1 Internet Engineering Task Force (IETF)                        P. Hoffman   
    2 Request for Comments: 9499                                         ICANN   
    3 BCP: 219                                                     K. Fujiwara   
    4 Obsoletes: 8499                                                     JPRS   
    5 Updates: 2308                                                 March 2024   
    6 Category: Best Current Practice                                            
    7 ISSN: 2070-1721                                                            
   10                             DNS Terminology                                
   12 Abstract                                                                   
   14    The Domain Name System (DNS) is defined in literally dozens of          
   15    different RFCs.  The terminology used by implementers and developers    
   16    of DNS protocols, and by operators of DNS systems, has changed in the   
   17    decades since the DNS was first defined.  This document gives current   
   18    definitions for many of the terms used in the DNS in a single           
   19    document.                                                               
   21    This document updates RFC 2308 by clarifying the definitions of         
   22    "forwarder" and "QNAME".  It obsoletes RFC 8499 by adding multiple      
   23    terms and clarifications.  Comprehensive lists of changed and new       
   24    definitions can be found in Appendices A and B.                         
   26 Status of This Memo                                                        
   28    This memo documents an Internet Best Current Practice.                  
   30    This document is a product of the Internet Engineering Task Force       
   31    (IETF).  It represents the consensus of the IETF community.  It has     
   32    received public review and has been approved for publication by the     
   33    Internet Engineering Steering Group (IESG).  Further information on     
   34    BCPs is available in Section 2 of RFC 7841.                             
   36    Information about the current status of this document, any errata,      
   37    and how to provide feedback on it may be obtained at                    
   38    https://www.rfc-editor.org/info/rfc9499.                                
   40 Copyright Notice                                                           
   42    Copyright (c) 2024 IETF Trust and the persons identified as the         
   43    document authors.  All rights reserved.                                 
   45    This document is subject to BCP 78 and the IETF Trust's Legal           
   46    Provisions Relating to IETF Documents                                   
   47    (https://trustee.ietf.org/license-info) in effect on the date of        
   48    publication of this document.  Please review these documents            
   49    carefully, as they describe your rights and restrictions with respect   
   50    to this document.  Code Components extracted from this document must    
   51    include Revised BSD License text as described in Section 4.e of the     
   52    Trust Legal Provisions and are provided without warranty as described   
   53    in the Revised BSD License.                                             
   55 Table of Contents                                                          
   57    1.  Introduction                                                        
   58    2.  Names                                                               
   59    3.  DNS Response Codes                                                  
   60    4.  DNS Transactions                                                    
   61    5.  Resource Records                                                    
   62    6.  DNS Servers and Clients                                             
   63    7.  Zones                                                               
   64    8.  Wildcards                                                           
   65    9.  Registration Model                                                  
   66    10. General DNSSEC                                                      
   67    11. DNSSEC States                                                       
   68    12. Security Considerations                                             
   69    13. IANA Considerations                                                 
   70    14. References                                                          
   71      14.1.  Normative References                                           
   72      14.2.  Informative References                                         
   73    Appendix A.  Definitions Updated by This Document                       
   74    Appendix B.  Definitions First Defined in This Document                 
   75    Acknowledgements                                                        
   76    Index                                                                   
   77    Authors' Addresses                                                      
   79 1.  Introduction                                                           
   81    The Domain Name System (DNS) is a simple query-response protocol        
   82    whose messages in both directions have the same format.  (Section 2     
   83    gives a definition of "global DNS", which is often what people mean     
   84    when they say "the DNS".)  The protocol and message format are          
   85    defined in [RFC1034] and [RFC1035].  These RFCs defined some terms,     
   86    and later documents defined others.  Some of the terms from [RFC1034]   
   87    and [RFC1035] have somewhat different meanings now than they did in     
   88    1987.                                                                   
   90    This document contains a collection of a wide variety of DNS-related    
   91    terms, organized loosely by topic.  Some of them have been precisely    
   92    defined in earlier RFCs, some have been loosely defined in earlier      
   93    RFCs, and some are not defined in an earlier RFC at all.                
   95    Other organizations sometimes define DNS-related terms in their own     
   96    way.  For example, the WHATWG defines "domain" at                       
   97    <https://url.spec.whatwg.org/>.  The Root Server System Advisory        
   98    Committee (RSSAC) has a good lexicon [RSSAC026].                        
  100    Most of the definitions listed here represent the consensus             
  101    definition of the DNS community -- both protocol developers and         
  102    operators.  Some of the definitions differ from earlier RFCs, and       
  103    those differences are noted.  In this document, where the consensus     
  104    definition is the same as the one in an RFC, that RFC is quoted.        
  105    Where the consensus definition has changed somewhat, the RFC is         
  106    mentioned but the new stand-alone definition is given.  See             
  107    Appendix A for a list of the definitions that this document updates.    
  109    It is important to note that, during the development of this            
  110    document, it became clear that some DNS-related terms are interpreted   
  111    quite differently by different DNS experts.  Further, some terms that   
  112    are defined in early DNS RFCs now have definitions that are generally   
  113    agreed to, but that are different from the original definitions.        
  114    This document is a small revision to [RFC8499]; that document was a     
  115    substantial revision to [RFC7719].                                      
  117    Note that there is no single consistent definition of "the DNS".  It    
  118    can be considered to be some combination of the following: a commonly   
  119    used naming scheme for objects on the Internet; a distributed           
  120    database representing the names and certain properties of these         
  121    objects; an architecture providing distributed maintenance,             
  122    resilience, and loose coherency for this database; and a simple         
  123    query-response protocol (as mentioned below) implementing this          
  124    architecture.  Section 2 defines "global DNS" and "private DNS" as a    
  125    way to deal with these differing definitions.                           
  127    Capitalization in DNS terms is often inconsistent among RFCs and        
  128    various DNS practitioners.  The capitalization used in this document    
  129    is a best guess at current practices, and is not meant to indicate      
  130    that other capitalization styles are wrong or archaic.  In some         
  131    cases, multiple styles of capitalization are used for the same term     
  132    due to quoting from different RFCs.                                     
  134    In this document, the words "byte" and "octet" are used                 
  135    interchangeably.  They appear here because they both appear in the      
  136    earlier RFCs that defined terms in the DNS.                             
  138    Readers should note that the terms in this document are grouped by      
  139    topic.  Someone who is not already familiar with the DNS probably       
  140    cannot learn about the DNS from scratch by reading this document from   
  141    front to back.  Instead, skipping around may be the only way to get     
  142    enough context to understand some of the definitions.  This document    
  143    has an index that might be useful for readers who are attempting to     
  144    learn the DNS by reading this document.                                 
  146 2.  Names                                                                  
  148    Naming system:  A naming system associates names with data.  Naming     
  149       systems have many significant facets that help differentiate them    
  150       from each other.  Some commonly identified facets include:           
  152       *  Composition of names                                              
  154       *  Format of names                                                   
  156       *  Administration of names                                           
  158       *  Types of data that can be associated with names                   
  160       *  Types of metadata for names                                       
  162       *  Protocol for getting data from a name                             
  164       *  Context for resolving a name                                      
  166       Note that this list is a small subset of facets that people have     
  167       identified over time for naming systems, and the IETF has yet to     
  168       agree on a good set of facets that can be used to compare naming     
  169       systems.  For example, other facets might include "protocol to       
  170       update data in a name", "privacy of names", and "privacy of data     
  171       associated with names", but those are not as well defined as the     
  172       ones listed above.  The list here is chosen because it helps         
  173       describe the DNS and naming systems similar to the DNS.              
  175    Domain name:  An ordered list of one or more labels.                    
  177       Note that this is a definition independent of the DNS RFCs           
  178       ([RFC1034] and [RFC1035]), and the definition here also applies to   
  179       systems other than the DNS.  [RFC1034] defines the "domain name      
  180       space" using mathematical trees and their nodes in graph theory,     
  181       and that definition has the same practical result as the             
  182       definition here.  Any path of a directed acyclic graph can be        
  183       represented by a domain name consisting of the labels of its         
  184       nodes, ordered by decreasing distance from the root(s) (which is     
  185       the normal convention within the DNS, including this document).  A   
  186       domain name whose last label identifies a root of the graph is       
  187       fully qualified; other domain names whose labels form a strict       
  188       prefix of a fully qualified domain name are relative to its first    
  189       omitted node.                                                        
  191       Also note that different IETF and non-IETF documents have used the   
  192       term "domain name" in many different ways.  It is common for         
  193       earlier documents to use "domain name" to mean "names that match     
  194       the syntax in [RFC1035]", but possibly with additional rules such    
  195       as "and are, or will be, resolvable in the global DNS" or "but       
  196       only using the presentation format".                                 
  198    Label:  An ordered list of zero or more octets that makes up a          
  199       portion of a domain name.  Using graph theory, a label identifies    
  200       one node in a portion of the graph of all possible domain names.     
  202    Global DNS:  Using the short set of facets listed in "Naming system",   
  203       the global DNS can be defined as follows.  Most of the rules here    
  204       come from [RFC1034] and [RFC1035], although the term "global DNS"    
  205       has not been defined before now.                                     
  207       Composition of names:  A name in the global DNS has one or more      
  208          labels.  The length of each label is between 0 and 63 octets      
  209          inclusive.  In a fully qualified domain name, the last label in   
  210          the ordered list is 0 octets long; it is the only label whose     
  211          length may be 0 octets, and it is called the "root" or "root      
  212          label".  A domain name in the global DNS has a maximum total      
  213          length of 255 octets in the wire format; the root represents      
  214          one octet for this calculation.  (Multicast DNS [RFC6762]         
  215          allows names up to 255 bytes plus a terminating zero byte based   
  216          on a different interpretation of RFC 1035 and what is included    
  217          in the 255 octets.)                                               
  219       Format of names:  Names in the global DNS are domain names.  There   
  220          are three formats: wire format, presentation format, and common   
  221          display.                                                          
  223          Wire format:  The basic wire format for names in the global DNS   
  224             is a list of labels ordered by decreasing distance from the    
  225             root, with the root label last.  Each label is preceded by a   
  226             length octet.  [RFC1035] also defines a compression scheme     
  227             that modifies this format.                                     
  229          Presentation format:  The presentation format for names in the    
  230             global DNS is a list of labels ordered by decreasing           
  231             distance from the root, encoded as ASCII, with a "."           
  232             character between each label.  In presentation format, a       
  233             fully qualified domain name includes the root label and the    
  234             associated separator dot.  For example, in presentation        
  235             format, a fully qualified domain name with two non-root        
  236             labels is always shown as "example.tld." instead of            
  237             "example.tld".  [RFC1035] defines a method for showing         
  238             octets that do not display in ASCII.                           
  240          Common display format:  The common display format is used in      
  241             applications and free text.  It is the same as the             
  242             presentation format, but showing the root label and the "."    
  243             before it is optional and is rarely done.  For example, in     
  244             common display format, a fully qualified domain name with      
  245             two non-root labels is usually shown as "example.tld"          
  246             instead of "example.tld.".  Names in the common display        
  247             format are normally written such that the directionality of    
  248             the writing system presents labels by decreasing distance      
  249             from the root (so, in both English and the C programming       
  250             language, the root or Top-Level Domain (TLD) label in the      
  251             ordered list is rightmost; but in Arabic, it may be            
  252             leftmost, depending on local conventions).                     
  254       Administration of names:  Administration is specified by             
  255          delegation (see the definition of "delegation" in Section 7).     
  256          Policies for administration of the root zone in the global DNS    
  257          are determined by the names operational community, which          
  258          convenes itself in the Internet Corporation for Assigned Names    
  259          and Numbers (ICANN).  The names operational community selects     
  260          the IANA Functions Operator for the global DNS root zone.  The    
  261          name servers that serve the root zone are provided by             
  262          independent root operators.  Other zones in the global DNS have   
  263          their own policies for administration.                            
  265       Types of data that can be associated with names:  A name can have    
  266          zero or more resource records associated with it.  There are      
  267          numerous types of resource records with unique data structures    
  268          defined in many different RFCs and in the IANA registry at        
  269          [IANA_Resource_Registry].                                         
  271       Types of metadata for names:  Any name that is published in the      
  272          DNS appears as a set of resource records (see the definition of   
  273          "RRset" in Section 5).  Some names do not, themselves, have       
  274          data associated with them in the DNS, but they "appear" in the    
  275          DNS anyway because they form part of a longer name that does      
  276          have data associated with it (see the definition of "empty non-   
  277          terminals" in Section 7).                                         
  279       Protocol for getting data from a name:  The protocol described in    
  280          [RFC1035].                                                        
  282       Context for resolving a name:  The global DNS root zone              
  283          distributed by Public Technical Identifiers (PTI).                
  285    Private DNS:  Names that use the protocol described in [RFC1035] but    
  286       do not rely on the global DNS root zone or names that are            
  287       otherwise not generally available on the Internet but are using      
  288       the protocol described in [RFC1035].  A system can use both the      
  289       global DNS and one or more private DNS systems; for example, see     
  290       "Split DNS" in Section 6.                                            
  292       Note that domain names that do not appear in the DNS and that are    
  293       intended never to be looked up using the DNS protocol are not part   
  294       of the global DNS or a private DNS, even though they are domain      
  295       names.                                                               
  297    Multicast DNS (mDNS):  "Multicast DNS (mDNS) provides the ability to    
  298       perform DNS-like operations on the local link in the absence of      
  299       any conventional Unicast DNS server.  In addition, Multicast DNS     
  300       designates a portion of the DNS namespace to be free for local       
  301       use, without the need to pay any annual fee, and without the need    
  302       to set up delegations or otherwise configure a conventional DNS      
  303       server to answer for those names."  (Quoted from [RFC6762],          
  304       Abstract) Although it uses a compatible wire format, mDNS is,        
  305       strictly speaking, a different protocol than DNS.  Also, where the   
  306       above quote says "a portion of the DNS namespace", it would be       
  307       clearer to say "a portion of the domain name space".  The names in   
  308       mDNS are not intended to be looked up in the DNS.                    
  310    Locally served DNS zone:  A locally served DNS zone is a special case   
  311       of private DNS.  Names are resolved using the DNS protocol in a      
  312       local context.  [RFC6303] defines subdomains of IN-ADDR.ARPA that    
  313       are locally served zones.  Resolution of names through locally       
  314       served zones may result in ambiguous results.  For example, the      
  315       same name may resolve to different results in different locally      
  316       served DNS zone contexts.  The context for a locally served DNS      
  317       zone may be explicit, such as those that are listed in [RFC6303]     
  318       and [RFC7793], or implicit, such as those defined by local DNS       
  319       administration and not known to the resolution client.               
  321    Fully Qualified Domain Name (FQDN):  This is often just a clear way     
  322       of saying the same thing as "domain name of a node", as outlined     
  323       above.  However, the term is ambiguous.  Strictly speaking, a        
  324       fully qualified domain name would include every label, including     
  325       the zero-length label of the root; such a name would be written      
  326       "www.example.net." (note the terminating dot).  But, because every   
  327       name eventually shares the common root, names are often written      
  328       relative to the root (such as "www.example.net") and are still       
  329       called "fully qualified".  This term first appeared in [RFC819].     
