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rfc:rfc8499

Internet Engineering Task Force (IETF) P. Hoffman Request for Comments: 8499 ICANN BCP: 219 A. Sullivan Obsoletes: 7719 Updates: 2308 K. Fujiwara Category: Best Current Practice JPRS ISSN: 2070-1721 January 2019

                          DNS Terminology

Abstract

 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 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.
 This document obsoletes RFC 7719 and updates RFC 2308.

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/rfc8499.

Hoffman, et al. Best Current Practice [Page 1] RFC 8499 DNS Terminology January 2019

Copyright Notice

 Copyright (c) 2019 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 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Names . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
 3.  DNS Response Codes  . . . . . . . . . . . . . . . . . . . . .  10
 4.  DNS Transactions  . . . . . . . . . . . . . . . . . . . . . .  11
 5.  Resource Records  . . . . . . . . . . . . . . . . . . . . . .  14
 6.  DNS Servers and Clients . . . . . . . . . . . . . . . . . . .  16
 7.  Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
 8.  Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . .  27
 9.  Registration Model  . . . . . . . . . . . . . . . . . . . . .  28
 10. General DNSSEC  . . . . . . . . . . . . . . . . . . . . . . .  30
 11. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . .  34
 12. Security Considerations . . . . . . . . . . . . . . . . . . .  36
 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
 14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
   14.1.  Normative References . . . . . . . . . . . . . . . . . .  36
   14.2.  Informative References . . . . . . . . . . . . . . . . .  39
 Appendix A.  Definitions Updated by This Document . . . . . . . .  44
 Appendix B.  Definitions First Defined in This Document . . . . .  44
 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  46
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  50
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  50

Hoffman, et al. Best Current Practice [Page 2] RFC 8499 DNS Terminology January 2019

1. 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 "public 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 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.
 Therefore, this document is 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.

Hoffman, et al. Best Current Practice [Page 3] RFC 8499 DNS Terminology January 2019

 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.
 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.

Hoffman, et al. Best Current Practice [Page 4] RFC 8499 DNS Terminology January 2019

 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.)

Hoffman, et al. Best Current Practice [Page 5] RFC 8499 DNS Terminology January 2019

    Format of names: Names in the global DNS are domain names.  There
    are three formats: wire format, presentation format, and common
    display.
       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.
       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.
       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.  At the time of writing, that
    operator is Public Technical Identifiers (PTI).  (See
    <https://pti.icann.org/> for more information about PTI operating
    the IANA Functions.)  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.

Hoffman, et al. Best Current Practice [Page 6] RFC 8499 DNS Terminology January 2019

    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 PTI.
 Private DNS:  Names that use the protocol described in [RFC1035] but
    that 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.

Hoffman, et al. Best Current Practice [Page 7] RFC 8499 DNS Terminology January 2019

 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

Hoffman, et al. Best Current Practice [Page 8] RFC 8499 DNS Terminology January 2019

    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".

Hoffman, et al. Best Current Practice [Page 9] RFC 8499 DNS Terminology January 2019

 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

Hoffman, et al. Best Current Practice [Page 10] RFC 8499 DNS Terminology January 2019

    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: the discussion of inverse queries in
    Section 6.4.1 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.)

Hoffman, et al. Best Current Practice [Page 11] RFC 8499 DNS Terminology January 2019

    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 a kind of 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.

Hoffman, et al. Best Current Practice [Page 12] RFC 8499 DNS Terminology January 2019

    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

Hoffman, et al. Best Current Practice [Page 13] RFC 8499 DNS Terminology January 2019

    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.

Hoffman, et al. Best Current Practice [Page 14] RFC 8499 DNS Terminology January 2019

 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 the same document 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, such as 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 RFC
    1035.  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].

Hoffman, et al. Best Current Practice [Page 15] RFC 8499 DNS Terminology January 2019

 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 some class.
    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 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 public 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 public 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".

Hoffman, et al. Best Current Practice [Page 16] RFC 8499 DNS Terminology January 2019

 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.

Hoffman, et al. Best Current Practice [Page 17] RFC 8499 DNS Terminology January 2019

 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.
 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 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.

Hoffman, et al. Best Current Practice [Page 18] RFC 8499 DNS Terminology January 2019

 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 transfer 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".

Hoffman, et al. Best Current Practice [Page 19] RFC 8499 DNS Terminology January 2019

 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) 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 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]).

Hoffman, et al. Best Current Practice [Page 20] RFC 8499 DNS Terminology January 2019

 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.  The effect of this is that a domain name
    that is notionally globally unique nevertheless has different
    meanings for different network users.  This can sometimes be the
    result of a "view" configuration, 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

Hoffman, et al. Best Current Practice [Page 21] RFC 8499 DNS Terminology January 2019

    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].

