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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 21, 2019 JPRS 8 August 20, 2018 10 DNS Terminology 11 draft-ietf-dnsop-terminology-bis-13 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 obsoletes RFC 7719 and updates RFC 2308. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at https://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on February 21, 2019. 41 Copyright Notice 43 Copyright (c) 2018 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (https://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3. DNS Response Codes . . . . . . . . . . . . . . . . . . . . . 9 61 4. DNS Transactions . . . . . . . . . . . . . . . . . . . . . . 10 62 5. Resource Records . . . . . . . . . . . . . . . . . . . . . . 13 63 6. DNS Servers and Clients . . . . . . . . . . . . . . . . . . . 15 64 7. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 65 8. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . 26 66 9. Registration Model . . . . . . . . . . . . . . . . . . . . . 27 67 10. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 28 68 11. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 33 69 12. Security Considerations . . . . . . . . . . . . . . . . . . . 34 70 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34 71 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 72 14.1. Normative References . . . . . . . . . . . . . . . . . . 34 73 14.2. Informative References . . . . . . . . . . . . . . . . . 37 74 Appendix A. Definitions Updated by this Document . . . . . . . . 41 75 Appendix B. Definitions First Defined in this Document . . . . . 41 76 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 77 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 47 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48 80 1. Introduction 82 The Domain Name System (DNS) is a simple query-response protocol 83 whose messages in both directions have the same format. (Section 2 84 gives a definition of "public DNS", which is often what people mean 85 when they say "the DNS".) The protocol and message format are 86 defined in [RFC1034] and [RFC1035]. These RFCs defined some terms, 87 but later documents defined others. Some of the terms from [RFC1034] 88 and [RFC1035] now have somewhat different meanings than they did in 89 1987. 91 This document collects a wide variety of DNS-related terms. Some of 92 them have been precisely defined in earlier RFCs, some have been 93 loosely defined in earlier RFCs, and some are not defined in any 94 earlier RFC at all. 96 Most of the definitions here are the consensus definition of the DNS 97 community -- both protocol developers and operators. Some of the 98 definitions differ from earlier RFCs, and those differences are 99 noted. In this document, where the consensus definition is the same 100 as the one in an RFC, that RFC is quoted. Where the consensus 101 definition has changed somewhat, the RFC is mentioned but the new 102 stand-alone definition is given. See Appendix A for a list of the 103 definitions that this document updates. 105 It is important to note that, during the development of this 106 document, it became clear that some DNS-related terms are interpreted 107 quite differently by different DNS experts. Further, some terms that 108 are defined in early DNS RFCs now have definitions that are generally 109 agreed to, but that are different from the original definitions. 110 Therefore, this document is a substantial revision to [RFC7719]. 112 The terms are organized loosely by topic. Some definitions are for 113 new terms for things that are commonly talked about in the DNS 114 community but that never had terms defined for them. 116 Other organizations sometimes define DNS-related terms their own way. 117 For example, the WHATWG defines "domain" at 118 . The Root Server System Advisory 119 Committee (RSSAC) has a good lexicon [RSSAC026]. 121 Note that there is no single consistent definition of "the DNS". It 122 can be considered to be some combination of the following: a commonly 123 used naming scheme for objects on the Internet; a distributed 124 database representing the names and certain properties of these 125 objects; an architecture providing distributed maintenance, 126 resilience, and loose coherency for this database; and a simple 127 query-response protocol (as mentioned below) implementing this 128 architecture. Section 2 defines "global DNS" and "private DNS" as a 129 way to deal with these differing definitions. 131 Capitalization in DNS terms is often inconsistent among RFCs and 132 various DNS practitioners. The capitalization used in this document 133 is a best guess at current practices, and is not meant to indicate 134 that other capitalization styles are wrong or archaic. In some 135 cases, multiple styles of capitalization are used for the same term 136 due to quoting from different RFCs. 138 Readers should note that the terms in this document are grouped by 139 topic. Someone who is not already familiar with the DNS can probably 140 not learn about the DNS from scratch by reading this document from 141 front to back. Instead, skipping around may be the only way to get 142 enough context to understand some of the definitions. This document 143 has an index that might be useful for readers who are attempting to 144 learn the DNS by reading this document. 146 2. Names 148 Naming system: A naming system associates names with data. Naming 149 systems have many significant facets that help differentiate them. 150 Some commonly-identified facets include: 152 * Composition of names 154 * Format of names 156 * Administration of names 158 * Types of data that can be associated with names 160 * Types of metadata for names 162 * Protocol for getting data from a name 164 * Context for resolving a name 166 Note that this list is a small subset of facets that people have 167 identified over time for naming systems, and the IETF has yet to 168 agree on a good set of facets that can be used to compare naming 169 systems. For example, other facets might include "protocol to 170 update data in a name", "privacy of names", and "privacy of data 171 associated with names", but those are not as well-defined as the 172 ones listed above. The list here is chosen because it helps 173 describe the DNS and naming systems similar to the DNS. 175 Domain name: An ordered list of one or more labels. 177 Note that this is a definition independent of the DNS RFCs, and 178 the definition here also applies to systems other than the DNS. 179 [RFC1034] defines the "domain name space" using mathematical trees 180 and their nodes in graph theory, and this definition has the same 181 practical result as the definition here. Using graph theory, a 182 domain name is a list of labels identifying a portion along one 183 edge of a directed acyclic graph. A domain name can be relative 184 to parts of the tree, or it can be fully qualified (in which case, 185 it begins at the common root of the graph). 187 Also note that different IETF and non-IETF documents have used the 188 term "domain name" in many different ways. It is common for 189 earlier documents to use "domain name" to mean "names that match 190 the syntax in [RFC1035]", but possibly with additional rules such 191 as "and are, or will be, resolvable in the global DNS" or "but 192 only using the presentation format". 194 Label: An ordered list of zero or more octets that makes up a 195 portion of a domain name. Using graph theory, a label identifies 196 one node in a portion of the graph of all possible domain names. 198 Global DNS: Using the short set of facets listed in "Naming system", 199 the global DNS can be defined as follows. Most of the rules here 200 come from [RFC1034] and [RFC1035], although the term "global DNS" 201 has not been defined before now. 203 Composition of names -- A name in the global DNS has one or more 204 labels. The length of each label is between 0 and 63 octets 205 inclusive. In a fully-qualified domain name, the first label in 206 the ordered list is 0 octets long; it is the only label whose 207 length may be 0 octets, and it is called the "root" or "root 208 label". A domain name in the global DNS has a maximum total 209 length of 255 octets in the wire format; the root represents one 210 octet for this calculation. (Multicast DNS [RFC6762] allows names 211 up to 255 bytes plus a terminating zero byte based on a different 212 interpretation of RFC 1035 and what is included in the 255 213 octets.) 215 Format of names -- Names in the global DNS are domain names. 216 There are three formats: wire format, presentation format, and 217 common display. 219 The basic wire format for names in the global DNS is a list of 220 labels ordered by decreasing distance from the root, with the root 221 label last. Each label is preceded by a length octet. [RFC1035] 222 also defines a compression scheme that modifies this format. 224 The presentation format for names in the global DNS is a list of 225 labels ordered by decreasing distance from the root, encoded as 226 ASCII, with a "." character between each label. In presentation 227 format, a fully-qualified domain name includes the root label and 228 the associated separator dot. For example, in presentation 229 format, a fully-qualified domain name with two non-root labels is 230 always shown as "example.tld." instead of "example.tld". 231 [RFC1035] defines a method for showing octets that do not display 232 in ASCII. 234 The common display format is used in applications and free text. 235 It is the same as the presentation format, but showing the root 236 label and the "." before it is optional and is rarely done. For 237 example, in common display format, a fully-qualified domain name 238 with two non-root labels is usually shown as "example.tld" instead 239 of "example.tld.". Names in the common display format are 240 normally written such that the directionality of the writing 241 system presents labels by decreasing distance from the root (so, 242 in both English and C the root or TLD label in the ordered list is 243 right-most; but in Arabic it may be left-most, depending on local 244 conventions). 246 Administration of names -- Administration is specified by 247 delegation (see the definition of "delegation" in Section 7). 248 Policies for administration of the root zone in the global DNS are 249 determined by the names operational community, which convenes 250 itself in the Internet Corporation for Assigned Names and Numbers 251 (ICANN). The names operational community selects the IANA 252 Functions Operator for the global DNS root zone. At the time this 253 document is published, that operator is Public Technical 254 Identifiers (PTI). (See for more 255 information about PTI operating the IANA Functions.) The name 256 servers that serve the root zone are provided by independent root 257 operators. Other zones in the global DNS have their own policies 258 for administration. 260 Types of data that can be associated with names -- A name can have 261 zero or more resource records associated with it. There are 262 numerous types of resource records with unique data structures 263 defined in many different RFCs and in the IANA registry at 264 [IANA_Resource_Registry]. 266 Types of metadata for names -- Any name that is published in the 267 DNS appears as a set of resource records (see the definition of 268 "RRset" in Section 5). Some names do not themselves have data 269 associated with them in the DNS, but "appear" in the DNS anyway 270 because they form part of a longer name that does have data 271 associated with it (see the definition of "empty non-terminals" in 272 Section 7). 274 Protocol for getting data from a name -- The protocol described in 275 [RFC1035]. 277 Context for resolving a name -- The global DNS root zone 278 distributed by PTI. 280 Private DNS: Names that use the protocol described in [RFC1035] but 281 that do not rely on the global DNS root zone, or names that are 282 otherwise not generally available on the Internet but are using 283 the protocol described in [RFC1035]. A system can use both the 284 global DNS and one or more private DNS systems; for example, see 285 "Split DNS" in Section 6. 287 Note that domain names that do not appear in the DNS, and that are 288 intended never to be looked up using the DNS protocol, are not 289 part of the global DNS or a private DNS even though they are 290 domain names. 