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2 Network Working Group P. Hoffman
3 Internet-Draft ICANN
4 Obsoletes: 7719 (if approved) A. Sullivan
5 Updates: 2308 (if approved)
6 Intended status: Best Current Practice K. Fujiwara
7 Expires: March 17, 2019 JPRS
8 September 13, 2018
10 DNS Terminology
11 draft-ietf-dnsop-terminology-bis-14
13 Abstract
15 The domain name system (DNS) is defined in literally dozens of
16 different RFCs. The terminology used by implementers and developers
17 of DNS protocols, and by operators of DNS systems, has sometimes
18 changed in the decades since the DNS was first defined. This
19 document gives current definitions for many of the terms used in the
20 DNS in a single document.
22 This document obsoletes RFC 7719 and updates RFC 2308.
24 Status of This Memo
26 This Internet-Draft is submitted in full conformance with the
27 provisions of BCP 78 and BCP 79.
29 Internet-Drafts are working documents of the Internet Engineering
30 Task Force (IETF). Note that other groups may also distribute
31 working documents as Internet-Drafts. The list of current Internet-
32 Drafts is at https://datatracker.ietf.org/drafts/current/.
34 Internet-Drafts are draft documents valid for a maximum of six months
35 and may be updated, replaced, or obsoleted by other documents at any
36 time. It is inappropriate to use Internet-Drafts as reference
37 material or to cite them other than as "work in progress."
39 This Internet-Draft will expire on March 17, 2019.
41 Copyright Notice
43 Copyright (c) 2018 IETF Trust and the persons identified as the
44 document authors. All rights reserved.
46 This document is subject to BCP 78 and the IETF Trust's Legal
47 Provisions Relating to IETF Documents
48 (https://trustee.ietf.org/license-info) in effect on the date of
49 publication of this document. Please review these documents
50 carefully, as they describe your rights and restrictions with respect
51 to this document. Code Components extracted from this document must
52 include Simplified BSD License text as described in Section 4.e of
53 the Trust Legal Provisions and are provided without warranty as
54 described in the Simplified BSD License.
56 Table of Contents
58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
59 2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
60 3. DNS Response Codes . . . . . . . . . . . . . . . . . . . . . 9
61 4. DNS Transactions . . . . . . . . . . . . . . . . . . . . . . 10
62 5. Resource Records . . . . . . . . . . . . . . . . . . . . . . 13
63 6. DNS Servers and Clients . . . . . . . . . . . . . . . . . . . 15
64 7. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
65 8. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . 26
66 9. Registration Model . . . . . . . . . . . . . . . . . . . . . 27
67 10. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 29
68 11. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 33
69 12. Security Considerations . . . . . . . . . . . . . . . . . . . 35
70 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
71 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
72 14.1. Normative References . . . . . . . . . . . . . . . . . . 35
73 14.2. Informative References . . . . . . . . . . . . . . . . . 38
74 Appendix A. Definitions Updated by this Document . . . . . . . . 42
75 Appendix B. Definitions First Defined in this Document . . . . . 43
76 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
77 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 48
78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 49
80 1. Introduction
82 The Domain Name System (DNS) is a simple query-response protocol
83 whose messages in both directions have the same format. (Section 2
84 gives a definition of "public DNS", which is often what people mean
85 when they say "the DNS".) The protocol and message format are
86 defined in [RFC1034] and [RFC1035]. These RFCs defined some terms,
87 but later documents defined others. Some of the terms from [RFC1034]
88 and [RFC1035] now have somewhat different meanings than they did in
89 1987.
91 This document collects a wide variety of DNS-related terms. Some of
92 them have been precisely defined in earlier RFCs, some have been
93 loosely defined in earlier RFCs, and some are not defined in any
94 earlier RFC at all.
96 Most of the definitions here are the consensus definition of the DNS
97 community -- both protocol developers and operators. Some of the
98 definitions differ from earlier RFCs, and those differences are
99 noted. In this document, where the consensus definition is the same
100 as the one in an RFC, that RFC is quoted. Where the consensus
101 definition has changed somewhat, the RFC is mentioned but the new
102 stand-alone definition is given. See Appendix A for a list of the
103 definitions that this document updates.
105 It is important to note that, during the development of this
106 document, it became clear that some DNS-related terms are interpreted
107 quite differently by different DNS experts. Further, some terms that
108 are defined in early DNS RFCs now have definitions that are generally
109 agreed to, but that are different from the original definitions.
110 Therefore, this document is a substantial revision to [RFC7719].
112 The terms are organized loosely by topic. Some definitions are for
113 new terms for things that are commonly talked about in the DNS
114 community but that never had terms defined for them.
116 Other organizations sometimes define DNS-related terms their own way.
117 For example, the WHATWG defines "domain" at
118 . The Root Server System Advisory
119 Committee (RSSAC) has a good lexicon [RSSAC026].
121 Note that there is no single consistent definition of "the DNS". It
122 can be considered to be some combination of the following: a commonly
123 used naming scheme for objects on the Internet; a distributed
124 database representing the names and certain properties of these
125 objects; an architecture providing distributed maintenance,
126 resilience, and loose coherency for this database; and a simple
127 query-response protocol (as mentioned below) implementing this
128 architecture. Section 2 defines "global DNS" and "private DNS" as a
129 way to deal with these differing definitions.
131 Capitalization in DNS terms is often inconsistent among RFCs and
132 various DNS practitioners. The capitalization used in this document
133 is a best guess at current practices, and is not meant to indicate
134 that other capitalization styles are wrong or archaic. In some
135 cases, multiple styles of capitalization are used for the same term
136 due to quoting from different RFCs.
138 Readers should note that the terms in this document are grouped by
139 topic. Someone who is not already familiar with the DNS can probably
140 not learn about the DNS from scratch by reading this document from
141 front to back. Instead, skipping around may be the only way to get
142 enough context to understand some of the definitions. This document
143 has an index that might be useful for readers who are attempting to
144 learn the DNS by reading this document.
146 2. Names
148 Naming system: A naming system associates names with data. Naming
149 systems have many significant facets that help differentiate them
150 from each other. Some commonly-identified facets include:
152 * Composition of names
154 * Format of names
156 * Administration of names
158 * Types of data that can be associated with names
160 * Types of metadata for names
162 * Protocol for getting data from a name
164 * Context for resolving a name
166 Note that this list is a small subset of facets that people have
167 identified over time for naming systems, and the IETF has yet to
168 agree on a good set of facets that can be used to compare naming
169 systems. For example, other facets might include "protocol to
170 update data in a name", "privacy of names", and "privacy of data
171 associated with names", but those are not as well-defined as the
172 ones listed above. The list here is chosen because it helps
173 describe the DNS and naming systems similar to the DNS.
175 Domain name: An ordered list of one or more labels.
177 Note that this is a definition independent of the DNS RFCs, and
178 the definition here also applies to systems other than the DNS.
179 [RFC1034] defines the "domain name space" using mathematical trees
180 and their nodes in graph theory, and this definition has the same
181 practical result as the definition here. Any path of a directed
182 acyclic graph can be represented by a domain name consisting of
183 the labels of its nodes, ordered by decreasing distance from the
184 root(s) (which is the normal convention within the DNS, including
185 this document). A domain name whose last label identifies a root
186 of the graph is fully qualified; other domain names whose labels
187 form a strict prefix of a fully qualified domain name are relative
188 to its first omitted node.
190 Also note that different IETF and non-IETF documents have used the
191 term "domain name" in many different ways. It is common for
192 earlier documents to use "domain name" to mean "names that match
193 the syntax in [RFC1035]", but possibly with additional rules such
194 as "and are, or will be, resolvable in the global DNS" or "but
195 only using the presentation format".
197 Label: An ordered list of zero or more octets that makes up a
198 portion of a domain name. Using graph theory, a label identifies
199 one node in a portion of the graph of all possible domain names.
201 Global DNS: Using the short set of facets listed in "Naming system",
202 the global DNS can be defined as follows. Most of the rules here
203 come from [RFC1034] and [RFC1035], although the term "global DNS"
204 has not been defined before now.
206 Composition of names -- A name in the global DNS has one or more
207 labels. The length of each label is between 0 and 63 octets
208 inclusive. In a fully-qualified domain name, the last label in
209 the ordered list is 0 octets long; it is the only label whose
210 length may be 0 octets, and it is called the "root" or "root
211 label". A domain name in the global DNS has a maximum total
212 length of 255 octets in the wire format; the root represents one
213 octet for this calculation. (Multicast DNS [RFC6762] allows names
214 up to 255 bytes plus a terminating zero byte based on a different
215 interpretation of RFC 1035 and what is included in the 255
216 octets.)
218 Format of names -- Names in the global DNS are domain names.
219 There are three formats: wire format, presentation format, and
220 common display.
222 The basic wire format for names in the global DNS is a list of
223 labels ordered by decreasing distance from the root, with the root
224 label last. Each label is preceded by a length octet. [RFC1035]
225 also defines a compression scheme that modifies this format.
227 The presentation format for names in the global DNS is a list of
228 labels ordered by decreasing distance from the root, encoded as
229 ASCII, with a "." character between each label. In presentation
230 format, a fully-qualified domain name includes the root label and
231 the associated separator dot. For example, in presentation
232 format, a fully-qualified domain name with two non-root labels is
233 always shown as "example.tld." instead of "example.tld".
234 [RFC1035] defines a method for showing octets that do not display
235 in ASCII.
237 The common display format is used in applications and free text.
238 It is the same as the presentation format, but showing the root
239 label and the "." before it is optional and is rarely done. For
240 example, in common display format, a fully-qualified domain name
241 with two non-root labels is usually shown as "example.tld" instead
242 of "example.tld.". Names in the common display format are
243 normally written such that the directionality of the writing
244 system presents labels by decreasing distance from the root (so,
245 in both English and the C programming language the root or TLD
246 label in the ordered list is right-most; but in Arabic it may be
247 left-most, depending on local conventions).
249 Administration of names -- Administration is specified by
250 delegation (see the definition of "delegation" in Section 7).
251 Policies for administration of the root zone in the global DNS are
252 determined by the names operational community, which convenes
253 itself in the Internet Corporation for Assigned Names and Numbers
254 (ICANN). The names operational community selects the IANA
255 Functions Operator for the global DNS root zone. At the time this
256 document is published, that operator is Public Technical
257 Identifiers (PTI). (See for more
258 information about PTI operating the IANA Functions.) The name
259 servers that serve the root zone are provided by independent root
260 operators. Other zones in the global DNS have their own policies
261 for administration.
263 Types of data that can be associated with names -- A name can have
264 zero or more resource records associated with it. There are
265 numerous types of resource records with unique data structures
266 defined in many different RFCs and in the IANA registry at
267 [IANA_Resource_Registry].
269 Types of metadata for names -- Any name that is published in the
270 DNS appears as a set of resource records (see the definition of
271 "RRset" in Section 5). Some names do not themselves have data
272 associated with them in the DNS, but "appear" in the DNS anyway
273 because they form part of a longer name that does have data
274 associated with it (see the definition of "empty non-terminals" in
275 Section 7).
277 Protocol for getting data from a name -- The protocol described in
278 [RFC1035].
280 Context for resolving a name -- The global DNS root zone
281 distributed by PTI.
