Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Best Current Practice                       A. Sullivan
Expires: October 31, November 27, 2015                                           Dyn
                                                             K. Fujiwara
                                                                    JPRS
                                                          April 29,
                                                            May 26, 2015

                            DNS Terminology
                   draft-ietf-dnsop-dns-terminology-01
                    draft-ietf-dnsop-dns-terminology-02

Abstract

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 31, November 27, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Names . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  DNS Header and Response Codes . . . . . . . . . . . . . . . .   4   5
   4.  Resource Records  . . . . . . . . . . . . . . . . . . . . . .   5   6
   5.  DNS Servers . . . . . . . . . . . . . . . . . . . . . . . . .   7   8
   6.  Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10  11
   7.  Registration Model  . . . . . . . . . . . . . . . . . . . . .  13  14
   8.  General DNSSEC  . . . . . . . . . . . . . . . . . . . . . . .  14  15
   9.  DNSSEC States . . . . . . . . . . . . . . . . . . . . . . . .  15  17
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17  19
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17  19
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17  19
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18  20
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  18  20
     13.2.  Informative References . . . . . . . . . . . . . . . . .  19  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20  22

1.  Introduction

   The domain name system (DNS) is a simple query-response protocol
   whose messages in both directions have the same format.  The protocol
   and message format are defined in [RFC1034] and [RFC1035].  These
   RFCs defined some terms, but later documents defined others.  Some of
   the terms from RFCs 1034 and 1035 now have somewhat different
   meanings than they did in 1987.

   This document collects a wide variety of DNS-related terms.  Some of
   them have been precisely defined in earlier RFCs, some have been
   loosely defined in earlier RFCs, and some are not defined in any
   earlier RFC at all.

   The definitions here are believed to be the consensus definition of
   the DNS community, both protocol developers and operators.  Some of
   the definitions differ from earlier RFCs, and those differences are
   noted.  The terms are organized loosely by topic.  Some definitions
   are for new terms for things that are commonly talked about in the
   DNS community but that never had terms defined for them.

   In this document, where the consensus definition is the same as the
   one in an RFC, that RFC is quoted.  Where the consensus definition
   has changed somewhat, the RFC is mentioned but the new stand-alone
   definition is given.

   Other organizations sometimes define DNS-related terms their own way.
   For example, the W3C defines "domain" at
   https://specs.webplatform.org/url/webspecs/develop/.

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

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

2.  Names

   Domain name -- name:  Section 3.1 of [RFC1034] talks of "the domain name
      space" as a tree structure.  "Each node has a label, which is zero
      to 63 octets in length. ... The domain name of a node is the list
      of the labels on the path from the node to the root of the tree.
      ... To simplify implementations, the total number of octets that
      represent a domain name (i.e., the sum of all label octets and
      label lengths) is limited to 255."

   Fully-qualified domain name (FQDN) -- (FQDN):  This is often just a clear way
      of saying the same thing as "domain name of a node", as outlined
      above.  However, the term is ambiguous.  Strictly speaking, a fully-
   qualified
      fully-qualified name would include every label, including the
      final, zero-
   length zero-length label of the root zone: such a name would be
      written "www.example.net." (note the terminating dot).  But
      because every name eventually shares the common root, names are
      often written relative to the root (such as "www.example.net") and
      are still called "fully qualified".
      This term first appeared in [RFC1206].

      The need for the term "fully-qualified domain name" comes from the
      existence of partially-qualified domain names, which are names
      where some of the right-most names are left off and are understood
      only by context.

   Label:  The identifier of an individual node in the sequence of nodes
      that comprise a fully-qualified domain name.

   Host name -- name:  This term and its equivalent, "hostname", have been
      widely used but are not defined in [RFC1034], [RFC1035],
      [RFC1123], or [RFC2181].  The DNS was originally deployed into the
      Host Tables environment as outlined in [RFC0952], and it is likely
      that the term followed informally from the definition there.  Over
      time, the definition seems to have shifted.  "Host name" is often
      meant to be a domain name that follows the rules in Section 3.5 of
      [RFC1034], the "preferred name syntax".  Note that any label in
      any domain name can contain any octet value; hostnames are
      generally considered to be domain names where every label follows
      the rules in the "preferred name syntax", with the amendment that
      labels can start with ASCII digits (this amendment comes from
      Section 2.1 of [RFC1123]).

      People also sometimes use the term hostname to refer to just the
      first label of an FQDN.  In addition, people sometimes use this
      term to describe any name that refers to a machine, and those
      might include labels that do not conform to the "preferred name
      syntax".

