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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) Oracle
6 Intended status: Best Current Practice K. Fujiwara
7 Expires: October 29, 2018 JPRS
8 April 27, 2018
10 DNS Terminology
11 draft-ietf-dnsop-terminology-bis-10
13 Abstract
15 The domain name system (DNS) is defined in literally dozens of
16 different RFCs. The terminology used by implementers and developers
17 of DNS protocols, and by operators of DNS systems, has sometimes
18 changed in the decades since the DNS was first defined. This
19 document gives current definitions for many of the terms used in the
20 DNS in a single document.
22 This document will be the successor to RFC 7719, and thus will
23 obsolete RFC 7719. It will also update RFC 2308.
25 Status of This Memo
27 This Internet-Draft is submitted in full conformance with the
28 provisions of BCP 78 and BCP 79.
30 Internet-Drafts are working documents of the Internet Engineering
31 Task Force (IETF). Note that other groups may also distribute
32 working documents as Internet-Drafts. The list of current Internet-
33 Drafts is at https://datatracker.ietf.org/drafts/current/.
35 Internet-Drafts are draft documents valid for a maximum of six months
36 and may be updated, replaced, or obsoleted by other documents at any
37 time. It is inappropriate to use Internet-Drafts as reference
38 material or to cite them other than as "work in progress."
40 This Internet-Draft will expire on October 29, 2018.
42 Copyright Notice
44 Copyright (c) 2018 IETF Trust and the persons identified as the
45 document authors. All rights reserved.
47 This document is subject to BCP 78 and the IETF Trust's Legal
48 Provisions Relating to IETF Documents
49 (https://trustee.ietf.org/license-info) in effect on the date of
50 publication of this document. Please review these documents
51 carefully, as they describe your rights and restrictions with respect
52 to this document. Code Components extracted from this document must
53 include Simplified BSD License text as described in Section 4.e of
54 the Trust Legal Provisions and are provided without warranty as
55 described in the Simplified BSD License.
57 Table of Contents
59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
60 2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
61 3. DNS Response Codes . . . . . . . . . . . . . . . . . . . . . 9
62 4. DNS Transactions . . . . . . . . . . . . . . . . . . . . . . 10
63 5. Resource Records . . . . . . . . . . . . . . . . . . . . . . 12
64 6. DNS Servers and Clients . . . . . . . . . . . . . . . . . . . 14
65 7. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
66 8. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . 25
67 9. Registration Model . . . . . . . . . . . . . . . . . . . . . 26
68 10. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 28
69 11. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 32
70 12. Security Considerations . . . . . . . . . . . . . . . . . . . 34
71 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
72 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
73 14.1. Normative References . . . . . . . . . . . . . . . . . . 34
74 14.2. Informative References . . . . . . . . . . . . . . . . . 37
75 Appendix A. Definitions Updated by this Document . . . . . . . . 41
76 Appendix B. Definitions First Defined in this Document . . . . . 41
77 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
78 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 47
79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47
81 1. Introduction
83 The Domain Name System (DNS) is a simple query-response protocol
84 whose messages in both directions have the same format. (Section 2
85 gives a definition of "public DNS", which is often what people mean
86 when they say "the DNS".) The protocol and message format are
87 defined in [RFC1034] and [RFC1035]. These RFCs defined some terms,
88 but later documents defined others. Some of the terms from [RFC1034]
89 and [RFC1035] now have somewhat different meanings than they did in
90 1987.
92 This document collects a wide variety of DNS-related terms. Some of
93 them have been precisely defined in earlier RFCs, some have been
94 loosely defined in earlier RFCs, and some are not defined in any
95 earlier RFC at all.
97 Most of the definitions here are the consensus definition of the DNS
98 community -- both protocol developers and operators. Some of the
99 definitions differ from earlier RFCs, and those differences are
100 noted. In this document, where the consensus definition is the same
101 as the one in an RFC, that RFC is quoted. Where the consensus
102 definition has changed somewhat, the RFC is mentioned but the new
103 stand-alone definition is given. See Appendix A for a list of the
104 definitions that this document updates.
106 It is important to note that, during the development of this
107 document, it became clear that some DNS-related terms are interpreted
108 quite differently by different DNS experts. Further, some terms that
109 are defined in early DNS RFCs now have definitions that are generally
110 agreed to, but that are different from the original definitions.
111 Therefore, this document is a substantial revision to [RFC7719].
113 The terms are organized loosely by topic. Some definitions are for
114 new terms for things that are commonly talked about in the DNS
115 community but that never had terms defined for them.
117 Other organizations sometimes define DNS-related terms their own way.
118 For example, the W3C defines "domain" at
119 . The Root Server System Advisory
120 Committee (RSSAC) has a good lexicon [RSSAC026].
122 Note that there is no single consistent definition of "the DNS". It
123 can be considered to be some combination of the following: a commonly
124 used naming scheme for objects on the Internet; a distributed
125 database representing the names and certain properties of these
126 objects; an architecture providing distributed maintenance,
127 resilience, and loose coherency for this database; and a simple
128 query-response protocol (as mentioned below) implementing this
129 architecture. Section 2 defines "global DNS" and "private DNS" as a
130 way to deal with these differing definitions.
132 Capitalization in DNS terms is often inconsistent among RFCs and
133 various DNS practitioners. The capitalization used in this document
134 is a best guess at current practices, and is not meant to indicate
135 that other capitalization styles are wrong or archaic. In some
136 cases, multiple styles of capitalization are used for the same term
137 due to quoting from different RFCs.
139 2. Names
141 Naming system: A naming system associates names with data. Naming
142 systems have many significant facets that help differentiate them.
143 Some commonly-identified facets include:
145 * Composition of names
147 * Format of names
149 * Administration of names
151 * Types of data that can be associated with names
153 * Types of metadata for names
155 * Protocol for getting data from a name
157 * Context for resolving a name
159 Note that this list is a small subset of facets that people have
160 identified over time for naming systems, and the IETF has yet to
161 agree on a good set of facets that can be used to compare naming
162 systems. For example, other facets might include "protocol to
163 update data in a name", "privacy of names", and "privacy of data
164 associated with names", but those are not as well-defined as the
165 ones listed above. The list here is chosen because it helps
166 describe the DNS and naming systems similar to the DNS.
168 Domain name: An ordered list of one or more labels.
170 Note that this is a definition independent of the DNS RFCs, and
171 the definition here also applies to systems other than the DNS.
172 [RFC1034] defines the "domain name space" using mathematical trees
173 and their nodes in graph theory, and this definition has the same
174 practical result as the definition here. Using graph theory, a
175 domain name is a list of labels identifying a portion along one
176 edge of a directed acyclic graph. A domain name can be relative
177 to parts of the tree, or it can be fully qualified (in which case,
178 it begins at the common root of the graph).
180 Also note that different IETF and non-IETF documents have used the
181 term "domain name" in many different ways. It is common for
182 earlier documents to use "domain name" to mean "names that match
183 the syntax in [RFC1035]", but possibly with additional rules such
184 as "and are, or will be, resolvable in the global DNS" or "but
185 only using the presentation format".
187 Label: An ordered list of zero or more octets and which makes up a
188 portion of a domain name. Using graph theory, a label identifies
189 one node in a portion of the graph of all possible domain names.
191 Global DNS: Using the short set of facets listed in "Naming system",
192 the global DNS can be defined as follows. Most of the rules here
193 come from [RFC1034] and [RFC1035], although the term "global DNS"
194 has not been defined before now.
196 Composition of names -- A name in the global DNS has one or more
197 labels. The length of each label is between 0 and 63 octets
198 inclusive. In a fully-qualified domain name, the first label in
199 the ordered list is 0 octets long; it is the only label whose
200 length may be 0 octets, and it is called the "root" or "root
201 label". A domain name in the global DNS has a maximum total
202 length of 255 octets in the wire format; the root represents one
203 octet for this calculation. (Multicast DNS [RFC6762] allows names
204 up to 255 bytes plus a terminating zero byte based on a different
205 interpretation of RFC 1035 and what is included in the 255
206 octets.)
208 Format of names -- Names in the global DNS are domain names.
209 There are three formats: wire format, presentation format, and
210 common display.
212 The basic wire format for names in the global DNS is a list of
213 labels ordered by decreasing distance from the root, with the root
214 label last. Each label is preceded by a length octet. [RFC1035]
215 also defines a compression scheme that modifies this format.
217 The presentation format for names in the global DNS is a list of
218 labels ordered by decreasing distance from the root, encoded as
219 ASCII, with a "." character between each label. In presentation
220 format, a fully-qualified domain name includes the root label and
221 the associated separator dot. For example, in presentation
222 format, a fully-qualified domain name with two non-root labels is
223 always shown as "example.tld." instead of "example.tld".
224 [RFC1035] defines a method for showing octets that do not display
225 in ASCII.
227 The common display format is used in applications and free text.
228 It is the same as the presentation format, but showing the root
229 label and the "." before it is optional and is rarely done. For
230 example, in common display format, a fully-qualified domain name
231 with two non-root labels is usually shown as "example.tld" instead
232 of "example.tld.". Names in the common display format are
233 normally written such that the directionality of the writing
234 system presents labels by decreasing distance from the root (so,
235 in both English and C the root or TLD label in the ordered list is
236 right-most; but in Arabic it may be left-most, depending on local
237 conventions).
239 Administration of names -- Administration is specified by
240 delegation (see the definition of "delegation" in Section 7).
242 Policies for administration of the root zone in the global DNS are
243 determined by the names operational community, which convenes
244 itself in the Internet Corporation for Assigned Names and Numbers
245 (ICANN). The names operational community selects the IANA
246 Functions Operator for the global DNS root zone. At the time this
247 document is published, that operator is Public Technical
248 Identifiers (PTI). The name servers that serve the root zone are
249 provided by independent root operators. Other zones in the global
250 DNS have their own policies for administration.
252 Types of data that can be associated with names -- A name can have
253 zero or more resource records associated with it. There are
254 numerous types of resource records with unique data structures
255 defined in many different RFCs and in the IANA registry at
256 [IANA_Resource_Registry].
258 Types of metadata for names -- Any name that is published in the
259 DNS appears as a set of resource records (see the definition of
260 "RRset" in Section 5). Some names do not themselves have data
261 associated with them in the DNS, but "appear" in the DNS anyway
262 because they form part of a longer name that does have data
263 associated with it (see the definition of "empty non-terminals" in
264 Section 7).
266 Protocol for getting data from a name -- The protocol described in
267 [RFC1035].
269 Context for resolving a name -- The global DNS root zone
270 distributed by PTI.
272 Private DNS: Names that use the protocol described in [RFC1035] but
273 that do not rely on the global DNS root zone, or names that are
274 otherwise not generally available on the Internet but are using
275 the protocol described in [RFC1035]. A system can use both the
276 global DNS and one or more private DNS systems; for example, see
277 "Split DNS" in Section 6.