  330       In this document, names are often written relative to the root.      
  332       The need for the term "fully qualified domain name" comes from the   
  333       existence of partially qualified domain names, which are names       
  334       where one or more of the last labels in the ordered list are         
  335       omitted (for example, a domain name of "www" relative to             
  336       "example.net" identifies "www.example.net").  Such relative names    
  337       are understood only by context.                                      
  339    Host name:  This term and its equivalent, "hostname", have been         
  340       widely used but are not defined in [RFC1034], [RFC1035],             
  341       [RFC1123], or [RFC2181].  The DNS was originally deployed into the   
  342       Host Tables environment as outlined in [RFC952], and it is likely    
  343       that the term followed informally from the definition there.  Over   
  344       time, the definition seems to have shifted.  "Host name" is often    
  345       meant to be a domain name that follows the rules in Section 3.5 of   
  346       [RFC1034], which is also called the "preferred name syntax".  (In    
  347       that syntax, every character in each label is a letter, a digit,     
  348       or a hyphen).  Note that any label in a domain name can contain      
  349       any octet value; hostnames are generally considered to be domain     
  350       names where every label follows the rules in the "preferred name     
  351       syntax", with the amendment that labels can start with ASCII         
  352       digits (this amendment comes from Section 2.1 of [RFC1123]).         
  354       People also sometimes use the term "hostname" to refer to just the   
  355       first label of an FQDN, such as "printer" in                         
  356       "printer.admin.example.com".  (Sometimes this is formalized in       
  357       configuration in operating systems.)  In addition, people            
  358       sometimes use this term to describe any name that refers to a        
  359       machine, and those might include labels that do not conform to the   
  360       "preferred name syntax".                                             
  362    Top-Level Domain (TLD):  A Top-Level Domain is a zone that is one       
  363       layer below the root, such as "com" or "jp".  There is nothing       
  364       special, from the point of view of the DNS, about TLDs.  Most of     
  365       them are also delegation-centric zones (defined in Section 7), and   
  366       there are significant policy issues around their operation.  TLDs    
  367       are often divided into sub-groups such as Country Code Top-Level     
  368       Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others;     
  369       the division is a matter of policy and beyond the scope of this      
  370       document.                                                            
  372    Internationalized Domain Name (IDN):  The Internationalized Domain      
  373       Names for Applications (IDNA) protocol is the standard mechanism     
  374       for handling domain names with non-ASCII characters in               
  375       applications in the DNS.  The current standard at the time of this   
  376       writing, normally called "IDNA2008", is defined in [RFC5890],        
  377       [RFC5891], [RFC5892], [RFC5893], and [RFC5894].  These documents     
  378       define many IDN-specific terms such as "LDH label", "A-label", and   
  379       "U-label".  [RFC6365] defines more terms that relate to              
  380       internationalization (some of which relate to IDNs); [RFC6055] has   
  381       a much more extensive discussion of IDNs, including some new         
  382       terminology.                                                         
  384    Subdomain:  "A domain is a subdomain of another domain if it is         
  385       contained within that domain.  This relationship can be tested by    
  386       seeing if the subdomain's name ends with the containing domain's     
  387       name."  (Quoted from [RFC1034], Section 3.1) For example, in the     
  388       host name "nnn.mmm.example.com", both "mmm.example.com" and          
  389       "nnn.mmm.example.com" are subdomains of "example.com".  Note that    
  390       the comparisons here are done on whole labels; that is,              
  391       "ooo.example.com" is not a subdomain of "oo.example.com".            
  393    Alias:  The owner of a CNAME resource record, or a subdomain of the     
  394       owner of a DNAME resource record (DNAME records are defined in       
  395       [RFC6672]).  See also "canonical name".                              
  397    Canonical name:  A CNAME resource record "identifies its owner name     
  398       as an alias, and specifies the corresponding canonical name in the   
  399       RDATA section of the RR."  (Quoted from [RFC1034], Section 3.6.2)    
  400       This usage of the word "canonical" is related to the mathematical    
  401       concept of "canonical form".                                         
  403    CNAME:  "It has been traditional to refer to the [owner] of a CNAME     
  404       record as 'a CNAME'.  This is unfortunate, as 'CNAME' is an          
  405       abbreviation of 'canonical name', and the [owner] of a CNAME         
  406       record is most certainly not a canonical name."  (Quoted from        
  407       [RFC2181], Section 10.1.1.  The quoted text has been changed from    
  408       "label" to "owner".)                                                 
  410 3.  DNS Response Codes                                                     
  412    Some of the response codes (RCODEs) that are defined in [RFC1035]       
  413    have acquired their own shorthand names.  All of the RCODEs are         
  414    listed at [IANA_Resource_Registry], although that list uses mixed-      
  415    case capitalization, while most documents use all caps.  Some of the    
  416    common names for values defined in [RFC1035] are described in this      
  417    section.  This section also includes an additional RCODE and a          
  418    general definition.  The official list of all RCODEs is in the IANA     
  419    registry.                                                               
  421    NOERROR:  This RCODE appears as "No error condition" in Section 4.1.1   
  422       of [RFC1035].                                                        
  424    FORMERR:  This RCODE appears as "Format error - The name server was     
  425       unable to interpret the query" in Section 4.1.1 of [RFC1035].        
  427    SERVFAIL:  This RCODE appears as "Server failure - The name server      
  428       was unable to process this query due to a problem with the name      
  429       server" in Section 4.1.1 of [RFC1035].                               
  431    NXDOMAIN:  This RCODE appears as "Name Error [...] this code            
  432       signifies that the domain name referenced in the query does not      
  433       exist." in Section 4.1.1 of [RFC1035].  [RFC2308] established        
  434       NXDOMAIN as a synonym for Name Error.                                
  436    NOTIMP:  This RCODE appears as "Not Implemented - The name server       
  437       does not support the requested kind of query" in Section 4.1.1 of    
  438       [RFC1035].                                                           
  440    REFUSED:  This RCODE appears as "Refused - The name server refuses to   
  441       perform the specified operation for policy reasons.  For example,    
  442       a name server may not wish to provide the information to the         
  443       particular requester, or a name server may not wish to perform a     
  444       particular operation (e.g., zone transfer) for particular data."     
  445       in Section 4.1.1 of [RFC1035].                                       
  447    NODATA:  "A pseudo RCODE which indicates that the name is valid, for    
  448       the given class, but [there] are no records of the given type.  A    
  449       NODATA response has to be inferred from the answer."  (Quoted from   
  450       [RFC2308], Section 1) "NODATA is indicated by an answer with the     
  451       RCODE set to NOERROR and no relevant answers in the Answer           
  452       section.  The Authority section will contain an SOA record, or       
  453       there will be no NS records there."  (Quoted from [RFC2308],         
  454       Section 2.2) Note that referrals have a similar format to NODATA     
  455       replies; [RFC2308] explains how to distinguish them.                 
  457       The term "NXRRSET" is sometimes used as a synonym for NODATA.        
  458       However, this is a mistake, given that NXRRSET is a specific error   
  459       code defined in [RFC2136].                                           
  461    Negative response:  A response that indicates that a particular RRset   
  462       does not exist or whose RCODE indicates that the nameserver cannot   
  463       answer.  Sections 2 and 7 of [RFC2308] describe the types of         
  464       negative responses in detail.                                        
  466 4.  DNS Transactions                                                       
  468    The header of a DNS message is its first 12 octets.  Many of the        
  469    fields and flags in the diagrams in Sections 4.1.1 through 4.1.3 of     
  470    [RFC1035] are referred to by their names in each diagram.  For          
  471    example, the response codes are called "RCODEs", the data for a         
  472    record is called the "RDATA", and the authoritative answer bit is       
  473    often called "the AA flag" or "the AA bit".                             
  475    Class:  A class "identifies a protocol family or instance of a          
  476       protocol".  (Quoted from [RFC1034], Section 3.6) "The DNS tags all   
  477       data with a class as well as the type, so that we can allow          
  478       parallel use of different formats for data of type address."         
  479       (Quoted from [RFC1034], Section 2.2) In practice, the class for      
  480       nearly every query is "IN" (the Internet).  There are some queries   
  481       for "CH" (the Chaos class), but they are usually for the purposes    
  482       of information about the server itself rather than for a different   
  483       type of address.                                                     
  485    QNAME:  The most commonly used rough definition is that the QNAME is    
  486       a field in the Question section of a query.  "A standard query       
  487       specifies a target domain name (QNAME), query type (QTYPE), and      
  488       query class (QCLASS) and asks for RRs which match."  (Quoted from    
  489       [RFC1034], Section 3.7.1) Strictly speaking, the definition comes    
  490       from [RFC1035], Section 4.1.2, where the QNAME is defined in         
  491       respect of the Question section.  This definition appears to be      
  492       applied consistently, as the discussion of inverse queries in        
  493       Section 6.4.1 of [RFC1035] refers to the "owner name of the query    
  494       RR and its TTL" because inverse queries populate the Answer          
  495       section and leave the Question section empty.  (Inverse queries      
  496       are deprecated in [RFC3425]; thus, relevant definitions do not       
  497       appear in this document.)                                            
  499       However, [RFC2308] has an alternate definition that puts the QNAME   
  500       in the answer (or series of answers) instead of the query.  It       
  501       defines QNAME as "...the name in the query section of an answer,     
  502       or where this resolves to a CNAME, or CNAME chain, the data field    
  503       of the last CNAME.  The last CNAME in this sense is that which       
  504       contains a value which does not resolve to another CNAME."  This     
  505       definition has a certain internal logic, because of the way CNAME    
  506       substitution works and the definition of CNAME.  If a name server    
  507       does not find an RRset that matches a query, but does find the       
  508       same name in the same class with a CNAME record, then the name       
  509       server "includes the CNAME record in the response and restarts the   
  510       query at the domain name specified in the data field of the CNAME    
  511       record."  (Quoted from [RFC1034], Section 3.6.2) This is made        
  512       explicit in the resolution algorithm outlined in Section 4.3.2 of    
  513       [RFC1034], which says to "change QNAME to the canonical name in      
  514       the CNAME RR, and go back to step 1" in the case of a CNAME RR.      
  515       Since a CNAME record explicitly declares that the owner name is      
  516       canonically named what is in the RDATA, then there is a way to       
  517       view the new name (i.e., the name that was in the RDATA of the       
  518       CNAME RR) as also being the QNAME.                                   
  520       However, this creates confusion because the response to a query      
  521       that results in CNAME processing contains in the echoed Question     
  522       section one QNAME (the name in the original query) and a second      
  523       QNAME that is in the data field of the last CNAME.  The confusion    
  524       comes from the iterative/recursive mode of resolution, which         
  525       finally returns an answer that need not actually have the same       
  526       owner name as the QNAME contained in the original query.             
  528       To address this potential confusion, it is helpful to distinguish    
  529       between three meanings:                                              
  531       QNAME (original):  The name actually sent in the Question section    
  532          in the original query, which is always echoed in the (final)      
  533          reply in the Question section when the QR bit is set to 1.        
  535       QNAME (effective):  A name actually resolved, which is either the    
  536          name originally queried or a name received in a CNAME chain       
  537          response.                                                         
  539       QNAME (final):  The name actually resolved, which is either the      
  540          name actually queried or else the last name in a CNAME chain      
  541          response.                                                         
  543       Note that, because the definition in [RFC2308] is actually for a     
  544       different concept than what was in [RFC1034], it would have been     
  545       better if [RFC2308] had used a different name for that concept.      
  546       In general use today, QNAME almost always means what is defined      
  547       above as "QNAME (original)".                                         
  549    Referrals:  A type of response in which a server, signaling that it     
  550       is not (completely) authoritative for an answer, provides the        
  551       querying resolver with an alternative place to send its query.       
  552       Referrals can be partial.                                            
  554       A referral arises when a server is not performing recursive          
  555       service while answering a query.  It appears in step 3(b) of the     
  556       algorithm in [RFC1034], Section 4.3.2.                               
  558       There are two types of referral response.  The first is a downward   
  559       referral (sometimes described as "delegation response"), where the   
  560       server is authoritative for some portion of the QNAME.  The          
  561       Authority section RRset's RDATA contains the name servers            
  562       specified at the referred-to zone cut.  In normal DNS operation,     
  563       this kind of response is required in order to find names beneath a   
  564       delegation.  The bare use of "referral" means this kind of           
  565       referral, and many people believe that this is the only legitimate   
  566       kind of referral in the DNS.                                         
  568       The second is an upward referral (sometimes described as "root       
  569       referral"), where the server is not authoritative for any portion    
  570       of the QNAME.  When this happens, the referred-to zone in the        
  571       Authority section is usually the root zone (".").  In normal DNS     
  572       operation, this kind of response is not required for resolution or   
  573       for correctly answering any query.  There is no requirement that     
  574       any server send upward referrals.  Some people regard upward         
  575       referrals as a sign of a misconfiguration or error.  Upward          
  576       referrals always need some sort of qualifier (such as "upward" or    
  577       "root") and are never identified simply by the word "referral".      
  579       A response that has only a referral contains an empty Answer         
  580       section.  It contains the NS RRset for the referred-to zone in the   
  581       Authority section.  It may contain RRs that provide addresses in     
  582       the Additional section.  The AA bit is clear.                        
  584       In the case where the query matches an alias, and the server is      
  585       not authoritative for the target of the alias but is authoritative   
  586       for some name above the target of the alias, the resolution          
  587       algorithm will produce a response that contains both the             
  588       authoritative answer for the alias and a referral.  Such a partial   
  589       answer and referral response has data in the Answer section.  It     
  590       has the NS RRset for the referred-to zone in the Authority           
  591       section.  It may contain RRs that provide addresses in the           
  592       Additional section.  The AA bit is set because the first name in     
  593       the Answer section matches the QNAME and the server is               
  594       authoritative for that answer (see [RFC1035], Section 4.1.1).        
  596 5.  Resource Records                                                       
  598    RR:  An acronym for resource record.  (See [RFC1034], Section 3.6.)     
  600    RRset:  A set of resource records "with the same label, class and       
  601       type, but with different data" (according to [RFC2181],              
  602       Section 5).  Also written as "RRSet" in some documents.  As a        
  603       clarification, "same label" in this definition means "same owner     
  604       name".  In addition, [RFC2181] states that "the TTLs of all RRs in   
  605       an RRSet must be the same".                                          
  607       Note that RRSIG resource records do not match this definition.       
  608       [RFC4035] says:                                                      
  610          An RRset MAY have multiple RRSIG RRs associated with it.  Note    
  611          that as RRSIG RRs are closely tied to the RRsets whose            
  612          signatures they contain, RRSIG RRs, unlike all other DNS RR       
  613          types, do not form RRsets.  In particular, the TTL values among   
  614          RRSIG RRs with a common owner name do not follow the RRset        
  615          rules described in [RFC2181].                                     