   TLD --

   TLD:  A Top-Level Domain, meaning a zone that is one layer below the
      root, such as .com or .jp.  There is nothing special, from the
      point of view of the DNS, about TLDs.  Most of them are also delegation-
   centric
      delegation-centric zones, and there are significant policy issues
      around their operation.

   ccTLD -- A TLD that  TLDs are often divided into sub-groups
      such as "ccTLDs", "gTLDs", and others; the division is allocated to a country.  Historically, these
   were two-letter TLDs, matter of
      policy, and were allocated to countries using beyond the two-
   letter code from scope of this document.

   IDN:  The common abbreviation for "internationalized domain name".
      IDNs are the ISO 3166-1 alpha-2 current standard [ISO3166].  In
   recent years, there have been allocations of TLDs that conform to
   IDNA2008 ([RFC5890], mechanism for handling domain names
      with non-ASCII characters in applications.  The current standard,
      normally called "IDNA2008", is defined in [RFC5890], [RFC5891],
      [RFC5892], [RFC5893], and [RFC5894]);
   these are still treated as ccTLDs for policy purposes.

   gTLD -- A "generic" TLD is [RFC5894].

   Alias:  The owner of a TLD that is not CNAME resource record, or a ccTLD, subdomain of the
      owner of a DNAME resource record [RFC6672].  See also "canonical
      name".

   Canonical name:  A CNAME resource record identifies its owner name as
      an alias, and specifies the corresponding canonical name in the
      RDATA section of the RR.  (Quoted from [RFC1034], section 3.6.2)
      This usage of the word "canonical" is not one related to the mathematical
      concept of "canonical form".

   CNAME:  It is traditional to refer to the small number owner of historical TLDs such a CNAME record as
      "a CNAME".  This is unfortunate, as .int "CNAME" is an abbreviation of
      "canonical name", and .arpa.  There the owner of a CNAME record is no precise definition for which TLDs that are an alias not ccTLDs are
   gTLDs.
      a canonical name.  (Quoted from [RFC2181], section 10.1.1)

   Public suffix -- suffix:  A domain under which subdomains can be registered,
      and on which HTTP cookies ([RFC6265]) should not be set.  There is
      no indication in a domaine domain name whether or not it is a public
      suffix; that can only be determined by outside means.  The IETF
      DBOUND Working Group [DBOUND] deals with issues with public
      suffixes.

      For example, at the time this document is published, .com.au is
      considered a public suffix, but .au is not.  Note  (Note that this term is
   controversial
      example might change in the DNS community future.)

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

3.  DNS Header and Response Codes

   The header of a DNS message is first 12 octets.  Many of the fields
   and flags in the header diagram in section 4.1.1 of [RFC1035] are
   referred to by their names in that diagram.  For example, the
   response codes are called "RCODEs", the data for a record is called
   the "RDATA" (sometimes also called "RRdata"), and the authoritative
   answer bit is often called "the AA flag" or "the AA bit".

   Some of response codes that are defined in [RFC1035] have gotten
   their own shorthand names.  Some common response code names that
   appear without reference to the numeric value are "FORMERR",
   "SERVFAIL", and "NXDOMAIN".  All of the RCODEs are listed at
   http://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml,
   although that site uses mixed-case capitalization, while most
   documents use all-caps.

   NODATA - This is not an actual response code, but is a particular
   type of response from a server that

   NODATA:  A pseudo RCODE which indicates that the queried domain name exists is valid for
      the given class, but are no records of the resource record type being
   queried for does not exist. given type.  A NODATA
      response is a combination of an
   RCODE of 0 (NOERROR) and an Answer section that is empty.  In
   addition, NODATA responses has to be inferred from authoritative servers have the
   authoritative answer.  (Quoted from
      [RFC2308], section 1.)  NODATA is indicated by an answer (AA bit) with the
      RCODE set to 1 NOERROR and include no relevant answers in the answer
      section.  The authority section will contain an SOA record.
   Section 1 of [RFC2308] defines NODATA as "a pseudo RCODE which
   indicates that the name is valid, for the given class, but are record, or
      there will be no NS records of the given type". there.  (Quoted from [RFC2308],
      section 2,2.)  Note that referrals have a similar format to NODATA
      replies; [RFC2308] explains how to distinguish them.

      The term "NXRRSET" is becoming more
   common sometimes used as a synonym for NODATA.
      However, this is a mistake, given that NXRRSET is a specific error
      code defined in [RFC2136].

   Negative response -- response:  A response whose RCODE which indicates that a particular
      RRset does not exist in the DNS exist, or a failure of a
   nameserver. whose RCODE indicates the nameserver
      cannot answer.  Sections 2 and 7 of [RFC2308] describes describe the types
      of negative responses in detail.