279 Note that domain names that do not appear in the DNS, and that are
280 intended never to be looked up using the DNS protocol, are not
281 part of the global DNS or a private DNS even though they are
282 domain names.
284 Multicast DNS: "Multicast DNS (mDNS) provides the ability to perform
285 DNS-like operations on the local link in the absence of any
286 conventional Unicast DNS server. In addition, Multicast DNS
287 designates a portion of the DNS namespace to be free for local
288 use, without the need to pay any annual fee, and without the need
289 to set up delegations or otherwise configure a conventional DNS
290 server to answer for those names." Quoted from [RFC6762],
291 Abstract) Although it uses a compatible wire format, mDNS is
292 strictly speaking a different protocol than DNS. Also, where the
293 above quote says "a portion of the DNS namespace", it would be
294 clearer to say "a portion of the domain name space" The names in
295 mDNS are not intended to be looked up in the DNS.
297 Locally served DNS zone: A locally served DNS zone is a special case
298 of private DNS. Names are resolved using the DNS protocol in a
299 local context. [RFC6303] defines subdomains of IN-ADDR.ARPA that
300 are locally served zones. Resolution of names through locally
301 served zones may result in ambiguous results. For example, the
302 same name may resolve to different results in different locally
303 served DNS zone contexts. The context through which a locally
304 served zone may be explicit, for example, as defined in [RFC6303],
305 or implicit, as defined by local DNS administration and not known
306 to the resolution client.
308 Fully qualified domain name (FQDN): This is often just a clear way
309 of saying the same thing as "domain name of a node", as outlined
310 above. However, the term is ambiguous. Strictly speaking, a
311 fully qualified domain name would include every label, including
312 the zero-length label of the root: such a name would be written
313 "www.example.net." (note the terminating dot). But because every
314 name eventually shares the common root, names are often written
315 relative to the root (such as "www.example.net") and are still
316 called "fully qualified". This term first appeared in [RFC0819].
317 In this document, names are often written relative to the root.
319 The need for the term "fully qualified domain name" comes from the
320 existence of partially qualified domain names, which are names
321 where one or more of the earliest labels in the ordered list are
322 omitted (for example, a name of "www" derived from
323 "www.example.net"). Such relative names are understood only by
324 context.
326 Host name: This term and its equivalent, "hostname", have been
327 widely used but are not defined in [RFC1034], [RFC1035],
328 [RFC1123], or [RFC2181]. The DNS was originally deployed into the
329 Host Tables environment as outlined in [RFC0952], and it is likely
330 that the term followed informally from the definition there. Over
331 time, the definition seems to have shifted. "Host name" is often
332 meant to be a domain name that follows the rules in Section 3.5 of
333 [RFC1034], the "preferred name syntax" (that is, every character
334 in each label are a letter, a digit, or a hyphen). Note that any
335 label in a domain name can contain any octet value; hostnames are
336 generally considered to be domain names where every label follows
337 the rules in the "preferred name syntax", with the amendment that
338 labels can start with ASCII digits (this amendment comes from
339 Section 2.1 of [RFC1123]).
341 People also sometimes use the term hostname to refer to just the
342 first label of an FQDN, such as "printer" in
343 "printer.admin.example.com". (Sometimes this is formalized in
344 configuration in operating systems.) In addition, people
345 sometimes use this term to describe any name that refers to a
346 machine, and those might include labels that do not conform to the
347 "preferred name syntax".
349 TLD: A Top-Level Domain, meaning a zone that is one layer below the
350 root, such as "com" or "jp". There is nothing special, from the
351 point of view of the DNS, about TLDs. Most of them are also
352 delegation-centric zones (defined in Section 7, and there are
353 significant policy issues around their operation. TLDs are often
354 divided into sub-groups such as Country Code Top-Level Domains
355 (ccTLDs), Generic Top-Level Domains (gTLDs), and others; the
356 division is a matter of policy, and beyond the scope of this
357 document.
359 IDN: The common abbreviation for "Internationalized Domain Name".
360 The IDNA protocol is the standard mechanism for handling domain
361 names with non-ASCII characters in applications in the DNS. The
362 current standard, normally called "IDNA2008", is defined in
363 [RFC5890], [RFC5891], [RFC5892], [RFC5893], and [RFC5894]. These
364 documents define many IDN-specific terms such as "LDH label",
365 "A-label", and "U-label". [RFC6365] defines more terms that
366 relate to internationalization (some of which relate to IDNs), and
367 [RFC6055] has a much more extensive discussion of IDNs, including
368 some new terminology.
370 Subdomain: "A domain is a subdomain of another domain if it is
371 contained within that domain. This relationship can be tested by
372 seeing if the subdomain's name ends with the containing domain's
373 name." (Quoted from [RFC1034], Section 3.1). For example, in the
374 host name "nnn.mmm.example.com", both "mmm.example.com" and
375 "nnn.mmm.example.com" are subdomains of "example.com".
377 Alias: The owner of a CNAME resource record, or a subdomain of the
378 owner of a DNAME resource record. See also "canonical name".
380 Canonical name: A CNAME resource record "identifies its owner name
381 as an alias, and specifies the corresponding canonical name in the
382 RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2)
383 This usage of the word "canonical" is related to the mathematical
384 concept of "canonical form".
386 CNAME: "It is traditional to refer to the owner of a CNAME record as
387 'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of
388 'canonical name', and the owner of a CNAME record is an alias, not
389 a canonical name." (Quoted from [RFC2181], Section 10.1.1)
391 3. DNS Response Codes
393 Some of response codes that are defined in [RFC1035] have acquired
394 their own shorthand names. All of the RCODEs are listed at
395 [IANA_Resource_Registry], although that site uses mixed-case
396 capitalization, while most documents use all-caps. Some of the
397 common names are described here, but the official list is in the IANA
398 registry.
400 NOERROR: "No error condition" (Quoted from [RFC1035],
401 Section 4.1.1.)
403 FORMERR: "Format error - The name server was unable to interpret the
404 query." (Quoted from [RFC1035], Section 4.1.1.)
406 SERVFAIL: "Server failure - The name server was unable to process
407 this query due to a problem with the name server." (Quoted from
408 [RFC1035], Section 4.1.1.)
410 NXDOMAIN: "Name Error - Meaningful only for responses from an
411 authoritative name server, this code signifies that the domain
412 name referenced in the query does not exist." (Quoted from
413 [RFC1035], Section 4.1.1.) [RFC2308] established NXDOMAIN as a
414 synonym for Name Error.
416 NOTIMP: "Not Implemented - The name server does not support the
417 requested kind of query." (Quoted from [RFC1035], Section 4.1.1.)
419 REFUSED: "Refused - The name server refuses to perform the specified
420 operation for policy reasons. For example, a name server may not
421 wish to provide the information to the particular requester, or a
422 name server may not wish to perform a particular operation (e.g.,
423 zone transfer) for particular data." (Quoted from [RFC1035],
424 Section 4.1.1.)
426 NODATA: "A pseudo RCODE which indicates that the name is valid for
427 the given class, but there are no records of the given type. A
428 NODATA response has to be inferred from the answer." (Quoted from
429 [RFC2308], Section 1.) "NODATA is indicated by an answer with the
430 RCODE set to NOERROR and no relevant answers in the answer
431 section. The authority section will contain an SOA record, or
432 there will be no NS records there." (Quoted from [RFC2308],
433 Section 2.2.) Note that referrals have a similar format to NODATA
434 replies; [RFC2308] explains how to distinguish them.
436 The term "NXRRSET" is sometimes used as a synonym for NODATA.
437 However, this is a mistake, given that NXRRSET is a specific error
438 code defined in [RFC2136].
440 Negative response: A response that indicates that a particular RRset
441 does not exist, or whose RCODE indicates the nameserver cannot
442 answer. Sections 2 and 7 of [RFC2308] describe the types of
443 negative responses in detail.
445 4. DNS Transactions
447 The header of a DNS message is its first 12 octets. Many of the
448 fields and flags in the header diagram in Sections 4.1.1 through
449 4.1.3 of [RFC1035] are referred to by their names in that diagram.
450 For example, the response codes are called "RCODEs", the data for a
451 record is called the "RDATA", and the authoritative answer bit is
452 often called "the AA flag" or "the AA bit".
454 QNAME: The most commonly-used rough definition is that the QNAME is
455 a field in the Question section of a query. "A standard query
456 specifies a target domain name (QNAME), query type (QTYPE), and
457 query class (QCLASS) and asks for RRs which match." (Quoted from
458 [RFC1034], Section 3.7.1.). Strictly speaking, the definition
459 comes from [RFC1035], Section 4.1.2, where the QNAME is defined in
460 respect of the Question Section. This definition appears to be
461 applied consistently: the discussion of inverse queries in section
462 6.4 refers to the "owner name of the query RR and its TTL",
463 because inverse queries populate the Answer Section and leave the
464 Question Section empty. (Inverse queries are deprecated in
465 [RFC3425], and so relevant definitions do not appear in this
466 document.)
468 [RFC2308], however, has an alternate definition that puts the
469 QNAME in the answer (or series of answers) instead of the query.
470 It defines QNAME as: "...the name in the query section of an
471 answer, or where this resolves to a CNAME, or CNAME chain, the
472 data field of the last CNAME. The last CNAME in this sense is
473 that which contains a value which does not resolve to another
474 CNAME." This definition has a certain internal logic, because of
475 the way CNAME substitution works and the definition of CNAME. If
476 a name server does not find an RRset that matches a query, but it
477 finds the same name in the same class with a CNAME record, then
478 the name server "includes the CNAME record in the response and
479 restarts the query at the domain name specified in the data field
480 of the CNAME record." ([RFC1034] Section 3.6.2). This is made
481 explicit in the resolution algorithm outlined in Section 4.3.2 of
482 [RFC1034], which says to "change QNAME to the canonical name in
483 the CNAME RR, and go back to step 1" in the case of a CNAME RR.
484 Since a CNAME record explicitly declares that the owner name is
485 canonically named what is in the RDATA, then there is a way to
486 view the new name (i.e. the name that was in the RDATA of the
487 CNAME RR) as also being the QNAME.
489 This creates a kind of confusion, however, because the answer to a
490 query that results in CNAME processing contains in the echoed
491 Question Section one QNAME (the name in the original query), and a
492 second QNAME that is in the data field of the last CNAME. The
493 confusion comes from the iterative/recursive mode of resolution,
494 which finally returns an answer that need not actually have the
495 same owner name as the QNAME contained in the original query.
497 To address this potential confusion, it is helpful to distinguish
498 between three meanings:
500 * QNAME (original): The name actually sent in the Question
501 Section in the original query, which is always echoed in the
502 (final) reply in the Question Section when the QR bit is set to
503 1.
505 * QNAME (effective): A name actually resolved, which is either
506 the name originally queried, or a name received in a CNAME
507 chain response.
509 * QNAME (final): The name actually resolved, which is either the
510 name actually queried or else the last name in a CNAME chain
511 response.
