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Checking references for intended status: Best Current Practice ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 882 (Obsoleted by RFC 1034, RFC 1035) ** Obsolete normative reference: RFC 1206 (Obsoleted by RFC 1325) ** Downref: Normative reference to an Informational RFC: RFC 6561 ** Downref: Normative reference to an Informational RFC: RFC 6781 ** Downref: Normative reference to an Informational RFC: RFC 6841 -- Obsolete informational reference (is this intentional?): RFC 4641 (Obsoleted by RFC 6781) Summary: 6 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Hoffman 3 Internet-Draft VPN Consortium 4 Intended status: Best Current Practice A. Sullivan 5 Expires: November 27, 2015 Dyn 6 K. Fujiwara 7 JPRS 8 May 26, 2015 10 DNS Terminology 11 draft-ietf-dnsop-dns-terminology-02 13 Abstract 15 The DNS is defined in literally dozens of different RFCs. The 16 terminology used in by implementers and developers of DNS protocols, 17 and 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 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on November 27, 2015. 39 Copyright Notice 41 Copyright (c) 2015 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 3. DNS Header and Response Codes . . . . . . . . . . . . . . . . 5 59 4. Resource Records . . . . . . . . . . . . . . . . . . . . . . 6 60 5. DNS Servers . . . . . . . . . . . . . . . . . . . . . . . . . 8 61 6. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 62 7. Registration Model . . . . . . . . . . . . . . . . . . . . . 14 63 8. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 15 64 9. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 17 65 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 66 11. Security Considerations . . . . . . . . . . . . . . . . . . . 19 67 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 68 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 69 13.1. Normative References . . . . . . . . . . . . . . . . . . 20 70 13.2. Informative References . . . . . . . . . . . . . . . . . 21 71 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 73 1. Introduction 75 The domain name system (DNS) is a simple query-response protocol 76 whose messages in both directions have the same format. The protocol 77 and message format are defined in [RFC1034] and [RFC1035]. These 78 RFCs defined some terms, but later documents defined others. Some of 79 the terms from RFCs 1034 and 1035 now have somewhat different 80 meanings than they did in 1987. 82 This document collects a wide variety of DNS-related terms. Some of 83 them have been precisely defined in earlier RFCs, some have been 84 loosely defined in earlier RFCs, and some are not defined in any 85 earlier RFC at all. 87 The definitions here are believed to be the consensus definition of 88 the DNS community, both protocol developers and operators. Some of 89 the definitions differ from earlier RFCs, and those differences are 90 noted. The terms are organized loosely by topic. Some definitions 91 are for new terms for things that are commonly talked about in the 92 DNS community but that never had terms defined for them. 94 In this document, where the consensus definition is the same as the 95 one in an RFC, that RFC is quoted. Where the consensus definition 96 has changed somewhat, the RFC is mentioned but the new stand-alone 97 definition is given. 99 Other organizations sometimes define DNS-related terms their own way. 100 For example, the W3C defines "domain" at 101 https://specs.webplatform.org/url/webspecs/develop/. 103 Note that there is no single consistent definition of "the DNS". It 104 can be considered to be some combination of the following: a 105 commonly-used naming scheme for objects on the Internet; a database 106 representing the names and certain properties of these objects; an 107 architecture providing distributed maintenance, resilience, and loose 108 coherency for this database; and a simple query-response protocol (as 109 mentioned below) implementing this architecture. 111 Capitalization in DNS terms is often inconsistent between RFCs and 112 between DNS practitioners. The capitalization used in this document 113 is a best guess at current practices, and is not meant to indicate 114 that other capitalization styles are wrong or archaic. In some 115 cases, multiple styles of capitalization are used for the same term 116 due to quoting from different RFCs. 118 2. Names 120 Domain name: Section 3.1 of [RFC1034] talks of "the domain name 121 space" as a tree structure. "Each node has a label, which is zero 122 to 63 octets in length. ... The domain name of a node is the list 123 of the labels on the path from the node to the root of the tree. 124 ... To simplify implementations, the total number of octets that 125 represent a domain name (i.e., the sum of all label octets and 126 label lengths) is limited to 255." 128 Fully-qualified domain name (FQDN): This is often just a clear way 129 of saying the same thing as "domain name of a node", as outlined 130 above. However, the term is ambiguous. Strictly speaking, a 131 fully-qualified name would include every label, including the 132 final, zero-length label of the root zone: such a name would be 133 written "www.example.net." (note the terminating dot). But 134 because every name eventually shares the common root, names are 135 often written relative to the root (such as "www.example.net") and 136 are still called "fully qualified". 137 This term first appeared in [RFC1206]. 139 The need for the term "fully-qualified domain name" comes from the 140 existence of partially-qualified domain names, which are names 141 where some of the right-most names are left off and are understood 142 only by context. 