292 Multicast DNS: "Multicast DNS (mDNS) provides the ability to perform 293 DNS-like operations on the local link in the absence of any 294 conventional Unicast DNS server. In addition, Multicast DNS 295 designates a portion of the DNS namespace to be free for local 296 use, without the need to pay any annual fee, and without the need 297 to set up delegations or otherwise configure a conventional DNS 298 server to answer for those names." (Quoted from [RFC6762], 299 Abstract) Although it uses a compatible wire format, mDNS is 300 strictly speaking a different protocol than DNS. Also, where the 301 above quote says "a portion of the DNS namespace", it would be 302 clearer to say "a portion of the domain name space" The names in 303 mDNS are not intended to be looked up in the DNS. 305 Locally served DNS zone: A locally served DNS zone is a special case 306 of private DNS. Names are resolved using the DNS protocol in a 307 local context. [RFC6303] defines subdomains of IN-ADDR.ARPA that 308 are locally served zones. Resolution of names through locally 309 served zones may result in ambiguous results. For example, the 310 same name may resolve to different results in different locally 311 served DNS zone contexts. The context for a locally served DNS 312 zone may be explicit, for example, as defined in [RFC6303], or 313 implicit, as defined by local DNS administration and not known to 314 the resolution client. 316 Fully qualified domain name (FQDN): This is often just a clear way 317 of saying the same thing as "domain name of a node", as outlined 318 above. However, the term is ambiguous. Strictly speaking, a 319 fully qualified domain name would include every label, including 320 the zero-length label of the root: such a name would be written 321 "www.example.net." (note the terminating dot). But because every 322 name eventually shares the common root, names are often written 323 relative to the root (such as "www.example.net") and are still 324 called "fully qualified". This term first appeared in [RFC0819]. 325 In this document, names are often written relative to the root. 327 The need for the term "fully qualified domain name" comes from the 328 existence of partially qualified domain names, which are names 329 where one or more of the earliest labels in the ordered list are 330 omitted (for example, a name of "www" derived from 331 "www.example.net"). Such relative names are understood only by 332 context. 334 Host name: This term and its equivalent, "hostname", have been 335 widely used but are not defined in [RFC1034], [RFC1035], 336 [RFC1123], or [RFC2181]. The DNS was originally deployed into the 337 Host Tables environment as outlined in [RFC0952], and it is likely 338 that the term followed informally from the definition there. Over 339 time, the definition seems to have shifted. "Host name" is often 340 meant to be a domain name that follows the rules in Section 3.5 of 341 [RFC1034], the "preferred name syntax" (that is, every character 342 in each label is a letter, a digit, or a hyphen). Note that any 343 label in a domain name can contain any octet value; hostnames are 344 generally considered to be domain names where every label follows 345 the rules in the "preferred name syntax", with the amendment that 346 labels can start with ASCII digits (this amendment comes from 347 Section 2.1 of [RFC1123]). 349 People also sometimes use the term hostname to refer to just the 350 first label of an FQDN, such as "printer" in 351 "printer.admin.example.com". (Sometimes this is formalized in 352 configuration in operating systems.) In addition, people 353 sometimes use this term to describe any name that refers to a 354 machine, and those might include labels that do not conform to the 355 "preferred name syntax". 357 TLD: A Top-Level Domain, meaning a zone that is one layer below the 358 root, such as "com" or "jp". There is nothing special, from the 359 point of view of the DNS, about TLDs. Most of them are also 360 delegation-centric zones (defined in Section 7, and there are 361 significant policy issues around their operation. TLDs are often 362 divided into sub-groups such as Country Code Top-Level Domains 363 (ccTLDs), Generic Top-Level Domains (gTLDs), and others; the 364 division is a matter of policy, and beyond the scope of this 365 document. 367 IDN: The common abbreviation for "Internationalized Domain Name". 368 The IDNA protocol is the standard mechanism for handling domain 369 names with non-ASCII characters in applications in the DNS. The 370 current standard, normally called "IDNA2008", is defined in 371 [RFC5890], [RFC5891], [RFC5892], [RFC5893], and [RFC5894]. These 372 documents define many IDN-specific terms such as "LDH label", 373 "A-label", and "U-label". [RFC6365] defines more terms that 374 relate to internationalization (some of which relate to IDNs), and 375 [RFC6055] has a much more extensive discussion of IDNs, including 376 some new terminology. 378 Subdomain: "A domain is a subdomain of another domain if it is 379 contained within that domain. This relationship can be tested by 380 seeing if the subdomain's name ends with the containing domain's 381 name." (Quoted from [RFC1034], Section 3.1). For example, in the 382 host name "nnn.mmm.example.com", both "mmm.example.com" and 383 "nnn.mmm.example.com" are subdomains of "example.com". 385 Alias: The owner of a CNAME resource record, or a subdomain of the 386 owner of a DNAME resource record (DNAME records are defined in 387 [RFC6672]). See also "canonical name". 389 Canonical name: A CNAME resource record "identifies its owner name 390 as an alias, and specifies the corresponding canonical name in the 391 RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2) 392 This usage of the word "canonical" is related to the mathematical 393 concept of "canonical form". 395 CNAME: "It is traditional to refer to the owner of a CNAME record as 396 'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of 397 'canonical name', and the owner of a CNAME record is an alias, not 398 a canonical name." (Quoted from [RFC2181], Section 10.1.1) 400 3. DNS Response Codes 402 Some of response codes that are defined in [RFC1035] have acquired 403 their own shorthand names. All of the RCODEs are listed at 404 [IANA_Resource_Registry], although that site uses mixed-case 405 capitalization, while most documents use all-caps. Some of the 406 common names are described here, but the official list is in the IANA 407 registry. 409 NOERROR: "No error condition" (Quoted from [RFC1035], 410 Section 4.1.1.) 412 FORMERR: "Format error - The name server was unable to interpret the 413 query." (Quoted from [RFC1035], Section 4.1.1.) 415 SERVFAIL: "Server failure - The name server was unable to process 416 this query due to a problem with the name server." (Quoted from 417 [RFC1035], Section 4.1.1.) 419 NXDOMAIN: "Name Error - This code signifies that the domain name 420 referenced in the query does not exist." (Quoted from [RFC1035], 421 Section 4.1.1.) [RFC2308] established NXDOMAIN as a synonym for 422 Name Error. 424 NOTIMP: "Not Implemented - The name server does not support the 425 requested kind of query." (Quoted from [RFC1035], Section 4.1.1.) 427 REFUSED: "Refused - The name server refuses to perform the specified 428 operation for policy reasons. For example, a name server may not 429 wish to provide the information to the particular requester, or a 430 name server may not wish to perform a particular operation (e.g., 431 zone transfer) for particular data." (Quoted from [RFC1035], 432 Section 4.1.1.) 434 NODATA: "A pseudo RCODE which indicates that the name is valid for 435 the given class, but there are no records of the given type. A 436 NODATA response has to be inferred from the answer." (Quoted from 437 [RFC2308], Section 1.) "NODATA is indicated by an answer with the 438 RCODE set to NOERROR and no relevant answers in the answer 439 section. The authority section will contain an SOA record, or 440 there will be no NS records there." (Quoted from [RFC2308], 441 Section 2.2.) Note that referrals have a similar format to NODATA 442 replies; [RFC2308] explains how to distinguish them. 444 The term "NXRRSET" is sometimes used as a synonym for NODATA. 445 However, this is a mistake, given that NXRRSET is a specific error 446 code defined in [RFC2136]. 448 Negative response: A response that indicates that a particular RRset 449 does not exist, or whose RCODE indicates the nameserver cannot 450 answer. Sections 2 and 7 of [RFC2308] describe the types of 451 negative responses in detail. 453 4. DNS Transactions 455 The header of a DNS message is its first 12 octets. Many of the 456 fields and flags in the header diagram in Sections 4.1.1 through 457 4.1.3 of [RFC1035] are referred to by their names in that diagram. 458 For example, the response codes are called "RCODEs", the data for a 459 record is called the "RDATA", and the authoritative answer bit is 460 often called "the AA flag" or "the AA bit". 462 QNAME: The most commonly-used rough definition is that the QNAME is 463 a field in the Question section of a query. "A standard query 464 specifies a target domain name (QNAME), query type (QTYPE), and 465 query class (QCLASS) and asks for RRs which match." (Quoted from 466 [RFC1034], Section 3.7.1.). Strictly speaking, the definition 467 comes from [RFC1035], Section 4.1.2, where the QNAME is defined in 468 respect of the Question Section. This definition appears to be 469 applied consistently: the discussion of inverse queries in section 470 6.4 refers to the "owner name of the query RR and its TTL", 471 because inverse queries populate the Answer Section and leave the 472 Question Section empty. (Inverse queries are deprecated in 473 [RFC3425], and so relevant definitions do not appear in this 474 document.) 476 [RFC2308], however, has an alternate definition that puts the 477 QNAME in the answer (or series of answers) instead of the query. 479 It defines QNAME as: "...the name in the query section of an 480 answer, or where this resolves to a CNAME, or CNAME chain, the 481 data field of the last CNAME. The last CNAME in this sense is 482 that which contains a value which does not resolve to another 483 CNAME." This definition has a certain internal logic, because of 484 the way CNAME substitution works and the definition of CNAME. If 485 a name server does not find an RRset that matches a query, but it 486 finds the same name in the same class with a CNAME record, then 487 the name server "includes the CNAME record in the response and 488 restarts the query at the domain name specified in the data field 489 of the CNAME record." ([RFC1034] Section 3.6.2). This is made 490 explicit in the resolution algorithm outlined in Section 4.3.2 of 491 [RFC1034], which says to "change QNAME to the canonical name in 492 the CNAME RR, and go back to step 1" in the case of a CNAME RR. 493 Since a CNAME record explicitly declares that the owner name is 494 canonically named what is in the RDATA, then there is a way to 495 view the new name (i.e. the name that was in the RDATA of the 496 CNAME RR) as also being the QNAME. 498 This creates a kind of confusion, however, because the response to 499 a query that results in CNAME processing contains in the echoed 500 Question Section one QNAME (the name in the original query), and a 501 second QNAME that is in the data field of the last CNAME. The 502 confusion comes from the iterative/recursive mode of resolution, 503 which finally returns an answer that need not actually have the 504 same owner name as the QNAME contained in the original query. 506 To address this potential confusion, it is helpful to distinguish 507 between three meanings: 509 * QNAME (original): The name actually sent in the Question 510 Section in the original query, which is always echoed in the 511 (final) reply in the Question Section when the QR bit is set to 512 1. 514 * QNAME (effective): A name actually resolved, which is either 515 the name originally queried, or a name received in a CNAME 516 chain response. 518 * QNAME (final): The name actually resolved, which is either the 519 name actually queried or else the last name in a CNAME chain 520 response. 522 Note that, because the definition in [RFC2308] is actually for a 523 different concept than what was in [RFC1034], it would have been 524 better if [RFC2308] had used a different name for that concept. 