513 Note that, because the definition in [RFC2308] is actually for a
514 different concept than what was in [RFC1034], it would have be
515 better if [RFC2308] had used a different name for that concept.
516 In general use today, QNAME almost always means what is defined
517 above as "QNAME (original)".
519 Referrals: A type of response in which a server, signaling that it
520 is not (completely) authoritative for an answer, provides the
521 querying resolver with an alternative place to send its query.
522 Referrals can be partial.
524 A referral arises when a server is not performing recursive
525 service while answering a query. It appears in step 3(b) of the
526 algorithm in [RFC1034], Section 4.3.2.
528 There are two types of referral response. The first is a downward
529 referral (sometimes described as "delegation response"), where the
530 server is authoritative for some portion of the QNAME. The
531 authority section RRset's RDATA contains the name servers
532 specified at the referred-to zone cut. In normal DNS operation,
533 this kind of response is required in order to find names beneath a
534 delegation. The bare use of "referral" means this kind of
535 referral, and many people believe that this is the only legitimate
536 kind of referral in the DNS.
538 The second is an upward referral (sometimes described as "root
539 referral"), where the server is not authoritative for any portion
540 of the QNAME. When this happens, the referred-to zone in the
541 authority section is usually the root zone (.). In normal DNS
542 operation, this kind of response is not required for resolution or
543 for correctly answering any query. There is no requirement that
544 any server send upward referrals. Some people regard upward
545 referrals as a sign of a misconfiguration or error. Upward
546 referrals always need some sort of qualifier (such as "upward" or
547 "root"), and are never identified by the bare word "referral".
549 A response that has only a referral contains an empty answer
550 section. It contains the NS RRset for the referred-to zone in the
551 authority section. It may contain RRs that provide addresses in
552 the additional section. The AA bit is clear.
554 In the case where the query matches an alias, and the server is
555 not authoritative for the target of the alias but it is
556 authoritative for some name above the target of the alias, the
557 resolution algorithm will produce a response that contains both
558 the authoritative answer for the alias, and also a referral. Such
559 a partial answer and referral response has data in the answer
560 section. It has the NS RRset for the referred-to zone in the
561 authority section. It may contain RRs that provide addresses in
562 the additional section. The AA bit is set, because the first name
563 in the answer section matches the QNAME and the server is
564 authoritative for that answer (see [RFC1035], Section 4.1.1).
566 5. Resource Records
568 RR: An acronym for resource record. ([RFC1034], Section 3.6.)
570 RRset: A set of resource records with the same label, class and
571 type, but with different data. (Definition from [RFC2181]) Also
572 spelled RRSet in some documents. As a clarification, "same label"
573 in this definition means "same owner name". In addition,
574 [RFC2181] states that "the TTLs of all RRs in an RRSet must be the
575 same".
577 Note that RRSIG resource records do not match this definition.
578 [RFC4035] says: "An RRset MAY have multiple RRSIG RRs associated
579 with it. Note that as RRSIG RRs are closely tied to the RRsets
580 whose signatures they contain, RRSIG RRs, unlike all other DNS RR
581 types, do not form RRsets. In particular, the TTL values among
582 RRSIG RRs with a common owner name do not follow the RRset rules
583 described in [RFC2181]."
585 Master file: "Master files are text files that contain RRs in text
586 form. Since the contents of a zone can be expressed in the form
587 of a list of RRs a master file is most often used to define a
588 zone, though it can be used to list a cache's contents."
589 ([RFC1035], Section 5.) Master files are sometimes called "zone
590 files".
592 Presentation format: The text format used in master files. This
593 format is shown but not formally defined in [RFC1034] and
594 [RFC1035]. The term "presentation format" first appears in
595 [RFC4034].
597 EDNS: The extension mechanisms for DNS, defined in [RFC6891].
598 Sometimes called "EDNS0" or "EDNS(0)" to indicate the version
599 number. EDNS allows DNS clients and servers to specify message
600 sizes larger than the original 512 octet limit, to expand the
601 response code space, and potentially to carry additional options
602 that affect the handling of a DNS query.
604 OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to
605 contain control information pertaining to the question-and-answer
606 sequence of a specific transaction. (Definition from [RFC6891],
607 Section 6.1.1) It is used by EDNS.
609 Owner: The domain name where a RR is found ([RFC1034], Section 3.6).
610 Often appears in the term "owner name".
612 SOA field names: DNS documents, including the definitions here,
613 often refer to the fields in the RDATA of an SOA resource record
614 by field name. Those fields are defined in Section 3.3.13 of
615 [RFC1035]. The names (in the order they appear in the SOA RDATA)
616 are MNAME, RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM.
617 Note that the meaning of MINIMUM field is updated in Section 4 of
618 [RFC2308]; the new definition is that the MINIMUM field is only
619 "the TTL to be used for negative responses". This document tends
620 to use field names instead of terms that describe the fields.
622 TTL: The maximum "time to live" of a resource record. "A TTL value
623 is an unsigned number, with a minimum value of 0, and a maximum
624 value of 2147483647. That is, a maximum of 2^31 - 1. When
625 transmitted, the TTL is encoded in the less significant 31 bits of
626 the 32 bit TTL field, with the most significant, or sign, bit set
627 to zero." (Quoted from [RFC2181], Section 8) (Note that [RFC1035]
628 erroneously stated that this is a signed integer; that was fixed
629 by [RFC2181].)
631 The TTL "specifies the time interval that the resource record may
632 be cached before the source of the information should again be
633 consulted". (Quoted from [RFC1035], Section 3.2.1) Also: "the
634 time interval (in seconds) that the resource record may be cached
635 before it should be discarded". (Quoted from [RFC1035],
636 Section 4.1.3). Despite being defined for a resource record, the
637 TTL of every resource record in an RRset is required to be the
638 same ([RFC2181], Section 5.2).
640 The reason that the TTL is the maximum time to live is that a
641 cache operator might decide to shorten the time to live for
642 operational purposes, such as if there is a policy to disallow TTL
643 values over a certain number. Some servers are known to ignore
644 the TTL on some RRsets (such as when the authoritative data has a
645 very short TTL) even though this is against the advice in RFC
646 1035. An RRset can be flushed from the cache before the end of
647 the TTL interval, at which point the value of the TTL becomes
648 unknown because the RRset with which it was associated no longer
649 exists.
651 There is also the concept of a "default TTL" for a zone, which can
652 be a configuration parameter in the server software. This is
653 often expressed by a default for the entire server, and a default
654 for a zone using the $TTL directive in a zone file. The $TTL
655 directive was added to the master file format by [RFC2308].
657 Class independent: A resource record type whose syntax and semantics
658 are the same for every DNS class. A resource record type that is
659 not class independent has different meanings depending on the DNS
660 class of the record, or the meaning is undefined for some class.
661 Most resorece record types are defined for class 1 (IN, the
662 Internet), but many are undefined for other classes.
664 Address records: Records whose type is A or AAAA. [RFC2181]
665 informally defines these as "(A, AAAA, etc)". Note that new types
666 of address records could be defined in the future.
668 6. DNS Servers and Clients
670 This section defines the terms used for the systems that act as DNS
671 clients, DNS servers, or both. In the RFCs, DNS servers are
672 sometimes called "name servers", "nameservers", or just "servers".
674 There is no formal definition of DNS server, but the RFCs generally
675 assume that it is an Internet server that listens for queries and
676 sends responses using the DNS protocol defined in [RFC1035] and its
677 successors.
679 It is important to note that the terms "DNS server" and "name server"
680 require context in order to understand the services being provided.
681 Both authoritative servers and recursive resolvers are often called
682 "DNS servers" and "name servers" even though they serve different
683 roles (and may be part of the same software package).
685 For terminology specific to the public DNS root server system, see
686 [RSSAC026]. That document defines terms such as "root server", "root
687 server operator", and terms that are specific to the way that the
688 root zone of the public DNS is served.
690 Resolver: A program "that extract[s] information from name servers
691 in response to client requests." (Quoted from [RFC1034],
692 Section 2.4) "The resolver is located on the same machine as the
693 program that requests the resolver's services, but it may need to
694 consult name servers on other hosts." (Quoted from [RFC1034],
695 Section 5.1) A resolver performs queries for a name, type, and
696 class, and receives answers. The logical function is called
697 "resolution". In practice, the term is usually referring to some
698 specific type of resolver (some of which are defined below), and
699 understanding the use of the term depends on understanding the
700 context.
702 A related term is "resolve", which is not formally defined in
703 [RFC1034] or [RFC1035]. An imputed definition might be "asking a
704 question that consists of a domain name, class, and type, and
705 receiving some sort of answer". Similarly, an imputed definition
706 of "resolution" might be "the answer received from resolving".
708 Stub resolver: A resolver that cannot perform all resolution itself.
709 Stub resolvers generally depend on a recursive resolver to
710 undertake the actual resolution function. Stub resolvers are
711 discussed but never fully defined in Section 5.3.1 of [RFC1034].
712 They are fully defined in Section 6.1.3.1 of [RFC1123].
714 Iterative mode: A resolution mode of a server that receives DNS
715 queries and responds with a referral to another server.
716 Section 2.3 of [RFC1034] describes this as "The server refers the
717 client to another server and lets the client pursue the query". A
718 resolver that works in iterative mode is sometimes called an
719 "iterative resolver". See also "iterative resolution" later in
720 this section.
722 Recursive mode: A resolution mode of a server that receives DNS
723 queries and either responds to those queries from a local cache or
724 sends queries to other servers in order to get the final answers
725 to the original queries. Section 2.3 of [RFC1034] describes this
726 as "The first server pursues the query for the client at another
727 server". Section 4.3.1 of of [RFC1034] says "in \[recursive\]
728 mode the name server acts in the role of a resolver and returns
729 either an error or the answer, but never referrals." That same
730 section also says "The recursive mode occurs when a query with RD
731 set arrives at a server which is willing to provide recursive
732 service; the client can verify that recursive mode was used by
733 checking that both RA and RD are set in the reply."
735 A server operating in recursive mode may be thought of as having a
736 name server side (which is what answers the query) and a resolver
737 side (which performs the resolution function). Systems operating
738 in this mode are commonly called "recursive servers". Sometimes
739 they are called "recursive resolvers". While strictly the
740 difference between these is that one of them sends queries to
741 another recursive server and the other does not, in practice it is
742 not possible to know in advance whether the server that one is
743 querying will also perform recursion; both terms can be observed
744 in use interchangeably.
746 Recursive resolver: A resolver that acts in recursive mode. In
747 general, a recursive resolver is expected to cache the answers it
748 receives (which would make it a full-service resolver), but some
749 recursive resolvers might not cache.
751 [RFC4697] tried to differentiate between a recursive resolver and
752 an iterative resolver.
754 Recursive query: A query with the Recursion Desired (RD) bit set to
755 1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive
756 service is available and is requested by the RD bit in the query,
757 the server uses its resolver to answer the query. (See
758 Section 4.3.2 of [RFC1035].)
760 Non-recursive query: A query with the Recursion Desired (RD) bit set
761 to 0 in the header. A server can answer non-recursive queries
762 using only local information: the response contains either an
763 error, the answer, or a referral to some other server "closer" to
764 the answer. (See Section 4.3.1 of [RFC1035].)
