RFC 9499: DNS Terminology

RFC 9499
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
changed in the decades since the DNS was first defined. This document gives current definitions
for many of the terms used in the DNS in a single document.
This document updates RFC 2308 by clarifying the definitions of "forwarder" and "QNAME". It
obsoletes RFC 8499 by adding multiple terms and clarifications. Comprehensive lists of changed
and new definitions can be found in Appendices A and B.
Stream:
RFC:
BCP:
Obsoletes:
Updates:
Category:
Published:
ISSN:
Authors:
Internet Engineering Task Force (IETF)
9499
219
8499
2308
Best Current Practice
March 2024
2070-1721
  P. Hoffman
ICANN
K. Fujiwara
JPRS
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force (IETF). It represents the
consensus of the IETF community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is
available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback
on it may be obtained at .https://www.rfc-editor.org/info/rfc9499
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights
reserved.
Hoffman & Fujiwara Best Current Practice Page 1

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF
Documents ( ) 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
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provided without warranty as described in the Revised BSD License.
https://trustee.ietf.org/license-info
Table of Contents
1.  Introduction
2.  Names
3.  DNS Response Codes
4.  DNS Transactions
5.  Resource Records
6.  DNS Servers and Clients
7.  Zones
8.  Wildcards
9.  Registration Model
10. General DNSSEC
11. DNSSEC States
12. Security Considerations
13. IANA Considerations
14. References
14.1.  Normative References
14.2.  Informative References
Appendix A.  Definitions Updated by This Document
Appendix B.  Definitions First Defined in This Document
Acknowledgements
Index
Authors' Addresses
3
4
8
9
11
13
18
22
23
24
28
29
29
29
29
32
35
35
37
37
45
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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 "global DNS", which is often what
people mean when they say "the DNS".) The protocol and message format are defined in
 and . These RFCs defined some terms, and later documents defined others.
Some of the terms from  and  have somewhat different meanings now than
they did in 1987.
This document contains a collection of a wide variety of DNS-related terms, organized loosely by
topic. Some of them have been precisely defined in earlier RFCs, some have been loosely defined
in earlier RFCs, and some are not defined in an earlier RFC at all.
Other organizations sometimes define DNS-related terms in their own way. For example, the
WHATWG defines "domain" at . The Root Server System Advisory
Committee (RSSAC) has a good lexicon .
Most of the definitions listed here represent the consensus definition of the DNS community --
both protocol developers and operators. Some of the definitions differ from earlier RFCs, and
those differences are noted. In this document, where the consensus definition is the same as the
one in an RFC, that RFC is quoted. Where the consensus definition has changed somewhat, the
RFC is mentioned but the new stand-alone definition is given. See Appendix A for a list of the
definitions that this document updates.
It is important to note that, during the development of this document, it became clear that some
DNS-related terms are interpreted quite differently by different DNS experts. Further, some
terms that are defined in early DNS RFCs now have definitions that are generally agreed to, but
that are different from the original definitions. This document is a small revision to ;
that document was a substantial revision to .
Note that there is no single consistent definition of "the DNS". It can be considered to be some
combination of the following: a commonly used naming scheme for objects on the Internet; a
distributed database representing the names and certain properties of these objects; an
architecture providing distributed maintenance, resilience, and loose coherency for this
database; and a simple query-response protocol (as mentioned below) implementing this
architecture. Section 2 defines "global DNS" and "private DNS" as a way to deal with these
differing definitions.
Capitalization in DNS terms is often inconsistent among RFCs and various DNS practitioners. The
capitalization used in this document is a best guess at current practices, and is not meant to
indicate that other capitalization styles are wrong or archaic. In some cases, multiple styles of
capitalization are used for the same term due to quoting from different RFCs.
In this document, the words "byte" and "octet" are used interchangeably. They appear here
because they both appear in the earlier RFCs that defined terms in the DNS.
[RFC1034] [RFC1035]
[RFC1034] [RFC1035]
<https://url.spec.whatwg.org/>
[RSSAC026]
[RFC8499]
[RFC7719]
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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.
Naming system:
Domain name:
2. Names
 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.
An ordered list of one or more labels.
Note that this is a definition independent of the DNS RFCs ( and ), and the
definition here also applies to systems other than the DNS.  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 ", but possibly with additional rules such as "and
are, or will be, resolvable in the global DNS" or "but only using the presentation format".
•
•
•
•
•
•
•
[RFC1034] [RFC1035]
[RFC1034]
[RFC1035]
RFC 9499 DNS Terminology March 2024
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Label:
Global DNS:
Composition of names:
Format of names:
Wire format:
Presentation format:
Common display format:
Administration of names:
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.
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  and , although the
term "global DNS" has not been defined before now.
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  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.)
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.  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".  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 is specified by delegation (see the definition of
"delegation" in Section 7). Policies for administration of the root zone in the global DNS are
determined by the names operational community, which convenes itself in the Internet
Corporation for Assigned Names and Numbers (ICANN). The names operational
community selects the IANA Functions Operator for the global DNS root zone. The name
servers that serve the root zone are provided by independent root operators. Other zones
in the global DNS have their own policies for administration.
[RFC1034] [RFC1035]
[RFC6762]
[RFC1035]
[RFC1035]
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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:
Private DNS:
Multicast DNS (mDNS):
Locally served DNS zone:
Fully Qualified Domain Name (FQDN):
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
.
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).
The protocol described in .
The global DNS root zone distributed by Public Technical
Identifiers (PTI).
Names that use the protocol described in  but do not rely on the global
DNS root zone or names that are otherwise not generally available on the Internet but are
using the protocol described in . 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) 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
, 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.
A locally served DNS zone is a special case of private DNS. Names are
resolved using the DNS protocol in a local context.  defines subdomains of IN-
ADDR.ARPA that are locally served zones. Resolution of names through locally served zones
may result in ambiguous results. For example, the same name may resolve to different results
in different locally served DNS zone contexts. The context for a locally served DNS zone may
be explicit, such as those that are listed in  and , or implicit, such as those
defined by local DNS administration and not known to the resolution client.
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
[IANA_Resource_Registry]
[RFC1035]
[RFC1035]
[RFC1035]
[RFC6762]
[RFC6303]
[RFC6303] [RFC7793]
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Host name:
Top-Level Domain (TLD):
Internationalized Domain Name (IDN):
Subdomain:
relative to the root (such as "www.example.net") and are still called "fully qualified". This
term first appeared in . 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.
This term and its equivalent, "hostname", have been widely used but are not
defined in , , , or . The DNS was originally deployed
into the Host Tables environment as outlined in , 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
, which is also called the "preferred name syntax". (In that syntax, every character
in each label is a letter, a digit, or a hyphen). Note that any label in a domain name can
contain any octet value; hostnames are generally considered to be domain names where
every label follows the rules in the "preferred name syntax", with the amendment that labels
can start with ASCII digits (this amendment comes from ).
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".
A Top-Level Domain is a zone that is one layer below the root, such as
"com" or "jp". There is nothing special, from the point of view of the DNS, about TLDs. Most of
them are also delegation-centric zones (defined in Section 7), and there are significant policy
issues around their operation. TLDs are often divided into sub-groups such as Country Code
Top-Level Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others; the division is a
matter of policy and beyond the scope of this document.
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 , , , , and
. These documents define many IDN-specific terms such as "LDH label", "A-label",
and "U-label".  defines more terms that relate to internationalization (some of which
relate to IDNs);  has a much more extensive discussion of IDNs, including some new
terminology.
"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 ) 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".
[RFC819]
[RFC1034] [RFC1035] [RFC1123] [RFC2181]
[RFC952]
Section 3.5 of
[RFC1034]
Section 2.1 of [RFC1123]
[RFC5890] [RFC5891] [RFC5892] [RFC5893]
[RFC5894]
[RFC6365]
[RFC6055]
[RFC1034], Section 3.1
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Alias:
Canonical name:
CNAME:
The owner of a CNAME resource record, or a subdomain of the owner of a DNAME
resource record (DNAME records are defined in ). See also "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
) This usage of the word "canonical" is related to the mathematical concept of
"canonical form".
"It has been traditional to refer to the [owner] of a CNAME record as 'a CNAME'. This is
unfortunate, as 'CNAME' is an abbreviation of 'canonical name', and the [owner] of a CNAME
record is most certainly not a canonical name." (Quoted from . The
quoted text has been changed from "label" to "owner".)
[RFC6672]
[RFC1034],
Section 3.6.2
[RFC2181], Section 10.1.1
NOERROR:
FORMERR:
SERVFAIL:
NXDOMAIN:
NOTIMP:
REFUSED:
NODATA:
3. DNS Response Codes
Some of the response codes (RCODEs) that are defined in  have acquired their own
shorthand names. All of the RCODEs are listed at , although that list
uses mixed-case capitalization, while most documents use all caps. Some of the common names
for values defined in  are described in this section. This section also includes an
additional RCODE and a general definition. The official list of all RCODEs is in the IANA registry.
This RCODE appears as "No error condition" in .
This RCODE appears as "Format error - The name server was unable to interpret the
query" in .
This RCODE appears as "Server failure - The name server was unable to process this
query due to a problem with the name server" in .
This RCODE appears as "Name Error [...] this code signifies that the domain name
referenced in the query does not exist." in .  established
NXDOMAIN as a synonym for Name Error.
This RCODE appears as "Not Implemented - The name server does not support the
requested kind of query" in .
This RCODE appears as "Refused - The name server refuses to perform the specified
operation for policy reasons. For example, a name server may not wish to provide the
information to the particular requester, or a name server may not wish to perform a
particular operation (e.g., zone transfer) for particular data." in .
"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 ) "NODATA is indicated by an answer with the
RCODE set to NOERROR and no relevant answers in the Answer section. The Authority section
will contain an SOA record, or there will be no NS records there." (Quoted from
) Note that referrals have a similar format to NODATA replies;  explains
how to distinguish them.
[RFC1035]
[IANA_Resource_Registry]
[RFC1035]
Section 4.1.1 of [RFC1035]
Section 4.1.1 of [RFC1035]
Section 4.1.1 of [RFC1035]
Section 4.1.1 of [RFC1035] [RFC2308]
Section 4.1.1 of [RFC1035]
Section 4.1.1 of [RFC1035]
[RFC2308], Section 1
[RFC2308],
Section 2.2 [RFC2308]
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Negative response:
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 .
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  describe
the types of negative responses in detail.
[RFC2136]
[RFC2308]
Class:
QNAME:
4. DNS Transactions
The header of a DNS message is its first 12 octets. Many of the fields and flags in the diagrams in
Sections 4.1.1 through 4.1.3 of  are referred to by their names in each diagram. For
example, the response codes are called "RCODEs", the data for a record is called the "RDATA", and
the authoritative answer bit is often called "the AA flag" or "the AA bit".
A class "identifies a protocol family or instance of a protocol". (Quoted from
) "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
) In practice, the class for nearly every query is "IN" (the Internet). There are some queries
for "CH" (the Chaos class), but they are usually for the purposes of information about the
server itself rather than for a different type of address.
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
) Strictly speaking, the definition comes from , where the
QNAME is defined in respect of the Question section. This definition appears to be applied
consistently, as the discussion of inverse queries in  refers to the
"owner name of the query RR and its TTL" because inverse queries populate the Answer
section and leave the Question section empty. (Inverse queries are deprecated in ;
thus, relevant definitions do not appear in this document.)
However,  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 )
This is made explicit in the resolution algorithm outlined in , 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.
[RFC1035]
[RFC1034],
Section 3.6
[RFC1034], Section
2.2
[RFC1034],
Section 3.7.1 [RFC1035], Section 4.1.2
Section 6.4.1 of [RFC1035]
[RFC3425]
[RFC2308]
[RFC1034], Section 3.6.2
Section 4.3.2 of [RFC1034]
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QNAME (original):
QNAME (effective):
QNAME (final):
Referrals:
However, this creates confusion because the response to a query that results in CNAME
processing contains in the echoed Question section one QNAME (the name in the original
query) and a second QNAME that is in the data field of the last CNAME. The confusion comes
from the iterative/recursive mode of resolution, which finally returns an answer that need
not actually have the same owner name as the QNAME contained in the original query.
To address this potential confusion, it is helpful to distinguish between three meanings:
The name actually sent in the Question section in the original query,
which is always echoed in the (final) reply in the Question section when the QR bit is set to
1.
A name actually resolved, which is either the name originally queried or
a name received in a CNAME chain response.
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  is actually for a different concept than what
was in , it would have been better if  had used a different name for that
concept. In general use today, QNAME almost always means what is defined above as
"QNAME (original)".
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 .
There are two types of referral response. The first is a downward referral (sometimes
described as "delegation response"), where the server is authoritative for some portion of the
QNAME. The Authority section RRset's RDATA contains the name servers specified at the
referred-to zone cut. In normal DNS operation, this kind of response is required in order to
find names beneath a delegation. The bare use of "referral" means this kind of referral, and
many people believe that this is the only legitimate kind of referral in the DNS.
The second is an upward referral (sometimes described as "root referral"), where the server is
not authoritative for any portion of the QNAME. When this happens, the referred-to zone in
the Authority section is usually the root zone ("."). In normal DNS operation, this kind of
response is not required for resolution or for correctly answering any query. There is no
requirement that any server send upward referrals. Some people regard upward referrals as
a sign of a misconfiguration or error. Upward referrals always need some sort of qualifier
(such as "upward" or "root") and are never identified simply by the word "referral".
A response that has only a referral contains an empty Answer section. It contains the NS
RRset for the referred-to zone in the Authority section. It may contain RRs that provide
addresses in the Additional section. The AA bit is clear.
[RFC2308]
[RFC1034] [RFC2308]
[RFC1034], Section 4.3.2
