The Domain Name System (DNS) is defined in literally dozens of different RFCs. The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has changed in the decades since the DNS was first defined. This document gives current definitions for many of the terms used in the DNS in a single document. This document updates RFC 2308 by clarifying the definitions of "forwarder" and "QNAME". It obsoletes RFC 8499 by adding multiple terms and clarifications. Comprehensive lists of changed and new definitions can be found in Appendices A and B.

This memo documents an Internet Best Current Practice. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9499.

Copyright (c) 2024 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 review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.

The Domain Name System (DNS) is a simple query-response protocol whose messages in both directions have the same format. (Section 2 gives a definition of "global DNS", which is often what people mean when they say "the DNS".) The protocol and message format are defined in [RFC 1034] and [RFC 1035]. These RFCs defined some terms, and later documents defined others. Some of the terms from [RFC 1034] and [RFC 1035] have somewhat different meanings now than they did in 1987. This document contains a collection of a wide variety of DNS-related terms, organized loosely by topic. Some of them have been precisely defined in earlier RFCs, some have been loosely defined in earlier RFCs, and some are not defined in an earlier RFC at all. Other organizations sometimes define DNS-related terms in their own way. For example, the WHATWG defines "domain" at <https://url.spec.whatwg.org/>. The Root Server System Advisory Committee (RSSAC) has a good lexicon [RSSAC026]. Most of the definitions listed here represent the consensus definition of the DNS community -- both protocol developers and operators. Some of the definitions differ from earlier RFCs, and those differences are noted. In this document, where the consensus definition is the same as the one in an RFC, that RFC is quoted. Where the consensus definition has changed somewhat, the RFC is mentioned but the new stand-alone definition is given. See Appendix A for a list of the definitions that this document updates. It is important to note that, during the development of this document, it became clear that some DNS-related terms are interpreted quite differently by different DNS experts. Further, some terms that are defined in early DNS RFCs now have definitions that are generally agreed to, but that are different from the original definitions. This document is a small revision to [RFC 8499]; that document was a substantial revision to [RFC 7719]. Note that there is no single consistent definition of "the DNS". It can be considered to be some combination of the following: a commonly used naming scheme for objects on the Internet; a distributed database representing the names and certain properties of these objects; an architecture providing distributed maintenance, resilience, and loose coherency for this database; and a simple query-response protocol (as mentioned below) implementing this architecture. Section 2 defines "global DNS" and "private DNS" as a way to deal with these differing definitions. Capitalization in DNS terms is often inconsistent among RFCs and various DNS practitioners. The capitalization used in this document is a best guess at current practices, and is not meant to indicate that other capitalization styles are wrong or archaic. In some cases, multiple styles of capitalization are used for the same term due to quoting from different RFCs. In this document, the words "byte" and "octet" are used interchangeably. They appear here because they both appear in the earlier RFCs that defined terms in the DNS. Readers should note that the terms in this document are grouped by topic. Someone who is not already familiar with the DNS probably cannot learn about the DNS from scratch by reading this document from front to back. Instead, skipping around may be the only way to get enough context to understand some of the definitions. This document has an index that might be useful for readers who are attempting to learn the DNS by reading this document.

Naming system:

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

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

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

Domain name:

An ordered list of one or more labels.

Note that this is a definition independent of the DNS RFCs ([RFC 1034] and [RFC 1035]), and the definition herealso applies to systems other than the DNS. [RFC 1034] defines the "domainname space" using mathematical trees and their nodes in graph theory, and that definitionhas the same practical result as the definition here. Any path of a directed acyclic graphcan be represented by a domain name consisting of the labels of its nodes, ordered bydecreasing distance from the root(s) (which is the normal convention within the DNS,including this document). A domain name whose last label identifies a root of the graph isfully qualified; other domain names whose labels form a strict prefix of a fully qualifieddomain name are relative to its first omitted node.

Also note that different IETF and non-IETF documents have used the term "domain name"in many different ways. It is common for earlier documents to use "domain name" to mean"names that match the syntax in [RFC 1035]", but possibly with additionalrules such as "and are, or will be, resolvable in the global DNS" or "but only using thepresentation 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 possibledomain names.

Global DNS:

Using the short set of facets listed in "Naming system", the global DNS can be defined asfollows. Most of the rules here come from [RFC 1034] and [RFC 1035], although the term "global DNS" has not been defined before now.

Composition of names:

A name in the global DNS has one or morelabels. The length of each label is between 0 and 63 octetsinclusive. In a fully qualified domain name, the last labelin the ordered list is 0 octets long; it is the only label whoselength may be 0 octets, and it is called the "root" or "root label".A domain name in the global DNS has a maximum total length of 255octets in the wire format; the root represents one octet for thiscalculation.(Multicast DNS [RFC 6762] allows names up to 255 bytes plus a terminating zero bytebased on a different interpretation of RFC 1035 and what is included in the 255 octets.)

Format of names:

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

Wire format:

The basic wire format for names in the global DNS is a list of labels ordered by decreasing distance from the root, with the root label last. Each label is preceded by a length octet. [RFC 1035] also defines a compression scheme that modifies this format.

Presentation format:

The presentation format for names in the global DNS is a list of labels ordered by decreasing distance from the root, encoded as ASCII, with a "." character between each label. In presentation format, a fully qualified domain name includes the root label and the associated separator dot. For example, in presentation format, a fully qualified domain name with two non-root labels is always shown as "example.tld." instead of "example.tld". [RFC 1035] defines a method for showing octets that do not display in ASCII.

Common display format:

The common display format is used in applications and free text. It is the same as the presentation format, but showing the root label and the "." before it is optional and is rarely done. For example, in common display format, a fully qualified domain name with two non-root labels is usually shown as "example.tld" instead of "example.tld.". Names in the common display format are normally written such that the directionality of the writing system presents labels by decreasing distance from the root (so, in both English and the C programming language, the root or Top-Level Domain (TLD) label in the ordered list is rightmost; but in Arabic, it may be leftmost, depending on local conventions).

Administration of names:

Administration is specified by delegation (see thedefinition of "delegation" in Section 7). Policies for administration ofthe root zone in the global DNS are determined by the names operational community, whichconvenes itself in the Internet Corporation for Assigned Names and Numbers (ICANN). Thenames operational community selects the IANA Functions Operator for the global DNS rootzone.The name servers that serve the root zone are provided by independentroot operators. Other zones in the global DNS have their own policies foradministration.

Types of data that can be associated with names:

A name can have zero or moreresource records associated with it. There are numerous types of resource records withunique data structures defined in many different RFCs and in the IANA registry at [IANA_Resource_Registry].

Types of metadata for names:

Any name that is published in the DNS appears as a setof resource records (see the definition of "RRset" in Section 5). Some namesdo not, themselves, have data associated with them in the DNS, but they "appear" in the DNSanyway because they form part of a longer name that does have data associated with it (seethe definition of "empty non-terminals" in Section 7).

Protocol for getting data from a name:

The protocol described in [RFC 1035].

Context for resolving a name:

The global DNS root zone distributed by Public Technical Identifiers (PTI).

Private DNS:

Names that use the protocol described in [RFC 1035] but do not rely onthe global DNS root zone or names that are otherwise not generally available on theInternet but are using the protocol described in [RFC 1035]. A system canuse both the global DNS and one or more private DNS systems; for example, see "Split DNS"in Section 6.

Note that domain names that do not appear in the DNS and that are intended never to belooked up using the DNS protocol are not part of the global DNS or a private DNS, eventhough they are domain names.

Multicast DNS (mDNS):

"Multicast DNS (mDNS) provides the ability to perform DNS-like operations on the local link in theabsence of any conventional Unicast DNS server. In addition, Multicast DNS designates a portion ofthe DNS namespace to be free for local use, without the need to pay any annual fee, and without theneed to set up delegations or otherwise configure a conventional DNS server to answer for thosenames." (Quoted from [RFC 6762], Abstract)Although it uses a compatible wire format, mDNS is, strictly speaking, a different protocol than DNS.Also, where the above quote says "a portion of the DNS namespace", it would be clearer to say "aportion of the domain name space". The names in mDNS are not intended to be looked up in theDNS.