283 Private DNS: Names that use the protocol described in [RFC1035] but
284 that do not rely on the global DNS root zone, or names that are
285 otherwise not generally available on the Internet but are using
286 the protocol described in [RFC1035]. A system can use both the
287 global DNS and one or more private DNS systems; for example, see
288 "Split DNS" in Section 6.
290 Note that domain names that do not appear in the DNS, and that are
291 intended never to be looked up using the DNS protocol, are not
292 part of the global DNS or a private DNS even though they are
293 domain names.
295 Multicast DNS: "Multicast DNS (mDNS) provides the ability to perform
296 DNS-like operations on the local link in the absence of any
297 conventional Unicast DNS server. In addition, Multicast DNS
298 designates a portion of the DNS namespace to be free for local
299 use, without the need to pay any annual fee, and without the need
300 to set up delegations or otherwise configure a conventional DNS
301 server to answer for those names." (Quoted from [RFC6762],
302 Abstract) Although it uses a compatible wire format, mDNS is
303 strictly speaking a different protocol than DNS. Also, where the
304 above quote says "a portion of the DNS namespace", it would be
305 clearer to say "a portion of the domain name space" The names in
306 mDNS are not intended to be looked up in the DNS.
308 Locally served DNS zone: A locally served DNS zone is a special case
309 of private DNS. Names are resolved using the DNS protocol in a
310 local context. [RFC6303] defines subdomains of IN-ADDR.ARPA that
311 are locally served zones. Resolution of names through locally
312 served zones may result in ambiguous results. For example, the
313 same name may resolve to different results in different locally
314 served DNS zone contexts. The context for a locally served DNS
315 zone may be explicit, for example, as defined in [RFC6303], or
316 implicit, as defined by local DNS administration and not known to
317 the resolution client.
319 Fully qualified domain name (FQDN): This is often just a clear way
320 of saying the same thing as "domain name of a node", as outlined
321 above. However, the term is ambiguous. Strictly speaking, a
322 fully qualified domain name would include every label, including
323 the zero-length label of the root: such a name would be written
324 "www.example.net." (note the terminating dot). But because every
325 name eventually shares the common root, names are often written
326 relative to the root (such as "www.example.net") and are still
327 called "fully qualified". This term first appeared in [RFC0819].
328 In this document, names are often written relative to the root.
330 The need for the term "fully qualified domain name" comes from the
331 existence of partially qualified domain names, which are names
332 where one or more of the last labels in the ordered list are
333 omitted (for example, a domain name of "www" relative to
334 "example.net" identifies "www.example.net"). Such relative names
335 are understood only by context.
337 Host name: This term and its equivalent, "hostname", have been
338 widely used but are not defined in [RFC1034], [RFC1035],
339 [RFC1123], or [RFC2181]. The DNS was originally deployed into the
340 Host Tables environment as outlined in [RFC0952], and it is likely
341 that the term followed informally from the definition there. Over
342 time, the definition seems to have shifted. "Host name" is often
343 meant to be a domain name that follows the rules in Section 3.5 of
344 [RFC1034], the "preferred name syntax" (that is, every character
345 in each label is a letter, a digit, or a hyphen). Note that any
346 label in a domain name can contain any octet value; hostnames are
347 generally considered to be domain names where every label follows
348 the rules in the "preferred name syntax", with the amendment that
349 labels can start with ASCII digits (this amendment comes from
350 Section 2.1 of [RFC1123]).
352 People also sometimes use the term hostname to refer to just the
353 first label of an FQDN, such as "printer" in
354 "printer.admin.example.com". (Sometimes this is formalized in
355 configuration in operating systems.) In addition, people
356 sometimes use this term to describe any name that refers to a
357 machine, and those might include labels that do not conform to the
358 "preferred name syntax".
360 TLD: A Top-Level Domain, meaning a zone that is one layer below the
361 root, such as "com" or "jp". There is nothing special, from the
362 point of view of the DNS, about TLDs. Most of them are also
363 delegation-centric zones (defined in Section 7, and there are
364 significant policy issues around their operation. TLDs are often
365 divided into sub-groups such as Country Code Top-Level Domains
366 (ccTLDs), Generic Top-Level Domains (gTLDs), and others; the
367 division is a matter of policy, and beyond the scope of this
368 document.
370 IDN: The common abbreviation for "Internationalized Domain Name".
371 The IDNA protocol is the standard mechanism for handling domain
372 names with non-ASCII characters in applications in the DNS. The
373 current standard at the time of this writing, normally called
374 "IDNA2008", is defined in [RFC5890], [RFC5891], [RFC5892],
375 [RFC5893], and [RFC5894]. These documents define many IDN-
376 specific terms such as "LDH label", "A-label", and "U-label".
377 [RFC6365] defines more terms that relate to internationalization
378 (some of which relate to IDNs), and [RFC6055] has a much more
379 extensive discussion of IDNs, including some new terminology.
381 Subdomain: "A domain is a subdomain of another domain if it is
382 contained within that domain. This relationship can be tested by
383 seeing if the subdomain's name ends with the containing domain's
384 name." (Quoted from [RFC1034], Section 3.1). For example, in the
385 host name "nnn.mmm.example.com", both "mmm.example.com" and
386 "nnn.mmm.example.com" are subdomains of "example.com". Note that
387 the comparisons here are done on whole labels; that is,
388 "ooo.example.com" is not a subdomain of "oo.example.com".
390 Alias: The owner of a CNAME resource record, or a subdomain of the
391 owner of a DNAME resource record (DNAME records are defined in
392 [RFC6672]). See also "canonical name".
394 Canonical name: A CNAME resource record "identifies its owner name
395 as an alias, and specifies the corresponding canonical name in the
396 RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2)
397 This usage of the word "canonical" is related to the mathematical
398 concept of "canonical form".
400 CNAME: "It is traditional to refer to the owner of a CNAME record as
401 'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of
402 'canonical name', and the owner of a CNAME record is an alias, not
403 a canonical name." (Quoted from [RFC2181], Section 10.1.1)
405 3. DNS Response Codes
407 Some of response codes that are defined in [RFC1035] have acquired
408 their own shorthand names. All of the RCODEs are listed at
409 [IANA_Resource_Registry], although that site uses mixed-case
410 capitalization, while most documents use all-caps. Some of the
411 common names are described here, but the official list is in the IANA
412 registry.
414 NOERROR: "No error condition" (Quoted from [RFC1035],
415 Section 4.1.1.)
417 FORMERR: "Format error - The name server was unable to interpret the
418 query." (Quoted from [RFC1035], Section 4.1.1.)
420 SERVFAIL: "Server failure - The name server was unable to process
421 this query due to a problem with the name server." (Quoted from
422 [RFC1035], Section 4.1.1.)
424 NXDOMAIN: "Name Error - This code signifies that the domain name
425 referenced in the query does not exist." (Quoted from [RFC1035],
426 Section 4.1.1.) [RFC2308] established NXDOMAIN as a synonym for
427 Name Error.
429 NOTIMP: "Not Implemented - The name server does not support the
430 requested kind of query." (Quoted from [RFC1035], Section 4.1.1.)
432 REFUSED: "Refused - The name server refuses to perform the specified
433 operation for policy reasons. For example, a name server may not
434 wish to provide the information to the particular requester, or a
435 name server may not wish to perform a particular operation (e.g.,
436 zone transfer) for particular data." (Quoted from [RFC1035],
437 Section 4.1.1.)
439 NODATA: "A pseudo RCODE which indicates that the name is valid for
440 the given class, but there are no records of the given type. A
441 NODATA response has to be inferred from the answer." (Quoted from
442 [RFC2308], Section 1.) "NODATA is indicated by an answer with the
443 RCODE set to NOERROR and no relevant answers in the answer
444 section. The authority section will contain an SOA record, or
445 there will be no NS records there." (Quoted from [RFC2308],
446 Section 2.2.) Note that referrals have a similar format to NODATA
447 replies; [RFC2308] explains how to distinguish them.
449 The term "NXRRSET" is sometimes used as a synonym for NODATA.
450 However, this is a mistake, given that NXRRSET is a specific error
451 code defined in [RFC2136].
453 Negative response: A response that indicates that a particular RRset
454 does not exist, or whose RCODE indicates the nameserver cannot
455 answer. Sections 2 and 7 of [RFC2308] describe the types of
456 negative responses in detail.
458 4. DNS Transactions
460 The header of a DNS message is its first 12 octets. Many of the
461 fields and flags in the header diagram in Sections 4.1.1 through
462 4.1.3 of [RFC1035] are referred to by their names in that diagram.
463 For example, the response codes are called "RCODEs", the data for a
464 record is called the "RDATA", and the authoritative answer bit is
465 often called "the AA flag" or "the AA bit".
467 Class: A class "identifies a protocol family or instance of a
468 protocol" (Quoted from [RFC1034], Section 3.6). "The DNS tags all
469 data with a class as well as the type, so that we can allow
470 parallel use of different formats for data of type address."
471 (Quoted from [RFC1034], Section 2.2). In practice, the class for
472 nearly every query is "IN". There are some queries for "CH", but
473 they are usually for the purposes of information about the server
474 itself rather than for a different type of address.
476 QNAME: The most commonly-used rough definition is that the QNAME is
477 a field in the Question section of a query. "A standard query
478 specifies a target domain name (QNAME), query type (QTYPE), and
479 query class (QCLASS) and asks for RRs which match." (Quoted from
480 [RFC1034], Section 3.7.1.). Strictly speaking, the definition
481 comes from [RFC1035], Section 4.1.2, where the QNAME is defined in
482 respect of the Question Section. This definition appears to be
483 applied consistently: the discussion of inverse queries in section
484 6.4 refers to the "owner name of the query RR and its TTL",
485 because inverse queries populate the Answer Section and leave the
486 Question Section empty. (Inverse queries are deprecated in
487 [RFC3425], and so relevant definitions do not appear in this
488 document.)
490 [RFC2308], however, has an alternate definition that puts the
491 QNAME in the answer (or series of answers) instead of the query.
492 It defines QNAME as: "...the name in the query section of an
493 answer, or where this resolves to a CNAME, or CNAME chain, the
494 data field of the last CNAME. The last CNAME in this sense is
495 that which contains a value which does not resolve to another
496 CNAME." This definition has a certain internal logic, because of
497 the way CNAME substitution works and the definition of CNAME. If
498 a name server does not find an RRset that matches a query, but it
499 finds the same name in the same class with a CNAME record, then
500 the name server "includes the CNAME record in the response and
501 restarts the query at the domain name specified in the data field
502 of the CNAME record." (Quoted from [RFC1034] Section 3.6.2).
503 This is made explicit in the resolution algorithm outlined in
504 Section 4.3.2 of [RFC1034], which says to "change QNAME to the
505 canonical name in the CNAME RR, and go back to step 1" in the case
506 of a CNAME RR. Since a CNAME record explicitly declares that the
507 owner name is canonically named what is in the RDATA, then there
508 is a way to view the new name (i.e. the name that was in the RDATA
509 of the CNAME RR) as also being the QNAME.
511 This creates a kind of confusion, however, because the response to
512 a query that results in CNAME processing contains in the echoed
513 Question Section one QNAME (the name in the original query), and a
514 second QNAME that is in the data field of the last CNAME. The
515 confusion comes from the iterative/recursive mode of resolution,
516 which finally returns an answer that need not actually have the
517 same owner name as the QNAME contained in the original query.