144 Label: The identifier of an individual node in the sequence of nodes 145 that comprise a fully-qualified domain name. 147 Host name: This term and its equivalent, "hostname", have been 148 widely used but are not defined in [RFC1034], [RFC1035], 149 [RFC1123], or [RFC2181]. The DNS was originally deployed into the 150 Host Tables environment as outlined in [RFC0952], and it is likely 151 that the term followed informally from the definition there. Over 152 time, the definition seems to have shifted. "Host name" is often 153 meant to be a domain name that follows the rules in Section 3.5 of 154 [RFC1034], the "preferred name syntax". Note that any label in 155 any domain name can contain any octet value; hostnames are 156 generally considered to be domain names where every label follows 157 the rules in the "preferred name syntax", with the amendment that 158 labels can start with ASCII digits (this amendment comes from 159 Section 2.1 of [RFC1123]). 161 People also sometimes use the term hostname to refer to just the 162 first label of an FQDN. In addition, people sometimes use this 163 term to describe any name that refers to a machine, and those 164 might include labels that do not conform to the "preferred name 165 syntax". 167 TLD: A Top-Level Domain, meaning a zone that is one layer below the 168 root, such as .com or .jp. There is nothing special, from the 169 point of view of the DNS, about TLDs. Most of them are also 170 delegation-centric zones, and there are significant policy issues 171 around their operation. TLDs are often divided into sub-groups 172 such as "ccTLDs", "gTLDs", and others; the division is a matter of 173 policy, and beyond the scope of this document. 175 IDN: The common abbreviation for "internationalized domain name". 176 IDNs are the current standard mechanism for handling domain names 177 with non-ASCII characters in applications. The current standard, 178 normally called "IDNA2008", is defined in [RFC5890], [RFC5891], 179 [RFC5892], [RFC5893], and [RFC5894]. 181 Alias: The owner of a CNAME resource record, or a subdomain of the 182 owner of a DNAME resource record [RFC6672]. See also "canonical 183 name". 185 Canonical name: A CNAME resource record identifies its owner name as 186 an alias, and specifies the corresponding canonical name in the 187 RDATA section of the RR. (Quoted from [RFC1034], section 3.6.2) 188 This usage of the word "canonical" is related to the mathematical 189 concept of "canonical form". 191 CNAME: It is traditional to refer to the owner of a CNAME record as 192 "a CNAME". This is unfortunate, as "CNAME" is an abbreviation of 193 "canonical name", and the owner of a CNAME record is an alias not 194 a canonical name. (Quoted from [RFC2181], section 10.1.1) 196 Public suffix: A domain under which subdomains can be registered, 197 and on which HTTP cookies ([RFC6265]) should not be set. There is 198 no indication in a domain name whether or not it is a public 199 suffix; that can only be determined by outside means. The IETF 200 DBOUND Working Group [DBOUND] deals with issues with public 201 suffixes. 203 For example, at the time this document is published, .com.au is 204 considered a public suffix, but .au is not. (Note that this 205 example might change in the future.) 207 Note that the term "public suffix" is controversial in the DNS 208 community for many reasons, and may be significantly changed in 209 the future. One example of the difficulty of calling a domain a 210 public suffix is that designation can change over time as the 211 registration policy for the zone changes, such as the case of the 212 .uk zone around the time this document is published. 214 3. DNS Header and Response Codes 216 The header of a DNS message is first 12 octets. Many of the fields 217 and flags in the header diagram in section 4.1.1 of [RFC1035] are 218 referred to by their names in that diagram. For example, the 219 response codes are called "RCODEs", the data for a record is called 220 the "RDATA" (sometimes also called "RRdata"), and the authoritative 221 answer bit is often called "the AA flag" or "the AA bit". 223 Some of response codes that are defined in [RFC1035] have gotten 224 their own shorthand names. Some common response code names that 225 appear without reference to the numeric value are "FORMERR", 226 "SERVFAIL", and "NXDOMAIN". All of the RCODEs are listed at 227 http://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml, 228 although that site uses mixed-case capitalization, while most 229 documents use all-caps. 231 NODATA: A pseudo RCODE which indicates that the name is valid for 232 the given class, but are no records of the given type. A NODATA 233 response has to be inferred from the answer. (Quoted from 234 [RFC2308], section 1.) NODATA is indicated by an answer with the 235 RCODE set to NOERROR and no relevant answers in the answer 236 section. The authority section will contain an SOA record, or 237 there will be no NS records there. (Quoted from [RFC2308], 238 section 2,2.) Note that referrals have a similar format to NODATA 239 replies; [RFC2308] explains how to distinguish them. 241 The term "NXRRSET" is sometimes used as a synonym for NODATA. 242 However, this is a mistake, given that NXRRSET is a specific error 243 code defined in [RFC2136]. 245 Negative response: A response which indicates that a particular 246 RRset does not exist, or whose RCODE indicates the nameserver 247 cannot answer. Sections 2 and 7 of [RFC2308] describe the types 248 of negative responses in detail. 250 Referrals: Data from the authority section of a non-authoritative 251 answer. [RFC1035] section 2.1 defines "authoritative" data. 252 However, referrals at zone cuts are not authoritative. Referrals 253 may be a zone cut NS resource records and their glue records. NS 254 records on the parent side of a zone cut are an authoritative 255 delegation, but are normally not treated as authoritative data by 256 the client. In general, a referral is a way for a server to send 257 an answer saying that the server does not know the answer, but 258 knows where the query should be directed in order to get an 259 answer. Historically, many authoritative servers answered with a 260 referral to the root zone when queried for a name for which they 261 were not authoritative, but this practice has declined. 