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RFC 7719

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Informational
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DNS Terminology

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Internet Engineering Task Force (IETF)                        P. Hoffman
Request for Comments: 7719                                         ICANN
Category: Informational                                      A. Sullivan
ISSN: 2070-1721                                                      Dyn
                                                             K. Fujiwara
                                                                    JPRS
                                                           December 2015


                            DNS Terminology

Abstract

   The 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 sometimes 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.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7719.

Page 2 
Copyright Notice

   Copyright (c) 2015 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
   (http://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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Names . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  DNS Header and Response Codes . . . . . . . . . . . . . . . .   6
   4.  Resource Records  . . . . . . . . . . . . . . . . . . . . . .   7
   5.  DNS Servers and Clients . . . . . . . . . . . . . . . . . . .   9
   6.  Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Registration Model  . . . . . . . . . . . . . . . . . . . . .  17
   8.  General DNSSEC  . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . .  20
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     11.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   The Domain Name System (DNS) is a simple query-response protocol
   whose messages in both directions have the same format.  The protocol
   and message format are defined in [RFC1034] and [RFC1035].  These
   RFCs defined some terms, but later documents defined others.  Some of
   the terms from RFCs 1034 and 1035 now have somewhat different
   meanings than they did in 1987.

   This document collects a wide variety of DNS-related terms.  Some of
   them have been precisely defined in earlier RFCs, some have been
   loosely defined in earlier RFCs, and some are not defined in any
   earlier RFC at all.

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   Most of the definitions here are 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.

   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.
   Therefore, the authors intend to follow this document with a
   substantial revision in the not-distant future.  That revision will
   probably have more in-depth discussion of some terms as well as new
   terms; it will also update some of the RFCs with new definitions.

   The terms are organized loosely by topic.  Some definitions are for
   new terms for things that are commonly talked about in the DNS
   community but that never had terms defined for them.

   Other organizations sometimes define DNS-related terms their own way.
   For example, the W3C defines "domain" at
   https://specs.webplatform.org/url/webspecs/develop/.

   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.

   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.

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2.  Names

   Domain name:  Section 3.1 of [RFC1034] talks of "the domain name
      space" as a tree structure.  "Each node has a label, which is zero
      to 63 octets in length. ... The domain name of a node is the list
      of the labels on the path from the node to the root of the tree.
      ... To simplify implementations, the total number of octets that
      represent a domain name (i.e., the sum of all label octets and
      label lengths) is limited to 255."  Any label in a domain name can
      contain any octet value.

   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 final, 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 some of the right-most names are left off and are understood
      only by context.

   Label:  The identifier of an individual node in the sequence of nodes
      identified by a fully qualified domain name.

   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], the "preferred name syntax".  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

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      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".

   TLD:  A Top-Level Domain, meaning 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, 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.

   IDN:  The common abbreviation for "Internationalized Domain Name".
      The IDNA protocol is the standard mechanism for handling domain
      names with non-ASCII characters in applications in the DNS.  The
      current standard, 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), and
      [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".

   Alias:  The owner of a CNAME resource record, or a subdomain of the
      owner of a DNAME resource record [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 is 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 an alias, not
      a canonical name."  (Quoted from [RFC2181], Section 10.1.1)

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   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, and on
      which HTTP cookies ([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.  At the
      time this document is published, the IETF DBOUND Working Group
      [DBOUND] is dealing with issues concerning 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
      (http://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 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 the case of the
      "uk" TLD around the time this document is published.

3.  DNS Header and Response Codes

   The header of a DNS message is its first 12 octets.  Many of the
   fields and flags in the header diagram in Sections 4.1.1 through
   4.1.3 of [RFC1035] are referred to by their names in that 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".

   Some of response codes that are defined in [RFC1035] have gotten
   their own shorthand names.  Some common response code names that
   appear without reference to the numeric value are "FORMERR",
   "SERVFAIL", and "NXDOMAIN" (the latter of which is also referred to
   as "Name Error").  All of the RCODEs are listed at
   http://www.iana.org/assignments/dns-parameters, although that site
   uses mixed-case capitalization, while most documents use all-caps.

   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

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      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 the nameserver cannot
      answer.  Sections 2 and 7 of [RFC2308] describe the types of
      negative responses in detail.

   Referrals:  Data from the authority section of a non-authoritative
      answer.  [RFC1035] Section 2.1 defines "authoritative" data.
      However, referrals at zone cuts (defined in Section 6) are not
      authoritative.  Referrals may be zone cut NS resource records and
      their glue records.  NS records on the parent side of a zone cut
      are an authoritative delegation, but are normally not treated as
      authoritative data.  In general, a referral is a way for a server
      to send an answer saying that the server does not know the answer,
      but knows where the query should be directed in order to get an
      answer.  Historically, many authoritative servers answered with a
      referral to the root zone when queried for a name for which they
      were not authoritative, but this practice has declined.

4.  Resource Records

   RR:  An acronym for resource record.  ([RFC1034], Section 3.6.)

   RRset:  A set of resource records with the same label, class and
      type, but with different data.  (Definition from [RFC2181]) Also
      spelled 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".  (This definition is definitely not the same as "the
      response one gets to a query for QTYPE=ANY", which is an
      unfortunate misunderstanding.)

   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 potentially to carry additional options
      that affect the handling of a DNS query.

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   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 from [RFC6891],
      Section 6.1.1) It is used by EDNS.

