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