263 Zone transfer: The act of a client requesting a copy of a zone and 264 an authoritative server sending the needed information. There are 265 two common standard ways to do zone transfers: the AXFR 266 ("Authoritative Transfer") mechanism to copy the full zone 267 (described in [RFC5936], and the IXFR ("Incremental Transfer") 268 mechanism to copy only parts of the zone that have changed 269 (described in [RFC1995]). Many systems use non-standard methods 270 for zone transfer outside the DNS protocol. 272 4. Resource Records 274 RR: A short form for resource record. ([RFC1034], section 3.6.) 276 RRset: A set of resource records with the same label, class and 277 type, but with different data. (Definition from [RFC2181]) Also 278 spelled RRSet in some documents. As a clarification, "same label" 279 in this definition means "same owner name". In addition, 280 [RFC2181] states that "the TTLs of all RRs in an RRSet must be the 281 same". 283 EDNS: The extension mechanisms for DNS, defined in [RFC6891]. 284 Sometimes called "EDNS0" or "EDNS(0)" to indicate the version 285 number. EDNS allows DNS clients and servers to specify message 286 sizes larger than the original 512 octet limit, to expand the 287 response code space, and to potentially carry additional options 288 that affect the handling of a DNS query. 290 OPT: A pseudo-RR (sometimes called a meta-RR) that is used only to 291 contain control information pertaining to the question-and-answer 292 sequence of a specific transaction. (Definition from [RFC6891], 293 section 6.1.1) It is used by EDNS. 295 Owner: The domain name where a RR is found ([RFC1034], section 3.6). 296 Often appears in the term "owner name". 298 SOA field names: DNS documents, including the definitions here, 299 often refer to the fields in the RDATA an SOA resource record by 300 field name. Those fields are defined in Section 3.3.13 of 301 [RFC1035]. The names (in the order they appear in the SOA RDATA) 302 are MNAME, RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. 303 Note that the meaning of MINIMUM field is updated in Section 4 of 304 [RFC2308]; the new definition is that the MINIMUM field is only 305 "the TTL to be used for negative responses". 307 TTL: The maximum "time to live" of a resource record. A TTL value 308 is an unsigned number, with a minimum value of 0, and a maximum 309 value of 2147483647. That is, a maximum of 2^31 - 1. When 310 transmitted, the TTL is encoded in the less significant 31 bits of 311 the 32 bit TTL field, with the most significant, or sign, bit set 312 to zero. (Quoted from [RFC2181], section 8) (Note that [RFC1035] 313 erroneously stated that this is a signed integer; it is fixed in 314 an erratum.) 316 The TTL "specifies the time interval that the resource record may 317 be cached before the source of the information should again be 318 consulted". (Quoted from [RFC1035], section 3.2.1) Also: "the 319 time interval (in seconds) that the resource record may be cached 320 before it should be discarded". (Quoted from [RFC1035], section 321 4.1.3). Despite being defined for a resource record, the TTL of 322 every resource record in an RRset is required to be the same 323 (RFC2181, section 5.2). 325 The reason that the TTL is the maximum time to live is that a 326 cache operator might decide to shorten the time to live for 327 operational purposes, such as if there is a policy to not allow 328 TTL values over a certain number. Also, if a value is flushed 329 from the cache when its value is still positive, the value 330 effectively becomes zero. Some servers do not honor the TTL on an 331 RRset from the authoritative servers, such as when when the 332 authoritative data has a very short TTL. 334 There is also the concept of a "default TTL" for a zone, which can 335 be a configuration parameter in the server software. This is 336 often expressed by a default for the entire server, and a default 337 for a zone using the $TTL directive in a zone file. The $TTL 338 directive was added to the master file format by [RFC2308]. 340 5. DNS Servers 342 This section defines the terms used for the systems that act as DNS 343 clients, DNS servers, or both. Some terms about servers describe 344 servers that do and do not use DNSSEC; see Section 8 for those 345 definitions. 347 Resolver: A program that extracts information from name servers in 348 response to client requests. (Quoted from [RFC1034], section 2.4) 349 The resolver is located on the same machine as the program that 350 requests the resolver's services, but it may need to consult name 351 servers on other hosts. (Quoted from [RFC1034], section 5.1) A 352 resolver performs queries for a name, type, and class, and 353 receives answers. The logical function is called "resolution". 354 In practice, the term is usually referring to some specific type 355 of resolver (some of which are defined below), and understanding 356 the use of the term depends on understanding the context. 358 Stub resolver: A resolver that cannot perform all resolution itself. 359 Stub resolvers generally depend on a recursive resolver to 360 undertake the actual resolution function. Stub resolvers are 361 discussed but never fully defined in Section 5.3.1 of [RFC1034]. 362 They are fully defined in Section 6.1.3.1 of [RFC1123]. 364 Iterative mode: A resolution mode of a server that receives DNS 365 queries and responds with a referral to another server. 366 Section 2.3 of [RFC1034] describes this as "The server refers the 367 client to another server and lets the client pursue the query". A 368 resolver that works in iterative mode is sometimes called an 369 "iterative resolver". 371 Recursive mode: A resolution mode of a server that receives DNS 372 queries and either responds to those queries from a local cache or 373 sends queries to other servers in order to get the final answers 374 to the original queries. Section 2.3 of [RFC1034] describes this 375 as "The first server pursues the query for the client at another 376 server". A server operating in recursive mode may be thought of 377 as having a name server side (which is what answers the query) and 378 a resolver side (which performs the resolution function). Systems 379 operating in this mode are commonly called "recursive servers". 