 . 1751 14.2. Informative References 1753 [IANA_Resource_Registry] 1754 Internet Assigned Numbers Authority, "Resource Record (RR) 1755 TYPEs", 2017, 1756 . 1758 [RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for 1759 Internet User Applications", RFC 819, 1760 DOI 10.17487/RFC0819, August 1982, 1761 . 1763 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet 1764 host table specification", RFC 952, DOI 10.17487/RFC0952, 1765 October 1985, . 1767 [RFC1713] Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713, 1768 DOI 10.17487/RFC1713, November 1994, 1769 . 1771 [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, 1772 DOI 10.17487/RFC1995, August 1996, 1773 . 1775 [RFC2133] Gilligan, R., Thomson, S., Bound, J., and W. Stevens, 1776 "Basic Socket Interface Extensions for IPv6", RFC 2133, 1777 DOI 10.17487/RFC2133, April 1997, 1778 . 1780 [RFC2775] Carpenter, B., "Internet Transparency", RFC 2775, 1781 DOI 10.17487/RFC2775, February 2000, 1782 . 1784 [RFC3172] Huston, G., Ed., "Management Guidelines & Operational 1785 Requirements for the Address and Routing Parameter Area 1786 Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172, 1787 September 2001, . 1789 [RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425, 1790 DOI 10.17487/RFC3425, November 2002, 1791 . 1793 [RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name 1794 System KEY (DNSKEY) Resource Record (RR) Secure Entry 1795 Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April 1796 2004, . 1798 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, 1799 DOI 10.17487/RFC3912, September 2004, 1800 . 1802 [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices", 1803 RFC 4641, DOI 10.17487/RFC4641, September 2006, 1804 . 1806 [RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution 1807 Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697, 1808 October 2006, . 1810 [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast 1811 Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, 1812 December 2006, . 1814 [RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security 1815 (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July 1816 2007, . 1818 [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", 1819 BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009, 1820 . 1822 [RFC5890] Klensin, J., "Internationalized Domain Names for 1823 Applications (IDNA): Definitions and Document Framework", 1824 RFC 5890, DOI 10.17487/RFC5890, August 2010, 1825 . 1827 [RFC5891] Klensin, J., "Internationalized Domain Names in 1828 Applications (IDNA): Protocol", RFC 5891, 1829 DOI 10.17487/RFC5891, August 2010, 1830 . 1832 [RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and 1833 Internationalized Domain Names for Applications (IDNA)", 1834 RFC 5892, DOI 10.17487/RFC5892, August 2010, 1835 . 1837 [RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts 1838 for Internationalized Domain Names for Applications 1839 (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010, 1840 . 1842 [RFC5894] Klensin, J., "Internationalized Domain Names for 1843 Applications (IDNA): Background, Explanation, and 1844 Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010, 1845 . 1847 [RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on 1848 Encodings for Internationalized Domain Names", RFC 6055, 1849 DOI 10.17487/RFC6055, February 2011, 1850 . 1852 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 1853 DOI 10.17487/RFC6265, April 2011, 1854 . 1856 [RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163, 1857 RFC 6303, DOI 10.17487/RFC6303, July 2011, 1858 . 1860 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1861 Cheshire, "Internet Assigned Numbers Authority (IANA) 1862 Procedures for the Management of the Service Name and 1863 Transport Protocol Port Number Registry", BCP 165, 1864 RFC 6335, DOI 10.17487/RFC6335, August 2011, 1865 . 1867 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in 1868 Internationalization in the IETF", BCP 166, RFC 6365, 1869 DOI 10.17487/RFC6365, September 2011, 1870 . 