RFC 9499

DNS Terminology

📅 published March 2024, from ietf

RFC 9499: DNS Terminology 🎀

okay bestie, let's get into it 💅 this is RFC 9499, titled "DNS Terminology", and it's a Best Current Practice (that's BCP 219 🌸). It came out in March 2024 from the Internet Engineering Task Force (IETF), and the authors are P. Hoffman (ICANN) and K. Fujiwara (JPRS). The ISSN is 2070-1721 ✨

the main drama: it obsoletes RFC 8499 (so that older doc is retired 😭) and it updates RFC 2308 by clarifying "forwarder" and "QNAME" 🧚

Abstract 🦋

The Domain Name System (DNS) is defined in literally dozens of different RFCs. The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has changed in the decades since the DNS was first defined. This document gives current definitions for many of the terms used in the DNS in a single document.

This document updates RFC 2308 by clarifying the definitions of "forwarder" and "QNAME". It obsoletes RFC 8499 by adding multiple terms and clarifications. Comprehensive lists of changed and new definitions can be found in Appendices A and B. 💖

Status of This Memo 📜

This memo documents an Internet Best Current Practice.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9499. 🔗

Copyright Notice 💜

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. ⭐

Table of Contents 🍓

1.  Introduction
2.  Names
3.  DNS Response Codes
4.  DNS Transactions
5.  Resource Records
6.  DNS Servers and Clients
7.  Zones
8.  Wildcards
9.  Registration Model
10. General DNSSEC
11. DNSSEC States
12. Security Considerations
13. IANA Considerations
14. References
  14.1.  Normative References
  14.2.  Informative References
Appendix A.  Definitions Updated by This Document
Appendix B.  Definitions First Defined in This Document
Acknowledgements
Index
Authors' Addresses

1. Introduction 🌈

The Domain Name System (DNS) is a simple query-response protocol whose messages in both directions have the same format. (Section 2 gives a definition of "global DNS", which is often what people mean when they say "the DNS".) The protocol and message format are defined in [RFC1034] and [RFC1035]. These RFCs defined some terms, and later documents defined others. Some of the terms from [RFC1034] and [RFC1035] have somewhat different meanings now than they did in 1987. 🕰️

This document contains a collection of a wide variety of DNS-related terms, organized loosely by topic. Some of them have been precisely defined in earlier RFCs, some have been loosely defined in earlier RFCs, and some are not defined in an earlier RFC at all. 📚

Other organizations sometimes define DNS-related terms in their own way. For example, the WHATWG defines "domain" at <https://url.spec.whatwg.org/>. The Root Server System Advisory Committee (RSSAC) has a good lexicon [RSSAC026]. 🧐

Most of the definitions listed here represent the consensus definition of the DNS community -- both protocol developers and operators. Some of the definitions differ from earlier RFCs, and those differences are noted. In this document, where the consensus definition is the same as the one in an RFC, that RFC is quoted. Where the consensus definition has changed somewhat, the RFC is mentioned but the new stand-alone definition is given. See Appendix A for a list of the definitions that this document updates. ✨

It is important to note that, during the development of this document, it became clear that some DNS-related terms are interpreted quite differently by different DNS experts. Further, some terms that are defined in early DNS RFCs now have definitions that are generally agreed to, but that are different from the original definitions. This document is a small revision to [RFC8499]; that document was a substantial revision to [RFC7719]. 👀

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

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

In this document, the words "byte" and "octet" are used interchangeably. They appear here because they both appear in the earlier RFCs that defined terms in the DNS. 🔁

Readers should note that the terms in this document are grouped by topic. Someone who is not already familiar with the DNS probably cannot learn about the DNS from scratch by reading this document from front to back. Instead, skipping around may be the only way to get enough context to understand some of the definitions. This document has an index that might be useful for readers who are attempting to learn the DNS by reading this document. 🗂️

2. Names 🎀

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

Composition of names
Format of names
Administration of names
Types of data that can be associated with names
Types of metadata for names
Protocol for getting data from a name
Context for resolving a name

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

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

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

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

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

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

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

Format of names: Names in the global DNS are domain names. There are three formats: wire format, presentation format, and common display. 🎨

Wire format: The basic wire format for names in the global DNS is a list of labels ordered by decreasing distance from the root, with the root label last. Each label is preceded by a length octet. [RFC1035] also defines a compression scheme that modifies this format. 📦

Presentation format: The presentation format for names in the global DNS is a list of labels ordered by decreasing distance from the root, encoded as ASCII, with a "." character between each label. In presentation format, a fully qualified domain name includes the root label and the associated separator dot. For example, in presentation format, a fully qualified domain name with two non-root labels is always shown as "example.tld." instead of "example.tld". [RFC1035] defines a method for showing octets that do not display in ASCII. 💅

Common display format 💖 is the one people actually use in apps and free text. It's the same as the presentation format, except showing the root label and the "." before it is optional, and honestly it's rarely done 🙈. So a fully qualified domain name with two non-root labels is usually shown as example.tld instead of example.tld.. The order depends on the writing system: in English and in the C programming language, the root or Top-Level Domain (TLD) label is rightmost in the ordered list, but in Arabic it may be leftmost, depending on local conventions 🌍✨.

Administration of names 👑 is handled by delegation (see the definition of "delegation" in Section 7). Policies for the root zone in the global DNS are set by the names operational community, which convenes itself in the Internet Corporation for Assigned Names and Numbers (ICANN). That community picks the IANA Functions Operator for the global DNS root zone 🧚. The name servers that serve the root zone come from independent root operators. Every other zone in the global DNS gets to have its own admin policies 💅.

Types of data that can be associated with names 🎀: a name can have zero or more resource records attached to it. There are tons of resource record types with their own unique data structures, defined across loads of different RFCs and in the IANA registry at [IANA_Resource_Registry] 📚.

Types of metadata for names 🦋: any name published in the DNS shows up as a set of resource records (see the definition of "RRset" in Section 5). Some names don't have data of their own in the DNS, but they still "appear" there because they're part of a longer name that does have data 🫶. (See the definition of "empty non-terminals" in Section 7.)

Protocol for getting data from a name 🍓: that's the protocol described in [RFC1035].

Context for resolving a name 🌸: the global DNS root zone, distributed by Public Technical Identifiers (PTI).

Private DNS 🔒: these are names that use the protocol described in [RFC1035], but they don't rely on the global DNS root zone. Or they're names that aren't generally available on the Internet but still use the protocol described in [RFC1035]. A system can use the global DNS and one or more private DNS systems at the same time. For example, check out "Split DNS" in Section 6 👀.

Note that domain names that don't appear in the DNS and are never meant to be looked up using the DNS protocol are not part of the global DNS or a private DNS, even though they are domain names. Still names, just not that kind 💀.

Multicast DNS (mDNS) 📡: "Multicast DNS (mDNS) provides the ability to perform DNS-like operations on the local link in the absence of any conventional Unicast DNS server. In addition, Multicast DNS designates a portion of the DNS namespace to be free for local use, without the need to pay any annual fee, and without the need to set up delegations or otherwise configure a conventional DNS server to answer for those names." (Quoted from [RFC6762], Abstract) Okay, the quote is giving everything 🤩. mDNS uses a compatible wire format, but strictly speaking it's a different protocol than DNS. Also, where the quote says "a portion of the DNS namespace", it would be clearer to say "a portion of the domain name space" 🙃. And the names in mDNS aren't meant to be looked up in the DNS.

Locally served DNS zone 🏡: this is a special case of private DNS. Names get resolved using the DNS protocol in a local context. [RFC6303] defines subdomains of IN-ADDR.ARPA that are locally served zones. Resolving names through locally served zones can give ambiguous results, because the same name might resolve to different results in different locally served DNS zone contexts 😭. The context for a locally served DNS zone can be explicit, like the ones listed in [RFC6303] and [RFC7793], or implicit, like the ones defined by local DNS administration and not known to the resolution client.

Fully Qualified Domain Name (FQDN) 💅: this is often just a clear way of saying the same thing as "domain name of a node", as outlined above. But the term is kinda ambiguous 🫠. Strictly speaking, a fully qualified domain name would include every label, including the zero-length label of the root. That name would be written www.example.net. (note the terminating dot). But because every name eventually shares the common root, names are often written relative to the root (like www.example.net) and still get called "fully qualified" ✨. This term first appeared in [RFC819]. In this document, names are often written relative to the root.

The need for the term "fully qualified domain name" comes from partially qualified domain names, which are names where one or more of the last labels in the ordered list are left out. For example, www relative to example.net identifies www.example.net. Those relative names are only understood by context 🧩.

Host name 🖥️: this term and its equivalent, "hostname", have been widely used but aren't defined in [RFC1034], [RFC1035], [RFC1123], or [RFC2181]. The DNS was originally deployed into the Host Tables environment, as outlined in [RFC952], and the term probably followed informally from the definition there. Over time the definition seems to have shifted 👀. "Host name" is often meant to be a domain name that follows the rules in Section 3.5 of [RFC1034], which is also called the "preferred name syntax". (In that syntax, every character in each label is a letter, a digit, or a hyphen.) Any label in a domain name can contain any octet value, though. Hostnames are generally considered to be domain names where every label follows the rules in the "preferred name syntax", with the amendment that labels can start with ASCII digits (this amendment comes from Section 2.1 of [RFC1123]) 🌈.