525 In general use today, QNAME almost always means what is defined 526 above as "QNAME (original)". 528 Referrals: A type of response in which a server, signaling that it 529 is not (completely) authoritative for an answer, provides the 530 querying resolver with an alternative place to send its query. 531 Referrals can be partial. 533 A referral arises when a server is not performing recursive 534 service while answering a query. It appears in step 3(b) of the 535 algorithm in [RFC1034], Section 4.3.2. 537 There are two types of referral response. The first is a downward 538 referral (sometimes described as "delegation response"), where the 539 server is authoritative for some portion of the QNAME. The 540 authority section RRset's RDATA contains the name servers 541 specified at the referred-to zone cut. In normal DNS operation, 542 this kind of response is required in order to find names beneath a 543 delegation. The bare use of "referral" means this kind of 544 referral, and many people believe that this is the only legitimate 545 kind of referral in the DNS. 547 The second is an upward referral (sometimes described as "root 548 referral"), where the server is not authoritative for any portion 549 of the QNAME. When this happens, the referred-to zone in the 550 authority section is usually the root zone (.). In normal DNS 551 operation, this kind of response is not required for resolution or 552 for correctly answering any query. There is no requirement that 553 any server send upward referrals. Some people regard upward 554 referrals as a sign of a misconfiguration or error. Upward 555 referrals always need some sort of qualifier (such as "upward" or 556 "root"), and are never identified by the bare word "referral". 558 A response that has only a referral contains an empty answer 559 section. It contains the NS RRset for the referred-to zone in the 560 authority section. It may contain RRs that provide addresses in 561 the additional section. The AA bit is clear. 563 In the case where the query matches an alias, and the server is 564 not authoritative for the target of the alias but it is 565 authoritative for some name above the target of the alias, the 566 resolution algorithm will produce a response that contains both 567 the authoritative answer for the alias, and also a referral. Such 568 a partial answer and referral response has data in the answer 569 section. It has the NS RRset for the referred-to zone in the 570 authority section. It may contain RRs that provide addresses in 571 the additional section. The AA bit is set, because the first name 572 in the answer section matches the QNAME and the server is 573 authoritative for that answer (see [RFC1035], Section 4.1.1). 575 5. Resource Records 577 RR: An acronym for resource record. ([RFC1034], Section 3.6.) 579 RRset: A set of resource records with the same label, class and 580 type, but with different data. (Definition from [RFC2181]) Also 581 spelled RRSet in some documents. As a clarification, "same label" 582 in this definition means "same owner name". In addition, 583 [RFC2181] states that "the TTLs of all RRs in an RRSet must be the 584 same". 586 Note that RRSIG resource records do not match this definition. 587 [RFC4035] says: "An RRset MAY have multiple RRSIG RRs associated 588 with it. Note that as RRSIG RRs are closely tied to the RRsets 589 whose signatures they contain, RRSIG RRs, unlike all other DNS RR 590 types, do not form RRsets. In particular, the TTL values among 591 RRSIG RRs with a common owner name do not follow the RRset rules 592 described in [RFC2181]." 594 Master file: "Master files are text files that contain RRs in text 595 form. Since the contents of a zone can be expressed in the form 596 of a list of RRs a master file is most often used to define a 597 zone, though it can be used to list a cache's contents." 598 ([RFC1035], Section 5.) Master files are sometimes called "zone 599 files". 601 Presentation format: The text format used in master files. This 602 format is shown but not formally defined in [RFC1034] and 603 [RFC1035]. The term "presentation format" first appears in 604 [RFC4034]. 606 EDNS: The extension mechanisms for DNS, defined in [RFC6891]. 607 Sometimes called "EDNS0" or "EDNS(0)" to indicate the version 608 number. EDNS allows DNS clients and servers to specify message 609 sizes larger than the original 512 octet limit, to expand the 610 response code space, and to carry additional options that affect 611 the handling of a DNS query. 613 OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to 614 contain control information pertaining to the question-and-answer 615 sequence of a specific transaction. (Definition from [RFC6891], 616 Section 6.1.1) It is used by EDNS. 618 Owner: The domain name where a RR is found ([RFC1034], Section 3.6). 619 Often appears in the term "owner name". 621 SOA field names: DNS documents, including the definitions here, 622 often refer to the fields in the RDATA of an SOA resource record 623 by field name. "SOA" stands for "start of a zone of authority". 624 Those fields are defined in Section 3.3.13 of [RFC1035]. The 625 names (in the order they appear in the SOA RDATA) are MNAME, 626 RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. Note that the 627 meaning of MINIMUM field is updated in Section 4 of [RFC2308]; the 628 new definition is that the MINIMUM field is only "the TTL to be 629 used for negative responses". This document tends to use field 630 names instead of terms that describe the fields. 632 TTL: The maximum "time to live" of a resource record. "A TTL value 633 is an unsigned number, with a minimum value of 0, and a maximum 634 value of 2147483647. That is, a maximum of 2^31 - 1. When 635 transmitted, the TTL is encoded in the less significant 31 bits of 636 the 32 bit TTL field, with the most significant, or sign, bit set 637 to zero." (Quoted from [RFC2181], Section 8) (Note that [RFC1035] 638 erroneously stated that this is a signed integer; that was fixed 639 by [RFC2181].) 641 The TTL "specifies the time interval that the resource record may 642 be cached before the source of the information should again be 643 consulted". (Quoted from [RFC1035], Section 3.2.1) Also: "the 644 time interval (in seconds) that the resource record may be cached 645 before it should be discarded". (Quoted from [RFC1035], 646 Section 4.1.3). Despite being defined for a resource record, the 647 TTL of every resource record in an RRset is required to be the 648 same ([RFC2181], Section 5.2). 650 The reason that the TTL is the maximum time to live is that a 651 cache operator might decide to shorten the time to live for 652 operational purposes, such as if there is a policy to disallow TTL 653 values over a certain number. Some servers are known to ignore 654 the TTL on some RRsets (such as when the authoritative data has a 655 very short TTL) even though this is against the advice in RFC 656 1035. An RRset can be flushed from the cache before the end of 657 the TTL interval, at which point the value of the TTL becomes 658 unknown because the RRset with which it was associated no longer 659 exists. 661 There is also the concept of a "default TTL" for a zone, which can 662 be a configuration parameter in the server software. This is 663 often expressed by a default for the entire server, and a default 664 for a zone using the $TTL directive in a zone file. The $TTL 665 directive was added to the master file format by [RFC2308]. 667 Class independent: A resource record type whose syntax and semantics 668 are the same for every DNS class. A resource record type that is 669 not class independent has different meanings depending on the DNS 670 class of the record, or the meaning is undefined for some class. 672 Most resource record types are defined for class 1 (IN, the 673 Internet), but many are undefined for other classes. 675 Address records: Records whose type is A or AAAA. [RFC2181] 676 informally defines these as "(A, AAAA, etc)". Note that new types 677 of address records could be defined in the future. 679 6. DNS Servers and Clients 681 This section defines the terms used for the systems that act as DNS 682 clients, DNS servers, or both. In the RFCs, DNS servers are 683 sometimes called "name servers", "nameservers", or just "servers". 684 There is no formal definition of DNS server, but the RFCs generally 685 assume that it is an Internet server that listens for queries and 686 sends responses using the DNS protocol defined in [RFC1035] and its 687 successors. 689 It is important to note that the terms "DNS server" and "name server" 690 require context in order to understand the services being provided. 691 Both authoritative servers and recursive resolvers are often called 692 "DNS servers" and "name servers" even though they serve different 693 roles (but may be part of the same software package). 695 For terminology specific to the public DNS root server system, see 696 [RSSAC026]. That document defines terms such as "root server", "root 697 server operator", and terms that are specific to the way that the 698 root zone of the public DNS is served. 700 Resolver: A program "that extract[s] information from name servers 701 in response to client requests." (Quoted from [RFC1034], 702 Section 2.4) A resolver performs queries for a name, type, and 703 class, and receives responses. The logical function is called 704 "resolution". In practice, the term is usually referring to some 705 specific type of resolver (some of which are defined below), and 706 understanding the use of the term depends on understanding the 707 context. 709 A related term is "resolve", which is not formally defined in 710 [RFC1034] or [RFC1035]. An imputed definition might be "asking a 711 question that consists of a domain name, class, and type, and 712 receiving some sort of response". Similarly, an imputed 713 definition of "resolution" might be "the response received from 714 resolving". 716 Stub resolver: A resolver that cannot perform all resolution itself. 717 Stub resolvers generally depend on a recursive resolver to 718 undertake the actual resolution function. Stub resolvers are 719 discussed but never fully defined in Section 5.3.1 of [RFC1034]. 720 They are fully defined in Section 6.1.3.1 of [RFC1123]. 722 Iterative mode: A resolution mode of a server that receives DNS 723 queries and responds with a referral to another server. 724 Section 2.3 of [RFC1034] describes this as "The server refers the 725 client to another server and lets the client pursue the query". A 726 resolver that works in iterative mode is sometimes called an 727 "iterative resolver". See also "iterative resolution" later in 728 this section. 730 Recursive mode: A resolution mode of a server that receives DNS 731 queries and either responds to those queries from a local cache or 732 sends queries to other servers in order to get the final answers 733 to the original queries. Section 2.3 of [RFC1034] describes this 734 as "The first server pursues the query for the client at another 735 server". Section 4.3.1 of of [RFC1034] says "in [recursive] mode 736 the name server acts in the role of a resolver and returns either 737 an error or the answer, but never referrals." That same section 738 also says "The recursive mode occurs when a query with RD set 739 arrives at a server which is willing to provide recursive service; 740 the client can verify that recursive mode was used by checking 741 that both RA and RD are set in the reply." 743 A server operating in recursive mode may be thought of as having a 744 name server side (which is what answers the query) and a resolver 745 side (which performs the resolution function). Systems operating 746 in this mode are commonly called "recursive servers". Sometimes 747 they are called "recursive resolvers". While strictly the 748 difference between these is that one of them sends queries to 749 another recursive server and the other does not, in practice it is 750 not possible to know in advance whether the server that one is 751 querying will also perform recursion; both terms can be observed 752 in use interchangeably. 754 Recursive resolver: A resolver that acts in recursive mode. In 755 general, a recursive resolver is expected to cache the answers it 756 receives (which would make it a full-service resolver), but some 757 recursive resolvers might not cache. 