Internet Engineering Task Force (IETF)                        P. Hoffman   
 Request for Comments: 9499                                         ICANN   
 BCP: 219                                                     K. Fujiwara   
 Obsoletes: 8499                                                     JPRS   
 Updates: 2308                                                 March 2024   
 Category: Best Current Practice                                            
 ISSN: 2070-1721                                                            
                                                                            
                                                                            
                             DNS Terminology                                
                                                                            
 Abstract                                                                   
                                                                            
    The Domain Name System (DNS) is defined in literally dozens of          
    different RFCs.  The terminology used by implementers and developers    
    of DNS protocols, and by operators of DNS systems, has changed in the   
    decades since the DNS was first defined.  This document gives current   
    definitions for many of the terms used in the DNS in a single           
    document.                                                               
                                                                            
    This document updates RFC 2308 by clarifying the definitions of         
    "forwarder" and "QNAME".  It obsoletes RFC 8499 by adding multiple      
    terms and clarifications.  Comprehensive lists of changed and new       
    definitions can be found in Appendices A and B.                         
                                                                            
 Status of This Memo                                                        
                                                                            
    This memo documents an Internet Best Current Practice.                  
                                                                            
    This document is a product of the Internet Engineering Task Force       
    (IETF).  It represents the consensus of the IETF community.  It has     
    received public review and has been approved for publication by the     
    Internet Engineering Steering Group (IESG).  Further information on     
    BCPs is available in Section 2 of RFC 7841.                             
                                                                            
    Information about the current status of this document, any errata,      
    and how to provide feedback on it may be obtained at                    
    https://www.rfc-editor.org/info/rfc9499.                                
                                                                            
 Copyright Notice                                                           
                                                                            
    Copyright (c) 2024 IETF Trust and the persons identified as the         
    document authors.  All rights reserved.                                 
                                                                            
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    in the Revised BSD License.                                             
                                                                            
 Table of Contents                                                          
                                                                            
    1.  Introduction                                                        
    2.  Names                                                               
    3.  DNS Response Codes                                                  
    4.  DNS Transactions                                                    
    5.  Resource Records                                                    
    6.  DNS Servers and Clients                                             
    7.  Zones                                                               
    8.  Wildcards                                                           
    9.  Registration Model                                                  
    10. General DNSSEC                                                      
    11. DNSSEC States                                                       
    12. Security Considerations                                             
    13. IANA Considerations                                                 
    14. References                                                          
      14.1.  Normative References                                           
      14.2.  Informative References                                         
    Appendix A.  Definitions Updated by This Document                       
    Appendix B.  Definitions First Defined in This Document                 
    Acknowledgements                                                        
    Index                                                                   
    Authors' Addresses                                                      
                                                                            
 .  Introduction                                                           
                                                                            
    The Domain Name System (DNS) is a simple query-response protocol        
    whose messages in both directions have the same format.  (Section 2     
    gives a definition of "global DNS", which is often what people mean     
    when they say "the DNS".)  The protocol and message format are          
    defined in [RFC1034] and [RFC1035].  These RFCs defined some terms,     
    and later documents defined others.  Some of the terms from [RFC1034]   
    and [RFC1035] have somewhat different meanings now than they did in     
    1987.                                                                   
                                                                            
    This document contains a collection of a wide variety of DNS-related    
    terms, organized loosely by topic.  Some of them have been precisely    
    defined in earlier RFCs, some have been loosely defined in earlier      
    RFCs, and some are not defined in an earlier RFC at all.                
                                                                            
    Other organizations sometimes define DNS-related terms in their own     
    way.  For example, the WHATWG defines "domain" at                       
    <https://url.spec.whatwg.org/>.  The Root Server System Advisory        
    Committee (RSSAC) has a good lexicon [RSSAC026].                        
                                                                            
    Most of the definitions listed here represent the consensus             
    definition of the DNS community -- both protocol developers and         
    operators.  Some of the definitions differ from earlier RFCs, and       
    those differences are noted.  In this document, where the consensus     
    definition is the same as the one in an RFC, that RFC is quoted.        
    Where the consensus definition has changed somewhat, the RFC is         
    mentioned but the new stand-alone definition is given.  See             
    Appendix A for a list of the definitions that this document updates.    
                                                                            
    It is important to note that, during the development of this            
    document, it became clear that some DNS-related terms are interpreted   
    quite differently by different DNS experts.  Further, some terms that   
    are defined in early DNS RFCs now have definitions that are generally   
    agreed to, but that are different from the original definitions.        
    This document is a small revision to [RFC8499]; that document was a     
    substantial revision to [RFC7719].                                      
                                                                            
    Note that there is no single consistent definition of "the DNS".  It    
    can be considered to be some combination of the following: a commonly   
    used naming scheme for objects on the Internet; a distributed           
    database representing the names and certain properties of these         
    objects; an architecture providing distributed maintenance,             
    resilience, and loose coherency for this database; and a simple         
    query-response protocol (as mentioned below) implementing this          
    architecture.  Section 2 defines "global DNS" and "private DNS" as a    
    way to deal with these differing definitions.                           
                                                                            
    Capitalization in DNS terms is often inconsistent among RFCs and        
    various DNS practitioners.  The capitalization used in this document    
    is a best guess at current practices, and is not meant to indicate      
    that other capitalization styles are wrong or archaic.  In some         
    cases, multiple styles of capitalization are used for the same term     
    due to quoting from different RFCs.                                     
                                                                            
    In this document, the words "byte" and "octet" are used                 
    interchangeably.  They appear here because they both appear in the      
    earlier RFCs that defined terms in the DNS.                             
                                                                            
    Readers should note that the terms in this document are grouped by      
    topic.  Someone who is not already familiar with the DNS probably       
    cannot learn about the DNS from scratch by reading this document from   
    front to back.  Instead, skipping around may be the only way to get     
    enough context to understand some of the definitions.  This document    
    has an index that might be useful for readers who are attempting to     
    learn the DNS by reading this document.                                 
                                                                            
 .  Names                                                                  
                                                                            
    Naming system:  A naming system associates names with data.  Naming     
       systems have many significant facets that help differentiate them    
       from each other.  Some commonly identified facets include:           
                                                                            
       *  Composition of names                                              
                                                                            
       *  Format of names                                                   
                                                                            
       *  Administration of names                                           
                                                                            
       *  Types of data that can be associated with names                   
                                                                            
       *  Types of metadata for names                                       
                                                                            
       *  Protocol for getting data from a name                             
                                                                            
       *  Context for resolving a name                                      
                                                                            
       Note that this list is a small subset of facets that people have     
       identified over time for naming systems, and the IETF has yet to     
       agree on a good set of facets that can be used to compare naming     
       systems.  For example, other facets might include "protocol to       
       update data in a name", "privacy of names", and "privacy of data     
       associated with names", but those are not as well defined as the     
       ones listed above.  The list here is chosen because it helps         
       describe the DNS and naming systems similar to the DNS.              
                                                                            
    Domain name:  An ordered list of one or more labels.                    
                                                                            
       Note that this is a definition independent of the DNS RFCs           
       ([RFC1034] and [RFC1035]), and the definition here also applies to   
       systems other than the DNS.  [RFC1034] defines the "domain name      
       space" using mathematical trees and their nodes in graph theory,     
       and that definition has the same practical result as the             
       definition here.  Any path of a directed acyclic graph can be        
       represented by a domain name consisting of the labels of its         
       nodes, ordered by decreasing distance from the root(s) (which is     
       the normal convention within the DNS, including this document).  A   
       domain name whose last label identifies a root of the graph is       
       fully qualified; other domain names whose labels form a strict       
       prefix of a fully qualified domain name are relative to its first    
       omitted node.                                                        
                                                                            
       Also note that different IETF and non-IETF documents have used the   
       term "domain name" in many different ways.  It is common for         
       earlier documents to use "domain name" to mean "names that match     
       the syntax in [RFC1035]", but possibly with additional rules such    
       as "and are, or will be, resolvable in the global DNS" or "but       
       only using the presentation format".                                 
                                                                            
    Label:  An ordered list of zero or more octets that makes up a          
       portion of a domain name.  Using graph theory, a label identifies    
       one node in a portion of the graph of all possible domain names.     
                                                                            
    Global DNS:  Using the short set of facets listed in "Naming system",   
       the global DNS can be defined as follows.  Most of the rules here    
       come from [RFC1034] and [RFC1035], although the term "global DNS"    
       has not been defined before now.                                     
                                                                            
       Composition of names:  A name in the global DNS has one or more      
          labels.  The length of each label is between 0 and 63 octets      
          inclusive.  In a fully qualified domain name, the last label in   
          the ordered list is 0 octets long; it is the only label whose     
          length may be 0 octets, and it is called the "root" or "root      
          label".  A domain name in the global DNS has a maximum total      
          length of 255 octets in the wire format; the root represents      
          one octet for this calculation.  (Multicast DNS [RFC6762]         
          allows names up to 255 bytes plus a terminating zero byte based   
          on a different interpretation of RFC 1035 and what is included    
          in the 255 octets.)                                               
                                                                            
       Format of names:  Names in the global DNS are domain names.  There   
          are three formats: wire format, presentation format, and common   
          display.                                                          
                                                                            
          Wire format:  The basic wire format for names in the global DNS   
             is a list of labels ordered by decreasing distance from the    
             root, with the root label last.  Each label is preceded by a   
             length octet.  [RFC1035] also defines a compression scheme     
             that modifies this format.                                     
                                                                            
          Presentation format:  The presentation format for names in the    
             global DNS is a list of labels ordered by decreasing           
             distance from the root, encoded as ASCII, with a "."           
             character between each label.  In presentation format, a       
             fully qualified domain name includes the root label and the    
             associated separator dot.  For example, in presentation        
             format, a fully qualified domain name with two non-root        
             labels is always shown as "example.tld." instead of            
             "example.tld".  [RFC1035] defines a method for showing         
             octets that do not display in ASCII.                           
                                                                            
          Common display format:  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 the definition of "delegation" in Section 7).     
          Policies for administration of the root zone in the global DNS    
          are determined by the names operational community, which          
          convenes itself in the Internet Corporation for Assigned Names    
          and Numbers (ICANN).  The names operational community selects     
          the IANA Functions Operator for the global DNS root zone.  The    
          name servers that serve the root zone are provided by             
          independent root operators.  Other zones in the global DNS have   
          their own policies for administration.                            
                                                                            
       Types of data that can be associated with names:  A name can have    
          zero or more resource records associated with it.  There are      
          numerous types of resource records with unique data structures    
          defined in many different RFCs and in the IANA registry at        
          [IANA_Resource_Registry].                                         
                                                                            
       Types of metadata for names:  Any name that is published in the      
          DNS appears as a set of resource records (see the definition of   
          "RRset" in Section 5).  Some names do not, themselves, have       
          data associated with them in the DNS, but they "appear" in the    
          DNS anyway because they form part of a longer name that does      
          have data associated with it (see the definition of "empty non-   
          terminals" in Section 7).                                         
                                                                            
       Protocol for getting data from a name:  The protocol described in    
          [RFC1035].                                                        
                                                                            
       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 [RFC1035] but    
       do not rely on the global DNS root zone or names that are            
       otherwise not generally available on the Internet but are using      
       the protocol described in [RFC1035].  A system can use both the      
       global DNS and one or more private DNS systems; for example, see     
       "Split DNS" in Section 6.                                            
                                                                            
       Note that domain names that do not appear in the DNS and that are    
       intended never to be looked up using the DNS protocol are not part   
       of the global DNS or a private DNS, even though they are domain      
       names.                                                               
                                                                            
    Multicast DNS (mDNS):  "Multicast DNS (mDNS) provides the ability to    
       perform DNS-like operations on the local link in the absence of      
       any conventional Unicast DNS server.  In addition, Multicast DNS     
       designates a portion of the DNS namespace to be free for local       
       use, without the need to pay any annual fee, and without the need    
       to set up delegations or otherwise configure a conventional DNS      
       server to answer for those names."  (Quoted from [RFC6762],          
       Abstract) Although it uses a compatible wire format, mDNS is,        
       strictly speaking, a different protocol than DNS.  Also, where the   
       above quote says "a portion of the DNS namespace", it would be       
       clearer to say "a portion of the domain name space".  The names in   
       mDNS are not intended to be looked up in the DNS.                    
                                                                            
    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.  [RFC6303] defines subdomains of IN-ADDR.ARPA that    
       are locally served zones.  Resolution of names through locally       
       served zones may result in ambiguous results.  For example, the      
       same name may resolve to different results in different locally      
       served DNS zone contexts.  The context for a locally served DNS      
       zone may be explicit, such as those that are listed in [RFC6303]     
       and [RFC7793], or implicit, such as those defined by local DNS       
       administration and not known to the resolution client.               
                                                                            
    Fully Qualified Domain Name (FQDN):  This is often just a clear way     
       of saying the same thing as "domain name of a node", as outlined     
       above.  However, the term is ambiguous.  Strictly speaking, a        
       fully qualified domain name would include every label, including     
       the zero-length label of the root; such a name would be written      
       "www.example.net." (note the terminating dot).  But, because every   
       name eventually shares the common root, names are often written      
       relative to the root (such as "www.example.net") and are still       
       called "fully qualified".  This term first appeared in [RFC819].     
       In this document, names are often written relative to the root.      
                                                                            