People also sometimes use "hostname" to mean just the first label of an FQDN, like printer in printer.admin.example.com. (Sometimes this gets formalized in operating system configuration.) And people sometimes use the term for any name that refers to a machine, and those might include labels that don't conform to the "preferred name syntax" 🤷‍♀️.

Top-Level Domain (TLD) 🌏: a Top-Level Domain is a zone that sits one layer below the root, like com or jp. From the point of view of the DNS, there's nothing special about TLDs ✨. Most of them are also delegation-centric zones (defined in Section 7), and there are big policy issues around how they run 📜. TLDs are often split into sub-groups like Country Code Top-Level Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others. That division is a matter of policy and out of scope for this document 🙅‍♀️.

Internationalized Domain Name (IDN) 🌐: the Internationalized Domain Names for Applications (IDNA) protocol is the standard mechanism for handling domain names with non-ASCII characters in applications in the DNS. The current standard at the time of this writing, normally called "IDNA2008", is defined in [RFC5890], [RFC5891], [RFC5892], [RFC5893], and [RFC5894]. These documents define lots of IDN-specific terms, like "LDH label", "A-label", and "U-label" 🧠. [RFC6365] defines more internationalization terms (some of which relate to IDNs), and [RFC6055] has a much more extensive discussion of IDNs, including some new terminology 📖.

Subdomain 🪆: "A domain is a subdomain of another domain if it is contained within that domain. This relationship can be tested by seeing if the subdomain's name ends with the containing domain's name." (Quoted from [RFC1034], Section 3.1) For example, in the host name nnn.mmm.example.com, both mmm.example.com and nnn.mmm.example.com are subdomains of example.com. The comparisons are done on whole labels, so ooo.example.com is not a subdomain of oo.example.com 🙅‍♀️.

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

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

CNAME 😤: "It has been traditional to refer to the [owner] of a CNAME record as 'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of 'canonical name', and the [owner] of a CNAME record is most certainly not a canonical name." (Quoted from [RFC2181], Section 10.1.1. The quoted text has been changed from "label" to "owner".) Honestly, this one is a whole mood 💀💅.

3. DNS Response Codes ✨

okay bestie, so RCODEs are the little response codes that DNS servers send back, and a bunch of them from [RFC1035] have gotten their own nicknames 💅 The full list lives at [IANA_Resource_Registry]. Heads up, that list uses mixed-case capitalization, but most documents write them in ALL CAPS, so don't be confused when you see both 🙈 Some of the common names for values defined in [RFC1035] are covered in this section. This section also adds one more RCODE and a general definition. The official list of every single RCODE is in the IANA registry, so that's the source of truth 🫶

NOERROR: This one shows up as "No error condition" in Section 4.1.1 of [RFC1035]. Everything's fine, no drama 🌸

FORMERR: This one shows up as "Format error - The name server was unable to interpret the query" in Section 4.1.1 of [RFC1035]. The server looked at your message and was like "wait, what is this?" 😭

SERVFAIL: This one shows up as "Server failure - The name server was unable to process this query due to a problem with the name server" in Section 4.1.1 of [RFC1035]. Oof, the server itself is having a rough day 💀

NXDOMAIN: This one shows up as "Name Error [...] this code signifies that the domain name referenced in the query does not exist." in Section 4.1.1 of [RFC1035]. Translation: that name is not real, bestie 🙅‍♀️ [RFC2308] made NXDOMAIN an official synonym for Name Error 🦋

NOTIMP: This one shows up as "Not Implemented - The name server does not support the requested kind of query" in Section 4.1.1 of [RFC1035]. The server just doesn't do that kind of query, and that's valid 🧚

REFUSED: This one shows up as "Refused - The name server refuses to perform the specified operation for policy reasons. For example, a name server may not wish to provide the information to the particular requester, or a name server may not wish to perform a particular operation (e.g., zone transfer) for particular data." in Section 4.1.1 of [RFC1035]. Main character energy from the server: it's choosing not to, and that's a policy decision 👑

NODATA: This is a pseudo RCODE, which basically means it's not a real RCODE on the wire, it's something you infer. The quote goes: "A pseudo RCODE which indicates that the name is valid, for the given class, but [there] are no records of the given type. A NODATA response has to be inferred from the answer." (Quoted from [RFC2308], Section 1) Then: "NODATA is indicated by an answer with the RCODE set to NOERROR and no relevant answers in the Answer section. The Authority section will contain an SOA record, or there will be no NS records there." (Quoted from [RFC2308], Section 2.2) Also, referrals look kind of similar to NODATA replies, and [RFC2308] explains how to tell them apart 🔍

Some people say "NXRRSET" when they mean NODATA, but that's a mistake ngl 🙈 NXRRSET is a specific error code defined in [RFC2136], so it's not the same thing as NODATA 🚫

Negative response: this is a response that says a particular RRset does not exist, or its RCODE tells you the nameserver can't answer. Sections 2 and 7 of [RFC2308] go into the details of the different types of negative responses 🌈

4. DNS Transactions 🦋

okay so the header of a DNS message is its first 12 octets 🎀 A lot of the fields and flags in the diagrams in Sections 4.1.1 through 4.1.3 of [RFC1035] are called by their names in each diagram. For example, response codes get called "RCODEs", the data for a record is called the "RDATA", and the authoritative answer bit is often called "the AA flag" or "the AA bit" 💖

Class: a class "identifies a protocol family or instance of a protocol". (Quoted from [RFC1034], Section 3.6) "The DNS tags all data with a class as well as the type, so that we can allow parallel use of different formats for data of type address." (Quoted from [RFC1034], Section 2.2) In practice, nearly every query uses the class "IN" (the Internet) ✨ There are some queries for "CH" (the Chaos class), but those are usually asking about the server itself, not some different type of address 🙈

QNAME: the most commonly used rough definition is that the QNAME is a field in the Question section of a query. "A standard query specifies a target domain name (QNAME), query type (QTYPE), and query class (QCLASS) and asks for RRs which match." (Quoted from [RFC1034], Section 3.7.1) Strictly speaking, the definition comes from [RFC1035], Section 4.1.2, where the QNAME is defined in respect of the Question section. This definition seems to be applied consistently, since the discussion of inverse queries in Section 6.4.1 of [RFC1035] refers to the "owner name of the query RR and its TTL" because inverse queries fill the Answer section and leave the Question section empty. (Inverse queries are deprecated in [RFC3425]; thus, relevant definitions do not appear in this document.) 🌸

However, [RFC2308] has an alternate definition that puts the QNAME in the answer (or series of answers) instead of the query. It defines QNAME as "...the name in the query section of an answer, or where this resolves to a CNAME, or CNAME chain, the data field of the last CNAME. The last CNAME in this sense is that which contains a value which does not resolve to another CNAME." This definition has a certain internal logic, because of the way CNAME substitution works and the definition of CNAME. If a name server does not find an RRset that matches a query, but does find the same name in the same class with a CNAME record, then the name server "includes the CNAME record in the response and restarts the query at the domain name specified in the data field of the CNAME record." (Quoted from [RFC1034], Section 3.6.2) 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, there is a way to view the new name (i.e., the name that was in the RDATA of the CNAME RR) as also being the QNAME. 🔁

However, this creates confusion, because the response to a query that results in CNAME processing contains, in the echoed Question section, one QNAME (the name in the original query) and a second QNAME that is in the data field of the last CNAME. The confusion comes from the iterative/recursive mode of resolution, which finally returns an answer that does not need to have the same owner name as the QNAME in the original query 😵‍💫

To address this potential confusion, it is helpful to distinguish between three meanings:

QNAME (original): The name actually sent in the Question section in the original query, which is always echoed in the (final) reply in the Question section when the QR bit is set to 1. 📮

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

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

Note that, because the definition in [RFC2308] is actually for a different concept than what was in [RFC1034], it would have been better if [RFC2308] had used a different name for that concept. In general use today, QNAME almost always means what is defined above as "QNAME (original)" 💅

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

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

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

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

A response that has only a referral contains an empty Answer section. It contains the NS RRset for the referred-to zone in the Authority section. It may contain RRs that provide addresses in the Additional section. The AA bit is clear 🫶

In the case where the query matches an alias, and the server is not authoritative for the target of the alias but is authoritative for some name above the target of the alias, the resolution algorithm will produce a response that contains both the authoritative answer for the alias and a referral. Such a partial answer and referral response has data in the Answer section. It has the NS RRset for the referred-to zone in the Authority section. It may contain RRs that provide addresses in the Additional section. The AA bit is set, because the first name in the Answer section matches the QNAME and the server is authoritative for that answer (see [RFC1035], Section 4.1.1). 🧚

5. Resource Records ✨

RR: Okay bestie, RR is short for resource record. 🎀 (See [RFC1034], Section 3.6.)