766 Iterative resolution: A name server may be presented with a query
767 that can only be answered by some other server. The two general
768 approaches to dealing with this problem are "recursive", in which
769 the first server pursues the query on behalf of the client at
770 another server, and "iterative", in which the server refers the
771 client to another server and lets the client pursue the query
772 there. (See Section 2.3 of [RFC1034].)
774 In iterative resolution, the client repeatedly makes non-recursive
775 queries and follows referrals and/or aliases. The iterative
776 resolution algorithm is described in Section 5.3.3 of [RFC1034].
778 Full resolver: This term is used in [RFC1035], but it is not defined
779 there. RFC 1123 defines a "full-service resolver" that may or may
780 not be what was intended by "full resolver" in [RFC1035]. This
781 term is not properly defined in any RFC.
783 Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this
784 term to mean a resolver that acts in recursive mode with a cache
785 (and meets other requirements).
787 Priming: "The act of finding the list of root servers from a
788 configuration that lists some or all of the purported IP addresses
789 of some or all of those root servers." (Quoted from [RFC8109],
790 Section 2.) In order to operate in recursive mode, a resolver
791 needs to know the address of at least one root server. Priming is
792 most often done from a configuration setting that contains a list
793 of authoritative servers for the root zone.
795 Root hints: "Operators who manage a DNS recursive resolver typically
796 need to configure a 'root hints file'. This file contains the
797 names and IP addresses of the authoritative name servers for the
798 root zone, so the software can bootstrap the DNS resolution
799 process. For many pieces of software, this list comes built into
800 the software." (Quoted from [IANA_RootFiles]) This file is often
801 used in priming.
803 Negative caching: "The storage of knowledge that something does not
804 exist, cannot give an answer, or does not give an answer."
805 (Quoted from [RFC2308], Section 1)
807 Authoritative server: "A server that knows the content of a DNS zone
808 from local knowledge, and thus can answer queries about that zone
809 without needing to query other servers." (Quoted from [RFC2182],
810 Section 2.) It is a system that responds to DNS queries with
811 information about zones for which it has been configured to answer
812 with the AA flag in the response header set to 1. It is a server
813 that has authority over one or more DNS zones. Note that it is
814 possible for an authoritative server to respond to a query without
815 the parent zone delegating authority to that server.
816 Authoritative servers also provide "referrals", usually to child
817 zones delegated from them; these referrals have the AA bit set to
818 0 and come with referral data in the Authority and (if needed) the
819 Additional sections.
821 Authoritative-only server: A name server that only serves
822 authoritative data and ignores requests for recursion. It will
823 "not normally generate any queries of its own. Instead, it
824 answers non-recursive queries from iterative resolvers looking for
825 information in zones it serves." (Quoted from [RFC4697],
826 Section 2.4)
828 Zone transfer: The act of a client requesting a copy of a zone and
829 an authoritative server sending the needed information. (See
830 Section 7 for a description of zones.) There are two common
831 standard ways to do zone transfers: the AXFR ("Authoritative
832 Transfer") mechanism to copy the full zone (described in
833 [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy
834 only parts of the zone that have changed (described in [RFC1995]).
835 Many systems use non-standard methods for zone transfer outside
836 the DNS protocol.
838 Secondary server: "An authoritative server which uses zone transfer
839 to retrieve the zone" (Quoted from [RFC1996], Section 2.1).
840 Secondary servers are also discussed in [RFC1034]. [RFC2182]
841 describes secondary servers in more detail. Although early DNS
842 RFCs such as [RFC1996] referred to this as a "slave", the current
843 common usage has shifted to calling it a "secondary".
845 Slave server: See secondary server.
847 Primary server: "Any authoritative server configured to be the
848 source of zone transfer for one or more [secondary] servers"
849 (Quoted from [RFC1996], Section 2.1) or, more specifically, "an
850 authoritative server configured to be the source of AXFR or IXFR
851 data for one or more [secondary] servers" (Quoted from [RFC2136]).
852 Primary servers are also discussed in [RFC1034]. Although early
853 DNS RFCs such as [RFC1996] referred to this as a "master", the
854 current common usage has shifted to "primary".
856 Master server: See primary server.
858 Primary master: "The primary master is named in the zone's SOA MNAME
859 field and optionally by an NS RR". (Quoted from [RFC1996],
860 Section 2.1). [RFC2136] defines "primary master" as "Master
861 server at the root of the AXFR/IXFR dependency graph. The primary
862 master is named in the zone's SOA MNAME field and optionally by an
863 NS RR. There is by definition only one primary master server per
864 zone." The idea of a primary master is only used by [RFC2136],
865 and is considered archaic in other parts of the DNS.
867 The idea of a primary master is only used in [RFC1996] and
868 [RFC2136]. A modern interpretation of the term "primary master"
869 is a server that is both authoritative for a zone and that gets
870 its updates to the zone from configuration (such as a master file)
871 or from UPDATE transactions.
873 Stealth server: This is "like a slave server except not listed in an
874 NS RR for the zone." (Quoted from [RFC1996], Section 2.1)
876 Hidden master: A stealth server that is a primary server for zone
877 transfers. "In this arrangement, the master name server that
878 processes the updates is unavailable to general hosts on the
879 Internet; it is not listed in the NS RRset." (Quoted from
880 [RFC6781], Section 3.4.3). An earlier RFC, [RFC4641], said that
881 the hidden master's name "appears in the SOA RRs MNAME field",
882 although in some setups, the name does not appear at all in the
883 public DNS. A hidden master can also be a secondary server for
884 the zone itself.
886 Forwarding: The process of one server sending a DNS query with the
887 RD bit set to 1 to another server to resolve that query.
888 Forwarding is a function of a DNS resolver; it is different than
889 simply blindly relaying queries.
891 [RFC5625] does not give a specific definition for forwarding, but
892 describes in detail what features a system that forwards needs to
893 support. Systems that forward are sometimes called "DNS proxies",
894 but that term has not yet been defined (even in [RFC5625]).
896 Forwarder: Section 1 of [RFC2308] describes a forwarder as "a
897 nameserver used to resolve queries instead of directly using the
898 authoritative nameserver chain". [RFC2308] further says "The
899 forwarder typically either has better access to the internet, or
900 maintains a bigger cache which may be shared amongst many
901 resolvers." That definition appears to suggest that forwarders
902 normally only query authoritative servers. In current use,
903 however, forwarders often stand between stub resolvers and
904 recursive servers. [RFC2308] is silent on whether a forwarder is
905 iterative-only or can be a full-service resolver.
907 Policy-implementing resolver: A resolver acting in recursive mode
908 that changes some of the answers that it returns based on policy
909 criteria, such as to prevent access to malware sites or
910 objectionable content. In general, a stub resolver has no idea
911 whether upstream resolvers implement such policy or, if they do,
912 the exact policy about what changes will be made. In some cases,
913 the user of the stub resolver has selected the policy-implementing
914 resolver with the explicit intention of using it to implement the
915 policies. In other cases, policies are imposed without the user
916 of the stub resolver being informed.
918 Open resolver: A full-service resolver that accepts and processes
919 queries from any (or nearly any) stub resolver. This is sometimes
920 also called a "public resolver", although the term "public
921 resolver" is used more with open resolvers that are meant to be
922 open, as compared to the vast majority of open resolvers that are
923 probably misconfigured to be open. Open resolvers are discussed
924 in [RFC5358]
926 Split DNS: The terms "split DNS" and "split-horizon DNS" have long
927 been used in the DNS community without formal definition. In
928 general, they refer to situations in which DNS servers that are
929 authoritative for a particular set of domains provide partly or
930 completely different answers in those domains depending on the
931 source of the query. The effect of this is that a domain name
932 that is notionally globally unique nevertheless has different
933 meanings for different network users. This can sometimes be the
934 result of a "view" configuration, described below.
936 [RFC2775], Section 3.8 gives a related definition that is too
937 specific to be generally useful.
939 View: A configuration for a DNS server that allows it to provide
940 different answers depending on attributes of the query, such as
941 for "split DNS". Typically, views differ by the source IP address
942 of a query, but can also be based on the destination IP address,
943 the type of query (such as AXFR), whether it is recursive, and so
944 on. Views are often used to provide more names or different
945 addresses to queries from "inside" a protected network than to
946 those "outside" that network. Views are not a standardized part
947 of the DNS, but they are widely implemented in server software.
949 Passive DNS: A mechanism to collect DNS data by storing DNS
950 responses from name servers. Some of these systems also collect
951 the DNS queries associated with the responses, although doing so
952 raises some privacy concerns. Passive DNS databases can be used
953 to answer historical questions about DNS zones such as which
954 values were present at a given time in the past, or when a name
955 was spotted first. Passive DNS databases allow searching of the
956 stored records on keys other than just the name and type, such as
957 "find all names which have A records of a particular value".
959 Anycast: "The practice of making a particular service address
960 available in multiple, discrete, autonomous locations, such that
961 datagrams sent are routed to one of several available locations."
962 (Quoted from [RFC4786], Section 2) See [RFC4786] for more detail
963 on Anycast and other terms that are specific to its use.
965 Instance: "When anycast routing is used to allow more than one
966 server to have the same IP address, each one of those servers is
967 commonly referred to as an 'instance'." "An instance of a server,
968 such as a root server, is often referred to as an 'Anycast
969 instance'." (Quoted from [RSSAC026])
971 Privacy-enabling DNS server: "A DNS server that implements DNS over
972 TLS [RFC7858] and may optionally implement DNS over DTLS
973 [RFC8094]." (Quoted from [RFC8310], Section 2)
975 7. Zones
977 This section defines terms that are used when discussing zones that
978 are being served or retrieved.
980 Zone: "Authoritative information is organized into units called
981 'zones', and these zones can be automatically distributed to the
982 name servers which provide redundant service for the data in a
983 zone." (Quoted from [RFC1034], Section 2.4)
985 Child: "The entity on record that has the delegation of the domain
986 from the Parent." (Quoted from [RFC7344], Section 1.1)
988 Parent: "The domain in which the Child is registered." (Quoted from
989 [RFC7344], Section 1.1) Earlier, "parent name server" was defined
990 in [RFC0882] as "the name server that has authority over the place
991 in the domain name space that will hold the new domain". (Note
992 that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].)
993 [RFC0819] also has some description of the relationship between
994 parents and children.
996 Origin:
998 (a) "The domain name that appears at the top of a zone (just below
999 the cut that separates the zone from its parent). The name of the
1000 zone is the same as the name of the domain at the zone's origin."
1001 (Quoted from [RFC2181], Section 6.) These days, this sense of
1002 "origin" and "apex" (defined below) are often used
1003 interchangeably.