Locally served DNS zone:

A locally served DNS zone is a special case of private DNS.Names are resolved using the DNS protocol in a local context. [RFC 6303] defines subdomains of IN-ADDR.ARPAthat are locally served zones.Resolution of names through locally served zones may result in ambiguous results.For example, the same name may resolve to different results in different locally served DNSzone contexts. The context for a locally served DNS zone may be explicit, such as those that are listed in [RFC 6303] and [RFC 7793], or implicit, such as those defined by local DNS administration and not known to theresolution client.

Fully Qualified Domain Name (FQDN):

This is often just a clear wayof saying the same thing as "domain name of a node", as outlinedabove. However, the term is ambiguous. Strictly speaking, a fully qualifieddomain name would include every label, including the zero-length labelof the root; such a name would be written "www.example.net."(note the terminating dot). But, because every name eventually sharesthe common root, names are often written relative to the root(such as "www.example.net") and are still called "fully qualified".This term first appeared in [RFC819]. In this document, namesare often written relative to the root.

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

Host name:

This term and its equivalent, "hostname", have beenwidely used but are not defined in [RFC 1034], [RFC 1035],[RFC 1123], or [RFC 2181]. TheDNS was originally deployed into the Host Tables environment asoutlined in [RFC952], and it is likely that the term followedinformally from the definition there.Over time, the definition seemsto have shifted. "Host name" is often meant to be a domain name that followsthe rules in Section 3.5 of RFC 1034, which is also called the "preferred namesyntax". (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 octetvalue; hostnames are generally considered to be domain names whereevery label follows the rules in the "preferred name syntax", with theamendment that labels can start with ASCII digits (this amendmentcomes from Section 2.1 of RFC 1123).

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

Top-Level Domain (TLD):

A Top-Level Domain is a zone that is one layer below theroot, such as "com" or "jp". There is nothing special, from the pointof view of the DNS, about TLDs. Most of them are alsodelegation-centric zones (defined in Section 7), and there are significant policy issuesaround their operation.TLDs are often divided into sub-groups such as Country Code Top-Level Domains(ccTLDs), Generic Top-Level Domains (gTLDs), and others; thedivision is a matter of policy and beyond the scope of this document.

Internationalized Domain Name (IDN):

The Internationalized Domain Names for Applications (IDNA) protocol isthe standard mechanism for handling domain names with non-ASCIIcharacters in applications in the DNS. The current standard at thetime of this writing, normally called "IDNA2008", is defined in [RFC 5890], [RFC 5891], [RFC 5892], [RFC 5893], and [RFC 5894]. These documents define many IDN-specific termssuch as "LDH label", "A-label", and "U-label". [RFC 6365] defines more terms that relate tointernationalization (some of which relate to IDNs); [RFC 6055] 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 byseeing if the subdomain's name ends with the containing domain's name." (Quotedfrom RFC 1034, Section 3.1)Forexample, in the host name "nnn.mmm.example.com", both "mmm.example.com" and "nnn.mmm.example.com" are subdomains of "example.com".Note that the comparisons here are done on whole labels; that is,"ooo.example.com" is not a subdomain of "oo.example.com".

Alias:

The owner of a CNAME resource record, or a subdomain of the owner of aDNAME resource record (DNAME records are defined in [RFC 6672]). See also "canonical name".

Canonical name:

A CNAME resource record "identifies its owner name as analias, and specifies the corresponding canonical name in the RDATAsection of the RR." (Quoted from RFC 1034, Section 3.6.2)This usage of the word "canonical" is related to the mathematicalconcept 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 RFC 2181, Section 10.1.1. The quotedtext has been changed from "label" to "owner".)

Some of the response codes (RCODEs) that are defined in [RFC 1035] have acquired their own shorthand names. All of the RCODEs are listed at [IANA_Resource_Registry], although that list uses mixed-case capitalization, while most documents use all caps. Some of the common names for values defined in [RFC 1035] are described in this section. This section also includes an additional RCODE and a general definition. The official list of all RCODEs is in the IANA registry.

NOERROR:

This RCODE appears as "No error condition" in Section 4.1.1 of RFC 1035.

FORMERR:

This RCODE appears as "Format error - The name server was unable to interpret the query" in Section 4.1.1 of RFC 1035.

SERVFAIL:

This RCODE appears as "Server failure - The name server was unable to process this query due to a problem with the nameserver" in Section 4.1.1 of RFC 1035.

NXDOMAIN:

This RCODE appears as "Name Error [...] this code signifies that the domain namereferenced in the query does not exist." in Section 4.1.1 of RFC 1035.[RFC 2308] established NXDOMAIN as a synonym for Name Error.

NOTIMP:

This RCODE appears as "Not Implemented - The name server does not support the requested kind of query" in Section 4.1.1 of RFC 1035.

REFUSED:

This RCODE appears as "Refused - The name server refuses to perform the specified operation for policy reasons. Forexample, a name server may not wish to provide the information to the particular requester, or aname server may not wish to perform a particular operation (e.g., zone transfer) for particulardata." in Section 4.1.1 of RFC 1035.

NODATA:

"A pseudo RCODE which indicates that the name is valid, forthe given class, but [there] are no records of the given type. A NODATAresponse has to be inferred from the answer." (Quoted from RFC 2308, Section 1)"NODATA is indicated by an answer with the RCODE set to NOERROR and norelevant answers in the Answer section. The Authority section willcontain an SOA record, or there will be no NS records there." (Quoted from RFC 2308, Section 2.2)Note that referrals have a similar format to NODATA replies; [RFC 2308]explains how to distinguish them.

The term "NXRRSET" is sometimes used as a synonym for NODATA. However, this is a mistake, giventhat NXRRSET is a specific error code defined in [RFC 2136].

Negative response:

A response that indicates that a particular RRset does not exist or whose RCODE indicates that the nameserver cannot answer. Sections 2 and 7 of [RFC 2308] describe the types of negative responses in detail.

The header of a DNS message is its first 12 octets. Many of the fields and flags in the diagrams in Sections 4.1.1 through 4.1.3 of [RFC 1035] are referred to by their names in each diagram. For example, the response codes are called "RCODEs", the data for a record is called the "RDATA", and the authoritative answer bit is often called "the AA flag" or "the AA bit".

Class:

A class "identifies a protocol family or instance of a protocol". (Quoted from RFC 1034, Section 3.6)"The DNS tags all data with a class as well as the type, so that we can allow parallel useof different formats for data of type address." (Quoted from RFC 1034, Section 2.2)In practice, the class for nearly every query is "IN" (the Internet).There are some queries for "CH" (the Chaos class), but they are usually for the purposes ofinformation about the server itself rather than for a different type of address.

QNAME:

The most commonly used rough definition is that the QNAME is a field in the Question section of aquery. "A standard query specifies a target domain name (QNAME), query type (QTYPE), and queryclass (QCLASS) and asks for RRs which match." (Quoted from RFC 1034, Section 3.7.1)Strictly speaking, the definition comes from RFC 1035, Section 4.1.2, where the QNAME is defined in respect of the Questionsection. This definition appears to be applied consistently, as the discussionof inverse queries in Section 6.4.1 of RFC 1035 refers to the "owner name ofthe query RR and its TTL" because inverse queries populate the Answer sectionand leave the Question section empty. (Inverse queriesare deprecated in [RFC 3425]; thus, relevantdefinitions do not appear in this document.)

