RFC 8499

DNS Terminology 💅

okay bestie, meet RFC 8499 🎀 It's a Best Current Practice (that's BCP 219 if you're fancy) from the IETF, and it's basically the one place where all the DNS vocab lives so nobody has to argue about what a word means ✨

• Request for Comments: 8499
• BCP: 219
• Category: Best Current Practice
• Date: January 2019 📅
• ISSN: 2070-1721
• Authors: P. Hoffman (ICANN), A. Sullivan, and K. Fujiwara (JPRS)
• Obsoletes: 7719
• Updates: 2308

It obsoletes RFC 7719 and updates RFC 2308, so if you've been using the old one, this is the new main character 🦋

Abstract ✨

so the DNS has been defined in literally dozens of different RFCs 😭 and the words people use for it have shifted over the decades since it first showed up. this doc gives current definitions for a whole bunch of DNS terms, all in one spot 💖

This document obsoletes RFC 7719 and updates RFC 2308.

Status of This Memo 📝

This memo documents an Internet Best Current Practice 🌸

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

Information about the current status of this document, any errata, and how to give feedback on it can be found at https://www.rfc-editor.org/info/rfc8499.

Copyright Notice ⚖️

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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

Table of Contents 🗺️

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

1. Introduction 🧚

okay so the Domain Name System (DNS) is a simple query-response protocol, and the messages going in both directions have the same format 💜 (Section 2 defines "public DNS", which is usually what people mean when they say "the DNS".) The protocol and message format are defined in [RFC1034] and [RFC1035]. Those RFCs defined some terms, and later documents defined others too. Some terms from [RFC1034] and [RFC1035] mean something a bit different now than they did in 1987 🙈

This document is a big collection of DNS-related terms, grouped loosely by topic. Some were defined precisely in earlier RFCs, some were loosely defined in earlier RFCs, and some never got defined in an earlier RFC at all 😭

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

most of the definitions here are the consensus definition of the DNS community, meaning both the protocol developers and the operators. some of them are different from earlier RFCs, and those differences are called out. when the consensus definition matches an RFC, this doc quotes that RFC. when it has changed a bit, the RFC gets mentioned but the new stand-alone definition is given instead. see Appendix A for the list of definitions this document updates ✨

it's important to know that while writing this document, it became clear that some DNS terms get interpreted quite differently by different DNS experts 💀 some terms defined in the early DNS RFCs now have definitions that everyone generally agrees on, but those definitions are different from the originals. so this document is a substantial revision of [RFC7719] 🔥

there's no single consistent definition of "the DNS" either 🌈 it can 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 all over the place across RFCs and across practitioners, honestly. the capitalization in this document is a best guess at current practice, and it doesn't mean other styles are wrong or outdated 🙈 in some cases, several capitalization styles show up for the same term because the text quotes different RFCs.

also, the terms here are grouped by topic, not by learning order. someone who doesn't already know the DNS probably can't learn it from scratch by reading front to back 😭 instead, you'll probably need to skip around to get enough context for some definitions. this document has an index that might help if you're trying to learn the DNS from it 📚

2. Names 🎀

Naming system: okay so a naming system associates names with data 💅 naming systems have lots of significant facets that help tell them apart. 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

this list is only a small subset of the facets people have identified over time, and the IETF hasn't agreed on a good set of facets for comparing naming systems yet 🤷‍♀️ other facets might include "protocol to update data in a name", "privacy of names", and "privacy of data associated with names", but those aren't as well defined as the ones above. this list is here because it helps describe the DNS and naming systems that are similar to it ✨