       The need for the term "fully qualified domain name" comes from the   
       existence of partially qualified domain names, which are names       
       where one or more of the last labels in the ordered list are         
       omitted (for example, a domain name of "www" relative to             
       "example.net" identifies "www.example.net").  Such relative names    
       are understood only by context.                                      
                                                                            
    Host name:  This term and its equivalent, "hostname", have been         
       widely used but are not defined in [RFC1034], [RFC1035],             
       [RFC1123], or [RFC2181].  The DNS was originally deployed into the   
       Host Tables environment as outlined in [RFC952], and it is likely    
       that the term followed informally from the definition there.  Over   
       time, the definition seems to have shifted.  "Host name" is often    
       meant to be a domain name that follows the rules in Section 3.5 of   
       [RFC1034], which is also called the "preferred name syntax".  (In    
       that syntax, every character in each label is a letter, a digit,     
       or a hyphen).  Note that any label in a domain name can contain      
       any octet value; hostnames are generally considered to be domain     
       names where every label follows the rules in the "preferred name     
       syntax", with the amendment that labels can start with ASCII         
       digits (this amendment comes from Section 2.1 of [RFC1123]).         
                                                                            
       People also sometimes use the term "hostname" to refer to just the   
       first label of an FQDN, such as "printer" in                         
       "printer.admin.example.com".  (Sometimes this is formalized in       
       configuration in operating systems.)  In addition, people            
       sometimes use this term to describe any name that refers to a        
       machine, and those might include labels that do not conform to the   
       "preferred name syntax".                                             
                                                                            
    Top-Level Domain (TLD):  A Top-Level Domain is a zone that is one       
       layer below the root, such as "com" or "jp".  There is nothing       
       special, from the point of view of the DNS, about TLDs.  Most of     
       them are also delegation-centric zones (defined in Section 7), and   
       there are significant policy issues around their operation.  TLDs    
       are often divided into sub-groups such as Country Code Top-Level     
       Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others;     
       the division is a matter of policy and beyond the scope of this      
       document.                                                            
                                                                            
    Internationalized Domain Name (IDN):  The Internationalized Domain      
       Names for Applications (IDNA) protocol is the standard mechanism     
       for handling domain names with non-ASCII characters in               
       applications in the DNS.  The current standard at the time of this   
       writing, normally called "IDNA2008", is defined in [RFC5890],        
       [RFC5891], [RFC5892], [RFC5893], and [RFC5894].  These documents     
       define many IDN-specific terms such as "LDH label", "A-label", and   
       "U-label".  [RFC6365] defines more terms that relate to              
       internationalization (some of which relate to IDNs); [RFC6055] has   
       a much more extensive discussion of IDNs, including some new         
       terminology.                                                         
                                                                            
    Subdomain:  "A domain is a subdomain of another domain if it is         
       contained within that domain.  This relationship can be tested by    
       seeing if the subdomain's name ends with the containing domain's     
       name."  (Quoted from [RFC1034], Section 3.1) For example, in the     
       host name "nnn.mmm.example.com", both "mmm.example.com" and          
       "nnn.mmm.example.com" are subdomains of "example.com".  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 a DNAME resource record (DNAME records are defined in       
       [RFC6672]).  See also "canonical name".                              
                                                                            
    Canonical name:  A CNAME resource record "identifies its owner name     
       as an alias, and specifies the corresponding canonical name in the   
       RDATA section of the RR."  (Quoted from [RFC1034], Section 3.6.2)    
       This usage of the word "canonical" is related to the mathematical    
       concept of "canonical form".                                         
                                                                            
    CNAME:  "It has been traditional to refer to the [owner] of a CNAME     
       record as 'a CNAME'.  This is unfortunate, as 'CNAME' is an          
       abbreviation of 'canonical name', and the [owner] of a CNAME         
       record is most certainly not a canonical name."  (Quoted from        
       [RFC2181], Section 10.1.1.  The quoted text has been changed from    
       "label" to "owner".)                                                 
                                                                            
 .  DNS Response Codes                                                     
                                                                            
    Some of the response codes (RCODEs) that are defined in [RFC1035]       
    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 [RFC1035] are described in this      
    section.  This section also includes an additional RCODE and a          
    general definition.  The official list of all RCODEs is in the IANA     
    registry.                                                               
                                                                            
    NOERROR:  This RCODE appears as "No error condition" in Section 4.1.1   
       of [RFC1035].                                                        
                                                                            
    FORMERR:  This RCODE appears as "Format error - The name server was     
       unable to interpret the query" in Section 4.1.1 of [RFC1035].        
                                                                            
    SERVFAIL:  This RCODE appears as "Server failure - The name server      
       was unable to process this query due to a problem with the name      
       server" in Section 4.1.1 of [RFC1035].                               
                                                                            
    NXDOMAIN:  This RCODE appears as "Name Error [...] this code            
       signifies that the domain name referenced in the query does not      
       exist." in Section 4.1.1 of [RFC1035].  [RFC2308] 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    
       [RFC1035].                                                           
                                                                            
    REFUSED:  This RCODE appears as "Refused - The name server refuses to   
       perform the specified operation for policy reasons.  For example,    
       a name server may not wish to provide the information to the         
       particular requester, or a name server may not wish to perform a     
       particular operation (e.g., zone transfer) for particular data."     
       in Section 4.1.1 of [RFC1035].                                       
                                                                            
    NODATA:  "A pseudo RCODE which indicates that the name is valid, for    
       the given class, but [there] are no records of the given type.  A    
       NODATA response has to be inferred from the answer."  (Quoted from   
       [RFC2308], Section 1) "NODATA is indicated by an answer with the     
       RCODE set to NOERROR and no relevant answers in the Answer           
       section.  The Authority section will contain an SOA record, or       
       there will be no NS records there."  (Quoted from [RFC2308],         
       Section 2.2) Note that referrals have a similar format to NODATA     
       replies; [RFC2308] explains how to distinguish them.                 
                                                                            
       The term "NXRRSET" is sometimes used as a synonym for NODATA.        
       However, this is a mistake, given that NXRRSET is a specific error   
       code defined in [RFC2136].                                           
                                                                            
    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 [RFC2308] describe the types of         
       negative responses in detail.                                        
                                                                            
 .  DNS Transactions                                                       
                                                                            
    The header of a DNS message is its first 12 octets.  Many of the        
    fields and flags in the diagrams in Sections 4.1.1 through 4.1.3 of     
    [RFC1035] 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 [RFC1034], Section 3.6) "The DNS tags all   
       data with a class as well as the type, so that we can allow          
       parallel use of different formats for data of type address."         
       (Quoted from [RFC1034], Section 2.2) In practice, the class for      
       nearly every query is "IN" (the Internet).  There are some queries   
       for "CH" (the Chaos class), but they are usually for the purposes    
       of information about the server itself rather than for a different   
       type of address.                                                     
                                                                            
    QNAME:  The most commonly used rough definition is that the QNAME is    
       a field in the Question section of a query.  "A standard query       
       specifies a target domain name (QNAME), query type (QTYPE), and      
       query class (QCLASS) and asks for RRs which match."  (Quoted from    
       [RFC1034], Section 3.7.1) Strictly speaking, the definition comes    
       from [RFC1035], Section 4.1.2, where the QNAME is defined in         
       respect of the Question section.  This definition appears to be      
       applied consistently, as the discussion of inverse queries in        
       Section 6.4.1 of [RFC1035] refers to the "owner name of the query    
       RR and its TTL" because inverse queries populate the Answer          
       section and leave the Question section empty.  (Inverse queries      
       are deprecated in [RFC3425]; thus, relevant definitions do not       
       appear in this document.)                                            
                                                                            
       However, [RFC2308] has an alternate definition that puts the QNAME   
       in the answer (or series of answers) instead of the query.  It       
       defines QNAME as "...the name in the query section of an answer,     
       or where this resolves to a CNAME, or CNAME chain, the data field    
       of the last CNAME.  The last CNAME in this sense is that which       
       contains a value which does not resolve to another CNAME."  This     
       definition has a certain internal logic, because of the way CNAME    
       substitution works and the definition of CNAME.  If a name server    
       does not find an RRset that matches a query, but does find the       
       same name in the same class with a CNAME record, then the name       
       server "includes the CNAME record in the response and restarts the   
       query at the domain name specified in the data field of the CNAME    
       record."  (Quoted from [RFC1034], Section 3.6.2) This is made        
       explicit in the resolution algorithm outlined in Section 4.3.2 of    
       [RFC1034], which says to "change QNAME to the canonical name in      
       the CNAME RR, and go back to step 1" in the case of a CNAME RR.      
       Since a CNAME record explicitly declares that the owner name is      
       canonically named what is in the RDATA, then there is a way to       
       view the new name (i.e., the name that was in the RDATA of the       
       CNAME RR) as also being the QNAME.                                   
                                                                            
       However, this creates confusion because the response to a query      
       that results in CNAME processing contains in the echoed Question     
       section one QNAME (the name in the original query) and a second      
       QNAME that is in the data field of the last CNAME.  The confusion    
       comes from the iterative/recursive mode of resolution, which         
       finally returns an answer that need not actually have the same       
       owner name as the QNAME contained in the original query.             
                                                                            
       To address this potential confusion, it is helpful to distinguish    
       between three meanings:                                              
                                                                            
       QNAME (original):  The name actually sent in the Question section    
          in the original query, which is always echoed in the (final)      
          reply in the Question section when the QR bit is set to 1.        
                                                                            
       QNAME (effective):  A name actually resolved, which is either the    
          name originally queried or a name received in a CNAME chain       
          response.                                                         
                                                                            
       QNAME (final):  The name actually resolved, which is either the      
          name actually queried or else the last name in a CNAME chain      
          response.                                                         
                                                                            
       Note that, because the definition in [RFC2308] is actually for a     
       different concept than what was in [RFC1034], it would have been     
       better if [RFC2308] had used a different name for that concept.      
       In general use today, QNAME almost always means what is defined      
       above as "QNAME (original)".                                         
                                                                            
    Referrals:  A type of response in which a server, signaling that it     
       is not (completely) authoritative for an answer, provides the        
       querying resolver with an alternative place to send its query.       
       Referrals can be partial.                                            
                                                                            
       A referral arises when a server is not performing recursive          
       service while answering a query.  It appears in step 3(b) of the     
       algorithm in [RFC1034], Section 4.3.2.                               
                                                                            
       There are two types of referral response.  The first is a downward   
       referral (sometimes described as "delegation response"), where the   
       server is authoritative for some portion of the QNAME.  The          
       Authority section RRset's RDATA contains the name servers            
       specified at the referred-to zone cut.  In normal DNS operation,     
       this kind of response is required in order to find names beneath a   
       delegation.  The bare use of "referral" means this kind of           
       referral, and many people believe that this is the only legitimate   
       kind of referral in the DNS.                                         
                                                                            
       The second is an upward referral (sometimes described as "root       
       referral"), where the server is not authoritative for any portion    
       of the QNAME.  When this happens, the referred-to zone in the        
       Authority section is usually the root zone (".").  In normal DNS     
       operation, this kind of response is not required for resolution or   
       for correctly answering any query.  There is no requirement that     
       any server send upward referrals.  Some people regard upward         
       referrals as a sign of a misconfiguration or error.  Upward          
       referrals always need some sort of qualifier (such as "upward" or    
       "root") and are never identified simply by the word "referral".      
                                                                            
       A response that has only a referral contains an empty Answer         
       section.  It contains the NS RRset for the referred-to zone in the   
       Authority section.  It may contain RRs that provide addresses in     
       the Additional section.  The AA bit is clear.                        
                                                                            
       In the case where the query matches an alias, and the server is      
       not authoritative for the target of the alias but is authoritative   
       for some name above the target of the alias, the resolution          
       algorithm will produce a response that contains both the             
       authoritative answer for the alias and a referral.  Such a partial   
       answer and referral response has data in the Answer section.  It     
       has the NS RRset for the referred-to zone in the Authority           
       section.  It may contain RRs that provide addresses in the           
       Additional section.  The AA bit is set because the first name in     
       the Answer section matches the QNAME and the server is               
       authoritative for that answer (see [RFC1035], Section 4.1.1).        
                                                                            
 .  Resource Records                                                       
                                                                            
    RR:  An acronym for resource record.  (See [RFC1034], Section 3.6.)     
                                                                            
    RRset:  A set of resource records "with the same label, class and       
       type, but with different data" (according to [RFC2181],              
       Section 5).  Also written as "RRSet" in some documents.  As a        
       clarification, "same label" in this definition means "same owner     
       name".  In addition, [RFC2181] states that "the TTLs of all RRs in   
       an RRSet must be the same".                                          
                                                                            
       Note that RRSIG resource records do not match this definition.       
       [RFC4035] says:                                                      
                                                                            
          An RRset MAY have multiple RRSIG RRs associated with it.  Note    
          that as RRSIG RRs are closely tied to the RRsets whose            
          signatures they contain, RRSIG RRs, unlike all other DNS RR       
          types, do not form RRsets.  In particular, the TTL values among   
          RRSIG RRs with a common owner name do not follow the RRset        
          rules described in [RFC2181].                                     
                                                                            