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In the case where the query matches an alias, and the server is not authoritative for the target
of the alias but is authoritative for some name above the target of the alias, the resolution
algorithm will produce a response that contains both the authoritative answer for the alias
and a referral. Such a partial answer and referral response has data in the Answer section. It
has the NS RRset for the referred-to zone in the Authority section. It may contain RRs that
provide addresses in the Additional section. The AA bit is set because the first name in the
Answer section matches the QNAME and the server is authoritative for that answer (see
). [RFC1035], Section 4.1.1
RR:
RRset:
Master file:
Presentation format:
EDNS:
OPT:
Owner:
5. Resource Records
An acronym for resource record. (See .)
A set of resource records "with the same label, class and type, but with different data"
(according to ). Also written as "RRSet" in some documents. As a
clarification, "same label" in this definition means "same owner name". In addition,
states that "the TTLs of all RRs in an RRSet must be the same".
Note that RRSIG resource records do not match this definition.  says:
An RRset  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 .
"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 ) Master
files are sometimes called "zone files".
The text format used in master files. This format is shown but not
formally defined in  or . The term "presentation format" first appears in
.
The extension mechanisms for DNS, defined in . 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.
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 .) It is used by EDNS.
"The domain name where the RR is found." (Quoted from ) Often
appears in the term "owner name".
[RFC1034], Section 3.6
[RFC2181], Section 5
[RFC2181]
[RFC4035]
MAY
[RFC2181]
[RFC1035], Section 5
[RFC1034] [RFC1035]
[RFC4034]
[RFC6891]
[RFC6891], Section 6.1.1
[RFC1034], Section 3.6
RFC 9499 DNS Terminology March 2024
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SOA field names:
TTL:
Class independent:
Address records:
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 . 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 ;
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.
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
) Note that  erroneously stated that this is a signed integer; that was fixed
by .
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 )
 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 ( ).
The reason that the TTL is the maximum time to live is that a cache operator might decide to
shorten the time to live for operational purposes, for example, if there is a policy to disallow
TTL values over a certain number. Some servers are known to ignore the TTL on some RRsets
(such as when the authoritative data has a very short TTL) even though this is against the
advice in . 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 .
A resource record type whose syntax and semantics are the same for every
DNS class. A resource record type that is not class independent has different meanings,
depending on the DNS class of the record or if the meaning is undefined for some classes.
Most resource record types are defined for class 1 (IN, the Internet), but many are undefined
for other classes.
Records whose type is either A or AAAA.  informally defines these as
"(A, AAAA, etc)". Note that new types of address records could be defined in the future.
Section 3.3.13 of [RFC1035]
Section 4 of [RFC2308]
[RFC2181],
Section 8 [RFC1035]
[RFC2181]
[RFC1035], Section 3.2.1Section
4.1.3 of [RFC1035]
[RFC2181], Section 5.2
[RFC1035]
[RFC2308]
[RFC2181]
RFC 9499 DNS Terminology March 2024
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Resolver:
Stub resolver:
Iterative mode:
Recursive mode:
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
and its successors.
It is important to note that the terms "DNS server" and "name server" require context in order to
understand the services being provided. Both authoritative servers and recursive resolvers are
often called "DNS servers" and "name servers" even though they serve different roles (but may
be part of the same software package).
For terminology specific to the global DNS root server system, see . That document
defines terms such as "root server", "root server operator", and terms that are specific to the way
that the root zone of the global DNS is served.
A program "that extract[s] information from name servers in response to client
requests." (Quoted from ) 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  or . 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".
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 . They are fully defined in
.
A resolution mode of a server that receives DNS queries and responds with a
referral to another server.  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.
 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.  describes this as "the first server
pursues the query for the client at another server".  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:
[RFC1035]
[RSSAC026]
[RFC1034], Section 2.4
[RFC1034] [RFC1035]
Section 5.3.1 of [RFC1034]
Section 6.1.3.1 of [RFC1123]
Section 2.3 of [RFC1034]
Section 2.3 of [RFC1034]
Section 4.3.1 of [RFC1034]
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Recursive resolver:
Recursive query:
Non-recursive query:
Iterative resolution:
Full resolver:
Full-service resolver:
Priming:
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.
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.
 tried to differentiate between a recursive resolver and an iterative resolver.
A query with the Recursion Desired (RD) bit set to 1 in the header. (See
.) 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 .)
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
.)
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 .)
In iterative resolution, the client repeatedly makes non-recursive queries and follows
referrals and/or aliases. The iterative resolution algorithm is described in
.
This term is used in , 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 .
This term is not properly defined in any RFC, and there is no consensus on what the term
means. The use of this term without proper context is discouraged.
 defines this term as a resolver that acts in
recursive mode with a cache (and meets other requirements).
"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
) 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.
[RFC4697]
Section
4.1.1 of [RFC1035]
Section 4.3.2 of [RFC1034]
Section 4.3.1 of [RFC1034]
Section 2.3 of [RFC1034]
Section 5.3.3 of
[RFC1034]
[RFC1035]
[RFC1035]
Section 6.1.3.1 of [RFC1123]
[RFC8109],
Section 2
RFC 9499 DNS Terminology March 2024
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Root hints:
Negative caching:
Authoritative server:
Authoritative-only server:
Zone transfer:
Slave server:
Secondary server:
Master server:
Primary server:
"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 ) This file is
often used in priming.
"The storage of knowledge that something does not exist, cannot or does not
give an answer." (Quoted from )
"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 ) 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.
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 ) In this case, "ignores requests for recursion"
means "responds to requests for recursion with responses indicating that recursion was not
performed".
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 , and the IXFR ("Incremental Transfer")
mechanism to copy only parts of the zone that have changed (described in ). Many
systems use non-standard methods for zone transfers outside the DNS protocol.
See "Secondary server".
"An authoritative server which uses zone transfer to retrieve the zone."
(Quoted from ) Secondary servers are also discussed in .
 describes secondary servers in more detail. Although early DNS RFCs such as
 referred to this as a "slave", the current common usage has shifted to calling it a
"secondary".
See "Primary server".
"Any authoritative server configured to be the source of zone transfer for one
or more [secondary] servers." (Quoted from ) Or, more specifically,
 calls it "an authoritative server configured to be the source of AXFR or IXFR data
[IANA_RootFiles]
[RFC2308], Section 1
[RFC2182], Section 2
[RFC4697], Section 2.4
[RFC5936]
[RFC1995]
[RFC1996], Section 2.1 [RFC1034]
[RFC2182]
[RFC1996]
[RFC1996], Section 2.1
[RFC2136]
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Primary master:
Stealth server:
Hidden master:
Forwarding:
Forwarder:
Policy-implementing resolver:
for one or more [secondary] servers". Primary servers are also discussed in .
Although early DNS RFCs such as  referred to this as a "master", the current
common usage has shifted to "primary".
"The primary master is named in the zone's SOA MNAME field and optionally
by an NS RR." (Quoted from )  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  and . 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.
This is "like a slave server except not listed in an NS RR for the zone." (Quoted
from )
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 )
 said that the hidden master's name "appears in the SOA RRs MNAME field";
however, the name does not appear at all in the global DNS in some setups. A hidden master
can also be a secondary server for the zone itself.
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.
 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 ).
 describes a forwarder as "a nameserver used to resolve
queries instead of directly using the authoritative nameserver chain".  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.  is silent on whether a
forwarder is iterative-only or can be a full-service 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.
[RFC1034]
[RFC1996]
[RFC1996], Section 2.1[RFC2136]
[RFC1996] [RFC2136]
[RFC1996], Section 2.1
[RFC6781], Section 3.4.3
[RFC4641]
[RFC5625]
[RFC5625]
Section 1 of [RFC2308]
[RFC2308]
[RFC2308]
RFC 9499 DNS Terminology March 2024
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Open resolver:
Split DNS:
View:
Passive DNS:
Anycast:
Instance:
Privacy-enabling DNS server:
DNS-over-TLS (DoT):
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 .
The terms "split DNS" and "split-horizon DNS" have long been used in the DNS
community without formal definition. In general, they refer to situations in which DNS
servers that are authoritative for a particular set of domains provide partly or completely
different answers in those domains depending on the source of the query. Nevertheless, the
effect of this is that a domain name that is notionally globally unique has different meanings
for different network users. This can sometimes be the result of a "view" configuration, as
described below.
 gives a related definition that is too specific to be generally useful.
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.
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".
"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 ) See  for more detail on Anycast and
other terms that are specific to its use.
"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 )
"A DNS server that implements DNS over TLS  and may
optionally implement DNS over DTLS ." (Quoted from ) Other
types of DNS servers might also be considered privacy-enabling, such as those running DNS-
over-HTTPS  or DNS-over-QUIC .
DNS over TLS as defined in  and its successors.
[RFC5358]
Section 3.8 of [RFC2775]
[RFC4786], Section 2 [RFC4786]
[RSSAC026]
[RFC7858]
[RFC8094] [RFC8310], Section 2
[RFC8484] [RFC9250]
[RFC7858]
RFC 9499 DNS Terminology March 2024
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DNS-over-HTTPS (DoH):
DNS-over-QUIC (DoQ):
Classic DNS:
Recursive DoT (RDoT):
Authoritative DoT (ADoT):
XFR-over-TLS (XoT):
DNS over HTTPS as defined in  and its successors.
DNS over QUIC as defined in  and its successors.
specifically defines DoQ as general-purpose transport for DNS that can be used in stub to
recursive, recursive to authoritative, and zone transfer scenarios.
DNS over UDP or DNS over TCP as defined in  and its successors. Classic
DNS applies to DNS communication between stub resolvers and recursive resolvers, and
between recursive resolvers and authoritative servers. This has sometimes been called
"Do53". Classic DNS is not encrypted.
RDoT specifically means DNS-over-TLS for transport between a stub
resolver and a recursive resolver, or between a recursive resolver and another recursive
resolver. This term is necessary because it is expected that DNS-over-TLS will later be defined
as a transport between recursive resolvers and authoritative servers.
If DNS-over-TLS is later defined as a transport between recursive
resolvers and authoritative servers, ADoT specifically means DNS-over-TLS for transport
between recursive resolvers and authoritative servers.
DNS zone transfer over TLS, as specified in . This term applies to
both AXFR over TLS (AXoT) and IXFR over TLS (IXoT).
[RFC8484]
[RFC9250] [RFC9250]
[RFC1035]
[RFC9103]
Zone:
Child:
Parent:
Origin:
(a)
7. Zones
This section defines terms that are used when discussing zones that are being served or
retrieved.
"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 )
"The entity on record that has the delegation of the domain from the Parent." (Quoted
from )
"The domain in which the Child is registered." (Quoted from )
Earlier, "parent name server" was defined in  as "the name server that has
authority over the place in the domain name space that will hold the new domain". (Note that
 was obsoleted by  and .)  also has some description of
the relationship between parents and children.
There are two different uses for this term:
"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 ) These days, this sense of
"origin" and "apex" (defined below) are often used interchangeably.
[RFC1034], Section 2.4
[RFC7344], Section 1.1
[RFC7344], Section 1.1
[RFC0882]
[RFC0882] [RFC1034] [RFC1035] [RFC819]
[RFC2181], Section 6
RFC 9499 DNS Terminology March 2024
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(b)
Apex:
Zone cut:
Delegation:
Authoritative data:
Lame delegation:
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
, 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.".
The point in the tree at an owner of an SOA and corresponding authoritative NS RRset.
This is also called the "zone apex".  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.  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.
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 ; note that this is barely an
ostensive definition.)  uses "cuts" instead of "zone cut".
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.
"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
) 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.
, defines "Authoritative RRset", which is related to authoritative data but
has a more precise definition.
"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 ) 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 )
[RFC1035], Section 5.1
[RFC4033]
[RFC1034]
[RFC2181], Section 6
Section 4.2 of [RFC1034]
[RFC1034], Section 4.2.1
[RFC4033], Section 2
[RFC1912], Section 2.8
[RFC1713], Section 2.3
RFC 9499 DNS Terminology March 2024
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Glue records:
These early definitions do not match the current use of the term "lame delegation", but there
is no consensus on what a lame delegation is. The term is used not only for the specific case
described above, but for a variety of other flaws in delegations that lead to non-authoritative
answers or no answers at all, such as:
a nameserver with an NS record for a zone that does not answer DNS queries;
a nameserver with an IP address that is not reachable by the resolver; and
a nameserver that responds to a query for a specific name with an error or without the
authoritative bit set.
Because the term in current usage has drifted from the original definition, and now is not
specific or clear as to the intended meaning, it should be considered historic and avoided in
favor of terms that are specific and clear.
"...[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 )
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 ) Although glue is sometimes used today with this wider
definition in mind, the context surrounding the definition in  suggests it is intended
to apply to the use of glue within the document itself and not necessarily beyond.
In an NS record, there are three types of relationships between the owner name of the record,
the name in the NS RDATA, and the zone origin: unrelated, in-domain, and sibling domain.
The application of these three types of relationships to glue records is defined in .
An unrelated relationship is one where the NS RDATA contains a name server that is not
subordinate to the zone origin and therefore is not part of the same zone.
An in-domain relationship is one where the NS RDATA contains a name server whose name is
either subordinate to or (rarely) the same as the owner name of the NS resource records. For
example, a delegation for "child.example.com" might have an in-domain name server called
"ns.child.example.com".
A sibling domain relationship is one where the NS RDATA contains a name server whose
name is either subordinate to or (rarely) the same as the zone origin of the parent and not
subordinate to or the same as the owner name of the NS resource records. For example, a
delegation for "child.example.com" in "example.com" zone might have a sibling domain name
server called "ns.another.example.com".
The following table shows examples of delegation types:
•
•
•
[RFC1034], Section 4.2.1
[RFC2181], Section 5.4.1
[RFC2181]
[RFC9471]
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 20

