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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 Intended status: Best Current Practice Dyn 6 Expires: September 14, 2017 K. Fujiwara 7 JPRS 8 March 13, 2017 10 DNS Terminology 11 draft-ietf-dnsop-terminology-bis-05 13 Abstract 15 The DNS is defined in literally dozens of different RFCs. The 16 terminology used by implementers and developers of DNS protocols, and 17 by operators of DNS systems, has sometimes changed in the decades 18 since the DNS was first defined. This document gives current 19 definitions for many of the terms used in the DNS in a single 20 document. 22 This document will be the successor to RFC 7719. 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 http://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 September 14, 2017. 41 Copyright Notice 43 Copyright (c) 2017 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 (http://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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 3. DNS Header and Response Codes . . . . . . . . . . . . . . . . 9 61 4. Resource Records . . . . . . . . . . . . . . . . . . . . . . 10 62 5. DNS Servers and Clients . . . . . . . . . . . . . . . . . . . 12 63 6. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 64 7. Registration Model . . . . . . . . . . . . . . . . . . . . . 21 65 8. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 22 66 9. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 26 67 10. Security Considerations . . . . . . . . . . . . . . . . . . . 28 68 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 69 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 70 12.1. Normative References . . . . . . . . . . . . . . . . . . 28 71 12.2. Informative References . . . . . . . . . . . . . . . . . 31 72 Appendix A. Definitions Updated by this Document . . . . . . . . 34 73 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 34 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 76 1. Introduction 78 The Domain Name System (DNS) is a simple query-response protocol 79 whose messages in both directions have the same format. (See 80 Section 2 for a fuller definition.) The protocol and message format 81 are defined in [RFC1034] and [RFC1035]. These RFCs defined some 82 terms, but later documents defined others. Some of the terms from 83 [RFC1034] and [RFC1035] now have somewhat different meanings than 84 they did in 1987. 86 This document collects a wide variety of DNS-related terms. Some of 87 them have been precisely defined in earlier RFCs, some have been 88 loosely defined in earlier RFCs, and some are not defined in any 89 earlier RFC at all. 91 Most of the definitions here are the consensus definition of the DNS 92 community -- both protocol developers and operators. Some of the 93 definitions differ from earlier RFCs, and those differences are 94 noted. In this document, where the consensus definition is the same 95 as the one in an RFC, that RFC is quoted. Where the consensus 96 definition has changed somewhat, the RFC is mentioned but the new 97 stand-alone definition is given. See Appendix A for a list of the 98 definitions that this document updates. 100 It is important to note that, during the development of this 101 document, it became clear that some DNS-related terms are interpreted 102 quite differently by different DNS experts. Further, some terms that 103 are defined in early DNS RFCs now have definitions that are generally 104 agreed to, but that are different from the original definitions. 105 Therefore, this document is a substantial revision to [RFC7719]. 107 The terms are organized loosely by topic. Some definitions are for 108 new terms for things that are commonly talked about in the DNS 109 community but that never had terms defined for them. 111 Other organizations sometimes define DNS-related terms their own way. 112 For example, the W3C defines "domain" at 113 https://specs.webplatform.org/url/webspecs/develop/. 115 Note that there is no single consistent definition of "the DNS". It 116 can be considered to be some combination of the following: a commonly 117 used naming scheme for objects on the Internet; a distributed 118 database representing the names and certain properties of these 119 objects; an architecture providing distributed maintenance, 120 resilience, and loose coherency for this database; and a simple 121 query-response protocol (as mentioned below) implementing this 122 architecture. Section 2 defines "global DNS" and "private DNS" as a 123 way to deal with these differing definitions. 125 Capitalization in DNS terms is often inconsistent among RFCs and 126 various DNS practitioners. The capitalization used in this document 127 is a best guess at current practices, and is not meant to indicate 128 that other capitalization styles are wrong or archaic. In some 129 cases, multiple styles of capitalization are used for the same term 130 due to quoting from different RFCs. 132 2. Names 134 Naming system: A naming system associates names with data. Naming 135 systems have many significant facets that help differentiate them. 136 Some commonly-identified facets include: 138 * Composition of names 140 * Format of names 142 * Administration of names 143 * Types of data that can be associated with names 145 * Types of metadata for names 147 * Protocol for getting data from a name 149 * Context for resolving a name 151 Note that this list is a small subset of facets that people have 152 identified over time for naming systems, and the IETF has yet to 153 agree on a good set of facets that can be used to compare naming 154 systems. For example, other facets might include "protocol to 155 update data in a name", "privacy of names", and "privacy of data 156 associated with names", but those do not not have a clear 157 definitions as the ones listed above. The list here is chosen 158 because it helps describe the DNS and naming systems similar to 159 the DNS. 161 Domain name: An ordered list of zero or more labels. 163 Note that this is a definition independent of the DNS RFCs, and 164 the definition here also applies to systems other than the DNS. 165 [RFC1034] defines the "domain name space" using mathematical trees 166 and their nodes in graph theory, and the definition in [RFC1034] 167 has the same practical result as the definition here. Using graph 168 theory, a domain name is a list of labels identifying a portion 169 along one edge of an acyclic directed graph. A domain name can be 170 relative to other parts of the tree, or it can be fully qualified 171 (in which case, it ends at the common root of the graph). 173 Also note that different IETF and non-IETF documents have used the 174 term "domain name" in many different ways. It is common for 175 earlier documents to use "domain name" to mean "names that match 176 the syntax in [RFC1035]", but possibly with additional rules such 177 as "and are, or will be, resolvable in the global DNS" or "but 178 only using the presentation format". 180 Label: An ordered list of zero or more octets and which makes up a 181 portion of a domain name. Using graph theory, a label identifies 182 one node in a portion of the graph of all possible domain names. 184 Global DNS: Using the short set of facets listed in "Naming system", 185 the global DNS can be defined as follows. Most of the rules here 186 come from [RFC1034] and [RFC1035], although the term "global DNS" 187 has not been defined before now. 189 Composition of names -- A name in the global DNS has one or more 190 labels. The length of each label is between 0 and 63 octets 191 inclusive. In a fully-qualified domain name, the first label is 0 192 octets long; it is the only label whose length may be 0 octets, 193 and it is called the "root" or "root label". A domain name in the 194 global DNS has a maximum total length of 255 octets in the wire 195 format; the root represents one octet for this calculation. 197 Format of names -- Names in the global DNS are domain names. 198 There are three formats: wire format, presentation format, and 199 common display. 201 The basic wire format for names in the global DNS is a list of 202 labels with the root label last. Each label is preceded by a 203 length octet. [RFC1035] also defines a compression scheme that 204 modifies this format. 206 The presentation format for names in the global DNS is a list of 207 labels, encoded as ASCII, with the root label last, and a "." 208 character between each label. In presentation format, a fully- 209 qualified domain name includes the root label and the associated 210 separator dot. In presentation format, a fully-qualified domain 211 name with two additional labels is always shown as "example.tld." 212 instead of "example.tld". [RFC1035] defines a method for showing 213 octets that do not display in ASCII. 215 The common display format is used in applications and free text. 216 It is the same as the presentation format, but showing the root 217 label and the "." before it is optional and is rarely done. In 218 common display format, a fully-qualified domain name with two 219 additional labels is usually shown as "example.tld" instead of 220 "example.tld.". Names in the common display format are normally 221 written such that the first label in the ordered list is in the 222 last position from the point of view of the directionality of the 223 writing system (so, in both English and C the first label is the 224 right-most label; but in Arabic it may be the left-most label, 225 depending on local conventions). 