1005 (b) The domain name within which a given relative domain name
1006 appears in zone files. Generally seen in the context of
1007 "$ORIGIN", which is a control entry defined in [RFC1035],
1008 Section 5.1, as part of the master file format. For example, if
1009 the $ORIGIN is set to "example.org.", then a master file line for
1010 "www" is in fact an entry for "www.example.org.".
1012 Apex: The point in the tree at an owner of an SOA and corresponding
1013 authoritative NS RRset. This is also called the "zone apex".
1014 [RFC4033] defines it as "the name at the child's side of a zone
1015 cut". The "apex" can usefully be thought of as a data-theoretic
1016 description of a tree structure, and "origin" is the name of the
1017 same concept when it is implemented in zone files. The
1018 distinction is not always maintained in use, however, and one can
1019 find uses that conflict subtly with this definition. [RFC1034]
1020 uses the term "top node of the zone" as a synonym of "apex", but
1021 that term is not widely used. These days, the first sense of
1022 "origin" (above) and "apex" are often used interchangeably.
1024 Zone cut: The delimitation point between two zones where the origin
1025 of one of the zones is the child of the other zone.
1027 "Zones are delimited by 'zone cuts'. Each zone cut separates a
1028 'child' zone (below the cut) from a 'parent' zone (above the
1029 cut)." (Quoted from [RFC2181], Section 6; note that this is
1030 barely an ostensive definition.) Section 4.2 of [RFC1034] uses
1031 "cuts" instead of "zone cut".
1033 Delegation: The process by which a separate zone is created in the
1034 name space beneath the apex of a given domain. Delegation happens
1035 when an NS RRset is added in the parent zone for the child origin.
1036 Delegation inherently happens at a zone cut. The term is also
1037 commonly a noun: the new zone that is created by the act of
1038 delegating.
1040 Authoritative data: "All of the RRs attached to all of the nodes
1041 from the top node of the zone down to leaf nodes or nodes above
1042 cuts around the bottom edge of the zone." (Quoted from [RFC1034],
1043 Section 4.2.1) Note that this definition might inadvertently also
1044 cause any NS records that appear in the zone to be included, even
1045 those that might not truly be authoritative because there are
1046 identical NS RRs below the zone cut. This reveals the ambiguity
1047 in the notion of authoritative data, because the parent-side NS
1048 records authoritatively indicate the delegation, even though they
1049 are not themselves authoritative data.
1051 Lame delegation: "A lame delegations exists when a nameserver is
1052 delegated responsibility for providing nameservice for a zone (via
1053 NS records) but is not performing nameservice for that zone
1054 (usually because it is not set up as a primary or secondary for
1055 the zone)." (Definition from [RFC1912], Section 2.8)
1057 Glue records: "[Resource records] which are not part of the
1058 authoritative data [of the zone], and are address resource records
1059 for the [name servers in subzones]. These RRs are only necessary
1060 if the name server's name is 'below' the cut, and are only used as
1061 part of a referral response." Without glue "we could be faced
1062 with the situation where the NS RRs tell us that in order to learn
1063 a name server's address, we should contact the server using the
1064 address we wish to learn." (Definition from [RFC1034],
1065 Section 4.2.1)
1067 A later definition is that glue "includes any record in a zone
1068 file that is not properly part of that zone, including nameserver
1069 records of delegated sub-zones (NS records), address records that
1070 accompany those NS records (A, AAAA, etc), and any other stray
1071 data that might appear" ([RFC2181], Section 5.4.1). Although glue
1072 is sometimes used today with this wider definition in mind, the
1073 context surrounding the [RFC2181] definition suggests it is
1074 intended to apply to the use of glue within the document itself
1075 and not necessarily beyond.
1077 Bailiwick: "In-bailiwick" is an adjective to describe a name server
1078 whose name is either a subdomain of or (rarely) the same as the
1079 origin of the zone that contains the delegation to the name
1080 server. In-bailiwick name servers may have glue records in their
1081 parent zone (using the first of the definitions of "glue records"
1082 in the definition above). (The term "bailiwick" means the
1083 district or territory where a bailiff or policeman has
1084 jurisdiction.)
1086 "In-bailiwick" names are divided into two type of name server
1087 names: "in-domain" names and "sibling domain" names.
1089 * In-domain: an adjective to describe a name server whose name is
1090 either subordinate to or (rarely) the same as the owner name of
1091 the NS resource records. An in-domain name server name MUST
1092 have glue records or name resolution fails. For example, a
1093 delegation for "child.example.com" may have "in-domain" name
1094 server name "ns.child.example.com".
1096 * Sibling domain: a name server's name that is either subordinate
1097 to or (rarely) the same as the zone origin and not subordinate
1098 to or the same as the owner name of the NS resource records.
1099 Glue records for sibling domains are allowed, but not
1100 necessary. For example, a delegation for "child.example.com"
1101 in "example.com" zone may have "sibling" name server name
1102 "ns.another.example.com".
1104 "Out-of-bailiwick" is the antonym of in-bailiwick. An adjective
1105 to describe a name server whose name is not subordinate to or the
1106 same as the zone origin. Glue records for out-of-bailiwick name
1107 servers are useless. Following table shows examples of delegation
1108 types.
1110 Delegation |Parent|Name Server Name | Type
1111 -----------+------+------------------+-----------------------------
1112 com | . |a.gtld-servers.net|in-bailiwick / sibling domain
1113 net | . |a.gtld-servers.net|in-bailiwick / in-domain
1114 example.org| org |ns.example.org |in-bailiwick / in-domain
1115 example.org| org |ns.ietf.org |in-bailiwick / sibling domain
1116 example.org| org |ns.example.com |out-of-bailiwick
1117 example.jp | jp |ns.example.jp |in-bailiwick / in-domain
1118 example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain
1119 example.jp | jp |ns.example.com |out-of-bailiwick
1121 Root zone: The zone of a DNS-based tree whose apex is the zero-
1122 length label. Also sometimes called "the DNS root".
1124 Empty non-terminals (ENT): "Domain names that own no resource
1125 records but have subdomains that do." (Quoted from [RFC4592],
1126 Section 2.2.2.) A typical example is in SRV records: in the name
1127 "_sip._tcp.example.com", it is likely that "_tcp.example.com" has
1128 no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV
1129 RRset.
1131 Delegation-centric zone: A zone that consists mostly of delegations
1132 to child zones. This term is used in contrast to a zone that
1133 might have some delegations to child zones, but also has many data
1134 resource records for the zone itself and/or for child zones. The
1135 term is used in [RFC4956] and [RFC5155], but is not defined there.
1137 Occluded name: "The addition of a delegation point via dynamic
1138 update will render all subordinate domain names to be in a limbo,
1139 still part of the zone, but not available to the lookup process.
1140 The addition of a DNAME resource record has the same impact. The
1141 subordinate names are said to be 'occluded'." (Quoted from
1142 [RFC5936], Section 3.5)
1144 Fast flux DNS: This "occurs when a domain is found in DNS using A
1145 records to multiple IP addresses, each of which has a very short
1146 Time-to-Live (TTL) value associated with it. This means that the
1147 domain resolves to varying IP addresses over a short period of
1148 time." (Quoted from [RFC6561], Section 1.1.5, with typo
1149 corrected) In addition to having legitimate uses, fast flux DNS
1150 can used to deliver malware. Because the addresses change so
1151 rapidly, it is difficult to ascertain all the hosts. It should be
1152 noted that the technique also works with AAAA records, but such
1153 use is not frequently observed on the Internet as of this writing.
1155 Reverse DNS, reverse lookup: "The process of mapping an address to a
1156 name is generally known as a 'reverse lookup', and the IN-
1157 ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse
1158 DNS'." (Quoted from [RFC5855], Section 1)
1160 Forward lookup: "Hostname-to-address translation". (Quoted from
1161 [RFC2133], Section 6)
1163 arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain
1164 was originally established as part of the initial deployment of
1165 the DNS, to provide a transition mechanism from the Host Tables
1166 that were common in the ARPANET, as well as a home for the IPv4
1167 reverse mapping domain. During 2000, the abbreviation was
1168 redesignated to 'Address and Routing Parameter Area' in the hope
1169 of reducing confusion with the earlier network name." (Quoted
1170 from [RFC3172], Section 2.) .arpa is an "infrastructure domain", a
1171 domain whose "role is to support the operating infrastructure of
1172 the Internet". (Quoted from [RFC3172], Section 2.) See [RFC3172]
1173 for more history of this name.
1175 Service name: "Service names are the unique key in the Service Name
1176 and Transport Protocol Port Number registry. This unique symbolic
1177 name for a service may also be used for other purposes, such as in
1178 DNS SRV records." (Quoted from [RFC6335], Section 5.)
1180 8. Wildcards
1182 Wildcard: [RFC1034] defined "wildcard", but in a way that turned out
1183 to be confusing to implementers. For an extended discussion of
1184 wildcards, including clearer definitions, see [RFC4592]. Special
1185 treatment is given to RRs with owner names starting with the label
1186 "*". "Such RRs are called 'wildcards'. Wildcard RRs can be
1187 thought of as instructions for synthesizing RRs." (Quoted from
1188 [RFC1034], Section 4.3.3)
1190 Asterisk label: "The first octet is the normal label type and length
1191 for a 1-octet-long label, and the second octet is the ASCII
1192 representation for the '*' character. A descriptive name of a
1193 label equaling that value is an 'asterisk label'." (Quoted from
1194 [RFC4592], Section 2.1.1)
1196 Wildcard domain name: "A 'wildcard domain name' is defined by having
1197 its initial (i.e., leftmost or least significant) label be
1198 asterisk label." (Quoted from [RFC4592], Section 2.1.1)
1200 Closest encloser: "The longest existing ancestor of a name."
1201 (Quoted from [RFC5155], Section 1.3) An earlier definition is "The
1202 node in the zone's tree of existing domain names that has the most
1203 labels matching the query name (consecutively, counting from the
1204 root label downward). Each match is a 'label match' and the order
1205 of the labels is the same." (Quoted from [RFC4592],
1206 Section 3.3.1)
1208 Closest provable encloser: "The longest ancestor of a name that can
1209 be proven to exist. Note that this is only different from the
1210 closest encloser in an Opt-Out zone." (Quoted from [RFC5155],
1211 Section 1.3)
1213 Next closer name: "The name one label longer than the closest
1214 provable encloser of a name." (Quoted from [RFC5155],
1215 Section 1.3)
1217 Source of Synthesis: "The source of synthesis is defined in the
1218 context of a query process as that wildcard domain name
1219 immediately descending from the closest encloser, provided that
1220 this wildcard domain name exists. 'Immediately descending' means
1221 that the source of synthesis has a name of the form: .." (Quoted from [RFC4592],
1223 Section 3.3.1)
1225 9. Registration Model
1227 Registry: The administrative operation of a zone that allows
1228 registration of names within that zone. People often use this
1229 term to refer only to those organizations that perform
1230 registration in large delegation-centric zones (such as TLDs); but
1231 formally, whoever decides what data goes into a zone is the
1232 registry for that zone. This definition of "registry" is from a
1233 DNS point of view; for some zones, the policies that determine
1234 what can go in the zone are decided by superior zones and not the
1235 registry operator.