    Master file:  "Master files are text files that contain RRs in text     
       form.  Since the contents of a zone can be expressed in the form     
       of a list of RRs a master file is most often used to define a        
       zone, though it can be used to list a cache's contents."  (Quoted    
       from [RFC1035], Section 5) Master files are sometimes called "zone   
       files".                                                              
                                                                            
    Presentation format:  The text format used in master files.  This       
       format is shown but not formally defined in [RFC1034] or             
       [RFC1035].  The term "presentation format" first appears in          
       [RFC4034].                                                           
                                                                            
    EDNS:  The extension mechanisms for DNS, defined in [RFC6891].          
       Sometimes called "EDNS0" or "EDNS(0)" to indicate the version        
       number.  EDNS allows DNS clients and servers to specify message      
       sizes larger than the original 512-octet limit, to expand the        
       response code space, and to carry additional options that affect     
       the handling of a DNS query.                                         
                                                                            
    OPT:  A pseudo-RR (sometimes called a "meta-RR") that is used only to   
       contain control information pertaining to the question-and-answer    
       sequence of a specific transaction.  (Definition paraphrased from    
       [RFC6891], Section 6.1.1.)  It is used by EDNS.                      
                                                                            
    Owner:  "The domain name where the RR is found."  (Quoted from          
       [RFC1034], 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 the RDATA of an SOA resource record     
       by field name.  "SOA" stands for "start of a zone of authority".     
       Those fields are defined in Section 3.3.13 of [RFC1035].  The        
       names (in the order they appear in the SOA RDATA) are MNAME,         
       RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM.  Note that the   
       meaning of the MINIMUM field is updated in Section 4 of [RFC2308];   
       the new definition is that the MINIMUM field is only "the TTL to     
       be used for negative responses".  This document tends to use field   
       names instead of terms that describe the fields.                     
                                                                            
    TTL:  The maximum "time to live" of a resource record.  "A TTL value    
       is an unsigned number, with a minimum value of 0, and a maximum      
       value of 2147483647.  That is, a maximum of 2^31 - 1.  When          
       transmitted, this value shall be encoded in the less significant     
       31 bits of the 32 bit TTL field, with the most significant, or       
       sign, bit set to zero."  (Quoted from [RFC2181], Section 8) Note     
       that [RFC1035] erroneously stated that this is a signed integer;     
       that was fixed by [RFC2181].                                         
                                                                            
       The TTL "specifies the time interval that the resource record may    
       be cached before the source of the information should again be       
       consulted."  (Quoted from [RFC1035], Section 3.2.1) Section 4.1.3    
       of [RFC1035] states "the time interval (in seconds) that the         
       resource record may be cached before it should be discarded".        
       Despite being defined for a resource record, the TTL of every        
       resource record in an RRset is required to be the same ([RFC2181],   
       Section 5.2).                                                        
                                                                            
       The reason that the TTL is the maximum time to live is that a        
       cache operator might decide to shorten the time to live for          
       operational purposes, for example, if there is a policy to           
       disallow TTL values over a certain number.  Some servers are known   
       to ignore the TTL on some RRsets (such as when the authoritative     
       data has a very short TTL) even though this is against the advice    
       in [RFC1035].  An RRset can be flushed from the cache before the     
       end of the TTL interval, at which point, the value of the TTL        
       becomes unknown because the RRset with which it was associated no    
       longer exists.                                                       
                                                                            
       There is also the concept of a "default TTL" for a zone, which can   
       be a configuration parameter in the server software.  This is        
       often expressed by a default for the entire server, and a default    
       for a zone using the $TTL directive in a zone file.  The $TTL        
       directive was added to the master file format by [RFC2308].          
                                                                            
    Class independent:  A resource record type whose syntax and semantics   
       are the same for every DNS class.  A resource record type that is    
       not class independent has different meanings, depending on the DNS   
       class of the record or 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.  [RFC2181]    
       informally defines these as "(A, AAAA, etc)".  Note that new types   
       of address records could be defined in the future.                   
                                                                            
 .  DNS Servers and Clients                                                
                                                                            
    This section defines the terms used for the systems that act as DNS     
    clients, DNS servers, or both.  In past RFCs, DNS servers are           
    sometimes called "name servers", "nameservers", or just "servers".      
    There is no formal definition of "DNS server", but RFCs generally       
    assume that it is an Internet server that listens for queries and       
    sends responses using the DNS protocol defined in [RFC1035] 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 name servers     
       in response to client requests."  (Quoted from [RFC1034],            
       Section 2.4) A resolver performs queries for a name, type, and       
       class, and receives responses.  The logical function is called       
       "resolution".  In practice, the term is usually referring to some    
       specific type of resolver (some of which are defined below), and     
       understanding the use of the term depends on understanding the       
       context.                                                             
                                                                            
       A related term is "resolve", which is not formally defined in        
       [RFC1034] or [RFC1035].  An imputed definition might be "asking a    
       question that consists of a domain name, class, and type, and        
       receiving some sort of response".  Similarly, an imputed             
       definition of "resolution" might be "the response received from      
       resolving".                                                          
                                                                            
    Stub resolver:  A resolver that cannot perform all resolution itself.   
       Stub resolvers generally depend on a recursive resolver to           
       undertake the actual resolution function.  Stub resolvers are        
       discussed but never fully defined in Section 5.3.1 of [RFC1034].     
       They are fully defined in Section 6.1.3.1 of [RFC1123].              
                                                                            
    Iterative mode:  A resolution mode of a server that receives DNS        
       queries and responds with a referral to another server.              
       Section 2.3 of [RFC1034] describes this as "The server refers the    
       client to another server and lets the client pursue the query."  A   
       resolver that works in iterative mode is sometimes called an         
       "iterative resolver".  See also "iterative resolution" later in      
       this section.                                                        
                                                                            
    Recursive mode:  A resolution mode of a server that receives DNS        
       queries and either responds to those queries from a local cache or   
       sends queries to other servers in order to get the final answers     
       to the original queries.  Section 2.3 of [RFC1034] describes this    
       as "the first server pursues the query for the client at another     
       server".  Section 4.3.1 of [RFC1034] says: "in [recursive] mode      
       the name server acts in the role of a resolver and returns either    
       an error or the answer, but never referrals."  That same section     
       also says:                                                           
                                                                            
          The recursive mode occurs when a query with RD set arrives at a   
          server which is willing to provide recursive service; the         
          client can verify that recursive mode was used by checking that   
          both RA and RD are set in the reply.                              
                                                                            
       A server operating in recursive mode may be thought of as having a   
       name server side (which is what answers the query) and a resolver    
       side (which performs the resolution function).  Systems operating    
       in this mode are commonly called "recursive servers".  Sometimes     
       they are called "recursive resolvers".  In practice, it is not       
       possible to know in advance whether the server that one is           
       querying will also perform recursion; both terms can be observed     
       in use interchangeably.                                              
                                                                            
    Recursive resolver:  A resolver that acts in recursive mode.  In        
       general, a recursive resolver is expected to cache the answers it    
       receives (which would make it a full-service resolver), but some     
       recursive resolvers might not cache.                                 
                                                                            
       [RFC4697] tried to differentiate between a recursive resolver and    
       an iterative resolver.                                               
                                                                            
    Recursive query:  A query with the Recursion Desired (RD) bit set to    
       1 in the header.  (See Section 4.1.1 of [RFC1035].)  If recursive    
       service is available and is requested by the RD bit in the query,    
       the server uses its resolver to answer the query.  (See              
       Section 4.3.2 of [RFC1034].)                                         
                                                                            
    Non-recursive query:  A query with the Recursion Desired (RD) bit set   
       to 0 in the header.  A server can answer non-recursive queries       
       using only local information: the response contains either an        
       error, the answer, or a referral to some other server "closer" to    
       the answer.  (See Section 4.3.1 of [RFC1034].)                       
                                                                            
    Iterative resolution:  A name server may be presented with a query      
       that can only be answered by some other server.  The two general     
       approaches to dealing with this problem are "recursive", in which    
       the first server pursues the query on behalf of the client at        
       another server, and "iterative", in which the server refers the      
       client to another server and lets the client pursue the query        
       there.  (See Section 2.3 of [RFC1034].)                              
                                                                            
       In iterative resolution, the client repeatedly makes non-recursive   
       queries and follows referrals and/or aliases.  The iterative         
       resolution algorithm is described in Section 5.3.3 of [RFC1034].     
                                                                            
    Full resolver:  This term is used in [RFC1035], but it is not defined   
       there.  RFC 1123 defines a "full-service resolver" that may or may   
       not be what was intended by "full resolver" in [RFC1035].  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 [RFC1123] defines this       
       term as a resolver that acts in recursive mode with a cache (and     
       meets other requirements).                                           
                                                                            
    Priming:  "The act of finding the list of root servers from a           
       configuration that lists some or all of the purported IP addresses   
       of some or all of those root servers."  (Quoted from [RFC8109],      
       Section 2) In order to operate in recursive mode, a resolver needs   
       to know the address of at least one root server.  Priming is most    
       often done from a configuration setting that contains a list of      
       authoritative servers for the root zone.                             
                                                                            
    Root hints:  "Operators who manage a DNS recursive resolver typically   
       need to configure a 'root hints file'.  This file contains the       
       names and IP addresses of the authoritative name servers for the     
       root zone, so the software can bootstrap the DNS resolution          
       process.  For many pieces of software, this list comes built into    
       the software."  (Quoted from [IANA_RootFiles]) This file is often    
       used in priming.                                                     
                                                                            
    Negative caching:  "The storage of knowledge that something does not    
       exist, cannot or does not give an answer."  (Quoted from             
       [RFC2308], Section 1)                                                
                                                                            
    Authoritative server:  "A server that knows the content of a DNS zone   
       from local knowledge, and thus can answer queries about that zone    
       without needing to query other servers."  (Quoted from [RFC2182],    
       Section 2) An authoritative server is named in the NS ("name         
       server") record in a zone.  It is a system that responds to DNS      
       queries with information about zones for which it has been           
       configured to answer with the AA flag in the response header set     
       to 1.  It is a server that has authority over one or more DNS        
       zones.  Note that it is possible for an authoritative server to      
       respond to a query without the parent zone delegating authority to   
       that server.  Authoritative servers also provide "referrals",        
       usually to child zones delegated from them; these referrals have     
       the AA bit set to 0 and come with referral data in the Authority     
       and (if needed) the Additional sections.                             
                                                                            
    Authoritative-only server:  A name server that only serves              
       authoritative data and ignores requests for recursion.  It will      
       "not normally generate any queries of its own.  Instead it answers   
       non-recursive queries from iterative resolvers looking for           
       information in zones it serves."  (Quoted from [RFC4697],            
       Section 2.4) In this case, "ignores requests for recursion" means    
       "responds to requests for recursion with responses indicating that   
       recursion was not performed".                                        
                                                                            
    Zone transfer:  The act of a client requesting a copy of a zone and     
       an authoritative server sending the needed information.  (See        
       Section 7 for a description of zones.)  There are two common         
       standard ways to do zone transfers: the AXFR ("Authoritative         
       Transfer") mechanism to copy the full zone (described in             
       [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy   
       only parts of the zone that have changed (described in [RFC1995]).   
       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 the zone."  (Quoted from [RFC1996], Section 2.1)         
       Secondary servers are also discussed in [RFC1034].  [RFC2182]        
       describes secondary servers in more detail.  Although early DNS      
       RFCs such as [RFC1996] referred to this as a "slave", the current    
       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 transfer for one or more [secondary] servers."        
       (Quoted from [RFC1996], Section 2.1) Or, more specifically,          
       [RFC2136] calls it "an authoritative server configured to be the     
       source of AXFR or IXFR data for one or more [secondary] servers".    
       Primary servers are also discussed in [RFC1034].  Although early     
       DNS RFCs such as [RFC1996] referred to this as a "master", the       
       current common usage has shifted to "primary".                       
                                                                            
    Primary master:  "The primary master is named in the zone's SOA MNAME   
       field and optionally by an NS RR."  (Quoted from [RFC1996],          
       Section 2.1) [RFC2136] defines "primary master" as "Master server    
       at the root of the AXFR/IXFR dependency graph.  The primary master   
       is named in the zone's SOA MNAME field and optionally by an NS RR.   
       There is by definition only one primary master server per zone."     
                                                                            
       The idea of a primary master is only used in [RFC1996] and           
       [RFC2136].  A modern interpretation of the term "primary master"     
       is a server that is both authoritative for a zone and that gets      
       its updates to the zone from configuration (such as a master file)   
       or from UPDATE transactions.                                         
                                                                            
    Stealth server:  This is "like a slave server except not listed in an   
       NS RR for the zone."  (Quoted from [RFC1996], Section 2.1)           
                                                                            
    Hidden master:  A stealth server that is a primary server for zone      
       transfers.  "In this arrangement, the master name server that        
       processes the updates is unavailable to general hosts on the         
       Internet; it is not listed in the NS RRset."  (Quoted from           
       [RFC6781], Section 3.4.3) [RFC4641] said that the hidden master's    
       name "appears in the SOA RRs MNAME field"; however, the name does    
       not appear at all in the global DNS in some setups.  A hidden        
       master can also be a secondary server for the zone itself.           
                                                                            