Bailiwick:
Root zone:
Empty non-terminals (ENTs):
Delegation-centric zone:
Occluded name:
Fast flux DNS:
"In-bailiwick" and "Out-of-bailiwick" are modifiers used to describe the relationship
between a zone and the name servers for that zone. The dictionary definition of bailiwick has
been observed to cause more confusion than meaning for this use. These terms should be
considered historic in nature.
The zone of a DNS-based tree whose apex is the zero-length label. Also sometimes
called "the DNS root".
"Domain names that own no resource records but have
subdomains that do." (Quoted from ) 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.
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
and , but it is not defined in either document.
"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 )
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 , with a typo corrected) In addition to having legitimate uses, fast
flux DNS can be used to deliver malware. Because the addresses change so rapidly, it is
difficult to ascertain all the hosts. It should be noted that the technique also works with AAAA
records, but such use is not frequently observed on the Internet as of this writing.
DelegationParent Name Server NameType
com . a.gtld-servers.net sibling domain
net . a.gtld-servers.net in-domain
example.orgorg ns.example.org in-domain
example.orgorg ns.ietf.org sibling domain
example.orgorg ns.example.com unrelated
example.jp jp ns.example.jp in-domain
example.jp jp ns.example.ne.jp sibling domain
example.jp jp ns.example.com unrelated
Table 1
[RFC4592], Section 2.2.2
[RFC4956]
[RFC5155]
[RFC5936], Section 3.5
[RFC6561], Section 1.1.5
RFC 9499 DNS Terminology March 2024
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Reverse DNS, reverse lookup:
Forward lookup:
arpa (Address and Routing Parameter Area Domain):
Service name:
"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 )
"Hostname-to-address translation". (Quoted from )
"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 ) .arpa is an "infrastructure domain", a domain whose "role
is to support the operating infrastructure of the Internet". (Quoted from )
See  for more history of this 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 )
[RFC5855], Section 1
[RFC3493], Section 6
[RFC3172], Section 2
[RFC3172], Section 2
[RFC3172]
[RFC6335], Section 5
Wildcard:
Asterisk label:
Wildcard domain name:
Closest encloser:
Closest provable encloser:
Next closer name:
8. Wildcards
 defined "wildcard", but in a way that turned out to be confusing to
implementers. For an extended discussion of wildcards, including clearer definitions, see
. 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 )
"The first octet is the normal label type and length for a 1-octet-long label, and
the second octet is the ASCII representation  for the '*' character. A descriptive name
of a label equaling that value is an 'asterisk label'." (Quoted from )
"A 'wildcard domain name' is defined by having its initial (i.e., leftmost
or least significant) label, in binary format: 0000 0001 0010 1010 (binary) = 0x01 0x2a
(hexadecimal)". (Quoted from ) The second octet in this label is the
ASCII representation for the "*" character.
"The longest existing ancestor of a name." (Quoted from
) 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 )
"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
) See Section 10 for more on "opt-out".
"The name one label longer than the closest provable encloser of a name."
(Quoted from )
[RFC1034]
[RFC4592]
[RFC1034], Section 4.3.3
[RFC20]
[RFC4592], Section 2.1.1
[RFC4592], Section 2.1.1
[RFC5155], Section
1.3
[RFC4592], Section 3.3.1
[RFC5155], Section 1.3
[RFC5155], Section 1.3
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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
Registry:
Registrant:
Registrar:
EPP:
WHOIS:
RDAP:
DNS operator:
Public suffix:
9. Registration Model
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.
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.
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.
The Extensible Provisioning Protocol (EPP), which is commonly used for communication
of registration information between registries and registrars. EPP is defined in .
A protocol specified in , 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.
The Registration Data Access Protocol, defined in , , ,
, , and . The RDAP protocol and data format are meant as a
replacement for WHOIS.
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.
"A domain that is controlled by a public registry." (Quoted from
) 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
[RFC5730]
[RFC3912]
[RFC7480] [RFC7481] [RFC7485]
[RFC9082] [RFC9083] [RFC9224]
[RFC6265], Section
5.3
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Subordinate and Superordinate:
) 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
.
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.
These terms are introduced in  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
) These terms are strictly ways of referring to the relationship standing
of two domains where one is a subdomain of the other.
[RFC6265]
<https://publicsuffix.org/>
[RFC5731]
[RFC5731], Section 1.1
DNSSEC-aware and DNSSEC-unaware:
Signed zone:
10. General DNSSEC
Most DNSSEC terms are defined in , , , and . The terms
that have caused confusion in the DNS community are highlighted here.
These two terms, which are used in some RFCs, have not
been formally defined. However,  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.)
"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 ) 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.
[RFC4033] [RFC4034] [RFC4035] [RFC5155]
Section 2 of [RFC4033]
[RFC4033], Section 2
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Online signing:
Unsigned zone:
NSEC:
NSEC3:
Opt-out:
Insecure delegation:
It should also be noted that, since the publication of , NSEC records are no longer
required for signed zones: a signed zone might include NSEC3 records instead.
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.
 defines "on-line signing" (note the hyphen) as "generating and
signing these records on demand", where "these" was defined as NSEC records. The current
definition expands that to generating and signing RRSIG, NSEC, and NSEC3 records on
demand.
 defines this as "a zone that is not signed".
 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
 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  treat the child zone as if it were unsigned."
"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 ) 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 )
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 .
Note that  says that  "is now considered part of the DNS Security
Document Family as described by ". 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.
"The Opt-Out Flag indicates whether this NSEC3 RR may cover unsigned delegations."
(Quoted from ) 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 )
"A signed name containing a delegation (NS RRset), but lacking a DS RRset,
signifying a delegation to an unsigned subzone." (Quoted from )
[RFC6840]
[RFC7129]
[RFC4470]
Section 2 of [RFC4033] Section 2 of
[RFC4035]
Section 5.2 of
[RFC4035]
SHOULD
[RFC4033], Section 3.2
[RFC4034], Section 4
[RFC5155]
[RFC6840] [RFC5155]
Section 10 of [RFC4033]
[RFC5155], Section 3.1.2.1
[RFC7129], Section 5.1
[RFC4956], Section 2
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Zone enumeration:
Validation:
Validating resolver:
Key signing key (KSK):
"The practice of discovering the full content of a zone via successive
queries." (Quoted from ) 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, 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
).
, 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
.
 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. , 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
) DNS data or responses deemed to be authentic or validated have a security status
of "secure" ( ; ). "Authenticating both DNS keys and
data is a matter of local policy, which may extend or even override the [DNSSEC] protocol
extensions..." (Quoted from )
The term "verification", when used, is usually a synonym for "validation".
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.
DNSSEC keys that "only sign the apex DNSKEY RRset in a zone." (Quoted
from )
[RFC5155], Section 1.3
•
•
•
[RFC4035], Section
5
[RFC4033], Section 2
[RFC4035], Section
5.3
[RFC5155]
[RFC4033], Section 2
[RFC4035]
[RFC4035], Section 4.3[RFC4033], Section 5
[RFC4033], Section 3.1
[RFC6781], Section 3.1
RFC 9499 DNS Terminology March 2024
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Zone signing key (ZSK):
Combined signing key (CSK):
Secure Entry Point (SEP):
Trust anchor:
DNSSEC Policy (DP):
DNSSEC Practice Statement (DPS):
Hardware security module (HSM):
"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 )
Also note that a ZSK is sometimes used to sign the apex DNSKEY RRset.
"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 ) 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.
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 ) 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 . That definition clearly indicated that the SEP
was a key, not just a bit in the key. The abstract of  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 , and the
definition from  is consistent with .
"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 )
A statement that "sets forth the security requirements and standards to be
implemented for a DNSSEC-signed zone." (Quoted from )
"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 )
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.
[RFC6781], Section 3.1
[RFC6781], Section 3.1
[RFC6781], Section 3.2.3
[RFC3757]
[RFC3757]
[RFC4034]
[RFC6781] [RFC4034]
[RFC4033], Section 2
[RFC6841], Section 2
[RFC6841], Section 2
RFC 9499 DNS Terminology March 2024
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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.
Secure:
Insecure:
Bogus:
Indeterminate:
Secure:
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  and , although the
definitions in the two documents differ a bit. This document makes no effort to reconcile the
definitions in the two documents and takes no position as to whether they need to be reconciled.
 says:
A validating resolver can determine the following 4 states:
The validating resolver has a trust anchor, has a chain of trust, and is able to
verify all the signatures in the response.
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.
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.
There is no trust anchor that would indicate that a specific portion of
the tree is secure. This is the default operation mode.
 says:
A security-aware resolver must be able to distinguish between four cases:
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.
[RFC4033] [RFC4035]
Section 5 of [RFC4033]
Section 4.3 of [RFC4035]
RFC 9499 DNS Terminology March 2024
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[IANA_RootFiles]
[RFC0882]
[RFC1034]
[RFC1035]
14. References
14.1. Normative References
, , .
, , ,
, November 1983, .
, , , ,
, November 1987, .
, , ,
, , November 1987,
.
Insecure:
Bogus:
Indeterminate:
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.
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.
An RRset for which the resolver is not able to determine whether the
RRset should be signed, as the resolver is not able to obtain the necessary DNSSEC
RRs. This can occur when the security-aware resolver is not able to contact security-
aware name servers for the relevant zones.
12. Security Considerations
These definitions do not change any security considerations for either the global DNS or private
DNS.
13. IANA Considerations
References to RFC 8499 in the IANA registries have been replaced with references to this
document.
IANA"Root Files"<https://www.iana.org/domains/root/files>
Mockapetris, P."Domain names: Concepts and facilities"RFC 882DOI 10.17487/
RFC0882 <https://www.rfc-editor.org/info/rfc882>
Mockapetris, P."Domain names - concepts and facilities"STD 13RFC 1034DOI
10.17487/RFC1034 <https://www.rfc-editor.org/info/rfc1034>
Mockapetris, P."Domain names - implementation and specification"STD 13
RFC 1035DOI 10.17487/RFC1035 <https://www.rfc-editor.org/
info/rfc1035>
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 29