227 Administration of names -- Administration is specified by 228 delegation (see the definition of to "delegation" in Section 6). 229 Policies for administration of the root zone in the global DNS are 230 determined by the names operational community, which convenes 231 itself in the Internet Corporation for Assigned Names and Numbers 232 (ICANN). The names operational community selects the IANA 233 Functions Operator for the global DNS root zone. At the time this 234 document is published, that operator is Public Technical 235 Identifiers (PTI). The name servers that serve the root zone are 236 provided by independent root operators. Other zones in the global 237 DNS have their own policies for administration. 239 Types of data that can be associated with names -- A name can have 240 zero or more resource records associated with it. There are 241 numerous types of resource records with unique data structures 242 defined in many different RFCs and in the IANA registry at 243 [IANA_Resource_Registry]. 245 Types of metadata for names -- Any name that is published in the 246 DNS appears as a set of resource records (see the definition of 247 "RRset" in Section 4). Some names do not themselves have data 248 associated with them in the DNS, but "appear" in the DNS anyway 249 because they form part of a longer name that does have data 250 associated with it (see the defintion of "empty non-terminals" in 251 Section 6). 253 Protocol for getting data from a name -- The protocol described in 254 [RFC1035]. 256 Context for resolving a name -- The global DNS root zone 257 distributed by PTI. 259 Private DNS: Names that use the protocol described in [RFC1035] but 260 that do not rely on the global DNS root zone, or names that are 261 otherwise not generally available on the Internet but are using 262 the protocol described in [RFC1035]. A system can use both the 263 global DNS and one or more private DNS systems; for example, see 264 "Split DNS" in Section 7. 266 Note that domain names that do not appear in the DNS, and that are 267 intended never to be looked up using the DNS protocol, are not 268 part of the global DNS or a private DNS even though they are 269 domain names. 271 Locally served DNS zone: A locally served DNS zone is a special case 272 of private DNS. Names are resolved using the DNS protocol in a 273 local context. [RFC6303] defines subdomains of IN-ADDR.ARPA tha 274 are locally served zones. Resolution of names through locally 275 served zones may result in ambiguous results. For example, the 276 same name may resolve to different results in different locally 277 served DNS zone contexts. The context through which a locally 278 served zone may be explicit, for example, as defined in [RFC6303], 279 or implicit, as defined by local DNS administration and not known 280 to the resolution client. 282 Fully qualified domain name (FQDN): This is often just a clear way 283 of saying the same thing as "domain name of a node", as outlined 284 above. However, the term is ambiguous. Strictly speaking, a 285 fully qualified domain name would include every label, including 286 the final, zero-length label of the root: such a name would be 287 written "www.example.net." (note the terminating dot). But 288 because every name eventually shares the common root, names are 289 often written relative to the root (such as "www.example.net") and 290 are still called "fully qualified". This term first appeared in 291 [RFC0819]. In this document, names are often written relative to 292 the root. 294 The need for the term "fully qualified domain name" comes from the 295 existence of partially qualified domain names, which are names 296 where some of the right-most names are left off and are understood 297 only by context. 299 Host name: This term and its equivalent, "hostname", have been 300 widely used but are not defined in [RFC1034], [RFC1035], 301 [RFC1123], or [RFC2181]. The DNS was originally deployed into the 302 Host Tables environment as outlined in [RFC0952], and it is likely 303 that the term followed informally from the definition there. Over 304 time, the definition seems to have shifted. "Host name" is often 305 meant to be a domain name that follows the rules in Section 3.5 of 306 [RFC1034], the "preferred name syntax". Note that any label in a 307 domain name can contain any octet value; hostnames are generally 308 considered to be domain names where every label follows the rules 309 in the "preferred name syntax", with the amendment that labels can 310 start with ASCII digits (this amendment comes from Section 2.1 of 311 [RFC1123]). 313 People also sometimes use the term hostname to refer to just the 314 first label of an FQDN, such as "printer" in 315 "printer.admin.example.com". (Sometimes this is formalized in 316 configuration in operating systems.) In addition, people 317 sometimes use this term to describe any name that refers to a 318 machine, and those might include labels that do not conform to the 319 "preferred name syntax". 321 TLD: A Top-Level Domain, meaning a zone that is one layer below the 322 root, such as "com" or "jp". There is nothing special, from the 323 point of view of the DNS, about TLDs. Most of them are also 324 delegation-centric zones, and there are significant policy issues 325 around their operation. TLDs are often divided into sub-groups 326 such as Country Code Top-Level Domains (ccTLDs), Generic Top-Level 327 Domains (gTLDs), and others; the division is a matter of policy, 328 and beyond the scope of this document. 330 IDN: The common abbreviation for "Internationalized Domain Name". 331 The IDNA protocol is the standard mechanism for handling domain 332 names with non-ASCII characters in applications in the DNS. The 333 current standard, normally called "IDNA2008", is defined in 334 [RFC5890], [RFC5891], [RFC5892], [RFC5893], and [RFC5894]. These 335 documents define many IDN-specific terms such as "LDH label", 336 "A-label", and "U-label". [RFC6365] defines more terms that 337 relate to internationalization (some of which relate to IDNs), and 338 [RFC6055] has a much more extensive discussion of IDNs, including 339 some new terminology. 341 Subdomain: "A domain is a subdomain of another domain if it is 342 contained within that domain. This relationship can be tested by 343 seeing if the subdomain's name ends with the containing domain's 344 name." (Quoted from [RFC1034], Section 3.1). For example, in the 345 host name "nnn.mmm.example.com", both "mmm.example.com" and 346 "nnn.mmm.example.com" are subdomains of "example.com". 348 Alias: The owner of a CNAME resource record, or a subdomain of the 349 owner of a DNAME resource record [RFC6672]. See also "canonical 350 name". 352 Canonical name: A CNAME resource record "identifies its owner name 353 as an alias, and specifies the corresponding canonical name in the 354 RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2) 355 This usage of the word "canonical" is related to the mathematical 356 concept of "canonical form". 358 CNAME: "It is traditional to refer to the owner of a CNAME record as 359 'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of 360 'canonical name', and the owner of a CNAME record is an alias, not 361 a canonical name." (Quoted from [RFC2181], Section 10.1.1) 363 Public suffix: "A domain that is controlled by a public registry." 364 (Quoted from [RFC6265], Section 5.3) A common definition for this 365 term is a domain under which subdomains can be registered, and on 366 which HTTP cookies ([RFC6265]) should not be set. There is no 367 indication in a domain name whether it is a public suffix; that 368 can only be determined by outside means. In fact, both a domain 369 and a subdomain of that domain can be public suffixes. At the 370 time this document is published, the IETF DBOUND Working Group 371 [DBOUND] is dealing with issues concerning public suffixes. 373 There is nothing inherent in a domain name to indicate whether it 374 is a public suffix. One resource for identifying public suffixes 375 is the Public Suffix List (PSL) maintained by Mozilla 376 (http://publicsuffix.org/). 378 For example, at the time this document is published, the "com.au" 379 domain is listed as a public suffix in the PSL. (Note that this 380 example might change in the future.) 381 Note that the term "public suffix" is controversial in the DNS 382 community for many reasons, and may be significantly changed in 383 the future. One example of the difficulty of calling a domain a 384 public suffix is that designation can change over time as the 385 registration policy for the zone changes, such as the case of the 386 "uk" TLD around the time this document is published. 388 3. DNS Header and Response Codes 390 The header of a DNS message is its first 12 octets. Many of the 391 fields and flags in the header diagram in Sections 4.1.1 through 392 4.1.3 of [RFC1035] are referred to by their names in that diagram. 393 For example, the response codes are called "RCODEs", the data for a 394 record is called the "RDATA", and the authoritative answer bit is 395 often called "the AA flag" or "the AA bit". 397 QNAME The most commonly-used definitions are that the QNAME is a 398 field in the Question section of a query. "A standard query 399 specifies a target domain name (QNAME), query type (QTYPE), and 400 query class (QCLASS) and asks for RRs which match." (Quoted from 401 [RFC1034], Section 3.7.1.) 