1872 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 1873 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, 1874 . 1876 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 1877 DOI 10.17487/RFC6762, February 2013, 1878 . 1880 [RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of 1881 Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, 1882 February 2014, . 1884 [RFC7480] Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the 1885 Registration Data Access Protocol (RDAP)", RFC 7480, 1886 DOI 10.17487/RFC7480, March 2015, 1887 . 1889 [RFC7481] Hollenbeck, S. and N. Kong, "Security Services for the 1890 Registration Data Access Protocol (RDAP)", RFC 7481, 1891 DOI 10.17487/RFC7481, March 2015, 1892 . 1894 [RFC7482] Newton, A. and S. Hollenbeck, "Registration Data Access 1895 Protocol (RDAP) Query Format", RFC 7482, 1896 DOI 10.17487/RFC7482, March 2015, 1897 . 1899 [RFC7483] Newton, A. and S. Hollenbeck, "JSON Responses for the 1900 Registration Data Access Protocol (RDAP)", RFC 7483, 1901 DOI 10.17487/RFC7483, March 2015, 1902 . 1904 [RFC7484] Blanchet, M., "Finding the Authoritative Registration Data 1905 (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March 1906 2015, . 1908 [RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin, 1909 "Inventory and Analysis of WHOIS Registration Objects", 1910 RFC 7485, DOI 10.17487/RFC7485, March 2015, 1911 . 1913 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1914 and P. Hoffman, "Specification for DNS over Transport 1915 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1916 2016, . 1918 [RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram 1919 Transport Layer Security (DTLS)", RFC 8094, 1920 DOI 10.17487/RFC8094, February 2017, 1921 . 1923 [RFC8109] Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS 1924 Resolver with Priming Queries", BCP 209, RFC 8109, 1925 DOI 10.17487/RFC8109, March 2017, 1926 . 1928 [RSSAC026] 1929 Root Server System Advisory Committee (RSSAC), "RSSAC 1930 Lexicon", 2017, 1931 . 1934 Appendix A. Definitions Updated by this Document 1936 The following definitions from RFCs are updated by this document: 1938 o Forwarder in [RFC2308] 1940 o Secure Entry Point (SEP) in [RFC3757]; note, however, that this 1941 RFC is already obsolete 1943 Appendix B. Definitions First Defined in this Document 1945 The following definitions are first defined in this document: 1947 o "Alias" in Section 2 1949 o "Apex" in Section 7 1951 o "arpa" in Section 7 1952 o "Bailiwick" in Section 7 1954 o "Class independent" in Section 5 1956 o "Delegation-centric zone" in Section 7 1958 o "Delegation" in Section 7 1960 o "DNS operator" in Section 9 1962 o "DNSSEC-aware" in Section 10 1964 o "DNSSEC-unaware" in Section 10 1966 o "Forwarding" in Section 6 1968 o "Full resolver" in Section 6 1970 o "Fully qualified domain name" in Section 2 1972 o "Global DNS" in Section 2 1974 o "Hardware Security Module (HSM)" in Section 10 1976 o "Host name" in Section 2 1978 o "IDN" in Section 2 1980 o "In-bailiwick" in Section 7 1982 o "Iterative resolution" in Section 6 1984 o "Label" in Section 2 1986 o "Locally served DNS zone" in Section 2 1988 o "Naming system" in Section 2 1990 o "Negative response" in Section 3 1992 o "Non-recursive query" in Section 6 1994 o "Open resolver" in Section 6 1996 o "Out-of-bailiwick" in Section 7 1998 o "Passive DNS" in Section 6 1999 o "Policy-implementing resolver" in Section 6 2001 o "Presentation format" in Section 5 2003 o "Priming" in Section 6 2005 o "Private DNS" in Section 2 2007 o "Recursive resolver" in Section 6 2009 o "Referrals" in Section 4 2011 o "Registrant" in Section 9 2013 o "Registrar" in Section 9 2015 o "Registry" in Section 9 2017 o "Root zone" in Section 7 2019 o "Secure Entry Point (SEP)" in Section 10 2021 o "Signing software" in Section 10 2023 o "Split DNS" in Section 6 2025 o "Stub resolver" in Section 6 2027 o "Subordinate" in Section 8 2029 o "Superordinate" in Section 8 2031 o "TLD" in Section 2 2033 o "Validating resolver" in Section 10 2035 o "Validation" in Section 10 2037 o "View" in Section 6 2039 o "Zone transfer" in Section 6 2041 Index 2043 A 2044 Address records 14 2045 Alias 8 2046 