draft-ietf-dnsop-terminology-bis-14.txt   rfc8499.txt 
Network Working Group P. Hoffman Internet Engineering Task Force (IETF) P. Hoffman
Internet-Draft ICANN Request for Comments: 8499 ICANN
Obsoletes: 7719 (if approved) A. Sullivan BCP: 219 A. Sullivan
Updates: 2308 (if approved) Obsoletes: 7719
Intended status: Best Current Practice K. Fujiwara Updates: 2308 K. Fujiwara
Expires: March 17, 2019 JPRS Category: Best Current Practice JPRS
September 13, 2018 ISSN: 2070-1721 January 2019
DNS Terminology DNS Terminology
draft-ietf-dnsop-terminology-bis-14
Abstract Abstract
The domain name system (DNS) is defined in literally dozens of The Domain Name System (DNS) is defined in literally dozens of
different RFCs. The terminology used by implementers and developers different RFCs. The terminology used by implementers and developers
of DNS protocols, and by operators of DNS systems, has sometimes of DNS protocols, and by operators of DNS systems, has sometimes
changed in the decades since the DNS was first defined. This changed in the decades since the DNS was first defined. This
document gives current definitions for many of the terms used in the document gives current definitions for many of the terms used in the
DNS in a single document. DNS in a single document.
This document obsoletes RFC 7719 and updates RFC 2308. This document obsoletes RFC 7719 and updates RFC 2308.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This memo documents an Internet Best Current Practice.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
This Internet-Draft will expire on March 17, 2019. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8499.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. DNS Response Codes . . . . . . . . . . . . . . . . . . . . . 9 3. DNS Response Codes . . . . . . . . . . . . . . . . . . . . . 10
4. DNS Transactions . . . . . . . . . . . . . . . . . . . . . . 10 4. DNS Transactions . . . . . . . . . . . . . . . . . . . . . . 11
5. Resource Records . . . . . . . . . . . . . . . . . . . . . . 13 5. Resource Records . . . . . . . . . . . . . . . . . . . . . . 14
6. DNS Servers and Clients . . . . . . . . . . . . . . . . . . . 15 6. DNS Servers and Clients . . . . . . . . . . . . . . . . . . . 16
7. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 7. Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . 26 8. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9. Registration Model . . . . . . . . . . . . . . . . . . . . . 27 9. Registration Model . . . . . . . . . . . . . . . . . . . . . 28
10. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 29 10. General DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 30
11. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 33 11. DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . . 34
12. Security Considerations . . . . . . . . . . . . . . . . . . . 35 12. Security Considerations . . . . . . . . . . . . . . . . . . . 36
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
14.1. Normative References . . . . . . . . . . . . . . . . . . 35 14.1. Normative References . . . . . . . . . . . . . . . . . . 36
14.2. Informative References . . . . . . . . . . . . . . . . . 38 14.2. Informative References . . . . . . . . . . . . . . . . . 39
Appendix A. Definitions Updated by this Document . . . . . . . . 42 Appendix A. Definitions Updated by This Document . . . . . . . . 44
Appendix B. Definitions First Defined in this Document . . . . . 43 Appendix B. Definitions First Defined in This Document . . . . . 44
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 48 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 49 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction 1. Introduction
The Domain Name System (DNS) is a simple query-response protocol The Domain Name System (DNS) is a simple query-response protocol
whose messages in both directions have the same format. (Section 2 whose messages in both directions have the same format. (Section 2
gives a definition of "public DNS", which is often what people mean gives a definition of "public DNS", which is often what people mean
when they say "the DNS".) The protocol and message format are when they say "the DNS".) The protocol and message format are
defined in [RFC1034] and [RFC1035]. These RFCs defined some terms, defined in [RFC1034] and [RFC1035]. These RFCs defined some terms,
but later documents defined others. Some of the terms from [RFC1034] and later documents defined others. Some of the terms from [RFC1034]
and [RFC1035] now have somewhat different meanings than they did in and [RFC1035] have somewhat different meanings now than they did in
1987. 1987.
This document collects a wide variety of DNS-related terms. Some of This document contains a collection of a wide variety of DNS-related
them have been precisely defined in earlier RFCs, some have been terms, organized loosely by topic. Some of them have been precisely
loosely defined in earlier RFCs, and some are not defined in any defined in earlier RFCs, some have been loosely defined in earlier
earlier RFC at all. RFCs, and some are not defined in an earlier RFC at all.
Most of the definitions here are the consensus definition of the DNS Other organizations sometimes define DNS-related terms their own way.
community -- both protocol developers and operators. Some of the For example, the WHATWG defines "domain" at
definitions differ from earlier RFCs, and those differences are <https://url.spec.whatwg.org/>. The Root Server System Advisory
noted. In this document, where the consensus definition is the same Committee (RSSAC) has a good lexicon [RSSAC026].
as the one in an RFC, that RFC is quoted. Where the consensus
definition has changed somewhat, the RFC is mentioned but the new Most of the definitions listed here represent the consensus
stand-alone definition is given. See Appendix A for a list of the definition of the DNS community -- both protocol developers and
definitions that this document updates. 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 It is important to note that, during the development of this
document, it became clear that some DNS-related terms are interpreted document, it became clear that some DNS-related terms are interpreted
quite differently by different DNS experts. Further, some terms that quite differently by different DNS experts. Further, some terms that
are defined in early DNS RFCs now have definitions that are generally are defined in early DNS RFCs now have definitions that are generally
agreed to, but that are different from the original definitions. agreed to, but that are different from the original definitions.
Therefore, this document is a substantial revision to [RFC7719]. Therefore, this document is a substantial revision to [RFC7719].
The terms are organized loosely by topic. Some definitions are for
new terms for things that are commonly talked about in the DNS
community but that never had terms defined for them.
Other organizations sometimes define DNS-related terms their own way.
For example, the WHATWG defines "domain" at
<https://url.spec.whatwg.org/>. The Root Server System Advisory
Committee (RSSAC) has a good lexicon [RSSAC026].
Note that there is no single consistent definition of "the DNS". It Note that there is no single consistent definition of "the DNS". It
can be considered to be some combination of the following: a commonly can be considered to be some combination of the following: a commonly
used naming scheme for objects on the Internet; a distributed used naming scheme for objects on the Internet; a distributed
database representing the names and certain properties of these database representing the names and certain properties of these
objects; an architecture providing distributed maintenance, objects; an architecture providing distributed maintenance,
resilience, and loose coherency for this database; and a simple resilience, and loose coherency for this database; and a simple
query-response protocol (as mentioned below) implementing this query-response protocol (as mentioned below) implementing this
architecture. Section 2 defines "global DNS" and "private DNS" as a architecture. Section 2 defines "global DNS" and "private DNS" as a
way to deal with these differing definitions. way to deal with these differing definitions.
Capitalization in DNS terms is often inconsistent among RFCs and Capitalization in DNS terms is often inconsistent among RFCs and
various DNS practitioners. The capitalization used in this document various DNS practitioners. The capitalization used in this document
is a best guess at current practices, and is not meant to indicate is a best guess at current practices, and is not meant to indicate
that other capitalization styles are wrong or archaic. In some that other capitalization styles are wrong or archaic. In some
cases, multiple styles of capitalization are used for the same term cases, multiple styles of capitalization are used for the same term
due to quoting from different RFCs. due to quoting from different RFCs.
Readers should note that the terms in this document are grouped by Readers should note that the terms in this document are grouped by
topic. Someone who is not already familiar with the DNS can probably topic. Someone who is not already familiar with the DNS probably
not learn about the DNS from scratch by reading this document from 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 front to back. Instead, skipping around may be the only way to get
enough context to understand some of the definitions. This document enough context to understand some of the definitions. This document
has an index that might be useful for readers who are attempting to has an index that might be useful for readers who are attempting to
learn the DNS by reading this document. learn the DNS by reading this document.
2. Names 2. Names
Naming system: A naming system associates names with data. Naming Naming system: A naming system associates names with data. Naming
systems have many significant facets that help differentiate them systems have many significant facets that help differentiate them
from each other. Some commonly-identified facets include: from each other. Some commonly identified facets include:
* Composition of names * Composition of names
* Format of names * Format of names
* Administration of names * Administration of names
* Types of data that can be associated with names * Types of data that can be associated with names
* Types of metadata for names * Types of metadata for names
* Protocol for getting data from a name * Protocol for getting data from a name
* Context for resolving a name * Context for resolving a name
Note that this list is a small subset of facets that people have 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 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 agree on a good set of facets that can be used to compare naming
systems. For example, other facets might include "protocol to systems. For example, other facets might include "protocol to
update data in a name", "privacy of names", and "privacy of data update data in a name", "privacy of names", and "privacy of data
associated with names", but those are not as well-defined as the associated with names", but those are not as well defined as the
ones listed above. The list here is chosen because it helps ones listed above. The list here is chosen because it helps
describe the DNS and naming systems similar to the DNS. describe the DNS and naming systems similar to the DNS.
Domain name: An ordered list of one or more labels. Domain name: An ordered list of one or more labels.
Note that this is a definition independent of the DNS RFCs, and Note that this is a definition independent of the DNS RFCs
the definition here also applies to systems other than the DNS. ([RFC1034] and [RFC1035]), and the definition here also applies to
[RFC1034] defines the "domain name space" using mathematical trees systems other than the DNS. [RFC1034] defines the "domain name
and their nodes in graph theory, and this definition has the same space" using mathematical trees and their nodes in graph theory,
practical result as the definition here. Any path of a directed and that definition has the same practical result as the
acyclic graph can be represented by a domain name consisting of definition here. Any path of a directed acyclic graph can be
the labels of its nodes, ordered by decreasing distance from the represented by a domain name consisting of the labels of its
root(s) (which is the normal convention within the DNS, including nodes, ordered by decreasing distance from the root(s) (which is
this document). A domain name whose last label identifies a root the normal convention within the DNS, including this document). A
of the graph is fully qualified; other domain names whose labels domain name whose last label identifies a root of the graph is
form a strict prefix of a fully qualified domain name are relative fully qualified; other domain names whose labels form a strict
to its first omitted node. 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 Also note that different IETF and non-IETF documents have used the
term "domain name" in many different ways. It is common for term "domain name" in many different ways. It is common for
earlier documents to use "domain name" to mean "names that match earlier documents to use "domain name" to mean "names that match
the syntax in [RFC1035]", but possibly with additional rules such the syntax in [RFC1035]", but possibly with additional rules such
as "and are, or will be, resolvable in the global DNS" or "but as "and are, or will be, resolvable in the global DNS" or "but
only using the presentation format". only using the presentation format".
Label: An ordered list of zero or more octets that makes up a Label: An ordered list of zero or more octets that makes up a
portion of a domain name. Using graph theory, a label identifies portion of a domain name. Using graph theory, a label identifies
one node in a portion of the graph of all possible domain names. one node in a portion of the graph of all possible domain names.
Global DNS: Using the short set of facets listed in "Naming system", Global DNS: Using the short set of facets listed in "Naming system",
the global DNS can be defined as follows. Most of the rules here the global DNS can be defined as follows. Most of the rules here
come from [RFC1034] and [RFC1035], although the term "global DNS" come from [RFC1034] and [RFC1035], although the term "global DNS"
has not been defined before now. has not been defined before now.
Composition of names -- A name in the global DNS has one or more Composition of names: A name in the global DNS has one or more
labels. The length of each label is between 0 and 63 octets labels. The length of each label is between 0 and 63 octets
inclusive. In a fully-qualified domain name, the last label in inclusive. In a fully-qualified domain name, the last label in
the ordered list is 0 octets long; it is the only label whose 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 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 label". A domain name in the global DNS has a maximum total
length of 255 octets in the wire format; the root represents one length of 255 octets in the wire format; the root represents one
octet for this calculation. (Multicast DNS [RFC6762] allows names octet for this calculation. (Multicast DNS [RFC6762] allows names
up to 255 bytes plus a terminating zero byte based on a different up to 255 bytes plus a terminating zero byte based on a different
interpretation of RFC 1035 and what is included in the 255 interpretation of RFC 1035 and what is included in the 255
octets.) octets.)
Format of names: Names in the global DNS are domain names. There
are three formats: wire format, presentation format, and common
display.
Format of names -- Names in the global DNS are domain names. The basic wire format for names in the global DNS is a list of
There are three formats: wire format, presentation format, and labels ordered by decreasing distance from the root, with the
common display. root label last. Each label is preceded by a length octet.
[RFC1035] also defines a compression scheme that modifies this
The basic wire format for names in the global DNS is a list of format.
labels ordered by decreasing distance from the root, with the root
label last. Each label is preceded by a length octet. [RFC1035]
also defines a compression scheme that modifies this format.
The presentation format for names in the global DNS is a list of The presentation format for names in the global DNS is a list
labels ordered by decreasing distance from the root, encoded as of labels ordered by decreasing distance from the root, encoded
ASCII, with a "." character between each label. In presentation as ASCII, with a "." character between each label. In
format, a fully-qualified domain name includes the root label and presentation format, a fully-qualified domain name includes the
the associated separator dot. For example, in presentation root label and the associated separator dot. For example, in
format, a fully-qualified domain name with two non-root labels is presentation format, a fully-qualified domain name with two
always shown as "example.tld." instead of "example.tld". non-root labels is always shown as "example.tld." instead of
[RFC1035] defines a method for showing octets that do not display "example.tld". [RFC1035] defines a method for showing octets
in ASCII. that do not display in ASCII.
The common display format is used in applications and free text. The common display format is used in applications and free
It is the same as the presentation format, but showing the root text. It is the same as the presentation format, but showing
label and the "." before it is optional and is rarely done. For the root label and the "." before it is optional and is rarely
example, in common display format, a fully-qualified domain name done. For example, in common display format, a fully-qualified
with two non-root labels is usually shown as "example.tld" instead domain name with two non-root labels is usually shown as
of "example.tld.". Names in the common display format are "example.tld" instead of "example.tld.". Names in the common
normally written such that the directionality of the writing display format are normally written such that the
system presents labels by decreasing distance from the root (so, directionality of the writing system presents labels by
in both English and the C programming language the root or TLD decreasing distance from the root (so, in both English and the
label in the ordered list is right-most; but in Arabic it may be C programming language the root or Top-Level Domain (TLD) label
left-most, depending on local conventions). in the ordered list is rightmost; but in Arabic, it may be
leftmost, depending on local conventions).
Administration of names -- Administration is specified by Administration of names: Administration is specified by delegation
delegation (see the definition of "delegation" in Section 7). (see the definition of "delegation" in Section 7). Policies for
Policies for administration of the root zone in the global DNS are administration of the root zone in the global DNS are determined
determined by the names operational community, which convenes by the names operational community, which convenes itself in the
itself in the Internet Corporation for Assigned Names and Numbers Internet Corporation for Assigned Names and Numbers (ICANN). The
(ICANN). The names operational community selects the IANA names operational community selects the IANA Functions Operator
Functions Operator for the global DNS root zone. At the time this for the global DNS root zone. At the time of writing, that
document is published, that operator is Public Technical operator is Public Technical Identifiers (PTI). (See
Identifiers (PTI). (See <https://pti.icann.org/> for more <https://pti.icann.org/> for more information about PTI operating
information about PTI operating the IANA Functions.) The name the IANA Functions.) The name servers that serve the root zone
servers that serve the root zone are provided by independent root are provided by independent root operators. Other zones in the
operators. Other zones in the global DNS have their own policies global DNS have their own policies for administration.
for administration.
Types of data that can be associated with names -- A name can have Types of data that can be associated with names: A name can have
zero or more resource records associated with it. There are zero or more resource records associated with it. There are
numerous types of resource records with unique data structures numerous types of resource records with unique data structures
defined in many different RFCs and in the IANA registry at defined in many different RFCs and in the IANA registry at
[IANA_Resource_Registry]. [IANA_Resource_Registry].