[RFC1123]
[RFC1912]
[RFC1996]
[RFC2136]
[RFC2181]
[RFC2182]
[RFC2308]
[RFC4033]
[RFC4034]
[RFC4035]
[RFC4592]
[RFC5155]
[RFC5358]
[RFC5730]
, ,
, , , October 1989,
.
, , ,
, February 1996, .
, ,
, , August 1996,
.
, , , and ,
, , , April
1997, .
 and , , ,
, July 1997, .
, , , and ,
, , , , July 1997,
.
, , ,
, March 1998, .
, , , , and ,
, , , March 2005,
.
, , , , and ,
, , , March 2005,
.
, , , , and ,
, ,
, March 2005, .
, , ,
, July 2006, .
, , , and ,
, , , March
2008, .
 and ,
, , , , October 2008,
.
, , , ,
, August 2009, .
Braden, R., Ed."Requirements for Internet Hosts - Application and Support"
STD 3RFC 1123DOI 10.17487/RFC1123 <https://www.rfc-
editor.org/info/rfc1123>
Barr, D."Common DNS Operational and Configuration Errors"RFC 1912DOI
10.17487/RFC1912 <https://www.rfc-editor.org/info/rfc1912>
Vixie, P."A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)"
RFC 1996DOI 10.17487/RFC1996 <https://www.rfc-editor.org/info/
rfc1996>
Vixie, P., Ed.Thomson, S.Rekhter, Y. J. Bound"Dynamic Updates in the
Domain Name System (DNS UPDATE)"RFC 2136DOI 10.17487/RFC2136
<https://www.rfc-editor.org/info/rfc2136>
Elz, R. R. Bush"Clarifications to the DNS Specification"RFC 2181DOI
10.17487/RFC2181 <https://www.rfc-editor.org/info/rfc2181>
Elz, R.Bush, R.Bradner, S. M. Patton"Selection and Operation of
Secondary DNS Servers"BCP 16RFC 2182DOI 10.17487/RFC2182
<https://www.rfc-editor.org/info/rfc2182>
Andrews, M."Negative Caching of DNS Queries (DNS NCACHE)"RFC 2308DOI
10.17487/RFC2308 <https://www.rfc-editor.org/info/rfc2308>
Arends, R.Austein, R.Larson, M.Massey, D. S. Rose"DNS Security
Introduction and Requirements"RFC 4033DOI 10.17487/RFC4033
<https://www.rfc-editor.org/info/rfc4033>
Arends, R.Austein, R.Larson, M.Massey, D. S. Rose"Resource Records for
the DNS Security Extensions"RFC 4034DOI 10.17487/RFC4034
<https://www.rfc-editor.org/info/rfc4034>
Arends, R.Austein, R.Larson, M.Massey, D. S. Rose"Protocol
Modifications for the DNS Security Extensions"RFC 4035DOI 10.17487/
RFC4035 <https://www.rfc-editor.org/info/rfc4035>
Lewis, E."The Role of Wildcards in the Domain Name System"RFC 4592DOI
10.17487/RFC4592 <https://www.rfc-editor.org/info/rfc4592>
Laurie, B.Sisson, G.Arends, R. D. Blacka"DNS Security (DNSSEC) Hashed
Authenticated Denial of Existence"RFC 5155DOI 10.17487/RFC5155
<https://www.rfc-editor.org/info/rfc5155>
Damas, J. F. Neves"Preventing Use of Recursive Nameservers in Reflector
Attacks"BCP 140RFC 5358DOI 10.17487/RFC5358 <https://
www.rfc-editor.org/info/rfc5358>
Hollenbeck, S."Extensible Provisioning Protocol (EPP)"STD 69RFC 5730DOI
10.17487/RFC5730 <https://www.rfc-editor.org/info/rfc5730>
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 30