519 To address this potential confusion, it is helpful to distinguish
520 between three meanings:
522 * QNAME (original): The name actually sent in the Question
523 Section in the original query, which is always echoed in the
524 (final) reply in the Question Section when the QR bit is set to
525 1.
527 * QNAME (effective): A name actually resolved, which is either
528 the name originally queried, or a name received in a CNAME
529 chain response.
531 * QNAME (final): The name actually resolved, which is either the
532 name actually queried or else the last name in a CNAME chain
533 response.
535 Note that, because the definition in [RFC2308] is actually for a
536 different concept than what was in [RFC1034], it would have been
537 better if [RFC2308] had used a different name for that concept.
538 In general use today, QNAME almost always means what is defined
539 above as "QNAME (original)".
541 Referrals: A type of response in which a server, signaling that it
542 is not (completely) authoritative for an answer, provides the
543 querying resolver with an alternative place to send its query.
544 Referrals can be partial.
546 A referral arises when a server is not performing recursive
547 service while answering a query. It appears in step 3(b) of the
548 algorithm in [RFC1034], Section 4.3.2.
550 There are two types of referral response. The first is a downward
551 referral (sometimes described as "delegation response"), where the
552 server is authoritative for some portion of the QNAME. The
553 authority section RRset's RDATA contains the name servers
554 specified at the referred-to zone cut. In normal DNS operation,
555 this kind of response is required in order to find names beneath a
556 delegation. The bare use of "referral" means this kind of
557 referral, and many people believe that this is the only legitimate
558 kind of referral in the DNS.
560 The second is an upward referral (sometimes described as "root
561 referral"), where the server is not authoritative for any portion
562 of the QNAME. When this happens, the referred-to zone in the
563 authority section is usually the root zone (.). In normal DNS
564 operation, this kind of response is not required for resolution or
565 for correctly answering any query. There is no requirement that
566 any server send upward referrals. Some people regard upward
567 referrals as a sign of a misconfiguration or error. Upward
568 referrals always need some sort of qualifier (such as "upward" or
569 "root"), and are never identified by the bare word "referral".
571 A response that has only a referral contains an empty answer
572 section. It contains the NS RRset for the referred-to zone in the
573 authority section. It may contain RRs that provide addresses in
574 the additional section. The AA bit is clear.
576 In the case where the query matches an alias, and the server is
577 not authoritative for the target of the alias but it is
578 authoritative for some name above the target of the alias, the
579 resolution algorithm will produce a response that contains both
580 the authoritative answer for the alias, and also a referral. Such
581 a partial answer and referral response has data in the answer
582 section. It has the NS RRset for the referred-to zone in the
583 authority section. It may contain RRs that provide addresses in
584 the additional section. The AA bit is set, because the first name
585 in the answer section matches the QNAME and the server is
586 authoritative for that answer (see [RFC1035], Section 4.1.1).
588 5. Resource Records
590 RR: An acronym for resource record. ([RFC1034], Section 3.6.)
592 RRset: A set of resource records "with the same label, class and
593 type, but with different data". (Definition from [RFC2181],
594 Section 5) Also spelled RRSet in some documents. As a
595 clarification, "same label" in this definition means "same owner
596 name". In addition, [RFC2181] states that "the TTLs of all RRs in
597 an RRSet must be the same".
599 Note that RRSIG resource records do not match this definition.
600 [RFC4035] says: "An RRset MAY have multiple RRSIG RRs associated
601 with it. Note that as RRSIG RRs are closely tied to the RRsets
602 whose signatures they contain, RRSIG RRs, unlike all other DNS RR
603 types, do not form RRsets. In particular, the TTL values among
604 RRSIG RRs with a common owner name do not follow the RRset rules
605 described in [RFC2181]."
607 Master file: "Master files are text files that contain RRs in text
608 form. Since the contents of a zone can be expressed in the form
609 of a list of RRs a master file is most often used to define a
610 zone, though it can be used to list a cache's contents." (Quoted
611 from [RFC1035], Section 5.) Master files are sometimes called
612 "zone files".
614 Presentation format: The text format used in master files. This
615 format is shown but not formally defined in [RFC1034] and
616 [RFC1035]. The term "presentation format" first appears in
617 [RFC4034].
619 EDNS: The extension mechanisms for DNS, defined in [RFC6891].
620 Sometimes called "EDNS0" or "EDNS(0)" to indicate the version
621 number. EDNS allows DNS clients and servers to specify message
622 sizes larger than the original 512 octet limit, to expand the
623 response code space, and to carry additional options that affect
624 the handling of a DNS query.
626 OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to
627 contain control information pertaining to the question-and-answer
628 sequence of a specific transaction. (Definition from [RFC6891],
629 Section 6.1.1) It is used by EDNS.
631 Owner: "The domain name where a RR is found" (Quoted from [RFC1034],
632 Section 3.6). Often appears in the term "owner name".
634 SOA field names: DNS documents, including the definitions here,
635 often refer to the fields in the RDATA of an SOA resource record
636 by field name. "SOA" stands for "start of a zone of authority".
637 Those fields are defined in Section 3.3.13 of [RFC1035]. The
638 names (in the order they appear in the SOA RDATA) are MNAME,
639 RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. Note that the
640 meaning of MINIMUM field is updated in Section 4 of [RFC2308]; the
641 new definition is that the MINIMUM field is only "the TTL to be
642 used for negative responses". This document tends to use field
643 names instead of terms that describe the fields.
645 TTL: The maximum "time to live" of a resource record. "A TTL value
646 is an unsigned number, with a minimum value of 0, and a maximum
647 value of 2147483647. That is, a maximum of 2^31 - 1. When
648 transmitted, the TTL is encoded in the less significant 31 bits of
649 the 32 bit TTL field, with the most significant, or sign, bit set
650 to zero." (Quoted from [RFC2181], Section 8) (Note that [RFC1035]
651 erroneously stated that this is a signed integer; that was fixed
652 by [RFC2181].)
654 The TTL "specifies the time interval that the resource record may
655 be cached before the source of the information should again be
656 consulted". (Quoted from [RFC1035], Section 3.2.1) Also: "the
657 time interval (in seconds) that the resource record may be cached
658 before it should be discarded". (Quoted from [RFC1035],
659 Section 4.1.3). Despite being defined for a resource record, the
660 TTL of every resource record in an RRset is required to be the
661 same ([RFC2181], Section 5.2).
663 The reason that the TTL is the maximum time to live is that a
664 cache operator might decide to shorten the time to live for
665 operational purposes, such as if there is a policy to disallow TTL
666 values over a certain number. Some servers are known to ignore
667 the TTL on some RRsets (such as when the authoritative data has a
668 very short TTL) even though this is against the advice in RFC
669 1035. An RRset can be flushed from the cache before the end of
670 the TTL interval, at which point the value of the TTL becomes
671 unknown because the RRset with which it was associated no longer
672 exists.
674 There is also the concept of a "default TTL" for a zone, which can
675 be a configuration parameter in the server software. This is
676 often expressed by a default for the entire server, and a default
677 for a zone using the $TTL directive in a zone file. The $TTL
678 directive was added to the master file format by [RFC2308].
680 Class independent: A resource record type whose syntax and semantics
681 are the same for every DNS class. A resource record type that is
682 not class independent has different meanings depending on the DNS
683 class of the record, or the meaning is undefined for some class.
684 Most resource record types are defined for class 1 (IN, the
685 Internet), but many are undefined for other classes.
687 Address records: Records whose type is A or AAAA. [RFC2181]
688 informally defines these as "(A, AAAA, etc)". Note that new types
689 of address records could be defined in the future.
691 6. DNS Servers and Clients
693 This section defines the terms used for the systems that act as DNS
694 clients, DNS servers, or both. In the RFCs, DNS servers are
695 sometimes called "name servers", "nameservers", or just "servers".
696 There is no formal definition of DNS server, but the RFCs generally
697 assume that it is an Internet server that listens for queries and
698 sends responses using the DNS protocol defined in [RFC1035] and its
699 successors.
701 It is important to note that the terms "DNS server" and "name server"
702 require context in order to understand the services being provided.
703 Both authoritative servers and recursive resolvers are often called
704 "DNS servers" and "name servers" even though they serve different
705 roles (but may be part of the same software package).
707 For terminology specific to the public DNS root server system, see
708 [RSSAC026]. That document defines terms such as "root server", "root
709 server operator", and terms that are specific to the way that the
710 root zone of the public DNS is served.
712 Resolver: A program "that extract[s] information from name servers
713 in response to client requests." (Quoted from [RFC1034],
714 Section 2.4) A resolver performs queries for a name, type, and
715 class, and receives responses. The logical function is called
716 "resolution". In practice, the term is usually referring to some
717 specific type of resolver (some of which are defined below), and
718 understanding the use of the term depends on understanding the
719 context.
721 A related term is "resolve", which is not formally defined in
722 [RFC1034] or [RFC1035]. An imputed definition might be "asking a
723 question that consists of a domain name, class, and type, and
724 receiving some sort of response". Similarly, an imputed
725 definition of "resolution" might be "the response received from
726 resolving".
728 Stub resolver: A resolver that cannot perform all resolution itself.
729 Stub resolvers generally depend on a recursive resolver to
730 undertake the actual resolution function. Stub resolvers are
731 discussed but never fully defined in Section 5.3.1 of [RFC1034].
732 They are fully defined in Section 6.1.3.1 of [RFC1123].
734 Iterative mode: A resolution mode of a server that receives DNS
735 queries and responds with a referral to another server.
736 Section 2.3 of [RFC1034] describes this as "The server refers the
737 client to another server and lets the client pursue the query". A
738 resolver that works in iterative mode is sometimes called an
739 "iterative resolver". See also "iterative resolution" later in
740 this section.
742 Recursive mode: A resolution mode of a server that receives DNS
743 queries and either responds to those queries from a local cache or
744 sends queries to other servers in order to get the final answers
745 to the original queries. Section 2.3 of [RFC1034] describes this
746 as "The first server pursues the query for the client at another
747 server". Section 4.3.1 of [RFC1034] says "in [recursive] mode the
748 name server acts in the role of a resolver and returns either an
749 error or the answer, but never referrals." That same section also
750 says "The recursive mode occurs when a query with RD set arrives
751 at a server which is willing to provide recursive service; the
752 client can verify that recursive mode was used by checking that
753 both RA and RD are set in the reply."
755 A server operating in recursive mode may be thought of as having a
756 name server side (which is what answers the query) and a resolver
757 side (which performs the resolution function). Systems operating
758 in this mode are commonly called "recursive servers". Sometimes
759 they are called "recursive resolvers". In practice it is not
760 possible to know in advance whether the server that one is
761 querying will also perform recursion; both terms can be observed
762 in use interchangeably.
764 Recursive resolver: A resolver that acts in recursive mode. In
765 general, a recursive resolver is expected to cache the answers it
766 receives (which would make it a full-service resolver), but some
767 recursive resolvers might not cache.
769 [RFC4697] tried to differentiate between a recursive resolver and
770 an iterative resolver.