Types of metadata for names -- Any name that is published in the Types of metadata for names: Any name that is published in the DNS
DNS appears as a set of resource records (see the definition of appears as a set of resource records (see the definition of
"RRset" in Section 5). Some names do not themselves have data "RRset" in Section 5). Some names do not, themselves, have data
associated with them in the DNS, but "appear" in the DNS anyway associated with them in the DNS, but they "appear" in the DNS
because they form part of a longer name that does have data anyway because they form part of a longer name that does have data
associated with it (see the definition of "empty non-terminals" in associated with it (see the definition of "empty non-terminals" in
Section 7). Section 7).
Protocol for getting data from a name -- The protocol described in Protocol for getting data from a name: The protocol described in
[RFC1035]. [RFC1035].
Context for resolving a name -- The global DNS root zone Context for resolving a name: The global DNS root zone distributed
distributed by PTI. by PTI.
Private DNS: Names that use the protocol described in [RFC1035] but Private DNS: Names that use the protocol described in [RFC1035] but
that do not rely on the global DNS root zone, or names that are that do not rely on the global DNS root zone or names that are
otherwise not generally available on the Internet but are using otherwise not generally available on the Internet but are using
the protocol described in [RFC1035]. A system can use both the the protocol described in [RFC1035]. A system can use both the
global DNS and one or more private DNS systems; for example, see global DNS and one or more private DNS systems; for example, see
"Split DNS" in Section 6. "Split DNS" in Section 6.
Note that domain names that do not appear in the DNS, and that are 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 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 part of the global DNS or a private DNS even though they are
domain names. domain names.
Multicast DNS: "Multicast DNS (mDNS) provides the ability to perform Multicast DNS (mDNS): "Multicast DNS (mDNS) provides the ability to
DNS-like operations on the local link in the absence of any perform DNS-like operations on the local link in the absence of
conventional Unicast DNS server. In addition, Multicast DNS any conventional Unicast DNS server. In addition, Multicast DNS
designates a portion of the DNS namespace to be free for local 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 use, without the need to pay any annual fee, and without the need
to set up delegations or otherwise configure a conventional DNS to set up delegations or otherwise configure a conventional DNS
server to answer for those names." (Quoted from [RFC6762], server to answer for those names." (Quoted from [RFC6762],
Abstract) Although it uses a compatible wire format, mDNS is Abstract) Although it uses a compatible wire format, mDNS is,
strictly speaking a different protocol than DNS. Also, where the strictly speaking, a different protocol than DNS. Also, where the
above quote says "a portion of the DNS namespace", it would be 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 clearer to say "a portion of the domain name space". The names in
mDNS are not intended to be looked up in the DNS. mDNS are not intended to be looked up in the DNS.
Locally served DNS zone: A locally served DNS zone is a special case Locally served DNS zone: A locally served DNS zone is a special case
of private DNS. Names are resolved using the DNS protocol in a of private DNS. Names are resolved using the DNS protocol in a
local context. [RFC6303] defines subdomains of IN-ADDR.ARPA that local context. [RFC6303] defines subdomains of IN-ADDR.ARPA that
are locally served zones. Resolution of names through locally are locally served zones. Resolution of names through locally
served zones may result in ambiguous results. For example, the served zones may result in ambiguous results. For example, the
same name may resolve to different results in different locally same name may resolve to different results in different locally
served DNS zone contexts. The context for a locally served DNS served DNS zone contexts. The context for a locally served DNS
zone may be explicit, for example, as defined in [RFC6303], or zone may be explicit, such as those that are listed in [RFC6303]
implicit, as defined by local DNS administration and not known to and [RFC7793], or implicit, such as those defined by local DNS
the resolution client. administration and not known to the resolution client.
Fully qualified domain name (FQDN): This is often just a clear way Fully-Qualified Domain Name (FQDN): This is often just a clear way
of saying the same thing as "domain name of a node", as outlined of saying the same thing as "domain name of a node", as outlined
above. However, the term is ambiguous. Strictly speaking, a above. However, the term is ambiguous. Strictly speaking, a
fully qualified domain name would include every label, including fully-qualified domain name would include every label, including
the zero-length label of the root: such a name would be written the zero-length label of the root: such a name would be written
"www.example.net." (note the terminating dot). But because every "www.example.net." (note the terminating dot). But, because every
name eventually shares the common root, names are often written name eventually shares the common root, names are often written
relative to the root (such as "www.example.net") and are still relative to the root (such as "www.example.net") and are still
called "fully qualified". This term first appeared in [RFC0819]. called "fully qualified". This term first appeared in [RFC819].
In this document, names are often written relative to the root. In this document, names are often written relative to the root.
The need for the term "fully qualified domain name" comes from the The need for the term "fully-qualified domain name" comes from the
existence of partially qualified domain names, which are names existence of partially qualified domain names, which are names
where one or more of the last labels in the ordered list are where one or more of the last labels in the ordered list are
omitted (for example, a domain name of "www" relative to omitted (for example, a domain name of "www" relative to
"example.net" identifies "www.example.net"). Such relative names "example.net" identifies "www.example.net"). Such relative names
are understood only by context. are understood only by context.
Host name: This term and its equivalent, "hostname", have been Host name: This term and its equivalent, "hostname", have been
widely used but are not defined in [RFC1034], [RFC1035], widely used but are not defined in [RFC1034], [RFC1035],
[RFC1123], or [RFC2181]. The DNS was originally deployed into the [RFC1123], or [RFC2181]. The DNS was originally deployed into the
Host Tables environment as outlined in [RFC0952], and it is likely Host Tables environment as outlined in [RFC952], and it is likely
that the term followed informally from the definition there. Over that the term followed informally from the definition there. Over
time, the definition seems to have shifted. "Host name" is often time, the definition seems to have shifted. "Host name" is often
meant to be a domain name that follows the rules in Section 3.5 of meant to be a domain name that follows the rules in Section 3.5 of
[RFC1034], the "preferred name syntax" (that is, every character [RFC1034], which is also called the "preferred name syntax". (In
in each label is a letter, a digit, or a hyphen). Note that any that syntax, every character in each label is a letter, a digit,
label in a domain name can contain any octet value; hostnames are or a hyphen). Note that any label in a domain name can contain
generally considered to be domain names where every label follows any octet value; hostnames are generally considered to be domain
the rules in the "preferred name syntax", with the amendment that names where every label follows the rules in the "preferred name
labels can start with ASCII digits (this amendment comes from syntax", with the amendment that labels can start with ASCII
Section 2.1 of [RFC1123]). digits (this amendment comes from Section 2.1 of [RFC1123]).
People also sometimes use the term hostname to refer to just the People also sometimes use the term "hostname" to refer to just the
first label of an FQDN, such as "printer" in first label of an FQDN, such as "printer" in
"printer.admin.example.com". (Sometimes this is formalized in "printer.admin.example.com". (Sometimes this is formalized in
configuration in operating systems.) In addition, people configuration in operating systems.) In addition, people
sometimes use this term to describe any name that refers to a sometimes use this term to describe any name that refers to a
machine, and those might include labels that do not conform to the machine, and those might include labels that do not conform to the
"preferred name syntax". "preferred name syntax".
TLD: A Top-Level Domain, meaning a zone that is one layer below the Top-Level Domain (TLD): A Top-Level Domain is a zone that is one
root, such as "com" or "jp". There is nothing special, from the layer below the root, such as "com" or "jp". There is nothing
point of view of the DNS, about TLDs. Most of them are also special, from the point of view of the DNS, about TLDs. Most of
delegation-centric zones (defined in Section 7, and there are them are also delegation-centric zones (defined in Section 7), and
significant policy issues around their operation. TLDs are often there are significant policy issues around their operation. TLDs
divided into sub-groups such as Country Code Top-Level Domains are often divided into sub-groups such as Country Code Top-Level
(ccTLDs), Generic Top-Level Domains (gTLDs), and others; the Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others;
division is a matter of policy, and beyond the scope of this the division is a matter of policy and beyond the scope of this
document. document.
IDN: The common abbreviation for "Internationalized Domain Name". Internationalized Domain Name (IDN): The Internationalized Domain
The IDNA protocol is the standard mechanism for handling domain Names for Applications (IDNA) protocol is the standard mechanism
names with non-ASCII characters in applications in the DNS. The for handling domain names with non-ASCII characters in
current standard at the time of this writing, normally called applications in the DNS. The current standard at the time of this
"IDNA2008", is defined in [RFC5890], [RFC5891], [RFC5892], writing, normally called "IDNA2008", is defined in [RFC5890],
[RFC5893], and [RFC5894]. These documents define many IDN- [RFC5891], [RFC5892], [RFC5893], and [RFC5894]. These documents
specific terms such as "LDH label", "A-label", and "U-label". define many IDN-specific terms such as "LDH label", "A-label", and
[RFC6365] defines more terms that relate to internationalization "U-label". [RFC6365] defines more terms that relate to
(some of which relate to IDNs), and [RFC6055] has a much more internationalization (some of which relate to IDNs); [RFC6055] has
extensive discussion of IDNs, including some new terminology. a much more extensive discussion of IDNs, including some new
terminology.
Subdomain: "A domain is a subdomain of another domain if it is Subdomain: "A domain is a subdomain of another domain if it is
contained within that domain. This relationship can be tested by contained within that domain. This relationship can be tested by
seeing if the subdomain's name ends with the containing domain's seeing if the subdomain's name ends with the containing domain's
name." (Quoted from [RFC1034], Section 3.1). For example, in the name." (Quoted from [RFC1034], Section 3.1) For example, in the
host name "nnn.mmm.example.com", both "mmm.example.com" and host name "nnn.mmm.example.com", both "mmm.example.com" and
"nnn.mmm.example.com" are subdomains of "example.com". Note that "nnn.mmm.example.com" are subdomains of "example.com". Note that
the comparisons here are done on whole labels; that is, the comparisons here are done on whole labels; that is,
"ooo.example.com" is not a subdomain of "oo.example.com". "ooo.example.com" is not a subdomain of "oo.example.com".
Alias: The owner of a CNAME resource record, or a subdomain of the Alias: The owner of a CNAME resource record, or a subdomain of the
owner of a DNAME resource record (DNAME records are defined in owner of a DNAME resource record (DNAME records are defined in
[RFC6672]). See also "canonical name". [RFC6672]). See also "canonical name".
Canonical name: A CNAME resource record "identifies its owner name Canonical name: A CNAME resource record "identifies its owner name
as an alias, and specifies the corresponding canonical name in the as an alias, and specifies the corresponding canonical name in the
RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2) RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2)
This usage of the word "canonical" is related to the mathematical This usage of the word "canonical" is related to the mathematical
concept of "canonical form". concept of "canonical form".
CNAME: "It is traditional to refer to the owner of a CNAME record as CNAME: "It has been traditional to refer to the [owner] of a CNAME
'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of record as 'a CNAME'. This is unfortunate, as 'CNAME' is an
'canonical name', and the owner of a CNAME record is an alias, not abbreviation of 'canonical name', and the [owner] of a CNAME
a canonical name." (Quoted from [RFC2181], Section 10.1.1) record is most certainly not a canonical name." (Quoted from
[RFC2181], Section 10.1.1. The quoted text has been changed from
"label" to "owner".)
3. DNS Response Codes 3. DNS Response Codes
Some of response codes that are defined in [RFC1035] have acquired Some of the response codes (RCODEs) that are defined in [RFC1035]
their own shorthand names. All of the RCODEs are listed at have acquired their own shorthand names. All of the RCODEs are
[IANA_Resource_Registry], although that site uses mixed-case listed at [IANA_Resource_Registry], although that list uses mixed-
capitalization, while most documents use all-caps. Some of the case capitalization, while most documents use all caps. Some of the
common names are described here, but the official list is in the IANA common names for values defined in [RFC1035] are described in this
section. This section also includes an additional RCODE and a
general definition. The official list of all RCODEs is in the IANA
registry. registry.
NOERROR: "No error condition" (Quoted from [RFC1035], NOERROR: This RCODE appears as "No error condition" in Section 4.1.1
Section 4.1.1.) of [RFC1035].
FORMERR: "Format error - The name server was unable to interpret the FORMERR: This RCODE appears as "Format error - The name server was
query." (Quoted from [RFC1035], Section 4.1.1.) unable to interpret the query" in Section 4.1.1 of [RFC1035].
SERVFAIL: "Server failure - The name server was unable to process SERVFAIL: This RCODE appears as "Server failure - The name server
this query due to a problem with the name server." (Quoted from was unable to process this query due to a problem with the name
[RFC1035], Section 4.1.1.) server" in Section 4.1.1 of [RFC1035].
NXDOMAIN: "Name Error - This code signifies that the domain name NXDOMAIN: This RCODE appears as "Name Error [...] this code
referenced in the query does not exist." (Quoted from [RFC1035], signifies that the domain name referenced in the query does not
Section 4.1.1.) [RFC2308] established NXDOMAIN as a synonym for exist." in Section 4.1.1 of [RFC1035]. [RFC2308] established
Name Error. NXDOMAIN as a synonym for Name Error.
NOTIMP: "Not Implemented - The name server does not support the NOTIMP: This RCODE appears as "Not Implemented - The name server
requested kind of query." (Quoted from [RFC1035], Section 4.1.1.) does not support the requested kind of query" in Section 4.1.1 of
[RFC1035].
REFUSED: "Refused - The name server refuses to perform the specified REFUSED: This RCODE appears as "Refused - The name server refuses to
operation for policy reasons. For example, a name server may not perform the specified operation for policy reasons. For example,
wish to provide the information to the particular requester, or a a name server may not wish to provide the information to the
name server may not wish to perform a particular operation (e.g., particular requester, or a name server may not wish to perform a
zone transfer) for particular data." (Quoted from [RFC1035], particular operation (e.g., zone transfer) for particular data."
Section 4.1.1.) in Section 4.1.1 of [RFC1035].
NODATA: "A pseudo RCODE which indicates that the name is valid for NODATA: "A pseudo RCODE which indicates that the name is valid, for
the given class, but there are no records of the given type. A 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 response has to be inferred from the answer." (Quoted from
[RFC2308], Section 1.) "NODATA is indicated by an answer with the [RFC2308], Section 1) "NODATA is indicated by an answer with the
RCODE set to NOERROR and no relevant answers in the answer RCODE set to NOERROR and no relevant answers in the Answer
section. The authority section will contain an SOA record, or section. The authority section will contain an SOA record, or
there will be no NS records there." (Quoted from [RFC2308], there will be no NS records there." (Quoted from [RFC2308],
Section 2.2.) Note that referrals have a similar format to NODATA Section 2.2) Note that referrals have a similar format to NODATA
replies; [RFC2308] explains how to distinguish them. replies; [RFC2308] explains how to distinguish them.
The term "NXRRSET" is sometimes used as a synonym for NODATA. The term "NXRRSET" is sometimes used as a synonym for NODATA.
However, this is a mistake, given that NXRRSET is a specific error However, this is a mistake, given that NXRRSET is a specific error
code defined in [RFC2136]. code defined in [RFC2136].
Negative response: A response that indicates that a particular RRset Negative response: A response that indicates that a particular RRset
does not exist, or whose RCODE indicates the nameserver cannot does not exist or whose RCODE indicates that the nameserver cannot
answer. Sections 2 and 7 of [RFC2308] describe the types of answer. Sections 2 and 7 of [RFC2308] describe the types of
negative responses in detail. negative responses in detail.
4. DNS Transactions 4. DNS Transactions
The header of a DNS message is its first 12 octets. Many of the The header of a DNS message is its first 12 octets. Many of the
fields and flags in the header diagram in Sections 4.1.1 through fields and flags in the diagrams in Sections 4.1.1 through 4.1.3 of
4.1.3 of [RFC1035] are referred to by their names in that diagram. [RFC1035] are referred to by their names in each diagram. For
For example, the response codes are called "RCODEs", the data for a example, the response codes are called "RCODEs", the data for a
record is called the "RDATA", and the authoritative answer bit is record is called the "RDATA", and the authoritative answer bit is
often called "the AA flag" or "the AA bit". often called "the AA flag" or "the AA bit".