403 [RFC2308], however, has an alternate definition that puts the 404 QNAME in the answer (or series of answers) instead of the query. 405 It defines QNAME as: "...the name in the query section of an 406 answer, or where this resolves to a CNAME, or CNAME chain, the 407 data field of the last CNAME. The last CNAME in this sense is 408 that which contains a value which does not resolve to another 409 CNAME." 411 Some of response codes that are defined in [RFC1035] have acquired 412 their own shorthand names. Some common response code names that 413 appear without reference to the numeric value are "FORMERR", 414 "SERVFAIL", and "NXDOMAIN" (the latter of which is also referred to 415 as "Name Error"). All of the RCODEs are listed at 416 http://www.iana.org/assignments/dns-parameters, although that site 417 uses mixed-case capitalization, while most documents use all-caps. 419 NODATA: "A pseudo RCODE which indicates that the name is valid for 420 the given class, but there are no records of the given type. A 421 NODATA response has to be inferred from the answer." (Quoted from 422 [RFC2308], Section 1.) "NODATA is indicated by an answer with the 423 RCODE set to NOERROR and no relevant answers in the answer 424 section. The authority section will contain an SOA record, or 425 there will be no NS records there." (Quoted from [RFC2308], 426 Section 2.2.) Note that referrals have a similar format to NODATA 427 replies; [RFC2308] explains how to distinguish them. 429 The term "NXRRSET" is sometimes used as a synonym for NODATA. 430 However, this is a mistake, given that NXRRSET is a specific error 431 code defined in [RFC2136]. 433 Negative response: A response that indicates that a particular RRset 434 does not exist, or whose RCODE indicates the nameserver cannot 435 answer. Sections 2 and 7 of [RFC2308] describe the types of 436 negative responses in detail. 438 Referrals: Data from the authority section of a non-authoritative 439 answer. [RFC1035] Section 2.1 defines "authoritative" data. 440 However, referrals at zone cuts (defined in Section 6) are not 441 authoritative. Referrals may be zone cut NS resource records and 442 their glue records. NS records on the parent side of a zone cut 443 are an authoritative delegation, but are normally not treated as 444 authoritative data. In general, a referral is a way for a server 445 to send an answer saying that the server does not know the answer, 446 but knows where the query should be directed in order to get an 447 answer. Historically, many authoritative servers answered with a 448 referral to the root zone when queried for a name for which they 449 were not authoritative, but this practice has declined. 451 4. Resource Records 453 RR: An acronym for resource record. ([RFC1034], Section 3.6.) 455 RRset: A set of resource records with the same label, class and 456 type, but with different data. (Definition from [RFC2181]) Also 457 spelled RRSet in some documents. As a clarification, "same label" 458 in this definition means "same owner name". In addition, 459 [RFC2181] states that "the TTLs of all RRs in an RRSet must be the 460 same". (This definition is definitely not the same as "the 461 response one gets to a query for QTYPE=ANY", which is an 462 unfortunate misunderstanding.) 464 Master file: "Master files are text files that contain RRs in text 465 form. Since the contents of a zone can be expressed in the form 466 of a list of RRs a master file is most often used to define a 467 zone, though it can be used to list a cache's contents." 468 ([RFC1035], Section 5.) 470 Presentation format: The text format used in master files. This 471 format is shown but not formally defined in [RFC1034] and 472 [RFC1035]. The term "presentation format" first appears in 473 [RFC4034]. 475 EDNS: The extension mechanisms for DNS, defined in [RFC6891]. 476 Sometimes called "EDNS0" or "EDNS(0)" to indicate the version 477 number. EDNS allows DNS clients and servers to specify message 478 sizes larger than the original 512 octet limit, to expand the 479 response code space, and potentially to carry additional options 480 that affect the handling of a DNS query. 482 OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to 483 contain control information pertaining to the question-and-answer 484 sequence of a specific transaction. (Definition from [RFC6891], 485 Section 6.1.1) It is used by EDNS. 487 Owner: The domain name where a RR is found ([RFC1034], Section 3.6). 488 Often appears in the term "owner name". 490 SOA field names: DNS documents, including the definitions here, 491 often refer to the fields in the RDATA of an SOA resource record 492 by field name. Those fields are defined in Section 3.3.13 of 493 [RFC1035]. The names (in the order they appear in the SOA RDATA) 494 are MNAME, RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. 495 Note that the meaning of MINIMUM field is updated in Section 4 of 496 [RFC2308]; the new definition is that the MINIMUM field is only 497 "the TTL to be used for negative responses". This document tends 498 to use field names instead of terms that describe the fields. 500 TTL: The maximum "time to live" of a resource record. "A TTL value 501 is an unsigned number, with a minimum value of 0, and a maximum 502 value of 2147483647. That is, a maximum of 2^31 - 1. When 503 transmitted, the TTL is encoded in the less significant 31 bits of 504 the 32 bit TTL field, with the most significant, or sign, bit set 505 to zero." (Quoted from [RFC2181], Section 8) (Note that [RFC1035] 506 erroneously stated that this is a signed integer; that was fixed 507 by [RFC2181].) 509 The TTL "specifies the time interval that the resource record may 510 be cached before the source of the information should again be 511 consulted". (Quoted from [RFC1035], Section 3.2.1) Also: "the 512 time interval (in seconds) that the resource record may be cached 513 before it should be discarded". (Quoted from [RFC1035], 514 Section 4.1.3). Despite being defined for a resource record, the 515 TTL of every resource record in an RRset is required to be the 516 same ([RFC2181], Section 5.2). 518 The reason that the TTL is the maximum time to live is that a 519 cache operator might decide to shorten the time to live for 520 operational purposes, such as if there is a policy to disallow TTL 521 values over a certain number. Also, if a value is flushed from 522 the cache when its value is still positive, the value effectively 523 becomes zero. Some servers are known to ignore the TTL on some 524 RRsets (such as when the authoritative data has a very short TTL) 525 even though this is against the advice in RFC 1035. 527 There is also the concept of a "default TTL" for a zone, which can 528 be a configuration parameter in the server software. This is 529 often expressed by a default for the entire server, and a default 530 for a zone using the $TTL directive in a zone file. The $TTL 531 directive was added to the master file format by [RFC2308]. 533 Class independent: A resource record type whose syntax and semantics 534 are the same for every DNS class. A resource record type that is 535 not class independent has different meanings depending on the DNS 536 class of the record, or the meaning is undefined for classes other 537 than IN (class 1, the Internet). 539 5. DNS Servers and Clients 541 This section defines the terms used for the systems that act as DNS 542 clients, DNS servers, or both. 544 Resolver: A program "that extract[s] information from name servers 545 in response to client requests." (Quoted from [RFC1034], 546 Section 2.4) "The resolver is located on the same machine as the 547 program that requests the resolver's services, but it may need to 548 consult name servers on other hosts." (Quoted from [RFC1034], 549 Section 5.1) A resolver performs queries for a name, type, and 550 class, and receives answers. The logical function is called 551 "resolution". In practice, the term is usually referring to some 552 specific type of resolver (some of which are defined below), and 553 understanding the use of the term depends on understanding the 554 context. 556 Stub resolver: A resolver that cannot perform all resolution itself. 557 Stub resolvers generally depend on a recursive resolver to 558 undertake the actual resolution function. Stub resolvers are 559 discussed but never fully defined in Section 5.3.1 of [RFC1034]. 560 They are fully defined in Section 6.1.3.1 of [RFC1123]. 562 Iterative mode: A resolution mode of a server that receives DNS 563 queries and responds with a referral to another server. 564 Section 2.3 of [RFC1034] describes this as "The server refers the 565 client to another server and lets the client pursue the query". A 566 resolver that works in iterative mode is sometimes called an 567 "iterative resolver". 569 Recursive mode: A resolution mode of a server that receives DNS 570 queries and either responds to those queries from a local cache or 571 sends queries to other servers in order to get the final answers 572 to the original queries. Section 2.3 of [RFC1034] describes this 573 as "The first server pursues the query for the client at another 574 server". A server operating in recursive mode may be thought of 575 as having a name server side (which is what answers the query) and 576 a resolver side (which performs the resolution function). Systems 577 operating in this mode are commonly called "recursive servers". 