759 [RFC4697] tried to differentiate between a recursive resolver and 760 an iterative resolver. 762 Recursive query: A query with the Recursion Desired (RD) bit set to 763 1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive 764 service is available and is requested by the RD bit in the query, 765 the server uses its resolver to answer the query. (See 766 Section 4.3.2 of [RFC1035].) 768 Non-recursive query: A query with the Recursion Desired (RD) bit set 769 to 0 in the header. A server can answer non-recursive queries 770 using only local information: the response contains either an 771 error, the answer, or a referral to some other server "closer" to 772 the answer. (See Section 4.3.1 of [RFC1035].) 774 Iterative resolution: A name server may be presented with a query 775 that can only be answered by some other server. The two general 776 approaches to dealing with this problem are "recursive", in which 777 the first server pursues the query on behalf of the client at 778 another server, and "iterative", in which the server refers the 779 client to another server and lets the client pursue the query 780 there. (See Section 2.3 of [RFC1034].) 782 In iterative resolution, the client repeatedly makes non-recursive 783 queries and follows referrals and/or aliases. The iterative 784 resolution algorithm is described in Section 5.3.3 of [RFC1034]. 786 Full resolver: This term is used in [RFC1035], but it is not defined 787 there. RFC 1123 defines a "full-service resolver" that may or may 788 not be what was intended by "full resolver" in [RFC1035]. This 789 term is not properly defined in any RFC. 791 Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this 792 term to mean a resolver that acts in recursive mode with a cache 793 (and meets other requirements). 795 Priming: "The act of finding the list of root servers from a 796 configuration that lists some or all of the purported IP addresses 797 of some or all of those root servers." (Quoted from [RFC8109], 798 Section 2.) In order to operate in recursive mode, a resolver 799 needs to know the address of at least one root server. Priming is 800 most often done from a configuration setting that contains a list 801 of authoritative servers for the root zone. 803 Root hints: "Operators who manage a DNS recursive resolver typically 804 need to configure a 'root hints file'. This file contains the 805 names and IP addresses of the authoritative name servers for the 806 root zone, so the software can bootstrap the DNS resolution 807 process. For many pieces of software, this list comes built into 808 the software." (Quoted from [IANA_RootFiles]) This file is often 809 used in priming. 811 Negative caching: "The storage of knowledge that something does not 812 exist, cannot give an answer, or does not give an answer." 813 (Quoted from [RFC2308], Section 1) 815 Authoritative server: "A server that knows the content of a DNS zone 816 from local knowledge, and thus can answer queries about that zone 817 without needing to query other servers." (Quoted from [RFC2182], 818 Section 2.) An authoritative server is named in the NS ("name 819 server") record in a zone. It is a system that responds to DNS 820 queries with information about zones for which it has been 821 configured to answer with the AA flag in the response header set 822 to 1. It is a server that has authority over one or more DNS 823 zones. Note that it is possible for an authoritative server to 824 respond to a query without the parent zone delegating authority to 825 that server. Authoritative servers also provide "referrals", 826 usually to child zones delegated from them; these referrals have 827 the AA bit set to 0 and come with referral data in the Authority 828 and (if needed) the Additional sections. 830 Authoritative-only server: A name server that only serves 831 authoritative data and ignores requests for recursion. It will 832 "not normally generate any queries of its own. Instead, it 833 answers non-recursive queries from iterative resolvers looking for 834 information in zones it serves." (Quoted from [RFC4697], 835 Section 2.4) In this case, "ignores requests for recursion" means 836 "responds to requests for recursion with responses indicating that 837 recursion was not performed". 839 Zone transfer: The act of a client requesting a copy of a zone and 840 an authoritative server sending the needed information. (See 841 Section 7 for a description of zones.) There are two common 842 standard ways to do zone transfers: the AXFR ("Authoritative 843 Transfer") mechanism to copy the full zone (described in 844 [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy 845 only parts of the zone that have changed (described in [RFC1995]). 846 Many systems use non-standard methods for zone transfer outside 847 the DNS protocol. 849 Slave server: See "Secondary server". 851 Secondary server: "An authoritative server which uses zone transfer 852 to retrieve the zone" (Quoted from [RFC1996], Section 2.1). 853 Secondary servers are also discussed in [RFC1034]. [RFC2182] 854 describes secondary servers in more detail. Although early DNS 855 RFCs such as [RFC1996] referred to this as a "slave", the current 856 common usage has shifted to calling it a "secondary". 858 Master server: See "Primary server". 860 Primary server: "Any authoritative server configured to be the 861 source of zone transfer for one or more [secondary] servers" 862 (Quoted from [RFC1996], Section 2.1) or, more specifically, "an 863 authoritative server configured to be the source of AXFR or IXFR 864 data for one or more [secondary] servers" (Quoted from [RFC2136]). 865 Primary servers are also discussed in [RFC1034]. Although early 866 DNS RFCs such as [RFC1996] referred to this as a "master", the 867 current common usage has shifted to "primary". 869 Primary master: "The primary master is named in the zone's SOA MNAME 870 field and optionally by an NS RR". (Quoted from [RFC1996], 871 Section 2.1). [RFC2136] defines "primary master" as "Master 872 server at the root of the AXFR/IXFR dependency graph. The primary 873 master is named in the zone's SOA MNAME field and optionally by an 874 NS RR. There is by definition only one primary master server per 875 zone." The idea of a primary master is only used by [RFC2136], 876 and is considered archaic in other parts of the DNS. 878 The idea of a primary master is only used in [RFC1996] and 879 [RFC2136]. A modern interpretation of the term "primary master" 880 is a server that is both authoritative for a zone and that gets 881 its updates to the zone from configuration (such as a master file) 882 or from UPDATE transactions. 884 Stealth server: This is "like a slave server except not listed in an 885 NS RR for the zone." (Quoted from [RFC1996], Section 2.1) 887 Hidden master: A stealth server that is a primary server for zone 888 transfers. "In this arrangement, the master name server that 889 processes the updates is unavailable to general hosts on the 890 Internet; it is not listed in the NS RRset." (Quoted from 891 [RFC6781], Section 3.4.3). An earlier RFC, [RFC4641], said that 892 the hidden master's name "appears in the SOA RRs MNAME field", 893 although in some setups, the name does not appear at all in the 894 public DNS. A hidden master can also be a secondary server for 895 the zone itself. 897 Forwarding: The process of one server sending a DNS query with the 898 RD bit set to 1 to another server to resolve that query. 899 Forwarding is a function of a DNS resolver; it is different than 900 simply blindly relaying queries. 902 [RFC5625] does not give a specific definition for forwarding, but 903 describes in detail what features a system that forwards needs to 904 support. Systems that forward are sometimes called "DNS proxies", 905 but that term has not yet been defined (even in [RFC5625]). 907 Forwarder: Section 1 of [RFC2308] describes a forwarder as "a 908 nameserver used to resolve queries instead of directly using the 909 authoritative nameserver chain". [RFC2308] further says "The 910 forwarder typically either has better access to the internet, or 911 maintains a bigger cache which may be shared amongst many 912 resolvers." That definition appears to suggest that forwarders 913 normally only query authoritative servers. In current use, 914 however, forwarders often stand between stub resolvers and 915 recursive servers. [RFC2308] is silent on whether a forwarder is 916 iterative-only or can be a full-service resolver. 918 Policy-implementing resolver: A resolver acting in recursive mode 919 that changes some of the answers that it returns based on policy 920 criteria, such as to prevent access to malware sites or 921 objectionable content. In general, a stub resolver has no idea 922 whether upstream resolvers implement such policy or, if they do, 923 the exact policy about what changes will be made. In some cases, 924 the user of the stub resolver has selected the policy-implementing 925 resolver with the explicit intention of using it to implement the 926 policies. In other cases, policies are imposed without the user 927 of the stub resolver being informed. 929 Open resolver: A full-service resolver that accepts and processes 930 queries from any (or nearly any) client. This is sometimes also 931 called a "public resolver", although the term "public resolver" is 932 used more with open resolvers that are meant to be open, as 933 compared to the vast majority of open resolvers that are probably 934 misconfigured to be open. Open resolvers are discussed in 935 [RFC5358] 937 Split DNS: The terms "split DNS" and "split-horizon DNS" have long 938 been used in the DNS community without formal definition. In 939 general, they refer to situations in which DNS servers that are 940 authoritative for a particular set of domains provide partly or 941 completely different answers in those domains depending on the 942 source of the query. The effect of this is that a domain name 943 that is notionally globally unique nevertheless has different 944 meanings for different network users. This can sometimes be the 945 result of a "view" configuration, described below. 947 [RFC2775], Section 3.8 gives a related definition that is too 948 specific to be generally useful. 950 View: A configuration for a DNS server that allows it to provide 951 different responses depending on attributes of the query, such as 952 for "split DNS". Typically, views differ by the source IP address 953 of a query, but can also be based on the destination IP address, 954 the type of query (such as AXFR), whether it is recursive, and so 955 on. Views are often used to provide more names or different 956 addresses to queries from "inside" a protected network than to 957 those "outside" that network. Views are not a standardized part 958 of the DNS, but they are widely implemented in server software. 960 Passive DNS: A mechanism to collect DNS data by storing DNS 961 responses from name servers. Some of these systems also collect 962 the DNS queries associated with the responses, although doing so 963 raises some privacy concerns. Passive DNS databases can be used 964 to answer historical questions about DNS zones such as which 965 values were present at a given time in the past, or when a name 966 was spotted first. Passive DNS databases allow searching of the 967 stored records on keys other than just the name and type, such as 968 "find all names which have A records of a particular value". 970 Anycast: "The practice of making a particular service address 971 available in multiple, discrete, autonomous locations, such that 972 datagrams sent are routed to one of several available locations." 973 (Quoted from [RFC4786], Section 2) See [RFC4786] for more detail 974 on Anycast and other terms that are specific to its use. 976 Instance: "When anycast routing is used to allow more than one 977 server to have the same IP address, each one of those servers is 978 commonly referred to as an 'instance'." "An instance of a server, 979 such as a root server, is often referred to as an 'Anycast 980 instance'." (Quoted from [RSSAC026]) 982 Privacy-enabling DNS server: "A DNS server that implements DNS over 983 TLS [RFC7858] and may optionally implement DNS over DTLS 984 [RFC8094]." (Quoted from [RFC8310], Section 2) 986 7. Zones 988 This section defines terms that are used when discussing zones that 989 are being served or retrieved. 