Domain name: an ordered list of one or more labels 💖

this is a definition that doesn't depend on the DNS RFCs ([RFC1034] and [RFC1035]), and it also applies to systems other than the DNS 🦋 [RFC1034] defines the "domain name space" using mathematical trees and their nodes from graph theory, and that definition ends up with the same practical result as the one here. any path of a directed acyclic graph can be represented by a domain name made of the labels of its nodes, ordered by decreasing distance from the root(s). (that's the normal convention in the DNS, including in 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, different IETF and non-IETF documents have used the term "domain name" in lots of different ways 💀 earlier documents often use "domain name" to mean "names that match the syntax in [RFC1035]", but sometimes with extra rules, like "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 under "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" hadn't 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's the only label whose length may be 0 octets, and it's 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, and the root counts as one octet in that 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's included in the 255 octets.) ✨

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

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

the presentation format for names in the global DNS is a list of labels ordered by decreasing distance from the root, encoded as ASCII, with a "." character between each label. in presentation format, a fully-qualified domain name includes the root label and its separator dot. for example, 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 don't display in ASCII 🌈

Okay bestie, the common display format is the chill one 💅✨ It's the same as the presentation format, except showing the root label and the "." before it is optional and rarely done 🌸 So a fully-qualified domain name with two non-root labels is usually shown as example.tld instead of example.tld. 🫶 Names in this format are normally written so the writing system's direction shows labels going from farthest-from-root to closest. In English and the C programming language, the root or Top-Level Domain (TLD) label in the ordered list is rightmost 💖 In Arabic it may be leftmost, depending on local conventions 🌈

Administration of names: Administration is specified by delegation (see the definition of "delegation" in Section 7) 🦋 Policies for administering 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. At the time of writing, that operator is Public Technical Identifiers (PTI) 🎀 (For more on PTI running the IANA Functions, check https://pti.icann.org/.) The name servers that serve the root zone come from independent root operators 🧚 Other zones in the global DNS have their own administration policies 💜

Types of data that can be associated with names: A name can have zero or more resource records attached to it 🍓 There are loads of resource record types, each with its own unique data structure, defined in lots 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" in the DNS anyway, because they're part of a longer name that does have data (see the definition of "empty non-terminals" in Section 7) 🙈 Main character energy for the middle names, honestly 💅

Protocol for getting data from a name: The protocol described in [RFC1035] 🦋

Context for resolving a name: The global DNS root zone distributed by PTI 🌍

Private DNS: Names that use the protocol described in [RFC1035], but that don't rely on the global DNS root zone, or names that aren't generally available on the Internet but still use the protocol described in [RFC1035] 🔒 A system can use both the global DNS and one or more private DNS systems. For example, see "Split DNS" in Section 6 ✨

Note that domain names that don't appear in the DNS, and that 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 DNS names, bestie 💀

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) 🌸 It uses a compatible wire format, but mDNS is, strictly speaking, a different protocol than DNS 🫢 Also, where the quote above says "a portion of the DNS namespace", it would be clearer to say "a portion of the domain name space" 🧠 The names in mDNS aren't meant to be looked up in the DNS 🙅‍♀️

Locally served DNS zone: A locally served DNS zone 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 may give ambiguous results 😭 For example, the same name may resolve to different results in different locally served DNS zone contexts 🌀 The context for a locally served DNS zone may be explicit, like the ones listed in [RFC6303] and [RFC7793], or implicit, like those defined by local DNS administration and not known to the resolution client 🦋

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

The need for the term "fully-qualified domain name" comes from partially qualified domain names, which are names where one or more of the last labels in the ordered list are omitted. For example, a domain name of www relative to example.net identifies www.example.net 🧚 Such relative names are only understood by context 🌷

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

People also sometimes use "hostname" to mean just the first label of an FQDN, like printer in printer.admin.example.com. (Sometimes this is formalized in operating system configuration.) 🖥️ And people sometimes use the term to describe any name that refers to a machine, and those might include labels that don't conform to the "preferred name syntax" 🙈 The vibes are all over the place, iykyk 😭

Top-Level Domain (TLD): A Top-Level Domain is a zone one layer below the root, like com or jp 🌍 There's nothing special about TLDs from the point of view of the DNS 🤍 Most of them are also delegation-centric zones (defined in Section 7), and there are significant policy issues around how they operate 👑 TLDs are often divided into sub-groups such as Country Code Top-Level Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others. That division is a matter of policy and beyond the scope of this document 🫧