    Forwarding:  The process of one server sending a DNS query with the     
       RD bit set to 1 to another server to resolve that query.             
       Forwarding is a function of a DNS resolver; it is different than     
       simply blindly relaying queries.                                     
                                                                            
       [RFC5625] does not give a specific definition for forwarding, but    
       describes in detail what features a system that forwards needs to    
       support.  Systems that forward are sometimes called "DNS proxies",   
       but that term has not yet been defined (even in [RFC5625]).          
                                                                            
    Forwarder:  Section 1 of [RFC2308] describes a forwarder as "a          
       nameserver used to resolve queries instead of directly using the     
       authoritative nameserver chain".  [RFC2308] further says "The        
       forwarder typically either has better access to the internet, or     
       maintains a bigger cache which may be shared amongst many            
       resolvers."  That definition appears to suggest that forwarders      
       normally only query authoritative servers.  In current use,          
       however, forwarders often stand between stub resolvers and           
       recursive servers.  [RFC2308] is silent on whether a forwarder is    
       iterative-only or can be a full-service resolver.                    
                                                                            
    Policy-implementing resolver:  A resolver acting in recursive mode      
       that changes some of the answers that it returns based on policy     
       criteria, such as to prevent access to malware sites or              
       objectionable content.  In general, a stub resolver has no idea      
       whether upstream resolvers implement such policy or, if they do,     
       the exact policy about what changes will be made.  In some cases,    
       the user of the stub resolver has selected the policy-implementing   
       resolver with the explicit intention of using it to implement the    
       policies.  In other cases, policies are imposed without the user     
       of the stub resolver being informed.                                 
                                                                            
    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 open resolvers that are probably    
       misconfigured to be open.  Open resolvers are discussed in           
       [RFC5358].                                                           
                                                                            
    Split DNS:  The terms "split DNS" and "split-horizon DNS" have long     
       been used in the DNS community without formal definition.  In        
       general, they refer to situations in which DNS servers that are      
       authoritative for a particular set of domains provide partly or      
       completely different answers in those domains depending on the       
       source of the query.  Nevertheless, the effect of this is that a     
       domain name that is notionally globally unique has different         
       meanings for different network users.  This can sometimes be the     
       result of a "view" configuration, as described below.                
                                                                            
       Section 3.8 of [RFC2775] gives a related definition that is too      
       specific to be generally useful.                                     
                                                                            
    View:  A configuration for a DNS server that allows it to provide       
       different responses depending on attributes of the query, such as    
       for "split DNS".  Typically, views differ by the source IP address   
       of a query, but can also be based on the destination IP address,     
       the type of query (such as AXFR), whether it is recursive, and so    
       on.  Views are often used to provide more names or different         
       addresses to queries from "inside" a protected network than to       
       those "outside" that network.  Views are not a standardized part     
       of the DNS, but they are widely implemented in server software.      
                                                                            
    Passive DNS:  A mechanism to collect DNS data by storing DNS            
       responses from name servers.  Some of these systems also collect     
       the DNS queries associated with the responses, although doing so     
       raises some privacy concerns.  Passive DNS databases can be used     
       to answer historical questions about DNS zones, such as which        
       values were present at a given time in the past, or when a name      
       was spotted first.  Passive DNS databases allow searching of the     
       stored records on keys other than just the name and type, such as    
       "find all names which have A records of a particular value".         
                                                                            
    Anycast:  "The practice of making a particular service address          
       available in multiple, discrete, autonomous locations, such that     
       datagrams sent are routed to one of several available locations."    
       (Quoted from [RFC4786], Section 2) See [RFC4786] for more detail     
       on Anycast and other terms that are specific to its use.             
                                                                            
    Instance:  "When anycast routing is used to allow more than one         
       server to have the same IP address, each one of those servers is     
       commonly referred to as an 'instance'."  It goes on to say: "An      
       instance of a server, such as a root server, is often referred to    
       as an 'Anycast instance'."  (Quoted from [RSSAC026])                 
                                                                            
    Privacy-enabling DNS server:  "A DNS server that implements DNS over    
       TLS [RFC7858] and may optionally implement DNS over DTLS             
       [RFC8094]."  (Quoted from [RFC8310], Section 2) Other types of DNS   
       servers might also be considered privacy-enabling, such as those     
       running DNS-over-HTTPS [RFC8484] or DNS-over-QUIC [RFC9250].         
                                                                            
    DNS-over-TLS (DoT):  DNS over TLS as defined in [RFC7858] and its       
       successors.                                                          
                                                                            
    DNS-over-HTTPS (DoH):  DNS over HTTPS as defined in [RFC8484] and its   
       successors.                                                          
                                                                            
    DNS-over-QUIC (DoQ):  DNS over QUIC as defined in [RFC9250] and its     
       successors.  [RFC9250] specifically defines DoQ as general-purpose   
       transport for DNS that can be used in stub to recursive, recursive   
       to authoritative, and zone transfer scenarios.                       
                                                                            
    Classic DNS:  DNS over UDP or DNS over TCP as defined in [RFC1035]      
       and its successors.  Classic DNS applies to DNS communication        
       between stub resolvers and recursive resolvers, and between          
       recursive resolvers and authoritative servers.  This has sometimes   
       been called "Do53".  Classic DNS is not encrypted.                   
                                                                            
    Recursive DoT (RDoT):  RDoT specifically means DNS-over-TLS for         
       transport between a stub resolver and a recursive resolver, or       
       between a recursive resolver and another recursive resolver.  This   
       term is necessary because it is expected that DNS-over-TLS will      
       later be defined as a transport between recursive resolvers and      
       authoritative servers.                                               
                                                                            
    Authoritative DoT (ADoT):  If DNS-over-TLS is later defined as a        
       transport between recursive resolvers and authoritative servers,     
       ADoT specifically means DNS-over-TLS for transport between           
       recursive resolvers and authoritative servers.                       
                                                                            
    XFR-over-TLS (XoT):  DNS zone transfer over TLS, as specified in        
       [RFC9103].  This term applies to both AXFR over TLS (AXoT) and       
       IXFR over TLS (IXoT).                                                
                                                                            
 .  Zones                                                                  
                                                                            
    This section defines terms that are used when discussing zones that     
    are being served or retrieved.                                          
                                                                            
    Zone:  "Authoritative information is organized into units called        
       ZONEs, and these zones can be automatically distributed to the       
       name servers which provide redundant service for the data in a       
       zone."  (Quoted from [RFC1034], Section 2.4)                         
                                                                            
    Child:  "The entity on record that has the delegation of the domain     
       from the Parent."  (Quoted from [RFC7344], Section 1.1)              
                                                                            
    Parent:  "The domain in which the Child is registered."  (Quoted from   
       [RFC7344], Section 1.1) Earlier, "parent name server" was defined    
       in [RFC0882] as "the name server that has authority over the place   
       in the domain name space that will hold the new domain".  (Note      
       that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].)            
       [RFC819] also has some description of the relationship between       
       parents and children.                                                
                                                                            
    Origin:                                                                 
                                                                            
       There are two different uses for this term:                          
                                                                            
       (a)  "The domain name that appears at the top of a zone (just        
            below the cut that separates the zone from its parent)... The   
            name of the zone is the same as the name of the domain at the   
            zone's origin."  (Quoted from [RFC2181], Section 6) These       
            days, this sense of "origin" and "apex" (defined below) are     
            often used interchangeably.                                     
                                                                            
       (b)  The domain name within which a given relative domain name       
            appears in zone files.  Generally seen in the context of        
            "$ORIGIN", which is a control entry defined in [RFC1035],       
            Section 5.1, as part of the master file format.  For example,   
            if the $ORIGIN is set to "example.org.", then a master file     
            line for "www" is in fact an entry for "www.example.org.".      
                                                                            
    Apex:  The point in the tree at an owner of an SOA and corresponding    
       authoritative NS RRset.  This is also called the "zone apex".        
       [RFC4033] defines it as "the name at the child's side of a zone      
       cut".  The "apex" can usefully be thought of as a data-theoretic     
       description of a tree structure, and "origin" is the name of the     
       same concept when it is implemented in zone files.  The              
       distinction is not always maintained in use, however, and one can    
       find uses that conflict subtly with this definition.  [RFC1034]      
       uses the term "top node of the zone" as a synonym of "apex", but     
       that term is not widely used.  These days, the first sense of        
       "origin" (above) and "apex" are often used interchangeably.          
                                                                            
    Zone cut:  The delimitation point between two zones where the origin    
       of one of the zones is the child of the other zone.                  
                                                                            
       "Zones are delimited by 'zone cuts'.  Each zone cut separates a      
       'child' zone (below the cut) from a 'parent' zone (above the         
       cut)."  (Quoted from [RFC2181], Section 6; note that this is         
       barely an ostensive definition.)  Section 4.2 of [RFC1034] uses      
       "cuts" instead of "zone cut".                                        
                                                                            
    Delegation:  The process by which a separate zone is created in the     
       name space beneath the apex of a given domain.  Delegation happens   
       when an NS RRset is added in the parent zone for the child origin.   
       Delegation inherently happens at a zone cut.  The term is also       
       commonly a noun: the new zone that is created by the act of          
       delegating.                                                          
                                                                            
    Authoritative data:  "All of the RRs attached to all of the nodes       
       from the top node of the zone down to leaf nodes or nodes above      
       cuts around the bottom edge of the zone."  (Quoted from [RFC1034],   
       Section 4.2.1) Note that this definition might inadvertently also    
       cause any NS records that appear in the zone to be included, even    
       those that might not truly be authoritative, because there are       
       identical NS RRs below the zone cut.  This reveals the ambiguity     
       in the notion of authoritative data, because the parent-side NS      
       records authoritatively indicate the delegation, even though they    
       are not themselves authoritative data.                               
                                                                            
       [RFC4033], Section 2, defines "Authoritative RRset", which is        
       related to authoritative data but has a more precise definition.     
                                                                            
    Lame delegation:  "A lame delegations exists [sic] when a nameserver    
       is delegated responsibility for providing nameservice for a zone     
       (via NS records) but is not performing nameservice for that zone     
       (usually because it is not set up as a primary or secondary for      
       the zone)."  (Quoted from [RFC1912], Section 2.8) Another            
       definition is that a lame delegation "...happens when a name         
       server is listed in the NS records for some domain and in fact it    
       is not a server for that domain.  Queries are thus sent to the       
       wrong servers, who don't know nothing [sic] (at least not as         
       expected) about the queried domain.  Furthermore, sometimes these    
       hosts (if they exist!) don't even run name servers."  (Quoted from   
       [RFC1713], Section 2.3)                                              
                                                                            
       These early definitions do not match the current use of the term     
       "lame delegation", but there is no consensus on what a lame          
       delegation is.  The term is used not only for the specific case      
       described above, but for a variety of other flaws in delegations     
       that lead to non-authoritative answers or no answers at all, such    
       as:                                                                  
                                                                            
       *  a nameserver with an NS record for a zone that does not answer    
          DNS queries;                                                      
                                                                            
       *  a nameserver with an IP address that is not reachable by the      
          resolver; and                                                     
                                                                            
       *  a nameserver that responds to a query for a specific name with    
          an error or without the authoritative bit set.                    
                                                                            
       Because the term in current usage has drifted from the original      
       definition, and now is not specific or clear as to the intended      
       meaning, it should be considered historic and avoided in favor of    
       terms that are specific and clear.                                   
                                                                            
    Glue records:  "...[Resource records] which are not part of the         
       authoritative data [of the zone], and are address RRs for the        
       [name] servers [in subzones].  These RRs are only necessary if the   
       name server's name is 'below' the cut, and are only used as part     
       of a referral response."  Without glue "we could be faced with the   
       situation where the NS RRs tell us that in order to learn a name     
       server's address, we should contact the server using the address     
       we wish to learn."  (Quoted from [RFC1034], Section 4.2.1)           
                                                                            
       A later definition is that glue "includes any record in a zone       
       file that is not properly part of that zone, including nameserver    
       records of delegated sub-zones (NS records), address records that    
       accompany those NS records (A, AAAA, etc), and any other stray       
       data that might appear."  (Quoted from [RFC2181], Section 5.4.1)     
       Although glue is sometimes used today with this wider definition     
       in mind, the context surrounding the definition in [RFC2181]         
       suggests it is intended to apply to the use of glue within the       
       document itself and not necessarily beyond.                          
                                                                            
       In an NS record, there are three types of relationships between      
       the owner name of the record, the name in the NS RDATA, and the      
       zone origin: unrelated, in-domain, and sibling domain.  The          
       application of these three types of relationships to glue records    
       is defined in [RFC9471].                                             
                                                                            
       An unrelated relationship is one where the NS RDATA contains a       
       name server that is not subordinate to the zone origin and           
       therefore is not part of the same zone.                              
                                                                            
       An in-domain relationship is one where the NS RDATA contains a       
       name server whose name is either subordinate to or (rarely) the      
       same as the owner name of the NS resource records.  For example, a   
       delegation for "child.example.com" might have an in-domain name      
       server called "ns.child.example.com".                                
                                                                            