772 Recursive query: A query with the Recursion Desired (RD) bit set to
773 1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive
774 service is available and is requested by the RD bit in the query,
775 the server uses its resolver to answer the query. (See
776 Section 4.3.2 of [RFC1035].)
778 Non-recursive query: A query with the Recursion Desired (RD) bit set
779 to 0 in the header. A server can answer non-recursive queries
780 using only local information: the response contains either an
781 error, the answer, or a referral to some other server "closer" to
782 the answer. (See Section 4.3.1 of [RFC1035].)
784 Iterative resolution: A name server may be presented with a query
785 that can only be answered by some other server. The two general
786 approaches to dealing with this problem are "recursive", in which
787 the first server pursues the query on behalf of the client at
788 another server, and "iterative", in which the server refers the
789 client to another server and lets the client pursue the query
790 there. (See Section 2.3 of [RFC1034].)
792 In iterative resolution, the client repeatedly makes non-recursive
793 queries and follows referrals and/or aliases. The iterative
794 resolution algorithm is described in Section 5.3.3 of [RFC1034].
796 Full resolver: This term is used in [RFC1035], but it is not defined
797 there. RFC 1123 defines a "full-service resolver" that may or may
798 not be what was intended by "full resolver" in [RFC1035]. This
799 term is not properly defined in any RFC.
801 Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this
802 term to mean a resolver that acts in recursive mode with a cache
803 (and meets other requirements).
805 Priming: "The act of finding the list of root servers from a
806 configuration that lists some or all of the purported IP addresses
807 of some or all of those root servers." (Quoted from [RFC8109],
808 Section 2.) In order to operate in recursive mode, a resolver
809 needs to know the address of at least one root server. Priming is
810 most often done from a configuration setting that contains a list
811 of authoritative servers for the root zone.
813 Root hints: "Operators who manage a DNS recursive resolver typically
814 need to configure a 'root hints file'. This file contains the
815 names and IP addresses of the authoritative name servers for the
816 root zone, so the software can bootstrap the DNS resolution
817 process. For many pieces of software, this list comes built into
818 the software." (Quoted from [IANA_RootFiles]) This file is often
819 used in priming.
821 Negative caching: "The storage of knowledge that something does not
822 exist, cannot give an answer, or does not give an answer."
823 (Quoted from [RFC2308], Section 1)
825 Authoritative server: "A server that knows the content of a DNS zone
826 from local knowledge, and thus can answer queries about that zone
827 without needing to query other servers." (Quoted from [RFC2182],
828 Section 2.) An authoritative server is named in the NS ("name
829 server") record in a zone. It is a system that responds to DNS
830 queries with information about zones for which it has been
831 configured to answer with the AA flag in the response header set
832 to 1. It is a server that has authority over one or more DNS
833 zones. Note that it is possible for an authoritative server to
834 respond to a query without the parent zone delegating authority to
835 that server. Authoritative servers also provide "referrals",
836 usually to child zones delegated from them; these referrals have
837 the AA bit set to 0 and come with referral data in the Authority
838 and (if needed) the Additional sections.
840 Authoritative-only server: A name server that only serves
841 authoritative data and ignores requests for recursion. It will
842 "not normally generate any queries of its own. Instead, it
843 answers non-recursive queries from iterative resolvers looking for
844 information in zones it serves." (Quoted from [RFC4697],
845 Section 2.4) In this case, "ignores requests for recursion" means
846 "responds to requests for recursion with responses indicating that
847 recursion was not performed".
849 Zone transfer: The act of a client requesting a copy of a zone and
850 an authoritative server sending the needed information. (See
851 Section 7 for a description of zones.) There are two common
852 standard ways to do zone transfers: the AXFR ("Authoritative
853 Transfer") mechanism to copy the full zone (described in
854 [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy
855 only parts of the zone that have changed (described in [RFC1995]).
856 Many systems use non-standard methods for zone transfer outside
857 the DNS protocol.
859 Slave server: See "Secondary server".
861 Secondary server: "An authoritative server which uses zone transfer
862 to retrieve the zone" (Quoted from [RFC1996], Section 2.1).
863 Secondary servers are also discussed in [RFC1034]. [RFC2182]
864 describes secondary servers in more detail. Although early DNS
865 RFCs such as [RFC1996] referred to this as a "slave", the current
866 common usage has shifted to calling it a "secondary".
868 Master server: See "Primary server".
870 Primary server: "Any authoritative server configured to be the
871 source of zone transfer for one or more [secondary] servers"
872 (Quoted from [RFC1996], Section 2.1) or, more specifically, "an
873 authoritative server configured to be the source of AXFR or IXFR
874 data for one or more [secondary] servers" (Quoted from [RFC2136]).
875 Primary servers are also discussed in [RFC1034]. Although early
876 DNS RFCs such as [RFC1996] referred to this as a "master", the
877 current common usage has shifted to "primary".
879 Primary master: "The primary master is named in the zone's SOA MNAME
880 field and optionally by an NS RR". (Quoted from [RFC1996],
881 Section 2.1). [RFC2136] defines "primary master" as "Master
882 server at the root of the AXFR/IXFR dependency graph. The primary
883 master is named in the zone's SOA MNAME field and optionally by an
884 NS RR. There is by definition only one primary master server per
885 zone."
887 The idea of a primary master is only used in [RFC1996] and
888 [RFC2136]. A modern interpretation of the term "primary master"
889 is a server that is both authoritative for a zone and that gets
890 its updates to the zone from configuration (such as a master file)
891 or from UPDATE transactions.
893 Stealth server: This is "like a slave server except not listed in an
894 NS RR for the zone." (Quoted from [RFC1996], Section 2.1)
896 Hidden master: A stealth server that is a primary server for zone
897 transfers. "In this arrangement, the master name server that
898 processes the updates is unavailable to general hosts on the
899 Internet; it is not listed in the NS RRset." (Quoted from
900 [RFC6781], Section 3.4.3). An earlier RFC, [RFC4641], said that
901 the hidden master's name "appears in the SOA RRs MNAME field",
902 although in some setups, the name does not appear at all in the
903 public DNS. A hidden master can also be a secondary server for
904 the zone itself.
906 Forwarding: The process of one server sending a DNS query with the
907 RD bit set to 1 to another server to resolve that query.
909 Forwarding is a function of a DNS resolver; it is different than
910 simply blindly relaying queries.
912 [RFC5625] does not give a specific definition for forwarding, but
913 describes in detail what features a system that forwards needs to
914 support. Systems that forward are sometimes called "DNS proxies",
915 but that term has not yet been defined (even in [RFC5625]).
917 Forwarder: Section 1 of [RFC2308] describes a forwarder as "a
918 nameserver used to resolve queries instead of directly using the
919 authoritative nameserver chain". [RFC2308] further says "The
920 forwarder typically either has better access to the internet, or
921 maintains a bigger cache which may be shared amongst many
922 resolvers." That definition appears to suggest that forwarders
923 normally only query authoritative servers. In current use,
924 however, forwarders often stand between stub resolvers and
925 recursive servers. [RFC2308] is silent on whether a forwarder is
926 iterative-only or can be a full-service resolver.
928 Policy-implementing resolver: A resolver acting in recursive mode
929 that changes some of the answers that it returns based on policy
930 criteria, such as to prevent access to malware sites or
931 objectionable content. In general, a stub resolver has no idea
932 whether upstream resolvers implement such policy or, if they do,
933 the exact policy about what changes will be made. In some cases,
934 the user of the stub resolver has selected the policy-implementing
935 resolver with the explicit intention of using it to implement the
936 policies. In other cases, policies are imposed without the user
937 of the stub resolver being informed.
939 Open resolver: A full-service resolver that accepts and processes
940 queries from any (or nearly any) client. This is sometimes also
941 called a "public resolver", although the term "public resolver" is
942 used more with open resolvers that are meant to be open, as
943 compared to the vast majority of open resolvers that are probably
944 misconfigured to be open. Open resolvers are discussed in
945 [RFC5358]
947 Split DNS: The terms "split DNS" and "split-horizon DNS" have long
948 been used in the DNS community without formal definition. In
949 general, they refer to situations in which DNS servers that are
950 authoritative for a particular set of domains provide partly or
951 completely different answers in those domains depending on the
952 source of the query. The effect of this is that a domain name
953 that is notionally globally unique nevertheless has different
954 meanings for different network users. This can sometimes be the
955 result of a "view" configuration, described below.
957 [RFC2775], Section 3.8 gives a related definition that is too
958 specific to be generally useful.
960 View: A configuration for a DNS server that allows it to provide
961 different responses depending on attributes of the query, such as
962 for "split DNS". Typically, views differ by the source IP address
963 of a query, but can also be based on the destination IP address,
964 the type of query (such as AXFR), whether it is recursive, and so
965 on. Views are often used to provide more names or different
966 addresses to queries from "inside" a protected network than to
967 those "outside" that network. Views are not a standardized part
968 of the DNS, but they are widely implemented in server software.
970 Passive DNS: A mechanism to collect DNS data by storing DNS
971 responses from name servers. Some of these systems also collect
972 the DNS queries associated with the responses, although doing so
973 raises some privacy concerns. Passive DNS databases can be used
974 to answer historical questions about DNS zones such as which
975 values were present at a given time in the past, or when a name
976 was spotted first. Passive DNS databases allow searching of the
977 stored records on keys other than just the name and type, such as
978 "find all names which have A records of a particular value".
980 Anycast: "The practice of making a particular service address
981 available in multiple, discrete, autonomous locations, such that
982 datagrams sent are routed to one of several available locations."
983 (Quoted from [RFC4786], Section 2) See [RFC4786] for more detail
984 on Anycast and other terms that are specific to its use.
986 Instance: "When anycast routing is used to allow more than one
987 server to have the same IP address, each one of those servers is
988 commonly referred to as an 'instance'." "An instance of a server,
989 such as a root server, is often referred to as an 'Anycast
990 instance'." (Quoted from [RSSAC026])
992 Privacy-enabling DNS server: "A DNS server that implements DNS over
993 TLS [RFC7858] and may optionally implement DNS over DTLS
994 [RFC8094]." (Quoted from [RFC8310], Section 2) Other types of DNS
995 servers might also be considerd privacy-enabling, such as those
996 running DNS over HTTPS [I-D.ietf-doh-dns-over-https].
998 7. Zones
1000 This section defines terms that are used when discussing zones that
1001 are being served or retrieved.
1003 Zone: "Authoritative information is organized into units called
1004 'zones', and these zones can be automatically distributed to the
1005 name servers which provide redundant service for the data in a
1006 zone." (Quoted from [RFC1034], Section 2.4)
1008 Child: "The entity on record that has the delegation of the domain
1009 from the Parent." (Quoted from [RFC7344], Section 1.1)
1011 Parent: "The domain in which the Child is registered." (Quoted from
1012 [RFC7344], Section 1.1) Earlier, "parent name server" was defined
1013 in [RFC0882] as "the name server that has authority over the place
1014 in the domain name space that will hold the new domain". (Note
1015 that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].)
1016 [RFC0819] also has some description of the relationship between
1017 parents and children.
1019 Origin:
1021 There are two different uses for this term:
1023 (a) "The domain name that appears at the top of a zone (just below
1024 the cut that separates the zone from its parent). The name of the
1025 zone is the same as the name of the domain at the zone's origin."
1026 (Quoted from [RFC2181], Section 6.) These days, this sense of
1027 "origin" and "apex" (defined below) are often used
1028 interchangeably.