991 Zone: "Authoritative information is organized into units called 992 'zones', and these zones can be automatically distributed to the 993 name servers which provide redundant service for the data in a 994 zone." (Quoted from [RFC1034], Section 2.4) 996 Child: "The entity on record that has the delegation of the domain 997 from the Parent." (Quoted from [RFC7344], Section 1.1) 999 Parent: "The domain in which the Child is registered." (Quoted from 1000 [RFC7344], Section 1.1) Earlier, "parent name server" was defined 1001 in [RFC0882] as "the name server that has authority over the place 1002 in the domain name space that will hold the new domain". (Note 1003 that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].) 1004 [RFC0819] also has some description of the relationship between 1005 parents and children. 1007 Origin: 1009 (a) "The domain name that appears at the top of a zone (just below 1010 the cut that separates the zone from its parent). The name of the 1011 zone is the same as the name of the domain at the zone's origin." 1012 (Quoted from [RFC2181], Section 6.) These days, this sense of 1013 "origin" and "apex" (defined below) are often used 1014 interchangeably. 1016 (b) The domain name within which a given relative domain name 1017 appears in zone files. Generally seen in the context of 1018 "$ORIGIN", which is a control entry defined in [RFC1035], 1019 Section 5.1, as part of the master file format. For example, if 1020 the $ORIGIN is set to "example.org.", then a master file line for 1021 "www" is in fact an entry for "www.example.org.". 1023 Apex: The point in the tree at an owner of an SOA and corresponding 1024 authoritative NS RRset. This is also called the "zone apex". 1025 [RFC4033] defines it as "the name at the child's side of a zone 1026 cut". The "apex" can usefully be thought of as a data-theoretic 1027 description of a tree structure, and "origin" is the name of the 1028 same concept when it is implemented in zone files. The 1029 distinction is not always maintained in use, however, and one can 1030 find uses that conflict subtly with this definition. [RFC1034] 1031 uses the term "top node of the zone" as a synonym of "apex", but 1032 that term is not widely used. These days, the first sense of 1033 "origin" (above) and "apex" are often used interchangeably. 1035 Zone cut: The delimitation point between two zones where the origin 1036 of one of the zones is the child of the other zone. 1038 "Zones are delimited by 'zone cuts'. Each zone cut separates a 1039 'child' zone (below the cut) from a 'parent' zone (above the 1040 cut)." (Quoted from [RFC2181], Section 6; note that this is 1041 barely an ostensive definition.) Section 4.2 of [RFC1034] uses 1042 "cuts" instead of "zone cut". 1044 Delegation: The process by which a separate zone is created in the 1045 name space beneath the apex of a given domain. Delegation happens 1046 when an NS RRset is added in the parent zone for the child origin. 1047 Delegation inherently happens at a zone cut. The term is also 1048 commonly a noun: the new zone that is created by the act of 1049 delegating. 1051 Authoritative data: "All of the RRs attached to all of the nodes 1052 from the top node of the zone down to leaf nodes or nodes above 1053 cuts around the bottom edge of the zone." (Quoted from [RFC1034], 1054 Section 4.2.1) Note that this definition might inadvertently also 1055 cause any NS records that appear in the zone to be included, even 1056 those that might not truly be authoritative because there are 1057 identical NS RRs below the zone cut. This reveals the ambiguity 1058 in the notion of authoritative data, because the parent-side NS 1059 records authoritatively indicate the delegation, even though they 1060 are not themselves authoritative data. 1062 [RFC4033], Section 2, defines "Authoritative RRset" which is 1063 related to authoritative data but has a more precise definition. 1065 Lame delegation: "A lame delegations exists when a nameserver is 1066 delegated responsibility for providing nameservice for a zone (via 1067 NS records) but is not performing nameservice for that zone 1068 (usually because it is not set up as a primary or secondary for 1069 the zone)." (Quoted from [RFC1912], Section 2.8) 1071 Another definition is that a lame delegation "happens when a name 1072 server is listed in the NS records for some domain and in fact it 1073 is not a server for that domain. Queries are thus sent to the 1074 wrong servers, who don't know nothing (at least not as expected) 1075 about the queried domain. Furthermore, sometimes these hosts (if 1076 they exist!) don't even run name servers." (Quoted from 1077 [RFC1713], Section 2.3) 1079 Glue records: "[Resource records] which are not part of the 1080 authoritative data [of the zone], and are address resource records 1081 for the [name servers in subzones]. These RRs are only necessary 1082 if the name server's name is 'below' the cut, and are only used as 1083 part of a referral response." Without glue "we could be faced 1084 with the situation where the NS RRs tell us that in order to learn 1085 a name server's address, we should contact the server using the 1086 address we wish to learn." (Definition from [RFC1034], 1087 Section 4.2.1) 1089 A later definition is that glue "includes any record in a zone 1090 file that is not properly part of that zone, including nameserver 1091 records of delegated sub-zones (NS records), address records that 1092 accompany those NS records (A, AAAA, etc), and any other stray 1093 data that might appear" ([RFC2181], Section 5.4.1). Although glue 1094 is sometimes used today with this wider definition in mind, the 1095 context surrounding the [RFC2181] definition suggests it is 1096 intended to apply to the use of glue within the document itself 1097 and not necessarily beyond. 1099 Bailiwick: "In-bailiwick" is an adjective to describe a name server 1100 whose name is either a subdomain of or (rarely) the same as the 1101 origin of the zone that contains the delegation to the name 1102 server. In-bailiwick name servers may have glue records in their 1103 parent zone (using the first of the definitions of "glue records" 1104 in the definition above). (The term "bailiwick" means the 1105 district or territory where a bailiff or policeman has 1106 jurisdiction.) 1108 "In-bailiwick" names are divided into two type of name server 1109 names: "in-domain" names and "sibling domain" names. 1111 * In-domain: an adjective to describe a name server whose name is 1112 either subordinate to or (rarely) the same as the owner name of 1113 the NS resource records. An in-domain name server name MUST 1114 have glue records or name resolution fails. For example, a 1115 delegation for "child.example.com" may have "in-domain" name 1116 server name "ns.child.example.com". 1118 * Sibling domain: a name server's name that is either subordinate 1119 to or (rarely) the same as the zone origin and not subordinate 1120 to or the same as the owner name of the NS resource records. 1121 Glue records for sibling domains are allowed, but not 1122 necessary. For example, a delegation for "child.example.com" 1123 in "example.com" zone may have "sibling" name server name 1124 "ns.another.example.com". 1126 "Out-of-bailiwick" is the antonym of in-bailiwick. An adjective 1127 to describe a name server whose name is not subordinate to or the 1128 same as the zone origin. Glue records for out-of-bailiwick name 1129 servers are useless. Following table shows examples of delegation 1130 types. 1132 Delegation |Parent|Name Server Name | Type 1133 -----------+------+------------------+----------------------------- 1134 com | . |a.gtld-servers.net|in-bailiwick / sibling domain 1135 net | . |a.gtld-servers.net|in-bailiwick / in-domain 1136 example.org| org |ns.example.org |in-bailiwick / in-domain 1137 example.org| org |ns.ietf.org |in-bailiwick / sibling domain 1138 example.org| org |ns.example.com |out-of-bailiwick 1139 example.jp | jp |ns.example.jp |in-bailiwick / in-domain 1140 example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain 1141 example.jp | jp |ns.example.com |out-of-bailiwick 1143 Root zone: The zone of a DNS-based tree whose apex is the zero- 1144 length label. Also sometimes called "the DNS root". 1146 Empty non-terminals (ENT): "Domain names that own no resource 1147 records but have subdomains that do." (Quoted from [RFC4592], 1148 Section 2.2.2.) A typical example is in SRV records: in the name 1149 "_sip._tcp.example.com", it is likely that "_tcp.example.com" has 1150 no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV 1151 RRset. 1153 Delegation-centric zone: A zone that consists mostly of delegations 1154 to child zones. This term is used in contrast to a zone that 1155 might have some delegations to child zones, but also has many data 1156 resource records for the zone itself and/or for child zones. The 1157 term is used in [RFC4956] and [RFC5155], but is not defined there. 1159 Occluded name: "The addition of a delegation point via dynamic 1160 update will render all subordinate domain names to be in a limbo, 1161 still part of the zone, but not available to the lookup process. 1162 The addition of a DNAME resource record has the same impact. The 1163 subordinate names are said to be 'occluded'." (Quoted from 1164 [RFC5936], Section 3.5) 1166 Fast flux DNS: This "occurs when a domain is found in DNS using A 1167 records to multiple IP addresses, each of which has a very short 1168 Time-to-Live (TTL) value associated with it. This means that the 1169 domain resolves to varying IP addresses over a short period of 1170 time." (Quoted from [RFC6561], Section 1.1.5, with typo 1171 corrected) In addition to having legitimate uses, fast flux DNS 1172 can used to deliver malware. Because the addresses change so 1173 rapidly, it is difficult to ascertain all the hosts. It should be 1174 noted that the technique also works with AAAA records, but such 1175 use is not frequently observed on the Internet as of this writing. 1177 Reverse DNS, reverse lookup: "The process of mapping an address to a 1178 name is generally known as a 'reverse lookup', and the IN- 1179 ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse 1180 DNS'." (Quoted from [RFC5855], Section 1) 1182 Forward lookup: "Hostname-to-address translation". (Quoted from 1183 [RFC2133], Section 6) 1185 arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain 1186 was originally established as part of the initial deployment of 1187 the DNS, to provide a transition mechanism from the Host Tables 1188 that were common in the ARPANET, as well as a home for the IPv4 1189 reverse mapping domain. During 2000, the abbreviation was 1190 redesignated to 'Address and Routing Parameter Area' in the hope 1191 of reducing confusion with the earlier network name." (Quoted 1192 from [RFC3172], Section 2.) .arpa is an "infrastructure domain", a 1193 domain whose "role is to support the operating infrastructure of 1194 the Internet". (Quoted from [RFC3172], Section 2.) See [RFC3172] 1195 for more history of this name. 1197 Service name: "Service names are the unique key in the Service Name 1198 and Transport Protocol Port Number registry. This unique symbolic 1199 name for a service may also be used for other purposes, such as in 1200 DNS SRV records." (Quoted from [RFC6335], Section 5.) 1202 8. Wildcards 1204 Wildcard: [RFC1034] defined "wildcard", but in a way that turned out 1205 to be confusing to implementers. For an extended discussion of 1206 wildcards, including clearer definitions, see [RFC4592]. Special 1207 treatment is given to RRs with owner names starting with the label 1208 "*". "Such RRs are called 'wildcards'. Wildcard RRs can be 1209 thought of as instructions for synthesizing RRs." (Quoted from 1210 [RFC1034], Section 4.3.3) 1212 Asterisk label: "The first octet is the normal label type and length 1213 for a 1-octet-long label, and the second octet is the ASCII 1214 representation for the '*' character. A descriptive name of a 1215 label equaling that value is an 'asterisk label'." (Quoted from 1216 [RFC4592], Section 2.1.1) 1218 Wildcard domain name: "A 'wildcard domain name' is defined by having 1219 its initial (i.e., leftmost or least significant) label be 1220 asterisk label." (Quoted from [RFC4592], Section 2.1.1) 1222 Closest encloser: "The longest existing ancestor of a name." 1223 (Quoted from [RFC5155], Section 1.3) An earlier definition is "The 1224 node in the zone's tree of existing domain names that has the most 1225 labels matching the query name (consecutively, counting from the 1226 root label downward). Each match is a 'label match' and the order 1227 of the labels is the same." (Quoted from [RFC4592], 1228 Section 3.3.1) 1230 Closest provable encloser: "The longest ancestor of a name that can 1231 be proven to exist. Note that this is only different from the 1232 closest encloser in an Opt-Out zone." (Quoted from [RFC5155], 1233 Section 1.3) 1235 Next closer name: "The name one label longer than the closest 1236 provable encloser of a name." (Quoted from [RFC5155], 1237 Section 1.3) 1239 Source of Synthesis: "The source of synthesis is defined in the 1240 context of a query process as that wildcard domain name 1241 immediately descending from the closest encloser, provided that 1242 this wildcard domain name exists. 