However, [RFC 2308] has an alternate definition thatputs the QNAME in the answer (or series of answers) instead of thequery. It defines QNAME as"...the name in the query section of ananswer, or where this resolves to a CNAME, or CNAME chain, the datafield of the last CNAME. The last CNAME in this sense is that whichcontains a value which does not resolve to another CNAME."This definition has a certain internal logic, because of the way CNAMEsubstitution works and the definition of CNAME. If a name server doesnot find an RRset that matches a query, but does find the same name inthe same class with a CNAME record, then the name server "includes theCNAME record in the response and restarts the query at the domain namespecified in the data field of the CNAME record." (Quoted from RFC 1034, Section 3.6.2) This is made explicit in theresolution algorithm outlined in Section 4.3.2 of RFC 1034, which says to "change QNAME to the canonical namein 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 iscanonically named what is in the RDATA, then there is a way to viewthe new name (i.e., the name that was in the RDATA of the CNAME RR) asalso being the QNAME.

However, this creates confusion because the response to aquery that results in CNAME processing contains in the echoed Questionsection one QNAME (the name in the original query) and a second QNAMEthat is in the data field of the last CNAME. The confusion comes fromthe iterative/recursive mode of resolution, which finally returns ananswer that need not actually have the same owner name as the QNAMEcontained in the original query.

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

QNAME (original):

The name actually sent in the Question section in the original query, which isalways 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 queriedor a name received in a CNAME chain response.

QNAME (final):

The name actually resolved, which is either the name actually queried or elsethe last name in a CNAME chain response.

Note that, because the definition in [RFC 2308] isactually for a different concept than what was in [RFC 1034], it would have been better if [RFC 2308] had used a different name for that concept. Ingeneral use today, QNAME almost always means what is defined above as"QNAME (original)".

Referrals:

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

A referral arises when a server is not performing recursive servicewhile answering a query. It appears in step 3(b) of the algorithm inRFC 1034, Section 4.3.2.

There are two types of referral response. The first is a downwardreferral (sometimes described as "delegation response"), where theserver is authoritative for some portion of the QNAME. The Authoritysection RRset's RDATA contains the name servers specified at thereferred-to zone cut. In normal DNS operation, this kind of responseis required in order to find names beneath a delegation. The bareuse of "referral" means this kind of referral, and many people believethat this is the only legitimate kind of referral in the DNS.

The second is an upward referral (sometimes described as "rootreferral"), where the server is not authoritative for any portion ofthe QNAME. When this happens, the referred-to zone in the Authoritysection is usually the root zone ("."). In normal DNS operation, thiskind of response is not required for resolution or for correctlyanswering any query. There is no requirement that any server sendupward referrals. Some people regard upward referrals as a sign of amisconfiguration or error. Upward referrals always need some sort ofqualifier (such as "upward" or "root") and are never identified simply bythe word "referral".

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

In the case where the query matches an alias, and the server is notauthoritative for the target of the alias but is authoritative forsome name above the target of the alias, the resolution algorithm willproduce a response that contains both the authoritative answer for thealias and a referral. Such a partial answer and referralresponse has data in the Answer section. It has the NS RRset for thereferred-to zone in the Authority section. It may contain RRs thatprovide addresses in the Additional section. The AA bit is setbecause the first name in the Answer section matches the QNAME and theserver is authoritative for that answer (see RFC 1035, Section 4.1.1).

RR:

An acronym for resource record. (See RFC 1034, Section 3.6.)

RRset:

A set of resource records "with the same label, class and type, but with differentdata" (according to RFC 2181, Section 5). Also written as "RRSet" in some documents. As a clarification,"same label" in this definition means "same owner name".In addition, [RFC 2181] states that "the TTLs of all RRs in an RRSet must be the same".

Note that RRSIG resource records do not match this definition. [RFC 4035] says:

An RRset MAY have multiple RRSIG RRs associated with it. Note that as RRSIG RRs are closely tied to the RRsets whose signatures they contain, RRSIG RRs, unlike all other DNS RR types, do not form RRsets. In particular, the TTL values among RRSIG RRs with a common owner name do not follow the RRset rules described in [RFC 2181].

Master file:

"Master files are text files that contain RRs in text form. Since the contents of a zonecan be expressed in the form of a list of RRs a master file is most often used to define azone, though it can be used to list a cache's contents." (Quoted from RFC 1035, Section 5)Master files are sometimes called "zone files".

Presentation format:

The text format used in master files. This format is shown but not formally defined in[RFC 1034] or [RFC 1035]. The term "presentation format"first appears in [RFC 4034].

EDNS:

The extension mechanisms for DNS, defined in [RFC 6891]. Sometimes called "EDNS0" or "EDNS(0)"to indicate the version number. EDNS allows DNS clients and servers to specify messagesizes larger than the original 512-octet limit, to expand the response code space, andto carry additional options that affect the handling of a DNS query.

OPT:

A pseudo-RR (sometimes called a "meta-RR") that is used only to containcontrol information pertaining to the question-and-answer sequence of a specifictransaction. (Definition paraphrased from RFC 6891, Section 6.1.1.) It is used by EDNS.

Owner:

"The domain name where the RR is found." (Quoted from RFC 1034, Section 3.6) Often appears in the term "owner name".

SOA field names:

DNS documents, including the definitions here, often refer to the fields in theRDATA of an SOA resource record by field name."SOA" stands for "start of a zone of authority".Those fields are defined in Section 3.3.13 of RFC 1035.The names (in the order they appear in the SOA RDATA) are MNAME, RNAME, SERIAL, REFRESH, RETRY,EXPIRE, and MINIMUM.Note that the meaning of the MINIMUM field is updated in Section 4 of RFC 2308; the new definitionis that the MINIMUM field is only "the TTL to be used for negative responses".This document tends to use field names instead of terms that describe the fields.

TTL:

The maximum "time to live" of a resource record. "A TTL value is an unsignednumber, with a minimum value of 0, and a maximum value of 2147483647. That is, amaximum of 2^31 - 1. When transmitted, this value shall be encoded in the lesssignificant 31 bits of the 32 bit TTL field, with the most significant, or sign,bit set to zero." (Quoted from RFC 2181, Section 8)Note that [RFC 1035]erroneously stated that this is a signed integer; that was fixed by [RFC 2181].

The TTL "specifies the time interval that the resource record may be cachedbefore the source of the information should again be consulted." (Quoted fromRFC 1035, Section 3.2.1) Section 4.1.3 of RFC 1035 states "the time interval (in seconds) that the resourcerecord may be cached before it should be discarded". Despite being defined for a resource record, the TTL of everyresource record in an RRset is required to be the same (RFC 2181, Section 5.2).

The reason that the TTL is the maximum time to live is that a cache operatormight decide to shorten the time to live for operational purposes, for example, ifthere is a policy to disallow TTL values over a certain number.Some servers are known to ignore the TTL on some RRsets (such as when the authoritative datahas a very short TTL) even though this is against the advice in [RFC 1035].An RRset can be flushed from the cache before the end of the TTL interval,at which point, the value of the TTL becomes unknown because the RRsetwith which it was associated no longer exists.

There is also the concept of a "default TTL" for a zone, which can be a configurationparameter in the server software. This is often expressed by a default for theentire server, and a default for a zone using the $TTL directivein a zone file. The $TTL directive was added to the master fileformat by [RFC 2308].

Class independent:

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

Address records:

Records whose type is either A or AAAA. [RFC 2181] informally defines these as "(A, AAAA, etc)".Note that new types of address records could be defined in the future.

This section defines the terms used for the systems that act as DNS clients, DNS servers, or both. In past RFCs, DNS servers are sometimes called "name servers", "nameservers", or just "servers". There is no formal definition of "DNS server", but RFCs generally assume that it is an Internet server that listens for queries and sends responses using the DNS protocol defined in [RFC 1035] and its successors. It is important to note that the terms "DNS server" and "name server" require context in order to understand the services being provided. Both authoritative servers and recursive resolvers are often called "DNS servers" and "name servers" even though they serve different roles (but may be part of the same software package). For terminology specific to the global DNS root server system, see [RSSAC026]. That document defines terms such as "root server", "root server operator", and terms that are specific to the way that the root zone of the global DNS is served.