   Referrals:  Data from the authority section of a non-authoritative
      answer.  [RFC1035] section 2.1 defines "authoritative" data.
      However, referrals at zone cuts are not authoritative.  Referrals
      may be a zone cut NS resource records and their glue records.  NS
      records on the parent side of a zone cut are an authoritative
      delegation, but are normally not treated as authoritative data by
      the client.  In general, a referral is a way for a server to send
      an answer saying that the server does not know the answer, but
      knows where the query should be directed in order to get an
      answer.  Historically, many authoritative servers answered with a
      referral to the root zone when queried for a name for which they
      were not authoritative, but this practice has declined.

   Zone transfer:  The act of a client requesting a copy of a zone and
      an authoritative server sending the needed information.  There are
      two common standard ways to do zone transfers: the AXFR
      ("Authoritative Transfer") mechanism to copy the full zone
      (described in [RFC5936], and the IXFR ("Incremental Transfer")
      mechanism to copy only parts of the zone that have changed
      (described in [RFC1995]).  Many systems use non-standard methods
      for zone transfer outside the DNS protocol.

4.  Resource Records

   RR --

   RR:  A short form for resource record.  ([RFC1034], section 3.6.)

   RRset --

   RRset:  A set of resource records with the same label, class and
      type, but with different data.  (Definition from [RFC2181]) Also
      spelled RRSet in some documents.  As a clarification, "same label"
      in this definition means "same owner name".  In addition,
      [RFC2181] states that "the TTLs of all RRs in an RRSet must be the
      same".

   EDNS -- Also commonly called "EDNS0", this is the

   EDNS:  The extension mechanisms for DNS.  The extension mechanism, DNS, defined in [RFC6891], [RFC6891].
      Sometimes called "EDNS0" or "EDNS(0)" to indicate the version
      number.  EDNS allows DNS clients and servers to specify message
      sizes larger than the original 512 octet limit, to expand the
      response code space, and to potentially carry additional options
      that affect the handling of a DNS query.

   OPT --

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

   Owner --

   Owner:  The domain name where a RR is found ([RFC1034], section 3.6).
      Often appears in the term "owner name".

   SOA field names -- names:  DNS documents, including the definitions here,
      often refer to the fields in the RDATA an SOA resource record by
      field name.  Those fields are defined in Section 3.3.13 of
      [RFC1035].  The names (in the order they appear in the SOA RDATA)
      are MNAME, RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM.
      Note that the meaning of MINIMUM field is updated in Section 4 of
      [RFC2308]; the new definition is that the MINIMUM field is only
      "the TTL to be used for negative responses".

   TTL --

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

      The TTL "specifies the time interval that the resource record may
      be cached before the source of the information should again be
      consulted".  (Quoted from [RFC1035], section 3.2.1) Also: "the
      time interval (in seconds) that the resource record may be cached
      before it should be discarded".  (Quoted from [RFC1035], section
      4.1.3).  Despite being defined for a resource record, the TTL of
      every resource record in an RRset is required to be the same
      (RFC2181, section 5.2).

      The reason that the TTL is the maximum time to live is that a
      cache operator might decide to shorten the time to live for
      operational purposes, such as if there is a policy to not allow
      TTL values over a certain number.  Also, if a value is flushed
      from the cache when its value is still positive, the value
      effectively becomes zero.  Some servers do not honor the TTL on an
      RRset from the authoritative servers, such as when when the
      authoritative data has a very short TTL.

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

5.  DNS Servers

   This section defines the terms used for the systems that act as DNS
   clients, DNS servers, or both.  Some terms about servers describe
   servers that do and do not use DNSSEC; see Section 8 for those
   definitions.

   [[ There is a request to "first describe the iterative and recursive
   resolution processes, and mention the expected values of the RD,RA,AA
   bits.  Then you can describe the distinctions between recursive and
   iterative clients, and between recursive and authoritative servers,
   in terms of the roles they play in the different resolution
   processes."  This would require the section to be quite different
   than the other sections in the document. ]]

   Resolver --

   Resolver:  A program that extracts information from name servers in
      response to client requests.  (Quoted from [RFC1034], section 2.4) It
   is a program that interfaces user programs to domain name servers.
      The resolver is located on the same machine as the program that
      requests the resolver's services. services, but it may need to consult name
      servers on other hosts.  (Quoted from [RFC1034], section 5.1) A
      resolver performs queries for a name, type, and class, and
      receives answers.  The logical function is called "resolution".
      In practice, the term is usually referring to some specific type
      of resolver (some of which are defined below), and understanding
      the use of the term depends on understanding the context.