       A sibling domain relationship is one where the NS RDATA contains a   
       name server whose name is either subordinate to or (rarely) the      
       same as the zone origin of the parent and not subordinate to or      
       the same as the owner name of the NS resource records.  For          
       example, a delegation for "child.example.com" in "example.com"       
       zone might have a sibling domain name server called                  
       "ns.another.example.com".                                            
                                                                            
       The following table shows examples of delegation types:              
                                                                            
                                                                            
         +=============+========+====================+================+     
         | Delegation  | Parent | Name Server Name   | Type           |     
         +=============+========+====================+================+     
         | com         | .      | a.gtld-servers.net | sibling domain |     
         +-------------+--------+--------------------+----------------+     
         | net         | .      | a.gtld-servers.net | in-domain      |     
         +-------------+--------+--------------------+----------------+     
         | example.org | org    | ns.example.org     | in-domain      |     
         +-------------+--------+--------------------+----------------+     
         | example.org | org    | ns.ietf.org        | sibling domain |     
         +-------------+--------+--------------------+----------------+     
         | example.org | org    | ns.example.com     | unrelated      |     
         +-------------+--------+--------------------+----------------+     
         | example.jp  | jp     | ns.example.jp      | in-domain      |     
         +-------------+--------+--------------------+----------------+     
         | example.jp  | jp     | ns.example.ne.jp   | sibling domain |     
         +-------------+--------+--------------------+----------------+     
         | example.jp  | jp     | ns.example.com     | unrelated      |     
         +-------------+--------+--------------------+----------------+     
                                                                            
                                    Table 1                                 
                                                                            
    Bailiwick:  "In-bailiwick" and "Out-of-bailiwick" are modifiers used    
       to describe the relationship between a zone and the name servers     
       for that zone.  The dictionary definition of bailiwick has been      
       observed to cause more confusion than meaning for this use.  These   
       terms should be considered historic in nature.                       
                                                                            
    Root zone:  The zone of a DNS-based tree whose apex is the zero-        
       length label.  Also sometimes called "the DNS root".                 
                                                                            
    Empty non-terminals (ENTs):  "Domain names that own no resource         
       records but have subdomains that do."  (Quoted from [RFC4592],       
       Section 2.2.2) A typical example is in SRV records: in the name      
       "_sip._tcp.example.com", it is likely that "_tcp.example.com" has    
       no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV    
       RRset.                                                               
                                                                            
    Delegation-centric zone:  A zone that consists mostly of delegations    
       to child zones.  This term is used in contrast to a zone that        
       might have some delegations to child zones but also has many data    
       resource records for the zone itself and/or for child zones.  The    
       term is used in [RFC4956] and [RFC5155], but it is not defined in    
       either document.                                                     
                                                                            
    Occluded name:  "The addition of a delegation point via dynamic         
       update will render all subordinate domain names to be in a limbo,    
       still part of the zone but not available to the lookup process.      
       The addition of a DNAME resource record has the same impact.  The    
       subordinate names are said to be 'occluded'."  (Quoted from          
       [RFC5936], Section 3.5)                                              
                                                                            
    Fast flux DNS:  This "occurs when a domain is [found] in DNS using A    
       records to multiple IP addresses, each of which has a very short     
       Time-to-Live (TTL) value associated with it.  This means that the    
       domain resolves to varying IP addresses over a short period of       
       time."  (Quoted from [RFC6561], Section 1.1.5, with a typo           
       corrected) In addition to having legitimate uses, fast flux DNS      
       can be used to deliver malware.  Because the addresses change so     
       rapidly, it is difficult to ascertain all the hosts.  It should be   
       noted that the technique also works with AAAA records, but such      
       use is not frequently observed on the Internet as of this writing.   
                                                                            
    Reverse DNS, reverse lookup:  "The process of mapping an address to a   
       name is generally known as a 'reverse lookup', and the IN-           
       ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse        
       DNS'."  (Quoted from [RFC5855], Section 1)                           
                                                                            
    Forward lookup:  "Hostname-to-address translation".  (Quoted from       
       [RFC3493], Section 6)                                                
                                                                            
    arpa (Address and Routing Parameter Area Domain):  "The 'arpa' domain   
       was originally established as part of the initial deployment of      
       the DNS to provide a transition mechanism from the Host Tables       
       that were common in the ARPANET, as well as a home for the IPv4      
       reverse mapping domain.  During 2000, the abbreviation was           
       redesignated to 'Address and Routing Parameter Area' in the hope     
       of reducing confusion with the earlier network name."  (Quoted       
       from [RFC3172], Section 2) .arpa is an "infrastructure domain", a    
       domain whose "role is to support the operating infrastructure of     
       the Internet".  (Quoted from [RFC3172], Section 2) See [RFC3172]     
       for more history of this name.                                       
                                                                            
    Service name:  "Service names are the unique key in the Service Name    
       and Transport Protocol Port Number registry.  This unique symbolic   
       name for a service may also be used for other purposes, such as in   
       DNS SRV records."  (Quoted from [RFC6335], Section 5)                
                                                                            
 .  Wildcards                                                              
                                                                            
    Wildcard:  [RFC1034] defined "wildcard", but in a way that turned out   
       to be confusing to implementers.  For an extended discussion of      
       wildcards, including clearer definitions, see [RFC4592].  Special    
       treatment is given to RRs with owner names starting with the label   
       "*".  "Such RRs are called 'wildcards'.  Wildcard RRs can be         
       thought of as instructions for synthesizing RRs."  (Quoted from      
       [RFC1034], Section 4.3.3)                                            
                                                                            
    Asterisk label:  "The first octet is the normal label type and length   
       for a 1-octet-long label, and the second octet is the ASCII          
       representation [RFC20] for the '*' character.  A descriptive name    
       of a label equaling that value is an 'asterisk label'."  (Quoted     
       from [RFC4592], Section 2.1.1)                                       
                                                                            
    Wildcard domain name:  "A 'wildcard domain name' is defined by having   
       its initial (i.e., leftmost or least significant) label, in binary   
       format: 0000 0001 0010 1010 (binary) = 0x01 0x2a (hexadecimal)".     
       (Quoted from [RFC4592], Section 2.1.1) The second octet in this      
       label is the ASCII representation for the "*" character.             
                                                                            
    Closest encloser:  "The longest existing ancestor of a name."           
       (Quoted from [RFC5155], Section 1.3) An earlier definition is "The   
       node in the zone's tree of existing domain names that has the most   
       labels matching the query name (consecutively, counting from the     
       root label downward).  Each match is a 'label match' and the order   
       of the labels is the same."  (Quoted from [RFC4592],                 
       Section 3.3.1)                                                       
                                                                            
    Closest provable encloser:  "The longest ancestor of a name that can    
       be proven to exist.  Note that this is only different from the       
       closest encloser in an Opt-Out zone."  (Quoted from [RFC5155],       
       Section 1.3) See Section 10 for more on "opt-out".                   
                                                                            
    Next closer name:  "The name one label longer than the closest          
       provable encloser of a name."  (Quoted from [RFC5155],               
       Section 1.3)                                                         
                                                                            
    Source of Synthesis:  "The source of synthesis is defined in the        
       context of a query process as that wildcard domain name              
       immediately descending from the closest encloser, provided that      
       this wildcard domain name exists.  'Immediately descending' means    
       that the source of synthesis has a name of the form:                 
                                                                            
       <asterisk label>.<closest encloser>."                                
                                                                            
       (Quoted from [RFC4592], Section 3.3.1)                               
                                                                            
 .  Registration Model                                                     
                                                                            
    Registry:  The administrative operation of a zone that allows           
       registration of names within that zone.  People often use this       
       term to refer only to those organizations that perform               
       registration in large delegation-centric zones (such as TLDs); but   
       formally, whoever decides what data goes into a zone is the          
       registry for that zone.  This definition of "registry" is from a     
       DNS point of view; for some zones, the policies that determine       
       what can go in the zone are decided by zones that are                
       superordinate and not the registry operator.                         
                                                                            
    Registrant:  An individual or organization on whose behalf a name in    
       a zone is registered by the registry.  In many zones, the registry   
       and the registrant may be the same entity, but in TLDs they often    
       are not.                                                             
                                                                            
    Registrar:  A service provider that acts as a go-between for            
       registrants and registries.  Not all registrations require a         
       registrar, though it is common to have registrars involved in        
       registrations in TLDs.                                               
                                                                            
    EPP:  The Extensible Provisioning Protocol (EPP), which is commonly     
       used for communication of registration information between           
       registries and registrars.  EPP is defined in [RFC5730].             
                                                                            
    WHOIS:  A protocol specified in [RFC3912], often used for querying      
       registry databases.  WHOIS data is frequently used to associate      
       registration data (such as zone management contacts) with domain     
       names.  The term "WHOIS data" is often used as a synonym for the     
       registry database, even though that database may be served by        
       different protocols, particularly RDAP.  The WHOIS protocol is       
       also used with IP address registry data.                             
                                                                            
    RDAP:  The Registration Data Access Protocol, defined in [RFC7480],     
       [RFC7481], [RFC7485], [RFC9082], [RFC9083], and [RFC9224].  The      
       RDAP protocol and data format are meant as a replacement for         
       WHOIS.                                                               
                                                                            
    DNS operator:  An entity responsible for running DNS servers.  For a    
       zone's authoritative servers, the registrant may act as their own    
       DNS operator, their registrar may do it on their behalf, or they     
       may use a third-party operator.  For some zones, the registry        
       function is performed by the DNS operator plus other entities who    
       decide about the allowed contents of the zone.                       
                                                                            
    Public suffix:  "A domain that is controlled by a public registry."     
       (Quoted from [RFC6265], Section 5.3) A common definition for this    
       term is a domain under which subdomains can be registered by third   
       parties and on which HTTP cookies (which are described in detail     
       in [RFC6265]) should not be set.  There is no indication in a        
       domain name whether it is a public suffix; that can only be          
       determined by outside means.  In fact, both a domain and a           
       subdomain of that domain can be public suffixes.                     
                                                                            
       There is nothing inherent in a domain name to indicate whether it    
       is a public suffix.  One resource for identifying public suffixes    
       is the Public Suffix List (PSL) maintained by Mozilla                
       <https://publicsuffix.org/>.                                         
                                                                            
       For example, at the time this document is published, the "com.au"    
       domain is listed as a public suffix in the PSL.  (Note that this     
       example might change in the future.)                                 
                                                                            
       Note that the term "public suffix" is controversial in the DNS       
       community for many reasons, and it may be significantly changed in   
       the future.  One example of the difficulty of calling a domain a     
       public suffix is that designation can change over time as the        
       registration policy for the zone changes, such as was the case       
       with the "uk" TLD in 2014.                                           
                                                                            
    Subordinate and Superordinate:  These terms are introduced in           
       [RFC5731] for use in the registration model, but not defined         
       there.  Instead, they are given in examples.  "For example, domain   
       name 'example.com' has a superordinate relationship to host name     
       ns1.example.com'...  For example, host ns1.example1.com is a         
       subordinate host of domain example1.com, but it is a not a           
       subordinate host of domain example2.com."  (Quoted from [RFC5731],   
       Section 1.1) These terms are strictly ways of referring to the       
       relationship standing of two domains where one is a subdomain of     
       the other.                                                           
                                                                            
 .  General DNSSEC                                                        
                                                                            
    Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and   
    [RFC5155].  The terms that have caused confusion in the DNS community   
    are highlighted here.                                                   
                                                                            
    DNSSEC-aware and DNSSEC-unaware:  These two terms, which are used in    
       some RFCs, have not been formally defined.  However, Section 2 of    
       [RFC4033] defines many types of resolvers and validators,            
       including "non-validating security-aware stub resolver", "non-       
       validating stub resolver", "security-aware name server",             
       "security-aware recursive name server", "security-aware resolver",   
       "security-aware stub resolver", and "security-oblivious              
       'anything'".  (Note that the term "validating resolver", which is    
       used in some places in DNSSEC-related documents, is also not         
       defined in those RFCs, but is defined below.)                        
                                                                            
    Signed zone:  "A zone whose RRsets are signed and that contains         
       properly constructed DNSKEY, Resource Record Signature (RRSIG),      
       Next Secure (NSEC), and (optionally) DS records."  (Quoted from      
       [RFC4033], Section 2) It has been noted in other contexts that the   
       zone itself is not really signed, but all the relevant RRsets in     
       the zone are signed.  Nevertheless, if a zone that should be         
       signed contains any RRsets that are not signed (or opted out),       
       those RRsets will be treated as bogus, so the whole zone needs to    
       be handled in some way.                                              
                                                                            
       It should also be noted that, since the publication of [RFC6840],    
       NSEC records are no longer required for signed zones: a signed       
       zone might include NSEC3 records instead.  [RFC7129] provides        
       additional background commentary and some context for the NSEC and   
       NSEC3 mechanisms used by DNSSEC to provide authenticated denial-     
       of-existence responses.  NSEC and NSEC3 are described below.         
                                                                            
    Online signing:  [RFC4470] defines "on-line signing" (note the          
       hyphen) as "generating and signing these records on demand", where   
       "these" was defined as NSEC records.  The current definition         
       expands that to generating and signing RRSIG, NSEC, and NSEC3        
       records on demand.                                                   
                                                                            