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)

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 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".

Hoffman, et al. Best Current Practice [Page 23] RFC 8499 DNS Terminology January 2019

 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)
 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)

Hoffman, et al. Best Current Practice [Page 24] RFC 8499 DNS Terminology January 2019

    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.
 Bailiwick:  "In-bailiwick" is a modifier to describe a name server
    whose name is either a subdomain of or (rarely) the same as the
    origin of the zone that contains the delegation to the name
    server.  In-bailiwick name servers may have glue records in their
    parent zone (using the first of the definitions of "glue records"
    in the definition above).  (The word "bailiwick" means the
    district or territory where a bailiff or policeman has
    jurisdiction.)
    "In-bailiwick" names are divided into two types of names for name
    servers: "in-domain" names and "sibling domain" names.
  • In-domain: a modifier to describe a name server whose name is

either subordinate to or (rarely) the same as the owner name of

       the NS resource records.  An in-domain name server name needs
       to have glue records or name resolution fails.  For example, a
       delegation for "child.example.com" may have "in-domain" name
       server name "ns.child.example.com".
  • Sibling domain: a name server's name that is either subordinate

to or (rarely) the same as the zone origin and not subordinate

       to or the same as the owner name of the NS resource records.
       Glue records for sibling domains are allowed, but not
       necessary.  For example, a delegation for "child.example.com"
       in "example.com" zone may have "sibling" name server name
       "ns.another.example.com".
    "Out-of-bailiwick" is the antonym of "in-bailiwick".  It is a
    modifier to describe a name server whose name is not subordinate
    to or the same as the zone origin.  Glue records for out-of-
    bailiwick name servers are useless.  The following table shows
    examples of delegation types.
 Delegation |Parent|Name Server Name  | Type
 -----------+------+------------------+-----------------------------
 com        | .    |a.gtld-servers.net|in-bailiwick / sibling domain
 net        | .    |a.gtld-servers.net|in-bailiwick / in-domain
 example.org| org  |ns.example.org    |in-bailiwick / in-domain
 example.org| org  |ns.ietf.org       |in-bailiwick / sibling domain
 example.org| org  |ns.example.com    |out-of-bailiwick
 example.jp | jp   |ns.example.jp     |in-bailiwick / in-domain
 example.jp | jp   |ns.example.ne.jp  |in-bailiwick / sibling domain
 example.jp | jp   |ns.example.com    |out-of-bailiwick

Hoffman, et al. Best Current Practice [Page 25] RFC 8499 DNS Terminology January 2019

 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 (ENT):  "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 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

Hoffman, et al. Best Current Practice [Page 26] RFC 8499 DNS Terminology January 2019

    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".

Hoffman, et al. Best Current Practice [Page 27] RFC 8499 DNS Terminology January 2019

 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.

Hoffman, et al. Best Current Practice [Page 28] RFC 8499 DNS Terminology January 2019

 RDAP:  The Registration Data Access Protocol, defined in [RFC7480],
    [RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485].  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
    (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 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.

Hoffman, et al. Best Current Practice [Page 29] RFC 8499 DNS Terminology January 2019

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.
 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."

Hoffman, et al. Best Current Practice [Page 30] RFC 8499 DNS Terminology January 2019

 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
    Section 4 of RFC 4034)
 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.

Hoffman, et al. Best Current Practice [Page 31] RFC 8499 DNS Terminology January 2019

 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".

Hoffman, et al. Best Current Practice [Page 32] RFC 8499 DNS Terminology January 2019

 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

Hoffman, et al. Best Current Practice [Page 33] RFC 8499 DNS Terminology January 2019

    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.

Hoffman, et al. Best Current Practice [Page 34] RFC 8499 DNS Terminology January 2019

 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

Hoffman, et al. Best Current Practice [Page 35] RFC 8499 DNS Terminology January 2019

       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 the
 DNS.

13. IANA Considerations

 This document has no IANA actions.

14. References

14.1. 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>.

Hoffman, et al. Best Current Practice [Page 36] RFC 8499 DNS Terminology January 2019

 [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>.

Hoffman, et al. Best Current Practice [Page 37] RFC 8499 DNS Terminology January 2019

 [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>.

Hoffman, et al. Best Current Practice [Page 38] RFC 8499 DNS Terminology January 2019

 [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>.

14.2. Informative References

 [IANA_Resource_Registry]
            IANA, "Resource Record (RR) TYPEs",
            <https://www.iana.org/assignments/dns-parameters/>.
 [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>.

