wdiff rfc8499v1.txt rfc8499.txt

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 November December 2018

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.

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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3
3. DNS Response Codes . . . . . . . . . . . . . . . . . . . . . 9
4. DNS Transactions . . . . . . . . . . . . . . . . . . . . . . 10
5. Resource Records . . . . . . . . . . . . . . . . . . . . . . 13
6. DNS Servers and Clients . . . . . . . . . . . . . . . . . . . 15
7. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 21
8. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9. Registration Model . . . . . . . . . . . . . . . . . . . . . 27
10. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 29
11. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 34 33
12. Security Considerations . . . . . . . . . . . . . . . . . . . 35
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.1. Normative References . . . . . . . . . . . . . . . . . . 35
14.2. Informative References . . . . . . . . . . . . . . . . . 38
Appendix A. Definitions Updated by This Document . . . . . . . . 42
Appendix B. Definitions First Defined in This Document . . . . . 43
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 49

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,
but
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.
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].

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

Note that there is no single consistent definition of "the DNS". It
can be considered to be some combination of the following: a commonly
used naming scheme for objects on the Internet; a distributed
database representing the names and certain properties of these
objects; an architecture providing distributed maintenance,
resilience, and loose coherency for this database; and a simple
query-response protocol (as mentioned below) implementing this
architecture. Section 2 defines "global DNS" and "private DNS" as a
way to deal with these differing definitions.

Capitalization in DNS terms is often inconsistent among RFCs and
various DNS practitioners. The capitalization used in this document
is a best guess at current practices, and is not meant to indicate
that other capitalization styles are wrong or archaic. In some
cases, multiple styles of capitalization are used for the same term
due to quoting from different RFCs.

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.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. Names

Naming system: A naming system associates names with data. Naming
systems have many significant facets that help differentiate them
from each other. Some commonly identified facets include:

* Composition of names

* Format of names

* Administration of names

* Types of data that can be associated with names

* Types of metadata for names

* Protocol for getting data from a name

* Context for resolving a name

Note that this list is a small subset of facets that people have
identified over time for naming systems, and the IETF has yet to
agree on a good set of facets that can be used to compare naming
systems. For example, other facets might include "protocol to
update data in a name", "privacy of names", and "privacy of data
associated with names", but those are not as well defined as the
ones listed above. The list here is chosen because it helps
describe the DNS and naming systems similar to the DNS.

Domain name: An ordered list of one or more labels.

Note that this is a definition independent of the DNS RFCs, RFCs
([RFC1034] and [RFC1035]), and the definition here also applies to
systems other than the DNS. [RFC1034] defines the "domain name
space" using mathematical trees and their nodes in graph theory,
and that definition has the same practical result as the
definition here. Any path of a directed acyclic graph can be
represented by a domain name consisting of the labels of its
nodes, ordered by decreasing distance from the root(s) (which is
the normal convention within the DNS, including this document). A
domain name whose last label identifies a root of the graph is
fully qualified; other domain names whose labels form a strict
prefix of a fully-qualified domain name are relative to its first
omitted node.

Also note that different IETF and non-IETF documents have used the
term "domain name" in many different ways. It is common for
earlier documents to use "domain name" to mean "names that match
the syntax in [RFC1035]", but possibly with additional rules such
as "and are, or will be, resolvable in the global DNS" or "but
only using the presentation format".

Label: An ordered list of zero or more octets that makes up a
portion of a domain name. Using graph theory, a label identifies
one node in a portion of the graph of all possible domain names.

Global DNS: Using the short set of facets listed in "Naming system",
the global DNS can be defined as follows. Most of the rules here
come from [RFC1034] and [RFC1035], although the term "global DNS"
has not been defined before now.

Composition of names: A name in the global DNS has one or more
labels. The length of each label is between 0 and 63 octets
inclusive. In a fully-qualified domain name, the last label in
the ordered list is 0 octets long; it is the only label whose
length may be 0 octets, and it is called the "root" or "root
label". A domain name in the global DNS has a maximum total
length of 255 octets in the wire format; the root represents one
octet for this calculation. (Multicast DNS [RFC6762] allows names
up to 255 bytes plus a terminating zero byte based on a different
interpretation of RFC 1035 and what is included in the 255
octets.)
Format of names: Names in the global DNS are domain names. There
are three formats: wire format, presentation format, and common
display.

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.

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.

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, for example, such as defined those that are listed in [RFC6303], [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 [RFC0819]. [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 [RFC0952], [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" (that is, syntax". (In
that syntax, every character in each label is a letter, a digit,
or a hyphen). Note that any label in a domain name can contain
any octet value; hostnames are generally considered to be domain
names where every label follows the rules in the "preferred name
syntax", with the amendment that labels can start with ASCII
digits (this amendment comes from Section 2.1 of [RFC1123]).

