Internet Engineering Task Force (IETF)                        P. HoffmanRequest for Comments: 9499                                         ICANNBCP: 219                                                     K. FujiwaraObsoletes: 8499                                                     JPRSUpdates: 2308                                                 March 2024Category: Best Current Practice                                         ISSN: 2070-1721                            DNS TerminologyAbstract   The Domain Name System (DNS) is defined in literally dozens of   different RFCs.  The terminology used by implementers and developers   of DNS protocols, and by operators of DNS systems, has changed in the   decades since the DNS was first defined.  This document gives current   definitions for many of the terms used in the DNS in a single   document.   This document updates RFC 2308 by clarifying the definitions of   "forwarder" and "QNAME".  It obsoletes RFC 8499 by adding multiple   terms and clarifications.  Comprehensive lists of changed and new   definitions can be found in Appendices A and B.Status of This Memo   This memo documents an Internet Best Current Practice.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   BCPs is available in Section 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained at   https://www.rfc-editor.org/info/rfc9499.Copyright Notice   Copyright (c) 2024 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject to BCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Revised BSD License text as described in Section 4.e of the   Trust Legal Provisions and are provided without warranty as described   in the Revised BSD License.Table of Contents   1.  Introduction   2.  Names   3.  DNS Response Codes   4.  DNS Transactions   5.  Resource Records   6.  DNS Servers and Clients   7.  Zones   8.  Wildcards   9.  Registration Model   10. General DNSSEC   11. DNSSEC States   12. Security Considerations   13. IANA Considerations   14. References     14.1.  Normative References     14.2.  Informative References   Appendix A.  Definitions Updated by This Document   Appendix B.  Definitions First Defined in This Document   Acknowledgements   Index   Authors' Addresses1.  Introduction   The Domain Name System (DNS) is a simple query-response protocol   whose messages in both directions have the same format.  (Section 2   gives a definition of "global DNS", which is often what people mean   when they say "the DNS".)  The protocol and message format are   defined in [RFC1034] and [RFC1035].  These RFCs defined some terms,   and later documents defined others.  Some of the terms from [RFC1034]   and [RFC1035] have somewhat different meanings now than they did in   1987.   This document contains a collection of a wide variety of DNS-related   terms, organized loosely by topic.  Some of them have been precisely   defined in earlier RFCs, some have been loosely defined in earlier   RFCs, and some are not defined in an earlier RFC at all.   Other organizations sometimes define DNS-related terms in their own   way.  For example, the WHATWG defines "domain" at   <https://url.spec.whatwg.org/>.  The Root Server System Advisory   Committee (RSSAC) has a good lexicon [RSSAC026].   Most of the definitions listed here represent the consensus   definition of the DNS community -- both protocol developers and   operators.  Some of the definitions differ from earlier RFCs, and   those differences are noted.  In this document, where the consensus   definition is the same as the one in an RFC, that RFC is quoted.   Where the consensus definition has changed somewhat, the RFC is   mentioned but the new stand-alone definition is given.  See   Appendix A for a list of the definitions that this document updates.   It is important to note that, during the development of this   document, it became clear that some DNS-related terms are interpreted   quite differently by different DNS experts.  Further, some terms that   are defined in early DNS RFCs now have definitions that are generally   agreed to, but that are different from the original definitions.   This document is a small revision to [RFC8499]; that document was a   substantial revision to [RFC7719].   Note that there is no single consistent definition of "the DNS".  It   can be considered to be some combination of the following: a commonly   used naming scheme for objects on the Internet; a distributed   database representing the names and certain properties of these   objects; an architecture providing distributed maintenance,   resilience, and loose coherency for this database; and a simple   query-response protocol (as mentioned below) implementing this   architecture.  Section 2 defines "global DNS" and "private DNS" as a   way to deal with these differing definitions.   Capitalization in DNS terms is often inconsistent among RFCs and   various DNS practitioners.  The capitalization used in this document   is a best guess at current practices, and is not meant to indicate   that other capitalization styles are wrong or archaic.  In some   cases, multiple styles of capitalization are used for the same term   due to quoting from different RFCs.   In this document, the words "byte" and "octet" are used   interchangeably.  They appear here because they both appear in the   earlier RFCs that defined terms in the DNS.   Readers should note that the terms in this document are grouped by   topic.  Someone who is not already familiar with the DNS probably   cannot learn about the DNS from scratch by reading this document from   front to back.  Instead, skipping around may be the only way to get   enough context to understand some of the definitions.  This document   has an index that might be useful for readers who are attempting to   learn the DNS by reading this document.2.  Names   Naming system:  A naming system associates names with data.  Naming      systems have many significant facets that help differentiate them      from each other.  Some commonly identified facets include:      *  Composition of names      *  Format of names      *  Administration of names      *  Types of data that can be associated with names      *  Types of metadata for names      *  Protocol for getting data from a name      *  Context for resolving a name      Note that this list is a small subset of facets that people have      identified over time for naming systems, and the IETF has yet to      agree on a good set of facets that can be used to compare naming      systems.  For example, other facets might include "protocol to      update data in a name", "privacy of names", and "privacy of data      associated with names", but those are not as well defined as the      ones listed above.  The list here is chosen because it helps      describe the DNS and naming systems similar to the DNS.   Domain name:  An ordered list of one or more labels.      Note that this is a definition independent of the DNS RFCs      ([RFC1034] and [RFC1035]), and the definition here also applies to      systems other than the DNS.  [RFC1034] defines the "domain name      space" using mathematical trees and their nodes in graph theory,      and that definition has the same practical result as the      definition here.  Any path of a directed acyclic graph can be      represented by a domain name consisting of the labels of its      nodes, ordered by decreasing distance from the root(s) (which is      the normal convention within the DNS, including this document).  A      domain name whose last label identifies a root of the graph is      fully qualified; other domain names whose labels form a strict      prefix of a fully qualified domain name are relative to its first      omitted node.      Also note that different IETF and non-IETF documents have used the      term "domain name" in many different ways.  It is common for      earlier documents to use "domain name" to mean "names that match      the syntax in [RFC1035]", but possibly with additional rules such      as "and are, or will be, resolvable in the global DNS" or "but      only using the presentation format".   Label:  An ordered list of zero or more octets that makes up a      portion of a domain name.  Using graph theory, a label identifies      one node in a portion of the graph of all possible domain names.   Global DNS:  Using the short set of facets listed in "Naming system",      the global DNS can be defined as follows.  Most of the rules here      come from [RFC1034] and [RFC1035], although the term "global DNS"      has not been defined before now.      Composition of names:  A name in the global DNS has one or more         labels.  The length of each label is between 0 and 63 octets         inclusive.  In a fully qualified domain name, the last label in         the ordered list is 0 octets long; it is the only label whose         length may be 0 octets, and it is called the "root" or "root         label".  A domain name in the global DNS has a maximum total         length of 255 octets in the wire format; the root represents         one octet for this calculation.  (Multicast DNS [RFC6762]         allows names up to 255 bytes plus a terminating zero byte based         on a different interpretation of RFC 1035 and what is included         in the 255 octets.)      Format of names:  Names in the global DNS are domain names.  There         are three formats: wire format, presentation format, and common         display.         Wire format:  The basic wire format for names in the global DNS            is a list of labels ordered by decreasing distance from the            root, with the root label last.  Each label is preceded by a            length octet.  [RFC1035] also defines a compression scheme            that modifies this format.         Presentation format:  The presentation format for names in the            global DNS is a list of labels ordered by decreasing            distance from the root, encoded as ASCII, with a "."            character between each label.  In presentation format, a            fully qualified domain name includes the root label and the            associated separator dot.  For example, in presentation            format, a fully qualified domain name with two non-root            labels is always shown as "example.tld." instead of            "example.tld".  [RFC1035] defines a method for showing            octets that do not display in ASCII.         Common display format:  The common display format is used in            applications and free text.  It is the same as the            presentation format, but showing the root label and the "."            before it is optional and is rarely done.  For example, in            common display format, a fully qualified domain name with            two non-root labels is usually shown as "example.tld"            instead of "example.tld.".  Names in the common display            format are normally written such that the directionality of            the writing system presents labels by decreasing distance            from the root (so, in both English and the C programming            language, the root or Top-Level Domain (TLD) label in the            ordered list is rightmost; but in Arabic, it may be            leftmost, depending on local conventions).      Administration of names:  Administration is specified by         delegation (see the definition of "delegation" in Section 7).         Policies for administration of the root zone in the global DNS         are determined by the names operational community, which         convenes itself in the Internet Corporation for Assigned Names         and Numbers (ICANN).  The names operational community selects         the IANA Functions Operator for the global DNS root zone.  The         name servers that serve the root zone are provided by         independent root operators.  Other zones in the global DNS have         their own policies for administration.      Types of data that can be associated with names:  A name can have         zero or more resource records associated with it.  There are         numerous types of resource records with unique data structures         defined in many different RFCs and in the IANA registry at         [IANA_Resource_Registry].      Types of metadata for names:  Any name that is published in the         DNS appears as a set of resource records (see the definition of         "RRset" in Section 5).  Some names do not, themselves, have         data associated with them in the DNS, but they "appear" in the         DNS anyway because they form part of a longer name that does         have data associated with it (see the definition of "empty non-         terminals" in Section 7).      Protocol for getting data from a name:  The protocol described in         [RFC1035].      Context for resolving a name:  The global DNS root zone         distributed by Public Technical Identifiers (PTI).   Private DNS:  Names that use the protocol described in [RFC1035] but      do not rely on the global DNS root zone or names that are      otherwise not generally available on the Internet but are using      the protocol described in [RFC1035].  A system can use both the      global DNS and one or more private DNS systems; for example, see      "Split DNS" in Section 6.      Note that domain names that do not appear in the DNS and that are      intended never to be looked up using the DNS protocol are not part      of the global DNS or a private DNS, even though they are domain      names.   Multicast DNS (mDNS):  "Multicast DNS (mDNS) provides the ability to      perform DNS-like operations on the local link in the absence of      any conventional Unicast DNS server.  In addition, Multicast DNS      designates a portion of the DNS namespace to be free for local      use, without the need to pay any annual fee, and without the need      to set up delegations or otherwise configure a conventional DNS      server to answer for those names."  (Quoted from [RFC6762],      Abstract) Although it uses a compatible wire format, mDNS is,      strictly speaking, a different protocol than DNS.  Also, where the      above quote says "a portion of the DNS namespace", it would be      clearer to say "a portion of the domain name space".  The names in      mDNS are not intended to be looked up in the DNS.   