'Immediately descending' means 1243 that the source of synthesis has a name of the form: .." (Quoted from [RFC4592], 1245 Section 3.3.1) 1247 9. Registration Model 1249 Registry: The administrative operation of a zone that allows 1250 registration of names within that zone. People often use this 1251 term to refer only to those organizations that perform 1252 registration in large delegation-centric zones (such as TLDs); but 1253 formally, whoever decides what data goes into a zone is the 1254 registry for that zone. This definition of "registry" is from a 1255 DNS point of view; for some zones, the policies that determine 1256 what can go in the zone are decided by zones that are 1257 superordinate and not the registry operator. 1259 Registrant: An individual or organization on whose behalf a name in 1260 a zone is registered by the registry. In many zones, the registry 1261 and the registrant may be the same entity, but in TLDs they often 1262 are not. 1264 Registrar: A service provider that acts as a go-between for 1265 registrants and registries. Not all registrations require a 1266 registrar, though it is common to have registrars involved in 1267 registrations in TLDs. 1269 EPP: The Extensible Provisioning Protocol (EPP), which is commonly 1270 used for communication of registration information between 1271 registries and registrars. EPP is defined in [RFC5730]. 1273 WHOIS: A protocol specified in [RFC3912], often used for querying 1274 registry databases. WHOIS data is frequently used to associate 1275 registration data (such as zone management contacts) with domain 1276 names. The term "WHOIS data" is often used as a synonym for the 1277 registry database, even though that database may be served by 1278 different protocols, particularly RDAP. The WHOIS protocol is 1279 also used with IP address registry data. 1281 RDAP: The Registration Data Access Protocol, defined in [RFC7480], 1282 [RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The 1283 RDAP protocol and data format are meant as a replacement for 1284 WHOIS. 1286 DNS operator: An entity responsible for running DNS servers. For a 1287 zone's authoritative servers, the registrant may act as their own 1288 DNS operator, or their registrar may do it on their behalf, or 1289 they may use a third-party operator. For some zones, the registry 1290 function is performed by the DNS operator plus other entities who 1291 decide about the allowed contents of the zone. 1293 Public suffix: "A domain that is controlled by a public registry." 1294 (Quoted from [RFC6265], Section 5.3) A common definition for this 1295 term is a domain under which subdomains can be registered by third 1296 parties, and on which HTTP cookies (which are described in detail 1297 in [RFC6265]) should not be set. There is no indication in a 1298 domain name whether it is a public suffix; that can only be 1299 determined by outside means. In fact, both a domain and a 1300 subdomain of that domain can be public suffixes. 1302 There is nothing inherent in a domain name to indicate whether it 1303 is a public suffix. One resource for identifying public suffixes 1304 is the Public Suffix List (PSL) maintained by Mozilla 1305 (http://publicsuffix.org/). 1307 For example, at the time this document is published, the "com.au" 1308 domain is listed as a public suffix in the PSL. (Note that this 1309 example might change in the future.) 1311 Note that the term "public suffix" is controversial in the DNS 1312 community for many reasons, and may be significantly changed in 1313 the future. One example of the difficulty of calling a domain a 1314 public suffix is that designation can change over time as the 1315 registration policy for the zone changes, such as was the case 1316 with the "uk" TLD in 2014. 1318 Subordinate and Superordinate: These terms are introduced in 1319 [RFC3731] for use in the registration model, but not defined 1320 there. Instead, they are given in examples. "For example, domain 1321 name 'example.com' has a superordinate relationship to host name 1322 ns1.example.com'." "For example, host ns1.example1.com is a 1323 subordinate host of domain example1.com, but it is a not a 1324 subordinate host of domain example2.com." (Quoted from [RFC3731], 1325 Section 1.1.) These terms are strictly ways of referring to the 1326 relationship standing of two domains where one is a subdomain of 1327 the other. 1329 10. General DNSSEC 1331 Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and 1332 [RFC5155]. The terms that have caused confusion in the DNS community 1333 are highlighted here. 1335 DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in 1336 some RFCs, have not been formally defined. However, Section 2 of 1337 [RFC4033] defines many types of resolvers and validators, 1338 including "non-validating security-aware stub resolver", "non- 1339 validating stub resolver", "security-aware name server", 1340 "security-aware recursive name server", "security-aware resolver", 1341 "security-aware stub resolver", and "security-oblivious 1342 'anything'". (Note that the term "validating resolver", which is 1343 used in some places in DNSSEC-related documents, is also not 1344 defined in those RFCs, but is defined below.) 1346 Signed zone: "A zone whose RRsets are signed and that contains 1347 properly constructed DNSKEY, Resource Record Signature (RRSIG), 1348 Next Secure (NSEC), and (optionally) DS records." (Quoted from 1349 [RFC4033], Section 2.) It has been noted in other contexts that 1350 the zone itself is not really signed, but all the relevant RRsets 1351 in the zone are signed. Nevertheless, if a zone that should be 1352 signed contains any RRsets that are not signed (or opted out), 1353 those RRsets will be treated as bogus, so the whole zone needs to 1354 be handled in some way. 1356 It should also be noted that, since the publication of [RFC6840], 1357 NSEC records are no longer required for signed zones: a signed 1358 zone might include NSEC3 records instead. [RFC7129] provides 1359 additional background commentary and some context for the NSEC and 1360 NSEC3 mechanisms used by DNSSEC to provide authenticated denial- 1361 of-existence responses. NSEC and NSEC3 are described below. 1363 Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that 1364 is not signed". Section 2 of [RFC4035] defines this as "A zone 1365 that does not include these records [properly constructed DNSKEY, 1366 Resource Record Signature (RRSIG), Next Secure (NSEC), and 1367 (optionally) DS records] according to the rules in this section". 1368 There is an important note at the end of Section 5.2 of [RFC4035] 1369 that defines an additional situation in which a zone is considered 1370 unsigned: "If the resolver does not support any of the algorithms 1371 listed in an authenticated DS RRset, then the resolver will not be 1372 able to verify the authentication path to the child zone. In this 1373 case, the resolver SHOULD treat the child zone as if it were 1374 unsigned." 1376 NSEC: "The NSEC record allows a security-aware resolver to 1377 authenticate a negative reply for either name or type non- 1378 existence with the same mechanisms used to authenticate other DNS 1379 replies." (Quoted from [RFC4033], Section 3.2.) In short, an 1380 NSEC record provides authenticated denial of existence. 1382 "The NSEC resource record lists two separate things: the next 1383 owner name (in the canonical ordering of the zone) that contains 1384 authoritative data or a delegation point NS RRset, and the set of 1385 RR types present at the NSEC RR's owner name." (Quoted from 1386 Section 4 of RFC 4034) 1388 NSEC3: Like the NSEC record, the NSEC3 record also provides 1389 authenticated denial of existence; however, NSEC3 records mitigate 1390 against zone enumeration and support Opt-Out. NSEC3 resource 1391 records require associated NSEC3PARAM resource records. NSEC3 and 1392 NSEC3PARAM resource records are defined in [RFC5155]. 1394 Note that [RFC6840] says that [RFC5155] "is now considered part of 1395 the DNS Security Document Family as described by Section 10 of 1396 [RFC4033]". This means that some of the definitions from earlier 1397 RFCs that only talk about NSEC records should probably be 1398 considered to be talking about both NSEC and NSEC3. 1400 Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover 1401 unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1.) 1402 Opt-out tackles the high costs of securing a delegation to an 1403 insecure zone. When using Opt-Out, names that are an insecure 1404 delegation (and empty non-terminals that are only derived from 1405 insecure delegations) don't require an NSEC3 record or its 1406 corresponding RRSIG records. Opt-Out NSEC3 records are not able 1407 to prove or deny the existence of the insecure delegations. 1408 (Adapted from [RFC7129], Section 5.1) 1410 Insecure delegation: "A signed name containing a delegation (NS 1411 RRset), but lacking a DS RRset, signifying a delegation to an 1412 unsigned subzone." (Quoted from [RFC4956], Section 2.) 1414 Zone enumeration: "The practice of discovering the full content of a 1415 zone via successive queries." (Quoted from [RFC5155], 1416 Section 1.3.) This is also sometimes called "zone walking". Zone 1417 enumeration is different from zone content guessing where the 1418 guesser uses a large dictionary of possible labels and sends 1419 successive queries for them, or matches the contents of NSEC3 1420 records against such a dictionary. 1422 Validation: Validation, in the context of DNSSEC, refers to one of 1423 the following: 1425 * Checking the validity of DNSSEC signatures 1427 * Checking the validity of DNS responses, such as those including 1428 authenticated denial of existence 1430 * Building an authentication chain from a trust anchor to a DNS 1431 response or individual DNS RRsets in a response 1433 The first two definitions above consider only the validity of 1434 individual DNSSEC components such as the RRSIG validity or NSEC 1435 proof validity. The third definition considers the components of 1436 the entire DNSSEC authentication chain, and thus requires 1437 "configured knowledge of at least one authenticated DNSKEY or DS 1438 RR" (as described in [RFC4035], Section 5). 1440 [RFC4033], Section 2, says that a "Validating Security-Aware Stub 1441 Resolver... performs signature validation" and uses a trust anchor 1442 "as a starting point for building the authentication chain to a 1443 signed DNS response", and thus uses the first and third 1444 definitions above. The process of validating an RRSIG resource 1445 record is described in [RFC4035], Section 5.3. 1447 [RFC5155] refers to validating responses throughout the document, 1448 in the context of hashed authenticated denial of existence; this 1449 uses the second definition above. 1451 The term "authentication" is used interchangeably with 1452 "validation", in the sense of the third definition above. 1453 [RFC4033], Section 2, describes the chain linking trust anchor to 1454 DNS data as the "authentication chain". A response is considered 1455 to be authentic if "all RRsets in the Answer and Authority 1456 sections of the response [are considered] to be authentic" 1457 ([RFC4035]). DNS data or responses deemed to be authentic or 1458 validated have a security status of "secure" ([RFC4035], 1459 Section 4.3; [RFC4033], Section 5). "Authenticating both DNS keys 1460 and data is a matter of local policy, which may extend or even 1461 override the [DNSSEC] protocol extensions" ([RFC4033], 1462 Section 3.1). 