  617    Master file:  "Master files are text files that contain RRs in text     
  618       form.  Since the contents of a zone can be expressed in the form     
  619       of a list of RRs a master file is most often used to define a        
  620       zone, though it can be used to list a cache's contents."  (Quoted    
  621       from [RFC1035], Section 5) Master files are sometimes called "zone   
  622       files".                                                              
  624    Presentation format:  The text format used in master files.  This       
  625       format is shown but not formally defined in [RFC1034] or             
  626       [RFC1035].  The term "presentation format" first appears in          
  627       [RFC4034].                                                           
  629    EDNS:  The extension mechanisms for DNS, defined in [RFC6891].          
  630       Sometimes called "EDNS0" or "EDNS(0)" to indicate the version        
  631       number.  EDNS allows DNS clients and servers to specify message      
  632       sizes larger than the original 512-octet limit, to expand the        
  633       response code space, and to carry additional options that affect     
  634       the handling of a DNS query.                                         
  636    OPT:  A pseudo-RR (sometimes called a "meta-RR") that is used only to   
  637       contain control information pertaining to the question-and-answer    
  638       sequence of a specific transaction.  (Definition paraphrased from    
  639       [RFC6891], Section 6.1.1.)  It is used by EDNS.                      
  641    Owner:  "The domain name where the RR is found."  (Quoted from          
  642       [RFC1034], Section 3.6) Often appears in the term "owner name".      
  644    SOA field names:  DNS documents, including the definitions here,        
  645       often refer to the fields in the RDATA of an SOA resource record     
  646       by field name.  "SOA" stands for "start of a zone of authority".     
  647       Those fields are defined in Section 3.3.13 of [RFC1035].  The        
  648       names (in the order they appear in the SOA RDATA) are MNAME,         
  649       RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM.  Note that the   
  650       meaning of the MINIMUM field is updated in Section 4 of [RFC2308];   
  651       the new definition is that the MINIMUM field is only "the TTL to     
  652       be used for negative responses".  This document tends to use field   
  653       names instead of terms that describe the fields.                     
  655    TTL:  The maximum "time to live" of a resource record.  "A TTL value    
  656       is an unsigned number, with a minimum value of 0, and a maximum      
  657       value of 2147483647.  That is, a maximum of 2^31 - 1.  When          
  658       transmitted, this value shall be encoded in the less significant     
  659       31 bits of the 32 bit TTL field, with the most significant, or       
  660       sign, bit set to zero."  (Quoted from [RFC2181], Section 8) Note     
  661       that [RFC1035] erroneously stated that this is a signed integer;     
  662       that was fixed by [RFC2181].                                         
  664       The TTL "specifies the time interval that the resource record may    
  665       be cached before the source of the information should again be       
  666       consulted."  (Quoted from [RFC1035], Section 3.2.1) Section 4.1.3    
  667       of [RFC1035] states "the time interval (in seconds) that the         
  668       resource record may be cached before it should be discarded".        
  669       Despite being defined for a resource record, the TTL of every        
  670       resource record in an RRset is required to be the same ([RFC2181],   
  671       Section 5.2).                                                        
  673       The reason that the TTL is the maximum time to live is that a        
  674       cache operator might decide to shorten the time to live for          
  675       operational purposes, for example, if there is a policy to           
  676       disallow TTL values over a certain number.  Some servers are known   
  677       to ignore the TTL on some RRsets (such as when the authoritative     
  678       data has a very short TTL) even though this is against the advice    
  679       in [RFC1035].  An RRset can be flushed from the cache before the     
  680       end of the TTL interval, at which point, the value of the TTL        
  681       becomes unknown because the RRset with which it was associated no    
  682       longer exists.                                                       
  684       There is also the concept of a "default TTL" for a zone, which can   
  685       be a configuration parameter in the server software.  This is        
  686       often expressed by a default for the entire server, and a default    
  687       for a zone using the $TTL directive in a zone file.  The $TTL        
  688       directive was added to the master file format by [RFC2308].          
  690    Class independent:  A resource record type whose syntax and semantics   
  691       are the same for every DNS class.  A resource record type that is    
  692       not class independent has different meanings, depending on the DNS   
  693       class of the record or if the meaning is undefined for some          
  694       classes.  Most resource record types are defined for class 1 (IN,    
  695       the Internet), but many are undefined for other classes.             
  697    Address records:  Records whose type is either A or AAAA.  [RFC2181]    
  698       informally defines these as "(A, AAAA, etc)".  Note that new types   
  699       of address records could be defined in the future.                   
  701 6.  DNS Servers and Clients                                                
  703    This section defines the terms used for the systems that act as DNS     
  704    clients, DNS servers, or both.  In past RFCs, DNS servers are           
  705    sometimes called "name servers", "nameservers", or just "servers".      
  706    There is no formal definition of "DNS server", but RFCs generally       
  707    assume that it is an Internet server that listens for queries and       
  708    sends responses using the DNS protocol defined in [RFC1035] and its     
  709    successors.                                                             
  711    It is important to note that the terms "DNS server" and "name server"   
  712    require context in order to understand the services being provided.     
  713    Both authoritative servers and recursive resolvers are often called     
  714    "DNS servers" and "name servers" even though they serve different       
  715    roles (but may be part of the same software package).                   
  717    For terminology specific to the global DNS root server system, see      
  718    [RSSAC026].  That document defines terms such as "root server", "root   
  719    server operator", and terms that are specific to the way that the       
  720    root zone of the global DNS is served.                                  
  722    Resolver:  A program "that extract[s] information from name servers     
  723       in response to client requests."  (Quoted from [RFC1034],            
  724       Section 2.4) A resolver performs queries for a name, type, and       
  725       class, and receives responses.  The logical function is called       
  726       "resolution".  In practice, the term is usually referring to some    
  727       specific type of resolver (some of which are defined below), and     
  728       understanding the use of the term depends on understanding the       
  729       context.                                                             
  731       A related term is "resolve", which is not formally defined in        
  732       [RFC1034] or [RFC1035].  An imputed definition might be "asking a    
  733       question that consists of a domain name, class, and type, and        
  734       receiving some sort of response".  Similarly, an imputed             
  735       definition of "resolution" might be "the response received from      
  736       resolving".                                                          
  738    Stub resolver:  A resolver that cannot perform all resolution itself.   
  739       Stub resolvers generally depend on a recursive resolver to           
  740       undertake the actual resolution function.  Stub resolvers are        
  741       discussed but never fully defined in Section 5.3.1 of [RFC1034].     
  742       They are fully defined in Section of [RFC1123].              
  744    Iterative mode:  A resolution mode of a server that receives DNS        
  745       queries and responds with a referral to another server.              
  746       Section 2.3 of [RFC1034] describes this as "The server refers the    
  747       client to another server and lets the client pursue the query."  A   
  748       resolver that works in iterative mode is sometimes called an         
  749       "iterative resolver".  See also "iterative resolution" later in      
  750       this section.                                                        
  752    Recursive mode:  A resolution mode of a server that receives DNS        
  753       queries and either responds to those queries from a local cache or   
  754       sends queries to other servers in order to get the final answers     
  755       to the original queries.  Section 2.3 of [RFC1034] describes this    
  756       as "the first server pursues the query for the client at another     
  757       server".  Section 4.3.1 of [RFC1034] says: "in [recursive] mode      
  758       the name server acts in the role of a resolver and returns either    
  759       an error or the answer, but never referrals."  That same section     
  760       also says:                                                           
  762          The recursive mode occurs when a query with RD set arrives at a   
  763          server which is willing to provide recursive service; the         
  764          client can verify that recursive mode was used by checking that   
  765          both RA and RD are set in the reply.                              
  767       A server operating in recursive mode may be thought of as having a   
  768       name server side (which is what answers the query) and a resolver    
  769       side (which performs the resolution function).  Systems operating    
  770       in this mode are commonly called "recursive servers".  Sometimes     
  771       they are called "recursive resolvers".  In practice, it is not       
  772       possible to know in advance whether the server that one is           
  773       querying will also perform recursion; both terms can be observed     
  774       in use interchangeably.                                              
  776    Recursive resolver:  A resolver that acts in recursive mode.  In        
  777       general, a recursive resolver is expected to cache the answers it    
  778       receives (which would make it a full-service resolver), but some     
  779       recursive resolvers might not cache.                                 
  781       [RFC4697] tried to differentiate between a recursive resolver and    
  782       an iterative resolver.                                               
  784    Recursive query:  A query with the Recursion Desired (RD) bit set to    
  785       1 in the header.  (See Section 4.1.1 of [RFC1035].)  If recursive    
  786       service is available and is requested by the RD bit in the query,    
  787       the server uses its resolver to answer the query.  (See              
  788       Section 4.3.2 of [RFC1034].)                                         
  790    Non-recursive query:  A query with the Recursion Desired (RD) bit set   
  791       to 0 in the header.  A server can answer non-recursive queries       
  792       using only local information: the response contains either an        
  793       error, the answer, or a referral to some other server "closer" to    
  794       the answer.  (See Section 4.3.1 of [RFC1034].)                       
  796    Iterative resolution:  A name server may be presented with a query      
  797       that can only be answered by some other server.  The two general     
  798       approaches to dealing with this problem are "recursive", in which    
  799       the first server pursues the query on behalf of the client at        
  800       another server, and "iterative", in which the server refers the      
  801       client to another server and lets the client pursue the query        
  802       there.  (See Section 2.3 of [RFC1034].)                              
  804       In iterative resolution, the client repeatedly makes non-recursive   
  805       queries and follows referrals and/or aliases.  The iterative         
  806       resolution algorithm is described in Section 5.3.3 of [RFC1034].     
  808    Full resolver:  This term is used in [RFC1035], but it is not defined   
  809       there.  RFC 1123 defines a "full-service resolver" that may or may   
  810       not be what was intended by "full resolver" in [RFC1035].  This      
  811       term is not properly defined in any RFC, and there is no consensus   
  812       on what the term means.  The use of this term without proper         
  813       context is discouraged.                                              
  815    Full-service resolver:  Section of [RFC1123] defines this       
  816       term as a resolver that acts in recursive mode with a cache (and     
  817       meets other requirements).                                           
  819    Priming:  "The act of finding the list of root servers from a           
  820       configuration that lists some or all of the purported IP addresses   
  821       of some or all of those root servers."  (Quoted from [RFC8109],      
  822       Section 2) In order to operate in recursive mode, a resolver needs   
  823       to know the address of at least one root server.  Priming is most    
  824       often done from a configuration setting that contains a list of      
  825       authoritative servers for the root zone.                             
  827    Root hints:  "Operators who manage a DNS recursive resolver typically   
  828       need to configure a 'root hints file'.  This file contains the       
  829       names and IP addresses of the authoritative name servers for the     
  830       root zone, so the software can bootstrap the DNS resolution          
  831       process.  For many pieces of software, this list comes built into    
  832       the software."  (Quoted from [IANA_RootFiles]) This file is often    
  833       used in priming.                                                     
  835    Negative caching:  "The storage of knowledge that something does not    
  836       exist, cannot or does not give an answer."  (Quoted from             
  837       [RFC2308], Section 1)                                                
  839    Authoritative server:  "A server that knows the content of a DNS zone   
  840       from local knowledge, and thus can answer queries about that zone    
  841       without needing to query other servers."  (Quoted from [RFC2182],    
  842       Section 2) An authoritative server is named in the NS ("name         
  843       server") record in a zone.  It is a system that responds to DNS      
  844       queries with information about zones for which it has been           
  845       configured to answer with the AA flag in the response header set     
  846       to 1.  It is a server that has authority over one or more DNS        
  847       zones.  Note that it is possible for an authoritative server to      
  848       respond to a query without the parent zone delegating authority to   
  849       that server.  Authoritative servers also provide "referrals",        
  850       usually to child zones delegated from them; these referrals have     
  851       the AA bit set to 0 and come with referral data in the Authority     
  852       and (if needed) the Additional sections.                             
  854    Authoritative-only server:  A name server that only serves              
  855       authoritative data and ignores requests for recursion.  It will      
  856       "not normally generate any queries of its own.  Instead it answers   
  857       non-recursive queries from iterative resolvers looking for           
  858       information in zones it serves."  (Quoted from [RFC4697],            
  859       Section 2.4) In this case, "ignores requests for recursion" means    
  860       "responds to requests for recursion with responses indicating that   
  861       recursion was not performed".                                        
  863    Zone transfer:  The act of a client requesting a copy of a zone and     
  864       an authoritative server sending the needed information.  (See        
  865       Section 7 for a description of zones.)  There are two common         
  866       standard ways to do zone transfers: the AXFR ("Authoritative         
  867       Transfer") mechanism to copy the full zone (described in             
  868       [RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy   
  869       only parts of the zone that have changed (described in [RFC1995]).   
  870       Many systems use non-standard methods for zone transfers outside     
  871       the DNS protocol.                                                    
  873    Slave server:  See "Secondary server".                                  
  875    Secondary server:  "An authoritative server which uses zone transfer    
  876       to retrieve the zone."  (Quoted from [RFC1996], Section 2.1)         
  877       Secondary servers are also discussed in [RFC1034].  [RFC2182]        
  878       describes secondary servers in more detail.  Although early DNS      
  879       RFCs such as [RFC1996] referred to this as a "slave", the current    
  880       common usage has shifted to calling it a "secondary".                
  882    Master server:  See "Primary server".                                   
  884    Primary server:  "Any authoritative server configured to be the         
  885       source of zone transfer for one or more [secondary] servers."        
  886       (Quoted from [RFC1996], Section 2.1) Or, more specifically,          
  887       [RFC2136] calls it "an authoritative server configured to be the     
  888       source of AXFR or IXFR data for one or more [secondary] servers".    
  889       Primary servers are also discussed in [RFC1034].  Although early     
  890       DNS RFCs such as [RFC1996] referred to this as a "master", the       
  891       current common usage has shifted to "primary".                       
  893    Primary master:  "The primary master is named in the zone's SOA MNAME   
  894       field and optionally by an NS RR."  (Quoted from [RFC1996],          
  895       Section 2.1) [RFC2136] defines "primary master" as "Master server    
  896       at the root of the AXFR/IXFR dependency graph.  The primary master   
  897       is named in the zone's SOA MNAME field and optionally by an NS RR.   
  898       There is by definition only one primary master server per zone."     
  900       The idea of a primary master is only used in [RFC1996] and           
  901       [RFC2136].  A modern interpretation of the term "primary master"     
  902       is a server that is both authoritative for a zone and that gets      
  903       its updates to the zone from configuration (such as a master file)   
  904       or from UPDATE transactions.                                         
  906    Stealth server:  This is "like a slave server except not listed in an   
  907       NS RR for the zone."  (Quoted from [RFC1996], Section 2.1)           
  909    Hidden master:  A stealth server that is a primary server for zone      
  910       transfers.  "In this arrangement, the master name server that        
  911       processes the updates is unavailable to general hosts on the         
  912       Internet; it is not listed in the NS RRset."  (Quoted from           
  913       [RFC6781], Section 3.4.3) [RFC4641] said that the hidden master's    
  914       name "appears in the SOA RRs MNAME field"; however, the name does    
  915       not appear at all in the global DNS in some setups.  A hidden        
  916       master can also be a secondary server for the zone itself.           