RRset: A set of resource records "with the same label, class and type, but with different data" (according to [RFC2181], Section 5). It's also written as "RRSet" in some documents. Quick clarification: "same label" here means "same owner name" 💅. [RFC2181] also states that "the TTLs of all RRs in an RRSet must be the same". 🍓

Note that RRSIG resource records do not match this definition 🙈. [RFC4035] says:

An RRset MAY have multiple RRSIG RRs associated with it. Note that as RRSIG RRs are closely tied to the RRsets whose signatures they contain, RRSIG RRs, unlike all other DNS RR types, do not form RRsets. In particular, the TTL values among RRSIG RRs with a common owner name do not follow the RRset rules described in [RFC2181].

Master file: "Master files are text files that contain RRs in text form. Since the contents of a zone can be expressed in the form of a list of RRs a master file is most often used to define a zone, though it can be used to list a cache's contents." (Quoted from [RFC1035], Section 5) Master files are sometimes called "zone files" 📁✨.

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

EDNS: The extension mechanisms for DNS, defined in [RFC6891]. 🧚 It's sometimes called "EDNS0" or "EDNS(0)" to indicate the version number. EDNS lets DNS clients and servers specify message sizes larger than the original 512-octet limit, expand the response code space, and carry additional options that affect the handling of a DNS query. Iconic behaviour, honestly 👑.

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

Owner: "The domain name where the RR is found." (Quoted from [RFC1034], Section 3.6) This often shows up in the term "owner name" 🫶.

SOA field names: DNS documents, including the definitions here, often refer to the fields in the RDATA of an SOA resource record by field name. "SOA" stands for "start of a zone of authority" ⭐. Those fields are defined in Section 3.3.13 of [RFC1035]. The names, in the order they appear in the SOA RDATA, are MNAME, RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. Note that the meaning of the MINIMUM field is updated in Section 4 of [RFC2308]. The new definition is that the MINIMUM field is only "the TTL to be used for negative responses". 🎀 This document tends to use field names instead of terms that describe the fields.

TTL: The maximum "time to live" of a resource record. 💖 "A TTL value is an unsigned number, with a minimum value of 0, and a maximum value of 2147483647. That is, a maximum of 2^31 - 1. When transmitted, this value shall be encoded in the less significant 31 bits of the 32 bit TTL field, with the most significant, or sign, bit set to zero." (Quoted from [RFC2181], Section 8) Note that [RFC1035] erroneously stated that this is a signed integer, and [RFC2181] fixed that 🙈.

The TTL "specifies the time interval that the resource record may be cached before the source of the information should again be consulted." (Quoted from [RFC1035], Section 3.2.1) Section 4.1.3 of [RFC1035] states "the time interval (in seconds) that the resource record may be cached before it should be discarded". Even though it's defined for a resource record, the TTL of every resource record in an RRset is required to be the same ([RFC2181], Section 5.2). 🔥

The reason the TTL is a maximum time to live is that a cache operator might decide to shorten it for operational purposes, for example if there's a policy to disallow TTL values over a certain number 💅. Some servers are known to ignore the TTL on some RRsets (such as when the authoritative data has a very short TTL), even though this is against the advice in [RFC1035] 💀. An RRset can be flushed from the cache before the end of the TTL interval. At that point, the value of the TTL becomes unknown, because the RRset it was associated with no longer exists 🦋.

There's also the concept of a "default TTL" for a zone, which can be a configuration parameter in the server software. This is often expressed as a default for the entire server, and a default for a zone using the $TTL directive in a zone file. The $TTL directive was added to the master file format by [RFC2308] 🌈.

Class independent: A resource record type whose syntax and semantics are the same for every DNS class. A resource record type that is not class independent has different meanings depending on the DNS class of the record, or the meaning is undefined for some classes. Most resource record types are defined for class 1 (IN, the Internet), but many are undefined for other classes 🍓.

Address records: Records whose type is either A or AAAA. [RFC2181] informally defines these as "(A, AAAA, etc)". Note that new types of address records could be defined in the future 🧚.

6. DNS Servers and Clients ✨

This section defines the terms used for the systems that act as DNS clients, DNS servers, or both. 👑 In past RFCs, DNS servers are sometimes called "name servers", "nameservers", or just "servers". There's no formal definition of "DNS server", but RFCs generally assume it's an Internet server that listens for queries and sends responses using the DNS protocol defined in [RFC1035] and its successors 💜.

It's important to note that the terms "DNS server" and "name server" require context to understand the services being provided 🫶. Both authoritative servers and recursive resolvers are often called "DNS servers" and "name servers", even though they serve different roles (but may be part of the same software package).

For terminology specific to the global DNS root server system, see [RSSAC026]. That document defines terms such as "root server", "root server operator", and terms specific to the way the root zone of the global DNS is served 🛰️✨.

Resolver: A program "that extract[s] information from name servers in response to client requests." (Quoted from [RFC1034], Section 2.4) A resolver performs queries for a name, type, and class, and receives responses. The logical function is called "resolution". In practice, the term usually refers to some specific type of resolver (some of which are defined below), so understanding it depends on understanding the context 🎀.

A related term is "resolve", which isn't formally defined in [RFC1034] or [RFC1035]. An imputed definition might be "asking a question that consists of a domain name, class, and type, and receiving some sort of response". Similarly, an imputed definition of "resolution" might be "the response received from resolving" 🦋.

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

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

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

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

A server operating in recursive mode may be thought of as having a name server side (which answers the query) and a resolver side (which performs the resolution function) ✨. Systems operating in this mode are commonly called "recursive servers". Sometimes they're called "recursive resolvers". In practice, it's not possible to know in advance whether the server you're querying will also perform recursion, and both terms get used interchangeably 🤷‍♀️💖.

Recursive resolver: A resolver that acts in recursive mode. In general, a recursive resolver is expected to cache the answers it receives (which would make it a full-service resolver), but some recursive resolvers might not cache 🧁. [RFC4697] tried to differentiate between a recursive resolver and an iterative resolver.

Recursive query: A query with the Recursion Desired (RD) bit set to 1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive service is available and is requested by the RD bit in the query, the server uses its resolver to answer the query. (See Section 4.3.2 of [RFC1034].) 🎀

Non-recursive query: A query with the Recursion Desired (RD) bit set to 0 in the header. A server can answer non-recursive queries using only local information: the response contains either an error, the answer, or a referral to some other server "closer" to the answer. (See Section 4.3.1 of [RFC1034].) 🦋

Iterative resolution: A name server may be presented with a query that can only be answered by some other server. The two general approaches to dealing with this are "recursive", in which the first server pursues the query on behalf of the client at another server, and "iterative", in which the server refers the client to another server and lets the client pursue the query there. (See Section 2.3 of [RFC1034].) 🧚

In iterative resolution, the client repeatedly makes non-recursive queries and follows referrals and/or aliases. The iterative resolution algorithm is described in Section 5.3.3 of [RFC1034] 🌷.

Full resolver: This term is used in [RFC1035], but it isn't defined there. [RFC1123] defines a "full-service resolver" that may or may not be what was intended by "full resolver" in [RFC1035]. This term isn't properly defined in any RFC, and there's no consensus on what it means. Using this term without proper context is discouraged 🙅‍♀️.

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

okay bestie, we're deep in the vocab pile now 🧚 these terms show up everywhere later, so let's get them locked in 🔒

Priming: "The act of finding the list of root servers from a configuration that lists some or all of the purported IP addresses of some or all of those root servers." (Quoted from [RFC8109], Section 2) so basically, to work in recursive mode, a resolver needs to know the address of at least one root server 🌸 priming is usually done from a config setting that holds a list of authoritative servers for the root zone 💅

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

Negative caching: "The storage of knowledge that something does not exist, cannot or does not give an answer." (Quoted from [RFC2308], Section 1) iykyk, it's remembering the "nope" 🙈

Authoritative server: "A server that knows the content of a DNS zone from local knowledge, and thus can answer queries about that zone without needing to query other servers." (Quoted from [RFC2182], Section 2) an authoritative server is named in the NS ("name server") record in a zone 🦋 it's a system that answers DNS queries with info about zones it's been configured to answer for, with the AA flag in the response header set to 1 ✨ it has authority over one or more DNS zones, and it's possible for it to answer a query without the parent zone delegating authority to it, which is lowkey wild 👑 authoritative servers also send "referrals", usually to child zones delegated from them. these referrals have the AA bit set to 0 and come with referral data in the Authority and (if needed) the Additional sections 🧩

Authoritative-only server: a name server that only serves authoritative data and ignores requests for recursion 💀 it "not normally generate any queries of its own. Instead it answers non-recursive queries from iterative resolvers looking for information in zones it serves." (Quoted from [RFC4697], Section 2.4) and in this case, "ignores requests for recursion" means "responds to requests for recursion with responses indicating that recursion was not performed" 🫶

Zone transfer: the act of a client requesting a copy of a zone and an authoritative server sending the needed info. (See Section 7 for a description of zones.) there are two common standard ways to do it: AXFR ("Authoritative Transfer") copies the full zone (described in [RFC5936]), and IXFR ("Incremental Transfer") copies only the parts of the zone that changed (described in [RFC1995]) 🔥 many systems use non-standard methods for zone transfers outside the DNS protocol, which is very much a choice 💅