1464 The term "verification", when used, is usually synonym for 1465 "validation". 1467 Validating resolver: A security-aware recursive name server, 1468 security-aware resolver, or security-aware stub resolver that is 1469 applying at least one of the definitions of validation (above), as 1470 appropriate to the resolution context. For the same reason that 1471 the generic term "resolver" is sometimes ambiguous and needs to be 1472 evaluated in context (see Section 6), "validating resolver" is a 1473 context-sensitive term. 1475 Key signing key (KSK): DNSSEC keys that "only sign the apex DNSKEY 1476 RRset in a zone."(Quoted from [RFC6781], Section 3.1) 1478 Zone signing key (ZSK): "DNSSEC keys that can be used to sign all 1479 the RRsets in a zone that require signatures, other than the apex 1480 DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Also note 1481 that a ZSK is sometimes used to sign the apex DNSKEY RRset. 1483 Combined signing key (CSK): "In cases where the differentiation 1484 between the KSK and ZSK is not made, i.e., where keys have the 1485 role of both KSK and ZSK, we talk about a Single-Type Signing 1486 Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes 1487 called a "combined signing key" or CSK. It is operational 1488 practice, not protocol, that determines whether a particular key 1489 is a ZSK, a KSK, or a CSK. 1491 Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be 1492 used to distinguish between keys that are intended to be used as 1493 the secure entry point into the zone when building chains of 1494 trust, i.e., they are (to be) pointed to by parental DS RRs or 1495 configured as a trust anchor. Therefore, it is suggested that the 1496 SEP flag be set on keys that are used as KSKs and not on keys that 1497 are used as ZSKs, while in those cases where a distinction between 1498 a KSK and ZSK is not made (i.e., for a Single-Type Signing 1499 Scheme), it is suggested that the SEP flag be set on all keys." 1500 (Quoted from [RFC6781], Section 3.2.3.) Note that the SEP flag is 1501 only a hint, and its presence or absence may not be used to 1502 disqualify a given DNSKEY RR from use as a KSK or ZSK during 1503 validation. 1505 The original definition of SEPs was in [RFC3757]. That definition 1506 clearly indicated that the SEP was a key, not just a bit in the 1507 key. The abstract of [RFC3757] says: "With the Delegation Signer 1508 (DS) resource record (RR), the concept of a public key acting as a 1509 secure entry point (SEP) has been introduced. During exchanges of 1510 public keys with the parent there is a need to differentiate SEP 1511 keys from other public keys in the Domain Name System KEY (DNSKEY) 1512 resource record set. A flag bit in the DNSKEY RR is defined to 1513 indicate that DNSKEY is to be used as a SEP." That definition of 1514 the SEP as a key was made obsolete by [RFC4034], and the 1515 definition from [RFC6781] is consistent with [RFC4034]. 1517 Trust anchor: "A configured DNSKEY RR or DS RR hash of a DNSKEY RR. 1518 A validating security-aware resolver uses this public key or hash 1519 as a starting point for building the authentication chain to a 1520 signed DNS response. In general, a validating resolver will have 1521 to obtain the initial values of its trust anchors via some secure 1522 or trusted means outside the DNS protocol." (Quoted from 1523 [RFC4033], Section 2) 1525 DNSSEC Policy (DP): A statement that "sets forth the security 1526 requirements and standards to be implemented for a DNSSEC-signed 1527 zone." (Quoted from [RFC6841], Section 2) 1529 DNSSEC Practice Statement (DPS): "A practices disclosure document 1530 that may support and be a supplemental document to the DNSSEC 1531 Policy (if such exists), and it states how the management of a 1532 given zone implements procedures and controls at a high level." 1533 (Quoted from [RFC6841], Section 2) 1535 Hardware security module (HSM): A specialized piece of hardware that 1536 is used to create keys for signatures and to sign messages. In 1537 DNSSEC, HSMs are often used to hold the private keys for KSKs and 1538 ZSKs and to create the signatures used in RRSIG records at 1539 periodic intervals. 1541 Signing software: Authoritative DNS servers that support DNSSEC 1542 often contain software that facilitates the creation and 1543 maintenance of DNSSEC signatures in zones. There is also stand- 1544 alone software that can be used to sign a zone regardless of 1545 whether the authoritative server itself supports signing. 1546 Sometimes signing software can support particular HSMs as part of 1547 the signing process. 1549 11. DNSSEC States 1551 A validating resolver can determine that a response is in one of four 1552 states: secure, insecure, bogus, or indeterminate. These states are 1553 defined in [RFC4033] and [RFC4035], although the two definitions 1554 differ a bit. This document makes no effort to reconcile the two 1555 definitions, and takes no position as to whether they need to be 1556 reconciled. 1558 Section 5 of [RFC4033] says: 1560 A validating resolver can determine the following 4 states: 1562 Secure: The validating resolver has a trust anchor, has a chain 1563 of trust, and is able to verify all the signatures in the 1564 response. 1566 Insecure: The validating resolver has a trust anchor, a chain 1567 of trust, and, at some delegation point, signed proof of the 1568 non-existence of a DS record. This indicates that subsequent 1569 branches in the tree are provably insecure. A validating 1570 resolver may have a local policy to mark parts of the domain 1571 space as insecure. 1573 Bogus: The validating resolver has a trust anchor and a secure 1574 delegation indicating that subsidiary data is signed, but 1575 the response fails to validate for some reason: missing 1576 signatures, expired signatures, signatures with unsupported 1577 algorithms, data missing that the relevant NSEC RR says 1578 should be present, and so forth. 1580 Indeterminate: There is no trust anchor that would indicate that a 1581 specific portion of the tree is secure. This is the default 1582 operation mode. 1584 Section 4.3 of [RFC4035] says: 1586 A security-aware resolver must be able to distinguish between four 1587 cases: 1589 Secure: An RRset for which the resolver is able to build a chain 1590 of signed DNSKEY and DS RRs from a trusted security anchor to 1591 the RRset. In this case, the RRset should be signed and is 1592 subject to signature validation, as described above. 1594 Insecure: An RRset for which the resolver knows that it has no 1595 chain of signed DNSKEY and DS RRs from any trusted starting 1596 point to the RRset. This can occur when the target RRset lies 1597 in an unsigned zone or in a descendent [sic] of an unsigned 1598 zone. In this case, the RRset may or may not be signed, but 1599 the resolver will not be able to verify the signature. 1601 Bogus: An RRset for which the resolver believes that it ought to 1602 be able to establish a chain of trust but for which it is 1603 unable to do so, either due to signatures that for some reason 1604 fail to validate or due to missing data that the relevant 1605 DNSSEC RRs indicate should be present. This case may indicate 1606 an attack but may also indicate a configuration error or some 1607 form of data corruption. 1609 Indeterminate: An RRset for which the resolver is not able to 1610 determine whether the RRset should be signed, as the resolver 1611 is not able to obtain the necessary DNSSEC RRs. This can occur 1612 when the security-aware resolver is not able to contact 1613 security-aware name servers for the relevant zones. 1615 12. Security Considerations 1617 These definitions do not change any security considerations for the 1618 DNS. 1620 13. IANA Considerations 1622 None. 1624 14. References 1626 14.1. Normative References 1628 [IANA_RootFiles] 1629 Internet Assigned Numbers Authority, "IANA Root Files", 1630 2016, . 1632 [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities", 1633 RFC 882, DOI 10.17487/RFC0882, November 1983, 1634 . 1636 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1637 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1638 . 1640 [RFC1035] Mockapetris, P., "Domain names - implementation and 1641 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1642 November 1987, . 1644 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1645 Application and Support", STD 3, RFC 1123, 1646 DOI 10.17487/RFC1123, October 1989, 1647 . 1649 [RFC1912] Barr, D., "Common DNS Operational and Configuration 1650 Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996, 1651 . 1653 [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone 1654 Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996, 1655 August 1996, . 1657 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 1658 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1659 RFC 2136, DOI 10.17487/RFC2136, April 1997, 1660 . 1662 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1663 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1664 . 1666 [RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection 1667 and Operation of Secondary DNS Servers", BCP 16, RFC 2182, 1668 DOI 10.17487/RFC2182, July 1997, 1669 . 1671 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 1672 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 1673 . 1675 [RFC3731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP) 1676 Domain Name Mapping", RFC 3731, DOI 10.17487/RFC3731, 1677 March 2004, . 1679 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1680 Rose, "DNS Security Introduction and Requirements", 1681 RFC 4033, DOI 10.17487/RFC4033, March 2005, 1682 . 1684 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1685 Rose, "Resource Records for the DNS Security Extensions", 1686 RFC 4034, DOI 10.17487/RFC4034, March 2005, 1687 . 1689 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1690 Rose, "Protocol Modifications for the DNS Security 1691 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 1692 . 1694 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name 1695 System", RFC 4592, DOI 10.17487/RFC4592, July 2006, 1696 . 1698 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 1699 Security (DNSSEC) Hashed Authenticated Denial of 1700 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008, 1701 . 1703 [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive 1704 Nameservers in Reflector Attacks", BCP 140, RFC 5358, 1705 DOI 10.17487/RFC5358, October 2008, 1706 . 1708 [RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)", 1709 STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009, 1710 . 1712 [RFC5855] Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6 1713 Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855, 1714 May 2010, . 1716 [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol 1717 (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010, 1718 . 1720 [RFC6561] Livingood, J., Mody, N., and M. O'Reirdan, 1721 "Recommendations for the Remediation of Bots in ISP 1722 Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012, 1723 . 1725 [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC 1726 Operational Practices, Version 2", RFC 6781, 1727 DOI 10.17487/RFC6781, December 2012, 1728 . 1730 [RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and 1731 Implementation Notes for DNS Security (DNSSEC)", RFC 6840, 1732 DOI 10.17487/RFC6840, February 2013, 1733 . 1735 [RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A 1736 Framework for DNSSEC Policies and DNSSEC Practice 1737 Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013, 1738 . 1740 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 1741 for DNS (EDNS(0))", STD 75, RFC 6891, 1742 DOI 10.17487/RFC6891, April 2013, 1743 . 1745 [RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating 1746 DNSSEC Delegation Trust Maintenance", RFC 7344, 1747 DOI 10.17487/RFC7344, September 2014, 1748 . 1750 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1751 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 1752 2015, . 1754 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles 1755 for DNS over TLS and DNS over DTLS", RFC 8310, 1756 DOI 10.17487/RFC8310, March 2018, 1757 . 1759 14.2. Informative References 1761 [IANA_Resource_Registry] 1762 Internet Assigned Numbers Authority, "Resource Record (RR) 1763 TYPEs", 2017, 1764 . 