578 Sometimes they are called "recursive resolvers". While strictly 579 the difference between these is that one of them sends queries to 580 another recursive server and the other does not, in practice it is 581 not possible to know in advance whether the server that one is 582 querying will also perform recursion; both terms can be observed 583 in use interchangeably. 585 Full resolver: This term is used in [RFC1035], but it is not defined 586 there. RFC 1123 defines a "full-service resolver" that may or may 587 not be what was intended by "full resolver" in [RFC1035]. This 588 term is not properly defined in any RFC. 590 Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this 591 term to mean a resolver that acts in recursive mode with a cache 592 (and meets other requirements). 594 Recursive resolver: A resolver that acts in recursive mode. In 595 general, a recursive resolver is expected to cache the answers it 596 receives (which would make it a full-service resolver), but some 597 recursive resolvers might not cache. 599 Priming: The mechanism used by a resolver to determine where to send 600 queries before there is anything in the resolver's cache. Priming 601 is most often done from a configuration setting that contains a 602 list of authoritative servers for the root zone. 604 Root hints: "Operators who manage a DNS recursive resolver typically 605 need to configure a 'root hints file'. This file contains the 606 names and IP addresses of the authoritative name servers for the 607 root zone, so the software can bootstrap the DNS resolution 608 process. For many pieces of software, this list comes built into 609 the software." (Quoted from [IANA_RootFiles]) 611 Negative caching: "The storage of knowledge that something does not 612 exist, cannot give an answer, or does not give an answer." 613 (Quoted from [RFC2308], Section 1) 615 Authoritative server: "A server that knows the content of a DNS zone 616 from local knowledge, and thus can answer queries about that zone 617 without needing to query other servers." (Quoted from [RFC2182], 618 Section 2.) It is a system that responds to DNS queries with 619 information about zones for which it has been configured to answer 620 with the AA flag in the response header set to 1. It is a server 621 that has authority over one or more DNS zones. Note that it is 622 possible for an authoritative server to respond to a query without 623 the parent zone delegating authority to that server. 624 Authoritative servers also provide "referrals", usually to child 625 zones delegated from them; these referrals have the AA bit set to 626 0 and come with referral data in the Authority and (if needed) the 627 Additional sections. 629 Authoritative-only server: A name server that only serves 630 authoritative data and ignores requests for recursion. It will 631 "not normally generate any queries of its own. Instead, it 632 answers non-recursive queries from iterative resolvers looking for 633 information in zones it serves." (Quoted from [RFC4697], 634 Section 2.4) 636 Zone transfer: The act of a client requesting a copy of a zone and 637 an authoritative server sending the needed information. (See 638 Section 6 for a description of zones.) There are two common 639 standard ways to do zone transfers: the AXFR ("Authoritative 640 Transfer") mechanism to copy the full zone (described in 641 [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy 642 only parts of the zone that have changed (described in [RFC1995]). 643 Many systems use non-standard methods for zone transfer outside 644 the DNS protocol. 646 Secondary server: "An authoritative server which uses zone transfer 647 to retrieve the zone" (Quoted from [RFC1996], Section 2.1). 648 [RFC2182] describes secondary servers in detail. Although early 649 DNS RFCs such as [RFC1996] referred to this as a "slave", the 650 current common usage has shifted to calling it a "secondary". 651 Secondary servers are also discussed in [RFC1034]. 653 Slave server: See secondary server. 655 Primary server: "Any authoritative server configured to be the 656 source of zone transfer for one or more [secondary] servers" 657 (Quoted from [RFC1996], Section 2.1) or, more specifically, "an 658 authoritative server configured to be the source of AXFR or IXFR 659 data for one or more [secondary] servers" (Quoted from [RFC2136]). 660 Although early DNS RFCs such as [RFC1996] referred to this as a 661 "master", the current common usage has shifted to "primary". 662 Primary servers are also discussed in [RFC1034]. 664 Master server: See primary server. 666 Primary master: "The primary master is named in the zone's SOA MNAME 667 field and optionally by an NS RR". (Quoted from [RFC1996], 668 Section 2.1). [RFC2136] defines "primary master" as "Master 669 server at the root of the AXFR/IXFR dependency graph. The primary 670 master is named in the zone's SOA MNAME field and optionally by an 671 NS RR. There is by definition only one primary master server per 672 zone." The idea of a primary master is only used by [RFC2136], 673 and is considered archaic in other parts of the DNS. 675 Stealth server: This is "like a slave server except not listed in an 676 NS RR for the zone." (Quoted from [RFC1996], Section 2.1) 678 Hidden master: A stealth server that is a master for zone transfers. 679 "In this arrangement, the master name server that processes the 680 updates is unavailable to general hosts on the Internet; it is not 681 listed in the NS RRset." (Quoted from [RFC6781], Section 3.4.3.) 682 An earlier RFC, [RFC4641], said that the hidden master's name 683 appears in the SOA RRs MNAME field, although in some setups, the 684 name does not appear at all in the public DNS. A hidden master 685 can be either a secondary or a primary master. 687 Forwarding: The process of one server sending a DNS query with the 688 RD bit set to 1 to another server to resolve that query. 689 Forwarding is a function of a DNS resolver; it is different than 690 simply blindly relaying queries. 692 [RFC5625] does not give a specific definition for forwarding, but 693 describes in detail what features a system that forwards need to 694 support. Systems that forward are sometimes called "DNS proxies", 695 but that term has not yet been defined (even in [RFC5625]). 697 Forwarder: Section 1 of [RFC2308] describes a forwarder as "a 698 nameserver used to resolve queries instead of directly using the 699 authoritative nameserver chain". [RFC2308] further says "The 700 forwarder typically either has better access to the internet, or 701 maintains a bigger cache which may be shared amongst many 702 resolvers." That definition appears to suggest that forwarders 703 normally only query authoritative servers. In current use, 704 however, forwarders often stand between stub resolvers and 705 recursive servers. [RFC2308] is silent on whether a forwarder is 706 iterative-only or can be a full-service resolver. 708 Policy-implementing resolver: A resolver acting in recursive mode 709 that changes some of the answers that it returns based on policy 710 criteria, such as to prevent access to malware sites or 711 objectionable content. In general, a stub resolver has no idea 712 whether upstream resolvers implement such policy or, if they do, 713 the exact policy about what changes will be made. In some cases, 714 the user of the stub resolver has selected the policy-implementing 715 resolver with the explicit intention of using it to implement the 716 policies. In other cases, policies are imposed without the user 717 of the stub resolver being informed. 719 Open resolver: A full-service resolver that accepts and processes 720 queries from any (or nearly any) stub resolver. This is sometimes 721 also called a "public resolver", although the term "public 722 resolver" is used more with open resolvers that are meant to be 723 open, as compared to the vast majority of open resolvers that are 724 probably misconfigured to be open. 726 View: A configuration for a DNS server that allows it to provide 727 different answers depending on attributes of the query. 728 Typically, views differ by the source IP address of a query, but 729 can also be based on the destination IP address, the type of query 730 (such as AXFR), whether it is recursive, and so on. Views are 731 often used to provide more names or different addresses to queries 732 from "inside" a protected network than to those "outside" that 733 network. Views are not a standardized part of the DNS, but they 734 are widely implemented in server software. 736 Passive DNS: A mechanism to collect large amounts of DNS data by 737 storing DNS responses from servers. Some of these systems also 738 collect the DNS queries associated with the responses; this can 739 raise privacy issues. Passive DNS databases can be used to answer 740 historical questions about DNS zones such as which records were 741 available for them at what times in the past. Passive DNS 742 databases allow searching of the stored records on keys other than 743 just the name, such as "find all names which have A records of a 744 particular value". 746 Anycast: "The practice of making a particular service address 747 available in multiple, discrete, autonomous locations, such that 748 datagrams sent are routed to one of several available locations." 