Internationalized Domain Name (IDN): The Internationalized Domain Names for Applications (IDNA) protocol is the standard mechanism for handling domain names with non-ASCII characters in applications in the DNS 🌏 The current standard at the time of 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 here are done on whole labels, so ooo.example.com is not a subdomain of oo.example.com 🙅‍♀️ Whole labels only, bestie, no partial matches 💅

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".) 😭 Lowkey a naming crime, but we're fixing it with the right words now 🫶💖

3. DNS Response Codes ✨

okay so some of the response codes (RCODEs) from [RFC1035] have gotten their own shorthand names 💅 All of the RCODEs are listed at [IANA_Resource_Registry], but that list uses mixed-case capitalization, while most documents use all caps. This section covers some of the common names for values defined in [RFC1035]. It also includes one extra RCODE and a general definition. The official list of all RCODEs lives in the IANA registry 🧚

NOERROR: This RCODE shows up as "No error condition" in Section 4.1.1 of [RFC1035].

FORMERR: This RCODE shows up as "Format error - The name server was unable to interpret the query" in Section 4.1.1 of [RFC1035].

SERVFAIL: This RCODE 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] 😭

NXDOMAIN: This RCODE 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]. [RFC2308] made NXDOMAIN a synonym for Name Error, so now they're the same thing 💜

NOTIMP: This RCODE shows up as "Not Implemented - The name server does not support the requested kind of query" in Section 4.1.1 of [RFC1035].

REFUSED: This RCODE 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]. It's giving "I don't want to" energy, but make it policy 🙈

NODATA: This is a pseudo RCODE 👀 It means 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) It's also described like this: "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 a lot like NODATA replies, and [RFC2308] explains how to tell them apart 🌸

The term "NXRRSET" is sometimes used as a synonym for NODATA. Bestie, this is a mistake, because NXRRSET is a specific error code defined in [RFC2136] 🚫

Negative response: A response that says a particular RRset does not exist, or whose RCODE shows the nameserver can't answer. Sections 2 and 7 of [RFC2308] describe the types of negative responses in detail 🍓

4. DNS Transactions 🎀

The header of a DNS message is its first 12 octets. 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, the response codes are called "RCODEs", the data for a record is called the "RDATA", and the authoritative answer bit is often called "the AA flag" or "the AA bit" ✨

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

QNAME: The most commonly used rough definition is that the QNAME is a field in the Question section of a query. "A standard query specifies a target domain name (QNAME), query type (QTYPE), and query class (QCLASS) and asks for RRs which match." (Quoted from [RFC1034], Section 3.7.1) Strictly speaking, the definition comes from [RFC1035], Section 4.1.2, where the QNAME is defined in respect of the Question section. This definition seems to be applied consistently. The discussion of inverse queries in Section 6.4.1 refers to the "owner name of the query RR and its TTL", because inverse queries 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 a kind of confusion 🌀 The response to a query that results in CNAME processing contains, in the echoed Question section, one QNAME (the name in the original query) and a second QNAME that is in the data field of the last CNAME. The confusion comes from the iterative/recursive mode of resolution, which finally returns an answer that need not actually have the same owner name as the QNAME contained in the original query.

To address this potential confusion, it helps 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. This 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 💖: 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). Sometimes spelled "RRSet" in other docs. Quick clarification: "same label" here means "same owner name" 🫶. [RFC2181] also says "the TTLs of all RRs in an RRSet must be the same".

Okay, plot twist 😭: RRSIG resource records don't fit this definition at all. [RFC4035] spills the tea: "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. It gets shown but never formally defined in [RFC1034] or [RFC1035]. The term "presentation format" first shows up in [RFC4034].

EDNS ✨: the extension mechanisms for DNS, defined in [RFC6891]. Sometimes it's called "EDNS0" or "EDNS(0)" to point out the version number. EDNS lets DNS clients and servers specify message sizes bigger than the original 512 octet limit, expand the response code space, and carry extra options that affect how a DNS query gets handled. It's giving upgrade 🚀.