1237 Registrant: An individual or organization on whose behalf a name in
1238 a zone is registered by the registry. In many zones, the registry
1239 and the registrant may be the same entity, but in TLDs they often
1240 are not.
1242 Registrar: A service provider that acts as a go-between for
1243 registrants and registries. Not all registrations require a
1244 registrar, though it is common to have registrars involved in
1245 registrations in TLDs.
1247 EPP: The Extensible Provisioning Protocol (EPP), which is commonly
1248 used for communication of registration information between
1249 registries and registrars. EPP is defined in [RFC5730].
1251 WHOIS: A protocol specified in [RFC3912], often used for querying
1252 registry databases. WHOIS data is frequently used to associate
1253 registration data (such as zone management contacts) with domain
1254 names. The term "WHOIS data" is often used as a synonym for the
1255 registry database, even though that database may be served by
1256 different protocols, particularly RDAP. The WHOIS protocol is
1257 also used with IP address registry data.
1259 RDAP: The Registration Data Access Protocol, defined in [RFC7480],
1260 [RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The
1261 RDAP protocol and data format are meant as a replacement for
1262 WHOIS.
1264 DNS operator: An entity responsible for running DNS servers. For a
1265 zone's authoritative servers, the registrant may act as their own
1266 DNS operator, or their registrar may do it on their behalf, or
1267 they may use a third-party operator. For some zones, the registry
1268 function is performed by the DNS operator plus other entities who
1269 decide about the allowed contents of the zone.
1271 Public suffix: "A domain that is controlled by a public registry."
1272 (Quoted from [RFC6265], Section 5.3) A common definition for this
1273 term is a domain under which subdomains can be registered by third
1274 parties, and on which HTTP cookies (which are described in detail
1275 in [RFC6265]) should not be set. There is no indication in a
1276 domain name whether it is a public suffix; that can only be
1277 determined by outside means. In fact, both a domain and a
1278 subdomain of that domain can be public suffixes.
1280 There is nothing inherent in a domain name to indicate whether it
1281 is a public suffix. One resource for identifying public suffixes
1282 is the Public Suffix List (PSL) maintained by Mozilla
1283 (http://publicsuffix.org/).
1285 For example, at the time this document is published, the "com.au"
1286 domain is listed as a public suffix in the PSL. (Note that this
1287 example might change in the future.)
1289 Note that the term "public suffix" is controversial in the DNS
1290 community for many reasons, and may be significantly changed in
1291 the future. One example of the difficulty of calling a domain a
1292 public suffix is that designation can change over time as the
1293 registration policy for the zone changes, such as was the case
1294 with the "uk" TLD in 2014.
1296 Subordinate and Superordinate: These terms are introduced in
1297 [RFC3731] for use in the registration model, but not defined
1298 there. Instead, they are given in examples. "For example, domain
1299 name 'example.com' has a superordinate relationship to host name
1300 ns1.example.com'." "For example, host ns1.example1.com is a
1301 subordinate host of domain example1.com, but it is a not a
1302 subordinate host of domain example2.com." (Quoted from [RFC3731],
1303 Section 1.1.) These terms are strictly ways of referring to the
1304 relationship standing of two domains where one is a subdomain of
1305 the other.
1307 10. General DNSSEC
1309 Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and
1310 [RFC5155]. The terms that have caused confusion in the DNS community
1311 are highlighted here.
1313 DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in
1314 some RFCs, have not been formally defined. However, Section 2 of
1315 [RFC4033] defines many types of resolvers and validators,
1316 including "non-validating security-aware stub resolver", "non-
1317 validating stub resolver", "security-aware name server",
1318 "security-aware recursive name server", "security-aware resolver",
1319 "security-aware stub resolver", and "security-oblivious
1320 'anything'". (Note that the term "validating resolver", which is
1321 used in some places in DNSSEC-related documents, is also not
1322 defined in those RFCs, but is defined below.)
1324 Signed zone: "A zone whose RRsets are signed and that contains
1325 properly constructed DNSKEY, Resource Record Signature (RRSIG),
1326 Next Secure (NSEC), and (optionally) DS records." (Quoted from
1327 [RFC4033], Section 2.) It has been noted in other contexts that
1328 the zone itself is not really signed, but all the relevant RRsets
1329 in the zone are signed. Nevertheless, if a zone that should be
1330 signed contains any RRsets that are not signed (or opted out),
1331 those RRsets will be treated as bogus, so the whole zone needs to
1332 be handled in some way.
1334 It should also be noted that, since the publication of [RFC6840],
1335 NSEC records are no longer required for signed zones: a signed
1336 zone might include NSEC3 records instead. [RFC7129] provides
1337 additional background commentary and some context for the NSEC and
1338 NSEC3 mechanisms used by DNSSEC to provide authenticated denial-
1339 of-existence responses. NSEC and NSEC3 are described below.
1341 Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that
1342 is not signed". Section 2 of [RFC4035] defines this as "A zone
1343 that does not include these records [properly constructed DNSKEY,
1344 Resource Record Signature (RRSIG), Next Secure (NSEC), and
1345 (optionally) DS records] according to the rules in this section".
1346 There is an important note at the end of Section 5.2 of [RFC4035]
1347 that defines an additional situation in which a zone is considered
1348 unsigned: "If the resolver does not support any of the algorithms
1349 listed in an authenticated DS RRset, then the resolver will not be
1350 able to verify the authentication path to the child zone. In this
1351 case, the resolver SHOULD treat the child zone as if it were
1352 unsigned."
1354 NSEC: "The NSEC record allows a security-aware resolver to
1355 authenticate a negative reply for either name or type non-
1356 existence with the same mechanisms used to authenticate other DNS
1357 replies." (Quoted from [RFC4033], Section 3.2.) In short, an
1358 NSEC record provides authenticated denial of existence.
1360 "The NSEC resource record lists two separate things: the next
1361 owner name (in the canonical ordering of the zone) that contains
1362 authoritative data or a delegation point NS RRset, and the set of
1363 RR types present at the NSEC RR's owner name." (Quoted from
1364 Section 4 of RFC 4034)
1366 NSEC3: Like the NSEC record, the NSEC3 record also provides
1367 authenticated denial of existence; however, NSEC3 records mitigate
1368 against zone enumeration and support Opt-Out. NSEC resource
1369 records require associated NSEC3PARAM resource records. NSEC3 and
1370 NSEC3PARAM resource records are defined in [RFC5155].
1372 Note that [RFC6840] says that [RFC5155] "is now considered part of
1373 the DNS Security Document Family as described by Section 10 of
1374 [RFC4033]". This means that some of the definitions from earlier
1375 RFCs that only talk about NSEC records should probably be
1376 considered to be talking about both NSEC and NSEC3.
1378 Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover
1379 unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1.)
1380 Opt-out tackles the high costs of securing a delegation to an
1381 insecure zone. When using Opt-Out, names that are an insecure
1382 delegation (and empty non-terminals that are only derived from
1383 insecure delegations) don't require an NSEC3 record or its
1384 corresponding RRSIG records. Opt-Out NSEC3 records are not able
1385 to prove or deny the existence of the insecure delegations.
1386 (Adapted from [RFC7129], Section 5.1)
1388 Insecure delegation: "A signed name containing a delegation (NS
1389 RRset), but lacking a DS RRset, signifying a delegation to an
1390 unsigned subzone." (Quoted from [RFC4956], Section 2.)
1392 Zone enumeration: "The practice of discovering the full content of a
1393 zone via successive queries." (Quoted from [RFC5155],
1394 Section 1.3.) This is also sometimes called "zone walking". Zone
1395 enumeration is different from zone content guessing where the
1396 guesser uses a large dictionary of possible labels and sends
1397 successive queries for them, or matches the contents of NSEC3
1398 records against such a dictionary.
1400 Validation: Validation, in the context of DNSSEC, refers to one of
1401 the following:
1403 * Checking the validity of DNSSEC signatures
1405 * Checking the validity of DNS responses, such as those including
1406 authenticated denial of existence
1408 * Building an authentication chain from a trust anchor to a DNS
1409 response or individual DNS RRsets in a response
1411 The first two definitions above consider only the validity of
1412 individual DNSSEC components such as the RRSIG validity or NSEC
1413 proof validity. The third definition considers the components of
1414 the entire DNSSEC authentication chain, and thus requires
1415 "configured knowledge of at least one authenticated DNSKEY or DS
1416 RR" (as described in [RFC4035], Section 5).
1418 [RFC4033], Section 2, says that a "Validating Security-Aware Stub
1419 Resolver... performs signature validation" and uses a trust anchor
1420 "as a starting point for building the authentication chain to a
1421 signed DNS response", and thus uses the first and third
1422 definitions above. The process of validating an RRSIG resource
1423 record is described in [RFC4035], Section 5.3.
1425 [RFC5155] refers to validating responses throughout the document,
1426 in the context of hashed authenticated denial of existence; this
1427 uses the second definition above.
1429 The term "authentication" is used interchangeably with
1430 "validation", in the sense of the third definition above.
1431 [RFC4033], Section 2, describes the chain linking trust anchor to
1432 DNS data as the "authentication chain". A response is considered
1433 to be authentic if "all RRsets in the Answer and Authority
1434 sections of the response [are considered] to be authentic"
1435 ([RFC4035]). DNS data or responses deemed to be authentic or
1436 validated have a security status of "secure" ([RFC4035],
1437 Section 4.3; [RFC4033], Section 5). "Authenticating both DNS keys
1438 and data is a matter of local policy, which may extend or even
1439 override the [DNSSEC] protocol extensions" ([RFC4033],
1440 Section 3.1).
1442 The term "verification", when used, is usually synonym for
1443 "validation".
1445 Validating resolver: A security-aware recursive name server,
1446 security-aware resolver, or security-aware stub resolver that is
1447 applying at least one of the definitions of validation (above), as
1448 appropriate to the resolution context. For the same reason that
1449 the generic term "resolver" is sometimes ambiguous and needs to be
1450 evaluated in context (see Section 6), "validating resolver" is a
1451 context-sensitive term.
1453 Key signing key (KSK): DNSSEC keys that "only sign the apex DNSKEY
1454 RRset in a zone."(Quoted from [RFC6781], Section 3.1)
1456 Zone signing key (ZSK): "DNSSEC keys that can be used to sign all
1457 the RRsets in a zone that require signatures, other than the apex
1458 DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Also note
1459 that a ZSK is sometimes used to sign the apex DNSKEY RRset.
1461 Combined signing key (CSK): "In cases where the differentiation
1462 between the KSK and ZSK is not made, i.e., where keys have the
1463 role of both KSK and ZSK, we talk about a Single-Type Signing
1464 Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes
1465 called a "combined signing key" or CSK. It is operational
1466 practice, not protocol, that determines whether a particular key
1467 is a ZSK, a KSK, or a CSK.