Slave server: See "Secondary server". 🌈

Secondary server: "An authoritative server which uses zone transfer to retrieve the zone." (Quoted from [RFC1996], Section 2.1) secondary servers are also discussed in [RFC1034], and [RFC2182] describes them in more detail 🍓 early DNS RFCs such as [RFC1996] called this a "slave", but the current common usage has shifted to "secondary" 🫶

Master server: See "Primary server". 🌸

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

Primary master: "The primary master is named in the zone's SOA MNAME field and optionally by an NS RR." (Quoted from [RFC1996], Section 2.1) [RFC2136] defines "primary master" as "Master server at the root of the AXFR/IXFR dependency graph. The primary master is named in the zone's SOA MNAME field and optionally by an NS RR. There is by definition only one primary master server per zone."

the idea of a primary master is only used in [RFC1996] and [RFC2136]. a modern take on "primary master" is a server that is both authoritative for a zone and gets its updates to the zone from configuration (such as a master file) or from UPDATE transactions ✨

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

Hidden master: a stealth server that is a primary server for zone transfers. "In this arrangement, the master name server that processes the updates is unavailable to general hosts on the Internet; it is not listed in the NS RRset." (Quoted from [RFC6781], Section 3.4.3) [RFC4641] said that the hidden master's name "appears in the SOA RRs MNAME field"; however, the name does not appear at all in the global DNS in some setups 🕵️‍♀️ a hidden master can also be a secondary server for the zone itself 🦋

Forwarding: the process of one server sending a DNS query with the RD bit set to 1 to another server to resolve that query. forwarding is a function of a DNS resolver, and it's different from simply blindly relaying queries 💖

[RFC5625] doesn't give a specific definition for forwarding, but it describes in detail what features a system that forwards needs to support. systems that forward are sometimes called "DNS proxies", but that term has not yet been defined (even in [RFC5625]) 🧚

Forwarder: Section 1 of [RFC2308] describes a forwarder as "a nameserver used to resolve queries instead of directly using the authoritative nameserver chain". [RFC2308] further says "The forwarder typically either has better access to the internet, or maintains a bigger cache which may be shared amongst many resolvers." that definition seems to suggest forwarders normally only query authoritative servers. in current use, though, forwarders often stand between stub resolvers and recursive servers 🫧 [RFC2308] is silent on whether a forwarder is iterative-only or can be a full-service resolver 🤷‍♀️

Policy-implementing resolver: a resolver acting in recursive mode that changes some of the answers it returns based on policy criteria, such as to prevent access to malware sites or objectionable content 🛡️ in general, a stub resolver has no idea whether upstream resolvers implement such policy or, if they do, the exact policy about what changes will be made. in some cases, the user of the stub resolver picked the policy-implementing resolver on purpose to implement those policies. in other cases, policies are imposed without the user of the stub resolver being informed 😭

Open resolver: a full-service resolver that accepts and processes queries from any (or nearly any) client. this is sometimes also called a "public resolver", although "public resolver" is used more for open resolvers that are meant to be open, as compared to the vast majority of open resolvers that are probably misconfigured to be open 🌈 open resolvers are discussed in [RFC5358] 🎀

Split DNS: the terms "split DNS" and "split-horizon DNS" have long been used in the DNS community without formal definition. in general, they refer to situations where DNS servers that are authoritative for a particular set of domains provide partly or completely different answers for those domains depending on the source of the query 🪞 the effect is that a domain name that is notionally globally unique has different meanings for different network users 😳 this can sometimes be the result of a "view" configuration, as described below ✨

Section 3.8 of [RFC2775] gives a related definition that is too specific to be generally useful 🙃

View: a configuration for a DNS server that lets it provide different responses depending on attributes of the query, such as for "split DNS". typically, views differ by the source IP address of a query, but they can also be based on the destination IP address, the type of query (such as AXFR), whether it is recursive, and so on 🦋 views are often used to provide more names or different addresses to queries from "inside" a protected network than to those "outside" that network 🏰 views are not a standardized part of the DNS, but they are widely implemented in server software 💅

Passive DNS: a mechanism to collect DNS data by storing DNS responses from name servers. some of these systems also collect the DNS queries associated with the responses, although doing so raises some privacy concerns 🕰️ passive DNS databases can answer historical questions about DNS zones, such as which values were present at a given time in the past, or when a name was first spotted. they also allow searching of stored records on keys other than just the name and type, such as "find all names which have A records of a particular value" 🔍

Anycast: "The practice of making a particular service address available in multiple, discrete, autonomous locations, such that datagrams sent are routed to one of several available locations." (Quoted from [RFC4786], Section 2) see [RFC4786] for more detail on Anycast and other terms specific to its use 🛰️

Instance: "When anycast routing is used to allow more than one server to have the same IP address, each one of those servers is commonly referred to as an 'instance'." It goes on to say: "An instance of a server, such as a root server, is often referred to as an 'Anycast instance'." (Quoted from [RSSAC026]) 🧁

Privacy-enabling DNS server: "A DNS server that implements DNS over TLS [RFC7858] and may optionally implement DNS over DTLS [RFC8094]." (Quoted from [RFC8310], Section 2) other types of DNS servers might also be considered privacy-enabling, such as those running DNS-over-HTTPS [RFC8484] or DNS-over-QUIC [RFC9250] 🔐

DNS-over-TLS (DoT): DNS over TLS as defined in [RFC7858] and its successors. 🔒

DNS-over-HTTPS (DoH): DNS over HTTPS as defined in [RFC8484] and its successors. 🌐

DNS-over-QUIC (DoQ): DNS over QUIC as defined in [RFC9250] and its successors. [RFC9250] specifically defines DoQ as a general-purpose transport for DNS that can be used in stub to recursive, recursive to authoritative, and zone transfer scenarios. 🚀

Classic DNS: DNS over UDP or DNS over TCP as defined in [RFC1035] and its successors. classic DNS applies to DNS communication between stub resolvers and recursive resolvers, and between recursive resolvers and authoritative servers. this has sometimes been called "Do53". classic DNS is not encrypted 👀

Recursive DoT (RDoT): okay so RDoT specifically means DNS-over-TLS for transport between a stub resolver and a recursive resolver, or between a recursive resolver and another recursive resolver 💅. The name is needed because DNS-over-TLS is expected to be defined later as a transport between recursive resolvers and authoritative servers too, so we need a way to tell the two apart 🎀.

Authoritative DoT (ADoT): if DNS-over-TLS does get defined later as a transport between recursive resolvers and authoritative servers, then ADoT is the name for that. It specifically means DNS-over-TLS for transport between recursive resolvers and authoritative servers ✨.

XFR-over-TLS (XoT): this is DNS zone transfer over TLS, as specified in [RFC9103] 🦋. It covers both AXFR over TLS (AXoT) and IXFR over TLS (IXoT) 🫶.

7. Zones ✨

Okay, zones are next 🌸 This section defines the terms we use when we're talking about zones that are being served or retrieved.

Zone: "Authoritative information is organized into units called ZONEs, and these zones can be automatically distributed to the name servers which provide redundant service for the data in a zone." (Quoted from [RFC1034], Section 2.4)

Child: "The entity on record that has the delegation of the domain from the Parent." (Quoted from [RFC7344], Section 1.1)

Parent: the Parent is "The domain in which the Child is registered." (Quoted from [RFC7344], Section 1.1) Way back, "parent name server" was defined in [RFC0882] as "the name server that has authority over the place in the domain name space that will hold the new domain". (Note that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].) [RFC819] also has some description of the relationship between parents and children 👑.

Origin:

There are two different uses for this term 💜

(a) "The domain name that appears at the top of a zone (just below the cut that separates the zone from its parent)... The name of the zone is the same as the name of the domain at the zone's origin." (Quoted from [RFC2181], Section 6) These days, this sense of "origin" and "apex" (defined below) are often used interchangeably.
(b) The domain name within which a given relative domain name appears in zone files. This is generally seen in the context of $ORIGIN, which is a control entry defined in [RFC1035], Section 5.1, as part of the master file format. For example, if the $ORIGIN is set to example.org., then a master file line for www is in fact an entry for www.example.org. 🍓

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

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

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

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

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

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

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

These early definitions do not match the current use of the term "lame delegation", but there is no consensus on what a lame delegation is. The term is used not only for the specific case described above, but for a variety of other flaws in delegations that lead to non-authoritative answers or no answers at all, such as:

a nameserver with an NS record for a zone that does not answer DNS queries;
a nameserver with an IP address that is not reachable by the resolver; and
a nameserver that responds to a query for a specific name with an error or without the authoritative bit set.

Because the term in current usage has drifted from the original definition, and now is not specific or clear as to the intended meaning, it should be considered historic and avoided in favor of terms that are specific and clear 🙅‍♀️

Glue records: "...[Resource records] which are not part of the authoritative data [of the zone], and are address RRs for the [name] servers [in subzones]. These RRs are only necessary if the name server's name is 'below' the cut, and are only used as part of a referral response." Without glue "we could be faced with the situation where the NS RRs tell us that in order to learn a name server's address, we should contact the server using the address we wish to learn." (Quoted from [RFC1034], Section 4.2.1) 💖

A later definition is that glue "includes any record in a zone file that is not properly part of that zone, including nameserver records of delegated sub-zones (NS records), address records that accompany those NS records (A, AAAA, etc), and any other stray data that might appear." (Quoted from [RFC2181], Section 5.4.1) Although glue is sometimes used today with this wider definition in mind, the context surrounding the definition in [RFC2181] suggests it is intended to apply to the use of glue within the document itself and not necessarily beyond.