OPT 🧚: a pseudo-RR (sometimes called a "meta-RR") that only exists to carry control information about the question-and-answer sequence of one specific transaction. (Definition paraphrased from [RFC6891], Section 6.1.1.) EDNS is what uses it 💜.

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

SOA field names 👑: DNS docs, these ones included, often talk about the fields inside the RDATA of an SOA resource record by their field names. "SOA" stands for "start of a zone of authority" 🍓. Those fields are defined in Section 3.3.13 of [RFC1035]. In the order they show up in the SOA RDATA, the names are MNAME, RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM. Heads up 👀: the meaning of the MINIMUM field got updated in Section 4 of [RFC2308]. The new deal is that MINIMUM 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) Fun fact 🤓: [RFC1035] wrongly said this was a signed integer, and [RFC2181] fixed it. Iconic correction 💖.

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 that same document adds: "the time interval (in seconds) that the resource record may be cached before it should be discarded". Even though TTL is defined per resource record, the TTL of every resource record in an RRset is required to be the same ([RFC2181], Section 5.2) 🔥.

So why is the TTL a maximum? A cache operator might shorten it for operational reasons, like if there's a policy against TTL values above a certain number 🙈. Some servers are known to ignore the TTL on some RRsets (like when the authoritative data has a very short TTL), even though that goes against the advice in RFC 1035 💀. An RRset can get flushed from the cache before the TTL interval is over. At that point the TTL value becomes unknown, since the RRset it was attached to doesn't exist anymore 🫠.

There's also the idea of a "default TTL" for a zone 🌷. It can be a configuration parameter in server software. It's often set as a default for the whole server and a separate 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 type that isn't class independent means different things depending on the DNS class of the record, or its 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 A or AAAA. [RFC2181] informally defines these as "(A, AAAA, etc)". New types of address records could get defined in the future 👀.

---

6. DNS Servers and Clients 💅

This section defines the terms 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 🖥️.

Context matters here 🧠. The terms "DNS server" and "name server" need context to understand the services being provided. Both authoritative servers and recursive resolvers often get called "DNS servers" and "name servers", even though they play different roles (they may live in the same software package, though) 🎀.

For terminology specific to the public DNS root server system, see [RSSAC026]. That doc defines terms like "root server", "root server operator", and terms specific to the way the root zone of the public DNS is served 👑.

Resolver 🔍: a program "that extract[s] information from name servers in response to client requests." (Quoted from [RFC1034], Section 2.4) A resolver makes 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 defined below), so the context is key 🍓.

Related term alert 🚨: "resolve" 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 can't do all the resolution itself. Stub resolvers generally depend on a recursive resolver to handle the actual resolution function. They get 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 it as "The server refers the client to another server and lets the client pursue the query." A resolver working 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 answers them from a local cache or sends queries to other servers 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 in recursive mode can be thought of as having a name server side (the part that answers the query) and a resolver side (the part that does the resolution function) 🧩. Systems in this mode are commonly called "recursive servers", and sometimes "recursive resolvers". Honestly? You can't know in advance whether the server you're querying will also do recursion, and both terms get used interchangeably 🙃.

Recursive resolver 💫: a resolver that acts in recursive mode. Generally, 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 tell recursive resolvers and iterative resolvers apart 👀.

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 the RD bit in the query asks for it, 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].) Simple, but powerful 🫶.

Iterative resolution 🎢: a name server may get a query that only some other server can answer. The two general approaches are "recursive", where the first server pursues the query for the client at another server, and "iterative", where the server refers the client to another server and lets the client chase 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 gets used in [RFC1035], but it isn't defined there. [RFC1123] defines a "full-service resolver" that may or may not be what [RFC1035] meant by "full resolver". This term isn't properly defined in any RFC, so tread carefully 🙈.