Class: A class "identifies a protocol family or instance of a Class: A class "identifies a protocol family or instance of a
protocol" (Quoted from [RFC1034], Section 3.6). "The DNS tags all protocol". (Quoted from [RFC1034], Section 3.6) "The DNS tags all
data with a class as well as the type, so that we can allow data with a class as well as the type, so that we can allow
parallel use of different formats for data of type address." parallel use of different formats for data of type address."
(Quoted from [RFC1034], Section 2.2). In practice, the class for (Quoted from [RFC1034], Section 2.2) In practice, the class for
nearly every query is "IN". There are some queries for "CH", but nearly every query is "IN" (the Internet). There are some queries
they are usually for the purposes of information about the server for "CH" (the Chaos class), but they are usually for the purposes
itself rather than for a different type of address. of information about the server itself rather than for a different
type of address.
QNAME: The most commonly-used rough definition is that the QNAME is QNAME: The most commonly used rough definition is that the QNAME is
a field in the Question section of a query. "A standard query a field in the Question section of a query. "A standard query
specifies a target domain name (QNAME), query type (QTYPE), and specifies a target domain name (QNAME), query type (QTYPE), and
query class (QCLASS) and asks for RRs which match." (Quoted from query class (QCLASS) and asks for RRs which match." (Quoted from
[RFC1034], Section 3.7.1.). Strictly speaking, the definition [RFC1034], Section 3.7.1) Strictly speaking, the definition comes
comes from [RFC1035], Section 4.1.2, where the QNAME is defined in from [RFC1035], Section 4.1.2, where the QNAME is defined in
respect of the Question Section. This definition appears to be respect of the Question section. This definition appears to be
applied consistently: the discussion of inverse queries in section applied consistently: the discussion of inverse queries in
6.4 refers to the "owner name of the query RR and its TTL", Section 6.4.1 refers to the "owner name of the query RR and its
because inverse queries populate the Answer Section and leave the TTL", because inverse queries populate the Answer section and
Question Section empty. (Inverse queries are deprecated in leave the Question section empty. (Inverse queries are deprecated
[RFC3425], and so relevant definitions do not appear in this in [RFC3425]; thus, relevant definitions do not appear in this
document.) document.)
However, [RFC2308] has an alternate definition that puts the QNAME
in the answer (or series of answers) instead of the query. It
defines QNAME as "...the name in the query section of an answer,
or where this resolves to a CNAME, or CNAME chain, the data field
of the last CNAME. The last CNAME in this sense is that which
contains a value which does not resolve to another CNAME." This
definition has a certain internal logic, because of the way CNAME
substitution works and the definition of CNAME. If a name server
does not find an RRset that matches a query, but does find the
same name in the same class with a CNAME record, then the name
server "includes the CNAME record in the response and restarts the
query at the domain name specified in the data field of the CNAME
record." (Quoted from [RFC1034], Section 3.6.2) This is made
explicit in the resolution algorithm outlined in Section 4.3.2 of
[RFC1034], which says to "change QNAME to the canonical name in
the CNAME RR, and go back to step 1" in the case of a CNAME RR.
Since a CNAME record explicitly declares that the owner name is
canonically named what is in the RDATA, then there is a way to
view the new name (i.e., the name that was in the RDATA of the
CNAME RR) as also being the QNAME.
[RFC2308], however, has an alternate definition that puts the However, this creates a kind of confusion because the response to
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 it
finds the same name in the same class with a CNAME record, then
the name server "includes the CNAME record in the response and
restarts the query at the domain name specified in the data field
of the CNAME record." (Quoted from [RFC1034] Section 3.6.2).
This is made explicit in the resolution algorithm outlined in
Section 4.3.2 of [RFC1034], which says to "change QNAME to the
canonical name in the CNAME RR, and go back to step 1" in the case
of a CNAME RR. Since a CNAME record explicitly declares that the
owner name is canonically named what is in the RDATA, then there
is a way to view the new name (i.e. the name that was in the RDATA
of the CNAME RR) as also being the QNAME.
This creates a kind of confusion, however, because the response to
a query that results in CNAME processing contains in the echoed a query that results in CNAME processing contains in the echoed
Question Section one QNAME (the name in the original query), and a 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 second QNAME that is in the data field of the last CNAME. The
confusion comes from the iterative/recursive mode of resolution, confusion comes from the iterative/recursive mode of resolution,
which finally returns an answer that need not actually have the which finally returns an answer that need not actually have the
same owner name as the QNAME contained in the original query. same owner name as the QNAME contained in the original query.
To address this potential confusion, it is helpful to distinguish To address this potential confusion, it is helpful to distinguish
between three meanings: between three meanings:
* QNAME (original): The name actually sent in the Question * QNAME (original): The name actually sent in the Question
Section in the original query, which is always echoed in the section in the original query, which is always echoed in the
(final) reply in the Question Section when the QR bit is set to (final) reply in the Question section when the QR bit is set to
1. 1.
* QNAME (effective): A name actually resolved, which is either * QNAME (effective): A name actually resolved, which is either
the name originally queried, or a name received in a CNAME the name originally queried or a name received in a CNAME chain
chain response. response.
* QNAME (final): The name actually resolved, which is either the * QNAME (final): The name actually resolved, which is either the
name actually queried or else the last name in a CNAME chain name actually queried or else the last name in a CNAME chain
response. response.
Note that, because the definition in [RFC2308] is actually for a Note that, because the definition in [RFC2308] is actually for a
different concept than what was in [RFC1034], it would have been different concept than what was in [RFC1034], it would have been
better if [RFC2308] had used a different name for that concept. better if [RFC2308] had used a different name for that concept.
In general use today, QNAME almost always means what is defined In general use today, QNAME almost always means what is defined
above as "QNAME (original)". above as "QNAME (original)".
Referrals: A type of response in which a server, signaling that it Referrals: A type of response in which a server, signaling that it
is not (completely) authoritative for an answer, provides the is not (completely) authoritative for an answer, provides the
querying resolver with an alternative place to send its query. querying resolver with an alternative place to send its query.
Referrals can be partial. Referrals can be partial.
A referral arises when a server is not performing recursive A referral arises when a server is not performing recursive
service while answering a query. It appears in step 3(b) of the service while answering a query. It appears in step 3(b) of the
skipping to change at page 12, line 43 skipping to change at page 13, line 30
authority section RRset's RDATA contains the name servers authority section RRset's RDATA contains the name servers
specified at the referred-to zone cut. In normal DNS operation, specified at the referred-to zone cut. In normal DNS operation,
this kind of response is required in order to find names beneath a this kind of response is required in order to find names beneath a
delegation. The bare use of "referral" means this kind of delegation. The bare use of "referral" means this kind of
referral, and many people believe that this is the only legitimate referral, and many people believe that this is the only legitimate
kind of referral in the DNS. kind of referral in the DNS.
The second is an upward referral (sometimes described as "root The second is an upward referral (sometimes described as "root
referral"), where the server is not authoritative for any portion referral"), where the server is not authoritative for any portion
of the QNAME. When this happens, the referred-to zone in the of the QNAME. When this happens, the referred-to zone in the
authority section is usually the root zone (.). In normal DNS authority section is usually the root zone ("."). In normal DNS
operation, this kind of response is not required for resolution or operation, this kind of response is not required for resolution or
for correctly answering any query. There is no requirement that for correctly answering any query. There is no requirement that
any server send upward referrals. Some people regard upward any server send upward referrals. Some people regard upward
referrals as a sign of a misconfiguration or error. Upward referrals as a sign of a misconfiguration or error. Upward
referrals always need some sort of qualifier (such as "upward" or referrals always need some sort of qualifier (such as "upward" or
"root"), and are never identified by the bare word "referral". "root") and are never identified simply by the word "referral".
A response that has only a referral contains an empty answer A response that has only a referral contains an empty answer
section. It contains the NS RRset for the referred-to zone in the section. It contains the NS RRset for the referred-to zone in the
authority section. It may contain RRs that provide addresses in Authority section. It may contain RRs that provide addresses in
the additional section. The AA bit is clear. the additional section. The AA bit is clear.
In the case where the query matches an alias, and the server is In the case where the query matches an alias, and the server is
not authoritative for the target of the alias but it is not authoritative for the target of the alias but is authoritative
authoritative for some name above the target of the alias, the for some name above the target of the alias, the resolution
resolution algorithm will produce a response that contains both algorithm will produce a response that contains both the
the authoritative answer for the alias, and also a referral. Such authoritative answer for the alias and a referral. Such a partial
a partial answer and referral response has data in the answer answer and referral response has data in the Answer section. It
section. It has the NS RRset for the referred-to zone in the has the NS RRset for the referred-to zone in the Authority
authority section. It may contain RRs that provide addresses in section. It may contain RRs that provide addresses in the
the additional section. The AA bit is set, because the first name additional section. The AA bit is set, because the first name in
in the answer section matches the QNAME and the server is the Answer section matches the QNAME and the server is
authoritative for that answer (see [RFC1035], Section 4.1.1). authoritative for that answer (see [RFC1035], Section 4.1.1).
5. Resource Records 5. Resource Records
RR: An acronym for resource record. ([RFC1034], Section 3.6.) RR: An acronym for resource record. (See [RFC1034], Section 3.6.)
RRset: A set of resource records "with the same label, class and RRset: A set of resource records "with the same label, class and
type, but with different data". (Definition from [RFC2181], type, but with different data" (according to [RFC2181],
Section 5) Also spelled RRSet in some documents. As a Section 5). Also written as "RRSet" in some documents. As a
clarification, "same label" in this definition means "same owner clarification, "same label" in this definition means "same owner
name". In addition, [RFC2181] states that "the TTLs of all RRs in name". In addition, [RFC2181] states that "the TTLs of all RRs in
an RRSet must be the same". an RRSet must be the same".
Note that RRSIG resource records do not match this definition. Note that RRSIG resource records do not match this definition.
[RFC4035] says: "An RRset MAY have multiple RRSIG RRs associated [RFC4035] says:
with it. Note that as RRSIG RRs are closely tied to the RRsets
whose signatures they contain, RRSIG RRs, unlike all other DNS RR An RRset MAY have multiple RRSIG RRs associated with it. Note
types, do not form RRsets. In particular, the TTL values among that as RRSIG RRs are closely tied to the RRsets whose
RRSIG RRs with a common owner name do not follow the RRset rules signatures they contain, RRSIG RRs, unlike all other DNS RR
described in [RFC2181]." types, do not form RRsets. In particular, the TTL values among
RRSIG RRs with a common owner name do not follow the RRset
rules described in [RFC2181].
Master file: "Master files are text files that contain RRs in text Master file: "Master files are text files that contain RRs in text
form. Since the contents of a zone can be expressed in the form 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 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 zone, though it can be used to list a cache's contents." (Quoted
from [RFC1035], Section 5.) Master files are sometimes called from [RFC1035], Section 5) Master files are sometimes called "zone
"zone files". files".
Presentation format: The text format used in master files. This Presentation format: The text format used in master files. This
format is shown but not formally defined in [RFC1034] and format is shown but not formally defined in [RFC1034] or
[RFC1035]. The term "presentation format" first appears in [RFC1035]. The term "presentation format" first appears in
[RFC4034]. [RFC4034].
EDNS: The extension mechanisms for DNS, defined in [RFC6891]. EDNS: The extension mechanisms for DNS, defined in [RFC6891].
Sometimes called "EDNS0" or "EDNS(0)" to indicate the version Sometimes called "EDNS0" or "EDNS(0)" to indicate the version
number. EDNS allows DNS clients and servers to specify message number. EDNS allows DNS clients and servers to specify message
sizes larger than the original 512 octet limit, to expand the sizes larger than the original 512 octet limit, to expand the
response code space, and to carry additional options that affect response code space and to carry additional options that affect
the handling of a DNS query. the handling of a DNS query.
OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to
contain control information pertaining to the question-and-answer contain control information pertaining to the question-and-answer
sequence of a specific transaction. (Definition from [RFC6891], sequence of a specific transaction. (Definition paraphrased from
Section 6.1.1) It is used by EDNS. [RFC6891], Section 6.1.1.) It is used by EDNS.
Owner: "The domain name where a RR is found" (Quoted from [RFC1034], Owner: "The domain name where the RR is found." (Quoted from
Section 3.6). Often appears in the term "owner name". [RFC1034], Section 3.6) Often appears in the term "owner name".
SOA field names: DNS documents, including the definitions here, SOA field names: DNS documents, including the definitions here,
often refer to the fields in the RDATA of an SOA resource record 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". by field name. "SOA" stands for "start of a zone of authority".
Those fields are defined in Section 3.3.13 of [RFC1035]. The Those fields are defined in Section 3.3.13 of [RFC1035]. The
names (in the order they appear in the SOA RDATA) are MNAME, names (in the order they appear in the SOA RDATA) are MNAME,
RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. Note that the RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. Note that the
meaning of MINIMUM field is updated in Section 4 of [RFC2308]; the meaning of the MINIMUM field is updated in Section 4 of [RFC2308];
new definition is that the MINIMUM field is only "the TTL to be the new definition is that the MINIMUM field is only "the TTL to
used for negative responses". This document tends to use field be used for negative responses". This document tends to use field
names instead of terms that describe the fields. names instead of terms that describe the fields.
TTL: The maximum "time to live" of a resource record. "A TTL value TTL: The maximum "time to live" of a resource record. "A TTL value
is an unsigned number, with a minimum value of 0, and a maximum 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 value of 2147483647. That is, a maximum of 2^31 - 1. When
transmitted, the TTL is encoded in the less significant 31 bits of transmitted, this value shall be encoded in the less significant
the 32 bit TTL field, with the most significant, or sign, bit set 31 bits of the 32 bit TTL field, with the most significant, or
to zero." (Quoted from [RFC2181], Section 8) (Note that [RFC1035] sign, bit set to zero." (Quoted from [RFC2181], Section 8) (Note
erroneously stated that this is a signed integer; that was fixed that [RFC1035] erroneously stated that this is a signed integer;
by [RFC2181].) that was fixed by [RFC2181].)
The TTL "specifies the time interval that the resource record may The TTL "specifies the time interval that the resource record may
be cached before the source of the information should again be be cached before the source of the information should again be
consulted". (Quoted from [RFC1035], Section 3.2.1) Also: "the consulted." (Quoted from [RFC1035], Section 3.2.1) Section 4.1.3
time interval (in seconds) that the resource record may be cached of the same document states: "the time interval (in seconds) that
before it should be discarded". (Quoted from [RFC1035], the resource record may be cached before it should be discarded".
Section 4.1.3). Despite being defined for a resource record, the Despite being defined for a resource record, the TTL of every
TTL of every resource record in an RRset is required to be the resource record in an RRset is required to be the same ([RFC2181],
same ([RFC2181], Section 5.2). Section 5.2).
The reason that the TTL is the maximum time to live is that a 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 cache operator might decide to shorten the time to live for
operational purposes, such as if there is a policy to disallow TTL operational purposes, such as if there is a policy to disallow TTL
values over a certain number. Some servers are known to ignore values over a certain number. Some servers are known to ignore
the TTL on some RRsets (such as when the authoritative data has a the TTL on some RRsets (such as when the authoritative data has a
very short TTL) even though this is against the advice in RFC very short TTL) even though this is against the advice in RFC
1035. An RRset can be flushed from the cache before the end of 1035. An RRset can be flushed from the cache before the end of
the TTL interval, at which point the value of the TTL becomes the TTL interval, at which point, the value of the TTL becomes
unknown because the RRset with which it was associated no longer unknown because the RRset with which it was associated no longer
exists. exists.
There is also the concept of a "default TTL" for a zone, which can There is also the concept of a "default TTL" for a zone, which can
be a configuration parameter in the server software. This is be a configuration parameter in the server software. This is
often expressed by a default for the entire server, and a default 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 for a zone using the $TTL directive in a zone file. The $TTL
directive was added to the master file format by [RFC2308]. directive was added to the master file format by [RFC2308].