People also sometimes use the term "hostname" to refer to just the
first label of an FQDN, such as "printer" in
"printer.admin.example.com". (Sometimes this is formalized in
configuration in operating systems.) In addition, people
sometimes use this term to describe any name that refers to a
machine, and those might include labels that do not conform to the
"preferred name syntax".

Top-Level Domain (TLD): A Top-Level Domain is a zone that is one
layer below the root, such as "com" or "jp". There is nothing
special, from the point of view of the DNS, about TLDs. Most of
them are also delegation-centric zones (defined in Section 7, 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). 3.1) For example, in the
host name "nnn.mmm.example.com", both "mmm.example.com" and
"nnn.mmm.example.com" are subdomains of "example.com". Note that
the comparisons here are done on whole labels; that is,
"ooo.example.com" is not a subdomain of "oo.example.com".

Alias: The owner of a CNAME resource record, or a subdomain of the
owner of a DNAME resource record (DNAME records are defined in
[RFC6672]). See also "canonical name".

Canonical name: A CNAME resource record "identifies its owner name
as an alias, and specifies the corresponding canonical name in the
RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2)
This usage of the word "canonical" is related to the mathematical
concept of "canonical form".

CNAME: "It has been traditional to refer to the [owner] of a CNAME
record as "a CNAME". 'a CNAME'. This is unfortunate, as "CNAME" 'CNAME' is an
abbreviation of "canonical name", '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 here, but the 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" (Quoted from [RFC1035], in Section 4.1.1.) 4.1.1
of [RFC1035].

FORMERR: This RCODE appears as "Format error - The name server was
unable to interpret the
query." (Quoted from [RFC1035], query" in Section 4.1.1.) 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." (Quoted from
[RFC1035],
server" in Section 4.1.1.) 4.1.1 of [RFC1035].

NXDOMAIN: This RCODE appears as "Name Error - This [...] this code
signifies that the domain name referenced in the query does not
exist." (Quoted from [RFC1035], in Section 4.1.1.) 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." (Quoted from [RFC1035], query" in Section 4.1.1.) 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." (Quoted from [RFC1035],
in Section 4.1.1.) 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.) 1) "NODATA is indicated by an answer with the
RCODE set to NOERROR and no relevant answers in the answer Answer
section. The authority section will contain an SOA record, or
there will be no NS records there." (Quoted from [RFC2308],
Section 2.2.) 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 header diagram diagrams in Sections 4.1.1 through 4.1.3 of
[RFC1035] are referred to by their names in that 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). 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.). 2.2) In practice, the class for
nearly every query is "IN". "IN" (the Internet). There are some queries
for "CH", "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.). 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. 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 section and
leave the Question Section section empty. (Inverse queries are deprecated
in [RFC3425]; thus, relevant definitions do not appear in this
document.)
However, [RFC2308] has an alternate definition that puts the QNAME
in the answer (or series of answers) instead of the query. It
defines QNAME as "...the name in the query section of an answer,
or where this resolves to a CNAME, or CNAME chain, the data field
of the last CNAME. The last CNAME in this sense is that which
contains a value which does not resolve to another CNAME." This
definition has a certain internal logic, because of the way CNAME
substitution works and the definition of CNAME. If a name server
does not find an RRset that matches a query, but does find the
same name in the same class with a CNAME record, then the name
server "includes the CNAME record in the response and restarts the
query at the domain name specified in the data field of the CNAME
record." (Quoted from [RFC1034], Section 3.6.2). 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 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
section in the original query, which is always echoed in the
(final) reply in the Question Section section when the QR bit is set to
1.

* QNAME (effective): A name actually resolved, which is either
the name originally queried or a name received in a CNAME chain
response.

* QNAME (final): The name actually resolved, which is either the
name actually queried or else the last name in a CNAME chain
response.

Note that, because the definition in [RFC2308] is actually for a
different concept than what was in [RFC1034], it would have been
better if [RFC2308] had used a different name for that concept.

In general use today, QNAME almost always means what is defined
above as "QNAME (original)".

Referrals: A type of response in which a server, signaling that it
is not (completely) authoritative for an answer, provides the
querying resolver with an alternative place to send its query.
Referrals can be partial.

A referral arises when a server is not performing recursive
service while answering a query. It appears in step 3(b) of the
algorithm in [RFC1034], Section 4.3.2.

There are two types of referral response. The first is a downward
referral (sometimes described as "delegation response"), where the
server is authoritative for some portion of the QNAME. The
authority section RRset's RDATA contains the name servers
specified at the referred-to zone cut. In normal DNS operation,
this kind of response is required in order to find names beneath a
delegation. The bare use of "referral" means this kind of
referral, and many people believe that this is the only legitimate
kind of referral in the DNS.