[RFC5731]
[RFC5855]
[RFC5936]
[RFC6561]
[RFC6781]
[RFC6840]
[RFC6841]
[RFC6891]
[RFC7344]
[RFC7719]
[RFC8310]
[RFC8499]
[RFC9250]
, ,
, , , August 2009,
.
 and , ,
, , , May 2010,
.
 and , , ,
, June 2010, .
, , and ,
, , , March
2012, .
, , and ,
, , , December 2012,
.
 and ,
, , , February 2013,
.
, , and ,
, , ,
January 2013, .
, , and , ,
, , , April 2013,
.
, , and ,
, , , September 2014,
.
, , and , , ,
, December 2015, .
, , and ,
, , , March 2018,
.
, , and , , ,
, , January 2019,
.
, , and ,
, , , May 2022,
.
Hollenbeck, S."Extensible Provisioning Protocol (EPP) Domain Name Mapping"
STD 69RFC 5731DOI 10.17487/RFC5731 <https://www.rfc-
editor.org/info/rfc5731>
Abley, J. T. Manderson"Nameservers for IPv4 and IPv6 Reverse Zones"BCP
155RFC 5855DOI 10.17487/RFC5855 <https://www.rfc-editor.org/
info/rfc5855>
Lewis, E. A. Hoenes, Ed."DNS Zone Transfer Protocol (AXFR)"RFC 5936
DOI 10.17487/RFC5936 <https://www.rfc-editor.org/info/rfc5936>
Livingood, J.Mody, N. M. O'Reirdan"Recommendations for the
Remediation of Bots in ISP Networks"RFC 6561DOI 10.17487/RFC6561
<https://www.rfc-editor.org/info/rfc6561>
Kolkman, O.Mekking, W. R. Gieben"DNSSEC Operational Practices,
Version 2"RFC 6781DOI 10.17487/RFC6781 <https://www.rfc-
editor.org/info/rfc6781>
Weiler, S., Ed.D. Blacka, Ed."Clarifications and Implementation Notes for
DNS Security (DNSSEC)"RFC 6840DOI 10.17487/RFC6840
<https://www.rfc-editor.org/info/rfc6840>
Ljunggren, F.Eklund Lowinder, AM.T. Okubo"A Framework for DNSSEC
Policies and DNSSEC Practice Statements"RFC 6841DOI 10.17487/RFC6841
<https://www.rfc-editor.org/info/rfc6841>
Damas, J.Graff, M. P. Vixie"Extension Mechanisms for DNS (EDNS(0))"
STD 75RFC 6891DOI 10.17487/RFC6891 <https://www.rfc-
editor.org/info/rfc6891>
Kumari, W.Gudmundsson, O.G. Barwood"Automating DNSSEC Delegation
Trust Maintenance"RFC 7344DOI 10.17487/RFC7344 <https://
www.rfc-editor.org/info/rfc7344>
Hoffman, P.Sullivan, A. K. Fujiwara"DNS Terminology"RFC 7719DOI
10.17487/RFC7719 <https://www.rfc-editor.org/info/rfc7719>
Dickinson, S.Gillmor, D. T. Reddy"Usage Profiles for DNS over TLS and
DNS over DTLS"RFC 8310DOI 10.17487/RFC8310 <https://
www.rfc-editor.org/info/rfc8310>
Hoffman, P.Sullivan, A. K. Fujiwara"DNS Terminology"BCP 219RFC
8499DOI 10.17487/RFC8499 <https://www.rfc-editor.org/info/
rfc8499>
Huitema, C.Dickinson, S. A. Mankin"DNS over Dedicated QUIC
Connections"RFC 9250DOI 10.17487/RFC9250 <https://www.rfc-
editor.org/info/rfc9250>
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 31