Full-service resolver 👑: Section 6.1.3.1 of [RFC1123] defines this term to mean a resolver that acts in recursive mode with a cache (and meets other requirements). Main character energy, fully served 💖.

Priming 🧚 bestie, this one's giving "finding the front door before you can even get in." It means "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 a resolver in recursive mode NEEDS to know the address of at least one root server, and priming usually happens from a config setting holding a list of authoritative servers for the root zone ✨

Root hints 🗺️ this is the file that does the priming job. Quoting [IANA_RootFiles]: "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." So yes, it's often used in priming 🌸

Negative caching 🙈 the storage of knowledge that something does not exist. Per the quote, it is "The storage of knowledge that something does not exist, cannot or does not give an answer." (Quoted from [RFC2308], Section 1) iconic behaviour, honestly 💀

Authoritative server 👑 this is the main character of the zone. The definition is "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) It's named in the NS ("name server") record in a zone. It's a system that replies to 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. Bestie, it's possible for an authoritative server to respond to a query WITHOUT the parent zone delegating authority to that server. Also, authoritative servers hand out "referrals", usually to child zones delegated from them. Those referrals have the AA bit set to 0 and come with referral data in the Authority and (if needed) the Additional sections 🦋

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

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. First, the AXFR ("Authoritative Transfer") mechanism copies the full zone (described in [RFC5936]). Second, the IXFR ("Incremental Transfer") mechanism copies only the parts of the zone that have changed (described in [RFC1995]). Many systems use non-standard methods for zone transfer outside the DNS protocol, which is a whole other vibe 🌈

Slave server 🍑 see "Secondary server". Same energy, different name.

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

Master server 🍑 see "Primary server". Another name glow-up.

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 the term 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. Bestie, that's a whole glow-up 🎀

Stealth server 🕵️‍♀️ this is "like a slave server except not listed in an NS RR for the zone." (Quoted from [RFC1996], Section 2.1) Secret agent energy, literally 🦋

Hidden master 🫣 a stealth server that is a primary server for zone transfers. "In this arrangement, the master name server that processes the updates is unavailable to general hosts on the Internet; it is not listed in the NS RRset." (Quoted from [RFC6781], Section 3.4.3) An earlier RFC, [RFC4641], said the hidden master's name "appears in the SOA RRs MNAME field", although in some setups the name doesn't appear at all in the public DNS. Also, a hidden master can be a secondary server for the zone itself. Multi-tasking queen 👑

Forwarding 📨 the process of one server sending a DNS query with the RD bit set to 1 to another server to resolve that query. It's a function of a DNS resolver, and it's different from simply blindly relaying queries. Not the same thing, bestie 🙅‍♀️

[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]). Undefined but iconic 💅

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. Plot twist, it's a mystery 🌸

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. Generally, 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. Sometimes the user of the stub resolver picked the policy-implementing resolver on purpose to get those policies. Other times, policies are imposed WITHOUT the stub resolver's user being informed. That's kinda sus 😭

Open resolver 🌞 a full-service resolver that accepts and processes queries from any (or nearly any) client. It's 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 in which DNS servers that are authoritative for a particular set of domains provide partly or completely different answers in those domains depending on the source of the query. The effect is that a domain name that is notionally globally unique nevertheless has different meanings for different network users. Sometimes this is the result of a "view" configuration, described below.

Section 3.8 of [RFC2775] gives a related definition that is too specific to be generally useful. Real talk, it's a lot 🙃

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's 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're widely implemented in server software. Not standardized, still everywhere 🧚

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, so be careful bestie 🫶 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 the stored records on keys other than just the name and type, such as "find all names which have A records of a particular value". Time-travel detective vibes 🔍

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. Many places, one address, iconic 💖

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]) Clones with the same address, but make it aesthetic ✨

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]. Privacy queen behaviour, no cap 👑💜

7. Zones ✨

7.1. Zone Vocab Time 🎀

okay bestie, this section is where we get into the vocab for zones that are being served or retrieved. the terms are a lot but they're literally the building blocks, so let's go 💅