749 (Quoted from [RFC4786], Section 2) 751 Split DNS: "Where a corporate network serves up partly or completely 752 different DNS inside and outside its firewall. There are many 753 possible variants on this; the basic point is that the 754 correspondence between a given FQDN (fully qualified domain name) 755 and a given IPv4 address is no longer universal and stable over 756 long periods." (Quoted from [RFC2775], Section 3.8) 758 6. Zones 760 This section defines terms that are used when discussing zones that 761 are being served or retrieved. 763 Zone: "Authoritative information is organized into units called 764 'zones', and these zones can be automatically distributed to the 765 name servers which provide redundant service for the data in a 766 zone." (Quoted from [RFC1034], Section 2.4) 768 Child: "The entity on record that has the delegation of the domain 769 from the Parent." (Quoted from [RFC7344], Section 1.1) 771 Parent: "The domain in which the Child is registered." (Quoted from 772 [RFC7344], Section 1.1) Earlier, "parent name server" was defined 773 in [RFC0882] as "the name server that has authority over the place 774 in the domain name space that will hold the new domain". (Note 775 that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].) 776 [RFC0819] also has some description of the relationship between 777 parents and children. 779 Origin: 781 (a) "The domain name that appears at the top of a zone (just below 782 the cut that separates the zone from its parent). The name of the 783 zone is the same as the name of the domain at the zone's origin." 784 (Quoted from [RFC2181], Section 6.) These days, this sense of 785 "origin" and "apex" (defined below) are often used 786 interchangeably. 788 (b) The domain name within which a given relative domain name 789 appears in zone files. Generally seen in the context of 790 "$ORIGIN", which is a control entry defined in [RFC1035], 791 Section 5.1, as part of the master file format. For example, if 792 the $ORIGIN is set to "example.org.", then a master file line for 793 "www" is in fact an entry for "www.example.org.". 795 Apex: The point in the tree at an owner of an SOA and corresponding 796 authoritative NS RRset. This is also called the "zone apex". 797 [RFC4033] defines it as "the name at the child's side of a zone 798 cut". The "apex" can usefully be thought of as a data-theoretic 799 description of a tree structure, and "origin" is the name of the 800 same concept when it is implemented in zone files. The 801 distinction is not always maintained in use, however, and one can 802 find uses that conflict subtly with this definition. [RFC1034] 803 uses the term "top node of the zone" as a synonym of "apex", but 804 that term is not widely used. These days, the first sense of 805 "origin" (above) and "apex" are often used interchangeably. 807 Zone cut: The delimitation point between two zones where the origin 808 of one of the zones is the child of the other zone. 810 "Zones are delimited by 'zone cuts'. Each zone cut separates a 811 'child' zone (below the cut) from a 'parent' zone (above the cut). 812 (Quoted from [RFC2181], Section 6; note that this is barely an 813 ostensive definition.) Section 4.2 of [RFC1034] uses "cuts" as 814 'zone cut'." 816 Delegation: The process by which a separate zone is created in the 817 name space beneath the apex of a given domain. Delegation happens 818 when an NS RRset is added in the parent zone for the child origin. 819 Delegation inherently happens at a zone cut. The term is also 820 commonly a noun: the new zone that is created by the act of 821 delegating. 823 Glue records: "[Resource records] which are not part of the 824 authoritative data [of the zone], and are address resource records 825 for the [name servers in subzones]. These RRs are only necessary 826 if the name server's name is 'below' the cut, and are only used as 827 part of a referral response." Without glue "we could be faced 828 with the situation where the NS RRs tell us that in order to learn 829 a name server's address, we should contact the server using the 830 address we wish to learn." (Definition from [RFC1034], 831 Section 4.2.1) 833 A later definition is that glue "includes any record in a zone 834 file that is not properly part of that zone, including nameserver 835 records of delegated sub-zones (NS records), address records that 836 accompany those NS records (A, AAAA, etc), and any other stray 837 data that might appear" ([RFC2181], Section 5.4.1). Although glue 838 is sometimes used today with this wider definition in mind, the 839 context surrounding the [RFC2181] definition suggests it is 840 intended to apply to the use of glue within the document itself 841 and not necessarily beyond. 843 In-bailiwick: 845 (a) An adjective to describe a name server whose name is either 846 subordinate to or (rarely) the same as the zone origin. In- 847 bailiwick name servers require glue records in their parent zone 848 (using the first of the definitions of "glue records" in the 849 definition above). 851 (b) Data for which the server is either authoritative, or else 852 authoritative for an ancestor of the owner name. This sense of 853 the term normally is used when discussing the relevancy of glue 854 records in a response. For example, the server for the parent 855 zone "example.com" might reply with glue records for 856 "ns.child.example.com". Because the "child.example.com" zone is a 857 descendant of the "example.com" zone, the glue records are in- 858 bailiwick. 860 Out-of-bailiwick: The antonym of in-bailiwick. 862 Authoritative data: "All of the RRs attached to all of the nodes 863 from the top node of the zone down to leaf nodes or nodes above 864 cuts around the bottom edge of the zone." (Quoted from [RFC1034], 865 Section 4.2.1) It is noted that this definition might 866 inadvertently also include any NS records that appear in the zone, 867 even those that might not truly be authoritative because there are 868 identical NS RRs below the zone cut. This reveals the ambiguity 869 in the notion of authoritative data, because the parent-side NS 870 records authoritatively indicate the delegation, even though they 871 are not themselves authoritative data. 873 Root zone: The zone whose apex is the zero-length label. Also 874 sometimes called "the DNS root". 876 Empty non-terminals (ENT): "Domain names that own no resource 877 records but have subdomains that do." (Quoted from [RFC4592], 878 Section 2.2.2.) A typical example is in SRV records: in the name 879 "_sip._tcp.example.com", it is likely that "_tcp.example.com" has 880 no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV 881 RRset. 883 Delegation-centric zone: A zone that consists mostly of delegations 884 to child zones. This term is used in contrast to a zone that 885 might have some delegations to child zones, but also has many data 886 resource records for the zone itself and/or for child zones. The 887 term is used in [RFC4956] and [RFC5155], but is not defined there. 889 Wildcard: [RFC1034] defined "wildcard", but in a way that turned out 890 to be confusing to implementers. Special treatment is given to 891 RRs with owner names starting with the label "*". "Such RRs are 892 called 'wildcards'. Wildcard RRs can be thought of as 893 instructions for synthesizing RRs." (Quoted from [RFC1034], 894 Section 4.3.3) For an extended discussion of wildcards, including 895 clearer definitions, see [RFC4592]. 897 Asterisk label: "The first octet is the normal label type and length 898 for a 1-octet-long label, and the second octet is the ASCII 899 representation for the '*' character. A descriptive name of a 900 label equaling that value is an 'asterisk label'." (Quoted from 901 [RFC4592], Section 2.1.1) 903 Wildcard domain name: "A 'wildcard domain name' is defined by having 904 its initial (i.e., leftmost or least significant) label be 905 asterisk label." (Quoted from [RFC4592], Section 2.1.1) 907 Closest encloser: "The longest existing ancestor of a name." 908 (Quoted from [RFC5155], Section 1.3) An earlier definition is "The 909 node in the zone's tree of existing domain names that has the most 910 labels matching the query name (consecutively, counting from the 911 root label downward). Each match is a 'label match' and the order 912 of the labels is the same." (Quoted from [RFC4592], 913 Section 3.3.1) 915 Closest provable encloser: "The longest ancestor of a name that can 916 be proven to exist. Note that this is only different from the 917 closest encloser in an Opt-Out zone." (Quoted from [RFC5155], 918 Section 1.3) 920 Next closer name: "The name one label longer than the closest 921 provable encloser of a name." (Quoted from [RFC5155], 922 Section 1.3) 924 Source of Synthesis: "The source of synthesis is defined in the 925 context of a query process as that wildcard domain name 926 immediately descending from the closest encloser, provided that 927 this wildcard domain name exists. 'Immediately descending' means 928 that the source of synthesis has a name of the form: .." (Quoted from [RFC4592], 930 Section 3.3.1) 932 Occluded name: "The addition of a delegation point via dynamic 933 update will render all subordinate domain names to be in a limbo, 934 still part of the zone, but not available to the lookup process. 935 The addition of a DNAME resource record has the same impact. The 936 subordinate names are said to be 'occluded'." (Quoted from 937 [RFC5936], Section 3.5) 939 Fast flux DNS: This "occurs when a domain is found in DNS using A 940 records to multiple IP addresses, each of which has a very short 941 Time-to-Live (TTL) value associated with it. This means that the 942 domain resolves to varying IP addresses over a short period of 943 time." (Quoted from [RFC6561], Section 1.1.5, with typo 944 corrected) It is often used to deliver malware. Because the 945 addresses change so rapidly, it is difficult to ascertain all the 946 hosts. It should be noted that the technique also works with AAAA 947 records, but such use is not frequently observed on the Internet 948 as of this writing. 