Class independent: A resource record type whose syntax and semantics Class independent: A resource record type whose syntax and semantics
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Most resource record types are defined for class 1 (IN, the Most resource record types are defined for class 1 (IN, the
Internet), but many are undefined for other classes. Internet), but many are undefined for other classes.
Address records: Records whose type is A or AAAA. [RFC2181] Address records: Records whose type is A or AAAA. [RFC2181]
informally defines these as "(A, AAAA, etc)". Note that new types informally defines these as "(A, AAAA, etc)". Note that new types
of address records could be defined in the future. of address records could be defined in the future.
6. DNS Servers and Clients 6. DNS Servers and Clients
This section defines the terms used for the systems that act as DNS This section defines the terms used for the systems that act as DNS
clients, DNS servers, or both. In the RFCs, DNS servers are clients, DNS servers, or both. In past RFCs, DNS servers are
sometimes called "name servers", "nameservers", or just "servers". sometimes called "name servers", "nameservers", or just "servers".
There is no formal definition of DNS server, but the RFCs generally There is no formal definition of "DNS server", but RFCs generally
assume that it is an Internet server that listens for queries and assume that it is an Internet server that listens for queries and
sends responses using the DNS protocol defined in [RFC1035] and its sends responses using the DNS protocol defined in [RFC1035] and its
successors. successors.
It is important to note that the terms "DNS server" and "name server" It is important to note that the terms "DNS server" and "name server"
require context in order to understand the services being provided. require context in order to understand the services being provided.
Both authoritative servers and recursive resolvers are often called Both authoritative servers and recursive resolvers are often called
"DNS servers" and "name servers" even though they serve different "DNS servers" and "name servers" even though they serve different
roles (but may be part of the same software package). roles (but may be part of the same software package).
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Stub resolver: A resolver that cannot perform all resolution itself. Stub resolver: A resolver that cannot perform all resolution itself.
Stub resolvers generally depend on a recursive resolver to Stub resolvers generally depend on a recursive resolver to
undertake the actual resolution function. Stub resolvers are undertake the actual resolution function. Stub resolvers are
discussed but never fully defined in Section 5.3.1 of [RFC1034]. discussed but never fully defined in Section 5.3.1 of [RFC1034].
They are fully defined in Section 6.1.3.1 of [RFC1123]. They are fully defined in Section 6.1.3.1 of [RFC1123].
Iterative mode: A resolution mode of a server that receives DNS Iterative mode: A resolution mode of a server that receives DNS
queries and responds with a referral to another server. queries and responds with a referral to another server.
Section 2.3 of [RFC1034] describes this as "The server refers the Section 2.3 of [RFC1034] describes this as "The server refers the
client to another server and lets the client pursue the query". A client to another server and lets the client pursue the query." A
resolver that works in iterative mode is sometimes called an resolver that works in iterative mode is sometimes called an
"iterative resolver". See also "iterative resolution" later in "iterative resolver". See also "iterative resolution" later in
this section. this section.
Recursive mode: A resolution mode of a server that receives DNS Recursive mode: A resolution mode of a server that receives DNS
queries and either responds to those queries from a local cache or queries and either responds to those queries from a local cache or
sends queries to other servers in order to get the final answers sends queries to other servers in order to get the final answers
to the original queries. Section 2.3 of [RFC1034] describes this to the original queries. Section 2.3 of [RFC1034] describes this
as "The first server pursues the query for the client at another as "the first server pursues the query for the client at another
server". Section 4.3.1 of [RFC1034] says "in [recursive] mode the server". Section 4.3.1 of [RFC1034] says: "in [recursive] mode
name server acts in the role of a resolver and returns either an the name server acts in the role of a resolver and returns either
error or the answer, but never referrals." That same section also an error or the answer, but never referrals." That same section
says "The recursive mode occurs when a query with RD set arrives also says:
at a server which is willing to provide recursive service; the
client can verify that recursive mode was used by checking that The recursive mode occurs when a query with RD set arrives at a
both RA and RD are set in the reply." 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 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 name server side (which is what answers the query) and a resolver
side (which performs the resolution function). Systems operating side (which performs the resolution function). Systems operating
in this mode are commonly called "recursive servers". Sometimes in this mode are commonly called "recursive servers". Sometimes
they are called "recursive resolvers". In practice it is not they are called "recursive resolvers". In practice, it is not
possible to know in advance whether the server that one is possible to know in advance whether the server that one is
querying will also perform recursion; both terms can be observed querying will also perform recursion; both terms can be observed
in use interchangeably. in use interchangeably.
Recursive resolver: A resolver that acts in recursive mode. In Recursive resolver: A resolver that acts in recursive mode. In
general, a recursive resolver is expected to cache the answers it general, a recursive resolver is expected to cache the answers it
receives (which would make it a full-service resolver), but some receives (which would make it a full-service resolver), but some
recursive resolvers might not cache. recursive resolvers might not cache.
[RFC4697] tried to differentiate between a recursive resolver and [RFC4697] tried to differentiate between a recursive resolver and
an iterative resolver. an iterative resolver.
Recursive query: A query with the Recursion Desired (RD) bit set to Recursive query: A query with the Recursion Desired (RD) bit set to
1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive 1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive
service is available and is requested by the RD bit in the query, service is available and is requested by the RD bit in the query,
the server uses its resolver to answer the query. (See the server uses its resolver to answer the query. (See
Section 4.3.2 of [RFC1035].) Section 4.3.2 of [RFC1034].)
Non-recursive query: A query with the Recursion Desired (RD) bit set Non-recursive query: A query with the Recursion Desired (RD) bit set
to 0 in the header. A server can answer non-recursive queries to 0 in the header. A server can answer non-recursive queries
using only local information: the response contains either an using only local information: the response contains either an
error, the answer, or a referral to some other server "closer" to error, the answer, or a referral to some other server "closer" to
the answer. (See Section 4.3.1 of [RFC1035].) the answer. (See Section 4.3.1 of [RFC1034].)
Iterative resolution: A name server may be presented with a query Iterative resolution: A name server may be presented with a query
that can only be answered by some other server. The two general that can only be answered by some other server. The two general
approaches to dealing with this problem are "recursive", in which approaches to dealing with this problem are "recursive", in which
the first server pursues the query on behalf of the client at the first server pursues the query on behalf of the client at
another server, and "iterative", in which the server refers the another server, and "iterative", in which the server refers the
client to another server and lets the client pursue the query client to another server and lets the client pursue the query
there. (See Section 2.3 of [RFC1034].) there. (See Section 2.3 of [RFC1034].)
In iterative resolution, the client repeatedly makes non-recursive In iterative resolution, the client repeatedly makes non-recursive
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not be what was intended by "full resolver" in [RFC1035]. This not be what was intended by "full resolver" in [RFC1035]. This
term is not properly defined in any RFC. term is not properly defined in any RFC.
Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this
term to mean a resolver that acts in recursive mode with a cache term to mean a resolver that acts in recursive mode with a cache
(and meets other requirements). (and meets other requirements).
Priming: "The act of finding the list of root servers from a Priming: "The act of finding the list of root servers from a
configuration that lists some or all of the purported IP addresses configuration that lists some or all of the purported IP addresses
of some or all of those root servers." (Quoted from [RFC8109], of some or all of those root servers." (Quoted from [RFC8109],
Section 2.) In order to operate in recursive mode, a resolver Section 2) In order to operate in recursive mode, a resolver needs
needs to know the address of at least one root server. Priming is to know the address of at least one root server. Priming is most
most often done from a configuration setting that contains a list often done from a configuration setting that contains a list of
of authoritative servers for the root zone. authoritative servers for the root zone.
Root hints: "Operators who manage a DNS recursive resolver typically Root hints: "Operators who manage a DNS recursive resolver typically
need to configure a 'root hints file'. This file contains the need to configure a 'root hints file'. This file contains the
names and IP addresses of the authoritative name servers for the names and IP addresses of the authoritative name servers for the
root zone, so the software can bootstrap the DNS resolution root zone, so the software can bootstrap the DNS resolution
process. For many pieces of software, this list comes built into process. For many pieces of software, this list comes built into
the software." (Quoted from [IANA_RootFiles]) This file is often the software." (Quoted from [IANA_RootFiles]) This file is often
used in priming. used in priming.
Negative caching: "The storage of knowledge that something does not Negative caching: "The storage of knowledge that something does not
exist, cannot give an answer, or does not give an answer." exist, cannot or does not give an answer." (Quoted from
(Quoted from [RFC2308], Section 1) [RFC2308], Section 1)
Authoritative server: "A server that knows the content of a DNS zone Authoritative server: "A server that knows the content of a DNS zone
from local knowledge, and thus can answer queries about that zone from local knowledge, and thus can answer queries about that zone
without needing to query other servers." (Quoted from [RFC2182], without needing to query other servers." (Quoted from [RFC2182],
Section 2.) An authoritative server is named in the NS ("name Section 2) An authoritative server is named in the NS ("name
server") record in a zone. It is a system that responds to DNS server") record in a zone. It is a system that responds to DNS
queries with information about zones for which it has been queries with information about zones for which it has been
configured to answer with the AA flag in the response header set 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 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 zones. Note that it is possible for an authoritative server to
respond to a query without the parent zone delegating authority to respond to a query without the parent zone delegating authority to
that server. Authoritative servers also provide "referrals", that server. Authoritative servers also provide "referrals",
usually to child zones delegated from them; these referrals have usually to child zones delegated from them; these referrals have
the AA bit set to 0 and come with referral data in the Authority the AA bit set to 0 and come with referral data in the Authority
and (if needed) the Additional sections. and (if needed) the Additional sections.
Authoritative-only server: A name server that only serves Authoritative-only server: A name server that only serves
authoritative data and ignores requests for recursion. It will authoritative data and ignores requests for recursion. It will
"not normally generate any queries of its own. Instead, it "not normally generate any queries of its own. Instead it answers
answers non-recursive queries from iterative resolvers looking for non-recursive queries from iterative resolvers looking for
information in zones it serves." (Quoted from [RFC4697], information in zones it serves." (Quoted from [RFC4697],
Section 2.4) In this case, "ignores requests for recursion" means Section 2.4) In this case, "ignores requests for recursion" means
"responds to requests for recursion with responses indicating that "responds to requests for recursion with responses indicating that
recursion was not performed". recursion was not performed".
Zone transfer: The act of a client requesting a copy of a zone and Zone transfer: The act of a client requesting a copy of a zone and
an authoritative server sending the needed information. (See an authoritative server sending the needed information. (See
Section 7 for a description of zones.) There are two common Section 7 for a description of zones.) There are two common
standard ways to do zone transfers: the AXFR ("Authoritative standard ways to do zone transfers: the AXFR ("Authoritative
Transfer") mechanism to copy the full zone (described in Transfer") mechanism to copy the full zone (described in
[RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy
only parts of the zone that have changed (described in [RFC1995]). only parts of the zone that have changed (described in [RFC1995]).
Many systems use non-standard methods for zone transfer outside Many systems use non-standard methods for zone transfer outside
the DNS protocol. the DNS protocol.
Slave server: See "Secondary server". Slave server: See "Secondary server".
Secondary server: "An authoritative server which uses zone transfer Secondary server: "An authoritative server which uses zone transfer
to retrieve the zone" (Quoted from [RFC1996], Section 2.1). to retrieve the zone." (Quoted from [RFC1996], Section 2.1)
Secondary servers are also discussed in [RFC1034]. [RFC2182] Secondary servers are also discussed in [RFC1034]. [RFC2182]
describes secondary servers in more detail. Although early DNS describes secondary servers in more detail. Although early DNS
RFCs such as [RFC1996] referred to this as a "slave", the current RFCs such as [RFC1996] referred to this as a "slave", the current
common usage has shifted to calling it a "secondary". common usage has shifted to calling it a "secondary".
Master server: See "Primary server". Master server: See "Primary server".
Primary server: "Any authoritative server configured to be the Primary server: "Any authoritative server configured to be the
source of zone transfer for one or more [secondary] servers" source of zone transfer for one or more [secondary] servers."
(Quoted from [RFC1996], Section 2.1) or, more specifically, "an (Quoted from [RFC1996], Section 2.1) Or, more specifically,
authoritative server configured to be the source of AXFR or IXFR [RFC2136] calls it "an authoritative server configured to be the
data for one or more [secondary] servers" (Quoted from [RFC2136]). source of AXFR or IXFR data for one or more [secondary] servers".
Primary servers are also discussed in [RFC1034]. Although early Primary servers are also discussed in [RFC1034]. Although early
DNS RFCs such as [RFC1996] referred to this as a "master", the DNS RFCs such as [RFC1996] referred to this as a "master", the
current common usage has shifted to "primary". current common usage has shifted to "primary".
Primary master: "The primary master is named in the zone's SOA MNAME Primary master: "The primary master is named in the zone's SOA MNAME
field and optionally by an NS RR". (Quoted from [RFC1996], field and optionally by an NS RR." (Quoted from [RFC1996],
Section 2.1). [RFC2136] defines "primary master" as "Master Section 2.1) [RFC2136] defines "primary master" as "Master server
server at the root of the AXFR/IXFR dependency graph. The primary at the root of the AXFR/IXFR dependency graph. The primary master
master is named in the zone's SOA MNAME field and optionally by an is named in the zone's SOA MNAME field and optionally by an NS RR.
NS RR. There is by definition only one primary master server per There is by definition only one primary master server per zone."
zone."
The idea of a primary master is only used in [RFC1996] and The idea of a primary master is only used in [RFC1996] and
[RFC2136]. A modern interpretation of the term "primary master" [RFC2136]. A modern interpretation of the term "primary master"
is a server that is both authoritative for a zone and that gets 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) its updates to the zone from configuration (such as a master file)
or from UPDATE transactions. or from UPDATE transactions.
Stealth server: This is "like a slave server except not listed in an Stealth server: This is "like a slave server except not listed in an
NS RR for the zone." (Quoted from [RFC1996], Section 2.1) NS RR for the zone." (Quoted from [RFC1996], Section 2.1)
Hidden master: A stealth server that is a primary server for zone Hidden master: A stealth server that is a primary server for zone
transfers. "In this arrangement, the master name server that transfers. "In this arrangement, the master name server that
processes the updates is unavailable to general hosts on the processes the updates is unavailable to general hosts on the
Internet; it is not listed in the NS RRset." (Quoted from Internet; it is not listed in the NS RRset." (Quoted from
[RFC6781], Section 3.4.3). An earlier RFC, [RFC4641], said that [RFC6781], Section 3.4.3) An earlier RFC, [RFC4641], said that the
the hidden master's name "appears in the SOA RRs MNAME field", hidden master's name "appears in the SOA RRs MNAME field",
although in some setups, the name does not appear at all in the although, in some setups, the name does not appear at all in the
public DNS. A hidden master can also be a secondary server for public DNS. A hidden master can also be a secondary server for
the zone itself. the zone itself.
Forwarding: The process of one server sending a DNS query with the Forwarding: The process of one server sending a DNS query with the
RD bit set to 1 to another server to resolve that query. RD bit set to 1 to another server to resolve that query.
Forwarding is a function of a DNS resolver; it is different than Forwarding is a function of a DNS resolver; it is different than
simply blindly relaying queries. simply blindly relaying queries.
[RFC5625] does not give a specific definition for forwarding, but [RFC5625] does not give a specific definition for forwarding, but
describes in detail what features a system that forwards needs to describes in detail what features a system that forwards needs to
support. Systems that forward are sometimes called "DNS proxies", support. Systems that forward are sometimes called "DNS proxies",
but that term has not yet been defined (even in [RFC5625]). but that term has not yet been defined (even in [RFC5625]).
Forwarder: Section 1 of [RFC2308] describes a forwarder as "a Forwarder: Section 1 of [RFC2308] describes a forwarder as "a
nameserver used to resolve queries instead of directly using the nameserver used to resolve queries instead of directly using the
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resolver with the explicit intention of using it to implement the resolver with the explicit intention of using it to implement the
policies. In other cases, policies are imposed without the user policies. In other cases, policies are imposed without the user
of the stub resolver being informed. of the stub resolver being informed.