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

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

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

Origin:

There are two different uses for this term 👑

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

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

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

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

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

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

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

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

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

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

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

"In-bailiwick" names are divided into two types of names for name servers: "in-domain" names and "sibling domain" names. 💖

• In-domain: a modifier to describe a name server whose name is either subordinate to or (rarely) the same as the owner name of the NS resource records. An in-domain name server name needs to have glue records or name resolution fails. For example, a delegation for "child.example.com" may have "in-domain" name server name "ns.child.example.com". 👑
• Sibling domain: a name server's name that is either subordinate to or (rarely) the same as the zone origin and not subordinate to or the same as the owner name of the NS resource records. Glue records for sibling domains are allowed, but not necessary. For example, a delegation for "child.example.com" in "example.com" zone may have "sibling" name server name "ns.another.example.com". 🫶

"Out-of-bailiwick" is the antonym of "in-bailiwick". It is a modifier to describe a name server whose name is not subordinate to or the same as the zone origin. Glue records for out-of-bailiwick name servers are useless. The following table shows examples of delegation types. 🍓

Delegation |Parent|Name Server Name  | Type
-----------+------+------------------+-----------------------------
com        | .    |a.gtld-servers.net|in-bailiwick / sibling domain
net        | .    |a.gtld-servers.net|in-bailiwick / in-domain
example.org| org  |ns.example.org    |in-bailiwick / in-domain
example.org| org  |ns.ietf.org       |in-bailiwick / sibling domain
example.org| org  |ns.example.com    |out-of-bailiwick
example.jp | jp   |ns.example.jp     |in-bailiwick / in-domain
example.jp | jp   |ns.example.ne.jp  |in-bailiwick / sibling domain
example.jp | jp   |ns.example.com    |out-of-bailiwick

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

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

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

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

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

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

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

arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain was originally established as part of the initial deployment of the DNS, to provide a transition mechanism from the Host Tables that were common in the ARPANET, as well as a home for the IPv4 reverse mapping domain. During 2000, the abbreviation was 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: okay so [RFC1034] came up with the word "wildcard" 💅, but it was kinda confusing for implementers 😭. For a deeper dive, with clearer definitions, check out [RFC4592]. Owner names that start with the label "*" get special treatment 🎀. "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) 💖

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9. Registration Model 🌈

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

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

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

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

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

RDAP: the Registration Data Access Protocol, defined in [RFC7480], [RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The RDAP protocol and data format are meant as a replacement for WHOIS 🦋.

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

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

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

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

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

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

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10. General DNSSEC 🔥

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

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

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

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

Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that is not signed". Section 2 of [RFC4035] defines this as a "zone that does not include these records [properly constructed DNSKEY, Resource Record Signature (RRSIG), Next Secure (NSEC), and (optionally) DS records] according to the rules in this section..." There is an important note at the end of Section 5.2 of [RFC4035] that defines an additional situation in which a zone is considered unsigned: "If the resolver does not support any of the algorithms listed in an authenticated DS RRset, then the resolver will not be able to verify the authentication path to the child zone. In this case, the resolver SHOULD treat the child zone as if it were unsigned." ⚠️

Heads up bestie: that last bit is a SHOULD from the original text, so it's a real recommendation, not a suggestion from me 💅.

NSEC: "The NSEC record allows a security-aware resolver to authenticate a negative reply for either name or type non-existence with the same mechanisms used to authenticate other DNS replies." (Quoted from [RFC4033], Section 3.2) In short, an NSEC record provides authenticated denial of existence 🦋.

"The NSEC resource record lists two separate things: the next owner name (in the canonical ordering of the zone) that contains authoritative data or a delegation point NS RRset, and the set of RR types present at the NSEC RR's owner name." (Quoted from Section 4 of RFC 4034) 🌈

NSEC3: like the NSEC record, the NSEC3 record also provides authenticated denial of existence. However, NSEC3 records mitigate zone enumeration and support Opt-Out 🧁. NSEC3 resource records require associated NSEC3PARAM resource records. NSEC3 and NSEC3PARAM resource records are defined in [RFC5155].