[RFC9471]
[IANA_Resource_Registry]
[RFC20]
[RFC819]
[RFC952]
[RFC1713]
[RFC1995]
[RFC2775]
[RFC3172]
[RFC3425]
[RFC3493]
[RFC3757]
[RFC3912]
[RFC4470]
, , , and ,
, , , September 2023,
.
14.2. Informative References
, ,
.
, , , ,
, October 1969, .
 and ,
, , , August 1982,
.
, , and , ,
, , October 1985,
.
, , , , ,
November 1994, .
, , , ,
August 1996, .
, , , ,
February 2000, .
,
, , ,
, September 2001, .
, , , , November
2002, .
, , , , and ,
, , , February 2003,
.
, , and ,
, ,
, April 2004, .
, , , ,
September 2004, .
 and ,
, , , April 2006,
.
Andrews, M.Huque, S.Wouters, P. D. Wessels"DNS Glue Requirements in
Referral Responses"RFC 9471DOI 10.17487/RFC9471 <https://
www.rfc-editor.org/info/rfc9471>
IANA"Resource Record (RR) TYPEs"<https://www.iana.org/
assignments/dns-parameters/>
Cerf, V."ASCII format for network interchange"STD 80RFC 20DOI 10.17487/
RFC0020 <https://www.rfc-editor.org/info/rfc20>
Su, Z. J. Postel"The Domain Naming Convention for Internet User
Applications"RFC 819DOI 10.17487/RFC0819 <https://www.rfc-
editor.org/info/rfc819>
Harrenstien, K.Stahl, M. E. Feinler"DoD Internet host table specification"
RFC 952DOI 10.17487/RFC0952 <https://www.rfc-editor.org/info/
rfc952>
Romao, A."Tools for DNS debugging"FYI 27RFC 1713DOI 10.17487/RFC1713
<https://www.rfc-editor.org/info/rfc1713>
Ohta, M."Incremental Zone Transfer in DNS"RFC 1995DOI 10.17487/RFC1995
<https://www.rfc-editor.org/info/rfc1995>
Carpenter, B."Internet Transparency"RFC 2775DOI 10.17487/RFC2775
<https://www.rfc-editor.org/info/rfc2775>
Huston, G., Ed."Management Guidelines & Operational Requirements for the
Address and Routing Parameter Area Domain ("arpa")"BCP 52RFC 3172DOI
10.17487/RFC3172 <https://www.rfc-editor.org/info/rfc3172>
Lawrence, D."Obsoleting IQUERY"RFC 3425DOI 10.17487/RFC3425
<https://www.rfc-editor.org/info/rfc3425>
Gilligan, R.Thomson, S.Bound, J.McCann, J. W. Stevens"Basic Socket
Interface Extensions for IPv6"RFC 3493DOI 10.17487/RFC3493
<https://www.rfc-editor.org/info/rfc3493>
Kolkman, O.Schlyter, J. E. Lewis"Domain Name System KEY (DNSKEY)
Resource Record (RR) Secure Entry Point (SEP) Flag"RFC 3757DOI 10.17487/
RFC3757 <https://www.rfc-editor.org/info/rfc3757>
Daigle, L."WHOIS Protocol Specification"RFC 3912DOI 10.17487/RFC3912
<https://www.rfc-editor.org/info/rfc3912>
Weiler, S. J. Ihren"Minimally Covering NSEC Records and DNSSEC On-line
Signing"RFC 4470DOI 10.17487/RFC4470 <https://www.rfc-
editor.org/info/rfc4470>
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 32