1766 [RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for 1767 Internet User Applications", RFC 819, 1768 DOI 10.17487/RFC0819, August 1982, 1769 . 1771 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet 1772 host table specification", RFC 952, DOI 10.17487/RFC0952, 1773 October 1985, . 1775 [RFC1713] Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713, 1776 DOI 10.17487/RFC1713, November 1994, 1777 . 1779 [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, 1780 DOI 10.17487/RFC1995, August 1996, 1781 . 1783 [RFC2133] Gilligan, R., Thomson, S., Bound, J., and W. Stevens, 1784 "Basic Socket Interface Extensions for IPv6", RFC 2133, 1785 DOI 10.17487/RFC2133, April 1997, 1786 . 1788 [RFC2775] Carpenter, B., "Internet Transparency", RFC 2775, 1789 DOI 10.17487/RFC2775, February 2000, 1790 . 1792 [RFC3172] Huston, G., Ed., "Management Guidelines & Operational 1793 Requirements for the Address and Routing Parameter Area 1794 Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172, 1795 September 2001, . 1797 [RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425, 1798 DOI 10.17487/RFC3425, November 2002, 1799 . 1801 [RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name 1802 System KEY (DNSKEY) Resource Record (RR) Secure Entry 1803 Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April 1804 2004, . 1806 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, 1807 DOI 10.17487/RFC3912, September 2004, 1808 . 1810 [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices", 1811 RFC 4641, DOI 10.17487/RFC4641, September 2006, 1812 . 1814 [RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution 1815 Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697, 1816 October 2006, . 1818 [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast 1819 Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, 1820 December 2006, . 1822 [RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security 1823 (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July 1824 2007, . 1826 [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", 1827 BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009, 1828 . 1830 [RFC5890] Klensin, J., "Internationalized Domain Names for 1831 Applications (IDNA): Definitions and Document Framework", 1832 RFC 5890, DOI 10.17487/RFC5890, August 2010, 1833 . 1835 [RFC5891] Klensin, J., "Internationalized Domain Names in 1836 Applications (IDNA): Protocol", RFC 5891, 1837 DOI 10.17487/RFC5891, August 2010, 1838 . 1840 [RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and 1841 Internationalized Domain Names for Applications (IDNA)", 1842 RFC 5892, DOI 10.17487/RFC5892, August 2010, 1843 . 1845 [RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts 1846 for Internationalized Domain Names for Applications 1847 (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010, 1848 . 1850 [RFC5894] Klensin, J., "Internationalized Domain Names for 1851 Applications (IDNA): Background, Explanation, and 1852 Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010, 1853 . 1855 [RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on 1856 Encodings for Internationalized Domain Names", RFC 6055, 1857 DOI 10.17487/RFC6055, February 2011, 1858 . 1860 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 1861 DOI 10.17487/RFC6265, April 2011, 1862 . 1864 [RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163, 1865 RFC 6303, DOI 10.17487/RFC6303, July 2011, 1866 . 1868 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1869 Cheshire, "Internet Assigned Numbers Authority (IANA) 1870 Procedures for the Management of the Service Name and 1871 Transport Protocol Port Number Registry", BCP 165, 1872 RFC 6335, DOI 10.17487/RFC6335, August 2011, 1873 . 1875 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in 1876 Internationalization in the IETF", BCP 166, RFC 6365, 1877 DOI 10.17487/RFC6365, September 2011, 1878 . 1880 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 1881 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, 1882 . 1884 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 1885 DOI 10.17487/RFC6762, February 2013, 1886 . 1888 [RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of 1889 Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, 1890 February 2014, . 1892 [RFC7480] Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the 1893 Registration Data Access Protocol (RDAP)", RFC 7480, 1894 DOI 10.17487/RFC7480, March 2015, 1895 . 1897 [RFC7481] Hollenbeck, S. and N. Kong, "Security Services for the 1898 Registration Data Access Protocol (RDAP)", RFC 7481, 1899 DOI 10.17487/RFC7481, March 2015, 1900 . 1902 [RFC7482] Newton, A. and S. Hollenbeck, "Registration Data Access 1903 Protocol (RDAP) Query Format", RFC 7482, 1904 DOI 10.17487/RFC7482, March 2015, 1905 . 1907 [RFC7483] Newton, A. and S. Hollenbeck, "JSON Responses for the 1908 Registration Data Access Protocol (RDAP)", RFC 7483, 1909 DOI 10.17487/RFC7483, March 2015, 1910 . 1912 [RFC7484] Blanchet, M., "Finding the Authoritative Registration Data 1913 (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March 1914 2015, . 1916 [RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin, 1917 "Inventory and Analysis of WHOIS Registration Objects", 1918 RFC 7485, DOI 10.17487/RFC7485, March 2015, 1919 . 1921 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1922 and P. Hoffman, "Specification for DNS over Transport 1923 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1924 2016, . 1926 [RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram 1927 Transport Layer Security (DTLS)", RFC 8094, 1928 DOI 10.17487/RFC8094, February 2017, 1929 . 1931 [RFC8109] Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS 1932 Resolver with Priming Queries", BCP 209, RFC 8109, 1933 DOI 10.17487/RFC8109, March 2017, 1934 . 1936 [RSSAC026] 1937 Root Server System Advisory Committee (RSSAC), "RSSAC 1938 Lexicon", 2017, 1939 . 1942 Appendix A. Definitions Updated by this Document 1944 The following definitions from RFCs are updated by this document: 1946 o Forwarder in [RFC2308] 1948 o QNAME in [RFC2308] 1950 o Secure Entry Point (SEP) in [RFC3757]; note, however, that this 1951 RFC is already obsolete 1953 Appendix B. Definitions First Defined in this Document 1955 The following definitions are first defined in this document: 1957 o "Alias" in Section 2 1959 o "Apex" in Section 7 1960 o "arpa" in Section 7 1962 o "Bailiwick" in Section 7 1964 o "Class independent" in Section 5 1966 o "Delegation-centric zone" in Section 7 1968 o "Delegation" in Section 7 1970 o "DNS operator" in Section 9 1972 o "DNSSEC-aware" in Section 10 1974 o "DNSSEC-unaware" in Section 10 1976 o "Forwarding" in Section 6 1978 o "Full resolver" in Section 6 1980 o "Fully qualified domain name" in Section 2 1982 o "Global DNS" in Section 2 1984 o "Hardware Security Module (HSM)" in Section 10 1986 o "Host name" in Section 2 1988 o "IDN" in Section 2 1990 o "In-bailiwick" in Section 7 1992 o "Iterative resolution" in Section 6 1994 o "Label" in Section 2 1996 o "Locally served DNS zone" in Section 2 1998 o "Naming system" in Section 2 2000 o "Negative response" in Section 3 2002 o "Non-recursive query" in Section 6 2004 o "Open resolver" in Section 6 2006 o "Out-of-bailiwick" in Section 7 2007 o "Passive DNS" in Section 6 2009 o "Policy-implementing resolver" in Section 6 2011 o "Presentation format" in Section 5 2013 o "Priming" in Section 6 2015 o "Private DNS" in Section 2 2017 o "Recursive resolver" in Section 6 2019 o "Referrals" in Section 4 2021 o "Registrant" in Section 9 2023 o "Registrar" in Section 9 2025 o "Registry" in Section 9 2027 o "Root zone" in Section 7 2029 o "Secure Entry Point (SEP)" in Section 10 2031 o "Signing software" in Section 10 2033 o "Split DNS" in Section 6 2035 o "Stub resolver" in Section 6 2037 o "Subordinate" in Section 8 2039 o "Superordinate" in Section 8 2041 o "TLD" in Section 2 2043 o "Validating resolver" in Section 10 2045 o "Validation" in Section 10 2047 o "View" in Section 6 2049 o "Zone transfer" in Section 6 2051 Index 2053 A 2054 Address records 15 2055 Alias 9 2056 Anycast 21 2057 Apex 22 2058 Asterisk label 26 2059 Authoritative data 22 2060 Authoritative server 18 2061 Authoritative-only server 18 2062 arpa: Address and Routing Parameter Area Domain 25 2064 C 2065 CNAME 9 2066 Canonical name 9 2067 Child 21 2068 Class independent 14 2069 Closest encloser 26 2070 Closest provable encloser 26 2071 Combined signing key (CSK) 31 2073 D 2074 DNS operator 27 2075 DNSSEC Policy (DP) 32 2076 DNSSEC Practice Statement (DPS) 32 2077 DNSSEC-aware and DNSSEC-unaware 28 2078 Delegation 22 2079 Delegation-centric zone 25 2080 Domain name 4 2082 E 2083 EDNS 13 2084 EPP 27 2085 Empty non-terminals (ENT) 24 2087 F 2088 FORMERR 9 2089 Fast flux DNS 25 2090 Forward lookup 25 2091 Forwarder 19 2092 Forwarding 19 2093 Full resolver 17 2094 Full-service resolver 17 2095 Fully qualified domain name (FQDN) 7 2097 G 2098 Global DNS 5 2099 Glue records 23 2101 H 2102 Hardware security module (HSM) 33 2103 Hidden master 19 2104 Host name 8 2106 I 2107 IDN 8 2108 In-bailiwick 23 2109 Insecure delegation 30 2110 Instance 21 2111 Iterative mode 16 2112 Iterative resolution 17 2114 K 2115 Key signing key (KSK) 31 2117 L 2118 Label 5 2119 Lame delegation 23 2120 Locally served DNS zone 7 2122 M 2123 Master file 13 2124 Master server 18 2125 Multicast DNS 7 2127 N 2128 NODATA 10 2129 NOERROR 9 2130 NOTIMP 9 2131 NS 18 2132 NSEC 29 2133 NSEC3 29 2134 NXDOMAIN 9 2135 Naming system 4 2136 Negative caching 17 2137 Negative response 10 2138 Next closer name 26 2139 Non-recursive query 17 2141 O 2142 OPT 13 2143 Occluded name 25 2144 Open resolver 20 2145 Opt-out 30 2146 Origin 21 2147 Out-of-bailiwick 23 2148 Owner 13 2150 P 2151 Parent 21 2152 Passive DNS 21 2153 Policy-implementing resolver 20 2154 Presentation format 13 2155 Primary master 19 2156 Primary server 18 2157 Priming 17 2158 Privacy-enabling DNS server 21 2159 Private DNS 6 2160 Public suffix 27 2162 Q 2163 QNAME 10 2165 R 2166 RDAP 27 2167 REFUSED 9 2168 RR 13 2169 RRset 13 2170 Recursive mode 16 2171 Recursive query 16 2172 Recursive resolver 16 2173 Referrals 12 2174 Registrant 27 2175 Registrar 27 2176 Registry 27 2177 Resolver 15 2178 Reverse DNS, reverse lookup 25 2179 Root hints 17 2180 Root zone 24 2182 S 2183 SERVFAIL 9 2184 SOA 13 2185 SOA field names 13 2186 Secondary server 18 2187 Secure Entry Point (SEP) 32 2188 Service name 25 2189 Signed zone 29 2190 Signing software 33 2191 Slave server 18 2192 Source of Synthesis 26 2193 Split DNS 20 2194 Split-horizon DNS 20 2195 Stealth server 19 2196 Stub resolver 15 2197 Subdomain 8 2198 Subordinate 28 2199 Superordinate 28 2201 T 2202 TLD 8 2203 TTL 14 2204 Trust anchor 32 2206 U 2207 Unsigned zone 29 2209 V 2210 Validating resolver 31 2211 Validation 30 2212 View 20 2214 W 2215 WHOIS 27 2216 Wildcard 26 2217 Wildcard domain name 26 2219 Z 2220 Zone 21 2221 Zone cut 22 2222 Zone enumeration 30 2223 Zone signing key (ZSK) 31 2224 Zone transfer 18 2226 Acknowledgements 2228 The following is the Acknowledgements for RFC 7719. Additional 2229 acknowledgements may be added as this draft is worked on. 2231 The authors gratefully acknowledge all of the authors of DNS-related 2232 RFCs that proceed this one. Comments from Tony Finch, Stephane 2233 Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John 2234 Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman, 2235 David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis, 2236 John Klensin, David Black, and many others in the DNSOP Working Group 2237 helped shape RFC 7719. 2239 Most of the major changes between RFC 7719 and this document came 2240 from active discussion on the DNSOP WG. Specific people who 2241 contributed material to this document include: Bob Harold, Dick 2242 Franks, Evan Hunt, John Dickinson, Mark Andrews, Martin Hoffmann, 2243 Paul Vixie, Peter Koch, Duane Wessels, Allison Mankin, Giovane Moura, 2244 Roni Even, Dan Romascanu, and Vladmir Cunat. 2246 Authors' Addresses 2248 Paul Hoffman 2249 ICANN 2251 Email: paul.hoffman@icann.org 2253 Andrew Sullivan 2255 Email: ajs@anvilwalrusden.com 2257 Kazunori Fujiwara 2258 Japan Registry Services Co., Ltd. 2259 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda 2260 Chiyoda-ku, Tokyo 101-0065 2261 Japan 2263 Phone: +81 3 5215 8451 2264 Email: fujiwara@jprs.co.jp