950 Reverse DNS, reverse lookup: "The process of mapping an address to a 951 name is generally known as a 'reverse lookup', and the IN- 952 ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse 953 DNS'." (Quoted from [RFC5855], Section 1) 955 Forward lookup: "Hostname-to-address translation". (Quoted from 956 [RFC2133], Section 6) 958 arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain 959 was originally established as part of the initial deployment of 960 the DNS, to provide a transition mechanism from the Host Tables 961 that were common in the ARPANET, as well as a home for the IPv4 962 reverse mapping domain. During 2000, the abbreviation was 963 redesignated to 'Address and Routing Parameter Area' in the hope 964 of reducing confusion with the earlier network name." (Quoted 965 from [RFC3172], Section 2.) 967 Infrastructure domain: A domain whose "role is to support the 968 operating infrastructure of the Internet". (Quoted from 969 [RFC3172], Section 2.) 971 Service name: "Service names are the unique key in the Service Name 972 and Transport Protocol Port Number registry. This unique symbolic 973 name for a service may also be used for other purposes, such as in 974 DNS SRV records." (Quoted from [RFC6335], Section 5.) 976 7. Registration Model 978 Registry: The administrative operation of a zone that allows 979 registration of names within that zone. People often use this 980 term to refer only to those organizations that perform 981 registration in large delegation-centric zones (such as TLDs); but 982 formally, whoever decides what data goes into a zone is the 983 registry for that zone. This definition of "registry" is from a 984 DNS point of view; for some zones, the policies that determine 985 what can go in the zone are decided by superior zones and not the 986 registry operator. 988 Registrant: An individual or organization on whose behalf a name in 989 a zone is registered by the registry. In many zones, the registry 990 and the registrant may be the same entity, but in TLDs they often 991 are not. 993 Registrar: A service provider that acts as a go-between for 994 registrants and registries. Not all registrations require a 995 registrar, though it is common to have registrars involved in 996 registrations in TLDs. 998 EPP: The Extensible Provisioning Protocol (EPP), which is commonly 999 used for communication of registration information between 1000 registries and registrars. EPP is defined in [RFC5730]. 1002 WHOIS: A protocol specified in [RFC3912], often used for querying 1003 registry databases. WHOIS data is frequently used to associate 1004 registration data (such as zone management contacts) with domain 1005 names. The term "WHOIS data" is often used as a synonym for the 1006 registry database, even though that database may be served by 1007 different protocols, particularly RDAP. The WHOIS protocol is 1008 also used with IP address registry data. 1010 RDAP: The Registration Data Access Protocol, defined in [RFC7480], 1011 [RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The 1012 RDAP protocol and data format are meant as a replacement for 1013 WHOIS. 1015 DNS operator: An entity responsible for running DNS servers. For a 1016 zone's authoritative servers, the registrant may act as their own 1017 DNS operator, or their registrar may do it on their behalf, or 1018 they may use a third-party operator. For some zones, the registry 1019 function is performed by the DNS operator plus other entities who 1020 decide about the allowed contents of the zone. 1022 8. General DNSSEC 1024 Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and 1025 [RFC5155]. The terms that have caused confusion in the DNS community 1026 are highlighted here. 1028 DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in 1029 some RFCs, have not been formally defined. However, Section 2 of 1030 [RFC4033] defines many types of resolvers and validators, 1031 including "non-validating security-aware stub resolver", "non- 1032 validating stub resolver", "security-aware name server", 1033 "security-aware recursive name server", "security-aware resolver", 1034 "security-aware stub resolver", and "security-oblivious 1035 'anything'". (Note that the term "validating resolver", which is 1036 used in some places in DNSSEC-related documents, is also not 1037 defined.) 1039 Signed zone: "A zone whose RRsets are signed and that contains 1040 properly constructed DNSKEY, Resource Record Signature (RRSIG), 1041 Next Secure (NSEC), and (optionally) DS records." (Quoted from 1042 [RFC4033], Section 2.) It has been noted in other contexts that 1043 the zone itself is not really signed, but all the relevant RRsets 1044 in the zone are signed. Nevertheless, if a zone that should be 1045 signed contains any RRsets that are not signed (or opted out), 1046 those RRsets will be treated as bogus, so the whole zone needs to 1047 be handled in some way. 1049 It should also be noted that, since the publication of [RFC6840], 1050 NSEC records are no longer required for signed zones: a signed 1051 zone might include NSEC3 records instead. [RFC7129] provides 1052 additional background commentary and some context for the NSEC and 1053 NSEC3 mechanisms used by DNSSEC to provide authenticated denial- 1054 of-existence responses. NSEC and NSEC3 are described below. 1056 Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that 1057 is not signed". Section 2 of [RFC4035] defines this as "A zone 1058 that does not include these records [properly constructed DNSKEY, 1059 Resource Record Signature (RRSIG), Next Secure (NSEC), and 1060 (optionally) DS records] according to the rules in this section". 1061 There is an important note at the end of Section 5.2 of [RFC4035] 1062 that defines an additional situation in which a zone is considered 1063 unsigned: "If the resolver does not support any of the algorithms 1064 listed in an authenticated DS RRset, then the resolver will not be 1065 able to verify the authentication path to the child zone. In this 1066 case, the resolver SHOULD treat the child zone as if it were 1067 unsigned." 1069 NSEC: "The NSEC record allows a security-aware resolver to 1070 authenticate a negative reply for either name or type non- 1071 existence with the same mechanisms used to authenticate other DNS 1072 replies." (Quoted from [RFC4033], Section 3.2.) In short, an 1073 NSEC record provides authenticated denial of existence. 1075 "The NSEC resource record lists two separate things: the next 1076 owner name (in the canonical ordering of the zone) that contains 1077 authoritative data or a delegation point NS RRset, and the set of 1078 RR types present at the NSEC RR's owner name." (Quoted from 1079 Section 4 of RFC 4034) 1081 NSEC3: Like the NSEC record, the NSEC3 record also provides 1082 authenticated denial of existence; however, NSEC3 records mitigate 1083 against zone enumeration and support Opt-Out. NSEC resource 1084 records require associated NSEC3PARAM resource records. NSEC3 and 1085 NSEC3PARAM resource records are defined in [RFC5155]. 1087 Note that [RFC6840] says that [RFC5155] "is now considered part of 1088 the DNS Security Document Family as described by Section 10 of 1089 [RFC4033]". This means that some of the definitions from earlier 1090 RFCs that only talk about NSEC records should probably be 1091 considered to be talking about both NSEC and NSEC3. 1093 Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover 1094 unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1.) 1095 Opt-out tackles the high costs of securing a delegation to an 1096 insecure zone. When using Opt-Out, names that are an insecure 1097 delegation (and empty non-terminals that are only derived from 1098 insecure delegations) don't require an NSEC3 record or its 1099 corresponding RRSIG records. Opt-Out NSEC3 records are not able 1100 to prove or deny the existence of the insecure delegations. 1101 (Adapted from [RFC7129], Section 5.1) 1103 Zone enumeration: "The practice of discovering the full content of a 1104 zone via successive queries." (Quoted from [RFC5155], 1105 Section 1.3.) This is also sometimes called "zone walking". Zone 1106 enumeration is different from zone content guessing where the 1107 guesser uses a large dictionary of possible labels and sends 1108 successive queries for them, or matches the contents of NSEC3 1109 records against such a dictionary. 1111 Validation: Validation, in the context of DNSSEC, refers to the 1112 following: 1114 * Checking the validity of DNSSEC signatures 1116 * Checking the validity of DNS responses, such as those including 1117 authenticated denial of existence 1119 * Building an authentication chain from a trust anchor to a DNS 1120 response or individual DNS RRsets in a response 1122 The first two definitions above consider only the validity of 1123 individual DNSSEC components such as the RRSIG validity or NSEC 1124 proof validity. The third definition considers the components of 1125 the entire DNSSEC authentication chain, and thus requires 1126 "configured knowledge of at least one authenticated DNSKEY or DS 1127 RR" (as described in [RFC4035], Section 5). 