380 Sometimes they are called "recursive resolvers". While strictly 381 the difference between these is that one of them sends queries to 382 another recursive server and the other does not, in practice it is 383 not possible to know in advance whether the server that one is 384 querying will also perform recursion; both terms can be observed 385 in use interchangeably. 387 Full resolver: This term is used in [RFC1035], but it is not defined 388 there. RFC 1123 defines a "full-service resolver" that may or may 389 not be what was intended by "full resolver" in [RFC1035]. 391 Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this 392 term to mean a resolver that acts in recursive mode with a cache 393 (and meets other requirements). 395 Priming: The mechanism used by a resolver to determine where to send 396 queries before there is anything in the resolver's cache. Priming 397 is most often done from a configuration setting that contains a 398 list of authoritative servers for the DNS root zone. 400 Negative caching: The storage of knowledge that something does not 401 exist, cannot give an answer, or does not give an answer. (Quoted 402 from Section 1 of [RFC2308]) 404 Authoritative server: A server that knows the content of a DNS zone 405 from local knowledge, and thus can answer queries about that zone 406 without needing to query other servers. (Quoted from [RFC2182], 407 section 2.) It is a system that responds to DNS queries with 408 information about zones for which it has been configured to answer 409 with the AA flag in the response header set to 1. It is a server 410 that has authority over one or more DNS zones. Note that it is 411 possible for an authoritative server to respond to a query without 412 the parent zone delegating authority to that server. 413 Authoritative servers also provide "referrals", usually to child 414 zones delegated from them; these referrals have the AA bit set to 415 0 and come with referral data in the Authority and (if needed) the 416 Additional sections. 418 Secondary server: "An authoritative server which uses zone transfer 419 to retrieve the zone" (quoted from [RFC1996], section 2.1). 420 [RFC2182] describes secondary servers in detail. Although early 421 DNS RFCs such as [RFC1996] referred to this as a "slave", the 422 current common usage has shifted to calling it a "secondary". 424 Slave server: See secondary server. 426 Primary server: "Any authoritative server configured to be the 427 source of zone transfer for one or more [secondary] servers" 428 (quoted from [RFC1996], section 2.1) or, more specifically, "an 429 authoritative server configured to be the source of AXFR or IXFR 430 data for one or more [secondary] servers" (quoted from [RFC2136]). 431 Although early DNS RFCs such as [RFC1996] referred to this as a 432 "master", the current common usage has shifted to "primary". 434 Master server: See primary server. 436 Primary master: The primary master is named in the zone's SOA MNAME 437 field and optionally by an NS resource record. (Quoted from 438 [RFC1996], section 2.1) [RFC2136] defines "primary master" as 439 "Master server at the root of the AXFR/IXFR dependency graph. The 440 primary master is named in the zone's SOA MNAME field and 441 optionally by an NS RR. There is by definition only one primary 442 master server per zone." 444 Stealth server: This is the same as a slave server except that it is 445 not listed in an NS resource record for the zone. (Quoted from 446 [RFC1996], section 2.1) 448 Hidden master: A stealth server that is a master for zone transfers. 449 In this arrangement, the master name server that processes the 450 updates is unavailable to general hosts on the Internet; it is not 451 listed in the NS RRset. (Quoted from [RFC6781], section 3.4.3.) 452 An earlier RFC, [RFC4641], said that the hidden master's name 453 appears in the SOA RRs MNAME field, although in some setups, the 454 name does not appear at all in the public DNS. A hidden master 455 can be either a secondary or a primary master. 457 Forwarding: The process of one server sending a DNS query with the 458 RD bit set to 1 to another server to resolve that query. 459 Forwarding is a function of a DNS resolver; it is different than 460 simply blindly relaying queries. 462 [RFC5625] does not give a specific definition for forwarding, but 463 describes in detail what features a system that forwards need to 464 support. Systems that forward are sometimes called "DNS proxies", 465 but that term has not yet been defined (even in [RFC5625]). 467 Forwarder: Section 1 of [RFC2308] describes a forwarder as "a 468 nameserver used to resolve queries instead of directly using the 469 authoritative nameserver chain". [RFC2308] further says "The 470 forwarder typically either has better access to the internet, or 471 maintains a bigger cache which may be shared amongst many 472 resolvers." That definition appears to suggest that forwarders 473 normally only query authoritative servers. In current use, 474 however, forwarders often stand between stub resolvers and 475 recursive servers. [RFC2308] is silent on whether a forwarder is 476 iterative-only or can be a full-service resolver. 478 Policy-implementing resolver: A resolver acting in recursive mode 479 that changes some of the answers that it returns based on policy 480 criteria, such as to prevent access to malware sites or 481 objectionable content. In general, a stub resolver has no idea 482 whether or not upstream resolvers implement such policy or, if 483 they do, the exact policy about what changes will be made. In 484 some cases, the user of the stub resolver has selected the policy- 485 implementing resolver with the explicit intention of using it to 486 implement the policies. In other cases, policies are imposed 487 without the user of the stub resolver being informed. 