1030 (b) The domain name within which a given relative domain name
1031 appears in zone files. Generally seen in the context of
1032 "$ORIGIN", which is a control entry defined in [RFC1035],
1033 Section 5.1, as part of the master file format. For example, if
1034 the $ORIGIN is set to "example.org.", then a master file line for
1035 "www" is in fact an entry for "www.example.org.".
1037 Apex: The point in the tree at an owner of an SOA and corresponding
1038 authoritative NS RRset. This is also called the "zone apex".
1039 [RFC4033] defines it as "the name at the child's side of a zone
1040 cut". The "apex" can usefully be thought of as a data-theoretic
1041 description of a tree structure, and "origin" is the name of the
1042 same concept when it is implemented in zone files. The
1043 distinction is not always maintained in use, however, and one can
1044 find uses that conflict subtly with this definition. [RFC1034]
1045 uses the term "top node of the zone" as a synonym of "apex", but
1046 that term is not widely used. These days, the first sense of
1047 "origin" (above) and "apex" are often used interchangeably.
1049 Zone cut: The delimitation point between two zones where the origin
1050 of one of the zones is the child of the other zone.
1052 "Zones are delimited by 'zone cuts'. Each zone cut separates a
1053 'child' zone (below the cut) from a 'parent' zone (above the
1054 cut)." (Quoted from [RFC2181], Section 6; note that this is
1055 barely an ostensive definition.) Section 4.2 of [RFC1034] uses
1056 "cuts" instead of "zone cut".
1058 Delegation: The process by which a separate zone is created in the
1059 name space beneath the apex of a given domain. Delegation happens
1060 when an NS RRset is added in the parent zone for the child origin.
1061 Delegation inherently happens at a zone cut. The term is also
1062 commonly a noun: the new zone that is created by the act of
1063 delegating.
1065 Authoritative data: "All of the RRs attached to all of the nodes
1066 from the top node of the zone down to leaf nodes or nodes above
1067 cuts around the bottom edge of the zone." (Quoted from [RFC1034],
1068 Section 4.2.1) Note that this definition might inadvertently also
1069 cause any NS records that appear in the zone to be included, even
1070 those that might not truly be authoritative because there are
1071 identical NS RRs below the zone cut. This reveals the ambiguity
1072 in the notion of authoritative data, because the parent-side NS
1073 records authoritatively indicate the delegation, even though they
1074 are not themselves authoritative data.
1076 [RFC4033], Section 2, defines "Authoritative RRset" which is
1077 related to authoritative data but has a more precise definition.
1079 Lame delegation: "A lame delegations exists when a nameserver is
1080 delegated responsibility for providing nameservice for a zone (via
1081 NS records) but is not performing nameservice for that zone
1082 (usually because it is not set up as a primary or secondary for
1083 the zone)." (Quoted from [RFC1912], Section 2.8)
1085 Another definition is that a lame delegation "happens when a name
1086 server is listed in the NS records for some domain and in fact it
1087 is not a server for that domain. Queries are thus sent to the
1088 wrong servers, who don't know nothing (at least not as expected)
1089 about the queried domain. Furthermore, sometimes these hosts (if
1090 they exist!) don't even run name servers." (Quoted from
1091 [RFC1713], Section 2.3)
1093 Glue records: "[Resource records] which are not part of the
1094 authoritative data [of the zone], and are address resource records
1095 for the [name servers in subzones]. These RRs are only necessary
1096 if the name server's name is 'below' the cut, and are only used as
1097 part of a referral response." Without glue "we could be faced
1098 with the situation where the NS RRs tell us that in order to learn
1099 a name server's address, we should contact the server using the
1100 address we wish to learn." (Definition from [RFC1034],
1101 Section 4.2.1)
1103 A later definition is that glue "includes any record in a zone
1104 file that is not properly part of that zone, including nameserver
1105 records of delegated sub-zones (NS records), address records that
1106 accompany those NS records (A, AAAA, etc), and any other stray
1107 data that might appear" (Quoted from [RFC2181], Section 5.4.1).
1108 Although glue is sometimes used today with this wider definition
1109 in mind, the context surrounding the [RFC2181] definition suggests
1110 it is intended to apply to the use of glue within the document
1111 itself and not necessarily beyond.
1113 Bailiwick: "In-bailiwick" is an adjective to describe a name server
1114 whose name is either a subdomain of or (rarely) the same as the
1115 origin of the zone that contains the delegation to the name
1116 server. In-bailiwick name servers may have glue records in their
1117 parent zone (using the first of the definitions of "glue records"
1118 in the definition above). (The term "bailiwick" means the
1119 district or territory where a bailiff or policeman has
1120 jurisdiction.)
1122 "In-bailiwick" names are divided into two type of name server
1123 names: "in-domain" names and "sibling domain" names.
1125 * In-domain: an adjective to describe a name server whose name is
1126 either subordinate to or (rarely) the same as the owner name of
1127 the NS resource records. An in-domain name server name MUST
1128 have glue records or name resolution fails. For example, a
1129 delegation for "child.example.com" may have "in-domain" name
1130 server name "ns.child.example.com".
1132 * Sibling domain: a name server's name that is either subordinate
1133 to or (rarely) the same as the zone origin and not subordinate
1134 to or the same as the owner name of the NS resource records.
1135 Glue records for sibling domains are allowed, but not
1136 necessary. For example, a delegation for "child.example.com"
1137 in "example.com" zone may have "sibling" name server name
1138 "ns.another.example.com".
1140 "Out-of-bailiwick" is the antonym of in-bailiwick. An adjective
1141 to describe a name server whose name is not subordinate to or the
1142 same as the zone origin. Glue records for out-of-bailiwick name
1143 servers are useless. Following table shows examples of delegation
1144 types.
1146 Delegation |Parent|Name Server Name | Type
1147 -----------+------+------------------+-----------------------------
1148 com | . |a.gtld-servers.net|in-bailiwick / sibling domain
1149 net | . |a.gtld-servers.net|in-bailiwick / in-domain
1150 example.org| org |ns.example.org |in-bailiwick / in-domain
1151 example.org| org |ns.ietf.org |in-bailiwick / sibling domain
1152 example.org| org |ns.example.com |out-of-bailiwick
1153 example.jp | jp |ns.example.jp |in-bailiwick / in-domain
1154 example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain
1155 example.jp | jp |ns.example.com |out-of-bailiwick
1157 Root zone: The zone of a DNS-based tree whose apex is the zero-
1158 length label. Also sometimes called "the DNS root".
1160 Empty non-terminals (ENT): "Domain names that own no resource
1161 records but have subdomains that do." (Quoted from [RFC4592],
1162 Section 2.2.2.) A typical example is in SRV records: in the name
1163 "_sip._tcp.example.com", it is likely that "_tcp.example.com" has
1164 no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV
1165 RRset.
1167 Delegation-centric zone: A zone that consists mostly of delegations
1168 to child zones. This term is used in contrast to a zone that
1169 might have some delegations to child zones, but also has many data
1170 resource records for the zone itself and/or for child zones. The
1171 term is used in [RFC4956] and [RFC5155], but is not defined there.
1173 Occluded name: "The addition of a delegation point via dynamic
1174 update will render all subordinate domain names to be in a limbo,
1175 still part of the zone, but not available to the lookup process.
1176 The addition of a DNAME resource record has the same impact. The
1177 subordinate names are said to be 'occluded'." (Quoted from
1178 [RFC5936], Section 3.5)
1180 Fast flux DNS: This "occurs when a domain is found in DNS using A
1181 records to multiple IP addresses, each of which has a very short
1182 Time-to-Live (TTL) value associated with it. This means that the
1183 domain resolves to varying IP addresses over a short period of
1184 time." (Quoted from [RFC6561], Section 1.1.5, with typo
1185 corrected) In addition to having legitimate uses, fast flux DNS
1186 can used to deliver malware. Because the addresses change so
1187 rapidly, it is difficult to ascertain all the hosts. It should be
1188 noted that the technique also works with AAAA records, but such
1189 use is not frequently observed on the Internet as of this writing.
1191 Reverse DNS, reverse lookup: "The process of mapping an address to a
1192 name is generally known as a 'reverse lookup', and the IN-
1193 ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse
1194 DNS'." (Quoted from [RFC5855], Section 1)
1196 Forward lookup: "Hostname-to-address translation". (Quoted from
1197 [RFC2133], Section 6)
1199 arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain
1200 was originally established as part of the initial deployment of
1201 the DNS, to provide a transition mechanism from the Host Tables
1202 that were common in the ARPANET, as well as a home for the IPv4
1203 reverse mapping domain. During 2000, the abbreviation was
1204 redesignated to 'Address and Routing Parameter Area' in the hope
1205 of reducing confusion with the earlier network name." (Quoted
1206 from [RFC3172], Section 2.) .arpa is an "infrastructure domain", a
1207 domain whose "role is to support the operating infrastructure of
1208 the Internet". (Quoted from [RFC3172], Section 2.) See [RFC3172]
1209 for more history of this name.
1211 Service name: "Service names are the unique key in the Service Name
1212 and Transport Protocol Port Number registry. This unique symbolic
1213 name for a service may also be used for other purposes, such as in
1214 DNS SRV records." (Quoted from [RFC6335], Section 5.)
1216 8. Wildcards
1218 Wildcard: [RFC1034] defined "wildcard", but in a way that turned out
1219 to be confusing to implementers. For an extended discussion of
1220 wildcards, including clearer definitions, see [RFC4592]. Special
1221 treatment is given to RRs with owner names starting with the label
1222 "*". "Such RRs are called 'wildcards'. Wildcard RRs can be
1223 thought of as instructions for synthesizing RRs." (Quoted from
1224 [RFC1034], Section 4.3.3)
1226 Asterisk label: "The first octet is the normal label type and length
1227 for a 1-octet-long label, and the second octet is the ASCII
1228 representation for the '*' character. A descriptive name of a
1229 label equaling that value is an 'asterisk label'." (Quoted from
1230 [RFC4592], Section 2.1.1)
1232 Wildcard domain name: "A 'wildcard domain name' is defined by having
1233 its initial (i.e., leftmost or least significant) label be
1234 asterisk label." (Quoted from [RFC4592], Section 2.1.1)
1236 Closest encloser: "The longest existing ancestor of a name."
1237 (Quoted from [RFC5155], Section 1.3) An earlier definition is "The
1238 node in the zone's tree of existing domain names that has the most
1239 labels matching the query name (consecutively, counting from the
1240 root label downward). Each match is a 'label match' and the order
1241 of the labels is the same." (Quoted from [RFC4592],
1242 Section 3.3.1)
1244 Closest provable encloser: "The longest ancestor of a name that can
1245 be proven to exist. Note that this is only different from the
1246 closest encloser in an Opt-Out zone." (Quoted from [RFC5155],
1247 Section 1.3) See Section 10 for more on "opt-out".
1249 Next closer name: "The name one label longer than the closest
1250 provable encloser of a name." (Quoted from [RFC5155],
1251 Section 1.3)
1253 Source of Synthesis: "The source of synthesis is defined in the
1254 context of a query process as that wildcard domain name
1255 immediately descending from the closest encloser, provided that
1256 this wildcard domain name exists. 'Immediately descending' means
1257 that the source of synthesis has a name of the form: .." (Quoted from [RFC4592],
1259 Section 3.3.1)
1261 9. Registration Model
1263 Registry: The administrative operation of a zone that allows
1264 registration of names within that zone. People often use this
1265 term to refer only to those organizations that perform
1266 registration in large delegation-centric zones (such as TLDs); but
1267 formally, whoever decides what data goes into a zone is the
1268 registry for that zone. This definition of "registry" is from a
1269 DNS point of view; for some zones, the policies that determine
1270 what can go in the zone are decided by zones that are
1271 superordinate and not the registry operator.