1129 [RFC4033], Section 2, says that a "Validating Security-Aware Stub 1130 Resolver... performs signature validation" and uses a trust anchor 1131 "as a starting point for building the authentication chain to a 1132 signed DNS response", and thus uses the first and third 1133 definitions above. The process of validating an RRSIG RR is 1134 described in [RFC4035], Section 5.3. 1136 [RFC5155] refers to validating responses throughout the document, 1137 in the context of hashed authenticated denial of existence; this 1138 uses the second definition above. 1140 The term "authentication" is used interchangeably with 1141 "validation", in the sense of the third definition above. 1142 [RFC4033], Section 2, describes the chain linking trust anchor to 1143 DNS data as the "authentication chain". A response is considered 1144 to be authentic if "all RRsets in the Answer and Authority 1145 sections of the response [are considered] to be authentic" 1146 ([RFC4035]). DNS data or responses deemed to be authentic or 1147 validated have a security status of "secure" ([RFC4035], 1148 Section 4.3; [RFC4033], Section 5). "Authenticating both DNS keys 1149 and data is a matter of local policy, which may extend or even 1150 override the [DNSSEC] protocol extensions" ([RFC4033], 1151 Section 3.1). 1153 The term "verification", when used, is usually synonym for 1154 "validation". 1156 Key signing key (KSK): DNSSEC keys that "only sign the apex DNSKEY 1157 RRset in a zone."(Quoted from [RFC6781], Section 3.1) 1159 Zone signing key (ZSK): "DNSSEC keys that can be used to sign all 1160 the RRsets in a zone that require signatures, other than the apex 1161 DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Note that the 1162 roles KSK and ZSK are not mutually exclusive: a single key can be 1163 both KSK and ZSK at the same time. Also note that a ZSK is 1164 sometimes used to sign the apex DNSKEY RRset. 1166 Combined signing key (CSK): "In cases where the differentiation 1167 between the KSK and ZSK is not made, i.e., where keys have the 1168 role of both KSK and ZSK, we talk about a Single-Type Signing 1169 Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes 1170 called a "combined signing key" or CSK. It is operational 1171 practice, not protocol, that determines whether a particular key 1172 is a ZSK, a KSK, or a CSK. 1174 Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be 1175 used to distinguish between keys that are intended to be used as 1176 the secure entry point into the zone when building chains of 1177 trust, i.e., they are (to be) pointed to by parental DS RRs or 1178 configured as a trust anchor. Therefore, it is suggested that the 1179 SEP flag be set on keys that are used as KSKs and not on keys that 1180 are used as ZSKs, while in those cases where a distinction between 1181 a KSK and ZSK is not made (i.e., for a Single-Type Signing 1182 Scheme), it is suggested that the SEP flag be set on all keys." 1183 (Quoted from [RFC6781], Section 3.2.3.) Note that the SEP flag is 1184 only a hint, and its presence or absence may not be used to 1185 disqualify a given DNSKEY RR from use as a KSK or ZSK during 1186 validation. 1188 The original defintion of SEPs was in [RFC3757]. That definition 1189 clearly indicated that the SEP was a key, not just a bit in the 1190 key. The abstract of [RFC3757] says: "With the Delegation Signer 1191 (DS) resource record (RR), the concept of a public key acting as a 1192 secure entry point (SEP) has been introduced. During exchanges of 1193 public keys with the parent there is a need to differentiate SEP 1194 keys from other public keys in the Domain Name System KEY (DNSKEY) 1195 resource record set. A flag bit in the DNSKEY RR is defined to 1196 indicate that DNSKEY is to be used as a SEP." That definition of 1197 the SEP as a key was made obsolete by [RFC4034], and the 1198 definition from [RFC6781] is consistent with [RFC4034]. 1200 Trust anchor: "A configured DNSKEY RR or DS RR hash of a DNSKEY RR. 1201 A validating security-aware resolver uses this public key or hash 1202 as a starting point for building the authentication chain to a 1203 signed DNS response." (Quoted from [RFC4033], Section 2) 1205 DNSSEC Policy (DP): A statement that "sets forth the security 1206 requirements and standards to be implemented for a DNSSEC-signed 1207 zone." (Quoted from [RFC6841], Section 2) 1209 DNSSEC Practice Statement (DPS): "A practices disclosure document 1210 that may support and be a supplemental document to the DNSSEC 1211 Policy (if such exists), and it states how the management of a 1212 given zone implements procedures and controls at a high level." 1213 (Quoted from [RFC6841], Section 2) 1215 Hardware security module (HSM): A specialized piece of hardware that 1216 is used to create keys for signatures and to sign messages. In 1217 DNSSEC, HSMs are often used to hold the private keys for KSKs and 1218 ZSKs and to create the RRSIG records at periodic intervals. 1220 Signing software: Authoritative DNS servers that supports DNSSEC 1221 often contains software that facilitates the creation and 1222 maintenance of DNSSEC signatures in zones. There is also stand- 1223 alone software that can be used to sign a zone regardless of 1224 whether the authoritative server itself supports signing. 1225 Sometimes signing software can support particular HSMs as part of 1226 the signing process. 1228 9. DNSSEC States 1230 A validating resolver can determine that a response is in one of four 1231 states: secure, insecure, bogus, or indeterminate. These states are 1232 defined in [RFC4033] and [RFC4035], although the two definitions 1233 differ a bit. This document makes no effort to reconcile the two 1234 definitions, and takes no position as to whether they need to be 1235 reconciled. 1237 Section 5 of [RFC4033] says: 1239 A validating resolver can determine the following 4 states: 1241 Secure: The validating resolver has a trust anchor, has a chain 1242 of trust, and is able to verify all the signatures in the 1243 response. 1245 Insecure: The validating resolver has a trust anchor, a chain 1246 of trust, and, at some delegation point, signed proof of the 1247 non-existence of a DS record. This indicates that subsequent 1248 branches in the tree are provably insecure. A validating 1249 resolver may have a local policy to mark parts of the domain 1250 space as insecure. 1252 Bogus: The validating resolver has a trust anchor and a secure 1253 delegation indicating that subsidiary data is signed, but 1254 the response fails to validate for some reason: missing 1255 signatures, expired signatures, signatures with unsupported 1256 algorithms, data missing that the relevant NSEC RR says 1257 should be present, and so forth. 1259 Indeterminate: There is no trust anchor that would indicate that a 1260 specific portion of the tree is secure. This is the default 1261 operation mode. 1263 Section 4.3 of [RFC4035] says: 1265 A security-aware resolver must be able to distinguish between four 1266 cases: 1268 Secure: An RRset for which the resolver is able to build a chain 1269 of signed DNSKEY and DS RRs from a trusted security anchor to 1270 the RRset. In this case, the RRset should be signed and is 1271 subject to signature validation, as described above. 1273 Insecure: An RRset for which the resolver knows that it has no 1274 chain of signed DNSKEY and DS RRs from any trusted starting 1275 point to the RRset. This can occur when the target RRset lies 1276 in an unsigned zone or in a descendent [sic] of an unsigned 1277 zone. In this case, the RRset may or may not be signed, but 1278 the resolver will not be able to verify the signature. 1280 Bogus: An RRset for which the resolver believes that it ought to 1281 be able to establish a chain of trust but for which it is 1282 unable to do so, either due to signatures that for some reason 1283 fail to validate or due to missing data that the relevant 1284 DNSSEC RRs indicate should be present. This case may indicate 1285 an attack but may also indicate a configuration error or some 1286 form of data corruption. 1288 Indeterminate: An RRset for which the resolver is not able to 1289 determine whether the RRset should be signed, as the resolver 1290 is not able to obtain the necessary DNSSEC RRs. This can occur 1291 when the security-aware resolver is not able to contact 1292 security-aware name servers for the relevant zones. 1294 10. Security Considerations 1296 These definitions do not change any security considerations for the 1297 DNS. 1299 11. IANA Considerations 1301 None. 1303 12. References 1305 12.1. Normative References 1307 [IANA_RootFiles] 1308 Internet Assigned Numbers Authority, "IANA Root Files", 1309 2016, . 1311 [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities", 1312 RFC 882, DOI 10.17487/RFC0882, November 1983, 1313 . 1315 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1316 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1317 . 1319 [RFC1035] Mockapetris, P., "Domain names - implementation and 1320 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1321 November 1987, . 1323 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1324 Application and Support", STD 3, RFC 1123, 1325 DOI 10.17487/RFC1123, October 1989, 1326 . 1328 [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone 1329 Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996, 1330 August 1996, . 