489 Open resolver: A full-service resolver that accepts and processes 490 queries from any (or nearly any) stub resolver. This is sometimes 491 also called a "public resolver". 493 View: A configuration for a DNS server that allows it to provide 494 different answers depending on attributes of the query. 495 Typically, views differ by the source IP address of a query, but 496 can also be based on the destination IP address, the type of query 497 (such as AXFR), whether or not it is recursive, and so on. Views 498 are often used to provide more names or different addresses to 499 queries from "inside" a protected network than to those "outside" 500 that network. Views are not a standardized part of the DNS, but 501 they are widely implemented in server software. 503 Passive DNS: A mechanism to collect large amounts of DNS data by 504 storing DNS responses from servers. Some of these systems also 505 collect the DNS queries associated with the responses; this can 506 raise privacy issues. Passive DNS databases can be used to answer 507 historical questions about DNS zones such as which records were 508 available for them at what times in the past. Passive DNS 509 databases allow searching of the stored records on keys other than 510 just the name, such as "find all names which have A records of a 511 particular value". 513 Child-centric resolver: A DNS resolver that, instead of serving the 514 NS RRset and glue records that it obtained from the parent of a 515 zone, serves data from the authoritative servers for that zone. 516 The term "child-centric" is meant as the opposite of "parent- 517 centric", which means a resolver that simply serves the NS RRset 518 and glue records for a zone that it obtained from the zone's 519 parent, without checking the authoritative servers for that zone. 521 6. Zones 523 This section defines terms that are used when discussing zones that 524 are being served or retrieved. 526 Zone: A unit of organization of authoritative data. Zones can be 527 automatically distributed to the name servers which provide 528 redundant service for the data in a zone. (Quoted from [RFC1034], 529 section 2.4). 531 Child: The entity on record that has the delegation of the domain 532 from the Parent. (Quoted from [RFC7344], section 1.1) 534 Parent: The domain in which the Child is registered. (Quoted from 535 [RFC7344], section 1.1) Earlier, "parent name server" was defined 536 in [RFC0882] as "the name server that has authority over the place 537 in the domain name space that will hold the new domain". 539 Origin: 541 (a) The domain name that appears at the top of a zone (just below 542 the cut that separates the zone from its parent). The name of the 543 zone is the same as the name of the domain at the zone's origin. 544 (Quoted from [RFC2181], section 6.) 546 (b) The domain name within which a given relative domain name 547 appears in zone files. Generally seen in the context of 548 "$ORIGIN", which is a control entry defined in [RFC1035], section 549 5.1, as part of the master file format. For example, if the 550 $ORIGIN is set to "example.org.", then a master file line for 551 "www" is in fact an entry for "www.example.org.". 553 Zone cut: The delimitation point between two zones where the origin 554 of one of the zones is the child of the other zone. 556 Zones are delimited by "zone cuts". Each zone cut separates a 557 "child" zone (below the cut) from a "parent" zone (above the cut). 558 (Quoted from [RFC2181], section 6; note that this is barely an 559 ostensive definition.) Section 4.2 of [RFC1034] uses "cuts" as 560 "zone cut". 562 Apex: The point in the tree at an owner of an SOA and corresponding 563 authoritative NS RRset. This is also called the "zone apex". 564 [RFC4033] defines it as "the name at the child's side of a zone 565 cut". The "apex" can usefully be thought of as a data-theoretic 566 description of a tree structure, and "origin" is the name of the 567 same concept when it is implemented in zone files. The 568 distinction is not always maintained in use, however, and one can 569 find uses that conflict subtly with this definition. [RFC1034] 570 uses the term "top node of the zone" instead of "apex". 572 Delegation: The process by which a separate zone is created in the 573 name space beneath the apex of a given domain. Delegation happens 574 when an NS RRset is added in the parent zone for the child origin. 575 Delegation inherently happens at a zone cut. The term is also 576 commonly a noun: the new zone that is created by the act of 577 delegating. 579 Glue records: "[Resource records] which are not part of the 580 authoritative data [of the zone], and are address resource records 581 for the [name servers in subzones]. These RRs are only necessary 582 if the name server's name is 'below' the cut, and are only used as 583 part of a referral response." Without glue "we could be faced 584 with the situation where the NS RRs tell us that in order to learn 585 a name server's address, we should contact the server using the 586 address we wish to learn." (Definition from [RFC1034], section 587 4.2.1) 589 A later definition is that glue "includes any record in a zone 590 file that is not properly part of that zone, including nameserver 591 records of delegated sub-zones (NS records), address records that 592 accompany those NS records (A, AAAA, etc), and any other stray 593 data that might appear" ([RFC2181], section 5.4.1). Although glue 594 is sometimes used today with this wider definition in mind, the 595 context surrounding the [RFC2181] definition suggests it is 596 intended to apply to the use of glue within the document itself 597 and not necessarily beyond. 599 In-bailiwick: 601 (a) An adjective to describe a name server the name of which is 602 either subordinate to or (rarely) the same as the zone origin. 