    Unsigned zone:  Section 2 of [RFC4033] defines this as "a zone that     
       is not signed".  Section 2 of [RFC4035] defines this as a "zone      
       that does not include these records [properly constructed DNSKEY,    
       Resource Record Signature (RRSIG), Next Secure (NSEC), and           
       (optionally) DS records] according to the rules in this              
       section..." There is an important note at the end of Section 5.2     
       of [RFC4035] that defines an additional situation in which a zone    
       is considered unsigned: "If the resolver does not support any of     
       the algorithms listed in an authenticated DS RRset, then the         
       resolver will not be able to verify the authentication path to the   
       child zone.  In this case, the resolver SHOULD treat the child       
       zone as if it were unsigned."                                        
                                                                            
    NSEC:  "The NSEC record allows a security-aware resolver to             
       authenticate a negative reply for either name or type non-           
       existence with the same mechanisms used to authenticate other DNS    
       replies."  (Quoted from [RFC4033], Section 3.2) In short, an NSEC    
       record provides authenticated denial of existence.                   
                                                                            
       "The NSEC resource record lists two separate things: the next        
       owner name (in the canonical ordering of the zone) that contains     
       authoritative data or a delegation point NS RRset, and the set of    
       RR types present at the NSEC RR's owner name."  (Quoted from         
       [RFC4034], 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 [RFC5155].                
                                                                            
       Note that [RFC6840] says that [RFC5155] "is now considered part of   
       the DNS Security Document Family as described by Section 10 of       
       [RFC4033]".  This means that some of the definitions from earlier    
       RFCs that only talk about NSEC records should probably be            
       considered to be talking about both NSEC and NSEC3.                  
                                                                            
    Opt-out:  "The Opt-Out Flag indicates whether this NSEC3 RR may cover   
       unsigned delegations."  (Quoted from [RFC5155], Section 3.1.2.1)     
       Opt-out tackles the high costs of securing a delegation to an        
       insecure zone.  When using Opt-Out, names that are an insecure       
       delegation (and empty non-terminals that are only derived from       
       insecure delegations) don't require an NSEC3 record or its           
       corresponding RRSIG records.  Opt-Out NSEC3 records are not able     
       to prove or deny the existence of the insecure delegations.          
       (Adapted from [RFC7129], Section 5.1)                                
                                                                            
    Insecure delegation:  "A signed name containing a delegation (NS        
       RRset), but lacking a DS RRset, signifying a delegation to an        
       unsigned subzone."  (Quoted from [RFC4956], Section 2)               
                                                                            
    Zone enumeration:  "The practice of discovering the full content of a   
       zone via successive queries."  (Quoted from [RFC5155],               
       Section 1.3) This is also sometimes called "zone walking".  Zone     
       enumeration is different from zone content guessing where the        
       guesser uses a large dictionary of possible labels and sends         
       successive queries for them, or matches the contents of NSEC3        
       records against such a dictionary.                                   
                                                                            
    Validation:  Validation, in the context of DNSSEC, refers to one of     
       the following:                                                       
                                                                            
       *  Checking the validity of DNSSEC signatures,                       
                                                                            
       *  Checking the validity of DNS responses, such as those including   
          authenticated denial of existence, or                             
                                                                            
       *  Building an authentication chain from a trust anchor to a DNS     
          response or individual DNS RRsets in a response.                  
                                                                            
       The first two definitions above consider only the validity of        
       individual DNSSEC components, such as the RRSIG validity or NSEC     
       proof validity.  The third definition considers the components of    
       the entire DNSSEC authentication chain; thus, it requires            
       "configured knowledge of at least one authenticated DNSKEY or DS     
       RR" (as described in [RFC4035], Section 5).                          
                                                                            
       [RFC4033], Section 2, says that a "Validating Security-Aware Stub    
       Resolver... performs signature validation" and uses a trust anchor   
       "as a starting point for building the authentication chain to a      
       signed DNS response"; thus, it uses the first and third              
       definitions above.  The process of validating an RRSIG resource      
       record is described in [RFC4035], Section 5.3.                       
                                                                            
       [RFC5155] refers to validating responses throughout the document     
       in the context of hashed authenticated denial of existence; this     
       uses the second definition above.                                    
                                                                            
       The term "authentication" is used interchangeably with               
       "validation", in the sense of the third definition above.            
       [RFC4033], Section 2, describes the chain linking trust anchor to    
       DNS data as the "authentication chain".  A response is considered    
       to be authentic if "all RRsets in the Answer and Authority           
       sections of the response [are considered] to be authentic" (Quoted   
       from [RFC4035]) DNS data or responses deemed to be authentic or      
       validated have a security status of "secure" ([RFC4035],             
       Section 4.3; [RFC4033], Section 5).  "Authenticating both DNS keys   
       and data is a matter of local policy, which may extend or even       
       override the [DNSSEC] protocol extensions..." (Quoted from           
       [RFC4033], Section 3.1)                                              
                                                                            
       The term "verification", when used, is usually a synonym for         
       "validation".                                                        
                                                                            
    Validating resolver:  A security-aware recursive name server,           
       security-aware resolver, or security-aware stub resolver that is     
       applying at least one of the definitions of validation (above) as    
       appropriate to the resolution context.  For the same reason that     
       the generic term "resolver" is sometimes ambiguous and needs to be   
       evaluated in context (see Section 6), "validating resolver" is a     
       context-sensitive term.                                              
                                                                            
    Key signing key (KSK):  DNSSEC keys that "only sign the apex DNSKEY     
       RRset in a zone."  (Quoted from [RFC6781], Section 3.1)              
                                                                            
    Zone signing key (ZSK):  "DNSSEC keys that can be used to sign all      
       the RRsets in a zone that require signatures, other than the apex    
       DNSKEY RRset."  (Quoted from [RFC6781], Section 3.1) Also note       
       that a ZSK is sometimes used to sign the apex DNSKEY RRset.          
                                                                            
    Combined signing key (CSK):  "In cases where the differentiation        
       between the KSK and ZSK is not made, i.e., where keys have the       
       role of both KSK and ZSK, we talk about a Single-Type Signing        
       Scheme."  (Quoted from [RFC6781], Section 3.1) This is sometimes     
       called a "combined signing key" or "CSK".  It is operational         
       practice, not protocol, that determines whether a particular key     
       is a ZSK, a KSK, or a CSK.                                           
                                                                            
    Secure Entry Point (SEP):  A flag in the DNSKEY RDATA that "can be      
       used to distinguish between keys that are intended to be used as     
       the secure entry point into the zone when building chains of         
       trust, i.e., they are (to be) pointed to by parental DS RRs or       
       configured as a trust anchor....  Therefore, it is suggested that    
       the SEP flag be set on keys that are used as KSKs and not on keys    
       that are used as ZSKs, while in those cases where a distinction      
       between a KSK and ZSK is not made (i.e., for a Single-Type Signing   
       Scheme), it is suggested that the SEP flag be set on all keys."      
       (Quoted from [RFC6781], Section 3.2.3) Note that the SEP flag is     
       only a hint, and its presence or absence may not be used to          
       disqualify a given DNSKEY RR from use as a KSK or ZSK during         
       validation.                                                          
                                                                            
       The original definition of SEPs was in [RFC3757].  That definition   
       clearly indicated that the SEP was a key, not just a bit in the      
       key.  The abstract of [RFC3757] says: "With the Delegation Signer    
       (DS) resource record (RR), the concept of a public key acting as a   
       secure entry point (SEP) has been introduced.  During exchanges of   
       public keys with the parent there is a need to differentiate SEP     
       keys from other public keys in the Domain Name System KEY (DNSKEY)   
       resource record set.  A flag bit in the DNSKEY RR is defined to      
       indicate that DNSKEY is to be used as a SEP."  That definition of    
       the SEP as a key was made obsolete by [RFC4034], and the             
       definition from [RFC6781] is consistent with [RFC4034].              
                                                                            
    Trust anchor:  "A configured DNSKEY RR or DS RR hash of a DNSKEY RR.    
       A validating security-aware resolver uses this public key or hash    
       as a starting point for building the authentication chain to a       
       signed DNS response.  In general, a validating resolver will have    
       to obtain the initial values of its trust anchors via some secure    
       or trusted means outside the DNS protocol."  (Quoted from            
       [RFC4033], Section 2)                                                
                                                                            
    DNSSEC Policy (DP):  A statement that "sets forth the security          
       requirements and standards to be implemented for a DNSSEC-signed     
       zone."  (Quoted from [RFC6841], Section 2)                           
                                                                            
    DNSSEC Practice Statement (DPS):  "A practices disclosure document      
       that may support and be a supplemental document to the DNSSEC        
       Policy (if such exists), and it states how the management of a       
       given zone implements procedures and controls at a high level."      
       (Quoted from [RFC6841], Section 2)                                   
                                                                            
    Hardware security module (HSM):  A specialized piece of hardware that   
       is used to create keys for signatures and to sign messages without   
       ever disclosing the private key.  In DNSSEC, HSMs are often used     
       to hold the private keys for KSKs and ZSKs and to create the         
       signatures used in RRSIG records at periodic intervals.              
                                                                            
    Signing software:  Authoritative DNS servers that support DNSSEC        
       often contain software that facilitates the creation and             
       maintenance of DNSSEC signatures in zones.  There is also stand-     
       alone software that can be used to sign a zone regardless of         
       whether the authoritative server itself supports signing.            
       Sometimes signing software can support particular HSMs as part of    
       the signing process.                                                 
                                                                            
 .  DNSSEC States                                                         
                                                                            
    A validating resolver can determine that a response is in one of four   
    states: secure, insecure, bogus, or indeterminate.  These states are    
    defined in [RFC4033] and [RFC4035], although the definitions in the     
    two documents differ a bit.  This document makes no effort to           
    reconcile the definitions in the two documents and takes no position    
    as to whether they need to be reconciled.                               
                                                                            
    Section 5 of [RFC4033] says:                                            
                                                                            
    |  A validating resolver can determine the following 4 states:          
    |  Secure:  The validating resolver has a trust anchor, has a chain     
    |     of trust, and is able to verify all the signatures in the         
    |     response.                                                         
    |                                                                       
    |  Insecure:  The validating resolver has a trust anchor, a chain of    
    |     trust, and, at some delegation point, signed proof of the non-    
    |     existence of a DS record.  This indicates that subsequent         
    |     branches in the tree are provably insecure.  A validating         
    |     resolver may have a local policy to mark parts of the domain      
    |     space as insecure.                                                
    |                                                                       
    |  Bogus:  The validating resolver has a trust anchor and a secure      
    |     delegation indicating that subsidiary data is signed, but the     
    |     response fails to validate for some reason: missing signatures,   
    |     expired signatures, signatures with unsupported algorithms,       
    |     data missing that the relevant NSEC RR says should be present,    
    |     and so forth.                                                     
    |                                                                       
    |  Indeterminate:  There is no trust anchor that would indicate that    
    |     a specific portion of the tree is secure.  This is the default    
    |     operation mode.                                                   
                                                                            
    Section 4.3 of [RFC4035] says:                                          
                                                                            
    |  A security-aware resolver must be able to distinguish between four   
    |  cases:                                                               
    |  Secure:  An RRset for which the resolver is able to build a chain    
    |     of signed DNSKEY and DS RRs from a trusted security anchor to     
    |     the RRset.  In this case, the RRset should be signed and is       
    |     subject to signature validation, as described above.              
    |                                                                       
    |  Insecure:  An RRset for which the resolver knows that it has no      
    |     chain of signed DNSKEY and DS RRs from any trusted starting       
    |     point to the RRset.  This can occur when the target RRset lies    
    |     in an unsigned zone or in a descendent [sic] of an unsigned       
    |     zone.  In this case, the RRset may or may not be signed, but      
    |     the resolver will not be able to verify the signature.            
    |                                                                       
    |  Bogus:  An RRset for which the resolver believes that it ought to    
    |     be able to establish a chain of trust but for which it is         
    |     unable to do so, either due to signatures that for some reason    
    |     fail to validate or due to missing data that the relevant         
    |     DNSSEC RRs indicate should be present.  This case may indicate    
    |     an attack but may also indicate a configuration error or some     
    |     form of data corruption.                                          
    |                                                                       
    |  Indeterminate:  An RRset for which the resolver is not able to       
    |     determine whether the RRset should be signed, as the resolver     
    |     is not able to obtain the necessary DNSSEC RRs.  This can occur   
    |     when the security-aware resolver is not able to contact           
    |     security-aware name servers for the relevant zones.               
                                                                            
 .  Security Considerations                                               
                                                                            
    These definitions do not change any security considerations for         
    either the global DNS or private DNS.                                   
                                                                            
 .  IANA Considerations                                                   
                                                                            
    References to RFC 8499 in the IANA registries have been replaced with   
    references to this document.                                            
                                                                            
 .  References                                                            
                                                                            
 14.1.  Normative References                                                
                                                                            
    [IANA_RootFiles]                                                        
               IANA, "Root Files",                                          
               <https://www.iana.org/domains/root/files>.                   
                                                                            
    [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",    
               RFC 882, DOI 10.17487/RFC0882, November 1983,                
               <https://www.rfc-editor.org/info/rfc882>.                    
                                                                            