Note that [RFC6840] says that [RFC5155] "is now considered part of the DNS Security Document Family as described by Section 10 of [RFC4033]". This means that some of the definitions from earlier RFCs that only talk about NSEC records should probably be considered to be talking about both NSEC and NSEC3 🍓.

Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1) Opt-out tackles the high costs of securing a delegation to an insecure zone 💸. When using Opt-Out, names that are an insecure delegation (and empty non-terminals that are only derived from insecure delegations) don't require an NSEC3 record or its corresponding RRSIG records. Opt-Out NSEC3 records are not able to prove or deny the existence of the insecure delegations. (Adapted from [RFC7129], Section 5.1) 🫶

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

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

Validation: in DNSSEC land, validation means one of these three things 💅✨

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

The first two definitions above look at individual DNSSEC pieces, like the RRSIG validity or NSEC proof validity 🔍. The third one 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 that a "Validating Security-Aware Stub Resolver... performs signature validation" and uses a trust anchor "as a starting point for building the authentication chain to a signed DNS response"; so it uses the first and third definitions above 💖. The process of validating an RRSIG resource record is described in [RFC4035], Section 5.3 🧾.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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11. DNSSEC States 🌈

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

Section 5 of [RFC4033] says:

"A validating resolver can determine the following 4 states:"

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

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

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

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

Section 4.3 of [RFC4035] says:

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

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

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

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

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

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12. Security Considerations 🛡️

These definitions do not change any security considerations for the DNS. 🫶

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13. IANA Considerations 🦄

This document has no IANA actions. 💅

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14. References 📚

14.1. Normative References 🎀

okay bestie, this is the homework list 📚✨ these are the docs the spec leans on, so here they all are, every single one, 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>.

14.2. Informative References ✨

okay bestie, these are the extra-credit reading links 📚🧚 Nothing here is required reading, but if you want the full story behind the DNS stuff, this is your reading list. It's giving well-researched 💅

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

[RFC819] Su, Z. and J. Postel, "The Domain Naming Convention for Internet User Applications", RFC 819, DOI 10.17487/RFC0819, August 1982, <https://www.rfc-editor.org/info/rfc819>.

[RFC952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet host table specification", RFC 952, DOI 10.17487/RFC0952, October 1985, <https://www.rfc-editor.org/info/rfc952>.

[RFC1713] Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713, DOI 10.17487/RFC1713, November 1994, <https://www.rfc-editor.org/info/rfc1713>.

[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, DOI 10.17487/RFC1995, August 1996, <https://www.rfc-editor.org/info/rfc1995>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[RFC7480] Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the Registration Data Access Protocol (RDAP)", RFC 7480, DOI 10.17487/RFC7480, March 2015, <https://www.rfc-editor.org/info/rfc7480>.

[RFC7481] Hollenbeck, S. and N. Kong, "Security Services for the Registration Data Access Protocol (RDAP)", RFC 7481, DOI 10.17487/RFC7481, March 2015, <https://www.rfc-editor.org/info/rfc7481>.

[RFC7482] Newton, A. and S. Hollenbeck, "Registration Data Access Protocol (RDAP) Query Format", RFC 7482, DOI 10.17487/RFC7482, March 2015, <https://www.rfc-editor.org/info/rfc7482>.

[RFC7483] Newton, A. and S. Hollenbeck, "JSON Responses for the Registration Data Access Protocol (RDAP)", RFC 7483, DOI 10.17487/RFC7483, March 2015, <https://www.rfc-editor.org/info/rfc7483>.

[RFC7484] Blanchet, M., "Finding the Authoritative Registration Data (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March 2015, <https://www.rfc-editor.org/info/rfc7484>.

[RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin, "Inventory and Analysis of WHOIS Registration Objects", RFC 7485, DOI 10.17487/RFC7485, March 2015, <https://www.rfc-editor.org/info/rfc7485>.