603 In-bailiwick name servers require glue in their parent zone. 605 (b) Data for which the server is either authoritative, or else 606 authoritative for an ancestor of the owner name. This sense of 607 the term normally is used when discussing the relevancy of glue 608 records in a response. For example, the server for the parent 609 zone example.com might reply with glue records for 610 ns.child.example.com. Because the child.example.com zone is a 611 descendant of the example.com zone, the glue records are in- 612 bailiwick. 614 Out-of-bailiwick: The antonym of in-bailiwick. 616 Authoritative data: All of the RRs attached to all of the nodes from 617 the top node of the zone down to leaf nodes or nodes above cuts 618 around the bottom edge of the zone. (Quoted from Section 4.2.1 of 619 [RFC1034]) It is noted that this definition might inadvertently 620 also include any NS records that appear in the zone, even those 621 that might not truly be authoritative because there are identical 622 NS RRs below the zone cut. This reveals the ambiguity in the 623 notion of authoritative data, because the parent-size NS records 624 authoritatively indicate the delegation, even though they are not 625 themselves authoritative data. 627 Root zone: The zone whose origin is the zero-length label. Also 628 sometimes called "the DNS root". 630 Empty non-terminals: Domain names that own no resource records but 631 have subdomains that do. (Quoted from [RFC4592], section 2.2.2.) 632 A typical example is in SRV records: in the name 633 "_sip._tcp.example.com", it is likely that "_tcp.example.com" has 634 no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV 635 RRset. 637 Delegation-centric zone: A zone which consists mostly of delegations 638 to child zones. This term is used in contrast to a zone which 639 might have some delegations to child zones, but also has many data 640 resource records for the zone itself and/or for child zones. The 641 term is used in [RFC4956] and [RFC5155], but is not defined there. 643 Wildcard: [RFC1034] defined "wildcard", but in a way that turned out 644 to be confusing to implementers. For an extended discussion of 645 wildcards, including clearer definitions, see [RFC4592]. 647 Occluded name: The addition of a delegation point via dynamic update 648 will render all subordinate domain names to be in a limbo, still 649 part of the zone but not available to the lookup process. The 650 addition of a DNAME resource record has the same impact. The 651 subordinate names are said to be "occluded". (Quoted from 652 [RFC5936], Section 3.5) 654 Fast flux DNS: This occurs when a domain is bound in DNS using A 655 records to multiple IP addresses, each of which has a very short 656 Time-to-Live (TTL) value associated with it. This means that the 657 domain resolves to varying IP addresses over a short period of 658 time. (Quoted from [RFC6561], section 1.1.5) It is often to 659 deliver malware. Because the addresses change so rapidly, it is 660 difficult to definitively find all the hosts. It should be noted 661 that the technique also works with AAAA records, but such use is 662 not frequently observed on the Internet as of this writing. 664 7. Registration Model 666 Registry: The administrative operation of a zone that allows 667 registration of names within that zone. People often use this 668 term to refer only to those organizations that perform 669 registration in large delegation-centric zones (such as TLDs); but 670 formally, whoever decides what data goes into a zone is the 671 registry for that zone. 673 Registrant: An individual or organization on whose behalf a name in 674 a zone is registered by the registry. In many zones, the registry 675 and the registrant may be the same entity, but in TLDs they often 676 are not. 678 Registrar: A service provider that acts as a go-between for 679 registrants and registries. Not all registrations require a 680 registrar, though it is common to have registrars be involved in 681 registrations in TLDs. 683 EPP: The Extensible Provisioning Protocol (EPP), which is commonly 684 used for communication of registration information between 685 registries and registrars. EPP is defined in [RFC5730]. 687 WHOIS: A protocol specified in [RFC3912], often used for querying 688 registry databases. WHOIS data is frequently used to associate 689 registration data (such as zone management contacts) with domain 690 names. 692 8. General DNSSEC 694 Most DNSSEC terms are defined in [RFC4033], [RFC4034], and [RFC4035]. 695 The terms that have caused confusion in the DNS community are 696 highlighted here. 698 DNSSEC-aware and DNSSEC-unaware: Section 2 of [RFC4033] defines many 699 types of resolvers and validators. In specific, the terms "non- 700 validating security-aware stub resolver", "non-validating stub 701 resolver", "security-aware name server", "security-aware recursive 702 name server", "security-aware resolver", "security-aware stub 703 resolver", and "security-oblivious 'anything'" are all defined. 704 (Note that the term "validating resolver", which is used in some 705 places in those documents, is nevertheless not defined in that 706 section.) 708 Signed zone: A zone whose RRsets are signed and that contains 709 properly constructed DNSKEY, Resource Record Signature (RRSIG), 710 Next Secure (NSEC), and (optionally) DS records. (Quoted from 711 [RFC4033], section 2.) It has been noted in other contexts that 712 the zone itself is not really signed, but all the relevant RRsets 713 in the zone are signed. Nevertheless, if a zone that should be 714 signed contains any RRsets that are not signed (or opted out), 715 those RRsets will be treated as bogus, so the whole zone needs to 716 be handled in some way. It should also be noted that, since the 717 publication of [RFC6840], NSEC records are no longer required for 718 signed zones: a signed zone might include NSEC3 records instead. 720 Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that 721 is not signed". Section 2 of [RFC4035] defines this as "A zone 722 that does not include these records [properly constructed DNSKEY, 723 Resource Record Signature (RRSIG), Next Secure (NSEC), and 724 (optionally) DS records] according to the rules in this section". 