Anycast 21 2047 Apex 22 2048 Asterisk label 26 2049 Authoritative data 22 2050 Authoritative server 17 2051 Authoritative-only server 18 2052 arpa: Address and Routing Parameter Area Domain 25 2054 C 2055 CNAME 9 2056 Canonical name 8 2057 Child 21 2058 Class independent 14 2059 Closest encloser 26 2060 Closest provable encloser 26 2061 Combined signing key (CSK) 31 2063 D 2064 DNS operator 27 2065 DNSSEC Policy (DP) 32 2066 DNSSEC Practice Statement (DPS) 32 2067 DNSSEC-aware and DNSSEC-unaware 28 2068 Delegation 22 2069 Delegation-centric zone 24 2070 Domain name 4 2072 E 2073 EDNS 13 2074 EPP 27 2075 Empty non-terminals (ENT) 24 2077 F 2078 FORMERR 9 2079 Fast flux DNS 25 2080 Forward lookup 25 2081 Forwarder 19 2082 Forwarding 19 2083 Full resolver 17 2084 Full-service resolver 17 2085 Fully qualified domain name (FQDN) 7 2087 G 2088 Global DNS 4 2089 Glue records 23 2091 H 2092 Hardware security module (HSM) 32 2093 Hidden master 19 2094 Host name 7 2096 I 2097 IDN 8 2098 In-bailiwick 23 2099 Insecure delegation 30 2100 Instance 21 2101 Iterative mode 15 2102 Iterative resolution 17 2104 K 2105 Key signing key (KSK) 31 2107 L 2108 Label 4 2109 Lame delegation 23 2110 Locally served DNS zone 7 2112 M 2113 Master file 13 2114 Master server 18 2115 Multicast DNS 6 2117 N 2118 NODATA 9 2119 NOERROR 9 2120 NOTIMP 9 2121 NS 17 2122 NSEC 29 2123 NSEC3 29 2124 NXDOMAIN 9 2125 Naming system 3 2126 Negative caching 17 2127 Negative response 10 2128 Next closer name 26 2129 Non-recursive query 16 2131 O 2132 OPT 13 2133 Occluded name 25 2134 Open resolver 20 2135 Opt-out 30 2136 Origin 21 2137 Out-of-bailiwick 23 2138 Owner 13 2140 P 2141 Parent 21 2142 Passive DNS 20 2143 Policy-implementing resolver 20 2144 Presentation format 13 2145 Primary master 19 2146 Primary server 18 2147 Priming 17 2148 Privacy-enabling DNS server 21 2149 Private DNS 6 2150 Public suffix 27 2152 Q 2153 QNAME 10 2155 R 2156 RDAP 27 2157 REFUSED 9 2158 RR 12 2159 RRset 12 2160 Recursive mode 16 2161 Recursive query 16 2162 Recursive resolver 16 2163 Referrals 11 2164 Registrant 27 2165 Registrar 27 2166 Registry 26 2167 Resolver 15 2168 Reverse DNS, reverse lookup 25 2169 Root hints 17 2170 Root zone 24 2172 S 2173 SERVFAIL 9 2174 SOA 13 2175 SOA field names 13 2176 Secondary server 18 2177 Secure Entry Point (SEP) 32 2178 Service name 25 2179 Signed zone 28 2180 Signing software 33 2181 Slave server 18 2182 Source of Synthesis 26 2183 Split DNS 20 2184 Split-horizon DNS 20 2185 Stealth server 19 2186 Stub resolver 15 2187 Subdomain 8 2188 Subordinate 28 2189 Superordinate 28 2191 T 2192 TLD 8 2193 TTL 13 2194 Trust anchor 32 2196 U 2197 Unsigned zone 29 2199 V 2200 Validating resolver 31 2201 Validation 30 2202 View 20 2204 W 2205 WHOIS 27 2206 Wildcard 25 2207 Wildcard domain name 26 2209 Z 2210 Zone 21 2211 Zone cut 22 2212 Zone enumeration 30 2213 Zone signing key (ZSK) 31 2214 Zone transfer 18 2216 Acknowledgements 2218 The following is the Acknowledgements for RFC 7719. Additional 2219 acknowledgements may be added as this draft is worked on. 2221 The authors gratefully acknowledge all of the authors of DNS-related 2222 RFCs that proceed this one. Comments from Tony Finch, Stephane 2223 Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John 2224 Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman, 2225 David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis, 2226 John Klensin, David Black, and many others in the DNSOP Working Group 2227 helped shape RFC 7719. 2229 Additional people contributed to this document, including: Bob 2230 Harold, Dick Franks, Evan Hunt, John Dickinson, Mark Andrews, Martin 2231 Hoffmann, Paul Vixie, Peter Koch, Duane Wessels [[ MORE NAMES WILL 2232 APPEAR HERE AS FOLKS CONTRIBUTE]]. 2234 Authors' Addresses 2236 Paul Hoffman 2237 ICANN 2239 Email: paul.hoffman@icann.org 2241 Andrew Sullivan 2243 Email: ajs@anvilwalrusden.com 2245 Kazunori Fujiwara 2246 Japan Registry Services Co., Ltd. 2247 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda 2248 Chiyoda-ku, Tokyo 101-0065 2249 Japan 2251 Phone: +81 3 5215 8451 2252 Email: fujiwara@jprs.co.jp