Locally served DNS zone:  A locally served DNS zone is a special case      of private DNS.  Names are resolved using the DNS protocol in a      local context.  [RFC6303] defines subdomains of IN-ADDR.ARPA that      are locally served zones.  Resolution of names through locally      served zones may result in ambiguous results.  For example, the      same name may resolve to different results in different locally      served DNS zone contexts.  The context for a locally served DNS      zone may be explicit, such as those that are listed in [RFC6303]      and [RFC7793], or implicit, such as those defined by local DNS      administration and not known to the resolution client.   Fully Qualified Domain Name (FQDN):  This is often just a clear way      of saying the same thing as "domain name of a node", as outlined      above.  However, the term is ambiguous.  Strictly speaking, a      fully qualified domain name would include every label, including      the zero-length label of the root; such a name would be written      "www.example.net." (note the terminating dot).  But, because every      name eventually shares the common root, names are often written      relative to the root (such as "www.example.net") and are still      called "fully qualified".  This term first appeared in [RFC819].      In this document, names are often written relative to the root.      The need for the term "fully qualified domain name" comes from the      existence of partially qualified domain names, which are names      where one or more of the last labels in the ordered list are      omitted (for example, a domain name of "www" relative to      "example.net" identifies "www.example.net").  Such relative names      are understood only by context.   Host name:  This term and its equivalent, "hostname", have been      widely used but are not defined in [RFC1034], [RFC1035],      [RFC1123], or [RFC2181].  The DNS was originally deployed into the      Host Tables environment as outlined in [RFC952], and it is likely      that the term followed informally from the definition there.  Over      time, the definition seems to have shifted.  "Host name" is often      meant to be a domain name that follows the rules in Section 3.5 of      [RFC1034], which is also called the "preferred name syntax".  (In      that syntax, every character in each label is a letter, a digit,      or a hyphen).  Note that any label in a domain name can contain      any octet value; hostnames are generally considered to be domain      names where every label follows the rules in the "preferred name      syntax", with the amendment that labels can start with ASCII      digits (this amendment comes from Section 2.1 of [RFC1123]).      People also sometimes use the term "hostname" to refer to just the      first label of an FQDN, such as "printer" in      "printer.admin.example.com".  (Sometimes this is formalized in      configuration in operating systems.)  In addition, people      sometimes use this term to describe any name that refers to a      machine, and those might include labels that do not conform to the      "preferred name syntax".   Top-Level Domain (TLD):  A Top-Level Domain is a zone that is one      layer below the root, such as "com" or "jp".  There is nothing      special, from the point of view of the DNS, about TLDs.  Most of      them are also delegation-centric zones (defined in Section 7), and      there are significant policy issues around their operation.  TLDs      are often divided into sub-groups such as Country Code Top-Level      Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others;      the division is a matter of policy and beyond the scope of this      document.   Internationalized Domain Name (IDN):  The Internationalized Domain      Names for Applications (IDNA) protocol is the standard mechanism      for handling domain names with non-ASCII characters in      applications in the DNS.  The current standard at the time of this      writing, normally called "IDNA2008", is defined in [RFC5890],      [RFC5891], [RFC5892], [RFC5893], and [RFC5894].  These documents      define many IDN-specific terms such as "LDH label", "A-label", and      "U-label".  [RFC6365] defines more terms that relate to      internationalization (some of which relate to IDNs); [RFC6055] has      a much more extensive discussion of IDNs, including some new      terminology.   Subdomain:  "A domain is a subdomain of another domain if it is      contained within that domain.  This relationship can be tested by      seeing if the subdomain's name ends with the containing domain's      name."  (Quoted from [RFC1034], Section 3.1) For example, in the      host name "nnn.mmm.example.com", both "mmm.example.com" and      "nnn.mmm.example.com" are subdomains of "example.com".  Note that      the comparisons here are done on whole labels; that is,      "ooo.example.com" is not a subdomain of "oo.example.com".   Alias:  The owner of a CNAME resource record, or a subdomain of the      owner of a DNAME resource record (DNAME records are defined in      [RFC6672]).  See also "canonical name".   Canonical name:  A CNAME resource record "identifies its owner name      as an alias, and specifies the corresponding canonical name in the      RDATA section of the RR."  (Quoted from [RFC1034], Section 3.6.2)      This usage of the word "canonical" is related to the mathematical      concept of "canonical form".   CNAME:  "It has been traditional to refer to the [owner] of a CNAME      record as 'a CNAME'.  This is unfortunate, as 'CNAME' is an      abbreviation of 'canonical name', and the [owner] of a CNAME      record is most certainly not a canonical name."  (Quoted from      [RFC2181], Section 10.1.1.  The quoted text has been changed from      "label" to "owner".)3.  DNS Response Codes   Some of the response codes (RCODEs) that are defined in [RFC1035]   have acquired their own shorthand names.  All of the RCODEs are   listed at [IANA_Resource_Registry], although that list uses mixed-   case capitalization, while most documents use all caps.  Some of the   common names for values defined in [RFC1035] are described in this   section.  This section also includes an additional RCODE and a   general definition.  The official list of all RCODEs is in the IANA   registry.   NOERROR:  This RCODE appears as "No error condition" in Section 4.1.1      of [RFC1035].   FORMERR:  This RCODE appears as "Format error - The name server was      unable to interpret the query" in Section 4.1.1 of [RFC1035].   SERVFAIL:  This RCODE appears as "Server failure - The name server      was unable to process this query due to a problem with the name      server" in Section 4.1.1 of [RFC1035].   NXDOMAIN:  This RCODE appears as "Name Error [...] this code      signifies that the domain name referenced in the query does not      exist." in Section 4.1.1 of [RFC1035].  [RFC2308] established      NXDOMAIN as a synonym for Name Error.   NOTIMP:  This RCODE appears as "Not Implemented - The name server      does not support the requested kind of query" in Section 4.1.1 of      [RFC1035].   REFUSED:  This RCODE appears as "Refused - The name server refuses to      perform the specified operation for policy reasons.  For example,      a name server may not wish to provide the information to the      particular requester, or a name server may not wish to perform a      particular operation (e.g., zone transfer) for particular data."      in Section 4.1.1 of [RFC1035].   NODATA:  "A pseudo RCODE which indicates that the name is valid, for      the given class, but [there] are no records of the given type.  A      NODATA response has to be inferred from the answer."  (Quoted from      [RFC2308], Section 1) "NODATA is indicated by an answer with the      RCODE set to NOERROR and no relevant answers in the Answer      section.  The Authority section will contain an SOA record, or      there will be no NS records there."  (Quoted from [RFC2308],      Section 2.2) Note that referrals have a similar format to NODATA      replies; [RFC2308] explains how to distinguish them.      The term "NXRRSET" is sometimes used as a synonym for NODATA.      However, this is a mistake, given that NXRRSET is a specific error      code defined in [RFC2136].   Negative response:  A response that indicates that a particular RRset      does not exist or whose RCODE indicates that the nameserver cannot      answer.  Sections 2 and 7 of [RFC2308] describe the types of      negative responses in detail.4.  DNS Transactions   The header of a DNS message is its first 12 octets.  Many of the   fields and flags in the diagrams in Sections 4.1.1 through 4.1.3 of   [RFC1035] are referred to by their names in each diagram.  For   example, the response codes are called "RCODEs", the data for a   record is called the "RDATA", and the authoritative answer bit is   often called "the AA flag" or "the AA bit".   Class:  A class "identifies a protocol family or instance of a      protocol".  (Quoted from [RFC1034], Section 3.6) "The DNS tags all      data with a class as well as the type, so that we can allow      parallel use of different formats for data of type address."      (Quoted from [RFC1034], Section 2.2) In practice, the class for      nearly every query is "IN" (the Internet).  There are some queries      for "CH" (the Chaos class), but they are usually for the purposes      of information about the server itself rather than for a different      type of address.   QNAME:  The most commonly used rough definition is that the QNAME is      a field in the Question section of a query.  "A standard query      specifies a target domain name (QNAME), query type (QTYPE), and      query class (QCLASS) and asks for RRs which match."  (Quoted from      [RFC1034], Section 3.7.1) Strictly speaking, the definition comes      from [RFC1035], Section 4.1.2, where the QNAME is defined in      respect of the Question section.  This definition appears to be      applied consistently, as the discussion of inverse queries in      Section 6.4.1 of [RFC1035] refers to the "owner name of the query      RR and its TTL" because inverse queries populate the Answer      section and leave the Question section empty.  (Inverse queries      are deprecated in [RFC3425]; thus, relevant definitions do not      appear in this document.)      However, [RFC2308] has an alternate definition that puts the QNAME      in the answer (or series of answers) instead of the query.  It      defines QNAME as "...the name in the query section of an answer,      or where this resolves to a CNAME, or CNAME chain, the data field      of the last CNAME.  The last CNAME in this sense is that which      contains a value which does not resolve to another CNAME."  This      definition has a certain internal logic, because of the way CNAME      substitution works and the definition of CNAME.  If a name server      does not find an RRset that matches a query, but does find the      same name in the same class with a CNAME record, then the name      server "includes the CNAME record in the response and restarts the      query at the domain name specified in the data field of the CNAME      record."  (Quoted from [RFC1034], Section 3.6.2) This is made      explicit in the resolution algorithm outlined in Section 4.3.2 of      [RFC1034], which says to "change QNAME to the canonical name in      the CNAME RR, and go back to step 1" in the case of a CNAME RR.      Since a CNAME record explicitly declares that the owner name is      canonically named what is in the RDATA, then there is a way to      view the new name (i.e., the name that was in the RDATA of the      CNAME RR) as also being the QNAME.      However, this creates confusion because the response to a query      that results in CNAME processing contains in the echoed Question      section one QNAME (the name in the original query) and a second      QNAME that is in the data field of the last CNAME.  The confusion      comes from the iterative/recursive mode of resolution, which      finally returns an answer that need not actually have the same      owner name as the QNAME contained in the original query.      To address this potential confusion, it is helpful to distinguish      between three meanings:      QNAME (original):  The name actually sent in the Question section         in the original query, which is always echoed in the (final)         reply in the Question section when the QR bit is set to 1.      QNAME (effective):  A name actually resolved, which is either the         name originally queried or a name received in a CNAME chain         response.      QNAME (final):  The name actually resolved, which is either the         name actually queried or else the last name in a CNAME chain         response.      Note that, because the definition in [RFC2308] is actually for a      different concept than what was in [RFC1034], it would have been      better if [RFC2308] had used a different name for that concept.      In general use today, QNAME almost always means what is defined      above as "QNAME (original)".   Referrals:  A type of response in which a server, signaling that it      is not (completely) authoritative for an answer, provides the      querying resolver with an alternative place to send its query.      Referrals can be partial.      A referral arises when a server is not performing recursive      service while answering a query.  It appears in step 3(b) of the      algorithm in [RFC1034], Section 4.3.2.      There are two types of referral response.  The first is a downward      referral (sometimes described as "delegation response"), where the      server is authoritative for some portion of the QNAME.  The      Authority section RRset's RDATA contains the name servers      specified at the referred-to zone cut.  In normal DNS operation,      this kind of response is required in order to find names beneath a      delegation.  The bare use of "referral" means this kind of      referral, and many people believe that this is the only legitimate      kind of referral in the DNS.      The second is an upward referral (sometimes described as "root      referral"), where the server is not authoritative for any portion      of the QNAME.  When this happens, the referred-to zone in the      Authority section is usually the root zone (".").  