  918    Forwarding:  The process of one server sending a DNS query with the     
  919       RD bit set to 1 to another server to resolve that query.             
  920       Forwarding is a function of a DNS resolver; it is different than     
  921       simply blindly relaying queries.                                     
  923       [RFC5625] does not give a specific definition for forwarding, but    
  924       describes in detail what features a system that forwards needs to    
  925       support.  Systems that forward are sometimes called "DNS proxies",   
  926       but that term has not yet been defined (even in [RFC5625]).          
  928    Forwarder:  Section 1 of [RFC2308] describes a forwarder as "a          
  929       nameserver used to resolve queries instead of directly using the     
  930       authoritative nameserver chain".  [RFC2308] further says "The        
  931       forwarder typically either has better access to the internet, or     
  932       maintains a bigger cache which may be shared amongst many            
  933       resolvers."  That definition appears to suggest that forwarders      
  934       normally only query authoritative servers.  In current use,          
  935       however, forwarders often stand between stub resolvers and           
  936       recursive servers.  [RFC2308] is silent on whether a forwarder is    
  937       iterative-only or can be a full-service resolver.                    
  939    Policy-implementing resolver:  A resolver acting in recursive mode      
  940       that changes some of the answers that it returns based on policy     
  941       criteria, such as to prevent access to malware sites or              
  942       objectionable content.  In general, a stub resolver has no idea      
  943       whether upstream resolvers implement such policy or, if they do,     
  944       the exact policy about what changes will be made.  In some cases,    
  945       the user of the stub resolver has selected the policy-implementing   
  946       resolver with the explicit intention of using it to implement the    
  947       policies.  In other cases, policies are imposed without the user     
  948       of the stub resolver being informed.                                 
  950    Open resolver:  A full-service resolver that accepts and processes      
  951       queries from any (or nearly any) client.  This is sometimes also     
  952       called a "public resolver", although the term "public resolver" is   
  953       used more with open resolvers that are meant to be open, as          
  954       compared to the vast majority of open resolvers that are probably    
  955       misconfigured to be open.  Open resolvers are discussed in           
  956       [RFC5358].                                                           
  958    Split DNS:  The terms "split DNS" and "split-horizon DNS" have long     
  959       been used in the DNS community without formal definition.  In        
  960       general, they refer to situations in which DNS servers that are      
  961       authoritative for a particular set of domains provide partly or      
  962       completely different answers in those domains depending on the       
  963       source of the query.  Nevertheless, the effect of this is that a     
  964       domain name that is notionally globally unique has different         
  965       meanings for different network users.  This can sometimes be the     
  966       result of a "view" configuration, as described below.                
  968       Section 3.8 of [RFC2775] gives a related definition that is too      
  969       specific to be generally useful.                                     
  971    View:  A configuration for a DNS server that allows it to provide       
  972       different responses depending on attributes of the query, such as    
  973       for "split DNS".  Typically, views differ by the source IP address   
  974       of a query, but can also be based on the destination IP address,     
  975       the type of query (such as AXFR), whether it is recursive, and so    
  976       on.  Views are often used to provide more names or different         
  977       addresses to queries from "inside" a protected network than to       
  978       those "outside" that network.  Views are not a standardized part     
  979       of the DNS, but they are widely implemented in server software.      
  981    Passive DNS:  A mechanism to collect DNS data by storing DNS            
  982       responses from name servers.  Some of these systems also collect     
  983       the DNS queries associated with the responses, although doing so     
  984       raises some privacy concerns.  Passive DNS databases can be used     
  985       to answer historical questions about DNS zones, such as which        
  986       values were present at a given time in the past, or when a name      
  987       was spotted first.  Passive DNS databases allow searching of the     
  988       stored records on keys other than just the name and type, such as    
  989       "find all names which have A records of a particular value".         
  991    Anycast:  "The practice of making a particular service address          
  992       available in multiple, discrete, autonomous locations, such that     
  993       datagrams sent are routed to one of several available locations."    
  994       (Quoted from [RFC4786], Section 2) See [RFC4786] for more detail     
  995       on Anycast and other terms that are specific to its use.             
  997    Instance:  "When anycast routing is used to allow more than one         
  998       server to have the same IP address, each one of those servers is     
  999       commonly referred to as an 'instance'."  It goes on to say: "An      
 1000       instance of a server, such as a root server, is often referred to    
 1001       as an 'Anycast instance'."  (Quoted from [RSSAC026])                 
 1003    Privacy-enabling DNS server:  "A DNS server that implements DNS over    
 1004       TLS [RFC7858] and may optionally implement DNS over DTLS             
 1005       [RFC8094]."  (Quoted from [RFC8310], Section 2) Other types of DNS   
 1006       servers might also be considered privacy-enabling, such as those     
 1007       running DNS-over-HTTPS [RFC8484] or DNS-over-QUIC [RFC9250].         
 1009    DNS-over-TLS (DoT):  DNS over TLS as defined in [RFC7858] and its       
 1010       successors.                                                          
 1012    DNS-over-HTTPS (DoH):  DNS over HTTPS as defined in [RFC8484] and its   
 1013       successors.                                                          
 1015    DNS-over-QUIC (DoQ):  DNS over QUIC as defined in [RFC9250] and its     
 1016       successors.  [RFC9250] specifically defines DoQ as general-purpose   
 1017       transport for DNS that can be used in stub to recursive, recursive   
 1018       to authoritative, and zone transfer scenarios.                       
 1020    Classic DNS:  DNS over UDP or DNS over TCP as defined in [RFC1035]      
 1021       and its successors.  Classic DNS applies to DNS communication        
 1022       between stub resolvers and recursive resolvers, and between          
 1023       recursive resolvers and authoritative servers.  This has sometimes   
 1024       been called "Do53".  Classic DNS is not encrypted.                   
 1026    Recursive DoT (RDoT):  RDoT specifically means DNS-over-TLS for         
 1027       transport between a stub resolver and a recursive resolver, or       
 1028       between a recursive resolver and another recursive resolver.  This   
 1029       term is necessary because it is expected that DNS-over-TLS will      
 1030       later be defined as a transport between recursive resolvers and      
 1031       authoritative servers.                                               
 1033    Authoritative DoT (ADoT):  If DNS-over-TLS is later defined as a        
 1034       transport between recursive resolvers and authoritative servers,     
 1035       ADoT specifically means DNS-over-TLS for transport between           
 1036       recursive resolvers and authoritative servers.                       
 1038    XFR-over-TLS (XoT):  DNS zone transfer over TLS, as specified in        
 1039       [RFC9103].  This term applies to both AXFR over TLS (AXoT) and       
 1040       IXFR over TLS (IXoT).                                                
 1042 7.  Zones                                                                  
 1044    This section defines terms that are used when discussing zones that     
 1045    are being served or retrieved.                                          
 1047    Zone:  "Authoritative information is organized into units called        
 1048       ZONEs, and these zones can be automatically distributed to the       
 1049       name servers which provide redundant service for the data in a       
 1050       zone."  (Quoted from [RFC1034], Section 2.4)                         
 1052    Child:  "The entity on record that has the delegation of the domain     
 1053       from the Parent."  (Quoted from [RFC7344], Section 1.1)              
 1055    Parent:  "The domain in which the Child is registered."  (Quoted from   
 1056       [RFC7344], Section 1.1) Earlier, "parent name server" was defined    
 1057       in [RFC0882] as "the name server that has authority over the place   
 1058       in the domain name space that will hold the new domain".  (Note      
 1059       that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].)            
 1060       [RFC819] also has some description of the relationship between       
 1061       parents and children.                                                
 1063    Origin:                                                                 
 1065       There are two different uses for this term:                          
 1067       (a)  "The domain name that appears at the top of a zone (just        
 1068            below the cut that separates the zone from its parent)... The   
 1069            name of the zone is the same as the name of the domain at the   
 1070            zone's origin."  (Quoted from [RFC2181], Section 6) These       
 1071            days, this sense of "origin" and "apex" (defined below) are     
 1072            often used interchangeably.                                     
 1074       (b)  The domain name within which a given relative domain name       
 1075            appears in zone files.  Generally seen in the context of        
 1076            "$ORIGIN", which is a control entry defined in [RFC1035],       
 1077            Section 5.1, as part of the master file format.  For example,   
 1078            if the $ORIGIN is set to "example.org.", then a master file     
 1079            line for "www" is in fact an entry for "www.example.org.".      
 1081    Apex:  The point in the tree at an owner of an SOA and corresponding    
 1082       authoritative NS RRset.  This is also called the "zone apex".        
 1083       [RFC4033] defines it as "the name at the child's side of a zone      
 1084       cut".  The "apex" can usefully be thought of as a data-theoretic     
 1085       description of a tree structure, and "origin" is the name of the     
 1086       same concept when it is implemented in zone files.  The              
 1087       distinction is not always maintained in use, however, and one can    
 1088       find uses that conflict subtly with this definition.  [RFC1034]      
 1089       uses the term "top node of the zone" as a synonym of "apex", but     
 1090       that term is not widely used.  These days, the first sense of        
 1091       "origin" (above) and "apex" are often used interchangeably.          
 1093    Zone cut:  The delimitation point between two zones where the origin    
 1094       of one of the zones is the child of the other zone.                  
 1096       "Zones are delimited by 'zone cuts'.  Each zone cut separates a      
 1097       'child' zone (below the cut) from a 'parent' zone (above the         
 1098       cut)."  (Quoted from [RFC2181], Section 6; note that this is         
 1099       barely an ostensive definition.)  Section 4.2 of [RFC1034] uses      
 1100       "cuts" instead of "zone cut".                                        
 1102    Delegation:  The process by which a separate zone is created in the     
 1103       name space beneath the apex of a given domain.  Delegation happens   
 1104       when an NS RRset is added in the parent zone for the child origin.   
 1105       Delegation inherently happens at a zone cut.  The term is also       
 1106       commonly a noun: the new zone that is created by the act of          
 1107       delegating.                                                          
 1109    Authoritative data:  "All of the RRs attached to all of the nodes       
 1110       from the top node of the zone down to leaf nodes or nodes above      
 1111       cuts around the bottom edge of the zone."  (Quoted from [RFC1034],   
 1112       Section 4.2.1) Note that this definition might inadvertently also    
 1113       cause any NS records that appear in the zone to be included, even    
 1114       those that might not truly be authoritative, because there are       
 1115       identical NS RRs below the zone cut.  This reveals the ambiguity     
 1116       in the notion of authoritative data, because the parent-side NS      
 1117       records authoritatively indicate the delegation, even though they    
 1118       are not themselves authoritative data.                               
 1120       [RFC4033], Section 2, defines "Authoritative RRset", which is        
 1121       related to authoritative data but has a more precise definition.     
 1123    Lame delegation:  "A lame delegations exists [sic] when a nameserver    
 1124       is delegated responsibility for providing nameservice for a zone     
 1125       (via NS records) but is not performing nameservice for that zone     
 1126       (usually because it is not set up as a primary or secondary for      
 1127       the zone)."  (Quoted from [RFC1912], Section 2.8) Another            
 1128       definition is that a lame delegation "...happens when a name         
 1129       server is listed in the NS records for some domain and in fact it    
 1130       is not a server for that domain.  Queries are thus sent to the       
 1131       wrong servers, who don't know nothing [sic] (at least not as         
 1132       expected) about the queried domain.  Furthermore, sometimes these    
 1133       hosts (if they exist!) don't even run name servers."  (Quoted from   
 1134       [RFC1713], Section 2.3)                                              
 1136       These early definitions do not match the current use of the term     
 1137       "lame delegation", but there is no consensus on what a lame          
 1138       delegation is.  The term is used not only for the specific case      
 1139       described above, but for a variety of other flaws in delegations     
 1140       that lead to non-authoritative answers or no answers at all, such    
 1141       as:                                                                  
 1143       *  a nameserver with an NS record for a zone that does not answer    
 1144          DNS queries;                                                      
 1146       *  a nameserver with an IP address that is not reachable by the      
 1147          resolver; and                                                     
 1149       *  a nameserver that responds to a query for a specific name with    
 1150          an error or without the authoritative bit set.                    
 1152       Because the term in current usage has drifted from the original      
 1153       definition, and now is not specific or clear as to the intended      
 1154       meaning, it should be considered historic and avoided in favor of    
 1155       terms that are specific and clear.                                   
 1157    Glue records:  "...[Resource records] which are not part of the         
 1158       authoritative data [of the zone], and are address RRs for the        
 1159       [name] servers [in subzones].  These RRs are only necessary if the   
 1160       name server's name is 'below' the cut, and are only used as part     
 1161       of a referral response."  Without glue "we could be faced with the   
 1162       situation where the NS RRs tell us that in order to learn a name     
 1163       server's address, we should contact the server using the address     
 1164       we wish to learn."  (Quoted from [RFC1034], Section 4.2.1)           
 1166       A later definition is that glue "includes any record in a zone       
 1167       file that is not properly part of that zone, including nameserver    
 1168       records of delegated sub-zones (NS records), address records that    
 1169       accompany those NS records (A, AAAA, etc), and any other stray       
 1170       data that might appear."  (Quoted from [RFC2181], Section 5.4.1)     
 1171       Although glue is sometimes used today with this wider definition     
 1172       in mind, the context surrounding the definition in [RFC2181]         
 1173       suggests it is intended to apply to the use of glue within the       
 1174       document itself and not necessarily beyond.                          
 1176       In an NS record, there are three types of relationships between      
 1177       the owner name of the record, the name in the NS RDATA, and the      
 1178       zone origin: unrelated, in-domain, and sibling domain.  The          
 1179       application of these three types of relationships to glue records    
 1180       is defined in [RFC9471].                                             
 1182       An unrelated relationship is one where the NS RDATA contains a       
 1183       name server that is not subordinate to the zone origin and           
 1184       therefore is not part of the same zone.                              
 1186       An in-domain relationship is one where the NS RDATA contains a       
 1187       name server whose name is either subordinate to or (rarely) the      
 1188       same as the owner name of the NS resource records.  For example, a   
 1189       delegation for "child.example.com" might have an in-domain name      
 1190       server called "ns.child.example.com".                                
 1192       A sibling domain relationship is one where the NS RDATA contains a   
 1193       name server whose name is either subordinate to or (rarely) the      
 1194       same as the zone origin of the parent and not subordinate to or      
 1195       the same as the owner name of the NS resource records.  For          
 1196       example, a delegation for "child.example.com" in "example.com"       
 1197       zone might have a sibling domain name server called                  
 1198       "ns.another.example.com".                                            
 1200       The following table shows examples of delegation types:              
 1203         +=============+========+====================+================+     
 1204         | Delegation  | Parent | Name Server Name   | Type           |     
 1205         +=============+========+====================+================+     
 1206         | com         | .      | a.gtld-servers.net | sibling domain |     
 1207         +-------------+--------+--------------------+----------------+     
 1208         | net         | .      | a.gtld-servers.net | in-domain      |     
 1209         +-------------+--------+--------------------+----------------+     
 1210         | example.org | org    | ns.example.org     | in-domain      |     
 1211         +-------------+--------+--------------------+----------------+     
 1212         | example.org | org    | ns.ietf.org        | sibling domain |     
 1213         +-------------+--------+--------------------+----------------+     
 1214         | example.org | org    | ns.example.com     | unrelated      |     
 1215         +-------------+--------+--------------------+----------------+     
 1216         | example.jp  | jp     | ns.example.jp      | in-domain      |     
 1217         +-------------+--------+--------------------+----------------+     
 1218         | example.jp  | jp     | ns.example.ne.jp   | sibling domain |     
 1219         +-------------+--------+--------------------+----------------+     
 1220         | example.jp  | jp     | ns.example.com     | unrelated      |     
 1221         +-------------+--------+--------------------+----------------+     
 1223                                    Table 1                                 
 1225    Bailiwick:  "In-bailiwick" and "Out-of-bailiwick" are modifiers used    
 1226       to describe the relationship between a zone and the name servers     
 1227       for that zone.  The dictionary definition of bailiwick has been      
 1228       observed to cause more confusion than meaning for this use.  These   
 1229       terms should be considered historic in nature.                       