Open resolver: A full-service resolver that accepts and processes Open resolver: A full-service resolver that accepts and processes
queries from any (or nearly any) client. This is sometimes also queries from any (or nearly any) client. This is sometimes also
called a "public resolver", although the term "public resolver" is called a "public resolver", although the term "public resolver" is
used more with open resolvers that are meant to be open, as used more with open resolvers that are meant to be open, as
compared to the vast majority of open resolvers that are probably compared to the vast majority of open resolvers that are probably
misconfigured to be open. Open resolvers are discussed in misconfigured to be open. Open resolvers are discussed in
[RFC5358] [RFC5358].
Split DNS: The terms "split DNS" and "split-horizon DNS" have long Split DNS: The terms "split DNS" and "split-horizon DNS" have long
been used in the DNS community without formal definition. In been used in the DNS community without formal definition. In
general, they refer to situations in which DNS servers that are general, they refer to situations in which DNS servers that are
authoritative for a particular set of domains provide partly or authoritative for a particular set of domains provide partly or
completely different answers in those domains depending on the completely different answers in those domains depending on the
source of the query. The effect of this is that a domain name source of the query. The effect of this is that a domain name
that is notionally globally unique nevertheless has different that is notionally globally unique nevertheless has different
meanings for different network users. This can sometimes be the meanings for different network users. This can sometimes be the
result of a "view" configuration, described below. result of a "view" configuration, described below.
[RFC2775], Section 3.8 gives a related definition that is too Section 3.8 of [RFC2775] gives a related definition that is too
specific to be generally useful. specific to be generally useful.
View: A configuration for a DNS server that allows it to provide View: A configuration for a DNS server that allows it to provide
different responses depending on attributes of the query, such as different responses depending on attributes of the query, such as
for "split DNS". Typically, views differ by the source IP address for "split DNS". Typically, views differ by the source IP address
of a query, but can also be based on the destination 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 the type of query (such as AXFR), whether it is recursive, and so
on. Views are often used to provide more names or different on. Views are often used to provide more names or different
addresses to queries from "inside" a protected network than to addresses to queries from "inside" a protected network than to
those "outside" that network. Views are not a standardized part those "outside" that network. Views are not a standardized part
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"find all names which have A records of a particular value". "find all names which have A records of a particular value".
Anycast: "The practice of making a particular service address Anycast: "The practice of making a particular service address
available in multiple, discrete, autonomous locations, such that available in multiple, discrete, autonomous locations, such that
datagrams sent are routed to one of several available locations." datagrams sent are routed to one of several available locations."
(Quoted from [RFC4786], Section 2) See [RFC4786] for more detail (Quoted from [RFC4786], Section 2) See [RFC4786] for more detail
on Anycast and other terms that are specific to its use. on Anycast and other terms that are specific to its use.
Instance: "When anycast routing is used to allow more than one Instance: "When anycast routing is used to allow more than one
server to have the same IP address, each one of those servers is server to have the same IP address, each one of those servers is
commonly referred to as an 'instance'." "An instance of a server, commonly referred to as an 'instance'." It goes on to say: "An
such as a root server, is often referred to as an 'Anycast instance of a server, such as a root server, is often referred to
instance'." (Quoted from [RSSAC026]) as an 'Anycast instance'." (Quoted from [RSSAC026])
Privacy-enabling DNS server: "A DNS server that implements DNS over Privacy-enabling DNS server: "A DNS server that implements DNS over
TLS [RFC7858] and may optionally implement DNS over DTLS TLS [RFC7858] and may optionally implement DNS over DTLS
[RFC8094]." (Quoted from [RFC8310], Section 2) Other types of DNS [RFC8094]." (Quoted from [RFC8310], Section 2) Other types of DNS
servers might also be considerd privacy-enabling, such as those servers might also be considered privacy-enabling, such as those
running DNS over HTTPS [I-D.ietf-doh-dns-over-https]. running DNS over HTTPS [RFC8484].
7. Zones 7. Zones
This section defines terms that are used when discussing zones that This section defines terms that are used when discussing zones that
are being served or retrieved. are being served or retrieved.
Zone: "Authoritative information is organized into units called Zone: "Authoritative information is organized into units called
'zones', and these zones can be automatically distributed to the ZONEs, and these zones can be automatically distributed to the
name servers which provide redundant service for the data in a name servers which provide redundant service for the data in a
zone." (Quoted from [RFC1034], Section 2.4) zone." (Quoted from [RFC1034], Section 2.4)
Child: "The entity on record that has the delegation of the domain Child: "The entity on record that has the delegation of the domain
from the Parent." (Quoted from [RFC7344], Section 1.1) from the Parent." (Quoted from [RFC7344], Section 1.1)
Parent: "The domain in which the Child is registered." (Quoted from Parent: "The domain in which the Child is registered." (Quoted from
[RFC7344], Section 1.1) Earlier, "parent name server" was defined [RFC7344], Section 1.1) Earlier, "parent name server" was defined
in [RFC0882] as "the name server that has authority over the place in [RFC0882] as "the name server that has authority over the place
in the domain name space that will hold the new domain". (Note in the domain name space that will hold the new domain". (Note
that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].) that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].)
[RFC0819] also has some description of the relationship between [RFC819] also has some description of the relationship between
parents and children. parents and children.
Origin: Origin:
There are two different uses for this term: There are two different uses for this term:
(a) "The domain name that appears at the top of a zone (just below (a) "The domain name that appears at the top of a zone (just
the cut that separates the zone from its parent). The name of the below the cut that separates the zone from its parent)... The
zone is the same as the name of the domain at the zone's origin." name of the zone is the same as the name of the domain at the
(Quoted from [RFC2181], Section 6.) These days, this sense of zone's origin." (Quoted from [RFC2181], Section 6) These
"origin" and "apex" (defined below) are often used days, this sense of "origin" and "apex" (defined below) are
interchangeably. often used interchangeably.
(b) The domain name within which a given relative domain name (b) The domain name within which a given relative domain name
appears in zone files. Generally seen in the context of appears in zone files. Generally seen in the context of
"$ORIGIN", which is a control entry defined in [RFC1035], "$ORIGIN", which is a control entry defined in [RFC1035],
Section 5.1, as part of the master file format. For example, if Section 5.1, as part of the master file format. For example,
the $ORIGIN is set to "example.org.", then a master file line for if the $ORIGIN is set to "example.org.", then a master file
"www" is in fact an entry for "www.example.org.". line for "www" is in fact an entry for "www.example.org.".
Apex: The point in the tree at an owner of an SOA and corresponding Apex: The point in the tree at an owner of an SOA and corresponding
authoritative NS RRset. This is also called the "zone apex". authoritative NS RRset. This is also called the "zone apex".
[RFC4033] defines it as "the name at the child's side of a zone [RFC4033] defines it as "the name at the child's side of a zone
cut". The "apex" can usefully be thought of as a data-theoretic cut". The "apex" can usefully be thought of as a data-theoretic
description of a tree structure, and "origin" is the name of the description of a tree structure, and "origin" is the name of the
same concept when it is implemented in zone files. The same concept when it is implemented in zone files. The
distinction is not always maintained in use, however, and one can distinction is not always maintained in use, however, and one can
find uses that conflict subtly with this definition. [RFC1034] find uses that conflict subtly with this definition. [RFC1034]
uses the term "top node of the zone" as a synonym of "apex", but uses the term "top node of the zone" as a synonym of "apex", but
skipping to change at page 23, line 29 skipping to change at page 24, line 23
from the top node of the zone down to leaf nodes or nodes above from the top node of the zone down to leaf nodes or nodes above
cuts around the bottom edge of the zone." (Quoted from [RFC1034], cuts around the bottom edge of the zone." (Quoted from [RFC1034],
Section 4.2.1) Note that this definition might inadvertently also Section 4.2.1) Note that this definition might inadvertently also
cause any NS records that appear in the zone to be included, even cause any NS records that appear in the zone to be included, even
those that might not truly be authoritative because there are those that might not truly be authoritative because there are
identical NS RRs below the zone cut. This reveals the ambiguity identical NS RRs below the zone cut. This reveals the ambiguity
in the notion of authoritative data, because the parent-side NS in the notion of authoritative data, because the parent-side NS
records authoritatively indicate the delegation, even though they records authoritatively indicate the delegation, even though they
are not themselves authoritative data. are not themselves authoritative data.
[RFC4033], Section 2, defines "Authoritative RRset" which is [RFC4033], Section 2, defines "Authoritative RRset", which is
related to authoritative data but has a more precise definition. related to authoritative data but has a more precise definition.
Lame delegation: "A lame delegations exists when a nameserver is Lame delegation: "A lame delegations exists [sic] when a nameserver
delegated responsibility for providing nameservice for a zone (via is delegated responsibility for providing nameservice for a zone
NS records) but is not performing nameservice for that 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 (usually because it is not set up as a primary or secondary for
the zone)." (Quoted from [RFC1912], Section 2.8) the zone)." (Quoted from [RFC1912], Section 2.8) Another
definition is that a lame delegation "...happens when a name
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 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 is not a server for that domain. Queries are thus sent to the
wrong servers, who don't know nothing (at least not as expected) wrong servers, who don't know nothing [sic] (at least not as
about the queried domain. Furthermore, sometimes these hosts (if expected) about the queried domain. Furthermore, sometimes these
they exist!) don't even run name servers." (Quoted from hosts (if they exist!) don't even run name servers." (Quoted from
[RFC1713], Section 2.3) [RFC1713], Section 2.3)
Glue records: "[Resource records] which are not part of the Glue records: "...[Resource records] which are not part of the
authoritative data [of the zone], and are address resource records authoritative data [of the zone], and are address RRs for the
for the [name servers in subzones]. These RRs are only necessary [name] servers [in subzones]. These RRs are only necessary if the
if the name server's name is 'below' the cut, and are only used as name server's name is 'below' the cut, and are only used as part
part of a referral response." Without glue "we could be faced of a referral response." Without glue "we could be faced with the
with the situation where the NS RRs tell us that in order to learn situation where the NS RRs tell us that in order to learn a name
a name server's address, we should contact the server using the server's address, we should contact the server using the address
address we wish to learn." (Definition from [RFC1034], we wish to learn." (Quoted from [RFC1034], Section 4.2.1)
Section 4.2.1)
A later definition is that glue "includes any record in a zone A later definition is that glue "includes any record in a zone
file that is not properly part of that zone, including nameserver file that is not properly part of that zone, including nameserver
records of delegated sub-zones (NS records), address records that records of delegated sub-zones (NS records), address records that
accompany those NS records (A, AAAA, etc), and any other stray accompany those NS records (A, AAAA, etc), and any other stray
data that might appear" (Quoted from [RFC2181], Section 5.4.1). data that might appear." (Quoted from [RFC2181], Section 5.4.1)
Although glue is sometimes used today with this wider definition Although glue is sometimes used today with this wider definition
in mind, the context surrounding the [RFC2181] definition suggests in mind, the context surrounding the definition in [RFC2181]
it is intended to apply to the use of glue within the document suggests it is intended to apply to the use of glue within the
itself and not necessarily beyond. document itself and not necessarily beyond.
Bailiwick: "In-bailiwick" is an adjective to describe a name server Bailiwick: "In-bailiwick" is a modifier to describe a name server
whose name is either a subdomain of or (rarely) the same as the whose name is either a subdomain of or (rarely) the same as the
origin of the zone that contains the delegation to the name origin of the zone that contains the delegation to the name
server. In-bailiwick name servers may have glue records in their server. In-bailiwick name servers may have glue records in their
parent zone (using the first of the definitions of "glue records" parent zone (using the first of the definitions of "glue records"
in the definition above). (The term "bailiwick" means the in the definition above). (The word "bailiwick" means the
district or territory where a bailiff or policeman has district or territory where a bailiff or policeman has
jurisdiction.) jurisdiction.)
"In-bailiwick" names are divided into two type of name server "In-bailiwick" names are divided into two types of names for name
names: "in-domain" names and "sibling domain" names. servers: "in-domain" names and "sibling domain" names.
* In-domain: an adjective to describe a name server whose name is * In-domain: a modifier to describe a name server whose name is
either subordinate to or (rarely) the same as the owner name of either subordinate to or (rarely) the same as the owner name of
the NS resource records. An in-domain name server name MUST the NS resource records. An in-domain name server name needs
have glue records or name resolution fails. For example, a to have glue records or name resolution fails. For example, a
delegation for "child.example.com" may have "in-domain" name delegation for "child.example.com" may have "in-domain" name
server name "ns.child.example.com". server name "ns.child.example.com".
* Sibling domain: a name server's name that is either subordinate * Sibling domain: a name server's name that is either subordinate
to or (rarely) the same as the zone origin and not subordinate to or (rarely) the same as the zone origin and not subordinate
to or the same as the owner name of the NS resource records. to or the same as the owner name of the NS resource records.
Glue records for sibling domains are allowed, but not Glue records for sibling domains are allowed, but not
necessary. For example, a delegation for "child.example.com" necessary. For example, a delegation for "child.example.com"
in "example.com" zone may have "sibling" name server name in "example.com" zone may have "sibling" name server name
"ns.another.example.com". "ns.another.example.com".
"Out-of-bailiwick" is the antonym of in-bailiwick. An adjective "Out-of-bailiwick" is the antonym of "in-bailiwick". It is a
to describe a name server whose name is not subordinate to or the modifier to describe a name server whose name is not subordinate
same as the zone origin. Glue records for out-of-bailiwick name to or the same as the zone origin. Glue records for out-of-
servers are useless. Following table shows examples of delegation bailiwick name servers are useless. The following table shows
types. examples of delegation types.
Delegation |Parent|Name Server Name | Type Delegation |Parent|Name Server Name | Type
-----------+------+------------------+----------------------------- -----------+------+------------------+-----------------------------
com | . |a.gtld-servers.net|in-bailiwick / sibling domain com | . |a.gtld-servers.net|in-bailiwick / sibling domain
net | . |a.gtld-servers.net|in-bailiwick / in-domain net | . |a.gtld-servers.net|in-bailiwick / in-domain
example.org| org |ns.example.org |in-bailiwick / in-domain example.org| org |ns.example.org |in-bailiwick / in-domain
example.org| org |ns.ietf.org |in-bailiwick / sibling domain example.org| org |ns.ietf.org |in-bailiwick / sibling domain
example.org| org |ns.example.com |out-of-bailiwick example.org| org |ns.example.com |out-of-bailiwick
example.jp | jp |ns.example.jp |in-bailiwick / in-domain example.jp | jp |ns.example.jp |in-bailiwick / in-domain
example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain
skipping to change at page 25, line 15 skipping to change at page 26, line 4
Delegation |Parent|Name Server Name | Type Delegation |Parent|Name Server Name | Type
-----------+------+------------------+----------------------------- -----------+------+------------------+-----------------------------
com | . |a.gtld-servers.net|in-bailiwick / sibling domain com | . |a.gtld-servers.net|in-bailiwick / sibling domain
net | . |a.gtld-servers.net|in-bailiwick / in-domain net | . |a.gtld-servers.net|in-bailiwick / in-domain
example.org| org |ns.example.org |in-bailiwick / in-domain example.org| org |ns.example.org |in-bailiwick / in-domain
example.org| org |ns.ietf.org |in-bailiwick / sibling domain example.org| org |ns.ietf.org |in-bailiwick / sibling domain
example.org| org |ns.example.com |out-of-bailiwick example.org| org |ns.example.com |out-of-bailiwick
example.jp | jp |ns.example.jp |in-bailiwick / in-domain example.jp | jp |ns.example.jp |in-bailiwick / in-domain
example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain
example.jp | jp |ns.example.com |out-of-bailiwick example.jp | jp |ns.example.com |out-of-bailiwick
Root zone: The zone of a DNS-based tree whose apex is the zero- Root zone: The zone of a DNS-based tree whose apex is the zero-
length label. Also sometimes called "the DNS root". length label. Also sometimes called "the DNS root".