Hoffman, et al. Best Current Practice [Page 39] RFC 8499 DNS Terminology January 2019

 [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>.
 [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>.

Hoffman, et al. Best Current Practice [Page 40] RFC 8499 DNS Terminology January 2019

 [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>.

Hoffman, et al. Best Current Practice [Page 41] RFC 8499 DNS Terminology January 2019

 [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)", 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)", RFC 7481,
            DOI 10.17487/RFC7481, March 2015,
            <https://www.rfc-editor.org/info/rfc7481>.
 [RFC7482]  Newton, A. and S. Hollenbeck, "Registration Data Access
            Protocol (RDAP) Query Format", RFC 7482,
            DOI 10.17487/RFC7482, March 2015,
            <https://www.rfc-editor.org/info/rfc7482>.
 [RFC7483]  Newton, A. and S. Hollenbeck, "JSON Responses for the
            Registration Data Access Protocol (RDAP)", RFC 7483,
            DOI 10.17487/RFC7483, March 2015,
            <https://www.rfc-editor.org/info/rfc7483>.
 [RFC7484]  Blanchet, M., "Finding the Authoritative Registration Data
            (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March
            2015, <https://www.rfc-editor.org/info/rfc7484>.
 [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>.

Hoffman, et al. Best Current Practice [Page 42] RFC 8499 DNS Terminology January 2019

 [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>.
 [RSSAC026] Root Server System Advisory Committee (RSSAC), "RSSAC
            Lexicon", 2017,
            <https://www.icann.org/en/system/files/files/
            rssac-026-14mar17-en.pdf>.

Hoffman, et al. Best Current Practice [Page 43] RFC 8499 DNS Terminology January 2019

Appendix A. Definitions Updated by This Document

 The following definitions from RFCs are updated by this document:
 o  Forwarder in [RFC2308]
 o  QNAME in [RFC2308]
 o  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:
 o  "Alias" in Section 2
 o  "Apex" in Section 7
 o  "arpa" in Section 7
 o  "Bailiwick" in Section 7
 o  "Class independent" in Section 5
 o  "Delegation-centric zone" in Section 7
 o  "Delegation" in Section 7
 o  "DNS operator" in Section 9
 o  "DNSSEC-aware" in Section 10
 o  "DNSSEC-unaware" in Section 10
 o  "Forwarding" in Section 6
 o  "Full resolver" in Section 6
 o  "Fully-qualified domain name" in Section 2
 o  "Global DNS" in Section 2
 o  "Hardware Security Module (HSM)" in Section 10
 o  "Host name" in Section 2
 o  "IDN" in Section 2

Hoffman, et al. Best Current Practice [Page 44] RFC 8499 DNS Terminology January 2019

 o  "In-bailiwick" in Section 7
 o  "Iterative resolution" in Section 6
 o  "Label" in Section 2
 o  "Locally served DNS zone" in Section 2
 o  "Naming system" in Section 2
 o  "Negative response" in Section 3
 o  "Non-recursive query" in Section 6
 o  "Open resolver" in Section 6
 o  "Out-of-bailiwick" in Section 7
 o  "Passive DNS" in Section 6
 o  "Policy-implementing resolver" in Section 6
 o  "Presentation format" in Section 5
 o  "Priming" in Section 6
 o  "Private DNS" in Section 2
 o  "Recursive resolver" in Section 6
 o  "Referrals" in Section 4
 o  "Registrant" in Section 9
 o  "Registrar" in Section 9
 o  "Registry" in Section 9
 o  "Root zone" in Section 7
 o  "Secure Entry Point (SEP)" in Section 10
 o  "Signing software" in Section 10
 o  "Split DNS" in Section 6
 o  "Stub resolver" in Section 6

Hoffman, et al. Best Current Practice [Page 45] RFC 8499 DNS Terminology January 2019

 o  "Subordinate" in Section 8
 o  "Superordinate" in Section 8
 o  "TLD" in Section 2
 o  "Validating resolver" in Section 10
 o  "Validation" in Section 10
 o  "View" in Section 6
 o  "Zone transfer" in Section 6

Index

 A
    Address records  16
    Alias  9
    Anycast  22
    Apex  23
    Asterisk label  27
    Authoritative data  24
    Authoritative server  19
    Authoritative-only server  19
    arpa: Address and Routing Parameter Area Domain  26
 C
    CNAME  10
    Canonical name  9
    Child  22
    Class  11
    Class independent  16
    Closest encloser  27
    Closest provable encloser  27
    Combined signing key (CSK)  33
 D
    DNS operator  29
    DNSSEC Policy (DP)  34
    DNSSEC Practice Statement (DPS)  34
    DNSSEC-aware and DNSSEC-unaware  30
    Delegation  24
    Delegation-centric zone  26
    Domain name  5