 1231    Root zone:  The zone of a DNS-based tree whose apex is the zero-        
 1232       length label.  Also sometimes called "the DNS root".                 
 1234    Empty non-terminals (ENTs):  "Domain names that own no resource         
 1235       records but have subdomains that do."  (Quoted from [RFC4592],       
 1236       Section 2.2.2) A typical example is in SRV records: in the name      
 1237       "_sip._tcp.example.com", it is likely that "_tcp.example.com" has    
 1238       no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV    
 1239       RRset.                                                               
 1241    Delegation-centric zone:  A zone that consists mostly of delegations    
 1242       to child zones.  This term is used in contrast to a zone that        
 1243       might have some delegations to child zones but also has many data    
 1244       resource records for the zone itself and/or for child zones.  The    
 1245       term is used in [RFC4956] and [RFC5155], but it is not defined in    
 1246       either document.                                                     
 1248    Occluded name:  "The addition of a delegation point via dynamic         
 1249       update will render all subordinate domain names to be in a limbo,    
 1250       still part of the zone but not available to the lookup process.      
 1251       The addition of a DNAME resource record has the same impact.  The    
 1252       subordinate names are said to be 'occluded'."  (Quoted from          
 1253       [RFC5936], Section 3.5)                                              
 1255    Fast flux DNS:  This "occurs when a domain is [found] in DNS using A    
 1256       records to multiple IP addresses, each of which has a very short     
 1257       Time-to-Live (TTL) value associated with it.  This means that the    
 1258       domain resolves to varying IP addresses over a short period of       
 1259       time."  (Quoted from [RFC6561], Section 1.1.5, with a typo           
 1260       corrected) In addition to having legitimate uses, fast flux DNS      
 1261       can be used to deliver malware.  Because the addresses change so     
 1262       rapidly, it is difficult to ascertain all the hosts.  It should be   
 1263       noted that the technique also works with AAAA records, but such      
 1264       use is not frequently observed on the Internet as of this writing.   
 1266    Reverse DNS, reverse lookup:  "The process of mapping an address to a   
 1267       name is generally known as a 'reverse lookup', and the IN-           
 1268       ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse        
 1269       DNS'."  (Quoted from [RFC5855], Section 1)                           
 1271    Forward lookup:  "Hostname-to-address translation".  (Quoted from       
 1272       [RFC3493], Section 6)                                                
 1274    arpa (Address and Routing Parameter Area Domain):  "The 'arpa' domain   
 1275       was originally established as part of the initial deployment of      
 1276       the DNS to provide a transition mechanism from the Host Tables       
 1277       that were common in the ARPANET, as well as a home for the IPv4      
 1278       reverse mapping domain.  During 2000, the abbreviation was           
 1279       redesignated to 'Address and Routing Parameter Area' in the hope     
 1280       of reducing confusion with the earlier network name."  (Quoted       
 1281       from [RFC3172], Section 2) .arpa is an "infrastructure domain", a    
 1282       domain whose "role is to support the operating infrastructure of     
 1283       the Internet".  (Quoted from [RFC3172], Section 2) See [RFC3172]     
 1284       for more history of this name.                                       
 1286    Service name:  "Service names are the unique key in the Service Name    
 1287       and Transport Protocol Port Number registry.  This unique symbolic   
 1288       name for a service may also be used for other purposes, such as in   
 1289       DNS SRV records."  (Quoted from [RFC6335], Section 5)                
 1291 8.  Wildcards                                                              
 1293    Wildcard:  [RFC1034] defined "wildcard", but in a way that turned out   
 1294       to be confusing to implementers.  For an extended discussion of      
 1295       wildcards, including clearer definitions, see [RFC4592].  Special    
 1296       treatment is given to RRs with owner names starting with the label   
 1297       "*".  "Such RRs are called 'wildcards'.  Wildcard RRs can be         
 1298       thought of as instructions for synthesizing RRs."  (Quoted from      
 1299       [RFC1034], Section 4.3.3)                                            
 1301    Asterisk label:  "The first octet is the normal label type and length   
 1302       for a 1-octet-long label, and the second octet is the ASCII          
 1303       representation [RFC20] for the '*' character.  A descriptive name    
 1304       of a label equaling that value is an 'asterisk label'."  (Quoted     
 1305       from [RFC4592], Section 2.1.1)                                       
 1307    Wildcard domain name:  "A 'wildcard domain name' is defined by having   
 1308       its initial (i.e., leftmost or least significant) label, in binary   
 1309       format: 0000 0001 0010 1010 (binary) = 0x01 0x2a (hexadecimal)".     
 1310       (Quoted from [RFC4592], Section 2.1.1) The second octet in this      
 1311       label is the ASCII representation for the "*" character.             
 1313    Closest encloser:  "The longest existing ancestor of a name."           
 1314       (Quoted from [RFC5155], Section 1.3) An earlier definition is "The   
 1315       node in the zone's tree of existing domain names that has the most   
 1316       labels matching the query name (consecutively, counting from the     
 1317       root label downward).  Each match is a 'label match' and the order   
 1318       of the labels is the same."  (Quoted from [RFC4592],                 
 1319       Section 3.3.1)                                                       
 1321    Closest provable encloser:  "The longest ancestor of a name that can    
 1322       be proven to exist.  Note that this is only different from the       
 1323       closest encloser in an Opt-Out zone."  (Quoted from [RFC5155],       
 1324       Section 1.3) See Section 10 for more on "opt-out".                   
 1326    Next closer name:  "The name one label longer than the closest          
 1327       provable encloser of a name."  (Quoted from [RFC5155],               
 1328       Section 1.3)                                                         
 1330    Source of Synthesis:  "The source of synthesis is defined in the        
 1331       context of a query process as that wildcard domain name              
 1332       immediately descending from the closest encloser, provided that      
 1333       this wildcard domain name exists.  'Immediately descending' means    
 1334       that the source of synthesis has a name of the form:                 
 1336       <asterisk label>.<closest encloser>."                                
 1338       (Quoted from [RFC4592], Section 3.3.1)                               
 1340 9.  Registration Model                                                     
 1342    Registry:  The administrative operation of a zone that allows           
 1343       registration of names within that zone.  People often use this       
 1344       term to refer only to those organizations that perform               
 1345       registration in large delegation-centric zones (such as TLDs); but   
 1346       formally, whoever decides what data goes into a zone is the          
 1347       registry for that zone.  This definition of "registry" is from a     
 1348       DNS point of view; for some zones, the policies that determine       
 1349       what can go in the zone are decided by zones that are                
 1350       superordinate and not the registry operator.                         
 1352    Registrant:  An individual or organization on whose behalf a name in    
 1353       a zone is registered by the registry.  In many zones, the registry   
 1354       and the registrant may be the same entity, but in TLDs they often    
 1355       are not.                                                             
 1357    Registrar:  A service provider that acts as a go-between for            
 1358       registrants and registries.  Not all registrations require a         
 1359       registrar, though it is common to have registrars involved in        
 1360       registrations in TLDs.                                               
 1362    EPP:  The Extensible Provisioning Protocol (EPP), which is commonly     
 1363       used for communication of registration information between           
 1364       registries and registrars.  EPP is defined in [RFC5730].             
 1366    WHOIS:  A protocol specified in [RFC3912], often used for querying      
 1367       registry databases.  WHOIS data is frequently used to associate      
 1368       registration data (such as zone management contacts) with domain     
 1369       names.  The term "WHOIS data" is often used as a synonym for the     
 1370       registry database, even though that database may be served by        
 1371       different protocols, particularly RDAP.  The WHOIS protocol is       
 1372       also used with IP address registry data.                             
 1374    RDAP:  The Registration Data Access Protocol, defined in [RFC7480],     
 1375       [RFC7481], [RFC7485], [RFC9082], [RFC9083], and [RFC9224].  The      
 1376       RDAP protocol and data format are meant as a replacement for         
 1377       WHOIS.                                                               
 1379    DNS operator:  An entity responsible for running DNS servers.  For a    
 1380       zone's authoritative servers, the registrant may act as their own    
 1381       DNS operator, their registrar may do it on their behalf, or they     
 1382       may use a third-party operator.  For some zones, the registry        
 1383       function is performed by the DNS operator plus other entities who    
 1384       decide about the allowed contents of the zone.                       
 1386    Public suffix:  "A domain that is controlled by a public registry."     
 1387       (Quoted from [RFC6265], Section 5.3) A common definition for this    
 1388       term is a domain under which subdomains can be registered by third   
 1389       parties and on which HTTP cookies (which are described in detail     
 1390       in [RFC6265]) should not be set.  There is no indication in a        
 1391       domain name whether it is a public suffix; that can only be          
 1392       determined by outside means.  In fact, both a domain and a           
 1393       subdomain of that domain can be public suffixes.                     
 1395       There is nothing inherent in a domain name to indicate whether it    
 1396       is a public suffix.  One resource for identifying public suffixes    
 1397       is the Public Suffix List (PSL) maintained by Mozilla                
 1398       <https://publicsuffix.org/>.                                         
 1400       For example, at the time this document is published, the "com.au"    
 1401       domain is listed as a public suffix in the PSL.  (Note that this     
 1402       example might change in the future.)                                 
 1404       Note that the term "public suffix" is controversial in the DNS       
 1405       community for many reasons, and it may be significantly changed in   
 1406       the future.  One example of the difficulty of calling a domain a     
 1407       public suffix is that designation can change over time as the        
 1408       registration policy for the zone changes, such as was the case       
 1409       with the "uk" TLD in 2014.                                           
 1411    Subordinate and Superordinate:  These terms are introduced in           
 1412       [RFC5731] for use in the registration model, but not defined         
 1413       there.  Instead, they are given in examples.  "For example, domain   
 1414       name 'example.com' has a superordinate relationship to host name     
 1415       ns1.example.com'...  For example, host ns1.example1.com is a         
 1416       subordinate host of domain example1.com, but it is a not a           
 1417       subordinate host of domain example2.com."  (Quoted from [RFC5731],   
 1418       Section 1.1) These terms are strictly ways of referring to the       
 1419       relationship standing of two domains where one is a subdomain of     
 1420       the other.                                                           
 1422 10.  General DNSSEC                                                        
 1424    Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and   
 1425    [RFC5155].  The terms that have caused confusion in the DNS community   
 1426    are highlighted here.                                                   
 1428    DNSSEC-aware and DNSSEC-unaware:  These two terms, which are used in    
 1429       some RFCs, have not been formally defined.  However, Section 2 of    
 1430       [RFC4033] defines many types of resolvers and validators,            
 1431       including "non-validating security-aware stub resolver", "non-       
 1432       validating stub resolver", "security-aware name server",             
 1433       "security-aware recursive name server", "security-aware resolver",   
 1434       "security-aware stub resolver", and "security-oblivious              
 1435       'anything'".  (Note that the term "validating resolver", which is    
 1436       used in some places in DNSSEC-related documents, is also not         
 1437       defined in those RFCs, but is defined below.)                        
 1439    Signed zone:  "A zone whose RRsets are signed and that contains         
 1440       properly constructed DNSKEY, Resource Record Signature (RRSIG),      
 1441       Next Secure (NSEC), and (optionally) DS records."  (Quoted from      
 1442       [RFC4033], Section 2) It has been noted in other contexts that the   
 1443       zone itself is not really signed, but all the relevant RRsets in     
 1444       the zone are signed.  Nevertheless, if a zone that should be         
 1445       signed contains any RRsets that are not signed (or opted out),       
 1446       those RRsets will be treated as bogus, so the whole zone needs to    
 1447       be handled in some way.                                              
 1449       It should also be noted that, since the publication of [RFC6840],    
 1450       NSEC records are no longer required for signed zones: a signed       
 1451       zone might include NSEC3 records instead.  [RFC7129] provides        
 1452       additional background commentary and some context for the NSEC and   
 1453       NSEC3 mechanisms used by DNSSEC to provide authenticated denial-     
 1454       of-existence responses.  NSEC and NSEC3 are described below.         
 1456    Online signing:  [RFC4470] defines "on-line signing" (note the          
 1457       hyphen) as "generating and signing these records on demand", where   
 1458       "these" was defined as NSEC records.  The current definition         
 1459       expands that to generating and signing RRSIG, NSEC, and NSEC3        
 1460       records on demand.                                                   
 1462    Unsigned zone:  Section 2 of [RFC4033] defines this as "a zone that     
 1463       is not signed".  Section 2 of [RFC4035] defines this as a "zone      
 1464       that does not include these records [properly constructed DNSKEY,    
 1465       Resource Record Signature (RRSIG), Next Secure (NSEC), and           
 1466       (optionally) DS records] according to the rules in this              
 1467       section..." There is an important note at the end of Section 5.2     
 1468       of [RFC4035] that defines an additional situation in which a zone    
 1469       is considered unsigned: "If the resolver does not support any of     
 1470       the algorithms listed in an authenticated DS RRset, then the         
 1471       resolver will not be able to verify the authentication path to the   
 1472       child zone.  In this case, the resolver SHOULD treat the child       
 1473       zone as if it were unsigned."                                        
 1475    NSEC:  "The NSEC record allows a security-aware resolver to             
 1476       authenticate a negative reply for either name or type non-           
 1477       existence with the same mechanisms used to authenticate other DNS    
 1478       replies."  (Quoted from [RFC4033], Section 3.2) In short, an NSEC    
 1479       record provides authenticated denial of existence.                   
 1481       "The NSEC resource record lists two separate things: the next        
 1482       owner name (in the canonical ordering of the zone) that contains     
 1483       authoritative data or a delegation point NS RRset, and the set of    
 1484       RR types present at the NSEC RR's owner name."  (Quoted from         
 1485       [RFC4034], Section 4)                                                
 1487    NSEC3:  Like the NSEC record, the NSEC3 record also provides            
 1488       authenticated denial of existence; however, NSEC3 records mitigate   
 1489       zone enumeration and support Opt-Out. NSEC3 resource records         
 1490       require associated NSEC3PARAM resource records.  NSEC3 and           
 1491       NSEC3PARAM resource records are defined in [RFC5155].                
 1493       Note that [RFC6840] says that [RFC5155] "is now considered part of   
 1494       the DNS Security Document Family as described by Section 10 of       
 1495       [RFC4033]".  This means that some of the definitions from earlier    
 1496       RFCs that only talk about NSEC records should probably be            
 1497       considered to be talking about both NSEC and NSEC3.                  