Resolver:

A program "that extract[s] information from nameservers in response to client requests." (Quoted from RFC 1034, Section 2.4) A resolver performsqueries for a name, type, and class, and receives responses. Thelogical function is called "resolution". In practice, the term isusually referring to some specific type of resolver(some of which are defined below), and understandingthe use of the term depends on understanding the context.

A related term is "resolve", which is not formally defined in [RFC 1034]or [RFC 1035]. An imputed definition might be "asking a question thatconsists of a domain name, class, and type, and receiving some sort of response".Similarly, an imputed definition of "resolution" might be "the response receivedfrom resolving".

Stub resolver:

A resolver that cannot perform all resolutionitself. Stub resolvers generally depend on a recursive resolver to undertake theactual resolution function. Stub resolvers are discussed but neverfully defined in Section 5.3.1 of RFC 1034.They are fully defined in Section 6.1.3.1 of RFC 1123.

Iterative mode:

A resolution mode of a server that receives DNSqueries and responds with a referral to another server. Section 2.3 of RFC 1034describes this as "The server refers the client toanother server and lets the client pursue the query."A resolver that works in iterative mode is sometimes called an "iterativeresolver".See also "iterative resolution" later in this section.

Recursive mode:

A resolution mode of a server that receives DNS queries and either responds to those queries from a local cache or sends queries to other servers in order to get the final answers to the original queries. Section 2.3 of RFC 1034 describes this as "the first server pursues the query for the client at another server". Section 4.3.1 of RFC 1034 says: "in [recursive] mode the name server acts in the role of a resolver and returns either an error or the answer, but never referrals." That same section also says:

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

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

Recursive resolver:

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

[RFC 4697] tried to differentiate between arecursive resolver and an iterative resolver.

Recursive query:

A query with the Recursion Desired (RD) bit set to 1 in the header. (See Section 4.1.1 of RFC 1035.) If recursive service is available and is requested by the RD bit in the query,the server uses its resolver to answer the query. (See Section 4.3.2 of RFC 1034.)

Non-recursive query:

A query with the Recursion Desired (RD) bit set to 0 in the header. A server can answernon-recursive queries using only local information: the response contains either an error, theanswer, or a referral to some other server "closer" to the answer.(See Section 4.3.1 of RFC 1034.)

Iterative resolution:

A name server may be presented with a query that can only be answered by some other server. The twogeneral approaches to dealing with this problem are "recursive", in which the first server pursuesthe query on behalf of the client at another server, and "iterative", in which the server refers the clientto another server and lets the client pursue the query there. (See Section 2.3 of RFC 1034.)

In iterative resolution, the client repeatedly makes non-recursive queries and follows referralsand/or aliases. The iterative resolution algorithm is described in Section 5.3.3 of RFC 1034.

Full resolver:

This term is used in [RFC 1035], but it is not defined there. RFC1123 defines a "full-service resolver" that may or may not be what was intendedby "full resolver" in [RFC 1035].This term is not properly defined in any RFC, and there is no consensus on what the term means.The use of this term without proper context is discouraged.

Full-service resolver:

Section 6.1.3.1 of RFC 1123 defines this termas a resolver that acts in recursive mode with a cache (and meetsother requirements).

Priming:

"The act of finding the list of root servers from aconfiguration that lists some or all of the purported IP addresses ofsome or all of those root servers." (Quoted from RFC 8109, Section 2)In order to operate in recursive mode, a resolver needs to know the address of at least one root server.Priming is most often done from a configuration setting that contains alist of authoritative servers for the root zone.

Root hints:

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

Negative caching:

"The storage of knowledge that something does not exist, cannotor does not give an answer." (Quoted from RFC 2308, Section 1)

Authoritative server:

"A server that knows the content of a DNS zone from local knowledge, and thus can answerqueries about that zone without needing to query other servers." (Quoted from RFC 2182, Section 2)An authoritative server is named in the NS ("name server") record in a zone.It is a system that responds to DNS queries with information aboutzones for which it has been configured to answer with the AA flag inthe response header set to 1. It is a server that has authority overone or more DNS zones. Note that it is possible for an authoritativeserver to respond to a query without the parent zone delegatingauthority to that server. Authoritative servers also provide"referrals", usually to child zones delegated from them; thesereferrals have the AA bit set to 0 and come with referral data in theAuthority 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-recursivequeries from iterative resolvers looking for information in zones it serves." (Quoted from RFC 4697, Section 2.4)In this case, "ignores requests for recursion" means "responds to requests forrecursion with responses indicating that recursion was not performed".

Zone transfer:

The act of a client requesting a copy of a zone and an authoritative serversending the needed information.(See Section 7 for a description of zones.)There are two common standard ways to do zone transfers:the AXFR ("Authoritative Transfer") mechanism to copy the full zone (described in [RFC 5936], and the IXFR ("Incremental Transfer") mechanism to copy only parts of the zone that have changed (described in [RFC 1995]).Many systems use non-standard methods for zone transfers outside the DNS protocol.

Slave server:

See "Secondary server".

Secondary server:

"An authoritative server which uses zone transfer to retrieve thezone." (Quoted from RFC 1996, Section 2.1)Secondary servers are also discussed in [RFC 1034].[RFC 2182] describes secondary servers inmore detail. Although early DNS RFCs such as [RFC 1996] referred to this as a "slave", thecurrent common usage has shifted to calling it a "secondary".

Master server:

See "Primary server".

Primary server:

"Any authoritative server configured to be the source of zone transferfor one or more [secondary] servers." (Quoted from RFC 1996, Section 2.1) Or, morespecifically, [RFC 2136] calls it "an authoritative server configured to be the source of AXFR or IXFR datafor one or more [secondary] servers".Primary servers are also discussed in [RFC 1034].Although early DNS RFCs such as [RFC 1996] referred to this as a "master", the currentcommon usage has shifted to "primary".

Primary master:

"The primary master is named in the zone's SOA MNAME field andoptionally by an NS RR." (Quoted from RFC 1996, Section 2.1)[RFC 2136] 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 bydefinition only one primary master server per zone."

The idea of a primary master is only used in [RFC 1996] and [RFC 2136].A modern interpretation of the term "primary master" is a server that is both authoritative for a zoneand that gets its updates to the zone from configuration (such as a master file) or from UPDATE transactions.

Stealth server:

This is "like a slave server except not listed in an NS RR forthe zone." (Quoted from RFC 1996, Section 2.1)

Hidden master:

A stealth server that is a primary server for zone transfers. "In this arrangement, themaster name server that processes the updates is unavailable to general hosts on theInternet; it is not listed in the NS RRset." (Quoted fromRFC 6781, Section 3.4.3) [RFC 4641] saidthat the hidden master's name "appears in the SOA RRs MNAME field"; however, the name does not appear at all in the global DNS in some setups. A hidden master can also be asecondary server for the zone itself.

Forwarding:

The process of one server sending a DNS query with theRD bit set to 1 to another server to resolve that query. Forwarding isa function of a DNS resolver; it is different than simply blindlyrelaying queries.

[RFC 5625] does not give a specific definition for forwarding, butdescribes in detail what features a system that forwards needs tosupport. Systems that forward are sometimes called "DNS proxies", butthat term has not yet been defined (even in [RFC 5625]).

Forwarder:

Section 1 of RFC 2308 describes a forwarder as "anameserver used to resolve queries instead of directly using theauthoritative nameserver chain". [RFC 2308] further says "Theforwarder typically either has better access to the internet, ormaintains a bigger cache which may be shared amongst many resolvers."That definition appears to suggest that forwardersnormally only query authoritative servers. In current use, however,forwarders often stand between stub resolvers and recursive servers.[RFC 2308] is silent on whether a forwarder is iterative-only orcan be a full-service resolver.