Hoffman, et al. Best Current Practice [Page 46] RFC 8499 DNS Terminology January 2019

 E
    EDNS  14
    EPP  28
    Empty non-terminals (ENT)  26
 F
    FORMERR  10
    Fast flux DNS  26
    Forward lookup  26
    Forwarder  21
    Forwarding  20
    Full resolver  18
    Full-service resolver  18
    Fully-qualified domain name (FQDN)  8
 G
    Global DNS  5
    Glue records  24
 H
    Hardware security module (HSM)  34
    Hidden master  20
    Host name  8
 I
    IDN  9
    In-bailiwick  25
    Insecure delegation  31
    Instance  22
    Internationalized Domain Name  9
    Iterative mode  17
    Iterative resolution  18
 K
    Key signing key (KSK)  33
 L
    Label  5
    Lame delegation  24
    Locally served DNS zone  8
 M
    Master file  14
    Master server  19
    Multicast DNS  7
    mDNS  7

Hoffman, et al. Best Current Practice [Page 47] RFC 8499 DNS Terminology January 2019

 N
    NODATA  10
    NOERROR  10
    NOTIMP  10
    NS  19
    NSEC  31
    NSEC3  31
    NXDOMAIN  10
    Naming system  4
    Negative caching  19
    Negative response  11
    Next closer name  28
    Non-recursive query  18
 O
    OPT  14
    Occluded name  26
    Open resolver  21
    Opt-out  31
    Origin  23
    Out-of-bailiwick  25
    Owner  15
 P
    Parent  23
    Passive DNS  22
    Policy-implementing resolver  21
    Presentation format  14
    Primary master  20
    Primary server  20
    Priming  18
    Privacy-enabling DNS server  22
    Private DNS  7
    Public suffix  29
 Q
    QNAME  11
 R
    RDAP  29
    REFUSED  10
    RR  14
    RRset  14
    Recursive mode  17
    Recursive query  18
    Recursive resolver  17
    Referrals  13
    Registrant  28

Hoffman, et al. Best Current Practice [Page 48] RFC 8499 DNS Terminology January 2019

    Registrar  28
    Registry  28
    Resolver  16
    Reverse DNS, reverse lookup  26
    Root hints  18
    Root zone  26
 S
    SERVFAIL  10
    SOA  14
    SOA field names  14
    Secondary server  19
    Secure Entry Point (SEP)  33
    Service name  27
    Signed zone  30
    Signing software  34
    Slave server  19
    Source of Synthesis  28
    Split DNS  21
    Split-horizon DNS  21
    Stealth server  20
    Stub resolver  17
    Subdomain  9
    Subordinate  29
    Superordinate  29
 T
    TLD  9
    TTL  15
    Trust anchor  34
 U
    Unsigned zone  30
 V
    Validating resolver  33
    Validation  32
    View  21
 W
    WHOIS  28
    Wildcard  27
    Wildcard domain name  27

Hoffman, et al. Best Current Practice [Page 49] RFC 8499 DNS Terminology January 2019

 Z
    Zone  22
    Zone cut  23
    Zone enumeration  31
    Zone signing key (ZSK)  33
    Zone transfer  19

Acknowledgements

 The following is the Acknowledgements section of RFC 7719.
    The authors gratefully acknowledge all of the authors of DNS-
    related RFCs that proceed this one.  Comments from Tony Finch,
    Stephane Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray
    Bellis, John Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque,
    Paul Ebersman, David Lawrence, Matthijs Mekking, Casey Deccio, Bob
    Harold, Ed Lewis, John Klensin, David Black, and many others in
    the DNSOP Working Group helped shape RFC 7719.
 Most of the major changes between RFC 7719 and this document came
 from active discussion on the DNSOP WG.  Specific people who
 contributed material to this document include: Bob Harold, Dick
 Franks, Evan Hunt, John Dickinson, Mark Andrews, Martin Hoffmann,
 Paul Vixie, Peter Koch, Duane Wessels, Allison Mankin, Giovane Moura,
 Roni Even, Dan Romascanu, and Vladmir Cunat.

Authors' Addresses

 Paul Hoffman
 ICANN
 Email: paul.hoffman@icann.org
 Andrew Sullivan
 Email: ajs@anvilwalrusden.com
 Kazunori Fujiwara
 Japan Registry Services Co., Ltd.
 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
 Chiyoda-ku, Tokyo  101-0065
 Japan
 Phone: +81 3 5215 8451
 Email: fujiwara@jprs.co.jp

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