[RFC4641]
[RFC4697]
[RFC4786]
[RFC4956]
[RFC5625]
[RFC5890]
[RFC5891]
[RFC5892]
[RFC5893]
[RFC5894]
[RFC6055]
[RFC6265]
[RFC6303]
[RFC6335]
 and , , ,
, September 2006, .
 and , , ,
, , October 2006,
.
 and , , , ,
, December 2006,
.
, , and , , ,
, July 2007, .
, , , ,
, August 2009, .
,
, , ,
August 2010, .
, ,
, , August 2010,
.
,
, , , August 2010,
.
 and ,
, , ,
August 2010, .
,
, , ,
August 2010, .
, , and ,
, , , February
2011, .
, , ,
, April 2011, .
, , , ,
, July 2011, .
, , , , and ,
, , ,
, August 2011, .
Kolkman, O. R. Gieben"DNSSEC Operational Practices"RFC 4641DOI
10.17487/RFC4641 <https://www.rfc-editor.org/info/rfc4641>
Larson, M. P. Barber"Observed DNS Resolution Misbehavior"BCP 123RFC
4697DOI 10.17487/RFC4697 <https://www.rfc-editor.org/info/
rfc4697>
Abley, J. K. Lindqvist"Operation of Anycast Services"BCP 126RFC 4786
DOI 10.17487/RFC4786 <https://www.rfc-editor.org/info/
rfc4786>
Arends, R.Kosters, M. D. Blacka"DNS Security (DNSSEC) Opt-In"RFC 4956
DOI 10.17487/RFC4956 <https://www.rfc-editor.org/info/rfc4956>
Bellis, R."DNS Proxy Implementation Guidelines"BCP 152RFC 5625DOI
10.17487/RFC5625 <https://www.rfc-editor.org/info/rfc5625>
Klensin, J."Internationalized Domain Names for Applications (IDNA):
Definitions and Document Framework"RFC 5890DOI 10.17487/RFC5890
<https://www.rfc-editor.org/info/rfc5890>
Klensin, J."Internationalized Domain Names in Applications (IDNA): Protocol"
RFC 5891DOI 10.17487/RFC5891 <https://www.rfc-editor.org/info/
rfc5891>
Faltstrom, P., Ed."The Unicode Code Points and Internationalized Domain
Names for Applications (IDNA)"RFC 5892DOI 10.17487/RFC5892
<https://www.rfc-editor.org/info/rfc5892>
Alvestrand, H., Ed.C. Karp"Right-to-Left Scripts for Internationalized
Domain Names for Applications (IDNA)"RFC 5893DOI 10.17487/RFC5893
<https://www.rfc-editor.org/info/rfc5893>
Klensin, J."Internationalized Domain Names for Applications (IDNA):
Background, Explanation, and Rationale"RFC 5894DOI 10.17487/RFC5894
<https://www.rfc-editor.org/info/rfc5894>
Thaler, D.Klensin, J. S. Cheshire"IAB Thoughts on Encodings for
Internationalized Domain Names"RFC 6055DOI 10.17487/RFC6055
<https://www.rfc-editor.org/info/rfc6055>
Barth, A."HTTP State Management Mechanism"RFC 6265DOI 10.17487/
RFC6265 <https://www.rfc-editor.org/info/rfc6265>
Andrews, M."Locally Served DNS Zones"BCP 163RFC 6303DOI 10.17487/
RFC6303 <https://www.rfc-editor.org/info/rfc6303>
Cotton, M.Eggert, L.Touch, J.Westerlund, M.S. Cheshire"Internet
Assigned Numbers Authority (IANA) Procedures for the Management of the
Service Name and Transport Protocol Port Number Registry"BCP 165RFC 6335
DOI 10.17487/RFC6335 <https://www.rfc-editor.org/info/rfc6335>
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 33

[RFC6365]
[RFC6672]
[RFC6762]
[RFC7129]
[RFC7480]
[RFC7481]
[RFC9082]
[RFC9083]
[RFC9224]
[RFC7485]
[RFC7793]
[RFC7858]
[RFC8094]
 and ,
, , , , September 2011,
.
 and , , ,
, June 2012, .
 and , , , ,
February 2013, .
 and , ,
, , February 2014,
.
, , and ,
, , , , March 2015,
.
 and ,
, , , , March 2015,
.
 and ,
, , , , June 2021,
.
 and ,
, , , , June 2021,
.
,
, , , , March 2022,
.
, , , , and ,
, , , March 2015,
.
,
, , , , May 2016,
.
, , , , , and ,
, ,
, May 2016, .
, , and ,
, , , February 2017,
.
Hoffman, P. J. Klensin"Terminology Used in Internationalization in the
IETF"BCP 166RFC 6365DOI 10.17487/RFC6365 <https://
www.rfc-editor.org/info/rfc6365>
Rose, S. W. Wijngaards"DNAME Redirection in the DNS"RFC 6672DOI
10.17487/RFC6672 <https://www.rfc-editor.org/info/rfc6672>
Cheshire, S. M. Krochmal"Multicast DNS"RFC 6762DOI 10.17487/RFC6762
<https://www.rfc-editor.org/info/rfc6762>
Gieben, R. W. Mekking"Authenticated Denial of Existence in the DNS"RFC
7129DOI 10.17487/RFC7129 <https://www.rfc-editor.org/info/
rfc7129>
Newton, A.Ellacott, B. N. Kong"HTTP Usage in the Registration Data
Access Protocol (RDAP)"STD 95RFC 7480DOI 10.17487/RFC7480
<https://www.rfc-editor.org/info/rfc7480>
Hollenbeck, S.N. Kong"Security Services for the Registration Data Access
Protocol (RDAP)"STD 95RFC 7481DOI 10.17487/RFC7481 <https://
www.rfc-editor.org/info/rfc7481>
Hollenbeck, S.A. Newton"Registration Data Access Protocol (RDAP) Query
Format"STD 95RFC 9082DOI 10.17487/RFC9082 <https://www.rfc-
editor.org/info/rfc9082>
Hollenbeck, S.A. Newton"JSON Responses for the Registration Data Access
Protocol (RDAP)"STD 95RFC 9083DOI 10.17487/RFC9083 <https://
www.rfc-editor.org/info/rfc9083>
Blanchet, M."Finding the Authoritative Registration Data Access Protocol
(RDAP) Service"STD 95RFC 9224DOI 10.17487/RFC9224 <https://
www.rfc-editor.org/info/rfc9224>
Zhou, L.Kong, N.Shen, S.Sheng, S. A. Servin"Inventory and Analysis of
WHOIS Registration Objects"RFC 7485DOI 10.17487/RFC7485
<https://www.rfc-editor.org/info/rfc7485>
Andrews, M."Adding 100.64.0.0/10 Prefixes to the IPv4 Locally-Served DNS
Zones Registry"BCP 163RFC 7793DOI 10.17487/RFC7793 <https://
www.rfc-editor.org/info/rfc7793>
Hu, Z.Zhu, L.Heidemann, J.Mankin, A.Wessels, D. P. Hoffman
"Specification for DNS over Transport Layer Security (TLS)"RFC 7858DOI
10.17487/RFC7858 <https://www.rfc-editor.org/info/rfc7858>
Reddy, T.Wing, D. P. Patil"DNS over Datagram Transport Layer Security
(DTLS)"RFC 8094DOI 10.17487/RFC8094 <https://www.rfc-
editor.org/info/rfc8094>
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 34