In an NS record, there are three types of relationships between the owner name of the record, the name in the NS RDATA, and the zone origin: unrelated, in-domain, and sibling domain. The application of these three types of relationships to glue records is defined in [RFC9471] 🦋

An unrelated relationship is one where the NS RDATA contains a name server that is not subordinate to the zone origin and therefore is not part of the same zone.

An in-domain relationship is one where the NS RDATA contains a name server whose name is either subordinate to or (rarely) the same as the owner name of the NS resource records. For example, a delegation for child.example.com might have an in-domain name server called ns.child.example.com.

A sibling domain relationship is one where the NS RDATA contains a name server whose name is either subordinate to or (rarely) the same as the zone origin of the parent and not subordinate to or the same as the owner name of the NS resource records. For example, a delegation for child.example.com in the example.com zone might have a sibling domain name server called ns.another.example.com 🌸

The following table shows examples of delegation types:

  +=============+========+====================+================+
  | Delegation  | Parent | Name Server Name   | Type           |
  +=============+========+====================+================+
  | com         | .      | a.gtld-servers.net | sibling domain |
  +-------------+--------+--------------------+----------------+
  | net         | .      | a.gtld-servers.net | in-domain      |
  +-------------+--------+--------------------+----------------+
  | example.org | org    | ns.example.org     | in-domain      |
  +-------------+--------+--------------------+----------------+
  | example.org | org    | ns.ietf.org        | sibling domain |
  +-------------+--------+--------------------+----------------+
  | example.org | org    | ns.example.com     | unrelated      |
  +-------------+--------+--------------------+----------------+
  | example.jp  | jp     | ns.example.jp      | in-domain      |
  +-------------+--------+--------------------+----------------+
  | example.jp  | jp     | ns.example.ne.jp   | sibling domain |
  +-------------+--------+--------------------+----------------+
  | example.jp  | jp     | ns.example.com     | unrelated      |
  +-------------+--------+--------------------+----------------+

                             Table 1

Bailiwick: "In-bailiwick" and "Out-of-bailiwick" are modifiers used to describe the relationship between a zone and the name servers for that zone. The dictionary definition of bailiwick has been observed to cause more confusion than meaning for this use. These terms should be considered historic in nature 🫠

Root zone: the zone of a DNS-based tree whose apex is the zero-length label. Also sometimes called "the DNS root" 🌍

Empty non-terminals (ENTs): "Domain names that own no resource records but have subdomains that do." (Quoted from [RFC4592], Section 2.2.2) A typical example is in SRV records: in the name _sip._tcp.example.com, it is likely that _tcp.example.com has no RRsets, but that _sip._tcp.example.com has (at least) an SRV RRset 🧠

Delegation-centric zone: a zone that consists mostly of delegations to child zones. This term is used in contrast to a zone that might have some delegations to child zones but also has many data resource records for the zone itself and/or for child zones. The term is used in [RFC4956] and [RFC5155], but it is not defined in either document ✨

Occluded name: "The addition of a delegation point via dynamic update will render all subordinate domain names to be in a limbo, still part of the zone but not available to the lookup process. The addition of a DNAME resource record has the same impact. The subordinate names are said to be 'occluded'." (Quoted from [RFC5936], Section 3.5) 🙈💀

Fast flux DNS: This "occurs when a domain is [found] in DNS using A records to multiple IP addresses, each of which has a very short Time-to-Live (TTL) value associated with it. This means that the domain resolves to varying IP addresses over a short period of time." (Quoted from [RFC6561], Section 1.1.5, with a typo corrected) In addition to having legitimate uses, fast flux DNS can be used to deliver malware. Because the addresses change so rapidly, it is difficult to ascertain all the hosts. It should be noted that the technique also works with AAAA records, but such use is not frequently observed on the Internet as of this writing. 🌈⚡

Reverse DNS, reverse lookup: "The process of mapping an address to a name is generally known as a 'reverse lookup', and the IN-ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse DNS'." (Quoted from [RFC5855], Section 1) 🔄✨

Forward lookup: "Hostname-to-address translation". (Quoted from [RFC3493], Section 6) ➡️🍓

arpa (Address and Routing Parameter Area Domain): "The 'arpa' domain was originally established as part of the initial deployment of the DNS to provide a transition mechanism from the Host Tables that were common in the ARPANET, as well as a home for the IPv4 reverse mapping domain. During 2000, the abbreviation was redesignated to 'Address and Routing Parameter Area' in the hope of reducing confusion with the earlier network name." (Quoted from [RFC3172], Section 2) .arpa is an "infrastructure domain", a domain whose "role is to support the operating infrastructure of the Internet". (Quoted from [RFC3172], Section 2) See [RFC3172] for more history of this name. 🏛️🧚

Service name: "Service names are the unique key in the Service Name and Transport Protocol Port Number registry. This unique symbolic name for a service may also be used for other purposes, such as in DNS SRV records." (Quoted from [RFC6335], Section 5) 🎀💜

8. Wildcards ✨

Wildcard: [RFC1034] defined "wildcard", but in a way that turned out to be confusing to implementers. For an extended discussion of wildcards, including clearer definitions, see [RFC4592]. Special treatment is given to RRs with owner names starting with the label "*". "Such RRs are called 'wildcards'. Wildcard RRs can be thought of as instructions for synthesizing RRs." (Quoted from [RFC1034], Section 4.3.3) 🃏🦋

Asterisk label: "The first octet is the normal label type and length for a 1-octet-long label, and the second octet is the ASCII representation [RFC20] for the '*' character. A descriptive name of a label equaling that value is an 'asterisk label'." (Quoted from [RFC4592], Section 2.1.1) ⭐

Wildcard domain name: "A 'wildcard domain name' is defined by having its initial (i.e., leftmost or least significant) label, in binary format: 0000 0001 0010 1010 (binary) = 0x01 0x2a (hexadecimal)". (Quoted from [RFC4592], Section 2.1.1) The second octet in this label is the ASCII representation for the "*" character. 💅

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

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

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

Source of Synthesis: "The source of synthesis is defined in the context of a query process as that wildcard domain name immediately descending from the closest encloser, provided that this wildcard domain name exists. 'Immediately descending' means that the source of synthesis has a name of the form:

<asterisk label>.<closest encloser>.

(Quoted from [RFC4592], Section 3.3.1) 🧠✨

9. Registration Model 💅

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

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

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

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

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

RDAP: The Registration Data Access Protocol, defined in [RFC7480], [RFC7481], [RFC7485], [RFC9082], [RFC9083], and [RFC9224]. The RDAP protocol and data format are meant as a replacement for WHOIS. 🆕✨

DNS operator: An entity responsible for running DNS servers. For a zone's authoritative servers, the registrant may act as their own DNS operator, their registrar may do it on their behalf, or they may use a third-party operator. For some zones, the registry function is performed by the DNS operator plus other entities who decide about the allowed contents of the zone. 🖥️🦋

Public suffix: "A domain that is controlled by a public registry." (Quoted from [RFC6265], Section 5.3) A common definition for this term is a domain under which subdomains can be registered by third parties and on which HTTP cookies (which are described in detail in [RFC6265]) should not be set. There is no indication in a domain name whether it is a public suffix; that can only be determined by outside means. In fact, both a domain and a subdomain of that domain can be public suffixes.

There is nothing inherent in a domain name to indicate whether it is a public suffix. One resource for identifying public suffixes is the Public Suffix List (PSL) maintained by Mozilla <https://publicsuffix.org/>.

For example, at the time this document is published, the "com.au" domain is listed as a public suffix in the PSL. (Note that this example might change in the future.)