Empty non-terminals (ENT): "Domain names that own no resource Empty non-terminals (ENT): "Domain names that own no resource
records but have subdomains that do." (Quoted from [RFC4592], records but have subdomains that do." (Quoted from [RFC4592],
Section 2.2.2.) A typical example is in SRV records: in the name Section 2.2.2) A typical example is in SRV records: in the name
"_sip._tcp.example.com", it is likely that "_tcp.example.com" has "_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 no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV
RRset. RRset.
Delegation-centric zone: A zone that consists mostly of delegations Delegation-centric zone: A zone that consists mostly of delegations
to child zones. This term is used in contrast to a zone that 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 might have some delegations to child zones but also has many data
resource records for the zone itself and/or for child zones. The resource records for the zone itself and/or for child zones. The
term is used in [RFC4956] and [RFC5155], but is not defined there. term is used in [RFC4956] and [RFC5155], but it is not defined in
either document.
Occluded name: "The addition of a delegation point via dynamic Occluded name: "The addition of a delegation point via dynamic
update will render all subordinate domain names to be in a limbo, update will render all subordinate domain names to be in a limbo,
still part of the zone, but not available to the lookup process. still part of the zone but not available to the lookup process.
The addition of a DNAME resource record has the same impact. The The addition of a DNAME resource record has the same impact. The
subordinate names are said to be 'occluded'." (Quoted from subordinate names are said to be 'occluded'." (Quoted from
[RFC5936], Section 3.5) [RFC5936], Section 3.5)
Fast flux DNS: This "occurs when a domain is found in DNS using A Fast flux DNS: This "occurs when a domain is [found] in DNS using A
records to multiple IP addresses, each of which has a very short records to multiple IP addresses, each of which has a very short
Time-to-Live (TTL) value associated with it. This means that the Time-to-Live (TTL) value associated with it. This means that the
domain resolves to varying IP addresses over a short period of domain resolves to varying IP addresses over a short period of
time." (Quoted from [RFC6561], Section 1.1.5, with typo time." (Quoted from [RFC6561], Section 1.1.5, with a typo
corrected) In addition to having legitimate uses, fast flux DNS corrected) In addition to having legitimate uses, fast flux DNS
can used to deliver malware. Because the addresses change so can used to deliver malware. Because the addresses change so
rapidly, it is difficult to ascertain all the hosts. It should be rapidly, it is difficult to ascertain all the hosts. It should be
noted that the technique also works with AAAA records, but such noted that the technique also works with AAAA records, but such
use is not frequently observed on the Internet as of this writing. use is not frequently observed on the Internet as of this writing.
Reverse DNS, reverse lookup: "The process of mapping an address to a Reverse DNS, reverse lookup: "The process of mapping an address to a
name is generally known as a 'reverse lookup', and the IN- name is generally known as a 'reverse lookup', and the
ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse IN-ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse
DNS'." (Quoted from [RFC5855], Section 1) DNS'." (Quoted from [RFC5855], Section 1)
Forward lookup: "Hostname-to-address translation". (Quoted from Forward lookup: "Hostname-to-address translation". (Quoted from
[RFC2133], Section 6) [RFC3493], Section 6)
arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain
was originally established as part of the initial deployment of was originally established as part of the initial deployment of
the DNS, to provide a transition mechanism from the Host Tables 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 that were common in the ARPANET, as well as a home for the IPv4
reverse mapping domain. During 2000, the abbreviation was reverse mapping domain. During 2000, the abbreviation was
redesignated to 'Address and Routing Parameter Area' in the hope redesignated to 'Address and Routing Parameter Area' in the hope
of reducing confusion with the earlier network name." (Quoted of reducing confusion with the earlier network name." (Quoted
from [RFC3172], Section 2.) .arpa is an "infrastructure domain", a from [RFC3172], Section 2) .arpa is an "infrastructure domain", a
domain whose "role is to support the operating infrastructure of domain whose "role is to support the operating infrastructure of
the Internet". (Quoted from [RFC3172], Section 2.) See [RFC3172] the Internet". (Quoted from [RFC3172], Section 2) See [RFC3172]
for more history of this name. for more history of this name.
Service name: "Service names are the unique key in the Service Name Service name: "Service names are the unique key in the Service Name
and Transport Protocol Port Number registry. This unique symbolic and Transport Protocol Port Number registry. This unique symbolic
name for a service may also be used for other purposes, such as in name for a service may also be used for other purposes, such as in
DNS SRV records." (Quoted from [RFC6335], Section 5.) DNS SRV records." (Quoted from [RFC6335], Section 5)
8. Wildcards 8. Wildcards
Wildcard: [RFC1034] defined "wildcard", but in a way that turned out Wildcard: [RFC1034] defined "wildcard", but in a way that turned out
to be confusing to implementers. For an extended discussion of to be confusing to implementers. For an extended discussion of
wildcards, including clearer definitions, see [RFC4592]. Special wildcards, including clearer definitions, see [RFC4592]. Special
treatment is given to RRs with owner names starting with the label treatment is given to RRs with owner names starting with the label
"*". "Such RRs are called 'wildcards'. Wildcard RRs can be "*". "Such RRs are called 'wildcards'. Wildcard RRs can be
thought of as instructions for synthesizing RRs." (Quoted from thought of as instructions for synthesizing RRs." (Quoted from
[RFC1034], Section 4.3.3) [RFC1034], Section 4.3.3)
Asterisk label: "The first octet is the normal label type and length Asterisk label: "The first octet is the normal label type and length
for a 1-octet-long label, and the second octet is the ASCII for a 1-octet-long label, and the second octet is the ASCII
representation for the '*' character. A descriptive name of a representation [RFC20] for the '*' character. A descriptive name
label equaling that value is an 'asterisk label'." (Quoted from of a label equaling that value is an 'asterisk label'." (Quoted
[RFC4592], Section 2.1.1) from [RFC4592], Section 2.1.1)
Wildcard domain name: "A 'wildcard domain name' is defined by having Wildcard domain name: "A 'wildcard domain name' is defined by having
its initial (i.e., leftmost or least significant) label be its initial (i.e., leftmost or least significant) label, in binary
asterisk label." (Quoted from [RFC4592], Section 2.1.1) format: 0000 0001 0010 1010 (binary) = 0x01 0x2a (hexadecimal)".
(Quoted from [RFC4592], Section 2.1.1) The second octet in this
label is the ASCII representation for the "*" character.
Closest encloser: "The longest existing ancestor of a name." Closest encloser: "The longest existing ancestor of a name."
(Quoted from [RFC5155], Section 1.3) An earlier definition is "The (Quoted from [RFC5155], Section 1.3) An earlier definition is "The
node in the zone's tree of existing domain names that has the most node in the zone's tree of existing domain names that has the most
labels matching the query name (consecutively, counting from the labels matching the query name (consecutively, counting from the
root label downward). Each match is a 'label match' and the order root label downward). Each match is a 'label match' and the order
of the labels is the same." (Quoted from [RFC4592], of the labels is the same." (Quoted from [RFC4592],
Section 3.3.1) Section 3.3.1)
Closest provable encloser: "The longest ancestor of a name that can Closest provable encloser: "The longest ancestor of a name that can
skipping to change at page 27, line 20 skipping to change at page 28, line 13
Section 1.3) See Section 10 for more on "opt-out". Section 1.3) See Section 10 for more on "opt-out".
Next closer name: "The name one label longer than the closest Next closer name: "The name one label longer than the closest
provable encloser of a name." (Quoted from [RFC5155], provable encloser of a name." (Quoted from [RFC5155],
Section 1.3) Section 1.3)
Source of Synthesis: "The source of synthesis is defined in the Source of Synthesis: "The source of synthesis is defined in the
context of a query process as that wildcard domain name context of a query process as that wildcard domain name
immediately descending from the closest encloser, provided that immediately descending from the closest encloser, provided that
this wildcard domain name exists. 'Immediately descending' means this wildcard domain name exists. 'Immediately descending' means
that the source of synthesis has a name of the form: <asterisk that the source of synthesis has a name of the form:
label>.<closest encloser>." (Quoted from [RFC4592], <asterisk label>.<closest encloser>."
Section 3.3.1) (Quoted from [RFC4592], Section 3.3.1)
9. Registration Model 9. Registration Model
Registry: The administrative operation of a zone that allows Registry: The administrative operation of a zone that allows
registration of names within that zone. People often use this registration of names within that zone. People often use this
term to refer only to those organizations that perform term to refer only to those organizations that perform
registration in large delegation-centric zones (such as TLDs); but registration in large delegation-centric zones (such as TLDs); but
formally, whoever decides what data goes into a zone is the formally, whoever decides what data goes into a zone is the
registry for that zone. This definition of "registry" is from a registry for that zone. This definition of "registry" is from a
DNS point of view; for some zones, the policies that determine DNS point of view; for some zones, the policies that determine
skipping to change at page 28, line 17 skipping to change at page 29, line 12
different protocols, particularly RDAP. The WHOIS protocol is different protocols, particularly RDAP. The WHOIS protocol is
also used with IP address registry data. also used with IP address registry data.
RDAP: The Registration Data Access Protocol, defined in [RFC7480], RDAP: The Registration Data Access Protocol, defined in [RFC7480],
[RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The [RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The
RDAP protocol and data format are meant as a replacement for RDAP protocol and data format are meant as a replacement for
WHOIS. WHOIS.
DNS operator: An entity responsible for running DNS servers. For a DNS operator: An entity responsible for running DNS servers. For a
zone's authoritative servers, the registrant may act as their own zone's authoritative servers, the registrant may act as their own
DNS operator, or their registrar may do it on their behalf, or DNS operator, their registrar may do it on their behalf, or they
they may use a third-party operator. For some zones, the registry may use a third-party operator. For some zones, the registry
function is performed by the DNS operator plus other entities who function is performed by the DNS operator plus other entities who
decide about the allowed contents of the zone. decide about the allowed contents of the zone.
Public suffix: "A domain that is controlled by a public registry." Public suffix: "A domain that is controlled by a public registry."
(Quoted from [RFC6265], Section 5.3) A common definition for this (Quoted from [RFC6265], Section 5.3) A common definition for this
term is a domain under which subdomains can be registered by third term is a domain under which subdomains can be registered by third
parties, and on which HTTP cookies (which are described in detail parties and on which HTTP cookies (which are described in detail
in [RFC6265]) should not be set. There is no indication in a in [RFC6265]) should not be set. There is no indication in a
domain name whether it is a public suffix; that can only be domain name whether it is a public suffix; that can only be
determined by outside means. In fact, both a domain and a determined by outside means. In fact, both a domain and a
subdomain of that domain can be public suffixes. subdomain of that domain can be public suffixes.
There is nothing inherent in a domain name to indicate whether it There is nothing inherent in a domain name to indicate whether it
is a public suffix. One resource for identifying public suffixes is a public suffix. One resource for identifying public suffixes
is the Public Suffix List (PSL) maintained by Mozilla is the Public Suffix List (PSL) maintained by Mozilla
(http://publicsuffix.org/). (http://publicsuffix.org/).
For example, at the time this document is published, the "com.au" 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 domain is listed as a public suffix in the PSL. (Note that this
example might change in the future.) example might change in the future.)
Note that the term "public suffix" is controversial in the DNS Note that the term "public suffix" is controversial in the DNS
community for many reasons, and may be significantly changed in community for many reasons, and it may be significantly changed in
the future. One example of the difficulty of calling a domain a the future. One example of the difficulty of calling a domain a
public suffix is that designation can change over time as the public suffix is that designation can change over time as the
registration policy for the zone changes, such as was the case registration policy for the zone changes, such as was the case
with the "uk" TLD in 2014. with the "uk" TLD in 2014.
Subordinate and Superordinate: These terms are introduced in Subordinate and Superordinate: These terms are introduced in
[RFC3731] for use in the registration model, but not defined [RFC5731] for use in the registration model, but not defined
there. Instead, they are given in examples. "For example, domain there. Instead, they are given in examples. "For example, domain
name 'example.com' has a superordinate relationship to host name name 'example.com' has a superordinate relationship to host name
ns1.example.com'." "For example, host ns1.example1.com is a 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 example1.com, but it is a not a
subordinate host of domain example2.com." (Quoted from [RFC3731], subordinate host of domain example2.com." (Quoted from [RFC5731],
Section 1.1.) These terms are strictly ways of referring to the Section 1.1) These terms are strictly ways of referring to the
relationship standing of two domains where one is a subdomain of relationship standing of two domains where one is a subdomain of
the other. the other.
10. General DNSSEC 10. General DNSSEC
Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and
[RFC5155]. The terms that have caused confusion in the DNS community [RFC5155]. The terms that have caused confusion in the DNS community
are highlighted here. are highlighted here.
DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in
some RFCs, have not been formally defined. However, Section 2 of some RFCs, have not been formally defined. However, Section 2 of
[RFC4033] defines many types of resolvers and validators, [RFC4033] defines many types of resolvers and validators,
including "non-validating security-aware stub resolver", "non- including "non-validating security-aware stub resolver",
validating stub resolver", "security-aware name server", "non-validating stub resolver", "security-aware name server",
"security-aware recursive name server", "security-aware resolver", "security-aware recursive name server", "security-aware resolver",
"security-aware stub resolver", and "security-oblivious "security-aware stub resolver", and "security-oblivious
'anything'". (Note that the term "validating resolver", which is 'anything'". (Note that the term "validating resolver", which is
used in some places in DNSSEC-related documents, is also not used in some places in DNSSEC-related documents, is also not
defined in those RFCs, but is defined below.) defined in those RFCs, but is defined below.)
Signed zone: "A zone whose RRsets are signed and that contains Signed zone: "A zone whose RRsets are signed and that contains
properly constructed DNSKEY, Resource Record Signature (RRSIG), properly constructed DNSKEY, Resource Record Signature (RRSIG),
Next Secure (NSEC), and (optionally) DS records." (Quoted from Next Secure (NSEC), and (optionally) DS records." (Quoted from
[RFC4033], Section 2.) It has been noted in other contexts that [RFC4033], Section 2) It has been noted in other contexts that the
the zone itself is not really signed, but all the relevant RRsets zone itself is not really signed, but all the relevant RRsets in
in the zone are signed. Nevertheless, if a zone that should be the zone are signed. Nevertheless, if a zone that should be
signed contains any RRsets that are not signed (or opted out), signed contains any RRsets that are not signed (or opted out),
those RRsets will be treated as bogus, so the whole zone needs to those RRsets will be treated as bogus, so the whole zone needs to
be handled in some way. be handled in some way.
It should also be noted that, since the publication of [RFC6840], It should also be noted that, since the publication of [RFC6840],
NSEC records are no longer required for signed zones: a signed NSEC records are no longer required for signed zones: a signed
zone might include NSEC3 records instead. [RFC7129] provides zone might include NSEC3 records instead. [RFC7129] provides
additional background commentary and some context for the NSEC and additional background commentary and some context for the NSEC and
NSEC3 mechanisms used by DNSSEC to provide authenticated denial- NSEC3 mechanisms used by DNSSEC to provide authenticated denial-
of-existence responses. NSEC and NSEC3 are described below. of-existence responses. NSEC and NSEC3 are described below.
Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that
is not signed". Section 2 of [RFC4035] defines this as "A zone is not signed". Section 2 of [RFC4035] defines this as a "zone
that does not include these records [properly constructed DNSKEY, that does not include these records [properly constructed DNSKEY,
Resource Record Signature (RRSIG), Next Secure (NSEC), and Resource Record Signature (RRSIG), Next Secure (NSEC), and
(optionally) DS records] according to the rules in this section". (optionally) DS records] according to the rules in this
There is an important note at the end of Section 5.2 of [RFC4035] section..." There is an important note at the end of Section 5.2
that defines an additional situation in which a zone is considered of [RFC4035] that defines an additional situation in which a zone
unsigned: "If the resolver does not support any of the algorithms is considered unsigned: "If the resolver does not support any of
listed in an authenticated DS RRset, then the resolver will not be the algorithms listed in an authenticated DS RRset, then the
able to verify the authentication path to the child zone. In this resolver will not be able to verify the authentication path to the
case, the resolver SHOULD treat the child zone as if it were child zone. In this case, the resolver SHOULD treat the child
unsigned." zone as if it were unsigned."