1273 Registrant: An individual or organization on whose behalf a name in
1274 a zone is registered by the registry. In many zones, the registry
1275 and the registrant may be the same entity, but in TLDs they often
1276 are not.
1278 Registrar: A service provider that acts as a go-between for
1279 registrants and registries. Not all registrations require a
1280 registrar, though it is common to have registrars involved in
1281 registrations in TLDs.
1283 EPP: The Extensible Provisioning Protocol (EPP), which is commonly
1284 used for communication of registration information between
1285 registries and registrars. EPP is defined in [RFC5730].
1287 WHOIS: A protocol specified in [RFC3912], often used for querying
1288 registry databases. WHOIS data is frequently used to associate
1289 registration data (such as zone management contacts) with domain
1290 names. The term "WHOIS data" is often used as a synonym for the
1291 registry database, even though that database may be served by
1292 different protocols, particularly RDAP. The WHOIS protocol is
1293 also used with IP address registry data.
1295 RDAP: The Registration Data Access Protocol, defined in [RFC7480],
1296 [RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The
1297 RDAP protocol and data format are meant as a replacement for
1298 WHOIS.
1300 DNS operator: An entity responsible for running DNS servers. For a
1301 zone's authoritative servers, the registrant may act as their own
1302 DNS operator, or their registrar may do it on their behalf, or
1303 they may use a third-party operator. For some zones, the registry
1304 function is performed by the DNS operator plus other entities who
1305 decide about the allowed contents of the zone.
1307 Public suffix: "A domain that is controlled by a public registry."
1308 (Quoted from [RFC6265], Section 5.3) A common definition for this
1309 term is a domain under which subdomains can be registered by third
1310 parties, and on which HTTP cookies (which are described in detail
1311 in [RFC6265]) should not be set. There is no indication in a
1312 domain name whether it is a public suffix; that can only be
1313 determined by outside means. In fact, both a domain and a
1314 subdomain of that domain can be public suffixes.
1316 There is nothing inherent in a domain name to indicate whether it
1317 is a public suffix. One resource for identifying public suffixes
1318 is the Public Suffix List (PSL) maintained by Mozilla
1319 (http://publicsuffix.org/).
1321 For example, at the time this document is published, the "com.au"
1322 domain is listed as a public suffix in the PSL. (Note that this
1323 example might change in the future.)
1325 Note that the term "public suffix" is controversial in the DNS
1326 community for many reasons, and may be significantly changed in
1327 the future. One example of the difficulty of calling a domain a
1328 public suffix is that designation can change over time as the
1329 registration policy for the zone changes, such as was the case
1330 with the "uk" TLD in 2014.
1332 Subordinate and Superordinate: These terms are introduced in
1333 [RFC3731] for use in the registration model, but not defined
1334 there. Instead, they are given in examples. "For example, domain
1335 name 'example.com' has a superordinate relationship to host name
1336 ns1.example.com'." "For example, host ns1.example1.com is a
1337 subordinate host of domain example1.com, but it is a not a
1338 subordinate host of domain example2.com." (Quoted from [RFC3731],
1339 Section 1.1.) These terms are strictly ways of referring to the
1340 relationship standing of two domains where one is a subdomain of
1341 the other.
1343 10. General DNSSEC
1345 Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and
1346 [RFC5155]. The terms that have caused confusion in the DNS community
1347 are highlighted here.
1349 DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in
1350 some RFCs, have not been formally defined. However, Section 2 of
1351 [RFC4033] defines many types of resolvers and validators,
1352 including "non-validating security-aware stub resolver", "non-
1353 validating stub resolver", "security-aware name server",
1354 "security-aware recursive name server", "security-aware resolver",
1355 "security-aware stub resolver", and "security-oblivious
1356 'anything'". (Note that the term "validating resolver", which is
1357 used in some places in DNSSEC-related documents, is also not
1358 defined in those RFCs, but is defined below.)
1360 Signed zone: "A zone whose RRsets are signed and that contains
1361 properly constructed DNSKEY, Resource Record Signature (RRSIG),
1362 Next Secure (NSEC), and (optionally) DS records." (Quoted from
1363 [RFC4033], Section 2.) It has been noted in other contexts that
1364 the zone itself is not really signed, but all the relevant RRsets
1365 in the zone are signed. Nevertheless, if a zone that should be
1366 signed contains any RRsets that are not signed (or opted out),
1367 those RRsets will be treated as bogus, so the whole zone needs to
1368 be handled in some way.
1370 It should also be noted that, since the publication of [RFC6840],
1371 NSEC records are no longer required for signed zones: a signed
1372 zone might include NSEC3 records instead. [RFC7129] provides
1373 additional background commentary and some context for the NSEC and
1374 NSEC3 mechanisms used by DNSSEC to provide authenticated denial-
1375 of-existence responses. NSEC and NSEC3 are described below.
1377 Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that
1378 is not signed". Section 2 of [RFC4035] defines this as "A zone
1379 that does not include these records [properly constructed DNSKEY,
1380 Resource Record Signature (RRSIG), Next Secure (NSEC), and
1381 (optionally) DS records] according to the rules in this section".
1382 There is an important note at the end of Section 5.2 of [RFC4035]
1383 that defines an additional situation in which a zone is considered
1384 unsigned: "If the resolver does not support any of the algorithms
1385 listed in an authenticated DS RRset, then the resolver will not be
1386 able to verify the authentication path to the child zone. In this
1387 case, the resolver SHOULD treat the child zone as if it were
1388 unsigned."
1390 NSEC: "The NSEC record allows a security-aware resolver to
1391 authenticate a negative reply for either name or type non-
1392 existence with the same mechanisms used to authenticate other DNS
1393 replies." (Quoted from [RFC4033], Section 3.2.) In short, an
1394 NSEC record provides authenticated denial of existence.
1396 "The NSEC resource record lists two separate things: the next
1397 owner name (in the canonical ordering of the zone) that contains
1398 authoritative data or a delegation point NS RRset, and the set of
1399 RR types present at the NSEC RR's owner name." (Quoted from
1400 Section 4 of RFC 4034)
1402 NSEC3: Like the NSEC record, the NSEC3 record also provides
1403 authenticated denial of existence; however, NSEC3 records mitigate
1404 against zone enumeration and support Opt-Out. NSEC3 resource
1405 records require associated NSEC3PARAM resource records. NSEC3 and
1406 NSEC3PARAM resource records are defined in [RFC5155].
1408 Note that [RFC6840] says that [RFC5155] "is now considered part of
1409 the DNS Security Document Family as described by Section 10 of
1410 [RFC4033]". This means that some of the definitions from earlier
1411 RFCs that only talk about NSEC records should probably be
1412 considered to be talking about both NSEC and NSEC3.
1414 Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover
1415 unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1.)
1416 Opt-out tackles the high costs of securing a delegation to an
1417 insecure zone. When using Opt-Out, names that are an insecure
1418 delegation (and empty non-terminals that are only derived from
1419 insecure delegations) don't require an NSEC3 record or its
1420 corresponding RRSIG records. Opt-Out NSEC3 records are not able
1421 to prove or deny the existence of the insecure delegations.
1422 (Adapted from [RFC7129], Section 5.1)
1424 Insecure delegation: "A signed name containing a delegation (NS
1425 RRset), but lacking a DS RRset, signifying a delegation to an
1426 unsigned subzone." (Quoted from [RFC4956], Section 2.)
1428 Zone enumeration: "The practice of discovering the full content of a
1429 zone via successive queries." (Quoted from [RFC5155],
1430 Section 1.3.) This is also sometimes called "zone walking". Zone
1431 enumeration is different from zone content guessing where the
1432 guesser uses a large dictionary of possible labels and sends
1433 successive queries for them, or matches the contents of NSEC3
1434 records against such a dictionary.
1436 Validation: Validation, in the context of DNSSEC, refers to one of
1437 the following:
1439 * Checking the validity of DNSSEC signatures
1441 * Checking the validity of DNS responses, such as those including
1442 authenticated denial of existence
1444 * Building an authentication chain from a trust anchor to a DNS
1445 response or individual DNS RRsets in a response
1447 The first two definitions above consider only the validity of
1448 individual DNSSEC components such as the RRSIG validity or NSEC
1449 proof validity. The third definition considers the components of
1450 the entire DNSSEC authentication chain, and thus requires
1451 "configured knowledge of at least one authenticated DNSKEY or DS
1452 RR" (as described in [RFC4035], Section 5).
1454 [RFC4033], Section 2, says that a "Validating Security-Aware Stub
1455 Resolver... performs signature validation" and uses a trust anchor
1456 "as a starting point for building the authentication chain to a
1457 signed DNS response", and thus uses the first and third
1458 definitions above. The process of validating an RRSIG resource
1459 record is described in [RFC4035], Section 5.3.
1461 [RFC5155] refers to validating responses throughout the document,
1462 in the context of hashed authenticated denial of existence; this
1463 uses the second definition above.
1465 The term "authentication" is used interchangeably with
1466 "validation", in the sense of the third definition above.
1467 [RFC4033], Section 2, describes the chain linking trust anchor to
1468 DNS data as the "authentication chain". A response is considered
1469 to be authentic if "all RRsets in the Answer and Authority
1470 sections of the response [are considered] to be authentic" (Quoted
1471 from [RFC4035]). DNS data or responses deemed to be authentic or
1472 validated have a security status of "secure" ([RFC4035],
1473 Section 4.3; [RFC4033], Section 5). "Authenticating both DNS keys
1474 and data is a matter of local policy, which may extend or even
1475 override the [DNSSEC] protocol extensions" (Quoted from [RFC4033],
1476 Section 3.1).
1478 The term "verification", when used, is usually synonym for
1479 "validation".
1481 Validating resolver: A security-aware recursive name server,
1482 security-aware resolver, or security-aware stub resolver that is
1483 applying at least one of the definitions of validation (above), as
1484 appropriate to the resolution context. For the same reason that
1485 the generic term "resolver" is sometimes ambiguous and needs to be
1486 evaluated in context (see Section 6), "validating resolver" is a
1487 context-sensitive term.
1489 Key signing key (KSK): DNSSEC keys that "only sign the apex DNSKEY
1490 RRset in a zone." (Quoted from [RFC6781], Section 3.1)
1492 Zone signing key (ZSK): "DNSSEC keys that can be used to sign all
1493 the RRsets in a zone that require signatures, other than the apex
1494 DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Also note
1495 that a ZSK is sometimes used to sign the apex DNSKEY RRset.
1497 Combined signing key (CSK): "In cases where the differentiation
1498 between the KSK and ZSK is not made, i.e., where keys have the
1499 role of both KSK and ZSK, we talk about a Single-Type Signing
1500 Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes
1501 called a "combined signing key" or CSK. It is operational
1502 practice, not protocol, that determines whether a particular key
1503 is a ZSK, a KSK, or a CSK.