Note that the term "public suffix" is controversial in the DNS community for many reasons, and it may be significantly changed in the future. One example of the difficulty of calling a domain a public suffix is that designation can change over time as the registration policy for the zone changes, such as was the case with the "uk" TLD in 2014. 🌍🍪

Subordinate and Superordinate: These terms are introduced in [RFC5731] for use in the registration model, but not defined there. Instead, they are given in examples. "For example, domain name 'example.com' has a superordinate relationship to host name ns1.example.com'... For example, host ns1.example1.com is a subordinate host of domain example1.com, but it is a not a subordinate host of domain example2.com." (Quoted from [RFC5731], Section 1.1) These terms are strictly ways of referring to the relationship standing of two domains where one is a subdomain of the other. 👯‍♀️💖

10. General DNSSEC 🔐

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

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

Signed zone: "A zone whose RRsets are signed and that contains properly constructed DNSKEY, Resource Record Signature (RRSIG), Next Secure (NSEC), and (optionally) DS records." (Quoted from [RFC4033], Section 2) It has been noted in other contexts that the zone itself is not really signed, but all the relevant RRsets in the zone are signed. Nevertheless, if a zone that should be signed contains any RRsets that are not signed (or opted out), those RRsets will be treated as bogus, so the whole zone needs to be handled in some way. 🔏

It should also be noted that, since the publication of [RFC6840], NSEC records are no longer required for signed zones: a signed zone might include NSEC3 records instead. [RFC7129] provides additional background commentary and some context for the NSEC and NSEC3 mechanisms used by DNSSEC to provide authenticated denial-of-existence responses. NSEC and NSEC3 are described below. 🙌

Online signing: [RFC4470] defines "on-line signing" (note the hyphen) as "generating and signing these records on demand", where "these" was defined as NSEC records. The current definition expands that to generating and signing RRSIG, NSEC, and NSEC3 records on demand. ⚡💫

Unsigned zone 🔓: okay so, [RFC4033] says a zone is unsigned if it is "a zone that is not signed". [RFC4035] goes a bit deeper and calls it a "zone that does not include these records [properly constructed DNSKEY, Resource Record Signature (RRSIG), Next Secure (NSEC), and (optionally) DS records] according to the rules in this section...". There's also a sneaky extra case at the end of Section 5.2 of [RFC4035]: "If the resolver does not support any of the algorithms listed in an authenticated DS RRset, then the resolver will not be able to verify the authentication path to the child zone. In this case, the resolver SHOULD treat the child zone as if it were unsigned." So a zone can count as unsigned just because your resolver can't handle the algorithms! Iconic 💀

NSEC 🕳️: the NSEC record lets a security-aware resolver authenticate a negative reply, meaning proof that a name or type doesn't exist, using the same mechanisms it uses for every other DNS reply. (Quoted from [RFC4033], Section 3.2.) Short version: NSEC gives you authenticated denial of existence. ✨

"The NSEC resource record lists two separate things: the next owner name (in the canonical ordering of the zone) that contains authoritative data or a delegation point NS RRset, and the set of RR types present at the NSEC RR's owner name." (Quoted from [RFC4034], Section 4.)

NSEC3 🔐: NSEC3 also gives you authenticated denial of existence, but it's extra careful because it mitigates zone enumeration and supports Opt-Out 🙈. NSEC3 resource records need associated NSEC3PARAM resource records. Both are defined in [RFC5155].

Note that [RFC6840] says [RFC5155] "is now considered part of the DNS Security Document Family as described by Section 10 of [RFC4033]". So any definitions from older RFCs that only talk about NSEC records should probably be read as talking about both NSEC and NSEC3. Main character energy for NSEC3 💜

Opt-out 🚪: "The Opt-Out Flag indicates whether this NSEC3 RR may cover unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1.) Opt-out is all about the high cost of securing a delegation to an insecure zone. When you use it, names that are an insecure delegation (and empty non-terminals that only come from insecure delegations) don't need an NSEC3 record or its corresponding RRSIG records. The catch: Opt-Out NSEC3 records can't prove or deny the existence of those insecure delegations. (Adapted from [RFC7129], Section 5.1.) Tradeoffs, bestie 🫶

Insecure delegation 🧩: "A signed name containing a delegation (NS RRset), but lacking a DS RRset, signifying a delegation to an unsigned subzone." (Quoted from [RFC4956], Section 2.)

Zone enumeration 🗺️: "The practice of discovering the full content of a zone via successive queries." (Quoted from [RFC5155], Section 1.3.) People also call this "zone walking". Zone enumeration is different from zone content guessing, where the guesser uses a big dictionary of possible labels and sends successive queries for them, or matches the contents of NSEC3 records against that dictionary. Two different vibes, both kinda sneaky 👀

Validation ✅: in DNSSEC, validation can mean one of the following:

Checking the validity of DNSSEC signatures,
Checking the validity of DNS responses, such as those including authenticated denial of existence, or
Building an authentication chain from a trust anchor to a DNS response or individual DNS RRsets in a response.

The first two only look at whether individual DNSSEC components are valid, like an RRSIG or an NSEC proof. The third looks at the whole DNSSEC authentication chain, so it needs "configured knowledge of at least one authenticated DNSKEY or DS RR" (as described in [RFC4035], Section 5). 💅

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

[RFC5155] talks about validating responses all over the place in the context of hashed authenticated denial of existence. That's the second definition.

"Authentication" gets used interchangeably with "validation" in the third sense. [RFC4033], Section 2, calls the chain linking a trust anchor to DNS data the "authentication chain". A response is considered authentic if "all RRsets in the Answer and Authority sections of the response [are considered] to be authentic" (Quoted from [RFC4035]). DNS data or responses that are deemed authentic or validated have a security status of "secure" ([RFC4035], Section 4.3; [RFC4033], Section 5). And "Authenticating both DNS keys and data is a matter of local policy, which may extend or even override the [DNSSEC] protocol extensions..." (Quoted from [RFC4033], Section 3.1). Policy is the boss here 👑

"Verification", when it shows up, is usually just a synonym for "validation".

Validating resolver 🧐: a security-aware recursive name server, security-aware resolver, or security-aware stub resolver that applies at least one of the validation definitions above, as fits the resolution context. The generic word "resolver" is sometimes ambiguous and needs to be judged in context (see Section 6), so "validating resolver" is a context-sensitive term too. Context is everything, fr fr 🌸

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

Zone signing key (ZSK) 🔑: "DNSSEC keys that can be used to sign all the RRsets in a zone that require signatures, other than the apex DNSKEY RRset." (Quoted from [RFC6781], Section 3.1.) Also, a ZSK is sometimes used to sign the apex DNSKEY RRset too, so it's not always a strict split 🙃

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

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

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

Trust anchor ⚓: "A configured DNSKEY RR or DS RR hash of a DNSKEY RR. A validating security-aware resolver uses this public key or hash as a starting point for building the authentication chain to a signed DNS response. In general, a validating resolver will have to obtain the initial values of its trust anchors via some secure or trusted means outside the DNS protocol." (Quoted from [RFC4033], Section 2.) So the anchor itself has to come from somewhere trustworthy that isn't DNS 🧭

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

DNSSEC Practice Statement (DPS) 🗂️: "A practices disclosure document that may support and be a supplemental document to the DNSSEC Policy (if such exists), and it states how the management of a given zone implements procedures and controls at a high level." (Quoted from [RFC6841], Section 2.) Policy says what should happen, and the practice statement says how it's actually done. Very organised, very iconic 💖

Hardware security module (HSM) 🔒: a specialised piece of hardware used to create keys for signatures and to sign messages without ever disclosing the private key. In DNSSEC, HSMs are often used to hold the private keys for KSKs and ZSKs and to create the signatures used in RRSIG records at periodic intervals. The private key never leaves the box, which is so secure it's giving fortress 🏰

Signing software ✍️: authoritative DNS servers that support DNSSEC often include software that helps create and maintain DNSSEC signatures in zones. There's also stand-alone software that can sign a zone whether or not the authoritative server itself supports signing. Sometimes signing software can support particular HSMs as part of the signing process. Tools for the win, periodt 🌈

11. DNSSEC States 🔐

okay so a validating resolver can end up with one of four states for any response: secure, insecure, bogus, or indeterminate 💅. [RFC4033] and [RFC4035] both define these, but the definitions are a little different between the two docs 🙈. This document doesn't try to merge them, and it doesn't take a stance on whether they should be merged. not our drama ✨

Section 5 of [RFC4033] describes the states like this:

Secure: the validating resolver has a trust anchor, has a chain of trust, and can verify all the signatures in the response 🔒
Insecure: the validating resolver has a trust anchor and a chain of trust, and at some delegation point it has signed proof that a DS record doesn't exist. that means the branches below are provably insecure 🫥. The resolver can also have a local policy that marks parts of the domain space as insecure.
Bogus: the validating resolver has a trust anchor and a secure delegation saying the subsidiary data is signed, but the response still fails validation. Reasons include missing signatures, expired signatures, signatures using unsupported algorithms, and data that's missing even though the relevant NSEC RR says it should be there, and so on 💀
Indeterminate: no trust anchor says a particular part of the tree is secure. this is the default operation mode 🌈

Section 4.3 of [RFC4035] says a security-aware resolver needs to be able to tell apart four cases:

Secure: an RRset where the resolver can build a chain of signed DNSKEY and DS RRs from a trusted security anchor down to that RRset. in this case the RRset should be signed and goes through signature validation, as described above 🫶
Insecure: an RRset where the resolver knows there's no chain of signed DNSKEY and DS RRs from any trusted starting point. this can happen when the target RRset is in an unsigned zone, or in a descendent [sic] of an unsigned zone. the RRset may or may not be signed, but the resolver won't be able to verify the signature 🙃
Bogus: an RRset where the resolver thinks it ought to be able to build a chain of trust, but can't. either the signatures fail to validate for some reason, or data is missing that the relevant DNSSEC RRs say should be there. this might be an attack, but it could also be a configuration error or some kind of data corruption 😱
Indeterminate: an RRset where the resolver can't tell whether it should be signed, because it can't get the DNSSEC RRs it needs. this can happen when the security-aware resolver can't reach security-aware name servers for the relevant zones 🌸

12. Security Considerations 🛡️

These definitions don't change any security considerations for either the global DNS or private DNS. no new security drama from this doc, but the usual DNS and DNSSEC concerns still apply everywhere 💜

13. IANA Considerations 📚

References to RFC 8499 in the IANA registries have been swapped out for references to this document. so if you're reading the registries, look for this doc instead ✨

14. References 🎀

14.1. Normative References

The full list of normative references is below, exactly as written 🦋

[IANA_RootFiles]
           IANA, "Root Files",
           <https://www.iana.org/domains/root/files>.

[RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",
           RFC 882, DOI 10.17487/RFC0882, November 1983,
           <https://www.rfc-editor.org/info/rfc882>.

[RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
           STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
           <https://www.rfc-editor.org/info/rfc1034>.

[RFC1035]  Mockapetris, P., "Domain names - implementation and
           specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
           November 1987, <https://www.rfc-editor.org/info/rfc1035>.

[RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
           Application and Support", STD 3, RFC 1123,
           DOI 10.17487/RFC1123, October 1989,
           <https://www.rfc-editor.org/info/rfc1123>.

[RFC1912]  Barr, D., "Common DNS Operational and Configuration
           Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,
           <https://www.rfc-editor.org/info/rfc1912>.

[RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
           Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
           August 1996, <https://www.rfc-editor.org/info/rfc1996>.

[RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
           "Dynamic Updates in the Domain Name System (DNS UPDATE)",
           RFC 2136, DOI 10.17487/RFC2136, April 1997,
           <https://www.rfc-editor.org/info/rfc2136>.

[RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
           Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
           <https://www.rfc-editor.org/info/rfc2181>.

[RFC2182]  Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection
           and Operation of Secondary DNS Servers", BCP 16, RFC 2182,
           DOI 10.17487/RFC2182, July 1997,
           <https://www.rfc-editor.org/info/rfc2182>.

[RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
           NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
           <https://www.rfc-editor.org/info/rfc2308>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
           for DNS over TLS and DNS over DTLS", RFC 8310,
           DOI 10.17487/RFC8310, March 2018,
           <https://www.rfc-editor.org/info/rfc8310>.

[RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
           Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
           January 2019, <https://www.rfc-editor.org/info/rfc8499>.

[RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
           Dedicated QUIC Connections", RFC 9250,
           DOI 10.17487/RFC9250, May 2022,
           <https://www.rfc-editor.org/info/rfc9250>.

[RFC9471]  Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS
           Glue Requirements in Referral Responses", RFC 9471,
           DOI 10.17487/RFC9471, September 2023,
           <https://www.rfc-editor.org/info/rfc9471>.

14.2. Informative References ✨📚

okay bestie, these are the receipts 🧾 the extra reading that backs everything up. Here's the whole list, in the exact order the document has it 💅

[IANA_Resource_Registry] IANA, "Resource Record (RR) TYPEs", <https://www.iana.org/assignments/dns-parameters/>.
[RFC20] Cerf, V., "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969, <https://www.rfc-editor.org/info/rfc20>.
[RFC819] Su, Z. and J. Postel, "The Domain Naming Convention for Internet User Applications", RFC 819, DOI 10.17487/RFC0819, August 1982, <https://www.rfc-editor.org/info/rfc819>.
[RFC952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet host table specification", RFC 952, DOI 10.17487/RFC0952, October 1985, <https://www.rfc-editor.org/info/rfc952>.
[RFC1713] Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713, DOI 10.17487/RFC1713, November 1994, <https://www.rfc-editor.org/info/rfc1713>.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, DOI 10.17487/RFC1995, August 1996, <https://www.rfc-editor.org/info/rfc1995>.
[RFC2775] Carpenter, B., "Internet Transparency", RFC 2775, DOI 10.17487/RFC2775, February 2000, <https://www.rfc-editor.org/info/rfc2775>.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational Requirements for the Address and Routing Parameter Area Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172, September 2001, <https://www.rfc-editor.org/info/rfc3172>.
[RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425, DOI 10.17487/RFC3425, November 2002, <https://www.rfc-editor.org/info/rfc3425>.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. Stevens, "Basic Socket Interface Extensions for IPv6", RFC 3493, DOI 10.17487/RFC3493, February 2003, <https://www.rfc-editor.org/info/rfc3493>.
[RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name System KEY (DNSKEY) Resource Record (RR) Secure Entry Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April 2004, <https://www.rfc-editor.org/info/rfc3757>.
[RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912, DOI 10.17487/RFC3912, September 2004, <https://www.rfc-editor.org/info/rfc3912>.
[RFC4470] Weiler, S. and J. Ihren, "Minimally Covering NSEC Records and DNSSEC On-line Signing", RFC 4470, DOI 10.17487/RFC4470, April 2006, <https://www.rfc-editor.org/info/rfc4470>.
[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices", RFC 4641, DOI 10.17487/RFC4641, September 2006, <https://www.rfc-editor.org/info/rfc4641>.
[RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697, October 2006, <https://www.rfc-editor.org/info/rfc4697>.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, December 2006, <https://www.rfc-editor.org/info/rfc4786>.
[RFC4956] Arends, R., Kosters, M., and D. Blacka, "DNS Security (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July 2007, <https://www.rfc-editor.org/info/rfc4956>.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009, <https://www.rfc-editor.org/info/rfc5625>.
[RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, August 2010, <https://www.rfc-editor.org/info/rfc5890>.
[RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/RFC5891, August 2010, <https://www.rfc-editor.org/info/rfc5891>.
[RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)", RFC 5892, DOI 10.17487/RFC5892, August 2010, <https://www.rfc-editor.org/info/rfc5892>.
[RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010, <https://www.rfc-editor.org/info/rfc5893>.
[RFC5894] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010, <https://www.rfc-editor.org/info/rfc5894>.
[RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on Encodings for Internationalized Domain Names", RFC 6055, DOI 10.17487/RFC6055, February 2011, <https://www.rfc-editor.org/info/rfc6055>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, DOI 10.17487/RFC6265, April 2011, <https://www.rfc-editor.org/info/rfc6265>.
[RFC6303] Andrews, M., "Locally Served DNS Zones", BCP 163, RFC 6303, DOI 10.17487/RFC6303, July 2011, <https://www.rfc-editor.org/info/rfc6303>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011, <https://www.rfc-editor.org/info/rfc6335>.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in Internationalization in the IETF", BCP 166, RFC 6365, DOI 10.17487/RFC6365, September 2011, <https://www.rfc-editor.org/info/rfc6365>.
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, <https://www.rfc-editor.org/info/rfc6672>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, DOI 10.17487/RFC6762, February 2013, <https://www.rfc-editor.org/info/rfc6762>.
[RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, February 2014, <https://www.rfc-editor.org/info/rfc7129>.
[RFC7480] Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the Registration Data Access Protocol (RDAP)", STD 95, RFC 7480, DOI 10.17487/RFC7480, March 2015, <https://www.rfc-editor.org/info/rfc7480>.
[RFC7481] Hollenbeck, S. and N. Kong, "Security Services for the Registration Data Access Protocol (RDAP)", STD 95, RFC 7481, DOI 10.17487/RFC7481, March 2015, <https://www.rfc-editor.org/info/rfc7481>.
[RFC9082] Hollenbeck, S. and A. Newton, "Registration Data Access Protocol (RDAP) Query Format", STD 95, RFC 9082, DOI 10.17487/RFC9082, June 2021, <https://www.rfc-editor.org/info/rfc9082>.
[RFC9083] Hollenbeck, S. and A. Newton, "JSON Responses for the Registration Data Access Protocol (RDAP)", STD 95, RFC 9083, DOI 10.17487/RFC9083, June 2021, <https://www.rfc-editor.org/info/rfc9083>.
[RFC9224] Blanchet, M., "Finding the Authoritative Registration Data Access Protocol (RDAP) Service", STD 95, RFC 9224, DOI 10.17487/RFC9224, March 2022, <https://www.rfc-editor.org/info/rfc9224>.
[RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin, "Inventory and Analysis of WHOIS Registration Objects", RFC 7485, DOI 10.17487/RFC7485, March 2015, <https://www.rfc-editor.org/info/rfc7485>.
[RFC7793] Andrews, M., "Adding 100.64.0.0/10 Prefixes to the IPv4 Locally-Served DNS Zones Registry", BCP 163, RFC 7793, DOI 10.17487/RFC7793, May 2016, <https://www.rfc-editor.org/info/rfc7793>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., and P. Hoffman, "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram Transport Layer Security (DTLS)", RFC 8094, DOI 10.17487/RFC8094, February 2017, <https://www.rfc-editor.org/info/rfc8094>.
[RFC8109] Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS Resolver with Priming Queries", BCP 209, RFC 8109, DOI 10.17487/RFC8109, March 2017, <https://www.rfc-editor.org/info/rfc8109>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, <https://www.rfc-editor.org/info/rfc8484>.
[RFC9103] Toorop, W., Dickinson, S., Sahib, S., Aras, P., and A. Mankin, "DNS Zone Transfer over TLS", RFC 9103, DOI 10.17487/RFC9103, August 2021, <https://www.rfc-editor.org/info/rfc9103>.
[RSSAC026] Root Server System Advisory Committee (RSSAC), "RSSAC0226 RSSAC Lexicon", 2017, <https://www.icann.org/en/system/files/files/rssac-026-14mar17-en.pdf>.