The second is an upward referral (sometimes described as "root
referral"), where the server is not authoritative for any portion
of the QNAME. When this happens, the referred-to zone in the
authority section is usually the root zone (.). ("."). In normal DNS
operation, this kind of response is not required for resolution or
for correctly answering any query. There is no requirement that
any server send upward referrals. Some people regard upward
referrals as a sign of a misconfiguration or error. Upward
referrals always need some sort of qualifier (such as "upward" or
"root") and are never identified simply by the word "referral".

A response that has only a referral contains an empty answer
section. It contains the NS RRset for the referred-to zone in the
authority
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 Answer section. It
has the NS RRset for the referred-to zone in the authority Authority
section. It may contain RRs that provide addresses in the
additional section. The AA bit is set, because the first name in
the answer Answer section matches the QNAME and the server is
authoritative for that answer (see [RFC1035], Section 4.1.1).

5. Resource Records

RR: An abbreviation 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.) 5) Master files are sometimes called "zone
files".

Presentation format: The text format used in master files. This
format is shown but not formally defined in [RFC1034] or
[RFC1035]. The term "presentation format" first appears in
[RFC4034].

EDNS: The extension mechanisms for DNS, defined in [RFC6891].
Sometimes called "EDNS0" or "EDNS(0)" to indicate the version
number. EDNS allows DNS clients and servers to specify message
sizes larger than the original 512 octet limit, to expand the
response code space and to carry additional options that affect
the handling of a DNS query.

OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to
contain control information pertaining to the question-and-answer
sequence of a specific transaction. (Definition paraphrased from
[RFC6891], Section 6.1.1.) It is used by EDNS.

Owner: "The domain name where the RR is found." (Quoted from
[RFC1034], Section 3.6). 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.) 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].

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".

Stub resolver: A resolver that cannot perform all resolution itself.
Stub resolvers generally depend on a recursive resolver to
undertake the actual resolution function. Stub resolvers are
discussed but never fully defined in Section 5.3.1 of [RFC1034].
They are fully defined in Section 6.1.3.1 of [RFC1123].

Iterative mode: A resolution mode of a server that receives DNS
queries and responds with a referral to another server.
Section 2.3 of [RFC1034] describes this as "The server refers the
client to another server and lets the client pursue the query." A
resolver that works in iterative mode is sometimes called an
"iterative resolver". See also "iterative resolution" later in
this section.

Recursive mode: A resolution mode of a server that receives DNS
queries and either responds to those queries from a local cache or
sends queries to other servers in order to get the final answers
to the original queries. Section 2.3 of [RFC1034] describes this
as "the first server pursues the query for the client at another
server". Section 4.3.1 of [RFC1034] says: "in [recursive] mode
the name server acts in the role of a resolver and returns either
an error or the answer, but never referrals." That same section
also says:

The recursive mode occurs when a query with RD set arrives at a
server which is willing to provide recursive service; the
client can verify that recursive mode was used by checking that
both RA and RD are set in the reply.

A server operating in recursive mode may be thought of as having a
name server side (which is what answers the query) and a resolver
side (which performs the resolution function). Systems operating
in this mode are commonly called "recursive servers". Sometimes
they are called "recursive resolvers". In practice, it is not
possible to know in advance whether the server that one is
querying will also perform recursion; both terms can be observed
in use interchangeably.

Recursive resolver: A resolver that acts in recursive mode. In
general, a recursive resolver is expected to cache the answers it
receives (which would make it a full-service resolver), but some
recursive resolvers might not cache.

[RFC4697] tried to differentiate between a recursive resolver and
an iterative resolver.

Recursive query: A query with the Recursion Desired (RD) bit set to
1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive
service is available and is requested by the RD bit in the query,
the server uses its resolver to answer the query. (See
Section 4.3.2 of [RFC1035].) [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 [RFC1035].) [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.) 2) In order to operate in recursive mode, a resolver needs
to know the address of at least one root server. Priming is most
often done from a configuration setting that contains a list of
authoritative servers for the root zone.

Root hints: "Operators who manage a DNS recursive resolver typically
need to configure a 'root hints file'. This file contains the
names and IP addresses of the authoritative name servers for the
root zone, so the software can bootstrap the DNS resolution
process. For many pieces of software, this list comes built into
the software." (Quoted from [IANA_RootFiles]) This file is often
used in priming.

Negative caching: "The storage of knowledge that something does not
exist, cannot or does not give an answer." (Quoted from
[RFC2308], Section 1)

Authoritative server: "A server that knows the content of a DNS zone
from local knowledge, and thus can answer queries about that zone
without needing to query other servers." (Quoted from [RFC2182],
Section 2.) 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). 2.1)
Secondary servers are also discussed in [RFC1034]. [RFC2182]
describes secondary servers in more detail. Although early DNS
RFCs such as [RFC1996] referred to this as a "slave", the current
common usage has shifted to calling it a "secondary".