 1499    Opt-out:  "The Opt-Out Flag indicates whether this NSEC3 RR may cover   
 1500       unsigned delegations."  (Quoted from [RFC5155], Section     
 1501       Opt-out tackles the high costs of securing a delegation to an        
 1502       insecure zone.  When using Opt-Out, names that are an insecure       
 1503       delegation (and empty non-terminals that are only derived from       
 1504       insecure delegations) don't require an NSEC3 record or its           
 1505       corresponding RRSIG records.  Opt-Out NSEC3 records are not able     
 1506       to prove or deny the existence of the insecure delegations.          
 1507       (Adapted from [RFC7129], Section 5.1)                                
 1509    Insecure delegation:  "A signed name containing a delegation (NS        
 1510       RRset), but lacking a DS RRset, signifying a delegation to an        
 1511       unsigned subzone."  (Quoted from [RFC4956], Section 2)               
 1513    Zone enumeration:  "The practice of discovering the full content of a   
 1514       zone via successive queries."  (Quoted from [RFC5155],               
 1515       Section 1.3) This is also sometimes called "zone walking".  Zone     
 1516       enumeration is different from zone content guessing where the        
 1517       guesser uses a large dictionary of possible labels and sends         
 1518       successive queries for them, or matches the contents of NSEC3        
 1519       records against such a dictionary.                                   
 1521    Validation:  Validation, in the context of DNSSEC, refers to one of     
 1522       the following:                                                       
 1524       *  Checking the validity of DNSSEC signatures,                       
 1526       *  Checking the validity of DNS responses, such as those including   
 1527          authenticated denial of existence, or                             
 1529       *  Building an authentication chain from a trust anchor to a DNS     
 1530          response or individual DNS RRsets in a response.                  
 1532       The first two definitions above consider only the validity of        
 1533       individual DNSSEC components, such as the RRSIG validity or NSEC     
 1534       proof validity.  The third definition considers the components of    
 1535       the entire DNSSEC authentication chain; thus, it requires            
 1536       "configured knowledge of at least one authenticated DNSKEY or DS     
 1537       RR" (as described in [RFC4035], Section 5).                          
 1539       [RFC4033], Section 2, says that a "Validating Security-Aware Stub    
 1540       Resolver... performs signature validation" and uses a trust anchor   
 1541       "as a starting point for building the authentication chain to a      
 1542       signed DNS response"; thus, it uses the first and third              
 1543       definitions above.  The process of validating an RRSIG resource      
 1544       record is described in [RFC4035], Section 5.3.                       
 1546       [RFC5155] refers to validating responses throughout the document     
 1547       in the context of hashed authenticated denial of existence; this     
 1548       uses the second definition above.                                    
 1550       The term "authentication" is used interchangeably with               
 1551       "validation", in the sense of the third definition above.            
 1552       [RFC4033], Section 2, describes the chain linking trust anchor to    
 1553       DNS data as the "authentication chain".  A response is considered    
 1554       to be authentic if "all RRsets in the Answer and Authority           
 1555       sections of the response [are considered] to be authentic" (Quoted   
 1556       from [RFC4035]) DNS data or responses deemed to be authentic or      
 1557       validated have a security status of "secure" ([RFC4035],             
 1558       Section 4.3; [RFC4033], Section 5).  "Authenticating both DNS keys   
 1559       and data is a matter of local policy, which may extend or even       
 1560       override the [DNSSEC] protocol extensions..." (Quoted from           
 1561       [RFC4033], Section 3.1)                                              
 1563       The term "verification", when used, is usually a synonym for         
 1564       "validation".                                                        
 1566    Validating resolver:  A security-aware recursive name server,           
 1567       security-aware resolver, or security-aware stub resolver that is     
 1568       applying at least one of the definitions of validation (above) as    
 1569       appropriate to the resolution context.  For the same reason that     
 1570       the generic term "resolver" is sometimes ambiguous and needs to be   
 1571       evaluated in context (see Section 6), "validating resolver" is a     
 1572       context-sensitive term.                                              
 1574    Key signing key (KSK):  DNSSEC keys that "only sign the apex DNSKEY     
 1575       RRset in a zone."  (Quoted from [RFC6781], Section 3.1)              
 1577    Zone signing key (ZSK):  "DNSSEC keys that can be used to sign all      
 1578       the RRsets in a zone that require signatures, other than the apex    
 1579       DNSKEY RRset."  (Quoted from [RFC6781], Section 3.1) Also note       
 1580       that a ZSK is sometimes used to sign the apex DNSKEY RRset.          
 1582    Combined signing key (CSK):  "In cases where the differentiation        
 1583       between the KSK and ZSK is not made, i.e., where keys have the       
 1584       role of both KSK and ZSK, we talk about a Single-Type Signing        
 1585       Scheme."  (Quoted from [RFC6781], Section 3.1) This is sometimes     
 1586       called a "combined signing key" or "CSK".  It is operational         
 1587       practice, not protocol, that determines whether a particular key     
 1588       is a ZSK, a KSK, or a CSK.                                           
 1590    Secure Entry Point (SEP):  A flag in the DNSKEY RDATA that "can be      
 1591       used to distinguish between keys that are intended to be used as     
 1592       the secure entry point into the zone when building chains of         
 1593       trust, i.e., they are (to be) pointed to by parental DS RRs or       
 1594       configured as a trust anchor....  Therefore, it is suggested that    
 1595       the SEP flag be set on keys that are used as KSKs and not on keys    
 1596       that are used as ZSKs, while in those cases where a distinction      
 1597       between a KSK and ZSK is not made (i.e., for a Single-Type Signing   
 1598       Scheme), it is suggested that the SEP flag be set on all keys."      
 1599       (Quoted from [RFC6781], Section 3.2.3) Note that the SEP flag is     
 1600       only a hint, and its presence or absence may not be used to          
 1601       disqualify a given DNSKEY RR from use as a KSK or ZSK during         
 1602       validation.                                                          
 1604       The original definition of SEPs was in [RFC3757].  That definition   
 1605       clearly indicated that the SEP was a key, not just a bit in the      
 1606       key.  The abstract of [RFC3757] says: "With the Delegation Signer    
 1607       (DS) resource record (RR), the concept of a public key acting as a   
 1608       secure entry point (SEP) has been introduced.  During exchanges of   
 1609       public keys with the parent there is a need to differentiate SEP     
 1610       keys from other public keys in the Domain Name System KEY (DNSKEY)   
 1611       resource record set.  A flag bit in the DNSKEY RR is defined to      
 1612       indicate that DNSKEY is to be used as a SEP."  That definition of    
 1613       the SEP as a key was made obsolete by [RFC4034], and the             
 1614       definition from [RFC6781] is consistent with [RFC4034].              
 1616    Trust anchor:  "A configured DNSKEY RR or DS RR hash of a DNSKEY RR.    
 1617       A validating security-aware resolver uses this public key or hash    
 1618       as a starting point for building the authentication chain to a       
 1619       signed DNS response.  In general, a validating resolver will have    
 1620       to obtain the initial values of its trust anchors via some secure    
 1621       or trusted means outside the DNS protocol."  (Quoted from            
 1622       [RFC4033], Section 2)                                                
 1624    DNSSEC Policy (DP):  A statement that "sets forth the security          
 1625       requirements and standards to be implemented for a DNSSEC-signed     
 1626       zone."  (Quoted from [RFC6841], Section 2)                           
 1628    DNSSEC Practice Statement (DPS):  "A practices disclosure document      
 1629       that may support and be a supplemental document to the DNSSEC        
 1630       Policy (if such exists), and it states how the management of a       
 1631       given zone implements procedures and controls at a high level."      
 1632       (Quoted from [RFC6841], Section 2)                                   
 1634    Hardware security module (HSM):  A specialized piece of hardware that   
 1635       is used to create keys for signatures and to sign messages without   
 1636       ever disclosing the private key.  In DNSSEC, HSMs are often used     
 1637       to hold the private keys for KSKs and ZSKs and to create the         
 1638       signatures used in RRSIG records at periodic intervals.              
 1640    Signing software:  Authoritative DNS servers that support DNSSEC        
 1641       often contain software that facilitates the creation and             
 1642       maintenance of DNSSEC signatures in zones.  There is also stand-     
 1643       alone software that can be used to sign a zone regardless of         
 1644       whether the authoritative server itself supports signing.            
 1645       Sometimes signing software can support particular HSMs as part of    
 1646       the signing process.                                                 
 1648 11.  DNSSEC States                                                         
 1650    A validating resolver can determine that a response is in one of four   
 1651    states: secure, insecure, bogus, or indeterminate.  These states are    
 1652    defined in [RFC4033] and [RFC4035], although the definitions in the     
 1653    two documents differ a bit.  This document makes no effort to           
 1654    reconcile the definitions in the two documents and takes no position    
 1655    as to whether they need to be reconciled.                               
 1657    Section 5 of [RFC4033] says:                                            
 1659    |  A validating resolver can determine the following 4 states:          
 1660    |  Secure:  The validating resolver has a trust anchor, has a chain     
 1661    |     of trust, and is able to verify all the signatures in the         
 1662    |     response.                                                         
 1663    |                                                                       
 1664    |  Insecure:  The validating resolver has a trust anchor, a chain of    
 1665    |     trust, and, at some delegation point, signed proof of the non-    
 1666    |     existence of a DS record.  This indicates that subsequent         
 1667    |     branches in the tree are provably insecure.  A validating         
 1668    |     resolver may have a local policy to mark parts of the domain      
 1669    |     space as insecure.                                                
 1670    |                                                                       
 1671    |  Bogus:  The validating resolver has a trust anchor and a secure      
 1672    |     delegation indicating that subsidiary data is signed, but the     
 1673    |     response fails to validate for some reason: missing signatures,   
 1674    |     expired signatures, signatures with unsupported algorithms,       
 1675    |     data missing that the relevant NSEC RR says should be present,    
 1676    |     and so forth.                                                     
 1677    |                                                                       
 1678    |  Indeterminate:  There is no trust anchor that would indicate that    
 1679    |     a specific portion of the tree is secure.  This is the default    
 1680    |     operation mode.                                                   
 1682    Section 4.3 of [RFC4035] says:                                          
 1684    |  A security-aware resolver must be able to distinguish between four   
 1685    |  cases:                                                               
 1686    |  Secure:  An RRset for which the resolver is able to build a chain    
 1687    |     of signed DNSKEY and DS RRs from a trusted security anchor to     
 1688    |     the RRset.  In this case, the RRset should be signed and is       
 1689    |     subject to signature validation, as described above.              
 1690    |                                                                       
 1691    |  Insecure:  An RRset for which the resolver knows that it has no      
 1692    |     chain of signed DNSKEY and DS RRs from any trusted starting       
 1693    |     point to the RRset.  This can occur when the target RRset lies    
 1694    |     in an unsigned zone or in a descendent [sic] of an unsigned       
 1695    |     zone.  In this case, the RRset may or may not be signed, but      
 1696    |     the resolver will not be able to verify the signature.            
 1697    |                                                                       
 1698    |  Bogus:  An RRset for which the resolver believes that it ought to    
 1699    |     be able to establish a chain of trust but for which it is         
 1700    |     unable to do so, either due to signatures that for some reason    
 1701    |     fail to validate or due to missing data that the relevant         
 1702    |     DNSSEC RRs indicate should be present.  This case may indicate    
 1703    |     an attack but may also indicate a configuration error or some     
 1704    |     form of data corruption.                                          
 1705    |                                                                       
 1706    |  Indeterminate:  An RRset for which the resolver is not able to       
 1707    |     determine whether the RRset should be signed, as the resolver     
 1708    |     is not able to obtain the necessary DNSSEC RRs.  This can occur   
 1709    |     when the security-aware resolver is not able to contact           
 1710    |     security-aware name servers for the relevant zones.               
 1712 12.  Security Considerations                                               
 1714    These definitions do not change any security considerations for         
 1715    either the global DNS or private DNS.                                   
 1717 13.  IANA Considerations                                                   
 1719    References to RFC 8499 in the IANA registries have been replaced with   
 1720    references to this document.                                            
 1722 14.  References                                                            
 1724 14.1.  Normative References                                                
 1726    [IANA_RootFiles]                                                        
 1727               IANA, "Root Files",                                          
 1728               <https://www.iana.org/domains/root/files>.                   
 1730    [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",    
 1731               RFC 882, DOI 10.17487/RFC0882, November 1983,                
 1732               <https://www.rfc-editor.org/info/rfc882>.                    
 1734    [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",   
 1735               STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,       
 1736               <https://www.rfc-editor.org/info/rfc1034>.                   
 1738    [RFC1035]  Mockapetris, P., "Domain names - implementation and          
 1739               specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,      
 1740               November 1987, <https://www.rfc-editor.org/info/rfc1035>.    
 1742    [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -          
 1743               Application and Support", STD 3, RFC 1123,                   
 1744               DOI 10.17487/RFC1123, October 1989,                          
 1745               <https://www.rfc-editor.org/info/rfc1123>.                   
 1747    [RFC1912]  Barr, D., "Common DNS Operational and Configuration          
 1748               Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,      
 1749               <https://www.rfc-editor.org/info/rfc1912>.                   
 1751    [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone      
 1752               Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,       
 1753               August 1996, <https://www.rfc-editor.org/info/rfc1996>.      
 1755    [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,      
 1756               "Dynamic Updates in the Domain Name System (DNS UPDATE)",    
 1757               RFC 2136, DOI 10.17487/RFC2136, April 1997,                  
 1758               <https://www.rfc-editor.org/info/rfc2136>.                   
 1760    [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS              
 1761               Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,   
 1762               <https://www.rfc-editor.org/info/rfc2181>.                   
 1764    [RFC2182]  Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection    
 1765               and Operation of Secondary DNS Servers", BCP 16, RFC 2182,   
 1766               DOI 10.17487/RFC2182, July 1997,                             
 1767               <https://www.rfc-editor.org/info/rfc2182>.                   
 1769    [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS           
 1770               NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,        
 1771               <https://www.rfc-editor.org/info/rfc2308>.                   
 1773    [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.      
 1774               Rose, "DNS Security Introduction and Requirements",          
 1775               RFC 4033, DOI 10.17487/RFC4033, March 2005,                  
 1776               <https://www.rfc-editor.org/info/rfc4033>.                   
 1778    [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.      
 1779               Rose, "Resource Records for the DNS Security Extensions",    
 1780               RFC 4034, DOI 10.17487/RFC4034, March 2005,                  
 1781               <https://www.rfc-editor.org/info/rfc4034>.                   
 1783    [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.      
 1784               Rose, "Protocol Modifications for the DNS Security           
 1785               Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,     
 1786               <https://www.rfc-editor.org/info/rfc4035>.                   
 1788    [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name         
 1789               System", RFC 4592, DOI 10.17487/RFC4592, July 2006,          
 1790               <https://www.rfc-editor.org/info/rfc4592>.                   