[RFC8109]
[RFC8484]
[RFC9103]
[RSSAC026]
, , and ,
, , , , March 2017,
.
 and , , ,
, October 2018, .
, , , , and ,
, , , August 2021,
.
, ,
2017, .
Koch, P.Larson, M. P. Hoffman"Initializing a DNS Resolver with Priming
Queries"BCP 209RFC 8109DOI 10.17487/RFC8109 <https://
www.rfc-editor.org/info/rfc8109>
Hoffman, P. P. McManus"DNS Queries over HTTPS (DoH)"RFC 8484DOI
10.17487/RFC8484 <https://www.rfc-editor.org/info/rfc8484>
Toorop, W.Dickinson, S.Sahib, S.Aras, P. A. Mankin"DNS Zone Transfer
over TLS"RFC 9103DOI 10.17487/RFC9103 <https://www.rfc-
editor.org/info/rfc9103>
Root Server System Advisory Committee (RSSAC)"RSSAC0226 RSSAC Lexicon"
<https://www.icann.org/en/system/files/files/rssac-026-14mar17-en.pdf>
Appendix A. Definitions Updated by This Document
The following definitions from RFCs are updated by this document:
Forwarder in
QNAME in
Secure Entry Point (SEP) in ; note, however, that this RFC is already obsolete (see
, , ).
• [RFC2308]
• [RFC2308]
• [RFC3757]
[RFC4033] [RFC4034] [RFC4035]
Appendix B. Definitions First Defined in This Document
The following definitions are first defined in this document:
"Alias" in Section 2
"Apex" in Section 7
"arpa" in Section 7
"Authoritative DoT (ADot)" in Section 6
"Bailiwick" in Section 7
"Class independent" in Section 5
"Classic DNS" in Section 6
"Delegation-centric zone" in Section 7
"Delegation" in Section 7
"DNS operator" in Section 9
"DNSSEC-aware" in Section 10
"DNSSEC-unaware" in Section 10
"Forwarding" in Section 6
"Full resolver" in Section 6
"Fully Qualified Domain Name" in Section 2
"Global DNS" in Section 2
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 35

"Hardware Security Module (HSM)" in Section 10
"Host name" in Section 2
"IDN" in Section 2
"In-domain" in Section 7
"Iterative resolution" in Section 6
"Label" in Section 2
"Locally served DNS zone" in Section 2
"Naming system" in Section 2
"Negative response" in Section 3
"Non-recursive query" in Section 6
"Open resolver" in Section 6
"Passive DNS" in Section 6
"Policy-implementing resolver" in Section 6
"Presentation format" in Section 5
"Priming" in Section 6
"Private DNS" in Section 2
"Recursive DoT (RDot)" in Section 6
"Recursive resolver" in Section 6
"Referrals" in Section 4
"Registrant" in Section 9
"Registrar" in Section 9
"Registry" in Section 9
"Root zone" in Section 7
"Secure Entry Point (SEP)" in Section 10
"Sibling domain" in Section 7
"Signing software" in Section 10
"Split DNS" in Section 6
"Stub resolver" in Section 6
"Subordinate" in Section 8
"Superordinate" in Section 8
"TLD" in Section 2
"Validating resolver" in Section 10
"Validation" in Section 10
"View" in Section 6
"Zone transfer" in Section 6
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 36

Address and Routing Parameter Area Domain (arpa)
Address records
ADoT
Alias
Anycast
Apex
Asterisk label
Authoritative data
Authoritative server
Authoritative-only server
AXoT
Bailiwick
Index

Section 7

Section 5

Section 6

Section 2

Section 6

Section 7

Section 8

Section 7

Section 6

Section 6

Section 6

Section 7
Acknowledgements
 and its predecessor, , were co-authored by . The current
document, which is a small update to , has just two authors. Andrew's work on the
earlier documents is greatly appreciated.
Numerous people made significant contributions to  and . Please see the
acknowledgements sections in those two documents for the extensive list of contributors.
Even though the current document is a small revision, many people in the DNSOP Working
Group have contributed to it, and their work is greatly appreciated.
[RFC8499] [RFC7719] Andrew Sullivan
[RFC8499]
[RFC8499] [RFC7719]
A B C D E F G H I K L M N O P Q R S T U V W X Z
A
B
RFC 9499 DNS Terminology March 2024
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Canonical name
Child
Class
Class independent
Classic DNS
Closest encloser
Closest provable encloser
CNAME
Combined signing key (CSK)
Delegation
Delegation-centric zone
DNS operator
DNS-over-HTTPS
DNS-over-QUIC
DNS-over-TLS
DNSSEC Policy (DP)
DNSSEC Practice Statement (DPS)
DNSSEC-aware and DNSSEC-unaware
DoH
Domain name
DoQ

Section 2

Section 7

Section 4

Section 5

Section 6

Section 8

Section 8

Section 2

Section 10

Section 7

Section 7

Section 9

Section 6

Section 6

Section 6

Section 10

Section 10

Section 10

Section 6

Section 2

C
D
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 38

DoT
EDNS
Empty non-terminals (ENTs)
EPP
Fast flux DNS
FORMERR
Forward lookup
Forwarder
Forwarding
Full resolver
Full-service resolver
Fully Qualified Domain Name (FQDN)
Global DNS
Glue records
Hardware security module (HSM)
Hidden master
Host name
Section 6

Section 6

Section 5

Section 7

Section 9

Section 7

Section 3

Section 7

Section 6

Section 6

Section 6

Section 6

Section 2

Section 2

Section 7

Section 10

Section 6

Section 2
E
F
G
H
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 39

IDN
In-bailiwick
In-domain
Insecure delegation
Instance
Internationalized Domain Name
Iterative mode
Iterative resolution
IXoT
Key signing key (KSK)
Label
Lame delegation
Locally served DNS zone
Master file
Master server
mDNS
Multicast DNS

Section 2

Section 7

Section 7

Section 10

Section 6

Section 2

Section 6

Section 6

Section 6

Section 10

Section 2

Section 7

Section 2

Section 5

Section 6

Section 2

Section 2
I
K
L
M
N
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 40

Naming system
Negative caching
Negative response
Next closer name
NODATA
NOERROR
Non-recursive query
NOTIMP
NS
NSEC
NSEC3
NXDOMAIN
Occluded name
on-line signing
online signing
Open resolver
OPT
Opt-out
Origin
Out-of-bailiwick
Owner

Section 2, Paragraph 1.2.1

Section 6

Section 3

Section 8

Section 3

Section 3

Section 6

Section 3

Section 6

Section 10

Section 10

Section 3

Section 7

Section 10

Section 10

Section 6

Section 5

Section 10

Section 7

Section 7

Section 5
O
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 41

Parent
Passive DNS
Policy-implementing resolver
Presentation format
Primary master
Primary server
Priming
Privacy-enabling DNS server
Private DNS
Public suffix
QNAME
RDAP
RDoT
Recursive DoT
Recursive mode
Recursive query
Recursive resolver
Referrals
REFUSED
Registrant

Section 7

Section 6

Section 6

Section 5

Section 6

Section 6

Section 6

Section 6

Section 2

Section 9

Section 4

Section 9

Section 6

Section 6

Section 6, Paragraph 4.10.1

Section 6

Section 6

Section 4

Section 3

P
Q
R
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 42

Registrar
Registry
Resolver
Reverse DNS, reverse lookup
Root hints
Root zone
RR
RRset
Secondary server
Secure Entry Point (SEP)
SERVFAIL
Service name
Sibling domain
Signed zone
Signing software
Slave server
SOA
SOA field names
Source of Synthesis
Split DNS
Split-horizon DNS
Section 9

Section 9

Section 9

Section 6

Section 7

Section 6

Section 7

Section 5

Section 5

Section 6

Section 10

Section 3

Section 7

Section 7

Section 10

Section 10

Section 6

Section 5

Section 5

Section 8, Paragraph 1.14.1

Section 6

Section 6
S
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 43

Stealth server
Stub resolver
Subdomain
Subordinate
Superordinate
TLD
Trust anchor
TTL
Unsigned zone
Validating resolver
Validation
View
WHOIS
Wildcard
Wildcard domain name
XoT

Section 6

Section 6

Section 2

Section 9

Section 9

Section 2

Section 10

Section 5

Section 10

Section 10

Section 10, Paragraph 2.26.1

Section 6

Section 9

Section 8

Section 8

Section 6
T
U
V
W
X
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 44

Zone
Zone cut
Zone enumeration
Zone signing key (ZSK)
Zone transfer

Section 7

Section 7

Section 10

Section 10

Section 6
Z
Authors' Addresses
Paul Hoffman
ICANN
 paul.hoffman@icann.org Email:
Kazunori Fujiwara
Japan Registry Services Co., Ltd.
 fujiwara@jprs.co.jp Email:
RFC 9499 DNS Terminology March 2024
Hoffman & Fujiwara Best Current Practice Page 45