Master server: See "Primary server".

Primary server: "Any authoritative server configured to be the
source of zone transfer for one or more [secondary] servers."
(Quoted from [RFC1996], Section 2.1) Or, more specifically,
[RFC2136] calls it "an authoritative server configured to be the
source of AXFR or IXFR data for one or more [secondary] servers".
Primary servers are also discussed in [RFC1034]. Although early
DNS RFCs such as [RFC1996] referred to this as a "master", the
current common usage has shifted to "primary".

Primary master: "The primary master is named in the zone's SOA MNAME
field and optionally by an NS RR." (Quoted from [RFC1996],
Section 2.1). 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). 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]).

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
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)
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].)
[RFC0819]
[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.) 6) These
days, this sense of "origin" and "apex" (defined below) are
often used interchangeably.

(b) The domain name within which a given relative domain name
appears in zone files. Generally seen in the context of
"$ORIGIN", which is a control entry defined in [RFC1035],
Section 5.1, as part of the master file format. For example,
if the $ORIGIN is set to "example.org.", then a master file
line for "www" is in fact an entry for "www.example.org.".

Apex: The point in the tree at an owner of an SOA and corresponding
authoritative NS RRset. This is also called the "zone apex".
[RFC4033] defines it as "the name at the child's side of a zone
cut". The "apex" can usefully be thought of as a data-theoretic
description of a tree structure, and "origin" is the name of the
same concept when it is implemented in zone files. The
distinction is not always maintained in use, however, and one can
find uses that conflict subtly with this definition. [RFC1034]
uses the term "top node of the zone" as a synonym of "apex", but
that term is not widely used. These days, the first sense of
"origin" (above) and "apex" are often used interchangeably.

Zone cut: The delimitation point between two zones where the origin
of one of the zones is the child of the other zone.

"Zones are delimited by 'zone cuts'. Each zone cut separates a
'child' zone (below the cut) from a 'parent' zone (above the
cut)." (Quoted from [RFC2181], Section 6; note that this is
barely an ostensive definition.) Section 4.2 of [RFC1034] uses
"cuts" instead of "zone cut".

Delegation: The process by which a separate zone is created in the
name space beneath the apex of a given domain. Delegation happens
when an NS RRset is added in the parent zone for the child origin.
Delegation inherently happens at a zone cut. The term is also
commonly a noun: the new zone that is created by the act of
delegating.

Authoritative data: "All of the RRs attached to all of the nodes
from the top node of the zone down to leaf nodes or nodes above
cuts around the bottom edge of the zone." (Quoted from [RFC1034],
Section 4.2.1) Note that this definition might inadvertently also
cause any NS records that appear in the zone to be included, even
those that might not truly be authoritative because there are
identical NS RRs below the zone cut. This reveals the ambiguity
in the notion of authoritative data, because the parent-side NS
records authoritatively indicate the delegation, even though they
are not themselves authoritative data.

[RFC4033], Section 2, defines "Authoritative RRset", which is
related to authoritative data but has a more precise definition.

Lame delegation: "A lame delegations exists [sic] when a nameserver
is delegated responsibility for providing nameservice for a zone
(via NS records) but is not performing nameservice for that zone
(usually because it is not set up as a primary or secondary for
the zone)." (Quoted from [RFC1912], Section 2.8) Another
definition is that a lame delegation "...happens when a name
server is listed in the NS records for some domain and in fact it
is not a server for that domain. Queries are thus sent to the
wrong servers, who don't know nothing [sic] (at least not as
expected) about the queried domain. Furthermore, sometimes these
hosts (if they exist!) don't even run name servers." (Quoted from
[RFC1713], Section 2.3)

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). 5.4.1)
Although glue is sometimes used today with this wider definition
in mind, the context surrounding the definition in [RFC2181]
suggests it is intended to apply to the use of glue within the
document itself and not necessarily beyond.

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 MUST 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.

Empty non-terminals (ENT): "Domain names that own no resource
records but have subdomains that do." (Quoted from [RFC4592],
Section 2.2.2.) 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
[RFC2133],
[RFC3493], Section 6)

arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain
was originally established as part of the initial deployment of
the DNS, to provide a transition mechanism from the Host Tables
that were common in the ARPANET, as well as a home for the IPv4
reverse mapping domain. During 2000, the abbreviation was
redesignated to 'Address and Routing Parameter Area' in the hope
of reducing confusion with the earlier network name." (Quoted
from [RFC3172], Section 2.) 2) .arpa is an "infrastructure domain", a
domain whose "role is to support the operating infrastructure of
the Internet". (Quoted from [RFC3172], Section 2.) 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.) 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 be
asterisk label." label, in binary
format: 0000 0001 0010 1010 (binary) = 0x01 0x2a (hexadecimal)".
(Quoted from [RFC4592], Section 2.1.1) The second octet in this
label is the ASCII representation for the "*" character.