 1792    [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS      
 1793               Security (DNSSEC) Hashed Authenticated Denial of             
 1794               Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,      
 1795               <https://www.rfc-editor.org/info/rfc5155>.                   
 1797    [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive         
 1798               Nameservers in Reflector Attacks", BCP 140, RFC 5358,        
 1799               DOI 10.17487/RFC5358, October 2008,                          
 1800               <https://www.rfc-editor.org/info/rfc5358>.                   
 1802    [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",    
 1803               STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,         
 1804               <https://www.rfc-editor.org/info/rfc5730>.                   
 1806    [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)      
 1807               Domain Name Mapping", STD 69, RFC 5731,                      
 1808               DOI 10.17487/RFC5731, August 2009,                           
 1809               <https://www.rfc-editor.org/info/rfc5731>.                   
 1811    [RFC5855]  Abley, J. and T. Manderson, "Nameservers for IPv4 and IPv6   
 1812               Reverse Zones", BCP 155, RFC 5855, DOI 10.17487/RFC5855,     
 1813               May 2010, <https://www.rfc-editor.org/info/rfc5855>.         
 1815    [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol    
 1816               (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,          
 1817               <https://www.rfc-editor.org/info/rfc5936>.                   
 1819    [RFC6561]  Livingood, J., Mody, N., and M. O'Reirdan,                   
 1820               "Recommendations for the Remediation of Bots in ISP          
 1821               Networks", RFC 6561, DOI 10.17487/RFC6561, March 2012,       
 1822               <https://www.rfc-editor.org/info/rfc6561>.                   
 1824    [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC             
 1825               Operational Practices, Version 2", RFC 6781,                 
 1826               DOI 10.17487/RFC6781, December 2012,                         
 1827               <https://www.rfc-editor.org/info/rfc6781>.                   
 1829    [RFC6840]  Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and      
 1830               Implementation Notes for DNS Security (DNSSEC)", RFC 6840,   
 1831               DOI 10.17487/RFC6840, February 2013,                         
 1832               <https://www.rfc-editor.org/info/rfc6840>.                   
 1834    [RFC6841]  Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A        
 1835               Framework for DNSSEC Policies and DNSSEC Practice            
 1836               Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,   
 1837               <https://www.rfc-editor.org/info/rfc6841>.                   
 1839    [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms    
 1840               for DNS (EDNS(0))", STD 75, RFC 6891,                        
 1841               DOI 10.17487/RFC6891, April 2013,                            
 1842               <https://www.rfc-editor.org/info/rfc6891>.                   
 1844    [RFC7344]  Kumari, W., Gudmundsson, O., and G. Barwood, "Automating     
 1845               DNSSEC Delegation Trust Maintenance", RFC 7344,              
 1846               DOI 10.17487/RFC7344, September 2014,                        
 1847               <https://www.rfc-editor.org/info/rfc7344>.                   
 1849    [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS             
 1850               Terminology", RFC 7719, DOI 10.17487/RFC7719, December       
 1851               2015, <https://www.rfc-editor.org/info/rfc7719>.             
 1853    [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles    
 1854               for DNS over TLS and DNS over DTLS", RFC 8310,               
 1855               DOI 10.17487/RFC8310, March 2018,                            
 1856               <https://www.rfc-editor.org/info/rfc8310>.                   
 1858    [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS             
 1859               Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,       
 1860               January 2019, <https://www.rfc-editor.org/info/rfc8499>.     
 1862    [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over         
 1863               Dedicated QUIC Connections", RFC 9250,                       
 1864               DOI 10.17487/RFC9250, May 2022,                              
 1865               <https://www.rfc-editor.org/info/rfc9250>.                   
 1867    [RFC9471]  Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS    
 1868               Glue Requirements in Referral Responses", RFC 9471,          
 1869               DOI 10.17487/RFC9471, September 2023,                        
 1870               <https://www.rfc-editor.org/info/rfc9471>.                   
 1872 14.2.  Informative References                                              
 1874    [IANA_Resource_Registry]                                                
 1875               IANA, "Resource Record (RR) TYPEs",                          
 1876               <https://www.iana.org/assignments/dns-parameters/>.          
 1878    [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,    
 1879               RFC 20, DOI 10.17487/RFC0020, October 1969,                  
 1880               <https://www.rfc-editor.org/info/rfc20>.                     
 1882    [RFC819]   Su, Z. and J. Postel, "The Domain Naming Convention for      
 1883               Internet User Applications", RFC 819,                        
 1884               DOI 10.17487/RFC0819, August 1982,                           
 1885               <https://www.rfc-editor.org/info/rfc819>.                    
 1887    [RFC952]   Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet    
 1888               host table specification", RFC 952, DOI 10.17487/RFC0952,    
 1889               October 1985, <https://www.rfc-editor.org/info/rfc952>.      
 1891    [RFC1713]  Romao, A., "Tools for DNS debugging", FYI 27, RFC 1713,      
 1892               DOI 10.17487/RFC1713, November 1994,                         
 1893               <https://www.rfc-editor.org/info/rfc1713>.                   
 1895    [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,      
 1896               DOI 10.17487/RFC1995, August 1996,                           
 1897               <https://www.rfc-editor.org/info/rfc1995>.                   
 1899    [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,            
 1900               DOI 10.17487/RFC2775, February 2000,                         
 1901               <https://www.rfc-editor.org/info/rfc2775>.                   
 1903    [RFC3172]  Huston, G., Ed., "Management Guidelines & Operational        
 1904               Requirements for the Address and Routing Parameter Area      
 1905               Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,    
 1906               September 2001, <https://www.rfc-editor.org/info/rfc3172>.   
 1908    [RFC3425]  Lawrence, D., "Obsoleting IQUERY", RFC 3425,                 
 1909               DOI 10.17487/RFC3425, November 2002,                         
 1910               <https://www.rfc-editor.org/info/rfc3425>.                   
 1912    [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.     
 1913               Stevens, "Basic Socket Interface Extensions for IPv6",       
 1914               RFC 3493, DOI 10.17487/RFC3493, February 2003,               
 1915               <https://www.rfc-editor.org/info/rfc3493>.                   
 1917    [RFC3757]  Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name        
 1918               System KEY (DNSKEY) Resource Record (RR) Secure Entry        
 1919               Point (SEP) Flag", RFC 3757, DOI 10.17487/RFC3757, April     
 1920               2004, <https://www.rfc-editor.org/info/rfc3757>.             
 1922    [RFC3912]  Daigle, L., "WHOIS Protocol Specification", RFC 3912,        
 1923               DOI 10.17487/RFC3912, September 2004,                        
 1924               <https://www.rfc-editor.org/info/rfc3912>.                   
 1926    [RFC4470]  Weiler, S. and J. Ihren, "Minimally Covering NSEC Records    
 1927               and DNSSEC On-line Signing", RFC 4470,                       
 1928               DOI 10.17487/RFC4470, April 2006,                            
 1929               <https://www.rfc-editor.org/info/rfc4470>.                   
 1931    [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",   
 1932               RFC 4641, DOI 10.17487/RFC4641, September 2006,              
 1933               <https://www.rfc-editor.org/info/rfc4641>.                   
 1935    [RFC4697]  Larson, M. and P. Barber, "Observed DNS Resolution           
 1936               Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,       
 1937               October 2006, <https://www.rfc-editor.org/info/rfc4697>.     
 1939    [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast            
 1940               Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,          
 1941               December 2006, <https://www.rfc-editor.org/info/rfc4786>.    
 1943    [RFC4956]  Arends, R., Kosters, M., and D. Blacka, "DNS Security        
 1944               (DNSSEC) Opt-In", RFC 4956, DOI 10.17487/RFC4956, July       
 1945               2007, <https://www.rfc-editor.org/info/rfc4956>.             
 1947    [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",           
 1948               BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,        
 1949               <https://www.rfc-editor.org/info/rfc5625>.                   
 1951    [RFC5890]  Klensin, J., "Internationalized Domain Names for             
 1952               Applications (IDNA): Definitions and Document Framework",    
 1953               RFC 5890, DOI 10.17487/RFC5890, August 2010,                 
 1954               <https://www.rfc-editor.org/info/rfc5890>.                   
 1956    [RFC5891]  Klensin, J., "Internationalized Domain Names in              
 1957               Applications (IDNA): Protocol", RFC 5891,                    
 1958               DOI 10.17487/RFC5891, August 2010,                           
 1959               <https://www.rfc-editor.org/info/rfc5891>.                   
 1961    [RFC5892]  Faltstrom, P., Ed., "The Unicode Code Points and             
 1962               Internationalized Domain Names for Applications (IDNA)",     
 1963               RFC 5892, DOI 10.17487/RFC5892, August 2010,                 
 1964               <https://www.rfc-editor.org/info/rfc5892>.                   
 1966    [RFC5893]  Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts      
 1967               for Internationalized Domain Names for Applications          
 1968               (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,        
 1969               <https://www.rfc-editor.org/info/rfc5893>.                   
 1971    [RFC5894]  Klensin, J., "Internationalized Domain Names for             
 1972               Applications (IDNA): Background, Explanation, and            
 1973               Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,     
 1974               <https://www.rfc-editor.org/info/rfc5894>.                   
 1976    [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on   
 1977               Encodings for Internationalized Domain Names", RFC 6055,     
 1978               DOI 10.17487/RFC6055, February 2011,                         
 1979               <https://www.rfc-editor.org/info/rfc6055>.                   
 1981    [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,      
 1982               DOI 10.17487/RFC6265, April 2011,                            
 1983               <https://www.rfc-editor.org/info/rfc6265>.                   
 1985    [RFC6303]  Andrews, M., "Locally Served DNS Zones", BCP 163,            
 1986               RFC 6303, DOI 10.17487/RFC6303, July 2011,                   
 1987               <https://www.rfc-editor.org/info/rfc6303>.                   
 1989    [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.    
 1990               Cheshire, "Internet Assigned Numbers Authority (IANA)        
 1991               Procedures for the Management of the Service Name and        
 1992               Transport Protocol Port Number Registry", BCP 165,           
 1993               RFC 6335, DOI 10.17487/RFC6335, August 2011,                 
 1994               <https://www.rfc-editor.org/info/rfc6335>.                   
 1996    [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in             
 1997               Internationalization in the IETF", BCP 166, RFC 6365,        
 1998               DOI 10.17487/RFC6365, September 2011,                        
 1999               <https://www.rfc-editor.org/info/rfc6365>.                   
 2001    [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the        
 2002               DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,             
 2003               <https://www.rfc-editor.org/info/rfc6672>.                   
 2005    [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,     
 2006               DOI 10.17487/RFC6762, February 2013,                         
 2007               <https://www.rfc-editor.org/info/rfc6762>.                   
 2009    [RFC7129]  Gieben, R. and W. Mekking, "Authenticated Denial of          
 2010               Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,       
 2011               February 2014, <https://www.rfc-editor.org/info/rfc7129>.    
 2013    [RFC7480]  Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the    
 2014               Registration Data Access Protocol (RDAP)", STD 95,           
 2015               RFC 7480, DOI 10.17487/RFC7480, March 2015,                  
 2016               <https://www.rfc-editor.org/info/rfc7480>.                   
 2018    [RFC7481]  Hollenbeck, S. and N. Kong, "Security Services for the       
 2019               Registration Data Access Protocol (RDAP)", STD 95,           
 2020               RFC 7481, DOI 10.17487/RFC7481, March 2015,                  
 2021               <https://www.rfc-editor.org/info/rfc7481>.                   
 2023    [RFC9082]  Hollenbeck, S. and A. Newton, "Registration Data Access      
 2024               Protocol (RDAP) Query Format", STD 95, RFC 9082,             
 2025               DOI 10.17487/RFC9082, June 2021,                             
 2026               <https://www.rfc-editor.org/info/rfc9082>.                   
 2028    [RFC9083]  Hollenbeck, S. and A. Newton, "JSON Responses for the        
 2029               Registration Data Access Protocol (RDAP)", STD 95,           
 2030               RFC 9083, DOI 10.17487/RFC9083, June 2021,                   
 2031               <https://www.rfc-editor.org/info/rfc9083>.                   
 2033    [RFC9224]  Blanchet, M., "Finding the Authoritative Registration Data   
 2034               Access Protocol (RDAP) Service", STD 95, RFC 9224,           
 2035               DOI 10.17487/RFC9224, March 2022,                            
 2036               <https://www.rfc-editor.org/info/rfc9224>.                   
 2038    [RFC7485]  Zhou, L., Kong, N., Shen, S., Sheng, S., and A. Servin,      
 2039               "Inventory and Analysis of WHOIS Registration Objects",      
 2040               RFC 7485, DOI 10.17487/RFC7485, March 2015,                  
 2041               <https://www.rfc-editor.org/info/rfc7485>.                   
 2043    [RFC7793]  Andrews, M., "Adding Prefixes to the IPv4      
 2044               Locally-Served DNS Zones Registry", BCP 163, RFC 7793,       
 2045               DOI 10.17487/RFC7793, May 2016,                              
 2046               <https://www.rfc-editor.org/info/rfc7793>.                   
 2048    [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,     
 2049               and P. Hoffman, "Specification for DNS over Transport        
 2050               Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May   
 2051               2016, <https://www.rfc-editor.org/info/rfc7858>.             
 2053    [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram        
 2054               Transport Layer Security (DTLS)", RFC 8094,                  
 2055               DOI 10.17487/RFC8094, February 2017,                         
 2056               <https://www.rfc-editor.org/info/rfc8094>.                   
 2058    [RFC8109]  Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS    
 2059               Resolver with Priming Queries", BCP 209, RFC 8109,           
 2060               DOI 10.17487/RFC8109, March 2017,                            
 2061               <https://www.rfc-editor.org/info/rfc8109>.                   
 2063    [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS          
 2064               (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,        
 2065               <https://www.rfc-editor.org/info/rfc8484>.                   
 2067    [RFC9103]  Toorop, W., Dickinson, S., Sahib, S., Aras, P., and A.       
 2068               Mankin, "DNS Zone Transfer over TLS", RFC 9103,              
 2069               DOI 10.17487/RFC9103, August 2021,                           
 2070               <https://www.rfc-editor.org/info/rfc9103>.                   