[RFC7793] Andrews, M., "Adding 100.64.0.0/10 Prefixes to the IPv4 Locally-Served DNS Zones Registry", BCP 163, RFC 7793, DOI 10.17487/RFC7793, May 2016, <https://www.rfc-editor.org/info/rfc7793>.

[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., and P. Hoffman, "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 2016, <https://www.rfc-editor.org/info/rfc7858>.

[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram Transport Layer Security (DTLS)", RFC 8094, DOI 10.17487/RFC8094, February 2017, <https://www.rfc-editor.org/info/rfc8094>.

[RFC8109] Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS Resolver with Priming Queries", BCP 209, RFC 8109, DOI 10.17487/RFC8109, March 2017, <https://www.rfc-editor.org/info/rfc8109>.

[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, <https://www.rfc-editor.org/info/rfc8484>.

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

---

Appendix A. Definitions Updated by This Document 🎀

The following definitions from RFCs are updated by this document, so these ones got a glow-up:

• 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 🌈

The following definitions are first defined in this document, so this is where they were born 🍓

• "Alias" in Section 2
• "Apex" in Section 7
• "arpa" in Section 7
• "Bailiwick" in Section 7
• "Class independent" in Section 5
• "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-bailiwick" 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
• "Out-of-bailiwick" in Section 7
• "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 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
• "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

that's the whole glossary, bestie 💖 you're now officially fluent in DNS-speak 🫶

Index 🧚

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

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

   D
      DNS operator  29
      DNSSEC Policy (DP)  34
      DNSSEC Practice Statement (DPS)  34
      DNSSEC-aware and DNSSEC-unaware  30
      Delegation  24
      Delegation-centric zone  26
      Domain name  5

   E
      EDNS  14
      EPP  28
      Empty non-terminals (ENT)  26

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

   G
      Global DNS  5
      Glue records  24

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

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

   K
      Key signing key (KSK)  33

   L
      Label  5
      Lame delegation  24
      Locally served DNS zone  8

   M
      Master file  14
      Master server  19
      Multicast DNS  7
      mDNS  7

   N
      NODATA  10
      NOERROR  10
      NOTIMP  10
      NS  19
      NSEC  31
      NSEC3  31
      NXDOMAIN  10
      Naming system  4
      Negative caching  19
      Negative response  11
      Next closer name  28
      Non-recursive query  18

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

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

   Q
      QNAME  11

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

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

   T
      TLD  9
      TTL  15
      Trust anchor  34

   U
      Unsigned zone  30

   V
      Validating resolver  33
      Validation  32
      View  21

   W
      WHOIS  28
      Wildcard  27
      Wildcard domain name  27

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

Acknowledgements 💜

The authors gratefully acknowledge all of the authors of DNS-related RFCs that proceed this one. Comments from Tony Finch, Stephane Bortzmeyer, Niall O'Reilly, Colm MacCarthaigh, Ray Bellis, John Kristoff, Robert Edmonds, Paul Wouters, Shumon Huque, Paul Ebersman, David Lawrence, Matthijs Mekking, Casey Deccio, Bob Harold, Ed Lewis, John Klensin, David Black, and many others in the DNSOP Working Group helped shape RFC 7719. 🫶

Most of the major changes between RFC 7719 and this document came from active discussion on the DNSOP WG. Specific people who contributed material to this document include: Bob Harold, Dick Franks, Evan Hunt, John Dickinson, Mark Andrews, Martin Hoffmann, Paul Vixie, Peter Koch, Duane Wessels, Allison Mankin, Giovane Moura, Roni Even, Dan Romascanu, and Vladmir Cunat. ✨👑

Authors' Addresses 🎀

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org


   Andrew Sullivan

   Email: ajs@anvilwalrusden.com


   Kazunori Fujiwara
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065
   Japan

   Phone: +81 3 5215 8451
   Email: fujiwara@jprs.co.jp

That's the whole RFC, bestie! 🌈💅