Closest encloser: "The longest existing ancestor of a name."
(Quoted from [RFC5155], Section 1.3) An earlier definition is "The
node in the zone's tree of existing domain names that has the most
labels matching the query name (consecutively, counting from the
root label downward). Each match is a 'label match' and the order
of the labels is the same." (Quoted from [RFC4592],
Section 3.3.1)

Closest provable encloser: "The longest ancestor of a name that can
be proven to exist. Note that this is only different from the
closest encloser in an Opt-Out zone." (Quoted from [RFC5155],
Section 1.3) See Section 10 for more on "opt-out".

Next closer name: "The name one label longer than the closest
provable encloser of a name." (Quoted from [RFC5155],
Section 1.3)

Source of Synthesis: "The source of synthesis is defined in the
context of a query process as that wildcard domain name
immediately descending from the closest encloser, provided that
this wildcard domain name exists. 'Immediately descending' means
that the source of synthesis has a name of the form:
<asterisk label>.<closest encloser>."
(Quoted from [RFC4592], Section 3.3.1)

9. Registration Model

Registry: The administrative operation of a zone that allows
registration of names within that zone. People often use this
term to refer only to those organizations that perform
registration in large delegation-centric zones (such as TLDs); but
formally, whoever decides what data goes into a zone is the
registry for that zone. This definition of "registry" is from a
DNS point of view; for some zones, the policies that determine
what can go in the zone are decided by zones that are
superordinate and not the registry operator.

Registrant: An individual or organization on whose behalf a name in
a zone is registered by the registry. In many zones, the registry
and the registrant may be the same entity, but in TLDs they often
are not.

Registrar: A service provider that acts as a go-between for
registrants and registries. Not all registrations require a
registrar, though it is common to have registrars involved in
registrations in TLDs.

EPP: The Extensible Provisioning Protocol (EPP), which is commonly
used for communication of registration information between
registries and registrars. EPP is defined in [RFC5730].

WHOIS: A protocol specified in [RFC3912], often used for querying
registry databases. WHOIS data is frequently used to associate
registration data (such as zone management contacts) with domain
names. The term "WHOIS data" is often used as a synonym for the
registry database, even though that database may be served by
different protocols, particularly RDAP. The WHOIS protocol is
also used with IP address registry data.

RDAP: The Registration Data Access Protocol, defined in [RFC7480],
[RFC7481], [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" 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
[RFC3731]
[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 [RFC3731], [RFC5731],
Section 1.1.) 1.1) These terms are strictly ways of referring to the
relationship standing of two domains where one is a subdomain of
the other.

10. General DNSSEC

Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and
[RFC5155]. The terms that have caused confusion in the DNS community
are highlighted here.

DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in
some RFCs, have not been formally defined. However, Section 2 of
[RFC4033] defines many types of resolvers and validators,
including "non-validating security-aware stub resolver", "non-
validating stub resolver", "security-aware name server",
"security-aware recursive name server", "security-aware resolver",
"security-aware stub resolver", and "security-oblivious
'anything'". (Note that the term "validating resolver", which is
used in some places in DNSSEC-related documents, is also not
defined in those RFCs, but is defined below.)

Signed zone: "A zone whose RRsets are signed and that contains
properly constructed DNSKEY, Resource Record Signature (RRSIG),
Next Secure (NSEC), and (optionally) DS records." (Quoted from
[RFC4033], Section 2.) 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."
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.) 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.) 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.) 2)

Zone enumeration: "The practice of discovering the full content of a
zone via successive queries." (Quoted from [RFC5155],
Section 1.3.) 1.3) This is also sometimes called "zone walking". Zone
enumeration is different from zone content guessing where the
guesser uses a large dictionary of possible labels and sends
successive queries for them, or matches the contents of NSEC3
records against such a dictionary.

Validation: Validation, in the context of DNSSEC, refers to one of
the following:

* Checking the validity of DNSSEC signatures,

* Checking the validity of DNS responses, such as those including
authenticated denial of existence, or

* Building an authentication chain from a trust anchor to a DNS
response or individual DNS RRsets in a response

The first two definitions above consider only the validity of
individual DNSSEC components such as the RRSIG validity or NSEC
proof validity. The third definition considers the components of
the entire DNSSEC authentication chain; thus, it requires
"configured knowledge of at least one authenticated DNSKEY or DS
RR" (as described in [RFC4035], Section 5).

[RFC4033], Section 2, says that a "Validating Security-Aware Stub
Resolver... performs signature validation" and uses a trust anchor
"as a starting point for building the authentication chain to a
signed DNS response"; thus, it uses the first and third
definitions above. The process of validating an RRSIG resource
record is described in [RFC4035], Section 5.3.