 2072    [RSSAC026] Root Server System Advisory Committee (RSSAC), "RSSAC0226    
 2073               RSSAC Lexicon", 2017,                                        
 2074               <https://www.icann.org/en/system/files/files/rssac-          
 2075               026-14mar17-en.pdf>.                                         
 2077 Appendix A.  Definitions Updated by This Document                          
 2079    The following definitions from RFCs are updated by this document:       
 2081    *  Forwarder in [RFC2308]                                               
 2083    *  QNAME in [RFC2308]                                                   
 2085    *  Secure Entry Point (SEP) in [RFC3757]; note, however, that this      
 2086       RFC is already obsolete (see [RFC4033], [RFC4034], [RFC4035]).       
 2088 Appendix B.  Definitions First Defined in This Document                    
 2090    The following definitions are first defined in this document:           
 2092    *  "Alias" in Section 2                                                 
 2094    *  "Apex" in Section 7                                                  
 2096    *  "arpa" in Section 7                                                  
 2098    *  "Authoritative DoT (ADot)" in Section 6                              
 2100    *  "Bailiwick" in Section 7                                             
 2102    *  "Class independent" in Section 5                                     
 2104    *  "Classic DNS" in Section 6                                           
 2106    *  "Delegation-centric zone" in Section 7                               
 2108    *  "Delegation" in Section 7                                            
 2110    *  "DNS operator" in Section 9                                          
 2112    *  "DNSSEC-aware" in Section 10                                         
 2114    *  "DNSSEC-unaware" in Section 10                                       
 2116    *  "Forwarding" in Section 6                                            
 2118    *  "Full resolver" in Section 6                                         
 2120    *  "Fully Qualified Domain Name" in Section 2                           
 2122    *  "Global DNS" in Section 2                                            
 2124    *  "Hardware Security Module (HSM)" in Section 10                       
 2126    *  "Host name" in Section 2                                             
 2128    *  "IDN" in Section 2                                                   
 2130    *  "In-domain" in Section 7                                             
 2132    *  "Iterative resolution" in Section 6                                  
 2134    *  "Label" in Section 2                                                 
 2136    *  "Locally served DNS zone" in Section 2                               
 2138    *  "Naming system" in Section 2                                         
 2140    *  "Negative response" in Section 3                                     
 2142    *  "Non-recursive query" in Section 6                                   
 2144    *  "Open resolver" in Section 6                                         
 2146    *  "Passive DNS" in Section 6                                           
 2148    *  "Policy-implementing resolver" in Section 6                          
 2150    *  "Presentation format" in Section 5                                   
 2152    *  "Priming" in Section 6                                               
 2154    *  "Private DNS" in Section 2                                           
 2156    *  "Recursive DoT (RDot)" in Section 6                                  
 2158    *  "Recursive resolver" in Section 6                                    
 2160    *  "Referrals" in Section 4                                             
 2162    *  "Registrant" in Section 9                                            
 2164    *  "Registrar" in Section 9                                             
 2166    *  "Registry" in Section 9                                              
 2168    *  "Root zone" in Section 7                                             
 2170    *  "Secure Entry Point (SEP)" in Section 10                             
 2172    *  "Sibling domain" in Section 7                                        
 2174    *  "Signing software" in Section 10                                     
 2176    *  "Split DNS" in Section 6                                             
 2178    *  "Stub resolver" in Section 6                                         
 2180    *  "Subordinate" in Section 8                                           
 2182    *  "Superordinate" in Section 8                                         
 2184    *  "TLD" in Section 2                                                   
 2186    *  "Validating resolver" in Section 10                                  
 2188    *  "Validation" in Section 10                                           
 2190    *  "View" in Section 6                                                  
 2192    *  "Zone transfer" in Section 6                                         
 2194 Acknowledgements                                                           
 2196    [RFC8499] and its predecessor, [RFC7719], were co-authored by Andrew    
 2197    Sullivan.  The current document, which is a small update to             
 2198    [RFC8499], has just two authors.  Andrew's work on the earlier          
 2199    documents is greatly appreciated.                                       
 2201    Numerous people made significant contributions to [RFC8499] and         
 2202    [RFC7719].  Please see the acknowledgements sections in those two       
 2203    documents for the extensive list of contributors.                       
 2205    Even though the current document is a small revision, many people in    
 2206    the DNSOP Working Group have contributed to it, and their work is       
 2207    greatly appreciated.                                                    
 2209 Index                                                                      
 2211    A B C D E F G H I K L M N O P Q R S T U V W X Z                         
 2213       A                                                                    
 2215          Address and Routing Parameter Area Domain (arpa)                  
 2216             Section 7                                                      
 2217          Address records                                                   
 2218             Section 5                                                      
 2219          ADoT                                                              
 2220             Section 6                                                      
 2221          Alias                                                             
 2222             Section 2                                                      
 2223          Anycast                                                           
 2224             Section 6                                                      
 2225          Apex                                                              
 2226             Section 7                                                      
 2227          Asterisk label                                                    
 2228             Section 8                                                      
 2229          Authoritative data                                                
 2230             Section 7                                                      
 2231          Authoritative server                                              
 2232             Section 6                                                      
 2233          Authoritative-only server                                         
 2234             Section 6                                                      
 2235          AXoT                                                              
 2236             Section 6                                                      
 2238       B                                                                    
 2240          Bailiwick                                                         
 2241             Section 7                                                      
 2243       C                                                                    
 2245          Canonical name                                                    
 2246             Section 2                                                      
 2247          Child                                                             
 2248             Section 7                                                      
 2249          Class                                                             
 2250             Section 4                                                      
 2251          Class independent                                                 
 2252             Section 5                                                      
 2253          Classic DNS                                                       
 2254             Section 6                                                      
 2255          Closest encloser                                                  
 2256             Section 8                                                      
 2257          Closest provable encloser                                         
 2258             Section 8                                                      
 2259          CNAME                                                             
 2260             Section 2                                                      
 2261          Combined signing key (CSK)                                        
 2262             Section 10                                                     
 2264       D                                                                    
 2266          Delegation                                                        
 2267             Section 7                                                      
 2268          Delegation-centric zone                                           
 2269             Section 7                                                      
 2270          DNS operator                                                      
 2271             Section 9                                                      
 2272          DNS-over-HTTPS                                                    
 2273             Section 6                                                      
 2274          DNS-over-QUIC                                                     
 2275             Section 6                                                      
 2276          DNS-over-TLS                                                      
 2277             Section 6                                                      
 2278          DNSSEC Policy (DP)                                                
 2279             Section 10                                                     
 2280          DNSSEC Practice Statement (DPS)                                   
 2281             Section 10                                                     
 2282          DNSSEC-aware and DNSSEC-unaware                                   
 2283             Section 10                                                     
 2284          DoH                                                               
 2285             Section 6                                                      
 2286          Domain name                                                       
 2287             Section 2                                                      
 2288          DoQ                                                               
 2289             Section 6                                                      
 2290          DoT                                                               
 2291             Section 6                                                      
 2293       E                                                                    
 2295          EDNS                                                              
 2296             Section 5                                                      
 2297          Empty non-terminals (ENTs)                                        
 2298             Section 7                                                      
 2299          EPP                                                               
 2300             Section 9                                                      
 2302       F                                                                    
 2304          Fast flux DNS                                                     
 2305             Section 7                                                      
 2306          FORMERR                                                           
 2307             Section 3                                                      
 2308          Forward lookup                                                    
 2309             Section 7                                                      
 2310          Forwarder                                                         
 2311             Section 6                                                      
 2312          Forwarding                                                        
 2313             Section 6                                                      
 2314          Full resolver                                                     
 2315             Section 6                                                      
 2316          Full-service resolver                                             
 2317             Section 6                                                      
 2318          Fully Qualified Domain Name (FQDN)                                
 2319             Section 2                                                      
 2321       G                                                                    
 2323          Global DNS                                                        
 2324             Section 2                                                      
 2325          Glue records                                                      
 2326             Section 7                                                      
 2328       H                                                                    
 2330          Hardware security module (HSM)                                    
 2331             Section 10                                                     
 2332          Hidden master                                                     
 2333             Section 6                                                      
 2334          Host name                                                         
 2335             Section 2                                                      
 2337       I                                                                    
 2339          IDN                                                               
 2340             Section 2                                                      
 2341          In-bailiwick                                                      
 2342             Section 7                                                      
 2343          In-domain                                                         
 2344             Section 7                                                      
 2345          Insecure delegation                                               
 2346             Section 10                                                     
 2347          Instance                                                          
 2348             Section 6                                                      
 2349          Internationalized Domain Name                                     
 2350             Section 2                                                      
 2351          Iterative mode                                                    
 2352             Section 6                                                      
 2353          Iterative resolution                                              
 2354             Section 6                                                      
 2355          IXoT                                                              
 2356             Section 6                                                      
 2358       K                                                                    
 2360          Key signing key (KSK)                                             
 2361             Section 10                                                     
 2363       L                                                                    
 2365          Label                                                             
 2366             Section 2                                                      
 2367          Lame delegation                                                   
 2368             Section 7                                                      
 2369          Locally served DNS zone                                           
 2370             Section 2                                                      
 2372       M                                                                    
 2374          Master file                                                       
 2375             Section 5                                                      
 2376          Master server                                                     
 2377             Section 6                                                      
 2378          mDNS                                                              
 2379             Section 2                                                      
 2380          Multicast DNS                                                     
 2381             Section 2                                                      
 2383       N                                                                    
 2385          Naming system                                                     
 2386             Section 2, Paragraph 1.2.1                                     
 2387          Negative caching                                                  
 2388             Section 6                                                      
 2389          Negative response                                                 
 2390             Section 3                                                      
 2391          Next closer name                                                  
 2392             Section 8                                                      
 2393          NODATA                                                            
 2394             Section 3                                                      
 2395          NOERROR                                                           
 2396             Section 3                                                      
 2397          Non-recursive query                                               
 2398             Section 6                                                      
 2399          NOTIMP                                                            
 2400             Section 3                                                      
 2401          NS                                                                
 2402             Section 6                                                      
 2403          NSEC                                                              
 2404             Section 10                                                     
 2405          NSEC3                                                             
 2406             Section 10                                                     
 2407          NXDOMAIN                                                          
 2408             Section 3                                                      
 2410       O                                                                    
 2412          Occluded name                                                     
 2413             Section 7                                                      
 2414          on-line signing                                                   
 2415             Section 10                                                     
 2416          online signing                                                    
 2417             Section 10                                                     
 2418          Open resolver                                                     
 2419             Section 6                                                      
 2420          OPT                                                               
 2421             Section 5                                                      
 2422          Opt-out                                                           
 2423             Section 10                                                     
 2424          Origin                                                            
 2425             Section 7                                                      
 2426          Out-of-bailiwick                                                  
 2427             Section 7                                                      
 2428          Owner                                                             
 2429             Section 5                                                      
 2431       P                                                                    
 2433          Parent                                                            
 2434             Section 7                                                      
 2435          Passive DNS                                                       
 2436             Section 6                                                      
 2437          Policy-implementing resolver                                      
 2438             Section 6                                                      
 2439          Presentation format                                               
 2440             Section 5                                                      
 2441          Primary master                                                    
 2442             Section 6                                                      
 2443          Primary server                                                    
 2444             Section 6                                                      
 2445          Priming                                                           
 2446             Section 6                                                      
 2447          Privacy-enabling DNS server                                       
 2448             Section 6                                                      
 2449          Private DNS                                                       
 2450             Section 2                                                      
 2451          Public suffix                                                     
 2452             Section 9                                                      
 2454       Q                                                                    
 2456          QNAME                                                             
 2457             Section 4                                                      
 2459       R                                                                    
 2461          RDAP                                                              
 2462             Section 9                                                      
 2463          RDoT                                                              
 2464             Section 6                                                      
 2465          Recursive DoT                                                     
 2466             Section 6                                                      
 2467          Recursive mode                                                    
 2468             Section 6, Paragraph 4.10.1                                    
 2469          Recursive query                                                   
 2470             Section 6                                                      
 2471          Recursive resolver                                                
 2472             Section 6                                                      
 2473          Referrals                                                         
 2474             Section 4                                                      
 2475          REFUSED                                                           
 2476             Section 3                                                      
 2477          Registrant                                                        
 2478             Section 9                                                      
 2479          Registrar                                                         
 2480             Section 9                                                      
 2481          Registry                                                          
 2482             Section 9                                                      
 2483          Resolver                                                          
 2484             Section 6                                                      
 2485          Reverse DNS, reverse lookup                                       
 2486             Section 7                                                      
 2487          Root hints                                                        
 2488             Section 6                                                      
 2489          Root zone                                                         
 2490             Section 7                                                      
 2491          RR                                                                
 2492             Section 5                                                      
 2493          RRset                                                             
 2494             Section 5                                                      
 2496       S                                                                    
 2498          Secondary server                                                  
 2499             Section 6                                                      
 2500          Secure Entry Point (SEP)                                          
 2501             Section 10                                                     
 2502          SERVFAIL                                                          
 2503             Section 3                                                      
 2504          Service name                                                      
 2505             Section 7                                                      
 2506          Sibling domain                                                    
 2507             Section 7                                                      
 2508          Signed zone                                                       
 2509             Section 10                                                     
 2510          Signing software                                                  
 2511             Section 10                                                     
 2512          Slave server                                                      
 2513             Section 6                                                      
 2514          SOA                                                               
 2515             Section 5                                                      
 2516          SOA field names                                                   
 2517             Section 5                                                      
 2518          Source of Synthesis                                               
 2519             Section 8, Paragraph 1.14.1                                    
 2520          Split DNS                                                         
 2521             Section 6                                                      
 2522          Split-horizon DNS                                                 
 2523             Section 6                                                      
 2524          Stealth server                                                    
 2525             Section 6                                                      
 2526          Stub resolver                                                     
 2527             Section 6                                                      
 2528          Subdomain                                                         
 2529             Section 2                                                      
 2530          Subordinate                                                       
 2531             Section 9                                                      
 2532          Superordinate                                                     
 2533             Section 9                                                      
 2535       T                                                                    
 2537          TLD                                                               
 2538             Section 2                                                      
 2539          Trust anchor                                                      
 2540             Section 10                                                     
 2541          TTL                                                               
 2542             Section 5                                                      
 2544       U                                                                    
 2546          Unsigned zone                                                     
 2547             Section 10                                                     
 2549       V                                                                    
 2551          Validating resolver                                               
 2552             Section 10                                                     
 2553          Validation                                                        
 2554             Section 10, Paragraph 2.26.1                                   
 2555          View                                                              
 2556             Section 6                                                      
 2558       W                                                                    
 2560          WHOIS                                                             
 2561             Section 9                                                      
 2562          Wildcard                                                          
 2563             Section 8                                                      
 2564          Wildcard domain name                                              
 2565             Section 8                                                      
 2567       X                                                                    
 2569          XoT                                                               
 2570             Section 6                                                      
 2572       Z                                                                    
 2574          Zone                                                              
 2575             Section 7                                                      
 2576          Zone cut                                                          
 2577             Section 7                                                      
 2578          Zone enumeration                                                  
 2579             Section 10                                                     
 2580          Zone signing key (ZSK)                                            
 2581             Section 10                                                     
 2582          Zone transfer                                                     
 2583             Section 6                                                      
 2585 Authors' Addresses                                                         
 2587    Paul Hoffman                                                            
 2588    ICANN                                                                   
 2589    Email: paul.hoffman@icann.org                                           
 2592    Kazunori Fujiwara                                                       
 2593    Japan Registry Services Co., Ltd.                                       
 2594    Email: fujiwara@jprs.co.jp                                              

The IETF is responsible for the creation and maintenance of the DNS RFCs. The ICANN DNS RFC annotation project provides a forum for collecting community annotations on these RFCs as an aid to understanding for implementers and any interested parties. The annotations displayed here are not the result of the IETF consensus process.

This RFC is included in the DNS RFCs annotation project whose home page is here.