Appendix A. Definitions Updated by This Document 🦋

okay so this is the bit where the definitions get a glow-up 🌸 the following definitions from RFCs are updated by this document:

Forwarder in [RFC2308]
QNAME in [RFC2308]
Secure Entry Point (SEP) in [RFC3757]; note, however, that this RFC is already obsolete (see [RFC4033], [RFC4034], [RFC4035]). 💀

Appendix B. Definitions First Defined in This Document

okay so this is the list of every term that gets its very first official definition in this RFC, bestie 📖✨ each one is tagged with the section where it lands, so you can go straight to the source 🦋

"Alias" in Section 2 💖
"Apex" in Section 7 🎀
"arpa" in Section 7 🌸
"Authoritative DoT (ADot)" in Section 6 ✨
"Bailiwick" in Section 7 💅
"Class independent" in Section 5 🫶
"Classic DNS" in Section 6 👑
"Delegation-centric zone" in Section 7 🧚
"Delegation" in Section 7 💜
"DNS operator" in Section 9 🍓
"DNSSEC-aware" in Section 10 🔥
"DNSSEC-unaware" in Section 10 🌈
"Forwarding" in Section 6 ⭐
"Full resolver" in Section 6 💀
"Fully Qualified Domain Name" in Section 2 🙈
"Global DNS" in Section 2 🦋
"Hardware Security Module (HSM)" in Section 10 🔐
"Host name" in Section 2 🌸
"IDN" in Section 2 💖
"In-domain" in Section 7 ✨
"Iterative resolution" in Section 6 🎀
"Label" in Section 2 💅
"Locally served DNS zone" in Section 2 🫶
"Naming system" in Section 2 👑
"Negative response" in Section 3 🧚
"Non-recursive query" in Section 6 💜
"Open resolver" in Section 6 🍓
"Passive DNS" in Section 6 🔥
"Policy-implementing resolver" in Section 6 🌈
"Presentation format" in Section 5 ⭐
"Priming" in Section 6 💀
"Private DNS" in Section 2 🙈
"Recursive DoT (RDot)" in Section 6 🦋
"Recursive resolver" in Section 6 🌸
"Referrals" in Section 4 💖
"Registrant" in Section 9 🎀
"Registrar" in Section 9 💅
"Registry" in Section 9 🫶
"Root zone" in Section 7 👑
"Secure Entry Point (SEP)" in Section 10 🧚
"Sibling domain" in Section 7 💜
"Signing software" in Section 10 🍓
"Split DNS" in Section 6 🔥
"Stub resolver" in Section 6 🌈
"Subordinate" in Section 8 ⭐
"Superordinate" in Section 8 💀
"TLD" in Section 2 🙈
"Validating resolver" in Section 10 🦋
"Validation" in Section 10 🌸
"View" in Section 6 💖
"Zone transfer" in Section 6 🎀

Acknowledgements 🫶

okay the people part, and it's so sweet 🥹 [RFC8499] and its predecessor, [RFC7719], were co-authored by Andrew Sullivan. The current document, which is a small update to [RFC8499], has just two authors. Andrew's work on the earlier documents is greatly appreciated 🙌

Numerous people made significant contributions to [RFC8499] and [RFC7719]. Please see the acknowledgements sections in those two documents for the extensive list of contributors ✨

Even though the current document is a small revision, many people in the DNSOP Working Group have contributed to it, and their work is greatly appreciated 💕 literally so many hands in this one 🧚

Index 📚

A B C D E F G H I K L M N O P Q R S T U V W X Z

A 🦋

Address and Routing Parameter Area Domain (arpa)
   Section 7
Address records
   Section 5
ADoT
   Section 6
Alias
   Section 2
Anycast
   Section 6
Apex
   Section 7
Asterisk label
   Section 8
Authoritative data
   Section 7
Authoritative server
   Section 6
Authoritative-only server
   Section 6
AXoT
   Section 6

B 🌸

Bailiwick
   Section 7

C 💖

Canonical name
   Section 2
Child
   Section 7
Class
   Section 4
Class independent
   Section 5
Classic DNS
   Section 6
Closest encloser
   Section 8
Closest provable encloser
   Section 8
CNAME
   Section 2
Combined signing key (CSK)
   Section 10

D 🎀

Delegation
   Section 7
Delegation-centric zone
   Section 7
DNS operator
   Section 9
DNS-over-HTTPS
   Section 6
DNS-over-QUIC
   Section 6
DNS-over-TLS
   Section 6
DNSSEC Policy (DP)
   Section 10
DNSSEC Practice Statement (DPS)
   Section 10
DNSSEC-aware and DNSSEC-unaware
   Section 10
DoH
   Section 6
Domain name
   Section 2
DoQ
   Section 6
DoT
   Section 6

E 🧚

EDNS
   Section 5
Empty non-terminals (ENTs)
   Section 7
EPP
   Section 9

F 💅

Fast flux DNS
   Section 7
FORMERR
   Section 3
Forward lookup
   Section 7
Forwarder
   Section 6
Forwarding
   Section 6
Full resolver
   Section 6
Full-service resolver
   Section 6
Fully Qualified Domain Name (FQDN)
   Section 2

G 🫶

Global DNS
   Section 2
Glue records
   Section 7

H 👑

Hardware security module (HSM)
   Section 10
Hidden master
   Section 6
Host name
   Section 2

I 🌈

IDN
   Section 2
In-bailiwick
   Section 7
In-domain
   Section 7
Insecure delegation
   Section 10
Instance
   Section 6
Internationalized Domain Name
   Section 2
Iterative mode
   Section 6
Iterative resolution
   Section 6
IXoT
   Section 6

K 🍓

Key signing key (KSK)
   Section 10

L 🔥

Label
   Section 2
Lame delegation
   Section 7
Locally served DNS zone
   Section 2

M ⭐

Master file
   Section 5
Master server
   Section 6
mDNS
   Section 2
Multicast DNS
   Section 2

N 💀

Naming system
   Section 2, Paragraph 1.2.1
Negative caching
   Section 6
Negative response
   Section 3
Next closer name
   Section 8
NODATA
   Section 3
NOERROR
   Section 3
Non-recursive query
   Section 6
NOTIMP
   Section 3
NS
   Section 6
NSEC
   Section 10
NSEC3
   Section 10
NXDOMAIN
   Section 3

O 🙈

Occluded name
   Section 7
on-line signing
   Section 10
online signing
   Section 10
Open resolver
   Section 6
OPT
   Section 5
Opt-out
   Section 10
Origin
   Section 7
Out-of-bailiwick
   Section 7
Owner
   Section 5

P 🦋

Parent
   Section 7
Passive DNS
   Section 6
Policy-implementing resolver
   Section 6
Presentation format
   Section 5
Primary master
   Section 6
Primary server
   Section 6
Priming
   Section 6
Privacy-enabling DNS server
   Section 6
Private DNS
   Section 2
Public suffix
   Section 9

Q 🌸

QNAME
   Section 4

R 💖

RDAP
   Section 9
RDoT
   Section 6
Recursive DoT
   Section 6
Recursive mode
   Section 6, Paragraph 4.10.1
Recursive query
   Section 6
Recursive resolver
   Section 6
Referrals
   Section 4
REFUSED
   Section 3
Registrant
   Section 9
Registrar
   Section 9
Registry
   Section 9
Resolver
   Section 6
Reverse DNS, reverse lookup
   Section 7
Root hints
   Section 6
Root zone
   Section 7
RR
   Section 5
RRset
   Section 5

S 🎀

Secondary server
   Section 6
Secure Entry Point (SEP)
   Section 10
SERVFAIL
   Section 3
Service name
   Section 7
Sibling domain
   Section 7
Signed zone
   Section 10
Signing software
   Section 10
Slave server
   Section 6
SOA
   Section 5
SOA field names
   Section 5
Source of Synthesis
   Section 8, Paragraph 1.14.1
Split DNS
   Section 6
Split-horizon DNS
   Section 6
Stealth server
   Section 6
Stub resolver
   Section 6
Subdomain
   Section 2
Subordinate
   Section 9
Superordinate
   Section 9

T 💅

TLD
   Section 2
Trust anchor
   Section 10
TTL
   Section 5

U 🫶

Unsigned zone
   Section 10

V 👑

Validating resolver
   Section 10
Validation
   Section 10, Paragraph 2.26.1
View
   Section 6

W 🧚

WHOIS
   Section 9
Wildcard
   Section 8
Wildcard domain name
   Section 8

X 💜

XoT
   Section 6

Z 🍓

Zone
   Section 7
Zone cut
   Section 7
Zone enumeration
   Section 10
Zone signing key (ZSK)
   Section 10
Zone transfer
   Section 6

Authors' Addresses 💌

Paul Hoffman ICANN Email: paul.hoffman@icann.org

Kazunori Fujiwara Japan Registry Services Co., Ltd. Email: fujiwara@jprs.co.jp

That's the whole RFC, bestie! 🌈✨ Thanks for hanging with me through all 11 parts 🫶💖

📦 where this page comes from

ietf: rfc9499 2024-03
💖 girlified by claude-haiku-5-5 on 2026-10-19: a language model's rewrite of the original, under its terms