[RFC5155] refers to validating responses throughout the document,
in the context of hashed authenticated denial of existence; this
uses the second definition above.

The term "authentication" is used interchangeably with
"validation", in the sense of the third definition above.
[RFC4033], Section 2, describes the chain linking trust anchor to
DNS data as the "authentication chain". A response is considered
to be authentic if "all RRsets in the Answer and Authority
sections of the response [are considered] to be authentic" (Quoted
from [RFC4035]). [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). 3.1)

The term "verification", when used, is usually a synonym for
"validation".

Validating resolver: A security-aware recursive name server,
security-aware resolver, or security-aware stub resolver that is
applying at least one of the definitions of validation (above), as
appropriate to the resolution context. For the same reason that
the generic term "resolver" is sometimes ambiguous and needs to be
evaluated in context (see Section 6), "validating resolver" is a
context-sensitive term.

Key signing key (KSK): DNSSEC keys that "only sign the apex DNSKEY
RRset in a zone." (Quoted from [RFC6781], Section 3.1)

Zone signing key (ZSK): "DNSSEC keys that can be used to sign all
the RRsets in a zone that require signatures, other than the apex
DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Also note
that a ZSK is sometimes used to sign the apex DNSKEY RRset.

Combined signing key (CSK): "In cases where the differentiation
between the KSK and ZSK is not made, i.e., where keys have the
role of both KSK and ZSK, we talk about a Single-Type Signing
Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes
called a "combined signing key" or "CSK". It is operational
practice, not protocol, that determines whether a particular key
is a ZSK, a KSK, or a CSK.

Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be
used to distinguish between keys that are intended to be used as
the secure entry point into the zone when building chains of
trust, i.e., they are (to be) pointed to by parental DS RRs or
configured as a trust anchor.... Therefore, it is suggested that
the SEP flag be set on keys that are used as KSKs and not on keys
that are used as ZSKs, while in those cases where a distinction
between a KSK and ZSK is not made (i.e., for a Single-Type Signing
Scheme), it is suggested that the SEP flag be set on all keys."
(Quoted from [RFC6781], Section 3.2.3.) 3.2.3) Note that the SEP flag is
only a hint, and its presence or absence may not be used to
disqualify a given DNSKEY RR from use as a KSK or ZSK during
validation.

The original definition of SEPs was in [RFC3757]. That definition
clearly indicated that the SEP was a key, not just a bit in the
key. The abstract of [RFC3757] says: "With the Delegation Signer
(DS) resource record (RR), the concept of a public key acting as a
secure entry point (SEP) has been introduced. During exchanges of
public keys with the parent there is a need to differentiate SEP
keys from other public keys in the Domain Name System KEY (DNSKEY)
resource record set. A flag bit in the DNSKEY RR is defined to
indicate that DNSKEY is to be used as a SEP." That definition of
the SEP as a key was made obsolete by [RFC4034], and the
definition from [RFC6781] is consistent with [RFC4034].

Trust anchor: "A configured DNSKEY RR or DS RR hash of a DNSKEY RR.
A validating security-aware resolver uses this public key or hash
as a starting point for building the authentication chain to a
signed DNS response. In general, a validating resolver will have
to obtain the initial values of its trust anchors via some secure
or trusted means outside the DNS protocol." (Quoted from
[RFC4033], Section 2)

DNSSEC Policy (DP): A statement that "sets forth the security
requirements and standards to be implemented for a DNSSEC-signed
zone." (Quoted from [RFC6841], Section 2)

DNSSEC Practice Statement (DPS): "A practices disclosure document
that may support and be a supplemental document to the DNSSEC
Policy (if such exists), and it states how the management of a
given zone implements procedures and controls at a high level."
(Quoted from [RFC6841], Section 2)

Hardware security module (HSM): A specialized piece of hardware that
is used to create keys for signatures and to sign messages without
ever disclosing the private key. In DNSSEC, HSMs are often used
to hold the private keys for KSKs and ZSKs and to create the
signatures used in RRSIG records at periodic intervals.

Signing software: Authoritative DNS servers that support DNSSEC
often contain software that facilitates the creation and
maintenance of DNSSEC signatures in zones. There is also stand-
alone software that can be used to sign a zone regardless of
whether the authoritative server itself supports signing.
Sometimes signing software can support particular HSMs as part of
the signing process.

11. DNSSEC States

A validating resolver can determine that a response is in one of four
states: secure, insecure, bogus, or indeterminate. These states are
defined in [RFC4033] and [RFC4035], although the two definitions in the
two documents differ a bit. This document makes no effort to
reconcile the definitions in the two
definitions, documents, and takes no position
as to whether they need to be reconciled.

Section 5 of [RFC4033] says:

A validating resolver can determine the following 4 states:

Secure: The validating resolver has a trust anchor, has a chain
of trust, and is able to verify all the signatures in the
response.