NSEC: "The NSEC record allows a security-aware resolver to NSEC: "The NSEC record allows a security-aware resolver to
authenticate a negative reply for either name or type non- authenticate a negative reply for either name or type
existence with the same mechanisms used to authenticate other DNS non-existence with the same mechanisms used to authenticate other
replies." (Quoted from [RFC4033], Section 3.2.) In short, an DNS replies." (Quoted from [RFC4033], Section 3.2) In short, an
NSEC record provides authenticated denial of existence. NSEC record provides authenticated denial of existence.
"The NSEC resource record lists two separate things: the next "The NSEC resource record lists two separate things: the next
owner name (in the canonical ordering of the zone) that contains owner name (in the canonical ordering of the zone) that contains
authoritative data or a delegation point NS RRset, and the set of authoritative data or a delegation point NS RRset, and the set of
RR types present at the NSEC RR's owner name." (Quoted from RR types present at the NSEC RR's owner name." (Quoted from
Section 4 of RFC 4034) Section 4 of RFC 4034)
NSEC3: Like the NSEC record, the NSEC3 record also provides NSEC3: Like the NSEC record, the NSEC3 record also provides
authenticated denial of existence; however, NSEC3 records mitigate authenticated denial of existence; however, NSEC3 records mitigate
against zone enumeration and support Opt-Out. NSEC3 resource zone enumeration and support Opt-Out. NSEC3 resource records
records require associated NSEC3PARAM resource records. NSEC3 and require associated NSEC3PARAM resource records. NSEC3 and
NSEC3PARAM resource records are defined in [RFC5155]. NSEC3PARAM resource records are defined in [RFC5155].
Note that [RFC6840] says that [RFC5155] "is now considered part of Note that [RFC6840] says that [RFC5155] "is now considered part of
the DNS Security Document Family as described by Section 10 of the DNS Security Document Family as described by Section 10 of
[RFC4033]". This means that some of the definitions from earlier [RFC4033]". This means that some of the definitions from earlier
RFCs that only talk about NSEC records should probably be RFCs that only talk about NSEC records should probably be
considered to be talking about both NSEC and NSEC3. considered to be talking about both NSEC and NSEC3.
Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover
unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1.) unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1)
Opt-out tackles the high costs of securing a delegation to an Opt-out tackles the high costs of securing a delegation to an
insecure zone. When using Opt-Out, names that are an insecure insecure zone. When using Opt-Out, names that are an insecure
delegation (and empty non-terminals that are only derived from delegation (and empty non-terminals that are only derived from
insecure delegations) don't require an NSEC3 record or its insecure delegations) don't require an NSEC3 record or its
corresponding RRSIG records. Opt-Out NSEC3 records are not able corresponding RRSIG records. Opt-Out NSEC3 records are not able
to prove or deny the existence of the insecure delegations. to prove or deny the existence of the insecure delegations.
(Adapted from [RFC7129], Section 5.1) (Adapted from [RFC7129], Section 5.1)
Insecure delegation: "A signed name containing a delegation (NS Insecure delegation: "A signed name containing a delegation (NS
RRset), but lacking a DS RRset, signifying a delegation to an RRset), but lacking a DS RRset, signifying a delegation to an
unsigned subzone." (Quoted from [RFC4956], Section 2.) unsigned subzone." (Quoted from [RFC4956], Section 2)
Zone enumeration: "The practice of discovering the full content of a Zone enumeration: "The practice of discovering the full content of a
zone via successive queries." (Quoted from [RFC5155], zone via successive queries." (Quoted from [RFC5155],
Section 1.3.) This is also sometimes called "zone walking". Zone Section 1.3) This is also sometimes called "zone walking". Zone
enumeration is different from zone content guessing where the enumeration is different from zone content guessing where the
guesser uses a large dictionary of possible labels and sends guesser uses a large dictionary of possible labels and sends
successive queries for them, or matches the contents of NSEC3 successive queries for them, or matches the contents of NSEC3
records against such a dictionary. records against such a dictionary.
Validation: Validation, in the context of DNSSEC, refers to one of Validation: Validation, in the context of DNSSEC, refers to one of
the following: the following:
* Checking the validity of DNSSEC signatures * Checking the validity of DNSSEC signatures,
* Checking the validity of DNS responses, such as those including * Checking the validity of DNS responses, such as those including
authenticated denial of existence authenticated denial of existence, or
* Building an authentication chain from a trust anchor to a DNS * Building an authentication chain from a trust anchor to a DNS
response or individual DNS RRsets in a response response or individual DNS RRsets in a response
The first two definitions above consider only the validity of The first two definitions above consider only the validity of
individual DNSSEC components such as the RRSIG validity or NSEC individual DNSSEC components such as the RRSIG validity or NSEC
proof validity. The third definition considers the components of proof validity. The third definition considers the components of
the entire DNSSEC authentication chain, and thus requires the entire DNSSEC authentication chain; thus, it requires
"configured knowledge of at least one authenticated DNSKEY or DS "configured knowledge of at least one authenticated DNSKEY or DS
RR" (as described in [RFC4035], Section 5). RR" (as described in [RFC4035], Section 5).
[RFC4033], Section 2, says that a "Validating Security-Aware Stub [RFC4033], Section 2, says that a "Validating Security-Aware Stub
Resolver... performs signature validation" and uses a trust anchor Resolver... performs signature validation" and uses a trust anchor
"as a starting point for building the authentication chain to a "as a starting point for building the authentication chain to a
signed DNS response", and thus uses the first and third signed DNS response"; thus, it uses the first and third
definitions above. The process of validating an RRSIG resource definitions above. The process of validating an RRSIG resource
record is described in [RFC4035], Section 5.3. record is described in [RFC4035], Section 5.3.
[RFC5155] refers to validating responses throughout the document, [RFC5155] refers to validating responses throughout the document,
in the context of hashed authenticated denial of existence; this in the context of hashed authenticated denial of existence; this
uses the second definition above. uses the second definition above.
The term "authentication" is used interchangeably with The term "authentication" is used interchangeably with
"validation", in the sense of the third definition above. "validation", in the sense of the third definition above.
[RFC4033], Section 2, describes the chain linking trust anchor to [RFC4033], Section 2, describes the chain linking trust anchor to
DNS data as the "authentication chain". A response is considered DNS data as the "authentication chain". A response is considered
to be authentic if "all RRsets in the Answer and Authority to be authentic if "all RRsets in the Answer and Authority
sections of the response [are considered] to be authentic" (Quoted sections of the response [are considered] to be authentic" (Quoted
from [RFC4035]). DNS data or responses deemed to be authentic or from [RFC4035]) DNS data or responses deemed to be authentic or
validated have a security status of "secure" ([RFC4035], validated have a security status of "secure" ([RFC4035],
Section 4.3; [RFC4033], Section 5). "Authenticating both DNS keys Section 4.3; [RFC4033], Section 5). "Authenticating both DNS keys
and data is a matter of local policy, which may extend or even and data is a matter of local policy, which may extend or even
override the [DNSSEC] protocol extensions" (Quoted from [RFC4033], override the [DNSSEC] protocol extensions..." (Quoted from
Section 3.1). [RFC4033], Section 3.1)
The term "verification", when used, is usually synonym for The term "verification", when used, is usually a synonym for
"validation". "validation".
Validating resolver: A security-aware recursive name server, Validating resolver: A security-aware recursive name server,
security-aware resolver, or security-aware stub resolver that is security-aware resolver, or security-aware stub resolver that is
applying at least one of the definitions of validation (above), as applying at least one of the definitions of validation (above), as
appropriate to the resolution context. For the same reason that appropriate to the resolution context. For the same reason that
the generic term "resolver" is sometimes ambiguous and needs to be the generic term "resolver" is sometimes ambiguous and needs to be
evaluated in context (see Section 6), "validating resolver" is a evaluated in context (see Section 6), "validating resolver" is a
context-sensitive term. context-sensitive term.
skipping to change at page 32, line 25 skipping to change at page 33, line 25
Zone signing key (ZSK): "DNSSEC keys that can be used to sign all Zone signing key (ZSK): "DNSSEC keys that can be used to sign all
the RRsets in a zone that require signatures, other than the apex the RRsets in a zone that require signatures, other than the apex
DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Also note DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Also note
that a ZSK is sometimes used to sign the apex DNSKEY RRset. that a ZSK is sometimes used to sign the apex DNSKEY RRset.
Combined signing key (CSK): "In cases where the differentiation Combined signing key (CSK): "In cases where the differentiation
between the KSK and ZSK is not made, i.e., where keys have the 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 role of both KSK and ZSK, we talk about a Single-Type Signing
Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes
called a "combined signing key" or CSK. It is operational called a "combined signing key" or "CSK". It is operational
practice, not protocol, that determines whether a particular key practice, not protocol, that determines whether a particular key
is a ZSK, a KSK, or a CSK. is a ZSK, a KSK, or a CSK.
Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be
used to distinguish between keys that are intended to be used as used to distinguish between keys that are intended to be used as
the secure entry point into the zone when building chains of 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 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 configured as a trust anchor.... Therefore, it is suggested that
SEP flag be set on keys that are used as KSKs and not on keys that the SEP flag be set on keys that are used as KSKs and not on keys
are used as ZSKs, while in those cases where a distinction between that are used as ZSKs, while in those cases where a distinction
a KSK and ZSK is not made (i.e., for a Single-Type Signing 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." Scheme), it is suggested that the SEP flag be set on all keys."
(Quoted from [RFC6781], Section 3.2.3.) Note that the SEP flag is (Quoted from [RFC6781], Section 3.2.3) Note that the SEP flag is
only a hint, and its presence or absence may not be used to 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 disqualify a given DNSKEY RR from use as a KSK or ZSK during
validation. validation.
The original definition of SEPs was in [RFC3757]. That definition The original definition of SEPs was in [RFC3757]. That definition
clearly indicated that the SEP was a key, not just a bit in the clearly indicated that the SEP was a key, not just a bit in the
key. The abstract of [RFC3757] says: "With the Delegation Signer key. The abstract of [RFC3757] says: "With the Delegation Signer
(DS) resource record (RR), the concept of a public key acting as a (DS) resource record (RR), the concept of a public key acting as a
secure entry point (SEP) has been introduced. During exchanges of secure entry point (SEP) has been introduced. During exchanges of
public keys with the parent there is a need to differentiate SEP public keys with the parent there is a need to differentiate SEP
skipping to change at page 33, line 43 skipping to change at page 34, line 44
maintenance of DNSSEC signatures in zones. There is also stand- maintenance of DNSSEC signatures in zones. There is also stand-
alone software that can be used to sign a zone regardless of alone software that can be used to sign a zone regardless of
whether the authoritative server itself supports signing. whether the authoritative server itself supports signing.
Sometimes signing software can support particular HSMs as part of Sometimes signing software can support particular HSMs as part of
the signing process. the signing process.
11. DNSSEC States 11. DNSSEC States
A validating resolver can determine that a response is in one of four A validating resolver can determine that a response is in one of four
states: secure, insecure, bogus, or indeterminate. These states are states: secure, insecure, bogus, or indeterminate. These states are
defined in [RFC4033] and [RFC4035], although the two definitions defined in [RFC4033] and [RFC4035], although the definitions in the
differ a bit. This document makes no effort to reconcile the two two documents differ a bit. This document makes no effort to
definitions, and takes no position as to whether they need to be reconcile the definitions in the two documents, and takes no position
reconciled. as to whether they need to be reconciled.
Section 5 of [RFC4033] says: Section 5 of [RFC4033] says:
A validating resolver can determine the following 4 states: A validating resolver can determine the following 4 states:
Secure: The validating resolver has a trust anchor, has a chain Secure: The validating resolver has a trust anchor, has a chain
of trust, and is able to verify all the signatures in the of trust, and is able to verify all the signatures in the
response. response.
Insecure: The validating resolver has a trust anchor, a chain Insecure: The validating resolver has a trust anchor, a chain
skipping to change at page 35, line 41 skipping to change at page 36, line 20
when the security-aware resolver is not able to contact when the security-aware resolver is not able to contact
security-aware name servers for the relevant zones. security-aware name servers for the relevant zones.
12. Security Considerations 12. Security Considerations
These definitions do not change any security considerations for the These definitions do not change any security considerations for the
DNS. DNS.
13. IANA Considerations 13. IANA Considerations
None. This document has no IANA actions.
14. References 14. References
14.1. Normative References 14.1. Normative References
[IANA_RootFiles] [IANA_RootFiles]
Internet Assigned Numbers Authority, "IANA Root Files", IANA, "Root Files",
2016, <http://www.iana.org/domains/root/files>. <https://www.iana.org/domains/root/files>.
[RFC0882] Mockapetris, P., "Domain names: Concepts and facilities", [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities",
RFC 882, DOI 10.17487/RFC0882, November 1983, RFC 882, DOI 10.17487/RFC0882, November 1983,
<https://www.rfc-editor.org/info/rfc882>. <https://www.rfc-editor.org/info/rfc882>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>. <https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
skipping to change at page 36, line 48 skipping to change at page 37, line 27
[RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection [RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection
and Operation of Secondary DNS Servers", BCP 16, RFC 2182, and Operation of Secondary DNS Servers", BCP 16, RFC 2182,
DOI 10.17487/RFC2182, July 1997, DOI 10.17487/RFC2182, July 1997,
<https://www.rfc-editor.org/info/rfc2182>. <https://www.rfc-editor.org/info/rfc2182>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/info/rfc2308>. <https://www.rfc-editor.org/info/rfc2308>.
[RFC3731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP) [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and
Domain Name Mapping", RFC 3731, DOI 10.17487/RFC3731, S. Rose, "DNS Security Introduction and Requirements",
March 2004, <https://www.rfc-editor.org/info/rfc3731>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005, RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>. <https://www.rfc-editor.org/info/rfc4033>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and
Rose, "Resource Records for the DNS Security Extensions", S. Rose, "Resource Records for the DNS Security
RFC 4034, DOI 10.17487/RFC4034, March 2005, Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>. <https://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and
Rose, "Protocol Modifications for the DNS Security S. Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>. <https://www.rfc-editor.org/info/rfc4035>.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006, System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
<https://www.rfc-editor.org/info/rfc4592>. <https://www.rfc-editor.org/info/rfc4592>.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008, Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
skipping to change at page 37, line 38 skipping to change at page 38, line 14
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358, Nameservers in Reflector Attacks", BCP 140, RFC 5358,
DOI 10.17487/RFC5358, October 2008, DOI 10.17487/RFC5358, October 2008,
<https://www.rfc-editor.org/info/rfc5358>. <https://www.rfc-editor.org/info/rfc5358>.
[RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)", [RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009, STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,
<https://www.rfc-editor.org/info/rfc5730>. <https://www.rfc-editor.org/info/rfc5730>.
[RFC5731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
Domain Name Mapping", STD 69, RFC 5731,
DOI 10.17487/RFC5731, August 2009,
<https://www.rfc-editor.org/info/rfc5731>.
[RFC5855] Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6 [RFC5855] Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6
Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855, Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855,
May 2010, <https://www.rfc-editor.org/info/rfc5855>. May 2010, <https://www.rfc-editor.org/info/rfc5855>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010, (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>. <https://www.rfc-editor.org/info/rfc5936>.
[RFC6561] Livingood, J., Mody, N., and M. O'Reirdan, [RFC6561] Livingood, J., Mody, N., and M. O'Reirdan,
"Recommendations for the Remediation of Bots in ISP "Recommendations for the Remediation of Bots in ISP
skipping to change at page 38, line 41 skipping to change at page 39, line 21
Terminology", RFC 7719, DOI 10.17487/RFC7719, December Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>. 2015, <https://www.rfc-editor.org/info/rfc7719>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310, for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018, DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>. <https://www.rfc-editor.org/info/rfc8310>.