In normal DNS      operation, this kind of response is not required for resolution or      for correctly answering any query.  There is no requirement that      any server send upward referrals.  Some people regard upward      referrals as a sign of a misconfiguration or error.  Upward      referrals always need some sort of qualifier (such as "upward" or      "root") and are never identified simply by the word "referral".      A response that has only a referral contains an empty Answer      section.  It contains the NS RRset for the referred-to zone in the      Authority section.  It may contain RRs that provide addresses in      the Additional section.  The AA bit is clear.      In the case where the query matches an alias, and the server is      not authoritative for the target of the alias but is authoritative      for some name above the target of the alias, the resolution      algorithm will produce a response that contains both the      authoritative answer for the alias and a referral.  Such a partial      answer and referral response has data in the Answer section.  It      has the NS RRset for the referred-to zone in the Authority      section.  It may contain RRs that provide addresses in the      Additional section.  The AA bit is set because the first name in      the Answer section matches the QNAME and the server is      authoritative for that answer (see [RFC1035], Section 4.1.1).5.  Resource Records   RR:  An acronym for resource record.  (See [RFC1034], Section 3.6.)   RRset:  A set of resource records "with the same label, class and      type, but with different data" (according to [RFC2181],      Section 5).  Also written as "RRSet" in some documents.  As a      clarification, "same label" in this definition means "same owner      name".  In addition, [RFC2181] states that "the TTLs of all RRs in      an RRSet must be the same".      Note that RRSIG resource records do not match this definition.      [RFC4035] says:         An RRset MAY have multiple RRSIG RRs associated with it.  Note         that as RRSIG RRs are closely tied to the RRsets whose         signatures they contain, RRSIG RRs, unlike all other DNS RR         types, do not form RRsets.  In particular, the TTL values among         RRSIG RRs with a common owner name do not follow the RRset         rules described in [RFC2181].   Master file:  "Master files are text files that contain RRs in text      form.  Since the contents of a zone can be expressed in the form      of a list of RRs a master file is most often used to define a      zone, though it can be used to list a cache's contents."  (Quoted      from [RFC1035], Section 5) Master files are sometimes called "zone      files".   Presentation format:  The text format used in master files.  This      format is shown but not formally defined in [RFC1034] or      [RFC1035].  The term "presentation format" first appears in      [RFC4034].   EDNS:  The extension mechanisms for DNS, defined in [RFC6891].      Sometimes called "EDNS0" or "EDNS(0)" to indicate the version      number.  EDNS allows DNS clients and servers to specify message      sizes larger than the original 512-octet limit, to expand the      response code space, and to carry additional options that affect      the handling of a DNS query.   OPT:  A pseudo-RR (sometimes called a "meta-RR") that is used only to      contain control information pertaining to the question-and-answer      sequence of a specific transaction.  (Definition paraphrased from      [RFC6891], Section 6.1.1.)  It is used by EDNS.   Owner:  "The domain name where the RR is found."  (Quoted from      [RFC1034], Section 3.6) Often appears in the term "owner name".   SOA field names:  DNS documents, including the definitions here,      often refer to the fields in the RDATA of an SOA resource record      by field name.  "SOA" stands for "start of a zone of authority".      Those fields are defined in Section 3.3.13 of [RFC1035].  The      names (in the order they appear in the SOA RDATA) are MNAME,      RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM.  Note that the      meaning of the MINIMUM field is updated in Section 4 of [RFC2308];      the new definition is that the MINIMUM field is only "the TTL to      be used for negative responses".  This document tends to use field      names instead of terms that describe the fields.   TTL:  The maximum "time to live" of a resource record.  "A TTL value      is an unsigned number, with a minimum value of 0, and a maximum      value of 2147483647.  That is, a maximum of 2^31 - 1.  When      transmitted, this value shall be encoded in the less significant      31 bits of the 32 bit TTL field, with the most significant, or      sign, bit set to zero."  (Quoted from [RFC2181], Section 8) Note      that [RFC1035] erroneously stated that this is a signed integer;      that was fixed by [RFC2181].      The TTL "specifies the time interval that the resource record may      be cached before the source of the information should again be      consulted."  (Quoted from [RFC1035], Section 3.2.1) Section 4.1.3      of [RFC1035] states "the time interval (in seconds) that the      resource record may be cached before it should be discarded".      Despite being defined for a resource record, the TTL of every      resource record in an RRset is required to be the same ([RFC2181],      Section 5.2).      The reason that the TTL is the maximum time to live is that a      cache operator might decide to shorten the time to live for      operational purposes, for example, if there is a policy to      disallow TTL values over a certain number.  Some servers are known      to ignore the TTL on some RRsets (such as when the authoritative      data has a very short TTL) even though this is against the advice      in [RFC1035].  An RRset can be flushed from the cache before the      end of the TTL interval, at which point, the value of the TTL      becomes unknown because the RRset with which it was associated no      longer exists.      There is also the concept of a "default TTL" for a zone, which can      be a configuration parameter in the server software.  This is      often expressed by a default for the entire server, and a default      for a zone using the $TTL directive in a zone file.  The $TTL      directive was added to the master file format by [RFC2308].   Class independent:  A resource record type whose syntax and semantics      are the same for every DNS class.  A resource record type that is      not class independent has different meanings, depending on the DNS      class of the record or if the meaning is undefined for some      classes.  Most resource record types are defined for class 1 (IN,      the Internet), but many are undefined for other classes.   Address records:  Records whose type is either A or AAAA.  [RFC2181]      informally defines these as "(A, AAAA, etc)".  Note that new types      of address records could be defined in the future.6.  DNS Servers and Clients   This section defines the terms used for the systems that act as DNS   clients, DNS servers, or both.  In past RFCs, DNS servers are   sometimes called "name servers", "nameservers", or just "servers".   There is no formal definition of "DNS server", but RFCs generally   assume that it is an Internet server that listens for queries and   sends responses using the DNS protocol defined in [RFC1035] and its   successors.   It is important to note that the terms "DNS server" and "name server"   require context in order to understand the services being provided.   Both authoritative servers and recursive resolvers are often called   "DNS servers" and "name servers" even though they serve different   roles (but may be part of the same software package).   For terminology specific to the global DNS root server system, see   [RSSAC026].  That document defines terms such as "root server", "root   server operator", and terms that are specific to the way that the   root zone of the global DNS is served.   Resolver:  A program "that extract[s] information from name servers      in response to client requests."  (Quoted from [RFC1034],      Section 2.4) A resolver performs queries for a name, type, and      class, and receives responses.  The logical function is called      "resolution".  In practice, the term is usually referring to some      specific type of resolver (some of which are defined below), and      understanding the use of the term depends on understanding the      context.      A related term is "resolve", which is not formally defined in      [RFC1034] or [RFC1035].  An imputed definition might be "asking a      question that consists of a domain name, class, and type, and      receiving some sort of response".  Similarly, an imputed      definition of "resolution" might be "the response received from      resolving".   Stub resolver:  A resolver that cannot perform all resolution itself.      Stub resolvers generally depend on a recursive resolver to      undertake the actual resolution function.  Stub resolvers are      discussed but never fully defined in Section 5.3.1 of [RFC1034].      They are fully defined in Section 6.1.3.1 of [RFC1123].   Iterative mode:  A resolution mode of a server that receives DNS      queries and responds with a referral to another server.      Section 2.3 of [RFC1034] describes this as "The server refers the      client to another server and lets the client pursue the query."  A      resolver that works in iterative mode is sometimes called an      "iterative resolver".  See also "iterative resolution" later in      this section.   Recursive mode:  A resolution mode of a server that receives DNS      queries and either responds to those queries from a local cache or      sends queries to other servers in order to get the final answers      to the original queries.  Section 2.3 of [RFC1034] describes this      as "the first server pursues the query for the client at another      server".  Section 4.3.1 of [RFC1034] says: "in [recursive] mode      the name server acts in the role of a resolver and returns either      an error or the answer, but never referrals."  That same section      also says:         The recursive mode occurs when a query with RD set arrives at a         server which is willing to provide recursive service; the         client can verify that recursive mode was used by checking that         both RA and RD are set in the reply.      A server operating in recursive mode may be thought of as having a      name server side (which is what answers the query) and a resolver      side (which performs the resolution function).  Systems operating      in this mode are commonly called "recursive servers".  Sometimes      they are called "recursive resolvers".  In practice, it is not      possible to know in advance whether the server that one is      querying will also perform recursion; both terms can be observed      in use interchangeably.   Recursive resolver:  A resolver that acts in recursive mode.  In      general, a recursive resolver is expected to cache the answers it      receives (which would make it a full-service resolver), but some      recursive resolvers might not cache.      [RFC4697] tried to differentiate between a recursive resolver and      an iterative resolver.   Recursive query:  A query with the Recursion Desired (RD) bit set to      1 in the header.  (See Section 4.1.1 of [RFC1035].)  If recursive      service is available and is requested by the RD bit in the query,      the server uses its resolver to answer the query.  (See      Section 4.3.2 of [RFC1034].)   Non-recursive query:  A query with the Recursion Desired (RD) bit set      to 0 in the header.  A server can answer non-recursive queries      using only local information: the response contains either an      error, the answer, or a referral to some other server "closer" to      the answer.  (See Section 4.3.1 of [RFC1034].)   Iterative resolution:  A name server may be presented with a query      that can only be answered by some other server.  The two general      approaches to dealing with this problem are "recursive", in which      the first server pursues the query on behalf of the client at      another server, and "iterative", in which the server refers the      client to another server and lets the client pursue the query      there.  (See Section 2.3 of [RFC1034].)      In iterative resolution, the client repeatedly makes non-recursive      queries and follows referrals and/or aliases.  The iterative      resolution algorithm is described in Section 5.3.3 of [RFC1034].   Full resolver:  This term is used in [RFC1035], but it is not defined      there.  RFC 1123 defines a "full-service resolver" that may or may      not be what was intended by "full resolver" in [RFC1035].  This      term is not properly defined in any RFC, and there is no consensus      on what the term means.  The use of this term without proper      context is discouraged.   Full-service resolver:  Section 6.1.3.1 of [RFC1123] defines this      term as a resolver that acts in recursive mode with a cache (and      meets other requirements).   Priming:  "The act of finding the list of root servers from a      configuration that lists some or all of the purported IP addresses      of some or all of those root servers."  (Quoted from [RFC8109],      Section 2) In order to operate in recursive mode, a resolver needs      to know the address of at least one root server.  Priming is most      often done from a configuration setting that contains a list of      authoritative servers for the root zone.   Root hints:  "Operators who manage a DNS recursive resolver typically      need to configure a 'root hints file'.  This file contains the      names and IP addresses of the authoritative name servers for the      root zone, so the software can bootstrap the DNS resolution      process.  For many pieces of software, this list comes built into      the software."  (Quoted from [IANA_RootFiles]) This file is often      used in priming.   Negative caching:  "The storage of knowledge that something does not      exist, cannot or does not give an answer."  (Quoted from      [RFC2308], Section 1)   Authoritative server:  "A server that knows the content of a DNS zone      from local knowledge, and thus can answer queries about that zone      without needing to query other servers."  (Quoted from [RFC2182],      Section 2) An authoritative server is named in the NS ("name      server") record in a zone.  It is a system that responds to DNS      queries with information about zones for which it has been      configured to answer with the AA flag in the response header set      to 1.  It is a server that has authority over one or more DNS      zones.  Note that it is possible for an authoritative server to      respond to a query without the parent zone delegating authority to      that server.  Authoritative servers also provide "referrals",      usually to child zones delegated from them; these referrals have      the AA bit set to 0 and come with referral data in the Authority      and (if needed) the Additional sections.   Authoritative-only server:  A name server that only serves      authoritative data and ignores requests for recursion.  It will      "not normally generate any queries of its own.  Instead it answers      non-recursive queries from iterative resolvers looking for      information in zones it serves."  (Quoted from [RFC4697],      Section 2.4) In this case, "ignores requests for recursion" means      "responds to requests for recursion with responses indicating that      recursion was not performed".   Zone transfer:  The act of a client requesting a copy of a zone and      an authoritative server sending the needed information.  (See      Section 7 for a description of zones.)  There are two common      standard ways to do zone transfers: the AXFR ("Authoritative      Transfer") mechanism to copy the full zone (described in      [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy      only parts of the zone that have changed (described in [RFC1995]).      Many systems use non-standard methods for zone transfers outside      the DNS protocol.   Slave server:  See "Secondary server".   Secondary server:  "An authoritative server which uses zone transfer      to retrieve the zone."  (Quoted from [RFC1996], Section 2.1)      Secondary servers are also discussed in [RFC1034].  [RFC2182]      describes secondary servers in more detail.  Although early DNS      RFCs such as [RFC1996] referred to this as a "slave", the current      common usage has shifted to calling it a "secondary".   Master server:  See "Primary server".   Primary server:  "Any authoritative server configured to be the      source of zone transfer for one or more [secondary] servers."      (Quoted from [RFC1996], Section 2.1) Or, more specifically,      [RFC2136] calls it "an authoritative server configured to be the      source of AXFR or IXFR data for one or more [secondary] servers".      Primary servers are also discussed in [RFC1034].  Although early      DNS RFCs such as [RFC1996] referred to this as a "master", the      current common usage has shifted to "primary".   Primary master:  "The primary master is named in the zone's SOA MNAME      field and optionally by an NS RR."  (Quoted from [RFC1996],      Section 2.1) [RFC2136] defines "primary master" as "Master server      at the root of the AXFR/IXFR dependency graph.  The primary master      is named in the zone's SOA MNAME field and optionally by an NS RR.      There is by definition only one primary master server per zone."      The idea of a primary master is only used in [RFC1996] and      [RFC2136].  A modern interpretation of the term "primary master"      is a server that is both authoritative for a zone and that gets      its updates to the zone from configuration (such as a master file)      or from UPDATE transactions.   Stealth server:  This is "like a slave server except not listed in an      NS RR for the zone."  (Quoted from [RFC1996], Section 2.1)   Hidden master:  A stealth server that is a primary server for zone      transfers.  "In this arrangement, the master name server that      processes the updates is unavailable to general hosts on the      Internet; it is not listed in the NS RRset."  (Quoted from      [RFC6781], Section 3.4.3) [RFC4641] said that the hidden master's      name "appears in the SOA RRs MNAME field"; however, the name does      not appear at all in the global DNS in some setups.  A hidden      master can also be a secondary server for the zone itself.   Forwarding:  The process of one server sending a DNS query with the      RD bit set to 1 to another server to resolve that query.      Forwarding is a function of a DNS resolver; it is different than      simply blindly relaying queries.      [RFC5625] does not give a specific definition for forwarding, but      describes in detail what features a system that forwards needs to      support.  Systems that forward are sometimes called "DNS proxies",      but that term has not yet been defined (even in [RFC5625]).   Forwarder:  Section 1 of [RFC2308] describes a forwarder as "a      nameserver used to resolve queries instead of directly using the      authoritative nameserver chain".  [RFC2308] further says "The      forwarder typically either has better access to the internet, or      maintains a bigger cache which may be shared amongst many      resolvers."  That definition appears to suggest that forwarders      normally only query authoritative servers.  In current use,      however, forwarders often stand between stub resolvers and      recursive servers.  [RFC2308] is silent on whether a forwarder is      iterative-only or can be a full-service resolver.   Policy-implementing resolver:  A resolver acting in recursive mode      that changes some of the answers that it returns based on policy      criteria, such as to prevent access to malware sites or      objectionable content.  In general, a stub resolver has no idea      whether upstream resolvers implement such policy or, if they do,      the exact policy about what changes will be made.  In some cases,      the user of the stub resolver has selected the policy-implementing      resolver with the explicit intention of using it to implement the      policies.  In other cases, policies are imposed without the user      of the stub resolver being informed.   Open resolver:  A full-service resolver that accepts and processes      queries from any (or nearly any) client.  This is sometimes also      called a "public resolver", although the term "public resolver" is      used more with open resolvers that are meant to be open, as      compared to the vast majority of open resolvers that are probably      misconfigured to be open.  Open resolvers are discussed in      [RFC5358].   Split DNS:  The terms "split DNS" and "split-horizon DNS" have long      been used in the DNS community without formal definition.  In      general, they refer to situations in which DNS servers that are      authoritative for a particular set of domains provide partly or      completely different answers in those domains depending on the      source of the query.  Nevertheless, the effect of this is that a      domain name that is notionally globally unique has different      meanings for different network users.  This can sometimes be the      result of a "view" configuration, as described below.      Section 3.8 of [RFC2775] gives a related definition that is too      specific to be generally useful.   View:  A configuration for a DNS server that allows it to provide      different responses depending on attributes of the query, such as      for "split DNS".  Typically, views differ by the source IP address      of a query, but can also be based on the destination IP address,      the type of query (such as AXFR), whether it is recursive, and so      on.  Views are often used to provide more names or different      addresses to queries from "inside" a protected network than to      those "outside" that network.  Views are not a standardized part      of the DNS, but they are widely implemented in server software.   Passive DNS:  A mechanism to collect DNS data by storing DNS      responses from name servers.  Some of these systems also collect      the DNS queries associated with the responses, although doing so      raises some privacy concerns.  Passive DNS databases can be used      to answer historical questions about DNS zones, such as which      values were present at a given time in the past, or when a name      was spotted first.  Passive DNS databases allow searching of the      stored records on keys other than just the name and type, such as      "find all names which have A records of a particular value".   Anycast:  "The practice of making a particular service address      available in multiple, discrete, autonomous locations, such that      datagrams sent are routed to one of several available locations."      (Quoted from [RFC4786], Section 2) See [RFC4786] for more detail      on Anycast and other terms that are specific to its use.   Instance:  "When anycast routing is used to allow more than one      server to have the same IP address, each one of those servers is      commonly referred to as an 'instance'."  It goes on to say: "An      instance of a server, such as a root server, is often referred to      as an 'Anycast instance'."  (Quoted from [RSSAC026])   Privacy-enabling DNS server:  "A DNS server that implements DNS over      TLS [RFC7858] and may optionally implement DNS over DTLS      [RFC8094]."  (Quoted from [RFC8310], Section 2) Other types of DNS      servers might also be considered privacy-enabling, such as those      running DNS-over-HTTPS [RFC8484] or DNS-over-QUIC [RFC9250].   DNS-over-TLS (DoT):  DNS over TLS as defined in [RFC7858] and its      successors.   DNS-over-HTTPS (DoH):  DNS over HTTPS as defined in [RFC8484] and its      successors.   DNS-over-QUIC (DoQ):  DNS over QUIC as defined in [RFC9250] and its      successors.  [RFC9250] specifically defines DoQ as general-purpose      transport for DNS that can be used in stub to recursive, recursive      to authoritative, and zone transfer scenarios.   Classic DNS:  DNS over UDP or DNS over TCP as defined in [RFC1035]      and its successors.  Classic DNS applies to DNS communication      between stub resolvers and recursive resolvers, and between      recursive resolvers and authoritative servers.  This has sometimes      been called "Do53".  Classic DNS is not encrypted.   Recursive DoT (RDoT):  RDoT specifically means DNS-over-TLS for      transport between a stub resolver and a recursive resolver, or      between a recursive resolver and another recursive resolver.  This      term is necessary because it is expected that DNS-over-TLS will      later be defined as a transport between recursive resolvers and      authoritative servers.   Authoritative DoT (ADoT):  If DNS-over-TLS is later defined as a      transport between recursive resolvers and authoritative servers,      ADoT specifically means DNS-over-TLS for transport between      recursive resolvers and authoritative servers.   XFR-over-TLS (XoT):  DNS zone transfer over TLS, as specified in      [RFC9103].  This term applies to both AXFR over TLS (AXoT) and      IXFR over TLS (IXoT).7.  Zones   This section defines terms that are used when discussing zones that   are being served or retrieved.   Zone:  "Authoritative information is organized into units called      ZONEs, and these zones can be automatically distributed to the      name servers which provide redundant service for the data in a      zone."  (Quoted from [RFC1034], Section 2.4)   Child:  "The entity on record that has the delegation of the domain      from the Parent."  (Quoted from [RFC7344], Section 1.1)   Parent:  "The domain in which the Child is registered."  (Quoted from      [RFC7344], Section 1.1) Earlier, "parent name server" was defined      in [RFC0882] as "the name server that has authority over the place      in the domain name space that will hold the new domain".  (Note      that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].)      [RFC819] also has some description of the relationship between      parents and children.   Origin:      There are two different uses for this term:      (a)  "The domain name that appears at the top of a zone (just           below the cut that separates the zone from its parent)... The           name of the zone is the same as the name of the domain at the           zone's origin."  (Quoted from [RFC2181], Section 6) These           days, this sense of "origin" and "apex" (defined below) are           often used interchangeably.      (b)  The domain name within which a given relative domain name           appears in zone files.  Generally seen in the context of           "$ORIGIN", which is a control entry defined in [RFC1035],           Section 5.1, as part of the master file format.  For example,           if the $ORIGIN is set to "example.org.", then a master file           line for "www" is in fact an entry for "www.example.org.".   Apex:  The point in the tree at an owner of an SOA and corresponding      authoritative NS RRset.  This is also called the "zone apex".      [RFC4033] defines it as "the name at the child's side of a zone      cut".  The "apex" can usefully be thought of as a data-theoretic      description of a tree structure, and "origin" is the name of the      same concept when it is implemented in zone files.  The      distinction is not always maintained in use, however, and one can      find uses that conflict subtly with this definition.  [RFC1034]      uses the term "top node of the zone" as a synonym of "apex", but      that term is not widely used.  These days, the first sense of      "origin" (above) and "apex" are often used interchangeably.   Zone cut:  The delimitation point between two zones where the origin      of one of the zones is the child of the other zone.      "Zones are delimited by 'zone cuts'.  Each zone cut separates a      'child' zone (below the cut) from a 'parent' zone (above the      cut)."  (Quoted from [RFC2181], Section 6; note that this is      barely an ostensive definition.)  Section 4.2 of [RFC1034] uses      "cuts" instead of "zone cut".   Delegation:  The process by which a separate zone is created in the      name space beneath the apex of a given domain.  Delegation happens      when an NS RRset is added in the parent zone for the child origin.      Delegation inherently happens at a zone cut.  The term is also      commonly a noun: the new zone that is created by the act of      delegating.   