   Owner:  The domain name where a RR is found ([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.  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 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, the TTL is 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) Also: "the
      time interval (in seconds) that the resource record may be cached
      before it should be discarded".  (Quoted from [RFC1035],
      Section 4.1.3).  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, such as if there is a policy to disallow TTL
      values over a certain number.  Also, if a value is flushed from
      the cache when its value is still positive, the value effectively
      becomes zero.  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 RFC 1035.

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      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 the meaning is undefined for classes other
      than IN (class 1, the Internet).

5.  DNS Servers and Clients

   This section defines the terms used for the systems that act as DNS
   clients, DNS servers, or both.

   Resolver:  A program "that extract[s] information from name servers
      in response to client requests."  (Quoted from [RFC1034],
      Section 2.4) "The resolver is located on the same machine as the
      program that requests the resolver's services, but it may need to
      consult name servers on other hosts."  (Quoted from [RFC1034],
      Section 5.1) A resolver performs queries for a name, type, and
      class, and receives answers.  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.

   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".

   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".  A server operating in recursive mode may be thought of

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      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".  While strictly
      the difference between these is that one of them sends queries to
      another recursive server and the other does not, 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.

   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.

   Full-service resolver:  Section 6.1.3.1 of [RFC1123] defines this
      term to mean a resolver that acts in recursive mode with a cache
      (and meets other requirements).

   Priming:  The mechanism used by a resolver to determine where to send
      queries before there is anything in the resolver's cache.  Priming
      is most often done from a configuration setting that contains a
      list of authoritative servers for the root zone.

   Negative caching:  "The storage of knowledge that something does not
      exist, cannot give an answer, 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.)  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)

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   Zone transfer:  The act of a client requesting a copy of a zone and
      an authoritative server sending the needed information.  (See
      Section 6 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 transfer outside
      the DNS protocol.

   Secondary server:  "An authoritative server which uses zone transfer
      to retrieve the zone" (Quoted from [RFC1996], Section 2.1).
      [RFC2182] describes secondary servers in 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".
      Secondary servers are also discussed in [RFC1034].

   Slave server:  See secondary 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, "an
      authoritative server configured to be the source of AXFR or IXFR
      data for one or more [secondary] servers" (Quoted from [RFC2136]).
      Although early DNS RFCs such as [RFC1996] referred to this as a
      "master", the current common usage has shifted to "primary".
      Primary servers are also discussed in [RFC1034].

   Master server:  See primary server.

   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 by [RFC2136],
      and is considered archaic in other parts of the DNS.

   Stealth server:  This is "like a slave server except not listed in an
      NS RR for the zone."  (Quoted from [RFC1996], Section 2.1)

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   Hidden master:  A stealth server that is a master 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.)
      An earlier RFC, [RFC4641], said that the hidden master's name
      appears in the SOA RRs MNAME field, although in some setups, the
      name does not appear at all in the public DNS.  A hidden master
      can be either a secondary or a primary master.

   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 need 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) stub resolver.  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.

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   View:  A configuration for a DNS server that allows it to provide
      different answers depending on attributes of the query.
      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 large amounts of DNS data by
      storing DNS responses from servers.  Some of these systems also
      collect the DNS queries associated with the responses; this can
      raise privacy issues.  Passive DNS databases can be used to answer
      historical questions about DNS zones such as which records were
      available for them at what times in the past.  Passive DNS
      databases allow searching of the stored records on keys other than
      just the name, 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)

6.  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 [RFC882] as "the name server that has authority over the place
      in the domain name space that will hold the new domain".  (Note
      that [RFC882] was obsoleted by [RFC1034] and [RFC1035].)  [RFC819]
      also has some description of the relationship between parents and
      children.

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   Origin:

      (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" as
      '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.

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   Glue records:  "[Resource records] which are not part of the
      authoritative data [of the zone], and are address resource records
      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."  (Definition 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" ([RFC2181], Section 5.4.1).  Although glue
      is sometimes used today with this wider definition in mind, the
      context surrounding the [RFC2181] definition suggests it is
      intended to apply to the use of glue within the document itself
      and not necessarily beyond.

   In-bailiwick:

      (a) An adjective to describe a name server whose name is either
      subordinate to or (rarely) the same as the zone origin.  In-
      bailiwick name servers require glue records in their parent zone
      (using the first of the definitions of "glue records" in the
      definition above).

      (b) Data for which the server is either authoritative, or else
      authoritative for an ancestor of the owner name.  This sense of
      the term normally is used when discussing the relevancy of glue
      records in a response.  For example, the server for the parent
      zone "example.com" might reply with glue records for
      "ns.child.example.com".  Because the "child.example.com" zone is a
      descendant of the "example.com" zone, the glue records are in-
      bailiwick.

   Out-of-bailiwick:  The antonym of in-bailiwick.

   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) It is noted that this definition might
      inadvertently also include any NS records that appear in the zone,
      even those that might not truly be authoritative because there are
      identical NS RRs below the zone cut.  This reveals the ambiguity

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      in the notion of authoritative data, because the parent-side NS
      records authoritatively indicate the delegation, even though they
      are not themselves authoritative data.

   Root zone:  The zone whose apex is the zero-length label.  Also
      sometimes called "the DNS root".

   Empty non-terminals:  "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 is not defined there.

   Wildcard:  [RFC1034] defined "wildcard", but in a way that turned out
      to be confusing to implementers.  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) For an extended discussion of wildcards, including
      clearer definitions, see [RFC4592].

   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 typo
      corrected) It is often 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.


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