1332 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 1333 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1334 RFC 2136, DOI 10.17487/RFC2136, April 1997, 1335 . 1337 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1338 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1339 . 1341 [RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection 1342 and Operation of Secondary DNS Servers", BCP 16, RFC 2182, 1343 DOI 10.17487/RFC2182, July 1997, 1344 . 1346 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 1347 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 1348 . 1350 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1351 Rose, "DNS Security Introduction and Requirements", 1352 RFC 4033, DOI 10.17487/RFC4033, March 2005, 1353 . 1355 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1356 Rose, "Resource Records for the DNS Security Extensions", 1357 RFC 4034, DOI 10.17487/RFC4034, March 2005, 1358 . 1360 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1361 Rose, "Protocol Modifications for the DNS Security 1362 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 1363 . 1365 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name 1366 System", RFC 4592, DOI 10.17487/RFC4592, July 2006, 1367 . 1369 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 1370 Security (DNSSEC) Hashed Authenticated Denial of 1371 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008, 1372 . 1374 [RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)", 1375 STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009, 1376 . 1378 [RFC5855] Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6 1379 Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855, 1380 May 2010, . 1382 [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol 1383 (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010, 1384 . 1386 [RFC6561] Livingood, J., Mody, N., and M. O'Reirdan, 1387 "Recommendations for the Remediation of Bots in ISP 1388 Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012, 1389 . 1391 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 1392 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, 1393 . 1395 [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC 1396 Operational Practices, Version 2", RFC 6781, 1397 DOI 10.17487/RFC6781, December 2012, 1398 . 1400 [RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and 1401 Implementation Notes for DNS Security (DNSSEC)", RFC 6840, 1402 DOI 10.17487/RFC6840, February 2013, 1403 . 1405 [RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A 1406 Framework for DNSSEC Policies and DNSSEC Practice 1407 Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013, 1408 . 1410 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 1411 for DNS (EDNS(0))", STD 75, RFC 6891, 1412 DOI 10.17487/RFC6891, April 2013, 1413 . 1415 [RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating 1416 DNSSEC Delegation Trust Maintenance", RFC 7344, 1417 DOI 10.17487/RFC7344, September 2014, 1418 . 1420 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1421 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 1422 2015, . 1424 12.2. Informative References 1426 [DBOUND] IETF, "Domain Boundaries (dbound) Working Group", 2016, 1427 . 1429 [IANA_Resource_Registry] 1430 Internet Assigned Numbers Authority, "Resource Record (RR) 1431 TYPEs", 2017, 1432 . 1434 [RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for 1435 Internet User Applications", RFC 819, 1436 DOI 10.17487/RFC0819, August 1982, 1437 . 1439 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet 1440 host table specification", RFC 952, DOI 10.17487/RFC0952, 1441 October 1985, . 1443 [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, 1444 DOI 10.17487/RFC1995, August 1996, 1445 . 1447 [RFC2133] Gilligan, R., Thomson, S., Bound, J., and W. Stevens, 1448 "Basic Socket Interface Extensions for IPv6", RFC 2133, 1449 DOI 10.17487/RFC2133, April 1997, 1450 . 1452 [RFC2775] Carpenter, B., "Internet Transparency", RFC 2775, 1453 DOI 10.17487/RFC2775, February 2000, 1454 . 1456 [RFC3172] Huston, G., Ed., "Management Guidelines & Operational 1457 Requirements for the Address and Routing Parameter Area 1458 Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172, 1459 September 2001, . 1461 [RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name 1462 System KEY (DNSKEY) Resource Record (RR) Secure Entry 1463 Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April 1464 2004, . 1466 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, 1467 DOI 10.17487/RFC3912, September 2004, 1468 . 1470 [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices", 1471 RFC 4641, DOI 10.17487/RFC4641, September 2006, 1472 . 1474 [RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution 1475 Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697, 1476 October 2006, . 1478 [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast 1479 Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, 1480 December 2006, . 1482 [RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security 1483 (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July 1484 2007, . 1486 [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", 1487 BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009, 1488 . 1490 [RFC5890] Klensin, J., "Internationalized Domain Names for 1491 Applications (IDNA): Definitions and Document Framework", 1492 RFC 5890, DOI 10.17487/RFC5890, August 2010, 1493 . 1495 [RFC5891] Klensin, J., "Internationalized Domain Names in 1496 Applications (IDNA): Protocol", RFC 5891, 1497 DOI 10.17487/RFC5891, August 2010, 1498 . 1500 [RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and 1501 Internationalized Domain Names for Applications (IDNA)", 1502 RFC 5892, DOI 10.17487/RFC5892, August 2010, 1503 . 1505 [RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts 1506 for Internationalized Domain Names for Applications 1507 (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010, 1508 . 1510 [RFC5894] Klensin, J., "Internationalized Domain Names for 1511 Applications (IDNA): Background, Explanation, and 1512 Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010, 1513 . 1515 [RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on 1516 Encodings for Internationalized Domain Names", RFC 6055, 1517 DOI 10.17487/RFC6055, February 2011, 1518 . 1520 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 1521 DOI 10.17487/RFC6265, April 2011, 1522 . 1524 [RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163, 1525 RFC 6303, DOI 10.17487/RFC6303, July 2011, 1526 . 1528 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1529 Cheshire, "Internet Assigned Numbers Authority (IANA) 1530 Procedures for the Management of the Service Name and 1531 Transport Protocol Port Number Registry", BCP 165, 1532 RFC 6335, DOI 10.17487/RFC6335, August 2011, 1533 . 1535 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in 1536 Internationalization in the IETF", BCP 166, RFC 6365, 1537 DOI 10.17487/RFC6365, September 2011, 1538 . 1540 [RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of 1541 Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, 1542 February 2014, . 1544 [RFC7480] Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the 1545 Registration Data Access Protocol (RDAP)", RFC 7480, 1546 DOI 10.17487/RFC7480, March 2015, 1547 . 1549 [RFC7481] Hollenbeck, S. and N. Kong, "Security Services for the 1550 Registration Data Access Protocol (RDAP)", RFC 7481, 1551 DOI 10.17487/RFC7481, March 2015, 1552 . 1554 [RFC7482] Newton, A. and S. Hollenbeck, "Registration Data Access 1555 Protocol (RDAP) Query Format", RFC 7482, 1556 DOI 10.17487/RFC7482, March 2015, 1557 . 1559 [RFC7483] Newton, A. and S. Hollenbeck, "JSON Responses for the 1560 Registration Data Access Protocol (RDAP)", RFC 7483, 1561 DOI 10.17487/RFC7483, March 2015, 1562 . 1564 [RFC7484] Blanchet, M., "Finding the Authoritative Registration Data 1565 (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March 1566 2015, . 1568 [RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin, 1569 "Inventory and Analysis of WHOIS Registration Objects", 1570 RFC 7485, DOI 10.17487/RFC7485, March 2015, 1571 . 1573 Appendix A. Definitions Updated by this Document 1575 The following definitions from RFCs are updated by this document: 1577 o Forwarder in [RFC2308] 1579 o Secure Entry Point (SEP) in [RFC3757] 1581 Acknowledgements 1583 The following is the Acknowledgements for RFC 7719. Additional 1584 acknowledgements may be added as this draft is worked on. 1586 The authors gratefully acknowledge all of the authors of DNS-related 1587 RFCs that proceed this one. Comments from Tony Finch, Stephane 1588 Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John 1589 Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman, 1590 David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis, 1591 John Klensin, David Black, and many others in the DNSOP Working Group 1592 helped shape RFC 7719. 1594 Additional people contributed to this document, including: John 1595 Dickinson, Bob Harold, [[ MORE NAMES WILL APPEAR HERE AS FOLKS 1596 CONTRIBUTE]]. 1598 Authors' Addresses 1600 Paul Hoffman 1601 ICANN 1603 Email: paul.hoffman@icann.org 1605 Andrew Sullivan 1606 Dyn 1607 150 Dow Street, Tower 2 1608 Manchester, NH 03101 1609 United States 1611 Email: asullivan@dyn.com 1613 Kazunori Fujiwara 1614 Japan Registry Services Co., Ltd. 1615 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda 1616 Chiyoda-ku, Tokyo 101-0065 1617 Japan 1619 Phone: +81 3 5215 8451 1620 Email: fujiwara@jprs.co.jp