Authoritative data:  "All of the RRs attached to all of the nodes      from the top node of the zone down to leaf nodes or nodes above      cuts around the bottom edge of the zone."  (Quoted from [RFC1034],      Section 4.2.1) Note that this definition might inadvertently also      cause any NS records that appear in the zone to be included, even      those that might not truly be authoritative, because there are      identical NS RRs below the zone cut.  This reveals the ambiguity      in the notion of authoritative data, because the parent-side NS      records authoritatively indicate the delegation, even though they      are not themselves authoritative data.      [RFC4033], Section 2, defines "Authoritative RRset", which is      related to authoritative data but has a more precise definition.   Lame delegation:  "A lame delegations exists [sic] when a nameserver      is delegated responsibility for providing nameservice for a zone      (via NS records) but is not performing nameservice for that zone      (usually because it is not set up as a primary or secondary for      the zone)."  (Quoted from [RFC1912], Section 2.8) Another      definition is that a lame delegation "...happens when a name      server is listed in the NS records for some domain and in fact it      is not a server for that domain.  Queries are thus sent to the      wrong servers, who don't know nothing [sic] (at least not as      expected) about the queried domain.  Furthermore, sometimes these      hosts (if they exist!) don't even run name servers."  (Quoted from      [RFC1713], Section 2.3)      These early definitions do not match the current use of the term      "lame delegation", but there is no consensus on what a lame      delegation is.  The term is used not only for the specific case      described above, but for a variety of other flaws in delegations      that lead to non-authoritative answers or no answers at all, such      as:      *  a nameserver with an NS record for a zone that does not answer         DNS queries;      *  a nameserver with an IP address that is not reachable by the         resolver; and      *  a nameserver that responds to a query for a specific name with         an error or without the authoritative bit set.      Because the term in current usage has drifted from the original      definition, and now is not specific or clear as to the intended      meaning, it should be considered historic and avoided in favor of      terms that are specific and clear.   Glue records:  "...[Resource records] which are not part of the      authoritative data [of the zone], and are address RRs for the      [name] servers [in subzones].  These RRs are only necessary if the      name server's name is 'below' the cut, and are only used as part      of a referral response."  Without glue "we could be faced with the      situation where the NS RRs tell us that in order to learn a name      server's address, we should contact the server using the address      we wish to learn."  (Quoted from [RFC1034], Section 4.2.1)      A later definition is that glue "includes any record in a zone      file that is not properly part of that zone, including nameserver      records of delegated sub-zones (NS records), address records that      accompany those NS records (A, AAAA, etc), and any other stray      data that might appear."  (Quoted from [RFC2181], Section 5.4.1)      Although glue is sometimes used today with this wider definition      in mind, the context surrounding the definition in [RFC2181]      suggests it is intended to apply to the use of glue within the      document itself and not necessarily beyond.      In an NS record, there are three types of relationships between      the owner name of the record, the name in the NS RDATA, and the      zone origin: unrelated, in-domain, and sibling domain.  The      application of these three types of relationships to glue records      is defined in [RFC9471].      An unrelated relationship is one where the NS RDATA contains a      name server that is not subordinate to the zone origin and      therefore is not part of the same zone.      An in-domain relationship is one where the NS RDATA contains a      name server whose name is either subordinate to or (rarely) the      same as the owner name of the NS resource records.  For example, a      delegation for "child.example.com" might have an in-domain name      server called "ns.child.example.com".      A sibling domain relationship is one where the NS RDATA contains a      name server whose name is either subordinate to or (rarely) the      same as the zone origin of the parent and not subordinate to or      the same as the owner name of the NS resource records.  For      example, a delegation for "child.example.com" in "example.com"      zone might have a sibling domain name server called      "ns.another.example.com".      The following table shows examples of delegation types:         +=============+========+====================+================+        | Delegation  | Parent | Name Server Name   | Type           |        +=============+========+====================+================+        | com         | .      | a.gtld-servers.net | sibling domain |        +-------------+--------+--------------------+----------------+        | net         | .      | a.gtld-servers.net | in-domain      |        +-------------+--------+--------------------+----------------+        | example.org | org    | ns.example.org     | in-domain      |        +-------------+--------+--------------------+----------------+        | example.org | org    | ns.ietf.org        | sibling domain |        +-------------+--------+--------------------+----------------+        | example.org | org    | ns.example.com     | unrelated      |        +-------------+--------+--------------------+----------------+        | example.jp  | jp     | ns.example.jp      | in-domain      |        +-------------+--------+--------------------+----------------+        | example.jp  | jp     | ns.example.ne.jp   | sibling domain |        +-------------+--------+--------------------+----------------+        | example.jp  | jp     | ns.example.com     | unrelated      |        +-------------+--------+--------------------+----------------+                                   Table 1   Bailiwick:  "In-bailiwick" and "Out-of-bailiwick" are modifiers used      to describe the relationship between a zone and the name servers      for that zone.  The dictionary definition of bailiwick has been      observed to cause more confusion than meaning for this use.  These      terms should be considered historic in nature.   Root zone:  The zone of a DNS-based tree whose apex is the zero-      length label.  Also sometimes called "the DNS root".   Empty non-terminals (ENTs):  "Domain names that own no resource      records but have subdomains that do."  (Quoted from [RFC4592],      Section 2.2.2) A typical example is in SRV records: in the name      "_sip._tcp.example.com", it is likely that "_tcp.example.com" has      no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV      RRset.   Delegation-centric zone:  A zone that consists mostly of delegations      to child zones.  This term is used in contrast to a zone that      might have some delegations to child zones but also has many data      resource records for the zone itself and/or for child zones.  The      term is used in [RFC4956] and [RFC5155], but it is not defined in      either document.   Occluded name:  "The addition of a delegation point via dynamic      update will render all subordinate domain names to be in a limbo,      still part of the zone but not available to the lookup process.      The addition of a DNAME resource record has the same impact.  The      subordinate names are said to be 'occluded'."  (Quoted from      [RFC5936], Section 3.5)   Fast flux DNS:  This "occurs when a domain is [found] in DNS using A      records to multiple IP addresses, each of which has a very short      Time-to-Live (TTL) value associated with it.  This means that the      domain resolves to varying IP addresses over a short period of      time."  (Quoted from [RFC6561], Section 1.1.5, with a typo      corrected) In addition to having legitimate uses, fast flux DNS      can be used to deliver malware.  Because the addresses change so      rapidly, it is difficult to ascertain all the hosts.  It should be      noted that the technique also works with AAAA records, but such      use is not frequently observed on the Internet as of this writing.   Reverse DNS, reverse lookup:  "The process of mapping an address to a      name is generally known as a 'reverse lookup', and the IN-      ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse      DNS'."  (Quoted from [RFC5855], Section 1)   Forward lookup:  "Hostname-to-address translation".  (Quoted from      [RFC3493], Section 6)   arpa (Address and Routing Parameter Area Domain):  "The 'arpa' domain      was originally established as part of the initial deployment of      the DNS to provide a transition mechanism from the Host Tables      that were common in the ARPANET, as well as a home for the IPv4      reverse mapping domain.  During 2000, the abbreviation was      redesignated to 'Address and Routing Parameter Area' in the hope      of reducing confusion with the earlier network name."  (Quoted      from [RFC3172], Section 2) .arpa is an "infrastructure domain", a      domain whose "role is to support the operating infrastructure of      the Internet".  (Quoted from [RFC3172], Section 2) See [RFC3172]      for more history of this name.   Service name:  "Service names are the unique key in the Service Name      and Transport Protocol Port Number registry.  This unique symbolic      name for a service may also be used for other purposes, such as in      DNS SRV records."  (Quoted from [RFC6335], Section 5)8.  Wildcards   Wildcard:  [RFC1034] defined "wildcard", but in a way that turned out      to be confusing to implementers.  For an extended discussion of      wildcards, including clearer definitions, see [RFC4592].  Special      treatment is given to RRs with owner names starting with the label      "*".  "Such RRs are called 'wildcards'.  Wildcard RRs can be      thought of as instructions for synthesizing RRs."  (Quoted from      [RFC1034], Section 4.3.3)   Asterisk label:  "The first octet is the normal label type and length      for a 1-octet-long label, and the second octet is the ASCII      representation [RFC20] for the '*' character.  A descriptive name      of a label equaling that value is an 'asterisk label'."  (Quoted      from [RFC4592], Section 2.1.1)   Wildcard domain name:  "A 'wildcard domain name' is defined by having      its initial (i.e., leftmost or least significant) label, in binary      format: 0000 0001 0010 1010 (binary) = 0x01 0x2a (hexadecimal)".      (Quoted from [RFC4592], Section 2.1.1) The second octet in this      label is the ASCII representation for the "*" character.   Closest encloser:  "The longest existing ancestor of a name."      (Quoted from [RFC5155], Section 1.3) An earlier definition is "The      node in the zone's tree of existing domain names that has the most      labels matching the query name (consecutively, counting from the      root label downward).  Each match is a 'label match' and the order      of the labels is the same."  (Quoted from [RFC4592],      Section 3.3.1)   Closest provable encloser:  "The longest ancestor of a name that can      be proven to exist.  Note that this is only different from the      closest encloser in an Opt-Out zone."  (Quoted from [RFC5155],      Section 1.3) See Section 10 for more on "opt-out".   Next closer name:  "The name one label longer than the closest      provable encloser of a name."  (Quoted from [RFC5155],      Section 1.3)   Source of Synthesis:  "The source of synthesis is defined in the      context of a query process as that wildcard domain name      immediately descending from the closest encloser, provided that      this wildcard domain name exists.  'Immediately descending' means      that the source of synthesis has a name of the form:      <asterisk label>.<closest encloser>."      (Quoted from [RFC4592], Section 3.3.1)9.  Registration Model   Registry:  The administrative operation of a zone that allows      registration of names within that zone.  People often use this      term to refer only to those organizations that perform      registration in large delegation-centric zones (such as TLDs); but      formally, whoever decides what data goes into a zone is the      registry for that zone.  This definition of "registry" is from a      DNS point of view; for some zones, the policies that determine      what can go in the zone are decided by zones that are      superordinate and not the registry operator.   Registrant:  An individual or organization on whose behalf a name in      a zone is registered by the registry.  In many zones, the registry      and the registrant may be the same entity, but in TLDs they often      are not.   Registrar:  A service provider that acts as a go-between for      registrants and registries.  Not all registrations require a      registrar, though it is common to have registrars involved in      registrations in TLDs.   EPP:  The Extensible Provisioning Protocol (EPP), which is commonly      used for communication of registration information between      registries and registrars.  EPP is defined in [RFC5730].   WHOIS:  A protocol specified in [RFC3912], often used for querying      registry databases.  WHOIS data is frequently used to associate      registration data (such as zone management contacts) with domain      names.  The term "WHOIS data" is often used as a synonym for the      registry database, even though that database may be served by      different protocols, particularly RDAP.  The WHOIS protocol is      also used with IP address registry data.   RDAP:  The Registration Data Access Protocol, defined in [RFC7480],      [RFC7481], [RFC7485], [RFC9082], [RFC9083], and [RFC9224].  The      RDAP protocol and data format are meant as a replacement for      WHOIS.   DNS operator:  An entity responsible for running DNS servers.  