Policy-implementing resolver:

A resolver acting in recursive mode that changes some of the answersthat it returns based on policy criteria, such as to prevent access tomalware sites or objectionable content. In general, a stub resolver has no ideawhether upstream resolvers implement such policy or, if they do, the exactpolicy about what changes will be made.In some cases, the user of the stub resolver has selected the policy-implementing resolverwith the explicit intention of using it to implement the policies. In other cases,policies are imposed without the user of the stub resolver being informed.

Open resolver:

A full-service resolver that accepts and processes queries from any (or nearly any) client.This is sometimes also called a "public resolver", although the term "public resolver"is used more with open resolvers that are meant to be open, as compared to the vast majority of openresolvers that are probably misconfigured to be open.Open resolvers are discussed in [RFC 5358].

Split DNS:

The terms "split DNS" and "split-horizon DNS" have long been used in the DNS community withoutformal definition. In general, they refer to situations in whichDNS servers that are authoritative for a particular set of domainsprovide partly or completely different answers in those domains dependingon the source of the query. Nevertheless, the effect of this is that a domain name thatis notionally globally unique has different meanings fordifferent network users. This can sometimes be the result of a "view"configuration, as described below.

Section 3.8 of RFC 2775 gives a related definition that is too specific to be generally useful.

View:

A configuration for a DNS server that allows it to providedifferent responses depending on attributes of the query, such as for "split DNS". Typically, views differby the source IP address of a query, but can also be based on the destination IP address,the type of query (such as AXFR), whether it is recursive, and so on.Views are often used toprovide more names or different addresses to queries from "inside" a protected networkthan to those "outside" that network. Views are not a standardizedpart of the DNS, but they are widely implemented in server software.

Passive DNS:

A mechanism to collect DNS data by storing DNS responses from name servers. Some of these systemsalso collect the DNS queries associated with the responses, although doing so raises some privacyconcerns. Passive DNS databases can be used to answer historical questions about DNS zones, such aswhich values were present at a given time in the past, or when a name was spotted first. Passive DNS databases allow searching of the stored records on keys other thanjust the name and type, such as "find all names which have A records of aparticular value".

Anycast:

"The practice of making a particular service address available in multiple, discrete, autonomouslocations, such that datagrams sent are routed to one of several available locations."(Quoted from RFC 4786, Section 2)See [RFC 4786] for more detail on Anycast and other terms that arespecific to its use.

Instance:

"When anycast routing is used to allow more than one server to have the same IPaddress, 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 'Anycastinstance'." (Quoted from [RSSAC026])

Privacy-enabling DNS server:

"A DNS server that implementsDNS over TLS [RFC 7858] and may optionally implement DNS over DTLS[RFC 8094]." (Quoted from RFC 8310, Section 2)Other types of DNS servers might also be considered privacy-enabling, such as thoserunning DNS-over-HTTPS [RFC 8484] or DNS-over-QUIC [RFC 9250].

DNS-over-TLS (DoT):

DNS over TLS as defined in [RFC 7858] and its successors.

DNS-over-HTTPS (DoH):

DNS over HTTPS as defined in [RFC 8484] and its successors.

DNS-over-QUIC (DoQ):

DNS over QUIC as defined in [RFC 9250] and its successors. [RFC 9250] specifically defines DoQ as general-purpose transportfor DNS that can be used in stub to recursive, recursive to authoritative, andzone transfer scenarios.

Classic DNS:

DNS over UDP or DNS over TCP as defined in [RFC 1035] and its successors.Classic DNS applies to DNS communication between stub resolvers and recursiveresolvers, and between recursive resolvers and authoritative servers.This has sometimes been called "Do53".Classic DNS is not encrypted.

Recursive DoT (RDoT):

RDoT specifically means DNS-over-TLS for transport between a stub resolver and arecursive resolver, or between a recursive resolver and another recursive resolver.This term is necessary because it is expected that DNS-over-TLS will later bedefined as a transport between recursive resolvers and authoritative servers.

Authoritative DoT (ADoT):

If DNS-over-TLS is later defined as a transport between recursive resolvers andauthoritative servers, ADoT specifically means DNS-over-TLS for transportbetween recursive resolvers and authoritative servers.

XFR-over-TLS (XoT):

DNS zone transfer over TLS, as specified in [RFC 9103].This term applies to both AXFR over TLS (AXoT) and IXFR over TLS (IXoT).

This section defines terms that are used when discussing zones that are being served or retrieved.

Zone:

"Authoritative information isorganized into units called ZONEs, and these zones can beautomatically distributed to the name servers which provideredundant service for the data in a zone." (Quoted from RFC 1034, Section 2.4)

Child:

"The entity on record that has the delegation of the domain from theParent." (Quoted from RFC 7344, Section 1.1)

Parent:

"The domain in which the Child is registered." (Quoted from RFC 7344, Section 1.1) Earlier,"parent name server" was defined in [RFC 0882] as "the name server that has authority over the placein the domain name space that will hold the new domain". (Notethat [RFC 0882] was obsoleted by [RFC 1034] and [RFC 1035].) [RFC819] also has some description ofthe 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 RFC 2181, 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 RFC 1035, 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".[RFC 4033] 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 inzone files. The distinction is not always maintained in use, however,and one can find uses that conflict subtly with this definition.[RFC 1034] 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 originof 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 RFC 2181, Section 6; note that this is barely an ostensivedefinition.)Section 4.2 of RFC 1034 uses "cuts" instead of "zone cut".

Delegation:

The process by which a separate zone is created in thename space beneath the apex of a given domain. Delegation happens when an NSRRset is added in the parent zone for the child origin. Delegationinherently 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 zonedown to leaf nodes or nodes above cuts around the bottom edge of the zone." (Quoted fromRFC 1034, Section 4.2.1)Note that this definition might inadvertently also cause any NS recordsthat appear in the zone to be included, even those that might not truly be authoritative, because there areidentical NS RRs below the zone cut. This reveals the ambiguity inthe notion of authoritative data, because the parent-side NS recordsauthoritatively indicate the delegation, even though they are notthemselves authoritative data.

RFC 4033, Section 2, defines "Authoritative RRset", which is relatedto authoritative data but has a more precise definition.

Lame delegation:

"A lame delegations exists [sic] when a nameserver is delegated responsibility for providing nameservicefor a zone (via NS records) but is not performing nameservice for that zone (usually because it isnot set up as a primary or secondary for the zone)." (Quoted from RFC 1912, 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 aserver for that domain. Queries are thus sent to the wrong servers, who don't know nothing [sic] (at leastnot as expected) about the queried domain. Furthermore, sometimes these hosts (if they exist!) don'teven run name servers." (Quoted from RFC 1713, Section 2.3)

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

• a nameserver with an NS record for a zone that does not answer DNS queries;
• a nameserver with an IP address that is not reachable by the resolver; and
• a nameserver that responds to a query for a specific name with an error orwithout the authoritative bit set.

Because the term in current usage has drifted from the original definition, and nowis not specific or clear as to the intended meaning, it should be considered historicand avoided in favor of terms that are specific and clear.

Glue records:

"...[Resource records] which are not part of the authoritative data [of the zone],and are address RRs for the [name] servers [in subzones]. These RRs are onlynecessary if the name server's name is 'below' the cut, and are only used as part of areferral response." Without glue "we could be faced with the situation where the NS RRstell us that in order to learn a name server's address, we should contact the server usingthe address we wish to learn." (Quoted from RFC 1034, Section 4.2.1)

A later definition is that glue "includes any record in a zone file that is not properlypart 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 datathat might appear." (Quoted from RFC 2181, Section 5.4.1)Although glue is sometimes used todaywith this wider definition in mind, the context surrounding the definition in [RFC 2181] suggests it is intended to apply to the use of glue within the document itself and notnecessarily beyond.

In an NS record, there are three types of relationships between the owner name of the record, the name in the NS RDATA, and the zone origin: unrelated, in-domain, and sibling domain.The application of these three types of relationships to glue records is defined in[RFC 9471].