14.2. Informative References 14.2. Informative References
[I-D.ietf-doh-dns-over-https]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", draft-ietf-doh-dns-over-https-14 (work in
progress), August 2018.
[IANA_Resource_Registry] [IANA_Resource_Registry]
Internet Assigned Numbers Authority, "Resource Record (RR) IANA, "Resource Record (RR) TYPEs",
TYPEs", 2017, <https://www.iana.org/assignments/dns-parameters/>.
<http://www.iana.org/assignments/dns-parameters/>.
[RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for [RFC819] Su, Z. and J. Postel, "The Domain Naming Convention for
Internet User Applications", RFC 819, Internet User Applications", RFC 819,
DOI 10.17487/RFC0819, August 1982, DOI 10.17487/RFC0819, August 1982,
<https://www.rfc-editor.org/info/rfc819>. <https://www.rfc-editor.org/info/rfc819>.
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet [RFC952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, DOI 10.17487/RFC0952, host table specification", RFC 952, DOI 10.17487/RFC0952,
October 1985, <https://www.rfc-editor.org/info/rfc952>. October 1985, <https://www.rfc-editor.org/info/rfc952>.
[RFC1713] Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713, [RFC1713] Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713,
DOI 10.17487/RFC1713, November 1994, DOI 10.17487/RFC1713, November 1994,
<https://www.rfc-editor.org/info/rfc1713>. <https://www.rfc-editor.org/info/rfc1713>.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, [RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996, DOI 10.17487/RFC1995, August 1996,
<https://www.rfc-editor.org/info/rfc1995>. <https://www.rfc-editor.org/info/rfc1995>.
[RFC2133] Gilligan, R., Thomson, S., Bound, J., and W. Stevens,
"Basic Socket Interface Extensions for IPv6", RFC 2133,
DOI 10.17487/RFC2133, April 1997,
<https://www.rfc-editor.org/info/rfc2133>.
[RFC2775] Carpenter, B., "Internet Transparency", RFC 2775, [RFC2775] Carpenter, B., "Internet Transparency", RFC 2775,
DOI 10.17487/RFC2775, February 2000, DOI 10.17487/RFC2775, February 2000,
<https://www.rfc-editor.org/info/rfc2775>. <https://www.rfc-editor.org/info/rfc2775>.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational [RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172, Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>. September 2001, <https://www.rfc-editor.org/info/rfc3172>.
[RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425, [RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425,
DOI 10.17487/RFC3425, November 2002, DOI 10.17487/RFC3425, November 2002,
<https://www.rfc-editor.org/info/rfc3425>. <https://www.rfc-editor.org/info/rfc3425>.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and
W. Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, DOI 10.17487/RFC3493, February 2003,
<https://www.rfc-editor.org/info/rfc3493>.
[RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name [RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name
System KEY (DNSKEY) Resource Record (RR) Secure Entry System KEY (DNSKEY) Resource Record (RR) Secure Entry
Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April
2004, <https://www.rfc-editor.org/info/rfc3757>. 2004, <https://www.rfc-editor.org/info/rfc3757>.
[RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, [RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912,
DOI 10.17487/RFC3912, September 2004, DOI 10.17487/RFC3912, September 2004,
<https://www.rfc-editor.org/info/rfc3912>. <https://www.rfc-editor.org/info/rfc3912>.
[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices", [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
skipping to change at page 42, line 19 skipping to change at page 42, line 38
[RFC7484] Blanchet, M., "Finding the Authoritative Registration Data [RFC7484] Blanchet, M., "Finding the Authoritative Registration Data
(RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March
2015, <https://www.rfc-editor.org/info/rfc7484>. 2015, <https://www.rfc-editor.org/info/rfc7484>.
[RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin, [RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin,
"Inventory and Analysis of WHOIS Registration Objects", "Inventory and Analysis of WHOIS Registration Objects",
RFC 7485, DOI 10.17487/RFC7485, March 2015, RFC 7485, DOI 10.17487/RFC7485, March 2015,
<https://www.rfc-editor.org/info/rfc7485>. <https://www.rfc-editor.org/info/rfc7485>.
[RFC7793] Andrews, M., "Adding 100.64.0.0/10 Prefixes to the IPv4
Locally-Served DNS Zones Registry", BCP 163, RFC 7793,
DOI 10.17487/RFC7793, May 2016,
<https://www.rfc-editor.org/info/rfc7793>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>. 2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram [RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094, Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017, DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>. <https://www.rfc-editor.org/info/rfc8094>.
[RFC8109] Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS [RFC8109] Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS
Resolver with Priming Queries", BCP 209, RFC 8109, Resolver with Priming Queries", BCP 209, RFC 8109,
DOI 10.17487/RFC8109, March 2017, DOI 10.17487/RFC8109, March 2017,
<https://www.rfc-editor.org/info/rfc8109>. <https://www.rfc-editor.org/info/rfc8109>.
[RSSAC026] [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
Root Server System Advisory Committee (RSSAC), "RSSAC (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RSSAC026] Root Server System Advisory Committee (RSSAC), "RSSAC
Lexicon", 2017, Lexicon", 2017,
<https://www.icann.org/en/system/files/files/ <https://www.icann.org/en/system/files/files/
rssac-026-14mar17-en.pdf>. rssac-026-14mar17-en.pdf>.
Appendix A. Definitions Updated by this Document Appendix A. Definitions Updated by This Document
The following definitions from RFCs are updated by this document: The following definitions from RFCs are updated by this document:
o Forwarder in [RFC2308] o Forwarder in [RFC2308]
o QNAME in [RFC2308] o QNAME in [RFC2308]
o Secure Entry Point (SEP) in [RFC3757]; note, however, that this o Secure Entry Point (SEP) in [RFC3757]; note, however, that this
RFC is already obsolete RFC is already obsolete (see [RFC4033], [RFC4034], [RFC4035]).
Appendix B. Definitions First Defined in this Document Appendix B. Definitions First Defined in This Document
The following definitions are first defined in this document: The following definitions are first defined in this document:
o "Alias" in Section 2 o "Alias" in Section 2
o "Apex" in Section 7 o "Apex" in Section 7
o "arpa" in Section 7 o "arpa" in Section 7
o "Bailiwick" in Section 7 o "Bailiwick" in Section 7
skipping to change at page 43, line 33 skipping to change at page 44, line 44
o "DNS operator" in Section 9 o "DNS operator" in Section 9
o "DNSSEC-aware" in Section 10 o "DNSSEC-aware" in Section 10
o "DNSSEC-unaware" in Section 10 o "DNSSEC-unaware" in Section 10
o "Forwarding" in Section 6 o "Forwarding" in Section 6
o "Full resolver" in Section 6 o "Full resolver" in Section 6
o "Fully qualified domain name" in Section 2 o "Fully-qualified domain name" in Section 2
o "Global DNS" in Section 2 o "Global DNS" in Section 2
o "Hardware Security Module (HSM)" in Section 10 o "Hardware Security Module (HSM)" in Section 10
o "Host name" in Section 2 o "Host name" in Section 2
o "IDN" in Section 2 o "IDN" in Section 2
o "In-bailiwick" in Section 7 o "In-bailiwick" in Section 7
o "Iterative resolution" in Section 6 o "Iterative resolution" in Section 6
o "Label" in Section 2 o "Label" in Section 2
o "Locally served DNS zone" in Section 2 o "Locally served DNS zone" in Section 2
o "Naming system" in Section 2 o "Naming system" in Section 2
o "Negative response" in Section 3 o "Negative response" in Section 3
o "Non-recursive query" in Section 6 o "Non-recursive query" in Section 6
o "Open resolver" in Section 6 o "Open resolver" in Section 6
o "Out-of-bailiwick" in Section 7 o "Out-of-bailiwick" in Section 7
o "Passive DNS" in Section 6 o "Passive DNS" in Section 6
skipping to change at page 44, line 41 skipping to change at page 46, line 4
o "Root zone" in Section 7 o "Root zone" in Section 7
o "Secure Entry Point (SEP)" in Section 10 o "Secure Entry Point (SEP)" in Section 10
o "Signing software" in Section 10 o "Signing software" in Section 10
o "Split DNS" in Section 6 o "Split DNS" in Section 6
o "Stub resolver" in Section 6 o "Stub resolver" in Section 6
o "Subordinate" in Section 8 o "Subordinate" in Section 8
o "Superordinate" in Section 8 o "Superordinate" in Section 8
o "TLD" in Section 2 o "TLD" in Section 2
o "Validating resolver" in Section 10 o "Validating resolver" in Section 10
o "Validation" in Section 10 o "Validation" in Section 10
o "View" in Section 6 o "View" in Section 6
o "Zone transfer" in Section 6 o "Zone transfer" in Section 6
Index Index
A A
Address records 15 Address records 16
Alias 9 Alias 9
Anycast 21 Anycast 22
Apex 22 Apex 23
Asterisk label 26 Asterisk label 27
Authoritative data 23 Authoritative data 24
Authoritative server 18 Authoritative server 19
Authoritative-only server 18 Authoritative-only server 19
arpa: Address and Routing Parameter Area Domain 26 arpa: Address and Routing Parameter Area Domain 26
C C
CNAME 9 CNAME 10
Canonical name 9 Canonical name 9
Child 22 Child 22
Class 10 Class 11
Class independent 15 Class independent 16
Closest encloser 26 Closest encloser 27
Closest provable encloser 27 Closest provable encloser 27
Combined signing key (CSK) 32 Combined signing key (CSK) 33
D D
DNS operator 28 DNS operator 29
DNSSEC Policy (DP) 33 DNSSEC Policy (DP) 34
DNSSEC Practice Statement (DPS) 33 DNSSEC Practice Statement (DPS) 34
DNSSEC-aware and DNSSEC-unaware 29 DNSSEC-aware and DNSSEC-unaware 30
Delegation 23 Delegation 24
Delegation-centric zone 25 Delegation-centric zone 26
Domain name 4 Domain name 5
E E
EDNS 14 EDNS 14
EPP 27 EPP 28
Empty non-terminals (ENT) 25 Empty non-terminals (ENT) 26
F F
FORMERR 9 FORMERR 10
Fast flux DNS 25 Fast flux DNS 26
Forward lookup 26 Forward lookup 26
Forwarder 20 Forwarder 21
Forwarding 19 Forwarding 20
Full resolver 17 Full resolver 18
Full-service resolver 17 Full-service resolver 18
Fully qualified domain name (FQDN) 7 Fully-qualified domain name (FQDN) 8
G G
Global DNS 5 Global DNS 5
Glue records 23 Glue records 24
H H
Hardware security module (HSM) 33 Hardware security module (HSM) 34
Hidden master 19 Hidden master 20
Host name 8 Host name 8
I I
IDN 8 IDN 9
In-bailiwick 24 In-bailiwick 25
Insecure delegation 30 Insecure delegation 31
Instance 21 Instance 22
Iterative mode 16 Internationalized Domain Name 9
Iterative resolution 17 Iterative mode 17
Iterative resolution 18
K K
Key signing key (KSK) 32 Key signing key (KSK) 33
L L
Label 5 Label 5
Lame delegation 23 Lame delegation 24
Locally served DNS zone 7 Locally served DNS zone 8
M M
Master file 13 Master file 14
Master server 19 Master server 19
Multicast DNS 7 Multicast DNS 7
mDNS 7
N N
NODATA 10 NODATA 10
NOERROR 9 NOERROR 10
NOTIMP 10 NOTIMP 10
NS 18 NS 19
NSEC 30 NSEC 31
NSEC3 30 NSEC3 31
NXDOMAIN 9 NXDOMAIN 10
Naming system 4 Naming system 4
Negative caching 18 Negative caching 19
Negative response 10 Negative response 11
Next closer name 27 Next closer name 28
Non-recursive query 17 Non-recursive query 18
O O
OPT 14 OPT 14
Occluded name 25 Occluded name 26
Open resolver 20 Open resolver 21
Opt-out 30 Opt-out 31
Origin 22 Origin 23
Out-of-bailiwick 24 Out-of-bailiwick 25
Owner 14 Owner 15
P P
Parent 22 Parent 23
Passive DNS 21 Passive DNS 22
Policy-implementing resolver 20 Policy-implementing resolver 21
Presentation format 13 Presentation format 14
Primary master 19 Primary master 20
Primary server 19 Primary server 20
Priming 17 Priming 18
Privacy-enabling DNS server 21 Privacy-enabling DNS server 22
Private DNS 6 Private DNS 7
Public suffix 28 Public suffix 29
Q Q
QNAME 11 QNAME 11
R R
RDAP 28 RDAP 29
REFUSED 10 REFUSED 10
RR 13 RR 14
RRset 13 RRset 14
Recursive mode 16 Recursive mode 17
Recursive query 17 Recursive query 18
Recursive resolver 17 Recursive resolver 17
Referrals 12 Referrals 13
Registrant 27 Registrant 28
Registrar 27 Registrar 28
Registry 27 Registry 28
Resolver 15 Resolver 16
Reverse DNS, reverse lookup 25 Reverse DNS, reverse lookup 26
Root hints 18 Root hints 18
Root zone 25 Root zone 26
S S
SERVFAIL 9 SERVFAIL 10
SOA 14 SOA 14
SOA field names 14 SOA field names 14
Secondary server 19 Secondary server 19
Secure Entry Point (SEP) 32 Secure Entry Point (SEP) 33
Service name 26 Service name 27
Signed zone 29 Signed zone 30
Signing software 33 Signing software 34
Slave server 18 Slave server 19
Source of Synthesis 27 Source of Synthesis 28
Split DNS 20 Split DNS 21
Split-horizon DNS 20 Split-horizon DNS 21
Stealth server 19 Stealth server 20
Stub resolver 16 Stub resolver 17
Subdomain 9 Subdomain 9
Subordinate 28 Subordinate 29
Superordinate 28 Superordinate 29
T T
TLD 8 TLD 9
TTL 14 TTL 15
Trust anchor 33 Trust anchor 34
U U
Unsigned zone 29 Unsigned zone 30
V V
Validating resolver 32 Validating resolver 33
Validation 31 Validation 32
View 21 View 21
W W
WHOIS 27 WHOIS 28
Wildcard 26 Wildcard 27
Wildcard domain name 26 Wildcard domain name 27
Z Z
Zone 21 Zone 22
Zone cut 22 Zone cut 23
Zone enumeration 30 Zone enumeration 31
Zone signing key (ZSK) 32 Zone signing key (ZSK) 33
Zone transfer 18 Zone transfer 19
Acknowledgements Acknowledgements
The following is the Acknowledgements for RFC 7719. The following is the Acknowledgements section of RFC 7719.
The authors gratefully acknowledge all of the authors of DNS-related The authors gratefully acknowledge all of the authors of DNS-
RFCs that proceed this one. Comments from Tony Finch, Stephane related RFCs that proceed this one. Comments from Tony Finch,
Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John Stephane Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray
Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman, Bellis, John Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque,
David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis, Paul Ebersman, David Lawrence, Matthijs Mekking, Casey Deccio, Bob
John Klensin, David Black, and many others in the DNSOP Working Group Harold, Ed Lewis, John Klensin, David Black, and many others in
helped shape RFC 7719. the DNSOP Working Group helped shape RFC 7719.
Most of the major changes between RFC 7719 and this document came Most of the major changes between RFC 7719 and this document came
from active discussion on the DNSOP WG. Specific people who from active discussion on the DNSOP WG. Specific people who
contributed material to this document include: Bob Harold, Dick contributed material to this document include: Bob Harold, Dick
Franks, Evan Hunt, John Dickinson, Mark Andrews, Martin Hoffmann, Franks, Evan Hunt, John Dickinson, Mark Andrews, Martin Hoffmann,
Paul Vixie, Peter Koch, Duane Wessels, Allison Mankin, Giovane Moura, Paul Vixie, Peter Koch, Duane Wessels, Allison Mankin, Giovane Moura,
Roni Even, Dan Romascanu, and Vladmir Cunat. Roni Even, Dan Romascanu, and Vladmir Cunat.
Authors' Addresses Authors' Addresses
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