For a      zone's authoritative servers, the registrant may act as their own      DNS operator, their registrar may do it on their behalf, or they      may use a third-party operator.  For some zones, the registry      function is performed by the DNS operator plus other entities who      decide about the allowed contents of the zone.   Public suffix:  "A domain that is controlled by a public registry."      (Quoted from [RFC6265], Section 5.3) A common definition for this      term is a domain under which subdomains can be registered by third      parties and on which HTTP cookies (which are described in detail      in [RFC6265]) should not be set.  There is no indication in a      domain name whether it is a public suffix; that can only be      determined by outside means.  In fact, both a domain and a      subdomain of that domain can be public suffixes.      There is nothing inherent in a domain name to indicate whether it      is a public suffix.  One resource for identifying public suffixes      is the Public Suffix List (PSL) maintained by Mozilla      <https://publicsuffix.org/>.      For example, at the time this document is published, the "com.au"      domain is listed as a public suffix in the PSL.  (Note that this      example might change in the future.)      Note that the term "public suffix" is controversial in the DNS      community for many reasons, and it may be significantly changed in      the future.  One example of the difficulty of calling a domain a      public suffix is that designation can change over time as the      registration policy for the zone changes, such as was the case      with the "uk" TLD in 2014.   Subordinate and Superordinate:  These terms are introduced in      [RFC5731] for use in the registration model, but not defined      there.  Instead, they are given in examples.  "For example, domain      name 'example.com' has a superordinate relationship to host name      ns1.example.com'...  For example, host ns1.example1.com is a      subordinate host of domain example1.com, but it is a not a      subordinate host of domain example2.com."  (Quoted from [RFC5731],      Section 1.1) These terms are strictly ways of referring to the      relationship standing of two domains where one is a subdomain of      the other.10.  General DNSSEC   Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and   [RFC5155].  The terms that have caused confusion in the DNS community   are highlighted here.   DNSSEC-aware and DNSSEC-unaware:  These two terms, which are used in      some RFCs, have not been formally defined.  However, Section 2 of      [RFC4033] defines many types of resolvers and validators,      including "non-validating security-aware stub resolver", "non-      validating stub resolver", "security-aware name server",      "security-aware recursive name server", "security-aware resolver",      "security-aware stub resolver", and "security-oblivious      'anything'".  (Note that the term "validating resolver", which is      used in some places in DNSSEC-related documents, is also not      defined in those RFCs, but is defined below.)   Signed zone:  "A zone whose RRsets are signed and that contains      properly constructed DNSKEY, Resource Record Signature (RRSIG),      Next Secure (NSEC), and (optionally) DS records."  (Quoted from      [RFC4033], Section 2) It has been noted in other contexts that the      zone itself is not really signed, but all the relevant RRsets in      the zone are signed.  Nevertheless, if a zone that should be      signed contains any RRsets that are not signed (or opted out),      those RRsets will be treated as bogus, so the whole zone needs to      be handled in some way.      It should also be noted that, since the publication of [RFC6840],      NSEC records are no longer required for signed zones: a signed      zone might include NSEC3 records instead.  [RFC7129] provides      additional background commentary and some context for the NSEC and      NSEC3 mechanisms used by DNSSEC to provide authenticated denial-      of-existence responses.  NSEC and NSEC3 are described below.   Online signing:  [RFC4470] defines "on-line signing" (note the      hyphen) as "generating and signing these records on demand", where      "these" was defined as NSEC records.  The current definition      expands that to generating and signing RRSIG, NSEC, and NSEC3      records on demand.   Unsigned zone:  Section 2 of [RFC4033] defines this as "a zone that      is not signed".  Section 2 of [RFC4035] defines this as a "zone      that does not include these records [properly constructed DNSKEY,      Resource Record Signature (RRSIG), Next Secure (NSEC), and      (optionally) DS records] according to the rules in this      section..." There is an important note at the end of Section 5.2      of [RFC4035] that defines an additional situation in which a zone      is considered unsigned: "If the resolver does not support any of      the algorithms listed in an authenticated DS RRset, then the      resolver will not be able to verify the authentication path to the      child zone.  In this case, the resolver SHOULD treat the child      zone as if it were unsigned."   NSEC:  "The NSEC record allows a security-aware resolver to      authenticate a negative reply for either name or type non-      existence with the same mechanisms used to authenticate other DNS      replies."  (Quoted from [RFC4033], Section 3.2) In short, an NSEC      record provides authenticated denial of existence.      "The NSEC resource record lists two separate things: the next      owner name (in the canonical ordering of the zone) that contains      authoritative data or a delegation point NS RRset, and the set of      RR types present at the NSEC RR's owner name."  (Quoted from      [RFC4034], Section 4)   NSEC3:  Like the NSEC record, the NSEC3 record also provides      authenticated denial of existence; however, NSEC3 records mitigate      zone enumeration and support Opt-Out. NSEC3 resource records      require associated NSEC3PARAM resource records.  NSEC3 and      NSEC3PARAM resource records are defined in [RFC5155].      Note that [RFC6840] says that [RFC5155] "is now considered part of      the DNS Security Document Family as described by Section 10 of      [RFC4033]".  This means that some of the definitions from earlier      RFCs that only talk about NSEC records should probably be      considered to be talking about both NSEC and NSEC3.   Opt-out:  "The Opt-Out Flag indicates whether this NSEC3 RR may cover      unsigned delegations."  (Quoted from [RFC5155], Section 3.1.2.1)      Opt-out tackles the high costs of securing a delegation to an      insecure zone.  When using Opt-Out, names that are an insecure      delegation (and empty non-terminals that are only derived from      insecure delegations) don't require an NSEC3 record or its      corresponding RRSIG records.  Opt-Out NSEC3 records are not able      to prove or deny the existence of the insecure delegations.      (Adapted from [RFC7129], Section 5.1)   Insecure delegation:  "A signed name containing a delegation (NS      RRset), but lacking a DS RRset, signifying a delegation to an      unsigned subzone."  (Quoted from [RFC4956], Section 2)   Zone enumeration:  "The practice of discovering the full content of a      zone via successive queries."  (Quoted from [RFC5155],      Section 1.3) This is also sometimes called "zone walking".  Zone      enumeration is different from zone content guessing where the      guesser uses a large dictionary of possible labels and sends      successive queries for them, or matches the contents of NSEC3      records against such a dictionary.   Validation:  Validation, in the context of DNSSEC, refers to one of      the following:      *  Checking the validity of DNSSEC signatures,      *  Checking the validity of DNS responses, such as those including         authenticated denial of existence, or      *  Building an authentication chain from a trust anchor to a DNS         response or individual DNS RRsets in a response.      The first two definitions above consider only the validity of      individual DNSSEC components, such as the RRSIG validity or NSEC      proof validity.  The third definition considers the components of      the entire DNSSEC authentication chain; thus, it requires      "configured knowledge of at least one authenticated DNSKEY or DS      RR" (as described in [RFC4035], Section 5).      [RFC4033], Section 2, says that a "Validating Security-Aware Stub      Resolver... performs signature validation" and uses a trust anchor      "as a starting point for building the authentication chain to a      signed DNS response"; thus, it uses the first and third      definitions above.  The process of validating an RRSIG resource      record is described in [RFC4035], Section 5.3.      [RFC5155] refers to validating responses throughout the document      in the context of hashed authenticated denial of existence; this      uses the second definition above.      The term "authentication" is used interchangeably with      "validation", in the sense of the third definition above.      [RFC4033], Section 2, describes the chain linking trust anchor to      DNS data as the "authentication chain".  A response is considered      to be authentic if "all RRsets in the Answer and Authority      sections of the response [are considered] to be authentic" (Quoted      from [RFC4035]) DNS data or responses deemed to be authentic or      validated have a security status of "secure" ([RFC4035],      Section 4.3; [RFC4033], Section 5).  "Authenticating both DNS keys      and data is a matter of local policy, which may extend or even      override the [DNSSEC] protocol extensions..." (Quoted from      [RFC4033], Section 3.1)      The term "verification", when used, is usually a synonym for      "validation".   Validating resolver:  A security-aware recursive name server,      security-aware resolver, or security-aware stub resolver that is      applying at least one of the definitions of validation (above) as      appropriate to the resolution context.  For the same reason that      the generic term "resolver" is sometimes ambiguous and needs to be      evaluated in context (see Section 6), "validating resolver" is a      context-sensitive term.   Key signing key (KSK):  DNSSEC keys that "only sign the apex DNSKEY      RRset in a zone."  (Quoted from [RFC6781], Section 3.1)   Zone signing key (ZSK):  "DNSSEC keys that can be used to sign all      the RRsets in a zone that require signatures, other than the apex      DNSKEY RRset."  (Quoted from [RFC6781], Section 3.1) Also note      that a ZSK is sometimes used to sign the apex DNSKEY RRset.   Combined signing key (CSK):  "In cases where the differentiation      between the KSK and ZSK is not made, i.e., where keys have the      role of both KSK and ZSK, we talk about a Single-Type Signing      Scheme."  (Quoted from [RFC6781], Section 3.1) This is sometimes      called a "combined signing key" or "CSK".  It is operational      practice, not protocol, that determines whether a particular key      is a ZSK, a KSK, or a CSK.   Secure Entry Point (SEP):  A flag in the DNSKEY RDATA that "can be      used to distinguish between keys that are intended to be used as      the secure entry point into the zone when building chains of      trust, i.e., they are (to be) pointed to by parental DS RRs or      configured as a trust anchor....  Therefore, it is suggested that      the SEP flag be set on keys that are used as KSKs and not on keys      that are used as ZSKs, while in those cases where a distinction      between a KSK and ZSK is not made (i.e., for a Single-Type Signing      Scheme), it is suggested that the SEP flag be set on all keys."      (Quoted from [RFC6781], Section 3.2.3) Note that the SEP flag is      only a hint, and its presence or absence may not be used to      disqualify a given DNSKEY RR from use as a KSK or ZSK during      validation.      The original definition of SEPs was in [RFC3757].  That definition      clearly indicated that the SEP was a key, not just a bit in the      key.  The abstract of [RFC3757] says: "With the Delegation Signer      (DS) resource record (RR), the concept of a public key acting as a      secure entry point (SEP) has been introduced.  During exchanges of      public keys with the parent there is a need to differentiate SEP      keys from other public keys in the Domain Name System KEY (DNSKEY)      resource record set.  A flag bit in the DNSKEY RR is defined to      indicate that DNSKEY is to be used as a SEP."  That definition of      the SEP as a key was made obsolete by [RFC4034], and the      definition from [RFC6781] is consistent with [RFC4034].   Trust anchor:  "A configured DNSKEY RR or DS RR hash of a DNSKEY RR.      A validating security-aware resolver uses this public key or hash      as a starting point for building the authentication chain to a      signed DNS response.  In general, a validating resolver will have      to obtain the initial values of its trust anchors via some secure      or trusted means outside the DNS protocol."  (Quoted from      [RFC4033], Section 2)   DNSSEC Policy (DP):  A statement that "sets forth the security      requirements and standards to be implemented for a DNSSEC-signed      zone."  (Quoted from [RFC6841], Section 2)   DNSSEC Practice Statement (DPS):  "A practices disclosure document      that may support and be a supplemental document to the DNSSEC      Policy (if such exists), and it states how the management of a      given zone implements procedures and controls at a high level."      (Quoted from [RFC6841], Section 2)   Hardware security module (HSM):  A specialized piece of hardware that      is used to create keys for signatures and to sign messages without      ever disclosing the private key.  In DNSSEC, HSMs are often used      to hold the private keys for KSKs and ZSKs and to create the      signatures used in RRSIG records at periodic intervals.   Signing software:  Authoritative DNS servers that support DNSSEC      often contain software that facilitates the creation and      maintenance of DNSSEC signatures in zones.  There is also stand-      alone software that can be used to sign a zone regardless of      whether the authoritative server itself supports signing.      Sometimes signing software can support particular HSMs as part of      the signing process.11.  