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

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

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

The following table shows examples of delegation types:

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

Table 1

Bailiwick:

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

Root zone:

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

Empty non-terminals (ENTs):

"Domain names that own no resource records but have subdomains that do."(Quoted from RFC 4592, 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, butthat "_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 isused in contrast to a zone that might have some delegations to child zones but also has many dataresource records for the zone itself and/or for child zones.The term is used in [RFC 4956] and [RFC 5155], but it is not defined in either document.

Occluded name:

"The addition of a delegation point via dynamic update will render all subordinatedomain names to be in a limbo, still part of the zone but not available to the lookup process. Theaddition of a DNAME resource record has the same impact. The subordinate names are said to be'occluded'." (Quoted from RFC 5936, 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 meansthat the domain resolves to varying IP addresses over a short period of time."(Quoted from RFC 6561, Section 1.1.5, with a typo corrected)In addition to having legitimate uses, fast flux DNS can be used to deliver malware.Because the addresses change so rapidly, it is difficult toascertain all the hosts. It should be noted that the technique also workswith AAAA records, but such use is not frequently observed on theInternet as of this writing.

Reverse DNS, reverse lookup:

"The process of mapping an address to a name isgenerally known as a 'reverse lookup', and the IN-ADDR.ARPA andIP6.ARPA zones are said to support the 'reverse DNS'."(Quoted from RFC 5855, Section 1)

Forward lookup:

"Hostname-to-address translation". (Quoted from RFC 3493, Section 6)

arpa (Address and Routing Parameter Area Domain):

"The 'arpa' domain was originally established as part of the initialdeployment of the DNS to provide a transition mechanism from theHost Tables that were common in the ARPANET, as well as a home forthe IPv4 reverse mapping domain. During 2000, the abbreviation wasredesignated to 'Address and Routing Parameter Area' in the hope ofreducing confusion with the earlier network name."(Quoted from RFC 3172, Section 2).arpa is an "infrastructure domain",a domain whose "role is tosupport the operating infrastructure of the Internet".(Quoted from RFC 3172, Section 2)See [RFC 3172] for more history of this name.

Service name:

"Service names are the unique key in the Service Name and TransportProtocol Port Number registry. This unique symbolic name for aservice may also be used for other purposes, such as in DNS SRVrecords." (Quoted from RFC 6335, Section 5)

Wildcard:

[RFC 1034] defined "wildcard", but in a way that turned out to beconfusing to implementers.For an extended discussion of wildcards, including clearer definitions, see [RFC 4592].Special treatment is given to RRs with owner names starting with the label "*". "Such RRsare called 'wildcards'. Wildcard RRs can be thought of as instructions for synthesizing RRs."(Quoted from RFC 1034, Section 4.3.3)

Asterisk label:

"The first octet is the normal label type and length for a 1-octet-longlabel, and the second octet is the ASCII representation [RFC20]for the '*' character.A descriptive name of a label equaling that value is an 'asterisklabel'." (Quoted from RFC 4592, 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 RFC 4592, 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 RFC 5155, Section 1.3)An earlier definition is "The node in the zone's tree of existingdomain names that has the most labels matching the query name(consecutively, counting from the root label downward). Each matchis a 'label match' and the order of the labels is the same."(Quoted from RFC 4592, Section 3.3.1)

Closest provable encloser:

"The longest ancestor of a name that canbe proven to exist. Note that this is only different from theclosest encloser in an Opt-Out zone."(Quoted from RFC 5155, Section 1.3)See Section 10 for more on "opt-out".

Next closer name:

"The name one label longer than the closestprovable encloser of a name."(Quoted from RFC 5155, 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 RFC 4592, Section 3.3.1)

Registry:

The administrative operation of a zone that allows registration of names within thatzone. People often use this term to refer only to those organizationsthat perform registration in large delegation-centric zones (such asTLDs); but formally, whoever decides what data goes into a zone is theregistry for that zone.This definition of "registry" is from a DNS point of view; for some zones, the policiesthat 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 ina zone is registered by the registry. In many zones, the registry andthe registrant may be the same entity, but in TLDs they often arenot.

Registrar:

A service provider that acts as a go-between forregistrants and registries. Not all registrations require aregistrar, though it is common to have registrars involved inregistrations in TLDs.

EPP:

The Extensible Provisioning Protocol (EPP), which is commonly used for communicationof registration information between registries and registrars. EPP is defined in [RFC 5730].

WHOIS:

A protocol specified in [RFC 3912], often used for querying registry databases.WHOIS data is frequently used to associate registration data (such as zone managementcontacts) with domain names.The term "WHOIS data" is often used as a synonym for the registry database, even thoughthat 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[RFC 7480], [RFC 7481], [RFC 7485], [RFC 9082], [RFC 9083], and[RFC 9224].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 registrantmay act as their own DNS operator, their registrar may do it on their behalf, or they may use athird-party operator.For some zones, the registry function is performed by the DNS operator plus other entitieswho decide about the allowed contents of the zone.

Public suffix:

"A domain that is controlled by a public registry." (Quoted from RFC 6265, 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 [RFC 6265]) should not be set.There is no indication in a domain name whether it is a public suffix; that can only bedetermined 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 isa public suffix. Oneresource for identifying public suffixes is the Public Suffix List (PSL)maintained by Mozilla <https://publicsuffix.org/>.

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

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

Subordinate and Superordinate:

These terms are introduced in [RFC 5731] 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 namens1.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 RFC 5731, Section 1.1)These terms are strictly ways of referring to the relationship standing of two domainswhere one is a subdomain of the other.

Most DNSSEC terms are defined in [RFC 4033], [RFC 4034], [RFC 4035], and [RFC 5155]. 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 RFC 4033 defines many types of resolvers andvalidators, including "non-validating security-aware stub resolver", "non-validatingstub 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 someplaces 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 containsproperly constructed DNSKEY, Resource Record Signature (RRSIG),Next Secure (NSEC), and (optionally) DS records." (Quoted from RFC 4033, Section 2)It has been noted in other contexts that the zone itself is notreally signed, but all the relevant RRsets in the zone are signed.Nevertheless, if a zone that should be signed contains any RRsets thatare 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 [RFC 6840], NSEC records are nolonger required for signed zones: a signed zone might include NSEC3 records instead.[RFC 7129] provides additional background commentary and some context for the NSEC andNSEC3 mechanisms used by DNSSEC to provide authenticated denial-of-existence responses.NSEC and NSEC3 are described below.

Online signing:

[RFC 4470] defines "on-line signing" (note the hyphen) as"generating and signing these records on demand", where "these" was definedas NSEC records. The current definition expands that togenerating and signing RRSIG, NSEC, and NSEC3 records on demand.

Unsigned zone:

Section 2 of RFC 4033 defines this as "a zone that is not signed". Section 2 of RFC 4035 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 therules in this section..." There is an important note at the end of Section 5.2 of RFC 4035 that defines anadditional situation in which a zone is considered unsigned:"If the resolver does not support any ofthe algorithms listed in an authenticated DS RRset, then the resolver will not be able to verify theauthentication path to the child zone. In this case, the resolver SHOULD treat the child zone as ifit were unsigned."

NSEC:

"The NSEC record allows a security-aware resolver to authenticate a negative reply foreither name or type non-existence with the same mechanisms used to authenticate other DNS replies."(Quoted from RFC 4033, Section 3.2) In short, an NSEC record provides authenticated denial ofexistence.

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

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 [RFC 5155].