725 There is an important note at the end of Section 5.2 of [RFC4035] 726 adding an additional situation when a zone is considered unsigned: 727 "If the resolver does not support any of the algorithms listed in 728 an authenticated DS RRset, then the resolver will not be able to 729 verify the authentication path to the child zone. In this case, 730 the resolver SHOULD treat the child zone as if it were unsigned." 732 NSEC: "The NSEC record allows a security-aware resolver to 733 authenticate a negative reply for either name or type non- 734 existence with the same mechanisms used to authenticate other DNS 735 replies." (Quoted from [RFC4033], section 3.2.) In short, an 736 NSEC record provides authenticated denial of existence. 738 The NSEC resource record lists two separate things: the next owner 739 name (in the canonical ordering of the zone) that contains 740 authoritative data or a delegation point NS RRset, and the set of 741 RR types present at the NSEC RR's owner name. (Quoted from 742 Section 4 of 4034) 744 NSEC3: The NSEC3 resource record is quite different than the NSEC 745 resource record. Like the NSEC record, the NSEC3 record also 746 provides authenticated denial of existence; however, NSEC3 records 747 mitigates against zone enumeration and support Opt-Out. NSEC3 748 resource records are defined in [RFC5155]. 750 Opt-out: The Opt-Out Flag indicates whether this NSEC3 RR may cover 751 unsigned delegations. (Quoted from [RFC5155], section 3.1.2.1.) 753 Zone enumeration: The practice of discovering the full content of a 754 zone via successive queries. (Quoted from [RFC5155], section 755 1.3.) This is also sometimes call "zone walking". Zone 756 enumeration is different from zone content guessing where the 757 guesser uses a large dictionary of possible labels and sends 758 successive queries for them, or matches the contents of NSEC3 759 records against such a dictionary. 761 Key signing key (KSK): DNSSEC keys that only sign the apex DNSKEY 762 RRset in a zone. (Quoted from [RFC6781], section 3.1.) 764 Zone signing key (ZSK): DNSSEC keys that can be used to sign all the 765 RRsets in a zone that require signatures, other than the apex 766 DNSKEY RRset. (Quoted from [RFC6781], section 3.1) Note that the 767 roles KSK and ZSK are not mutually exclusive: a single key can be 768 both KSK and ZSK at the same time. 770 Combined signing key (CSK): In cases where the differentiation 771 between the KSK and ZSK is not made, i.e., where keys have the 772 role of both KSK and ZSK, we talk about a Single-Type Signing 773 Scheme. (Quoted from [RFC6781], Section 3.1) This is sometimes 774 called a "combined signing key" or CSK. It is operational 775 practice, not protocol, that determines whether a particular key 776 is a ZSK, a KSK, or a CSK. 778 Secure Entry Point (SEP): A flag in the DNSKEY RRdata that can be 779 used to distinguish between keys that are intended to be used as 780 the secure entry point into the zone when building chains of 781 trust, i.e., they are (to be) pointed to by parental DS RRs or 782 configured as a trust anchor. (Quoted from [RFC6781], section 783 3.2.3.) Note that the SEP flag is only a hint, and its presence 784 or absence may not be used to disqualify a given DNSKEY RR from 785 use as a KSK or ZSK during validation. 787 DNSSEC Policy (DP): A statement that sets forth the security 788 requirements and standards to be implemented for a DNSSEC-signed 789 zone. (Quoted from [RFC6841], section 2) 791 DNSSEC Practice Statement (DPS): A practices disclosure document 792 that may support and be a supplemental document to the DNSSEC 793 Policy (if such exists), and it states how the management of a 794 given zone implements procedures and controls at a high level. 795 (Quoted from [RFC6841], section 2) 797 9. DNSSEC States 799 A validating resolver can determine that a response is in one of four 800 states: secure, insecure, bogus, or indeterminate. These states are 801 defined in [RFC4033] and [RFC4035], although the two definitions 802 differ a bit. 804 Section 5 of [RFC4033] says: 806 A validating resolver can determine the following 4 states: 808 Secure: The validating resolver has a trust anchor, has a chain of 809 trust, and is able to verify all the signatures in the response. 811 Insecure: The validating resolver has a trust anchor, a chain of 812 trust, and, at some delegation point, signed proof of the 813 non-existence of a DS record. This indicates that subsequent 814 branches in the tree are provably insecure. A validating resolver 815 may have a local policy to mark parts of the domain space as 816 insecure. 818 Bogus: The validating resolver has a trust anchor and a secure 819 delegation indicating that subsidiary data is signed, but the 820 response fails to validate for some reason: missing signatures, 821 expired signatures, signatures with unsupported algorithms, data 822 missing that the relevant NSEC RR says should be present, and so 823 forth. 825 Indeterminate: There is no trust anchor that would indicate that a 826 specific portion of the tree is secure. This is the default 827 operation mode. 829 Section 4.3 of [RFC4035] says: 831 A security-aware resolver must be able to distinguish between four 832 cases: 834 Secure: An RRset for which the resolver is able to build a chain of 835 signed DNSKEY and DS RRs from a trusted security anchor to the 836 RRset. In this case, the RRset should be signed and is subject to 837 signature validation, as described above. 839 Insecure: An RRset for which the resolver knows that it has no chain 840 of signed DNSKEY and DS RRs from any trusted starting point to the 841 RRset. This can occur when the target RRset lies in an unsigned 842 zone or in a descendent of an unsigned zone. In this case, the 843 RRset may or may not be signed, but the resolver will not be able 844 to verify the signature. 846 Bogus: An RRset for which the resolver believes that it ought to be 847 able to establish a chain of trust but for which it is unable to 848 do so, either due to signatures that for some reason fail to 849 validate or due to missing data that the relevant DNSSEC RRs 850 indicate should be present. This case may indicate an attack but 851 may also indicate a configuration error or some form of data 852 corruption. 