1469 Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be
1470 used to distinguish between keys that are intended to be used as
1471 the secure entry point into the zone when building chains of
1472 trust, i.e., they are (to be) pointed to by parental DS RRs or
1473 configured as a trust anchor. Therefore, it is suggested that the
1474 SEP flag be set on keys that are used as KSKs and not on keys that
1475 are used as ZSKs, while in those cases where a distinction between
1476 a KSK and ZSK is not made (i.e., for a Single-Type Signing
1477 Scheme), it is suggested that the SEP flag be set on all keys."
1478 (Quoted from [RFC6781], Section 3.2.3.) Note that the SEP flag is
1479 only a hint, and its presence or absence may not be used to
1480 disqualify a given DNSKEY RR from use as a KSK or ZSK during
1481 validation.
1483 The original definition of SEPs was in [RFC3757]. That definition
1484 clearly indicated that the SEP was a key, not just a bit in the
1485 key. The abstract of [RFC3757] says: "With the Delegation Signer
1486 (DS) resource record (RR), the concept of a public key acting as a
1487 secure entry point (SEP) has been introduced. During exchanges of
1488 public keys with the parent there is a need to differentiate SEP
1489 keys from other public keys in the Domain Name System KEY (DNSKEY)
1490 resource record set. A flag bit in the DNSKEY RR is defined to
1491 indicate that DNSKEY is to be used as a SEP." That definition of
1492 the SEP as a key was made obsolete by [RFC4034], and the
1493 definition from [RFC6781] is consistent with [RFC4034].
1495 Trust anchor: "A configured DNSKEY RR or DS RR hash of a DNSKEY RR.
1496 A validating security-aware resolver uses this public key or hash
1497 as a starting point for building the authentication chain to a
1498 signed DNS response. In general, a validating resolver will have
1499 to obtain the initial values of its trust anchors via some secure
1500 or trusted means outside the DNS protocol." (Quoted from
1501 [RFC4033], Section 2)
1503 DNSSEC Policy (DP): A statement that "sets forth the security
1504 requirements and standards to be implemented for a DNSSEC-signed
1505 zone." (Quoted from [RFC6841], Section 2)
1507 DNSSEC Practice Statement (DPS): "A practices disclosure document
1508 that may support and be a supplemental document to the DNSSEC
1509 Policy (if such exists), and it states how the management of a
1510 given zone implements procedures and controls at a high level."
1511 (Quoted from [RFC6841], Section 2)
1513 Hardware security module (HSM): A specialized piece of hardware that
1514 is used to create keys for signatures and to sign messages. In
1515 DNSSEC, HSMs are often used to hold the private keys for KSKs and
1516 ZSKs and to create the signatures used in RRSIG records at
1517 periodic intervals.
1519 Signing software: Authoritative DNS servers that support DNSSEC
1520 often contain software that facilitates the creation and
1521 maintenance of DNSSEC signatures in zones. There is also stand-
1522 alone software that can be used to sign a zone regardless of
1523 whether the authoritative server itself supports signing.
1524 Sometimes signing software can support particular HSMs as part of
1525 the signing process.
1527 11. DNSSEC States
1529 A validating resolver can determine that a response is in one of four
1530 states: secure, insecure, bogus, or indeterminate. These states are
1531 defined in [RFC4033] and [RFC4035], although the two definitions
1532 differ a bit. This document makes no effort to reconcile the two
1533 definitions, and takes no position as to whether they need to be
1534 reconciled.
1536 Section 5 of [RFC4033] says:
1538 A validating resolver can determine the following 4 states:
1540 Secure: The validating resolver has a trust anchor, has a chain
1541 of trust, and is able to verify all the signatures in the
1542 response.
1544 Insecure: The validating resolver has a trust anchor, a chain
1545 of trust, and, at some delegation point, signed proof of the
1546 non-existence of a DS record. This indicates that subsequent
1547 branches in the tree are provably insecure. A validating
1548 resolver may have a local policy to mark parts of the domain
1549 space as insecure.
1551 Bogus: The validating resolver has a trust anchor and a secure
1552 delegation indicating that subsidiary data is signed, but
1553 the response fails to validate for some reason: missing
1554 signatures, expired signatures, signatures with unsupported
1555 algorithms, data missing that the relevant NSEC RR says
1556 should be present, and so forth.
1558 Indeterminate: There is no trust anchor that would indicate that a
1559 specific portion of the tree is secure. This is the default
1560 operation mode.
1562 Section 4.3 of [RFC4035] says:
1564 A security-aware resolver must be able to distinguish between four
1565 cases:
1567 Secure: An RRset for which the resolver is able to build a chain
1568 of signed DNSKEY and DS RRs from a trusted security anchor to
1569 the RRset. In this case, the RRset should be signed and is
1570 subject to signature validation, as described above.
1572 Insecure: An RRset for which the resolver knows that it has no
1573 chain of signed DNSKEY and DS RRs from any trusted starting
1574 point to the RRset. This can occur when the target RRset lies
1575 in an unsigned zone or in a descendent [sic] of an unsigned
1576 zone. In this case, the RRset may or may not be signed, but
1577 the resolver will not be able to verify the signature.
1579 Bogus: An RRset for which the resolver believes that it ought to
1580 be able to establish a chain of trust but for which it is
1581 unable to do so, either due to signatures that for some reason
1582 fail to validate or due to missing data that the relevant
1583 DNSSEC RRs indicate should be present. This case may indicate
1584 an attack but may also indicate a configuration error or some
1585 form of data corruption.
1587 Indeterminate: An RRset for which the resolver is not able to
1588 determine whether the RRset should be signed, as the resolver
1589 is not able to obtain the necessary DNSSEC RRs. This can occur
1590 when the security-aware resolver is not able to contact
1591 security-aware name servers for the relevant zones.
1593 12. Security Considerations
1595 These definitions do not change any security considerations for the
1596 DNS.
1598 13. IANA Considerations
1600 None.
1602 14. References
1604 14.1. Normative References
1606 [IANA_RootFiles]
1607 Internet Assigned Numbers Authority, "IANA Root Files",
1608 2016, .
1610 [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities",
1611 RFC 882, DOI 10.17487/RFC0882, November 1983,
1612 .
1614 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
1615 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
1616 .
1618 [RFC1035] Mockapetris, P., "Domain names - implementation and
1619 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
1620 November 1987, .
1622 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
1623 Application and Support", STD 3, RFC 1123,
1624 DOI 10.17487/RFC1123, October 1989,
1625 .
1627 [RFC1912] Barr, D., "Common DNS Operational and Configuration
1628 Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,
1629 .
1631 [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
1632 Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
1633 August 1996, .
1635 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
1636 "Dynamic Updates in the Domain Name System (DNS UPDATE)",
1637 RFC 2136, DOI 10.17487/RFC2136, April 1997,
1638 .
1640 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
1641 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
1642 .
1644 [RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection
1645 and Operation of Secondary DNS Servers", BCP 16, RFC 2182,
1646 DOI 10.17487/RFC2182, July 1997,
1647 .
1649 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
1650 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
1651 .
1653 [RFC3731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
1654 Domain Name Mapping", RFC 3731, DOI 10.17487/RFC3731,
1655 March 2004, .
1657 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1658 Rose, "DNS Security Introduction and Requirements",
1659 RFC 4033, DOI 10.17487/RFC4033, March 2005,
1660 .
1662 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1663 Rose, "Resource Records for the DNS Security Extensions",
1664 RFC 4034, DOI 10.17487/RFC4034, March 2005,
1665 .
1667 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1668 Rose, "Protocol Modifications for the DNS Security
1669 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
1670 .
1672 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
1673 System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
1674 .
1676 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
1677 Security (DNSSEC) Hashed Authenticated Denial of
1678 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
1679 .
1681 [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
1682 Nameservers in Reflector Attacks", BCP 140, RFC 5358,
1683 DOI 10.17487/RFC5358, October 2008,
1684 .
1686 [RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
1687 STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,
1688 .
1690 [RFC5855] Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6
1691 Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855,
1692 May 2010, .
1694 [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
1695 (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
1696 .
1698 [RFC6561] Livingood, J., Mody, N., and M. O'Reirdan,
1699 "Recommendations for the Remediation of Bots in ISP
1700 Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012,
1701 .
1703 [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
1704 Operational Practices, Version 2", RFC 6781,
1705 DOI 10.17487/RFC6781, December 2012,
1706 .
1708 [RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
1709 Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
1710 DOI 10.17487/RFC6840, February 2013,
1711 .
1713 [RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
1714 Framework for DNSSEC Policies and DNSSEC Practice
1715 Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,
1716 .
1718 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
1719 for DNS (EDNS(0))", STD 75, RFC 6891,
1720 DOI 10.17487/RFC6891, April 2013,
1721 .
1723 [RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
1724 DNSSEC Delegation Trust Maintenance", RFC 7344,
1725 DOI 10.17487/RFC7344, September 2014,
1726 .
1728 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
1729 Terminology", RFC 7719, DOI 10.17487/RFC7719, December
1730 2015, .
1732 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
1733 for DNS over TLS and DNS over DTLS", RFC 8310,
1734 DOI 10.17487/RFC8310, March 2018,
1735 .
1737 14.2. Informative References
1739 [IANA_Resource_Registry]
1740 Internet Assigned Numbers Authority, "Resource Record (RR)
1741 TYPEs", 2017,
1742 .
1744 [RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for
1745 Internet User Applications", RFC 819,
1746 DOI 10.17487/RFC0819, August 1982,
1747 .
1749 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
1750 host table specification", RFC 952, DOI 10.17487/RFC0952,
1751 October 1985, .
1753 [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
1754 DOI 10.17487/RFC1995, August 1996,
1755 .
1757 [RFC2133] Gilligan, R., Thomson, S., Bound, J., and W. Stevens,
1758 "Basic Socket Interface Extensions for IPv6", RFC 2133,
1759 DOI 10.17487/RFC2133, April 1997,
1760 .
1762 [RFC2775] Carpenter, B., "Internet Transparency", RFC 2775,
1763 DOI 10.17487/RFC2775, February 2000,
1764 .
1766 [RFC3172] Huston, G., Ed., "Management Guidelines & Operational
1767 Requirements for the Address and Routing Parameter Area
1768 Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
1769 September 2001, .
1771 [RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425,
1772 DOI 10.17487/RFC3425, November 2002,
1773 .
1775 [RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name
1776 System KEY (DNSKEY) Resource Record (RR) Secure Entry
1777 Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April
1778 2004, .
1780 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912,
1781 DOI 10.17487/RFC3912, September 2004,
1782 .
1784 [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
1785 RFC 4641, DOI 10.17487/RFC4641, September 2006,
1786 .
1788 [RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution
1789 Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,
1790 October 2006, .
1792 [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
1793 Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
1794 December 2006, .
1796 [RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security
1797 (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July
1798 2007, .
1800 [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
1801 BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
1802 .