DNSSEC States   A validating resolver can determine that a response is in one of four   states: secure, insecure, bogus, or indeterminate.  These states are   defined in [RFC4033] and [RFC4035], although the definitions in the   two documents differ a bit.  This document makes no effort to   reconcile the definitions in the two documents and takes no position   as to whether they need to be reconciled.   Section 5 of [RFC4033] says:   |  A validating resolver can determine the following 4 states:   |  Secure:  The validating resolver has a trust anchor, has a chain   |     of trust, and is able to verify all the signatures in the   |     response.   |     |  Insecure:  The validating resolver has a trust anchor, a chain of   |     trust, and, at some delegation point, signed proof of the non-   |     existence of a DS record.  This indicates that subsequent   |     branches in the tree are provably insecure.  A validating   |     resolver may have a local policy to mark parts of the domain   |     space as insecure.   |     |  Bogus:  The validating resolver has a trust anchor and a secure   |     delegation indicating that subsidiary data is signed, but the   |     response fails to validate for some reason: missing signatures,   |     expired signatures, signatures with unsupported algorithms,   |     data missing that the relevant NSEC RR says should be present,   |     and so forth.   |     |  Indeterminate:  There is no trust anchor that would indicate that   |     a specific portion of the tree is secure.  This is the default   |     operation mode.   Section 4.3 of [RFC4035] says:   |  A security-aware resolver must be able to distinguish between four   |  cases:   |  Secure:  An RRset for which the resolver is able to build a chain   |     of signed DNSKEY and DS RRs from a trusted security anchor to   |     the RRset.  In this case, the RRset should be signed and is   |     subject to signature validation, as described above.   |     |  Insecure:  An RRset for which the resolver knows that it has no   |     chain of signed DNSKEY and DS RRs from any trusted starting   |     point to the RRset.  This can occur when the target RRset lies   |     in an unsigned zone or in a descendent [sic] of an unsigned   |     zone.  In this case, the RRset may or may not be signed, but   |     the resolver will not be able to verify the signature.   |     |  Bogus:  An RRset for which the resolver believes that it ought to   |     be able to establish a chain of trust but for which it is   |     unable to do so, either due to signatures that for some reason   |     fail to validate or due to missing data that the relevant   |     DNSSEC RRs indicate should be present.  This case may indicate   |     an attack but may also indicate a configuration error or some   |     form of data corruption.   |     |  Indeterminate:  An RRset for which the resolver is not able to   |     determine whether the RRset should be signed, as the resolver   |     is not able to obtain the necessary DNSSEC RRs.  This can occur   |     when the security-aware resolver is not able to contact   |     security-aware name servers for the relevant zones.12.  Security Considerations   These definitions do not change any security considerations for   either the global DNS or private DNS.13.  IANA Considerations   References to RFC 8499 in the IANA registries have been replaced with   references to this document.14.  References14.1.  Normative References   [IANA_RootFiles]              IANA, "Root Files",              <https://www.iana.org/domains/root/files>.   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",              RFC 882, DOI 10.17487/RFC0882, November 1983,              <https://www.rfc-editor.org/info/rfc882>.   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,              <https://www.rfc-editor.org/info/rfc1034>.   [RFC1035]  Mockapetris, P., "Domain names - implementation and              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,              November 1987, <https://www.rfc-editor.org/info/rfc1035>.   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -              Application and Support", STD 3, RFC 1123,              DOI 10.17487/RFC1123, October 1989,              <https://www.rfc-editor.org/info/rfc1123>.   [RFC1912]  Barr, D., "Common DNS Operational and Configuration              Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,              <https://www.rfc-editor.org/info/rfc1912>.   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,              August 1996, <https://www.rfc-editor.org/info/rfc1996>.   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,              "Dynamic Updates in the Domain Name System (DNS UPDATE)",              RFC 2136, DOI 10.17487/RFC2136, April 1997,              <https://www.rfc-editor.org/info/rfc2136>.   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,              <https://www.rfc-editor.org/info/rfc2181>.   [RFC2182]  Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection              and Operation of Secondary DNS Servers", BCP 16, RFC 2182,              DOI 10.17487/RFC2182, July 1997,              <https://www.rfc-editor.org/info/rfc2182>.   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,              <https://www.rfc-editor.org/info/rfc2308>.   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "DNS Security Introduction and Requirements",              RFC 4033, DOI 10.17487/RFC4033, March 2005,              <https://www.rfc-editor.org/info/rfc4033>.   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "Resource Records for the DNS Security Extensions",              RFC 4034, DOI 10.17487/RFC4034, March 2005,              <https://www.rfc-editor.org/info/rfc4034>.   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "Protocol Modifications for the DNS Security              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,              <https://www.rfc-editor.org/info/rfc4035>.   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,              <https://www.rfc-editor.org/info/rfc4592>.   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS              Security (DNSSEC) Hashed Authenticated Denial of              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,              <https://www.rfc-editor.org/info/rfc5155>.   [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive              Nameservers in Reflector Attacks", BCP 140, RFC 5358,              DOI 10.17487/RFC5358, October 2008,              <https://www.rfc-editor.org/info/rfc5358>.   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",              STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,              <https://www.rfc-editor.org/info/rfc5730>.   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)              Domain Name Mapping", STD 69, RFC 5731,              DOI 10.17487/RFC5731, August 2009,              <https://www.rfc-editor.org/info/rfc5731>.   [RFC5855]  Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6              Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855,              May 2010, <https://www.rfc-editor.org/info/rfc5855>.   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,              <https://www.rfc-editor.org/info/rfc5936>.   [RFC6561]  Livingood, J., Mody, N., and M. O'Reirdan,              "Recommendations for the Remediation of Bots in ISP              Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012,              <https://www.rfc-editor.org/info/rfc6561>.   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC              Operational Practices, Version 2", RFC 6781,              DOI 10.17487/RFC6781, December 2012,              <https://www.rfc-editor.org/info/rfc6781>.   [RFC6840]  Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and              Implementation Notes for DNS Security (DNSSEC)", RFC 6840,              DOI 10.17487/RFC6840, February 2013,              <https://www.rfc-editor.org/info/rfc6840>.   [RFC6841]  Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A              Framework for DNSSEC Policies and DNSSEC Practice              Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,              <https://www.rfc-editor.org/info/rfc6841>.   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms              for DNS (EDNS(0))", STD 75, RFC 6891,              DOI 10.17487/RFC6891, April 2013,              <https://www.rfc-editor.org/info/rfc6891>.   [RFC7344]  Kumari, W., Gudmundsson, O., and G. Barwood, "Automating              DNSSEC Delegation Trust Maintenance", RFC 7344,              DOI 10.17487/RFC7344, September 2014,              <https://www.rfc-editor.org/info/rfc7344>.   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS              Terminology", RFC 7719, DOI 10.17487/RFC7719, December              2015, <https://www.rfc-editor.org/info/rfc7719>.   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles              for DNS over TLS and DNS over DTLS", RFC 8310,              DOI 10.17487/RFC8310, March 2018,              <https://www.rfc-editor.org/info/rfc8310>.   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,              January 2019, <https://www.rfc-editor.org/info/rfc8499>.   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over              Dedicated QUIC Connections", RFC 9250,              DOI 10.17487/RFC9250, May 2022,              <https://www.rfc-editor.org/info/rfc9250>.   [RFC9471]  Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS              Glue Requirements in Referral Responses", RFC 9471,              DOI 10.17487/RFC9471, September 2023,              <https://www.rfc-editor.org/info/rfc9471>.14.2.  Informative References   [IANA_Resource_Registry]              IANA, "Resource Record (RR) TYPEs",              <https://www.iana.org/assignments/dns-parameters/>.   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,              RFC 20, DOI 10.17487/RFC0020, October 1969,              <https://www.rfc-editor.org/info/rfc20>.   [RFC819]   Su, Z. and J. Postel, "The Domain Naming Convention for              Internet User Applications", RFC 819,              DOI 10.17487/RFC0819, August 1982,              <https://www.rfc-editor.org/info/rfc819>.   [RFC952]   Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet              host table specification", RFC 952, DOI 10.17487/RFC0952,              October 1985, <https://www.rfc-editor.org/info/rfc952>.   [RFC1713]  Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713,              DOI 10.17487/RFC1713, November 1994,              <https://www.rfc-editor.org/info/rfc1713>.   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,              DOI 10.17487/RFC1995, August 1996,              <https://www.rfc-editor.org/info/rfc1995>.   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,              DOI 10.17487/RFC2775, February 2000,              <https://www.rfc-editor.org/info/rfc2775>.   [RFC3172]  Huston, G., Ed., "Management Guidelines & Operational              Requirements for the Address and Routing Parameter Area              Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,              September 2001, <https://www.rfc-editor.org/info/rfc3172>.   [RFC3425]  Lawrence, D., "Obsoleting IQUERY", RFC 3425,              DOI 10.17487/RFC3425, November 2002,              <https://www.rfc-editor.org/info/rfc3425>.   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.              Stevens, "Basic Socket Interface Extensions for IPv6",              RFC 3493, DOI 10.17487/RFC3493, February 2003,              <https://www.rfc-editor.org/info/rfc3493>.   [RFC3757]  Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name              System KEY (DNSKEY) Resource Record (RR) Secure Entry              Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April              2004, <https://www.rfc-editor.org/info/rfc3757>.   [RFC3912]  Daigle, L., "WHOIS Protocol Specification", RFC 3912,              DOI 10.17487/RFC3912, September 2004,              <https://www.rfc-editor.org/info/rfc3912>.   [RFC4470]  Weiler, S. and J. Ihren, "Minimally Covering NSEC Records              and DNSSEC On-line Signing", RFC 4470,              DOI 10.17487/RFC4470, April 2006,              <https://www.rfc-editor.org/info/rfc4470>.   [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",              RFC 4641, DOI 10.17487/RFC4641, September 2006,              <https://www.rfc-editor.org/info/rfc4641>.   [RFC4697]  Larson, M. and P. Barber, "Observed DNS Resolution              Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,              October 2006, <https://www.rfc-editor.org/info/rfc4697>.   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,              December 2006, <https://www.rfc-editor.org/info/rfc4786>.   [RFC4956]  Arends, R., Kosters, M., and D. Blacka, "DNS Security              (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July              2007, <https://www.rfc-editor.org/info/rfc4956>.   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",              BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,              <https://www.rfc-editor.org/info/rfc5625>.   [RFC5890]  Klensin, J., "Internationalized Domain Names for              Applications (IDNA): Definitions and Document Framework",              RFC 5890, DOI 10.17487/RFC5890, August 2010,              <https://www.rfc-editor.org/info/rfc5890>.   [RFC5891]  Klensin, J., "Internationalized Domain Names in              Applications (IDNA): Protocol", RFC 5891,              DOI 10.17487/RFC5891, August 2010,              <https://www.rfc-editor.org/info/rfc5891>.   [RFC5892]  Faltstrom, P., Ed., "The Unicode Code Points and              Internationalized Domain Names for Applications (IDNA)",              RFC 5892, DOI 10.17487/RFC5892, August 2010,              <https://www.rfc-editor.org/info/rfc5892>.   [RFC5893]  Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts              for Internationalized Domain Names for Applications              (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,              <https://www.rfc-editor.org/info/rfc5893>.   [RFC5894]  Klensin, J., "Internationalized Domain Names for              Applications (IDNA): Background, Explanation, and              Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,              <https://www.rfc-editor.org/info/rfc5894>.   [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on              Encodings for Internationalized Domain Names", RFC 6055,              DOI 10.17487/RFC6055, February 2011,              <https://www.rfc-editor.org/info/rfc6055>.   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,              DOI 10.17487/RFC6265, April 2011,              <https://www.rfc-editor.org/info/rfc6265>.   [RFC6303]  Andrews, M., "Locally Served DNS Zones", BCP 163,              RFC 6303, DOI 10.17487/RFC6303, July 2011,              <https://www.rfc-editor.org/info/rfc6303>.   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.              Cheshire, "Internet Assigned Numbers Authority (IANA)              Procedures for the Management of the Service Name and              Transport Protocol Port Number Registry", BCP 165,              RFC 6335, DOI 10.17487/RFC6335, August 2011,              <https://www.rfc-editor.org/info/rfc6335>.   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in              Internationalization in the IETF", BCP 166, RFC 6365,              DOI 10.17487/RFC6365, September 2011,              <https://www.rfc-editor.org/info/rfc6365>.   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the              DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,              <https://www.rfc-editor.org/info/rfc6672>.   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,              DOI 10.17487/RFC6762, February 2013,              <https://www.rfc-editor.org/info/rfc6762>.   [RFC7129]  Gieben, R. and W. Mekking, "Authenticated Denial of              Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,              February 2014, <https://www.rfc-editor.org/info/rfc7129>.   [RFC7480]  Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the              Registration Data Access Protocol (RDAP)", STD 95,              RFC 7480, DOI 10.17487/RFC7480, March 2015,              <https://www.rfc-editor.org/info/rfc7480>.   [RFC7481]  Hollenbeck, S. and N. Kong, "Security Services for the              Registration Data Access Protocol (RDAP)", STD 95,              RFC 7481, DOI 10.17487/RFC7481, March 2015,              <https://www.rfc-editor.org/info/rfc7481>.   [RFC9082]  Hollenbeck, S. and A. Newton, "Registration Data Access              Protocol (RDAP) Query Format", STD 95, RFC 9082,              DOI 10.17487/RFC9082, June 2021,              <https://www.rfc-editor.org/info/rfc9082>.   [RFC9083]  Hollenbeck, S. and A. Newton, "JSON Responses for the              Registration Data Access Protocol (RDAP)", STD 95,              RFC 9083, DOI 10.17487/RFC9083, June 2021,              <https://www.rfc-editor.org/info/rfc9083>.   [RFC9224]  Blanchet, M., "Finding the Authoritative Registration Data              Access Protocol (RDAP) Service", STD 95, RFC 9224,              DOI 10.17487/RFC9224, March 2022,              <https://www.rfc-editor.org/info/rfc9224>.   [RFC7485]  Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin,              "Inventory and Analysis of WHOIS Registration Objects",              RFC 7485, DOI 10.17487/RFC7485, March 2015,              <https://www.rfc-editor.org/info/rfc7485>.   [RFC7793]  Andrews, M., "Adding 100.64.0.0/10 Prefixes to the IPv4              Locally-Served DNS Zones Registry", BCP 163, RFC 7793,              DOI 10.17487/RFC7793, May 2016,              <https://www.rfc-editor.org/info/rfc7793>.   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,              and P. Hoffman, "Specification for DNS over Transport              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May              2016, <https://www.rfc-editor.org/info/rfc7858>.   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram              Transport Layer Security (DTLS)", RFC 8094,              DOI 10.17487/RFC8094, February 2017,              <https://www.rfc-editor.org/info/rfc8094>.   [RFC8109]  Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS              Resolver with Priming Queries", BCP 209, RFC 8109,              DOI 10.17487/RFC8109, March 2017,              <https://www.rfc-editor.org/info/rfc8109>.   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,              <https://www.rfc-editor.org/info/rfc8484>.   [RFC9103]  Toorop, W., Dickinson, S., Sahib, S., Aras, P., and A.              Mankin, "DNS Zone Transfer over TLS", RFC 9103,              DOI 10.17487/RFC9103, August 2021,              <https://www.rfc-editor.org/info/rfc9103>.   [RSSAC026] Root Server System Advisory Committee (RSSAC), "RSSAC0226              RSSAC Lexicon", 2017,              <https://www.icann.org/en/system/files/files/rssac-              026-14mar17-en.pdf>.Appendix A.  Definitions Updated by This Document   The following definitions from RFCs are updated by this document:   *  Forwarder in [RFC2308]   *  QNAME in [RFC2308]   *  Secure Entry Point (SEP) in [RFC3757]; note, however, that this      RFC is already obsolete (see [RFC4033], [RFC4034], [RFC4035]).Appendix B.  Definitions First Defined in This Document   The following definitions are first defined in this document:   *  "Alias" in Section 2   *  "Apex" in Section 7   *  "arpa" in Section 7   *  "Authoritative DoT (ADot)" in Section 6   *  "Bailiwick" in Section 7   *  "Class independent" in Section 5   *  "Classic DNS" in Section 6   *  "Delegation-centric zone" in Section 7   *  "Delegation" in Section 7   *  "DNS operator" in Section 9   *  "DNSSEC-aware" in Section 10   *  "DNSSEC-unaware" in Section 10   *  "Forwarding" in Section 6   *  "Full resolver" in Section 6   *  "Fully Qualified Domain Name" in Section 2   *  "Global DNS" in Section 2   *  "Hardware Security Module (HSM)" in Section 10   *  "Host name" in Section 2   *  "IDN" in Section 2   *  "In-domain" in Section 7   *  "Iterative resolution" in Section 6   *  "Label" in Section 2   *  "Locally served DNS zone" in Section 2   *  "Naming system" in Section 2   *  "Negative response" in Section 3   *  "Non-recursive query" in Section 6   *  "Open resolver" in Section 6   *  "Passive DNS" in Section 6   *  "Policy-implementing resolver" in Section 6   *  "Presentation format" in Section 5   *  "Priming" in Section 6   *  "Private DNS" in Section 2   *  "Recursive DoT (RDot)" in Section 6   *  "Recursive resolver" in Section 6   *  "Referrals" in Section 4   *  "Registrant" in Section 9   *  "Registrar" in Section 9   *  "Registry" in Section 9   *  "Root zone" in Section 7   *  "Secure Entry Point (SEP)" in Section 10   *  "Sibling domain" in Section 7   *  "Signing software" in Section 10   *  "Split DNS" in Section 6   *  "Stub resolver" in Section 6   *  "Subordinate" in Section 8   *  "Superordinate" in Section 8   *  "TLD" in Section 2   *  "Validating resolver" in Section 10   *  "Validation" in Section 10   *  "View" in Section 6   *  "Zone transfer" in Section 6Acknowledgements   [RFC8499] and its predecessor, [RFC7719], were co-authored by Andrew   Sullivan.  The current document, which is a small update to   [RFC8499], has just two authors.  Andrew's work on the earlier   documents is greatly appreciated.   Numerous people made significant contributions to [RFC8499] and   [RFC7719].  Please see the acknowledgements sections in those two   documents for the extensive list of contributors.   Even though the current document is a small revision, many people in   the DNSOP Working Group have contributed to it, and their work is   greatly appreciated.Index   A B C D E F G H I K L M N O P Q R S T U V W X Z      A         Address and Routing Parameter Area Domain (arpa)            Section 7         Address records            Section 5         ADoT            Section 6         Alias            Section 2         Anycast            Section 6         Apex            Section 7         Asterisk label            Section 8         Authoritative data            Section 7         Authoritative server            Section 6         Authoritative-only server            Section 6         AXoT            Section 6      B         Bailiwick            Section 7      C         Canonical name            Section 2         Child            Section 7         Class            Section 4         Class independent            Section 5         Classic DNS            Section 6         Closest encloser            Section 8         Closest provable encloser            Section 8         CNAME            Section 2         Combined signing key (CSK)            Section 10      D         Delegation            Section 7         Delegation-centric zone            Section 7         DNS operator            Section 9         DNS-over-HTTPS            Section 6         DNS-over-QUIC            Section 6         DNS-over-TLS            Section 6         DNSSEC Policy (DP)            Section 10         DNSSEC Practice Statement (DPS)            Section 10         DNSSEC-aware and DNSSEC-unaware            Section 10         DoH            Section 6         Domain name            Section 2         DoQ            Section 6         DoT            Section 6      E         EDNS            Section 5         Empty non-terminals (ENTs)            Section 7         EPP            Section 9      F         Fast flux DNS            Section 7         FORMERR            Section 3         Forward lookup            Section 7         Forwarder            Section 6         Forwarding            Section 6         Full resolver            Section 6         Full-service resolver            Section 6         Fully Qualified Domain Name (FQDN)            Section 2      G         Global DNS            Section 2         Glue records            Section 7      H         Hardware security module (HSM)            Section 10         Hidden master            Section 6         Host name            Section 2      I         IDN            Section 2         In-bailiwick            Section 7         In-domain            Section 7         Insecure delegation            Section 10         Instance            Section 6         Internationalized Domain Name            Section 2         Iterative mode            Section 6         Iterative resolution            Section 6         IXoT            Section 6      K         Key signing key (KSK)            Section 10      L         Label            Section 2         Lame delegation            Section 7         Locally served DNS zone            Section 2      M         Master file            Section 5         Master server            Section 6         mDNS            Section 2         Multicast DNS            Section 2      N         Naming system            Section 2, Paragraph 1.2.1         Negative caching            Section 6         Negative response            Section 3         Next closer name            Section 8         NODATA            Section 3         NOERROR            Section 3         Non-recursive query            Section 6         NOTIMP            Section 3         NS            Section 6         NSEC            Section 10         NSEC3            Section 10         NXDOMAIN            Section 3      O         Occluded name            Section 7         on-line signing            Section 10         online signing            Section 10         Open resolver            Section 6         OPT            Section 5         Opt-out            Section 10         Origin            Section 7         Out-of-bailiwick            Section 7         Owner            Section 5      P         Parent            Section 7         Passive DNS            Section 6         Policy-implementing resolver            Section 6         Presentation format            Section 5         Primary master            Section 6         Primary server            Section 6         Priming            Section 6         Privacy-enabling DNS server            Section 6         Private DNS            Section 2         Public suffix            Section 9      Q         QNAME            Section 4      R         RDAP            Section 9         RDoT            Section 6         Recursive DoT            Section 6         Recursive mode            Section 6, Paragraph 4.10.1         Recursive query            Section 6         Recursive resolver            Section 6         Referrals            Section 4         REFUSED            Section 3         Registrant            Section 9         Registrar            Section 9         Registry            Section 9         Resolver            Section 6         Reverse DNS, reverse lookup            Section 7         Root hints            Section 6         Root zone            Section 7         RR            Section 5         RRset            Section 5      S         Secondary server            Section 6         Secure Entry Point (SEP)            Section 10         SERVFAIL            Section 3         Service name            Section 7         Sibling domain            Section 7         Signed zone            Section 10         Signing software            Section 10         Slave server            Section 6         SOA            Section 5         SOA field names            Section 5         Source of Synthesis            Section 8, Paragraph 1.14.1         Split DNS            Section 6         Split-horizon DNS            Section 6         Stealth server            Section 6         Stub resolver            Section 6         Subdomain            Section 2         Subordinate            Section 9         Superordinate            Section 9      T         TLD            Section 2         Trust anchor            Section 10         TTL            Section 5      U         Unsigned zone            Section 10      V         Validating resolver            Section 10         Validation            Section 10, Paragraph 2.26.1         View            Section 6      W         WHOIS            Section 9         Wildcard            Section 8         Wildcard domain name            Section 8      X         XoT            Section 6      Z         Zone            Section 7         Zone cut            Section 7         Zone enumeration            Section 10         Zone signing key (ZSK)            Section 10         Zone transfer            Section 6Authors' Addresses   Paul Hoffman   ICANN   Email: paul.hoffman@icann.org   Kazunori Fujiwara   Japan Registry Services Co., Ltd.   Email: fujiwara@jprs.co.jp