    [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",   
               STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,       
               <https://www.rfc-editor.org/info/rfc1034>.                   
                                                                            
    [RFC1035]  Mockapetris, P., "Domain names - implementation and          
               specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,      
               November 1987, <https://www.rfc-editor.org/info/rfc1035>.    
                                                                            
    [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -          
               Application and Support", STD 3, RFC 1123,                   
               DOI 10.17487/RFC1123, October 1989,                          
               <https://www.rfc-editor.org/info/rfc1123>.                   
                                                                            
    [RFC1912]  Barr, D., "Common DNS Operational and Configuration          
               Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,      
               <https://www.rfc-editor.org/info/rfc1912>.                   
                                                                            
    [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone      
               Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,       
               August 1996, <https://www.rfc-editor.org/info/rfc1996>.      
                                                                            
    [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,      
               "Dynamic Updates in the Domain Name System (DNS UPDATE)",    
               RFC 2136, DOI 10.17487/RFC2136, April 1997,                  
               <https://www.rfc-editor.org/info/rfc2136>.                   
                                                                            
    [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS              
               Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,   
               <https://www.rfc-editor.org/info/rfc2181>.                   
                                                                            
    [RFC2182]  Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection    
               and Operation of Secondary DNS Servers", BCP 16, RFC 2182,   
               DOI 10.17487/RFC2182, July 1997,                             
               <https://www.rfc-editor.org/info/rfc2182>.                   
                                                                            
    [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS           
               NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,        
               <https://www.rfc-editor.org/info/rfc2308>.                   
                                                                            
    [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.      
               Rose, "DNS Security Introduction and Requirements",          
               RFC 4033, DOI 10.17487/RFC4033, March 2005,                  
               <https://www.rfc-editor.org/info/rfc4033>.                   
                                                                            
    [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.      
               Rose, "Resource Records for the DNS Security Extensions",    
               RFC 4034, DOI 10.17487/RFC4034, March 2005,                  
               <https://www.rfc-editor.org/info/rfc4034>.                   
                                                                            
    [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.      
               Rose, "Protocol Modifications for the DNS Security           
               Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,     
               <https://www.rfc-editor.org/info/rfc4035>.                   
                                                                            
    [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name         
               System", RFC 4592, DOI 10.17487/RFC4592, July 2006,          
               <https://www.rfc-editor.org/info/rfc4592>.                   
                                                                            
    [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS      
               Security (DNSSEC) Hashed Authenticated Denial of             
               Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,      
               <https://www.rfc-editor.org/info/rfc5155>.                   
                                                                            
    [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive         
               Nameservers in Reflector Attacks", BCP 140, RFC 5358,        
               DOI 10.17487/RFC5358, October 2008,                          
               <https://www.rfc-editor.org/info/rfc5358>.                   
                                                                            
    [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",    
               STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,         
               <https://www.rfc-editor.org/info/rfc5730>.                   
                                                                            
    [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)      
               Domain Name Mapping", STD 69, RFC 5731,                      
               DOI 10.17487/RFC5731, August 2009,                           
               <https://www.rfc-editor.org/info/rfc5731>.                   
                                                                            
    [RFC5855]  Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6   
               Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855,     
               May 2010, <https://www.rfc-editor.org/info/rfc5855>.         
                                                                            
    [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol    
               (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,          
               <https://www.rfc-editor.org/info/rfc5936>.                   
                                                                            
    [RFC6561]  Livingood, J., Mody, N., and M. O'Reirdan,                   
               "Recommendations for the Remediation of Bots in ISP          
               Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012,       
               <https://www.rfc-editor.org/info/rfc6561>.                   
                                                                            
    [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC             
               Operational Practices, Version 2", RFC 6781,                 
               DOI 10.17487/RFC6781, December 2012,                         
               <https://www.rfc-editor.org/info/rfc6781>.                   
                                                                            
    [RFC6840]  Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and      
               Implementation Notes for DNS Security (DNSSEC)", RFC 6840,   
               DOI 10.17487/RFC6840, February 2013,                         
               <https://www.rfc-editor.org/info/rfc6840>.                   
                                                                            
    [RFC6841]  Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A        
               Framework for DNSSEC Policies and DNSSEC Practice            
               Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,   
               <https://www.rfc-editor.org/info/rfc6841>.                   
                                                                            
    [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms    
               for DNS (EDNS(0))", STD 75, RFC 6891,                        
               DOI 10.17487/RFC6891, April 2013,                            
               <https://www.rfc-editor.org/info/rfc6891>.                   
                                                                            
    [RFC7344]  Kumari, W., Gudmundsson, O., and G. Barwood, "Automating     
               DNSSEC Delegation Trust Maintenance", RFC 7344,              
               DOI 10.17487/RFC7344, September 2014,                        
               <https://www.rfc-editor.org/info/rfc7344>.                   
                                                                            
    [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS             
               Terminology", RFC 7719, DOI 10.17487/RFC7719, December       
               2015, <https://www.rfc-editor.org/info/rfc7719>.             
                                                                            
    [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles    
               for DNS over TLS and DNS over DTLS", RFC 8310,               
               DOI 10.17487/RFC8310, March 2018,                            
               <https://www.rfc-editor.org/info/rfc8310>.                   
                                                                            
    [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS             
               Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,       
               January 2019, <https://www.rfc-editor.org/info/rfc8499>.     
                                                                            
    [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over         
               Dedicated QUIC Connections", RFC 9250,                       
               DOI 10.17487/RFC9250, May 2022,                              
               <https://www.rfc-editor.org/info/rfc9250>.                   
                                                                            
    [RFC9471]  Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS    
               Glue Requirements in Referral Responses", RFC 9471,          
               DOI 10.17487/RFC9471, September 2023,                        
               <https://www.rfc-editor.org/info/rfc9471>.                   
                                                                            
 14.2.  Informative References                                              
                                                                            
    [IANA_Resource_Registry]                                                
               IANA, "Resource Record (RR) TYPEs",                          
               <https://www.iana.org/assignments/dns-parameters/>.          
                                                                            
    [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,    
               RFC 20, DOI 10.17487/RFC0020, October 1969,                  
               <https://www.rfc-editor.org/info/rfc20>.                     
                                                                            
    [RFC819]   Su, Z. and J. Postel, "The Domain Naming Convention for      
               Internet User Applications", RFC 819,                        
               DOI 10.17487/RFC0819, August 1982,                           
               <https://www.rfc-editor.org/info/rfc819>.                    
                                                                            
    [RFC952]   Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet    
               host table specification", RFC 952, DOI 10.17487/RFC0952,    
               October 1985, <https://www.rfc-editor.org/info/rfc952>.      
                                                                            
    [RFC1713]  Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713,      
               DOI 10.17487/RFC1713, November 1994,                         
               <https://www.rfc-editor.org/info/rfc1713>.                   
                                                                            
    [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,      
               DOI 10.17487/RFC1995, August 1996,                           
               <https://www.rfc-editor.org/info/rfc1995>.                   
                                                                            
    [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,            
               DOI 10.17487/RFC2775, February 2000,                         
               <https://www.rfc-editor.org/info/rfc2775>.                   
                                                                            
    [RFC3172]  Huston, G., Ed., "Management Guidelines & Operational        
               Requirements for the Address and Routing Parameter Area      
               Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,    
               September 2001, <https://www.rfc-editor.org/info/rfc3172>.   
                                                                            
    [RFC3425]  Lawrence, D., "Obsoleting IQUERY", RFC 3425,                 
               DOI 10.17487/RFC3425, November 2002,                         
               <https://www.rfc-editor.org/info/rfc3425>.                   
                                                                            
    [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.     
               Stevens, "Basic Socket Interface Extensions for IPv6",       
               RFC 3493, DOI 10.17487/RFC3493, February 2003,               
               <https://www.rfc-editor.org/info/rfc3493>.                   
                                                                            
    [RFC3757]  Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name        
               System KEY (DNSKEY) Resource Record (RR) Secure Entry        
               Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April     
               2004, <https://www.rfc-editor.org/info/rfc3757>.             
                                                                            
    [RFC3912]  Daigle, L., "WHOIS Protocol Specification", RFC 3912,        
               DOI 10.17487/RFC3912, September 2004,                        
               <https://www.rfc-editor.org/info/rfc3912>.                   
                                                                            
    [RFC4470]  Weiler, S. and J. Ihren, "Minimally Covering NSEC Records    
               and DNSSEC On-line Signing", RFC 4470,                       
               DOI 10.17487/RFC4470, April 2006,                            
               <https://www.rfc-editor.org/info/rfc4470>.                   
                                                                            
    [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",   
               RFC 4641, DOI 10.17487/RFC4641, September 2006,              
               <https://www.rfc-editor.org/info/rfc4641>.                   
                                                                            
    [RFC4697]  Larson, M. and P. Barber, "Observed DNS Resolution           
               Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,       
               October 2006, <https://www.rfc-editor.org/info/rfc4697>.     
                                                                            
    [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast            
               Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,          
               December 2006, <https://www.rfc-editor.org/info/rfc4786>.    
                                                                            
    [RFC4956]  Arends, R., Kosters, M., and D. Blacka, "DNS Security        
               (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July       
               2007, <https://www.rfc-editor.org/info/rfc4956>.             
                                                                            
    [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",           
               BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,        
               <https://www.rfc-editor.org/info/rfc5625>.                   
                                                                            
    [RFC5890]  Klensin, J., "Internationalized Domain Names for             
               Applications (IDNA): Definitions and Document Framework",    
               RFC 5890, DOI 10.17487/RFC5890, August 2010,                 
               <https://www.rfc-editor.org/info/rfc5890>.                   
                                                                            
    [RFC5891]  Klensin, J., "Internationalized Domain Names in              
               Applications (IDNA): Protocol", RFC 5891,                    
               DOI 10.17487/RFC5891, August 2010,                           
               <https://www.rfc-editor.org/info/rfc5891>.                   
                                                                            
    [RFC5892]  Faltstrom, P., Ed., "The Unicode Code Points and             
               Internationalized Domain Names for Applications (IDNA)",     
               RFC 5892, DOI 10.17487/RFC5892, August 2010,                 
               <https://www.rfc-editor.org/info/rfc5892>.                   
                                                                            
    [RFC5893]  Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts      
               for Internationalized Domain Names for Applications          
               (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,        
               <https://www.rfc-editor.org/info/rfc5893>.                   
                                                                            
    [RFC5894]  Klensin, J., "Internationalized Domain Names for             
               Applications (IDNA): Background, Explanation, and            
               Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,     
               <https://www.rfc-editor.org/info/rfc5894>.                   
                                                                            
    [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on   
               Encodings for Internationalized Domain Names", RFC 6055,     
               DOI 10.17487/RFC6055, February 2011,                         
               <https://www.rfc-editor.org/info/rfc6055>.                   
                                                                            
    [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,      
               DOI 10.17487/RFC6265, April 2011,                            
               <https://www.rfc-editor.org/info/rfc6265>.                   
                                                                            
    [RFC6303]  Andrews, M., "Locally Served DNS Zones", BCP 163,            
               RFC 6303, DOI 10.17487/RFC6303, July 2011,                   
               <https://www.rfc-editor.org/info/rfc6303>.                   
                                                                            
    [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.    
               Cheshire, "Internet Assigned Numbers Authority (IANA)        
               Procedures for the Management of the Service Name and        
               Transport Protocol Port Number Registry", BCP 165,           
               RFC 6335, DOI 10.17487/RFC6335, August 2011,                 
               <https://www.rfc-editor.org/info/rfc6335>.                   
                                                                            
    [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in             
               Internationalization in the IETF", BCP 166, RFC 6365,        
               DOI 10.17487/RFC6365, September 2011,                        
               <https://www.rfc-editor.org/info/rfc6365>.                   
                                                                            
    [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the        
               DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,             
               <https://www.rfc-editor.org/info/rfc6672>.                   
                                                                            
    [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,     
               DOI 10.17487/RFC6762, February 2013,                         
               <https://www.rfc-editor.org/info/rfc6762>.                   
                                                                            
    [RFC7129]  Gieben, R. and W. Mekking, "Authenticated Denial of          
               Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,       
               February 2014, <https://www.rfc-editor.org/info/rfc7129>.    
                                                                            
    [RFC7480]  Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the    
               Registration Data Access Protocol (RDAP)", STD 95,           
               RFC 7480, DOI 10.17487/RFC7480, March 2015,                  
               <https://www.rfc-editor.org/info/rfc7480>.                   
                                                                            
    [RFC7481]  Hollenbeck, S. and N. Kong, "Security Services for the       
               Registration Data Access Protocol (RDAP)", STD 95,           
               RFC 7481, DOI 10.17487/RFC7481, March 2015,                  
               <https://www.rfc-editor.org/info/rfc7481>.                   
                                                                            
    [RFC9082]  Hollenbeck, S. and A. Newton, "Registration Data Access      
               Protocol (RDAP) Query Format", STD 95, RFC 9082,             
               DOI 10.17487/RFC9082, June 2021,                             
               <https://www.rfc-editor.org/info/rfc9082>.                   
                                                                            
    [RFC9083]  Hollenbeck, S. and A. Newton, "JSON Responses for the        
               Registration Data Access Protocol (RDAP)", STD 95,           
               RFC 9083, DOI 10.17487/RFC9083, June 2021,                   
               <https://www.rfc-editor.org/info/rfc9083>.                   
                                                                            