1505 Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be
1506 used to distinguish between keys that are intended to be used as
1507 the secure entry point into the zone when building chains of
1508 trust, i.e., they are (to be) pointed to by parental DS RRs or
1509 configured as a trust anchor. Therefore, it is suggested that the
1510 SEP flag be set on keys that are used as KSKs and not on keys that
1511 are used as ZSKs, while in those cases where a distinction between
1512 a KSK and ZSK is not made (i.e., for a Single-Type Signing
1513 Scheme), it is suggested that the SEP flag be set on all keys."
1514 (Quoted from [RFC6781], Section 3.2.3.) Note that the SEP flag is
1515 only a hint, and its presence or absence may not be used to
1516 disqualify a given DNSKEY RR from use as a KSK or ZSK during
1517 validation.
1519 The original definition of SEPs was in [RFC3757]. That definition
1520 clearly indicated that the SEP was a key, not just a bit in the
1521 key. The abstract of [RFC3757] says: "With the Delegation Signer
1522 (DS) resource record (RR), the concept of a public key acting as a
1523 secure entry point (SEP) has been introduced. During exchanges of
1524 public keys with the parent there is a need to differentiate SEP
1525 keys from other public keys in the Domain Name System KEY (DNSKEY)
1526 resource record set. A flag bit in the DNSKEY RR is defined to
1527 indicate that DNSKEY is to be used as a SEP." That definition of
1528 the SEP as a key was made obsolete by [RFC4034], and the
1529 definition from [RFC6781] is consistent with [RFC4034].
1531 Trust anchor: "A configured DNSKEY RR or DS RR hash of a DNSKEY RR.
1532 A validating security-aware resolver uses this public key or hash
1533 as a starting point for building the authentication chain to a
1534 signed DNS response. In general, a validating resolver will have
1535 to obtain the initial values of its trust anchors via some secure
1536 or trusted means outside the DNS protocol." (Quoted from
1537 [RFC4033], Section 2)
1539 DNSSEC Policy (DP): A statement that "sets forth the security
1540 requirements and standards to be implemented for a DNSSEC-signed
1541 zone." (Quoted from [RFC6841], Section 2)
1543 DNSSEC Practice Statement (DPS): "A practices disclosure document
1544 that may support and be a supplemental document to the DNSSEC
1545 Policy (if such exists), and it states how the management of a
1546 given zone implements procedures and controls at a high level."
1547 (Quoted from [RFC6841], Section 2)
1549 Hardware security module (HSM): A specialized piece of hardware that
1550 is used to create keys for signatures and to sign messages without
1551 ever disclosing the private key. In DNSSEC, HSMs are often used
1552 to hold the private keys for KSKs and ZSKs and to create the
1553 signatures used in RRSIG records at periodic intervals.
1555 Signing software: Authoritative DNS servers that support DNSSEC
1556 often contain software that facilitates the creation and
1557 maintenance of DNSSEC signatures in zones. There is also stand-
1558 alone software that can be used to sign a zone regardless of
1559 whether the authoritative server itself supports signing.
1560 Sometimes signing software can support particular HSMs as part of
1561 the signing process.
1563 11. DNSSEC States
1565 A validating resolver can determine that a response is in one of four
1566 states: secure, insecure, bogus, or indeterminate. These states are
1567 defined in [RFC4033] and [RFC4035], although the two definitions
1568 differ a bit. This document makes no effort to reconcile the two
1569 definitions, and takes no position as to whether they need to be
1570 reconciled.
1572 Section 5 of [RFC4033] says:
1574 A validating resolver can determine the following 4 states:
1576 Secure: The validating resolver has a trust anchor, has a chain
1577 of trust, and is able to verify all the signatures in the
1578 response.
1580 Insecure: The validating resolver has a trust anchor, a chain
1581 of trust, and, at some delegation point, signed proof of the
1582 non-existence of a DS record. This indicates that subsequent
1583 branches in the tree are provably insecure. A validating
1584 resolver may have a local policy to mark parts of the domain
1585 space as insecure.
1587 Bogus: The validating resolver has a trust anchor and a secure
1588 delegation indicating that subsidiary data is signed, but
1589 the response fails to validate for some reason: missing
1590 signatures, expired signatures, signatures with unsupported
1591 algorithms, data missing that the relevant NSEC RR says
1592 should be present, and so forth.
1594 Indeterminate: There is no trust anchor that would indicate that a
1595 specific portion of the tree is secure. This is the default
1596 operation mode.
1598 Section 4.3 of [RFC4035] says:
1600 A security-aware resolver must be able to distinguish between four
1601 cases:
1603 Secure: An RRset for which the resolver is able to build a chain
1604 of signed DNSKEY and DS RRs from a trusted security anchor to
1605 the RRset. In this case, the RRset should be signed and is
1606 subject to signature validation, as described above.
1608 Insecure: An RRset for which the resolver knows that it has no
1609 chain of signed DNSKEY and DS RRs from any trusted starting
1610 point to the RRset. This can occur when the target RRset lies
1611 in an unsigned zone or in a descendent [sic] of an unsigned
1612 zone. In this case, the RRset may or may not be signed, but
1613 the resolver will not be able to verify the signature.
1615 Bogus: An RRset for which the resolver believes that it ought to
1616 be able to establish a chain of trust but for which it is
1617 unable to do so, either due to signatures that for some reason
1618 fail to validate or due to missing data that the relevant
1619 DNSSEC RRs indicate should be present. This case may indicate
1620 an attack but may also indicate a configuration error or some
1621 form of data corruption.
1623 Indeterminate: An RRset for which the resolver is not able to
1624 determine whether the RRset should be signed, as the resolver
1625 is not able to obtain the necessary DNSSEC RRs. This can occur
1626 when the security-aware resolver is not able to contact
1627 security-aware name servers for the relevant zones.
1629 12. Security Considerations
1631 These definitions do not change any security considerations for the
1632 DNS.
1634 13. IANA Considerations
1636 None.
1638 14. References
1640 14.1. Normative References
1642 [IANA_RootFiles]
1643 Internet Assigned Numbers Authority, "IANA Root Files",
1644 2016, .
1646 [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities",
1647 RFC 882, DOI 10.17487/RFC0882, November 1983,
1648 .
1650 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
1651 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
1652 .
1654 [RFC1035] Mockapetris, P., "Domain names - implementation and
1655 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
1656 November 1987, .
1658 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
1659 Application and Support", STD 3, RFC 1123,
1660 DOI 10.17487/RFC1123, October 1989,
1661 .
1663 [RFC1912] Barr, D., "Common DNS Operational and Configuration
1664 Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,
1665 .
1667 [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
1668 Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
1669 August 1996, .
1671 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
1672 "Dynamic Updates in the Domain Name System (DNS UPDATE)",
1673 RFC 2136, DOI 10.17487/RFC2136, April 1997,
1674 .
1676 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
1677 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
1678 .
1680 [RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection
1681 and Operation of Secondary DNS Servers", BCP 16, RFC 2182,
1682 DOI 10.17487/RFC2182, July 1997,
1683 .
1685 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
1686 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
1687 .
1689 [RFC3731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
1690 Domain Name Mapping", RFC 3731, DOI 10.17487/RFC3731,
1691 March 2004, .
1693 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1694 Rose, "DNS Security Introduction and Requirements",
1695 RFC 4033, DOI 10.17487/RFC4033, March 2005,
1696 .
1698 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1699 Rose, "Resource Records for the DNS Security Extensions",
1700 RFC 4034, DOI 10.17487/RFC4034, March 2005,
1701 .
1703 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1704 Rose, "Protocol Modifications for the DNS Security
1705 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
1706 .
1708 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
1709 System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
1710 .
1712 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
1713 Security (DNSSEC) Hashed Authenticated Denial of
1714 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
1715 .
1717 [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
1718 Nameservers in Reflector Attacks", BCP 140, RFC 5358,
1719 DOI 10.17487/RFC5358, October 2008,
1720 .
1722 [RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
1723 STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,
1724 .
1726 [RFC5855] Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6
1727 Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855,
1728 May 2010, .
1730 [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
1731 (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
1732 .
1734 [RFC6561] Livingood, J., Mody, N., and M. O'Reirdan,
1735 "Recommendations for the Remediation of Bots in ISP
1736 Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012,
1737 .
1739 [RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
1740 Operational Practices, Version 2", RFC 6781,
1741 DOI 10.17487/RFC6781, December 2012,
1742 .
1744 [RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
1745 Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
1746 DOI 10.17487/RFC6840, February 2013,
1747 .
1749 [RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
1750 Framework for DNSSEC Policies and DNSSEC Practice
1751 Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,
1752 .
1754 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
1755 for DNS (EDNS(0))", STD 75, RFC 6891,
1756 DOI 10.17487/RFC6891, April 2013,
1757 .
1759 [RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
1760 DNSSEC Delegation Trust Maintenance", RFC 7344,
1761 DOI 10.17487/RFC7344, September 2014,
1762 .
1764 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
1765 Terminology", RFC 7719, DOI 10.17487/RFC7719, December
1766 2015, .
1768 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
1769 for DNS over TLS and DNS over DTLS", RFC 8310,
1770 DOI 10.17487/RFC8310, March 2018,
1771 .
1773 14.2. Informative References
1775 [I-D.ietf-doh-dns-over-https]
1776 Hoffman, P. and P. McManus, "DNS Queries over HTTPS
1777 (DoH)", draft-ietf-doh-dns-over-https-14 (work in
1778 progress), August 2018.
1780 [IANA_Resource_Registry]
1781 Internet Assigned Numbers Authority, "Resource Record (RR)
1782 TYPEs", 2017,
1783 .
1785 [RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for
1786 Internet User Applications", RFC 819,
1787 DOI 10.17487/RFC0819, August 1982,
1788 .
1790 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
1791 host table specification", RFC 952, DOI 10.17487/RFC0952,
1792 October 1985, .
1794 [RFC1713] Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713,
1795 DOI 10.17487/RFC1713, November 1994,
1796 .
1798 [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
1799 DOI 10.17487/RFC1995, August 1996,
1800 .
1802 [RFC2133] Gilligan, R., Thomson, S., Bound, J., and W. Stevens,
1803 "Basic Socket Interface Extensions for IPv6", RFC 2133,
1804 DOI 10.17487/RFC2133, April 1997,
1805 .
1807 [RFC2775] Carpenter, B., "Internet Transparency", RFC 2775,
1808 DOI 10.17487/RFC2775, February 2000,
1809 .
1811 [RFC3172] Huston, G., Ed., "Management Guidelines & Operational
1812 Requirements for the Address and Routing Parameter Area
1813 Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
1814 September 2001, .
1816 [RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425,
1817 DOI 10.17487/RFC3425, November 2002,
1818 .
1820 [RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name
1821 System KEY (DNSKEY) Resource Record (RR) Secure Entry
1822 Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April
1823 2004, .
1825 [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912,
1826 DOI 10.17487/RFC3912, September 2004,
1827 .
1829 [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
1830 RFC 4641, DOI 10.17487/RFC4641, September 2006,
1831 .
1833 [RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution
1834 Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,
1835 October 2006, .
1837 [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
1838 Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
1839 December 2006, .
1841 [RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security
1842 (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July
1843 2007, .
1845 [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
1846 BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
1847 .