854 Indeterminate: An RRset for which the resolver is not able to 855 determine whether the RRset should be signed, as the resolver is 856 not able to obtain the necessary DNSSEC RRs. This can occur when 857 the security-aware resolver is not able to contact security-aware 858 name servers for the relevant zones. 860 10. IANA Considerations 862 This document has no effect on IANA registries. 864 11. Security Considerations 866 These definitions do not change any security considerations for the 867 DNS. 869 12. Acknowledgements 871 The authors gratefully acknowledge all of the authors of DNS-related 872 RFCs that proceed this one. Comments from Tony Finch, Stephane 873 Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John 874 Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman, 875 David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis, 876 and many others in the DNSOP Working Group have helped shape this 877 document. 879 13. References 881 13.1. Normative References 883 [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities", 884 RFC 882, November 1983. 886 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 887 STD 13, RFC 1034, November 1987. 889 [RFC1035] Mockapetris, P., "Domain names - implementation and 890 specification", STD 13, RFC 1035, November 1987. 892 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 893 and Support", STD 3, RFC 1123, October 1989. 895 [RFC1206] Malkin, G. and A. Marine, "FYI on Questions and Answers: 896 Answers to commonly asked "new Internet user" questions", 897 RFC 1206, February 1991. 899 [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone 900 Changes (DNS NOTIFY)", RFC 1996, August 1996. 902 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, 903 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 904 RFC 2136, April 1997. 906 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 907 Specification", RFC 2181, July 1997. 909 [RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection 910 and Operation of Secondary DNS Servers", BCP 16, RFC 2182, 911 July 1997. 913 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 914 NCACHE)", RFC 2308, March 1998. 916 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 917 Rose, "DNS Security Introduction and Requirements", RFC 918 4033, March 2005. 920 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 921 Rose, "Resource Records for the DNS Security Extensions", 922 RFC 4034, March 2005. 924 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 925 Rose, "Protocol Modifications for the DNS Security 926 Extensions", RFC 4035, March 2005. 928 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name 929 System", RFC 4592, July 2006. 931 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 932 Security (DNSSEC) Hashed Authenticated Denial of 933 Existence", RFC 5155, March 2008. 935 [RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)", 936 STD 69, RFC 5730, August 2009. 938 [RFC5936] Lewis, E. and A. Hoenes, "DNS Zone Transfer Protocol 939 (AXFR)", RFC 5936, June 2010. 941 [RFC6561] Livingood, J., Mody, N., and M. O'Reirdan, 942 "Recommendations for the Remediation of Bots in ISP 943 Networks", RFC 6561, March 2012. 945 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 946 DNS", RFC 6672, June 2012. 948 [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC 949 Operational Practices, Version 2", RFC 6781, December 950 2012. 952 [RFC6840] Weiler, S. and D. Blacka, "Clarifications and 953 Implementation Notes for DNS Security (DNSSEC)", RFC 6840, 954 February 2013. 956 [RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A 957 Framework for DNSSEC Policies and DNSSEC Practice 958 Statements", RFC 6841, January 2013. 960 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 961 for DNS (EDNS(0))", STD 75, RFC 6891, April 2013. 963 [RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating 964 DNSSEC Delegation Trust Maintenance", RFC 7344, September 965 2014. 967 13.2. Informative References 969 [DBOUND] "DBOUND Working Group", 2015, 970 . 972 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet 973 host table specification", RFC 952, October 1985. 975 [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, 976 August 1996. 978 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, 979 September 2004. 981 [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices", 982 RFC 4641, September 2006. 984 [RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security 985 (DNSSEC) Opt-In", RFC 4956, July 2007. 987 [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP 988 152, RFC 5625, August 2009. 990 [RFC5890] Klensin, J., "Internationalized Domain Names for 991 Applications (IDNA): Definitions and Document Framework", 992 RFC 5890, August 2010. 994 [RFC5891] Klensin, J., "Internationalized Domain Names in 995 Applications (IDNA): Protocol", RFC 5891, August 2010. 997 [RFC5892] Faltstrom, P., "The Unicode Code Points and 998 Internationalized Domain Names for Applications (IDNA)", 999 RFC 5892, August 2010. 1001 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for 1002 Internationalized Domain Names for Applications (IDNA)", 1003 RFC 5893, August 2010. 1005 [RFC5894] Klensin, J., "Internationalized Domain Names for 1006 Applications (IDNA): Background, Explanation, and 1007 Rationale", RFC 5894, August 2010. 1009 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 1010 April 2011. 1012 Authors' Addresses 1014 Paul Hoffman 1015 VPN Consortium 1016 127 Segre Place 1017 Santa Cruz, CA 95060 1018 USA 1020 Email: paul.hoffman@vpnc.org 1021 Andrew Sullivan 1022 Dyn 1023 150 Dow St, Tower 2 1024 Manchester, NH 1604 1025 USA 1027 Email: asullivan@dyn.com 1029 Kazunori Fujiwara 1030 Japan Registry Services Co., Ltd. 1031 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda 1032 Chiyoda-ku, Tokyo 101-0065 1033 Japan 1035 Phone: +81 3 5215 8451 1036 Email: fujiwara@jprs.co.jp