    [RFC9224]  Blanchet, M., "Finding the Authoritative Registration Data   
               Access Protocol (RDAP) Service", STD 95, RFC 9224,           
               DOI 10.17487/RFC9224, March 2022,                            
               <https://www.rfc-editor.org/info/rfc9224>.                   
                                                                            
    [RFC7485]  Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin,      
               "Inventory and Analysis of WHOIS Registration Objects",      
               RFC 7485, DOI 10.17487/RFC7485, March 2015,                  
               <https://www.rfc-editor.org/info/rfc7485>.                   
                                                                            
    [RFC7793]  Andrews, M., "Adding 100.64.0.0/10 Prefixes to the IPv4      
               Locally-Served DNS Zones Registry", BCP 163, RFC 7793,       
               DOI 10.17487/RFC7793, May 2016,                              
               <https://www.rfc-editor.org/info/rfc7793>.                   
                                                                            
    [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,     
               and P. Hoffman, "Specification for DNS over Transport        
               Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May   
               2016, <https://www.rfc-editor.org/info/rfc7858>.             
                                                                            
    [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram        
               Transport Layer Security (DTLS)", RFC 8094,                  
               DOI 10.17487/RFC8094, February 2017,                         
               <https://www.rfc-editor.org/info/rfc8094>.                   
                                                                            
    [RFC8109]  Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS    
               Resolver with Priming Queries", BCP 209, RFC 8109,           
               DOI 10.17487/RFC8109, March 2017,                            
               <https://www.rfc-editor.org/info/rfc8109>.                   
                                                                            
    [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS          
               (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,        
               <https://www.rfc-editor.org/info/rfc8484>.                   
                                                                            
    [RFC9103]  Toorop, W., Dickinson, S., Sahib, S., Aras, P., and A.       
               Mankin, "DNS Zone Transfer over TLS", RFC 9103,              
               DOI 10.17487/RFC9103, August 2021,                           
               <https://www.rfc-editor.org/info/rfc9103>.                   
                                                                            
    [RSSAC026] Root Server System Advisory Committee (RSSAC), "RSSAC0226    
               RSSAC Lexicon", 2017,                                        
               <https://www.icann.org/en/system/files/files/rssac-          
               026-14mar17-en.pdf>.                                         
                                                                            
 Appendix A.  Definitions Updated by This Document                          
                                                                            
    The following definitions from RFCs are updated by this document:       
                                                                            
    *  Forwarder in [RFC2308]                                               
                                                                            
    *  QNAME in [RFC2308]                                                   
                                                                            
    *  Secure Entry Point (SEP) in [RFC3757]; note, however, that this      
       RFC is already obsolete (see [RFC4033], [RFC4034], [RFC4035]).       
                                                                            
 Appendix B.  Definitions First Defined in This Document                    
                                                                            
    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                                         
                                                                            
 Acknowledgements                                                           
                                                                            
    [RFC8499] and its predecessor, [RFC7719], were co-authored by Andrew    
    Sullivan.  The current document, which is a small update to             
    [RFC8499], has just two authors.  Andrew's work on the earlier          
    documents is greatly appreciated.                                       
                                                                            
    Numerous people made significant contributions to [RFC8499] and         
    [RFC7719].  Please see the acknowledgements sections in those two       
    documents for the extensive list of contributors.                       
                                                                            
    Even though the current document is a small revision, many people in    
    the DNSOP Working Group have contributed to it, and their work is       
    greatly appreciated.                                                    
                                                                            
 Index                                                                      
                                                                            
    A B C D E F G H I K L M N O P Q R S T U V W X Z                         
                                                                            
       A                                                                    
                                                                            
          Address and Routing Parameter Area Domain (arpa)                  
             Section 7                                                      
          Address records                                                   
             Section 5                                                      
          ADoT                                                              
             Section 6                                                      
          Alias                                                             
             Section 2                                                      
          Anycast                                                           
             Section 6                                                      
          Apex                                                              
             Section 7                                                      
          Asterisk label                                                    
             Section 8                                                      
          Authoritative data                                                
             Section 7                                                      
          Authoritative server                                              
             Section 6                                                      
          Authoritative-only server                                         
             Section 6                                                      
          AXoT                                                              
             Section 6                                                      
                                                                            
       B                                                                    
                                                                            
          Bailiwick                                                         
             Section 7                                                      
                                                                            
       C                                                                    
                                                                            
          Canonical name                                                    
             Section 2                                                      
          Child                                                             
             Section 7                                                      
          Class                                                             
             Section 4                                                      
          Class independent                                                 
             Section 5                                                      
          Classic DNS                                                       
             Section 6                                                      
          Closest encloser                                                  
             Section 8                                                      
          Closest provable encloser                                         
             Section 8                                                      
          CNAME                                                             
             Section 2                                                      
          Combined signing key (CSK)                                        
             Section 10                                                     
                                                                            
       D                                                                    
                                                                            
          Delegation                                                        
             Section 7                                                      
          Delegation-centric zone                                           
             Section 7                                                      
          DNS operator                                                      
             Section 9                                                      
          DNS-over-HTTPS                                                    
             Section 6                                                      
          DNS-over-QUIC                                                     
             Section 6                                                      
          DNS-over-TLS                                                      
             Section 6                                                      
          DNSSEC Policy (DP)                                                
             Section 10                                                     
          DNSSEC Practice Statement (DPS)                                   
             Section 10                                                     
          DNSSEC-aware and DNSSEC-unaware                                   
             Section 10                                                     
          DoH                                                               
             Section 6                                                      
          Domain name                                                       
             Section 2                                                      
          DoQ                                                               
             Section 6                                                      
          DoT                                                               
             Section 6                                                      
                                                                            
       E                                                                    
                                                                            
          EDNS                                                              
             Section 5                                                      
          Empty non-terminals (ENTs)                                        
             Section 7                                                      
          EPP                                                               
             Section 9                                                      
                                                                            
       F                                                                    
                                                                            
          Fast flux DNS                                                     
             Section 7                                                      
          FORMERR                                                           
             Section 3                                                      
          Forward lookup                                                    
             Section 7                                                      
          Forwarder                                                         
             Section 6                                                      
          Forwarding                                                        
             Section 6                                                      
          Full resolver                                                     
             Section 6                                                      
          Full-service resolver                                             
             Section 6                                                      
          Fully Qualified Domain Name (FQDN)                                
             Section 2                                                      
                                                                            
       G                                                                    
                                                                            
          Global DNS                                                        
             Section 2                                                      
          Glue records                                                      
             Section 7                                                      
                                                                            
       H                                                                    
                                                                            
          Hardware security module (HSM)                                    
             Section 10                                                     
          Hidden master                                                     
             Section 6                                                      
          Host name                                                         
             Section 2                                                      
                                                                            
       I                                                                    
                                                                            
          IDN                                                               
             Section 2                                                      
          In-bailiwick                                                      
             Section 7                                                      
          In-domain                                                         
             Section 7                                                      
          Insecure delegation                                               
             Section 10                                                     
          Instance                                                          
             Section 6                                                      
          Internationalized Domain Name                                     
             Section 2                                                      
          Iterative mode                                                    
             Section 6                                                      
          Iterative resolution                                              
             Section 6                                                      
          IXoT                                                              
             Section 6                                                      
                                                                            
       K                                                                    
                                                                            
          Key signing key (KSK)                                             
             Section 10                                                     
                                                                            
       L                                                                    
                                                                            
          Label                                                             
             Section 2                                                      
          Lame delegation                                                   
             Section 7                                                      
          Locally served DNS zone                                           
             Section 2                                                      
                                                                            
       M                                                                    
                                                                            
          Master file                                                       
             Section 5                                                      
          Master server                                                     
             Section 6                                                      
          mDNS                                                              
             Section 2                                                      
          Multicast DNS                                                     
             Section 2                                                      
                                                                            
       N                                                                    
                                                                            
          Naming system                                                     
             Section 2, Paragraph 1.2.1                                     
          Negative caching                                                  
             Section 6                                                      
          Negative response                                                 
             Section 3                                                      
          Next closer name                                                  
             Section 8                                                      
          NODATA                                                            
             Section 3                                                      
          NOERROR                                                           
             Section 3                                                      
          Non-recursive query                                               
             Section 6                                                      
          NOTIMP                                                            
             Section 3                                                      
          NS                                                                
             Section 6                                                      
          NSEC                                                              
             Section 10                                                     
          NSEC3                                                             
             Section 10                                                     
          NXDOMAIN                                                          
             Section 3                                                      
                                                                            
       O                                                                    
                                                                            
          Occluded name                                                     
             Section 7                                                      
          on-line signing                                                   
             Section 10                                                     
          online signing                                                    
             Section 10                                                     
          Open resolver                                                     
             Section 6                                                      
          OPT                                                               
             Section 5                                                      
          Opt-out                                                           
             Section 10                                                     
          Origin                                                            
             Section 7                                                      
          Out-of-bailiwick                                                  
             Section 7                                                      
          Owner                                                             
             Section 5                                                      
                                                                            
       P                                                                    
                                                                            
          Parent                                                            
             Section 7                                                      
          Passive DNS                                                       
             Section 6                                                      
          Policy-implementing resolver                                      
             Section 6                                                      
          Presentation format                                               
             Section 5                                                      
          Primary master                                                    
             Section 6                                                      
          Primary server                                                    
             Section 6                                                      
          Priming                                                           
             Section 6                                                      
          Privacy-enabling DNS server                                       
             Section 6                                                      
          Private DNS                                                       
             Section 2                                                      
          Public suffix                                                     
             Section 9                                                      
                                                                            
       Q                                                                    
                                                                            
          QNAME                                                             
             Section 4                                                      
                                                                            
       R                                                                    
                                                                            
          RDAP                                                              
             Section 9                                                      
          RDoT                                                              
             Section 6                                                      
          Recursive DoT                                                     
             Section 6                                                      
          Recursive mode                                                    
             Section 6, Paragraph 4.10.1                                    
          Recursive query                                                   
             Section 6                                                      
          Recursive resolver                                                
             Section 6                                                      
          Referrals                                                         
             Section 4                                                      
          REFUSED                                                           
             Section 3                                                      
          Registrant                                                        
             Section 9                                                      
          Registrar                                                         
             Section 9                                                      
          Registry                                                          
             Section 9                                                      
          Resolver                                                          
             Section 6                                                      
          Reverse DNS, reverse lookup                                       
             Section 7                                                      
          Root hints                                                        
             Section 6                                                      
          Root zone                                                         
             Section 7                                                      
          RR                                                                
             Section 5                                                      
          RRset                                                             
             Section 5                                                      
                                                                            
       S                                                                    
                                                                            
          Secondary server                                                  
             Section 6                                                      
          Secure Entry Point (SEP)                                          
             Section 10                                                     
          SERVFAIL                                                          
             Section 3                                                      
          Service name                                                      
             Section 7                                                      
          Sibling domain                                                    
             Section 7                                                      
          Signed zone                                                       
             Section 10                                                     
          Signing software                                                  
             Section 10                                                     
          Slave server                                                      
             Section 6                                                      
          SOA                                                               
             Section 5                                                      
          SOA field names                                                   
             Section 5                                                      
          Source of Synthesis                                               
             Section 8, Paragraph 1.14.1                                    
          Split DNS                                                         
             Section 6                                                      
          Split-horizon DNS                                                 
             Section 6                                                      
          Stealth server                                                    
             Section 6                                                      
          Stub resolver                                                     
             Section 6                                                      
          Subdomain                                                         
             Section 2                                                      
          Subordinate                                                       
             Section 9                                                      
          Superordinate                                                     
             Section 9                                                      
                                                                            
       T                                                                    
                                                                            
          TLD                                                               
             Section 2                                                      
          Trust anchor                                                      
             Section 10                                                     
          TTL                                                               
             Section 5                                                      
                                                                            
       U                                                                    
                                                                            
          Unsigned zone                                                     
             Section 10                                                     
                                                                            
       V                                                                    
                                                                            
          Validating resolver                                               
             Section 10                                                     
          Validation                                                        
             Section 10, Paragraph 2.26.1                                   
          View                                                              
             Section 6                                                      
                                                                            
       W                                                                    
                                                                            
          WHOIS                                                             
             Section 9                                                      
          Wildcard                                                          
             Section 8                                                      
          Wildcard domain name                                              
             Section 8                                                      
                                                                            
       X                                                                    
                                                                            
          XoT                                                               
             Section 6                                                      
                                                                            
       Z                                                                    
                                                                            
          Zone                                                              
             Section 7                                                      
          Zone cut                                                          
             Section 7                                                      
          Zone enumeration                                                  
             Section 10                                                     
          Zone signing key (ZSK)                                            
             Section 10                                                     
          Zone transfer                                                     
             Section 6                                                      
                                                                            
 Authors' Addresses                                                         
                                                                            
    Paul Hoffman                                                            
    ICANN                                                                   
    Email: paul.hoffman@icann.org                                           
                                                                            
                                                                            
    Kazunori Fujiwara                                                       
    Japan Registry Services Co., Ltd.                                       
    Email: fujiwara@jprs.co.jp