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

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

   Recursive mode -- mode:  A resolution mode of a server that receives DNS
      queries and either responds to those queries from a local cache or
      sends queries to other servers in order to get the final answers
      to the original queries.  Section 2.3 of [RFC1034] describes this
      as "The first server pursues the query for the client at another
      server".  A server operating in recursive mode may be thought of
      as having a name server side (which is what answers the query) and
      a resolver side (which performs the resolution function).  Systems
      operating in this mode are commonly called "recursive servers".
      Sometimes they are called "recursive resolvers".  While strictly
      the difference between these is that one of them sends queries to
      another recursive server and the other does not, in practice it is
      not possible to know in advance whether the server that one is
      querying will also perform recursion; both terms can be observed in use
   interchangeably.  Resolvers acting in recursive mode are also
   sometimes called "caching servers". terms can be observed
      in use interchangeably.

   Full resolver -- resolver:  This term is used in [RFC1035], but it is not defined
      there.  RFC 1123 defines a "full-service resolver" that may or may
      not be what was intended by "full resolver" in [RFC1035].

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

   Priming -- requirements).

   Priming:  The mechanism used by a resolver to determine where to send
      queries before there is anything in the resolver's cache.  Priming
      is most often done from a configuration setting that contains a
      list of authoritative servers for the DNS root zone.

   Negative caching -- caching:  The storage of knowledge that something does not
      exist, cannot give an answer, or does not give an answer.  (Quoted
      from Section 1 of [RFC2308])

   Authoritative server -- server:  A server that knows the content of a DNS zone
      from local knowledge, and thus can answer queries about that zone
      without needing to query other servers.  (Quoted from [RFC2182],
      section 2.)  It is a system that responds to DNS queries with
      information about zones for which it has been configured to answer
      with the AA flag in the response header set to 1.  It is a server
      that has authority over one or more DNS zones.  Note that it is
      possible for an authoritative server to respond to a query without
      the parent zone delegating authority to that server.
      Authoritative servers also provide "referrals", usually to child
      zones delegated from them; these referrals have the AA bit set to
      0 and come with referral data in the Authority and (if needed) the
      Additional sections.

   Secondary server -- server:  "An authoritative server which uses zone transfer
      to retrieve the zone" (quoted from [RFC1996], section 2.1).
      [RFC2182] describes secondary servers in detail.  Although early
      DNS RFCs such as [RFC1996] referred to this as a "slave", the
      current common usage has shifted to calling it a "secondary".

   Slave server -- server:  See secondary server.

   Primary server -- server:  "Any authoritative server configured to be the
      source of zone transfer for one or more [secondary] servers"
      (quoted from [RFC1996], section 2.1) or, more specifically, "an
      authoritative server configured to be the source of AXFR or IXFR
      data for one or more [secondary] servers" (quoted from [RFC2136]).
      Although early DNS RFCs such as [RFC1996] referred to this as a
      "master", the current common usage has shifted to "primary".

   Master server -- server:  See primary server.

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

   Stealth server -- server:  This is the same as a slave server except that it is
      not listed in an NS resource record for the zone.  (Quoted from
      [RFC1996], section 2.1)

   Hidden master:  A stealth server that is often actually a master for zone transfers, and in that case is called a "hidden master".

   Zone transfer -- The act of a client requesting a copy of a zone and
   an authoritative transfers.
      In this arrangement, the master name server sending that processes the needed information.  There are
   two common standard ways
      updates is unavailable to do zone transfers: general hosts on the AXFR
   ("Authoritative Transfer") mechanism to copy Internet; it is not
      listed in the full zone, and NS RRset.  (Quoted from [RFC6781], section 3.4.3.)
      An earlier RFC, [RFC4641], said that the
   IXFR ("Incremental Transfer") mechanism to copy only parts of hidden master's name
      appears in the
   zone that have changed.  Many systems use non-standard methods for
   zone transfer outside SOA RRs MNAME field, although in some setups, the DNS protocol.

   Forwarding --
      name does not appear at all in the public DNS.  A hidden master
      can be either a secondary or a primary master.

   Forwarding:  The process of one server sending a DNS query with the
      RD bit set to 1 to another server to resolve that query.
      Forwarding is a function of a DNS resolver; it is different than
      simply blindly relaying queries.

      [RFC5625] does not give a specific definition for forwarding, but
      describes in detail what features a system that forwards need to
      support.  Systems that forward are sometimes called "DNS proxies",
      but that term has not yet been defined (even in [RFC5625]).