Note that [RFC 6840] says that [RFC 5155] "is now considered part of the DNS Security Document Familyas described by Section 10 of RFC 4033". This means that some of the definitions from earlier RFCs thatonly 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 RFC 5155, Section 3.1.2.1)Opt-out tackles the high costs of securing a delegation to an insecure zone. When usingOpt-Out, names that are an insecure delegation (and empty non-terminals that are onlyderived from insecure delegations) don't require an NSEC3 record or its correspondingRRSIG records. Opt-Out NSEC3 records are not able to prove or deny the existence of theinsecure delegations. (Adapted from RFC 7129, 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 RFC 4956, Section 2)

Zone enumeration:

"The practice of discovering the full content of a zone via successive queries."(Quoted from RFC 5155, Section 1.3) This is also sometimes called "zone walking".Zone enumeration is different from zone content guessing where the guesser uses a large dictionaryof possible labels and sends successive queries for them, or matches the contents of NSEC3 recordsagainst such a dictionary.

Validation:

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

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

The first two definitions above consider only the validity of individual DNSSECcomponents, such as the RRSIG validity or NSEC proof validity. The third definitionconsiders the components of the entire DNSSEC authentication chain; thus, it requires"configured knowledge of at least one authenticated DNSKEY or DS RR" (as described inRFC 4035, Section 5).

RFC 4033, Section 2, says that a "Validating Security-Aware StubResolver... performs signature validation" and uses a trust anchor "as a starting pointfor building the authentication chain to a signed DNS response"; thus, it uses the firstand third definitions above. The process of validating an RRSIG resource record is described in RFC 4035, Section 5.3.

[RFC 5155] refers to validating responses throughout the document in thecontext of hashed authenticated denial of existence; this uses the second definitionabove.

The term "authentication" is used interchangeably with "validation", in the sense of thethird definition above.RFC 4033, Section 2, describes the chain linking trust anchor to DNS data as the "authentication chain". Aresponse is considered to be authentic if "all RRsets in the Answer and Authority sectionsof the response [are considered] to be authentic" (Quoted from [RFC 4035]) DNS data orresponses deemed to be authentic or validated have a security status of "secure" (RFC 4035, Section 4.3; RFC 4033, Section 5). "Authenticatingboth DNS keys and data is a matter of local policy, which may extend or even override the[DNSSEC] protocol extensions..." (Quoted from RFC 4033, 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 fromRFC 6781, Section 3.1)

Zone signing key (ZSK):

"DNSSEC keys that can be used to sign all the RRsets in a zone thatrequire signatures, other than the apex DNSKEY RRset." (Quoted from RFC 6781, 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 SigningScheme." (Quoted from RFC 6781, Section 3.1) This is sometimes called a "combinedsigning key" or "CSK". It is operational practice, not protocol, that determines whether aparticular 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 betweenkeys that are intended to be used as the secure entry point into the zone when buildingchains of trust, i.e., they are (to be) pointed to by parental DS RRs or configured as atrust anchor.... Therefore, it is suggested that the SEP flag be set on keys that are used as KSKs and not on keysthat 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 RFC 6781, Section 3.2.3) Note that theSEP flag is only a hint, and its presence or absence may not be used to disqualify a givenDNSKEY RR from use as a KSK or ZSK during validation.

The original definition of SEPs was in [RFC 3757]. That definitionclearly indicated that the SEP was a key, not just a bit in the key. Theabstract of [RFC 3757] says:"With the Delegation Signer (DS) resource record (RR), the concept ofa public key acting as a secure entry point (SEP) has beenintroduced. During exchanges of public keys with the parent there isa need to differentiate SEP keys from other public keys in the DomainName System KEY (DNSKEY) resource record set. A flag bit in theDNSKEY 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 [RFC 4034],and the definition from [RFC 6781] is consistent with [RFC 4034].

Trust anchor:

"A configured DNSKEY RR or DS RR hash of a DNSKEY RR. Avalidating security-aware resolver uses this public key or hash asa starting point for building the authentication chain to a signedDNS response. In general, a validating resolver will have toobtain the initial values of its trust anchors via some secure ortrusted means outside the DNS protocol." (Quoted from RFC 4033, Section 2)

DNSSEC Policy (DP):

A statement that "sets forth the security requirements andstandards to be implemented for a DNSSEC-signed zone." (Quoted from RFC 6841, Section 2)

DNSSEC Practice Statement (DPS):

"A practices disclosure document that maysupport and be a supplemental document to the DNSSEC Policy (if such exists),and it states how the management of a given zone implements procedures andcontrols at a high level." (Quoted from RFC 6841, Section 2)

Hardware security module (HSM):

A specialized piece of hardware that is used to create keys for signatures and tosign messages without ever disclosing the private key. In DNSSEC, HSMs are often used to hold the private keys forKSKs 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 thatfacilitates the creation and maintenance of DNSSEC signatures in zones.There is also stand-alone software that can be used to sign a zone regardlessof whether the authoritative server itself supports signing. Sometimessigning software can support particular HSMs as part of the signing process.

A validating resolver can determine that a response is in one of four states: secure, insecure, bogus, or indeterminate. These states are defined in [RFC 4033] and [RFC 4035], 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 RFC 4033 says:

---

---

Section 4.3 of RFC 4035 says:

---

---

These definitions do not change any security considerations for either the global DNS or private DNS.

References to RFC 8499 in the IANA registries have been replaced with references to this document.

[IANA_RootFiles]

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

[RFC0882]

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

[RFC1034]

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

[RFC1035]

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

[RFC1123]

R. Braden, "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]

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

[RFC1996]

P. Vixie, "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]

P. Vixie, S. Thomson, Y. Rekhter, 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]

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

[RFC2182]

R. Elz, R. Bush, S. Bradner, 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]

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

[RFC4033]

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

[RFC4034]

R. Arends, R. Austein, M. Larson, D. Massey, 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]

R. Arends, R. Austein, M. Larson, D. Massey, 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]

E. Lewis, "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]

B. Laurie, G. Sisson, R. Arends, 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]

J. Damas, 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]

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

[RFC5731]

S. Hollenbeck, "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]

J. Abley, 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]

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

[RFC6561]

J. Livingood, N. Mody, 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]

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

[RFC6840]

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

[RFC6841]

F. Ljunggren, AM. Eklund Lowinder, 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]

J. Damas, M. Graff, 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]

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

[RFC7719]

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

[RFC8310]

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

[RFC8499]

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

[RFC9250]

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

[RFC9471]

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

[IANA_Resource_Registry]

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

[RFC20]

V.G. Cerf, "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.

[RFC819]

Z. Su, 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]

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

[RFC1713]

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

[RFC1995]

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

[RFC2775]

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

[RFC3172]

G. Huston, "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]

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

[RFC3493]

R. Gilligan, S. Thomson, J. Bound, J. McCann, 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]

O. Kolkman, J. Schlyter, 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]

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

[RFC4470]

S. Weiler, and J. Ihren, "Minimally Covering NSEC Records and DNSSEC On-line Signing", RFC 4470, DOI 10.17487/RFC4470, April 2006,
<https://www.rfc-editor.org/info/rfc4470>.

[RFC4641]

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

[RFC4697]

M. Larson, 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]

J. Abley, 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]

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

[RFC5625]

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

[RFC5890]

J. Klensin, "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]

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

[RFC5892]

P. Faltstrom, "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]

H. Alvestrand, 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]

J. Klensin, "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]

D. Thaler, J. Klensin, 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]

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

[RFC6303]

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

[RFC6335]

M. Cotton, L. Eggert, J. Touch, M. Westerlund, 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]

P. Hoffman, 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]

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

[RFC6762]

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

[RFC7129]

R. Gieben, 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]

A. Newton, B. Ellacott, and N. Kong, "HTTP Usage in the Registration Data Access Protocol (RDAP)", STD 95, RFC 7480, DOI 10.17487/RFC7480, March 2015,
<https://www.rfc-editor.org/info/rfc7480>.

[RFC7481]

S. Hollenbeck, and N. Kong, "Security Services for the Registration Data Access Protocol (RDAP)", STD 95, RFC 7481, DOI 10.17487/RFC7481, March 2015,
<https://www.rfc-editor.org/info/rfc7481>.