1849 [RFC5890] Klensin, J., "Internationalized Domain Names for
1850 Applications (IDNA): Definitions and Document Framework",
1851 RFC 5890, DOI 10.17487/RFC5890, August 2010,
1852 .
1854 [RFC5891] Klensin, J., "Internationalized Domain Names in
1855 Applications (IDNA): Protocol", RFC 5891,
1856 DOI 10.17487/RFC5891, August 2010,
1857 .
1859 [RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and
1860 Internationalized Domain Names for Applications (IDNA)",
1861 RFC 5892, DOI 10.17487/RFC5892, August 2010,
1862 .
1864 [RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
1865 for Internationalized Domain Names for Applications
1866 (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
1867 .
1869 [RFC5894] Klensin, J., "Internationalized Domain Names for
1870 Applications (IDNA): Background, Explanation, and
1871 Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
1872 .
1874 [RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
1875 Encodings for Internationalized Domain Names", RFC 6055,
1876 DOI 10.17487/RFC6055, February 2011,
1877 .
1879 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
1880 DOI 10.17487/RFC6265, April 2011,
1881 .
1883 [RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163,
1884 RFC 6303, DOI 10.17487/RFC6303, July 2011,
1885 .
1887 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
1888 Cheshire, "Internet Assigned Numbers Authority (IANA)
1889 Procedures for the Management of the Service Name and
1890 Transport Protocol Port Number Registry", BCP 165,
1891 RFC 6335, DOI 10.17487/RFC6335, August 2011,
1892 .
1894 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
1895 Internationalization in the IETF", BCP 166, RFC 6365,
1896 DOI 10.17487/RFC6365, September 2011,
1897 .
1899 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
1900 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
1901 .
1903 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
1904 DOI 10.17487/RFC6762, February 2013,
1905 .
1907 [RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of
1908 Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
1909 February 2014, .
1911 [RFC7480] Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the
1912 Registration Data Access Protocol (RDAP)", RFC 7480,
1913 DOI 10.17487/RFC7480, March 2015,
1914 .
1916 [RFC7481] Hollenbeck, S. and N. Kong, "Security Services for the
1917 Registration Data Access Protocol (RDAP)", RFC 7481,
1918 DOI 10.17487/RFC7481, March 2015,
1919 .
1921 [RFC7482] Newton, A. and S. Hollenbeck, "Registration Data Access
1922 Protocol (RDAP) Query Format", RFC 7482,
1923 DOI 10.17487/RFC7482, March 2015,
1924 .
1926 [RFC7483] Newton, A. and S. Hollenbeck, "JSON Responses for the
1927 Registration Data Access Protocol (RDAP)", RFC 7483,
1928 DOI 10.17487/RFC7483, March 2015,
1929 .
1931 [RFC7484] Blanchet, M., "Finding the Authoritative Registration Data
1932 (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March
1933 2015, .
1935 [RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin,
1936 "Inventory and Analysis of WHOIS Registration Objects",
1937 RFC 7485, DOI 10.17487/RFC7485, March 2015,
1938 .
1940 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
1941 and P. Hoffman, "Specification for DNS over Transport
1942 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
1943 2016, .
1945 [RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
1946 Transport Layer Security (DTLS)", RFC 8094,
1947 DOI 10.17487/RFC8094, February 2017,
1948 .
1950 [RFC8109] Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS
1951 Resolver with Priming Queries", BCP 209, RFC 8109,
1952 DOI 10.17487/RFC8109, March 2017,
1953 .
1955 [RSSAC026]
1956 Root Server System Advisory Committee (RSSAC), "RSSAC
1957 Lexicon", 2017,
1958 .
1961 Appendix A. Definitions Updated by this Document
1963 The following definitions from RFCs are updated by this document:
1965 o Forwarder in [RFC2308]
1967 o QNAME in [RFC2308]
1969 o Secure Entry Point (SEP) in [RFC3757]; note, however, that this
1970 RFC is already obsolete
1972 Appendix B. Definitions First Defined in this Document
1974 The following definitions are first defined in this document:
1976 o "Alias" in Section 2
1978 o "Apex" in Section 7
1980 o "arpa" in Section 7
1982 o "Bailiwick" in Section 7
1984 o "Class independent" in Section 5
1986 o "Delegation-centric zone" in Section 7
1988 o "Delegation" in Section 7
1990 o "DNS operator" in Section 9
1992 o "DNSSEC-aware" in Section 10
1994 o "DNSSEC-unaware" in Section 10
1996 o "Forwarding" in Section 6
1998 o "Full resolver" in Section 6
2000 o "Fully qualified domain name" in Section 2
2002 o "Global DNS" in Section 2
2004 o "Hardware Security Module (HSM)" in Section 10
2006 o "Host name" in Section 2
2008 o "IDN" in Section 2
2010 o "In-bailiwick" in Section 7
2012 o "Iterative resolution" in Section 6
2014 o "Label" in Section 2
2016 o "Locally served DNS zone" in Section 2
2018 o "Naming system" in Section 2
2019 o "Negative response" in Section 3
2021 o "Non-recursive query" in Section 6
2023 o "Open resolver" in Section 6
2025 o "Out-of-bailiwick" in Section 7
2027 o "Passive DNS" in Section 6
2029 o "Policy-implementing resolver" in Section 6
2031 o "Presentation format" in Section 5
2033 o "Priming" in Section 6
2035 o "Private DNS" in Section 2
2037 o "Recursive resolver" in Section 6
2039 o "Referrals" in Section 4
2041 o "Registrant" in Section 9
2043 o "Registrar" in Section 9
2045 o "Registry" in Section 9
2047 o "Root zone" in Section 7
2049 o "Secure Entry Point (SEP)" in Section 10
2051 o "Signing software" in Section 10
2053 o "Split DNS" in Section 6
2055 o "Stub resolver" in Section 6
2057 o "Subordinate" in Section 8
2059 o "Superordinate" in Section 8
2061 o "TLD" in Section 2
2063 o "Validating resolver" in Section 10
2065 o "Validation" in Section 10
2066 o "View" in Section 6
2068 o "Zone transfer" in Section 6
2070 Index
2072 A
2073 Address records 15
2074 Alias 9
2075 Anycast 21
2076 Apex 22
2077 Asterisk label 26
2078 Authoritative data 23
2079 Authoritative server 18
2080 Authoritative-only server 18
2081 arpa: Address and Routing Parameter Area Domain 26
2083 C
2084 CNAME 9
2085 Canonical name 9
2086 Child 22
2087 Class 10
2088 Class independent 15
2089 Closest encloser 26
2090 Closest provable encloser 27
2091 Combined signing key (CSK) 32
2093 D
2094 DNS operator 28
2095 DNSSEC Policy (DP) 33
2096 DNSSEC Practice Statement (DPS) 33
2097 DNSSEC-aware and DNSSEC-unaware 29
2098 Delegation 23
2099 Delegation-centric zone 25
2100 Domain name 4
2102 E
2103 EDNS 14
2104 EPP 27
2105 Empty non-terminals (ENT) 25
2107 F
2108 FORMERR 9
2109 Fast flux DNS 25
2110 Forward lookup 26
2111 Forwarder 20
2112 Forwarding 19
2113 Full resolver 17
2114 Full-service resolver 17
2115 Fully qualified domain name (FQDN) 7
2117 G
2118 Global DNS 5
2119 Glue records 23
2121 H
2122 Hardware security module (HSM) 33
2123 Hidden master 19
2124 Host name 8
2126 I
2127 IDN 8
2128 In-bailiwick 24
2129 Insecure delegation 30
2130 Instance 21
2131 Iterative mode 16
2132 Iterative resolution 17
2134 K
2135 Key signing key (KSK) 32
2137 L
2138 Label 5
2139 Lame delegation 23
2140 Locally served DNS zone 7
2142 M
2143 Master file 13
2144 Master server 19
2145 Multicast DNS 7
2147 N
2148 NODATA 10
2149 NOERROR 9
2150 NOTIMP 10
2151 NS 18
2152 NSEC 30
2153 NSEC3 30
2154 NXDOMAIN 9
2155 Naming system 4
2156 Negative caching 18
2157 Negative response 10
2158 Next closer name 27
2159 Non-recursive query 17
2161 O
2162 OPT 14
2163 Occluded name 25
2164 Open resolver 20
2165 Opt-out 30
2166 Origin 22
2167 Out-of-bailiwick 24
2168 Owner 14
2170 P
2171 Parent 22
2172 Passive DNS 21
2173 Policy-implementing resolver 20
2174 Presentation format 13
2175 Primary master 19
2176 Primary server 19
2177 Priming 17
2178 Privacy-enabling DNS server 21
2179 Private DNS 6
2180 Public suffix 28
2182 Q
2183 QNAME 11
2185 R
2186 RDAP 28
2187 REFUSED 10
2188 RR 13
2189 RRset 13
2190 Recursive mode 16
2191 Recursive query 17
2192 Recursive resolver 17
2193 Referrals 12
2194 Registrant 27
2195 Registrar 27
2196 Registry 27
2197 Resolver 15
2198 Reverse DNS, reverse lookup 25
2199 Root hints 18
2200 Root zone 25
2202 S
2203 SERVFAIL 9
2204 SOA 14
2205 SOA field names 14
2206 Secondary server 19
2207 Secure Entry Point (SEP) 32
2208 Service name 26
2209 Signed zone 29
2210 Signing software 33
2211 Slave server 18
2212 Source of Synthesis 27
2213 Split DNS 20
2214 Split-horizon DNS 20
2215 Stealth server 19
2216 Stub resolver 16
2217 Subdomain 9
2218 Subordinate 28
2219 Superordinate 28
2221 T
2222 TLD 8
2223 TTL 14
2224 Trust anchor 33
2226 U
2227 Unsigned zone 29
2229 V
2230 Validating resolver 32
2231 Validation 31
2232 View 21
2234 W
2235 WHOIS 27
2236 Wildcard 26
2237 Wildcard domain name 26
2239 Z
2240 Zone 21
2241 Zone cut 22
2242 Zone enumeration 30
2243 Zone signing key (ZSK) 32
2244 Zone transfer 18
2246 Acknowledgements
2248 The following is the Acknowledgements for RFC 7719.
2250 The authors gratefully acknowledge all of the authors of DNS-related
2251 RFCs that proceed this one. Comments from Tony Finch, Stephane
2252 Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John
2253 Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman,
2254 David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis,
2255 John Klensin, David Black, and many others in the DNSOP Working Group
2256 helped shape RFC 7719.
2258 Most of the major changes between RFC 7719 and this document came
2259 from active discussion on the DNSOP WG. Specific people who
2260 contributed material to this document include: Bob Harold, Dick
2261 Franks, Evan Hunt, John Dickinson, Mark Andrews, Martin Hoffmann,
2262 Paul Vixie, Peter Koch, Duane Wessels, Allison Mankin, Giovane Moura,
2263 Roni Even, Dan Romascanu, and Vladmir Cunat.
2265 Authors' Addresses
2267 Paul Hoffman
2268 ICANN
2270 Email: paul.hoffman@icann.org
2272 Andrew Sullivan
2274 Email: ajs@anvilwalrusden.com
2276 Kazunori Fujiwara
2277 Japan Registry Services Co., Ltd.
2278 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
2279 Chiyoda-ku, Tokyo 101-0065
2280 Japan
2282 Phone: +81 3 5215 8451
2283 Email: fujiwara@jprs.co.jp