1804 [RFC5890] Klensin, J., "Internationalized Domain Names for
1805 Applications (IDNA): Definitions and Document Framework",
1806 RFC 5890, DOI 10.17487/RFC5890, August 2010,
1807 .
1809 [RFC5891] Klensin, J., "Internationalized Domain Names in
1810 Applications (IDNA): Protocol", RFC 5891,
1811 DOI 10.17487/RFC5891, August 2010,
1812 .
1814 [RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and
1815 Internationalized Domain Names for Applications (IDNA)",
1816 RFC 5892, DOI 10.17487/RFC5892, August 2010,
1817 .
1819 [RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
1820 for Internationalized Domain Names for Applications
1821 (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
1822 .
1824 [RFC5894] Klensin, J., "Internationalized Domain Names for
1825 Applications (IDNA): Background, Explanation, and
1826 Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
1827 .
1829 [RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
1830 Encodings for Internationalized Domain Names", RFC 6055,
1831 DOI 10.17487/RFC6055, February 2011,
1832 .
1834 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
1835 DOI 10.17487/RFC6265, April 2011,
1836 .
1838 [RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163,
1839 RFC 6303, DOI 10.17487/RFC6303, July 2011,
1840 .
1842 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
1843 Cheshire, "Internet Assigned Numbers Authority (IANA)
1844 Procedures for the Management of the Service Name and
1845 Transport Protocol Port Number Registry", BCP 165,
1846 RFC 6335, DOI 10.17487/RFC6335, August 2011,
1847 .
1849 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
1850 Internationalization in the IETF", BCP 166, RFC 6365,
1851 DOI 10.17487/RFC6365, September 2011,
1852 .
1854 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
1855 DOI 10.17487/RFC6762, February 2013,
1856 .
1858 [RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of
1859 Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
1860 February 2014, .
1862 [RFC7480] Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the
1863 Registration Data Access Protocol (RDAP)", RFC 7480,
1864 DOI 10.17487/RFC7480, March 2015,
1865 .
1867 [RFC7481] Hollenbeck, S. and N. Kong, "Security Services for the
1868 Registration Data Access Protocol (RDAP)", RFC 7481,
1869 DOI 10.17487/RFC7481, March 2015,
1870 .
1872 [RFC7482] Newton, A. and S. Hollenbeck, "Registration Data Access
1873 Protocol (RDAP) Query Format", RFC 7482,
1874 DOI 10.17487/RFC7482, March 2015,
1875 .
1877 [RFC7483] Newton, A. and S. Hollenbeck, "JSON Responses for the
1878 Registration Data Access Protocol (RDAP)", RFC 7483,
1879 DOI 10.17487/RFC7483, March 2015,
1880 .
1882 [RFC7484] Blanchet, M., "Finding the Authoritative Registration Data
1883 (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March
1884 2015, .
1886 [RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin,
1887 "Inventory and Analysis of WHOIS Registration Objects",
1888 RFC 7485, DOI 10.17487/RFC7485, March 2015,
1889 .
1891 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
1892 and P. Hoffman, "Specification for DNS over Transport
1893 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
1894 2016, .
1896 [RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
1897 Transport Layer Security (DTLS)", RFC 8094,
1898 DOI 10.17487/RFC8094, February 2017,
1899 .
1901 [RFC8109] Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS
1902 Resolver with Priming Queries", BCP 209, RFC 8109,
1903 DOI 10.17487/RFC8109, March 2017,
1904 .
1906 [RSSAC026]
1907 Root Server System Advisory Committee (RSSAC), "RSSAC
1908 Lexicon", 2017,
1909 .
1912 Appendix A. Definitions Updated by this Document
1914 The following definitions from RFCs are updated by this document:
1916 o Forwarder in [RFC2308]
1918 o Secure Entry Point (SEP) in [RFC3757]; note, however, that this
1919 RFC is already obsolete
1921 Appendix B. Definitions First Defined in this Document
1923 The following definitions are first defined in this document:
1925 o "Alias" in Section 2
1927 o "Apex" in Section 7
1929 o "arpa" in Section 7
1931 o "Class independent" in Section 5
1933 o "Delegation-centric zone" in Section 7
1935 o "Delegation" in Section 7
1937 o "DNS operator" in Section 9
1938 o "DNSSEC-aware" in Section 10
1940 o "DNSSEC-unaware" in Section 10
1942 o "Forwarding" in Section 6
1944 o "Full resolver" in Section 6
1946 o "Fully qualified domain name" in Section 2
1948 o "Global DNS" in Section 2
1950 o "Hardware Security Module (HSM)" in Section 10
1952 o "Host name" in Section 2
1954 o "IDN" in Section 2
1956 o "In-bailiwick" in Section 7
1958 o "Label" in Section 2
1960 o "Locally served DNS zone" in Section 2
1962 o "Naming system" in Section 2
1964 o "Negative response" in Section 3
1966 o "Open resolver" in Section 6
1968 o "Out-of-bailiwick" in Section 7
1970 o "Passive DNS" in Section 6
1972 o "Policy-implementing resolver" in Section 6
1974 o "Presentation format" in Section 5
1976 o "Priming" in Section 6
1978 o "Private DNS" in Section 2
1980 o "Recursive resolver" in Section 6
1982 o "Referrals" in Section 4
1984 o "Registrant" in Section 9
1985 o "Registrar" in Section 9
1987 o "Registry" in Section 9
1989 o "Root zone" in Section 7
1991 o "Secure Entry Point (SEP)" in Section 10
1993 o "Signing software" in Section 10
1995 o "Stub resolver" in Section 6
1997 o "TLD" in Section 2
1999 o "Validating resolver" in Section 10
2001 o "Validation" in Section 10
2003 o "View" in Section 6
2005 o "Zone transfer" in Section 6
2007 Index
2009 A
2010 Address records 14
2011 Alias 8
2012 Anycast 20
2013 Apex 22
2014 Asterisk label 25
2015 Authoritative data 22
2016 Authoritative server 17
2017 Authoritative-only server 18
2018 arpa: Address and Routing Parameter Area Domain 25
2020 C
2021 CNAME 9
2022 Canonical name 8
2023 Child 21
2024 Class independent 14
2025 Closest encloser 26
2026 Closest provable encloser 26
2027 Combined signing key (CSK) 31
2029 D
2030 DNS operator 27
2031 DNSSEC Policy (DP) 32
2032 DNSSEC Practice Statement (DPS) 32
2033 DNSSEC-aware and DNSSEC-unaware 28
2034 Delegation 22
2035 Delegation-centric zone 24
2036 Domain name 4
2038 E
2039 EDNS 13
2040 EPP 27
2041 Empty non-terminals (ENT) 24
2043 F
2044 FORMERR 9
2045 Fast flux DNS 24
2046 Forward lookup 25
2047 Forwarder 19
2048 Forwarding 19
2049 Full resolver 17
2050 Full-service resolver 17
2051 Fully qualified domain name (FQDN) 7
2053 G
2054 Global DNS 4
2055 Glue records 23
2057 H
2058 Hardware security module (HSM) 32
2059 Hidden master 19
2060 Host name 7
2062 I
2063 IDN 8
2064 In-bailiwick 23
2065 Insecure delegation 29
2066 Instance 21
2067 Iterative mode 15
2068 Iterative resolution 16
2070 K
2071 Key signing key (KSK) 31
2073 L
2074 Label 4
2075 Lame delegation 22
2076 Locally served DNS zone 7
2078 M
2079 Master file 13
2080 Master server 18
2081 Multicast DNS 6
2083 N
2084 NODATA 9
2085 NOERROR 9
2086 NOTIMP 9
2087 NSEC 29
2088 NSEC3 29
2089 NXDOMAIN 9
2090 Naming system 3
2091 Negative caching 17
2092 Negative response 10
2093 Next closer name 26
2094 Non-recursive query 16
2096 O
2097 OPT 13
2098 Occluded name 24
2099 Open resolver 20
2100 Opt-out 29
2101 Origin 21
2102 Out-of-bailiwick 23
2103 Owner 13
2105 P
2106 Parent 21
2107 Passive DNS 20
2108 Policy-implementing resolver 19
2109 Presentation format 13
2110 Primary master 18
2111 Primary server 18
2112 Priming 17
2113 Privacy-enabling DNS server 21
2114 Private DNS 6
2115 Public suffix 27
2117 Q
2118 QNAME 10
2120 R
2121 RDAP 27
2122 REFUSED 9
2123 RR 12
2124 RRset 12
2125 Recursive mode 16
2126 Recursive query 16
2127 Recursive resolver 16
2128 Referrals 11
2129 Registrant 26
2130 Registrar 26
2131 Registry 26
2132 Resolver 15
2133 Reverse DNS, reverse lookup 25
2134 Root hints 17
2135 Root zone 24
2137 S
2138 SERVFAIL 9
2139 SOA field names 13
2140 Secondary server 18
2141 Secure Entry Point (SEP) 31
2142 Service name 25
2143 Signed zone 28
2144 Signing software 32
2145 Slave server 18
2146 Source of Synthesis 26
2147 Split DNS 20
2148 Split-horizon DNS 20
2149 Stealth server 19
2150 Stub resolver 15
2151 Subdomain 8
2152 Subordinate 28
2153 Superordinate 28
2155 T
2156 TLD 8
2157 TTL 13
2158 Trust anchor 32
2160 U
2161 Unsigned zone 28
2163 V
2164 Validating resolver 31
2165 Validation 30
2166 View 20
2168 W
2169 WHOIS 27
2170 Wildcard 25
2171 Wildcard domain name 25
2173 Z
2174 Zone 21
2175 Zone cut 22
2176 Zone enumeration 30
2177 Zone signing key (ZSK) 31
2178 Zone transfer 18
2180 Acknowledgements
2182 The following is the Acknowledgements for RFC 7719. Additional
2183 acknowledgements may be added as this draft is worked on.
2185 The authors gratefully acknowledge all of the authors of DNS-related
2186 RFCs that proceed this one. Comments from Tony Finch, Stephane
2187 Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John
2188 Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman,
2189 David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis,
2190 John Klensin, David Black, and many others in the DNSOP Working Group
2191 helped shape RFC 7719.
2193 Additional people contributed to this document, including: Bob
2194 Harold, Dick Franks, Evan Hunt, John Dickinson, Mark Andrews, Martin
2195 Hoffmann, Paul Vixie, Peter Koch, Duane Wessels [[ MORE NAMES WILL
2196 APPEAR HERE AS FOLKS CONTRIBUTE]].
2198 Authors' Addresses
2200 Paul Hoffman
2201 ICANN
2203 Email: paul.hoffman@icann.org
2205 Andrew Sullivan
2206 Oracle
2207 100 Milverton Drive
2208 Mississauga, ON L5R 4H1
2209 Canada
2211 Email: andrew.s.sullivan@oracle.com
2213 Kazunori Fujiwara
2214 Japan Registry Services Co., Ltd.
2215 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
2216 Chiyoda-ku, Tokyo 101-0065
2217 Japan
2219 Phone: +81 3 5215 8451
2220 Email: fujiwara@jprs.co.jp