[RFC9082]

S. Hollenbeck, and A. Newton, "Registration Data Access Protocol (RDAP) Query Format", STD 95, RFC 9082, DOI 10.17487/RFC9082, June 2021,
<https://www.rfc-editor.org/info/rfc9082>.

[RFC9083]

S. Hollenbeck, and A. Newton, "JSON Responses for the Registration Data Access Protocol (RDAP)", STD 95, RFC 9083, DOI 10.17487/RFC9083, June 2021,
<https://www.rfc-editor.org/info/rfc9083>.

[RFC9224]

M. Blanchet, "Finding the Authoritative Registration Data Access Protocol (RDAP) Service", STD 95, RFC 9224, DOI 10.17487/RFC9224, March 2022,
<https://www.rfc-editor.org/info/rfc9224>.

[RFC7485]

L. Zhou, N. Kong, S. Shen, S. Sheng, 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]

M. Andrews, "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]

Z. Hu, L. Zhu, J. Heidemann, A. Mankin, D. Wessels, 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]

T. Reddy, D. Wing, 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]

P. Koch, M. Larson, 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]

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

[RFC9103]

W. Toorop, S. Dickinson, S. Sahib, P. Aras, and A. Mankin, "DNS Zone Transfer over TLS", RFC 9103, DOI 10.17487/RFC9103, August 2021,
<https://www.rfc-editor.org/info/rfc9103>.

[RSSAC026]

Root Server System Advisory Committee (RSSAC), "RSSAC0226 RSSAC Lexicon", 2017,
<https://www.icann.org/en/system/files/files/rssac-026-14mar17-en.pdf>.

The following definitions from RFCs are updated by this document:

• Forwarder in [RFC 2308]
• QNAME in [RFC 2308]
• Secure Entry Point (SEP) in [RFC 3757]; note, however, that this RFC is already obsolete (see [RFC 4033], [RFC 4034], [RFC 4035]).

The following definitions are first defined in this document:

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

[RFC 8499] and its predecessor, [RFC 7719], were co-authored by Andrew Sullivan. The current document, which is a small update to [RFC 8499], has just two authors. Andrew's work on the earlier documents is greatly appreciated. Numerous people made significant contributions to [RFC 8499] and [RFC 7719]. Please see the acknowledgements sections in those two documents for the extensive list of contributors. Even though the current document is a small revision, many people in the DNSOP Working Group have contributed to it, and their work is greatly appreciated.

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

• A

  • Address and Routing Parameter Area Domain (arpa)

    Section 7

    Address records

    Section 5

    ADoT

    Section 6

    Alias

    Section 2

    Anycast

    Section 6

    Apex

    Section 7

    Asterisk label

    Section 8

    Authoritative data

    Section 7

    Authoritative server

    Section 6

    Authoritative-only server

    Section 6

    AXoT

    Section 6
• B

  • Bailiwick

    Section 7
• C

  • Canonical name

    Section 2

    Child

    Section 7

    Class

    Section 4

    Class independent

    Section 5

    Classic DNS

    Section 6

    Closest encloser

    Section 8

    Closest provable encloser

    Section 8

    CNAME

    Section 2

    Combined signing key (CSK)

    Section 10
• D

  • Delegation

    Section 7

    Delegation-centric zone

    Section 7

    DNS operator

    Section 9

    DNS-over-HTTPS

    Section 6

    DNS-over-QUIC

    Section 6

    DNS-over-TLS

    Section 6

    DNSSEC Policy (DP)

    Section 10

    DNSSEC Practice Statement (DPS)

    Section 10

    DNSSEC-aware and DNSSEC-unaware

    Section 10

    DoH

    Section 6

    Domain name

    Section 2

    DoQ

    Section 6

    DoT

    Section 6
• E

  • EDNS

    Section 5

    Empty non-terminals (ENTs)

    Section 7

    EPP

    Section 9
• F

  • Fast flux DNS

    Section 7

    FORMERR

    Section 3

    Forward lookup

    Section 7

    Forwarder

    Section 6

    Forwarding

    Section 6

    Full resolver

    Section 6

    Full-service resolver

    Section 6

    Fully Qualified Domain Name (FQDN)

    Section 2
• G

  • Global DNS

    Section 2

    Glue records

    Section 7
• H

  • Hardware security module (HSM)

    Section 10

    Hidden master

    Section 6

    Host name

    Section 2
• I

  • IDN

    Section 2

    In-bailiwick

    Section 7

    In-domain

    Section 7

    Insecure delegation

    Section 10

    Instance

    Section 6

    Internationalized Domain Name

    Section 2

    Iterative mode

    Section 6

    Iterative resolution

    Section 6

    IXoT

    Section 6
• K

  • Key signing key (KSK)

    Section 10
• L

  • Label

    Section 2

    Lame delegation

    Section 7

    Locally served DNS zone

    Section 2
• M

  • Master file

    Section 5

    Master server

    Section 6

    mDNS

    Section 2

    Multicast DNS

    Section 2
• N

  • Naming system

    Section 2, Paragraph 1.2.1

    Negative caching

    Section 6

    Negative response

    Section 3

    Next closer name

    Section 8

    NODATA

    Section 3

    NOERROR

    Section 3

    Non-recursive query

    Section 6

    NOTIMP

    Section 3

    NS

    Section 6

    NSEC

    Section 10

    NSEC3

    Section 10

    NXDOMAIN

    Section 3
• O

  • Occluded name

    Section 7

    on-line signing

    Section 10

    online signing

    Section 10

    Open resolver

    Section 6

    OPT

    Section 5

    Opt-out

    Section 10

    Origin

    Section 7

    Out-of-bailiwick

    Section 7

    Owner

    Section 5
• P

  • Parent

    Section 7

    Passive DNS

    Section 6

    Policy-implementing resolver

    Section 6

    Presentation format

    Section 5

    Primary master

    Section 6

    Primary server

    Section 6

    Priming

    Section 6

    Privacy-enabling DNS server

    Section 6

    Private DNS

    Section 2

    Public suffix

    Section 9
• Q

  • QNAME

    Section 4
• R

  • RDAP

    Section 9

    RDoT

    Section 6

    Recursive DoT

    Section 6

    Recursive mode

    Section 6, Paragraph 4.10.1

    Recursive query

    Section 6

    Recursive resolver

    Section 6

    Referrals

    Section 4

    REFUSED

    Section 3

    Registrant

    Section 9

    Registrar

    Section 9

    Registry

    Section 9

    Resolver

    Section 6

    Reverse DNS, reverse lookup

    Section 7

    Root hints

    Section 6

    Root zone

    Section 7

    RR

    Section 5

    RRset

    Section 5
• S

  • Secondary server

    Section 6

    Secure Entry Point (SEP)

    Section 10

    SERVFAIL

    Section 3

    Service name

    Section 7

    Sibling domain

    Section 7

    Signed zone

    Section 10

    Signing software

    Section 10

    Slave server

    Section 6

    SOA

    Section 5

    SOA field names

    Section 5

    Source of Synthesis

    Section 8, Paragraph 1.14.1

    Split DNS

    Section 6

    Split-horizon DNS

    Section 6

    Stealth server

    Section 6

    Stub resolver

    Section 6

    Subdomain

    Section 2

    Subordinate

    Section 9

    Superordinate

    Section 9
• T

  • TLD

    Section 2

    Trust anchor

    Section 10

    TTL

    Section 5
• U

  • Unsigned zone

    Section 10
• V

  • Validating resolver

    Section 10

    Validation

    Section 10, Paragraph 2.26.1

    View

    Section 6
• W

  • WHOIS

    Section 9

    Wildcard

    Section 8

    Wildcard domain name

    Section 8
• X

  • XoT

    Section 6
• Z

  • Zone

    Section 7

    Zone cut

    Section 7

    Zone enumeration

    Section 10

    Zone signing key (ZSK)

    Section 10

    Zone transfer

    Section 6

Paul Hoffman

Kazunori Fujiwara