Insecure: The validating resolver has a trust anchor, a chain
of trust, and, at some delegation point, signed proof of the
non-existence of a DS record. This indicates that subsequent
branches in the tree are provably insecure. A validating
resolver may have a local policy to mark parts of the domain
space as insecure.

Bogus: The validating resolver has a trust anchor and a secure
delegation indicating that subsidiary data is signed, but
the response fails to validate for some reason: missing
signatures, expired signatures, signatures with unsupported
algorithms, data missing that the relevant NSEC RR says
should be present, and so forth.

Indeterminate: There is no trust anchor that would indicate that a
specific portion of the tree is secure. This is the default
operation mode.

Section 4.3 of [RFC4035] says:

A security-aware resolver must be able to distinguish between four
cases:

Secure: An RRset for which the resolver is able to build a chain
of signed DNSKEY and DS RRs from a trusted security anchor to
the RRset. In this case, the RRset should be signed and is
subject to signature validation, as described above.

Insecure: An RRset for which the resolver knows that it has no
chain of signed DNSKEY and DS RRs from any trusted starting
point to the RRset. This can occur when the target RRset lies
in an unsigned zone or in a descendent [sic] of an unsigned
zone. In this case, the RRset may or may not be signed, but
the resolver will not be able to verify the signature.

Bogus: An RRset for which the resolver believes that it ought to
be able to establish a chain of trust but for which it is
unable to do so, either due to signatures that for some reason
fail to validate or due to missing data that the relevant
DNSSEC RRs indicate should be present. This case may indicate
an attack but may also indicate a configuration error or some
form of data corruption.

Indeterminate: An RRset for which the resolver is not able to
determine whether the RRset should be signed, as the resolver
is not able to obtain the necessary DNSSEC RRs. This can occur
when the security-aware resolver is not able to contact
security-aware name servers for the relevant zones.

12. Security Considerations

These definitions do not change any security considerations for 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>.

[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>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[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>.

[RFC3731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
Domain Name Mapping", RFC 3731, DOI 10.17487/RFC3731,
March 2004, <https://www.rfc-editor.org/info/rfc3731>.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.

[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
<https://www.rfc-editor.org/info/rfc4592>.

[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<https://www.rfc-editor.org/info/rfc5155>.

[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
DOI 10.17487/RFC5358, October 2008,
<https://www.rfc-editor.org/info/rfc5358>.

[RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,
<https://www.rfc-editor.org/info/rfc5730>.

[RFC5731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
Domain Name Mapping", STD 69, RFC 5731,
DOI 10.17487/RFC5731, August 2009,
<https://www.rfc-editor.org/info/rfc5731>.

[RFC5855] Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6
Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855,
May 2010, <https://www.rfc-editor.org/info/rfc5855>.

[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.

[RFC6561] Livingood, J., Mody, N., and M. O'Reirdan,
"Recommendations for the Remediation of Bots in ISP
Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012,
<https://www.rfc-editor.org/info/rfc6561>.

[RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
Operational Practices, Version 2", RFC 6781,
DOI 10.17487/RFC6781, December 2012,
<https://www.rfc-editor.org/info/rfc6781>.

[RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
DOI 10.17487/RFC6840, February 2013,
<https://www.rfc-editor.org/info/rfc6840>.

[RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
Framework for DNSSEC Policies and DNSSEC Practice
Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,
<https://www.rfc-editor.org/info/rfc6841>.

[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.

[RFC7344] Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
DNSSEC Delegation Trust Maintenance", RFC 7344,
DOI 10.17487/RFC7344, September 2014,
<https://www.rfc-editor.org/info/rfc7344>.

[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[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/>.

[RFC0819]

[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>.

[RFC0952]

[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>.

[RFC2133] Gilligan, R., Thomson, S., Bound, J., and W. Stevens,
"Basic Socket Interface Extensions for IPv6", RFC 2133,
DOI 10.17487/RFC2133, April 1997,
<https://www.rfc-editor.org/info/rfc2133>.

[RFC2775] Carpenter, B., "Internet Transparency", RFC 2775,
DOI 10.17487/RFC2775, February 2000,
<https://www.rfc-editor.org/info/rfc2775>.

[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>.

[RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425,
DOI 10.17487/RFC3425, November 2002,
<https://www.rfc-editor.org/info/rfc3425>.

[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, DOI 10.17487/RFC3493, February 2003,
<https://www.rfc-editor.org/info/rfc3493>.

[RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name
System KEY (DNSKEY) Resource Record (RR) Secure Entry
Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April
2004, <https://www.rfc-editor.org/info/rfc3757>.

[RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912,
DOI 10.17487/RFC3912, September 2004,
<https://www.rfc-editor.org/info/rfc3912>.

[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
RFC 4641, DOI 10.17487/RFC4641, September 2006,
<https://www.rfc-editor.org/info/rfc4641>.

[RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution
Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,
October 2006, <https://www.rfc-editor.org/info/rfc4697>.

[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <https://www.rfc-editor.org/info/rfc4786>.

[RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security
(DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July
2007, <https://www.rfc-editor.org/info/rfc4956>.

[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
<https://www.rfc-editor.org/info/rfc5625>.

[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.

[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://www.rfc-editor.org/info/rfc5891>.

[RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, DOI 10.17487/RFC5892, August 2010,
<https://www.rfc-editor.org/info/rfc5892>.

[RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
for Internationalized Domain Names for Applications
(IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
<https://www.rfc-editor.org/info/rfc5893>.

[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
<https://www.rfc-editor.org/info/rfc5894>.

[RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
Encodings for Internationalized Domain Names", RFC 6055,
DOI 10.17487/RFC6055, February 2011,
<https://www.rfc-editor.org/info/rfc6055>.

[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.

[RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163,
RFC 6303, DOI 10.17487/RFC6303, July 2011,
<https://www.rfc-editor.org/info/rfc6303>.

[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.

[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
DOI 10.17487/RFC6365, September 2011,
<https://www.rfc-editor.org/info/rfc6365>.

[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
<https://www.rfc-editor.org/info/rfc6672>.

[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.

[RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of
Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
February 2014, <https://www.rfc-editor.org/info/rfc7129>.

[RFC7480] Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the
Registration Data Access Protocol (RDAP)", 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>.

[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>.

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
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
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 15 16
Alias 9
Anycast 21 22
Apex 23
Asterisk label 26 27
Authoritative data 23 24
Authoritative server 18 19
Authoritative-only server 18 19
arpa: Address and Routing Parameter Area Domain 26

C
CNAME 9 10
Canonical name 9
Child 22
Class 11
Class independent 15 16
Closest encloser 27
Closest provable encloser 27
Combined signing key (CSK) 32 33

D
DNS operator 28 29
DNSSEC Policy (DP) 33 34
DNSSEC Practice Statement (DPS) 33 34
DNSSEC-aware and DNSSEC-unaware 29 30
Delegation 23 24
Delegation-centric zone 25 26
Domain name 4 5
E
EDNS 14
EPP 28
Empty non-terminals (ENT) 25 26

F
FORMERR 9 10
Fast flux DNS 25 26
Forward lookup 26
Forwarder 20 21
Forwarding 20
Full resolver 18
Full-service resolver 18
Fully-qualified domain name (FQDN) 7 8

G
Global DNS 5
Glue records 24

H
Hardware security module (HSM) 33 34
Hidden master 20
Host name 8

I
IDN 8 9
In-bailiwick 24 25
Insecure delegation 31
Instance 21 22
Internationalized Domain Name 9
Iterative mode 16 17
Iterative resolution 17 18

K
Key signing key (KSK) 32 33

L
Label 5
Lame delegation 23 24
Locally served DNS zone 7 8

M
Master file 14
Master server 19
Multicast DNS 7
mDNS 7
N
NODATA 10
NOERROR 9 10
NOTIMP 10
NS 18
NSEC 30 31
NSEC3 30 31
NXDOMAIN 10
Naming system 4
Negative caching 18 19
Negative response 10 11
Next closer name 27 28
Non-recursive query 17 18

O
OPT 14
Occluded name 25 26
Open resolver 20 21
Opt-out 30 31
Origin 22 23
Out-of-bailiwick 24 25
Owner 14 15

P
Parent 22 23
Passive DNS 21 22
Policy-implementing resolver 20 21
Presentation format 14
Primary master 19 20
Primary server 19 20
Priming 18
Privacy-enabling DNS server 22
Private DNS 7
Public suffix 28 29

Q
QNAME 11

R
RDAP 28 29
REFUSED 10
RR 13 14
RRset 13 14
Recursive mode 16 17
Recursive query 17 18
Recursive resolver 17
Referrals 12 13
Registrant 27 28
Registrar 28
Registry 27 28
Resolver 16
Reverse DNS, reverse lookup 26
Root hints 18
Root zone 25 26

S
SERVFAIL 10
SOA 14
SOA field names 14
Secondary server 19
Secure Entry Point (SEP) 32 33
Service name 26 27
Signed zone 29 30
Signing software 33 34
Slave server 19
Source of Synthesis 27 28
Split DNS 21
Split-horizon DNS 21
Stealth server 20
Stub resolver 16 17
Subdomain 9
Subordinate 29
Superordinate 29

T
TLD 8 9
TTL 14 15
Trust anchor 33 34

U
Unsigned zone 30

V
Validating resolver 32 33
Validation 31 32
View 21

W
WHOIS 28
Wildcard 26 27
Wildcard domain name 27
Z
Zone 22
Zone cut 23
Zone enumeration 31
Zone signing key (ZSK) 32 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