   Forwarder --

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

   Policy-implementing resolver -- resolver:  A resolver acting in recursive mode
      that changes some of the answers that it returns based on policy
      criteria, such as to prevent access to malware sites or
      objectionable content.  In general, a stub resolver has no idea
      whether or not upstream resolvers implement such policy or, if
      they do, the exact policy about what changes will be made.  In
      some cases, the user of the stub resolver has selected the policy-implementing policy-
      implementing resolver with the explicit intention of using it to
      implement the policies.  In other cases, policies are imposed
      without the user of the stub resolver being informed.

   Open resolver -- resolver:  A full full-service resolver that accepts and processes
      queries from any (or nearly any) stub resolver.  This is sometimes
      also called a "public resolver".

   View --

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

   Passive DNS -- DNS:  A mechanism to collect large amounts of DNS data by
      storing DNS responses from servers.  Some of these systems also
      collect the DNS queries associated with the responses; this can
      raise privacy issues.  Passive DNS databases can be used to answer
      historical questions about DNS zones such as which records were
      available for them at what times in the past.  Passive DNS
      databases allow searching of the stored records on keys other than
      just the name, such as "find all names which have A records of a
      particular value".

   Child-centric resolver -- resolver:  A DNS resolver that, instead of serving the
      NS RRset and glue records that it obtained from the parent of a
      zone, serves data from the authoritative servers for that zone.
      The term "child-centric" is meant as the opposite of "parent-centric", "parent-
      centric", which means a resolver that simply serves the NS RRset
      and glue records for a zone that it obtained from the zone's
      parent, without checking the authoritative servers for that zone.

6.  Zones

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

   Zone --

   Zone:  A unit of organization of authoritative data.  Zones can be
      automatically distributed to the name servers which provide
      redundant service for the data in a zone.  (Quoted from [RFC1034],
      section 2.4).

   Child --

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

   Parent --

   Parent:  The domain in which the Child is registered.  (Quoted from
      [RFC7344], section 1.1) Earlier, "parent name server" was defined
      in [RFC0882] as "the name server that has authority over the place
      in the domain name space that will hold the new domain".

   Origin -- 1.

   Origin:

      (a) The domain name that appears at the top of a zone.  2. zone (just below
      the cut that separates the zone from its parent).  The name of the
      zone is the same as the name of the domain at the zone's origin.
      (Quoted from [RFC2181], section 6.)

      (b) The domain name within which a given relative domain name
      appears in zone files.  Generally seen in the context of
      "$ORIGIN", which is a control entry defined in [RFC1035], section
      5.1, as part of the master file format.  For example, if the
      $ORIGIN is set to "example.org.", then a master file line for
      "www" is in fact an entry for "www.example.org.".

   Zone cut -- cut:  The delimitation point between two zones where the origin
      of one of the zones is the child of the other zone.  (Section 6 of
   [RFC2181] uses

      Zones are delimited by "zone cuts".  Each zone cut separates a
      "child" zone (below the cut) from a "parent" zone (above the cut).
      (Quoted from [RFC2181], section 6; note that this term extensively, although never actually defines
   it.) is barely an
      ostensive definition.)  Section 4.2 of [RFC1034] uses "cuts" as
      "zone cut".

   Apex --

   Apex:  The point in the tree at an owner of an SOA and corresponding
      authoritative NS RRset.  This is also called the "zone apex".
      [RFC4033] defines it as "the name at the child's side of a zone
      cut".  The "apex" can usefully be thought of as a data-theoretic
      description of a tree structure, and "origin" is the name of the
      same concept when it is implemented in zone files.  The
      distinction is not always maintained in use, however, and one can
      find uses that conflict subtly with this definition.

   Delegation --  [RFC1034]
      uses the term "top node of the zone" instead of "apex".

   Delegation:  The process by which a separate zone is created in the
      name space beneath the apex of a given domain.  Delegation happens
      when an NS RRset is added in the parent zone for the child origin,
   and a corresponding zone apex is created at the child origin.
      Delegation inherently happens at a zone cut.

   Referrals -- Data from the authority section of a non-authoritative
   answer.  [RFC1035] section 2.1 defines "authoritative" data.
   However, referrals at zone cuts are not authoritative.  Referrals may
   be  The term is also
      commonly a zone cut NS resource records and their glue.  NS records on noun: the
   parent side of a new zone cut are an authoritative delegation, but are
   normally not treated as authoritative data by the client.  In
   general, a referral is a way for a server to send an answer saying that is created by the server does not know the answer, but knows where the query
   should be directed in order to get an answer.  Historically, many
   authoritative servers answered with a referral to the root zone when
   queried for a name for which they were not authoritative, but this
   practice has declined. act of
      delegating.

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

      A later definition is that glue "includes any record in a zone
      file that is not properly part of that zone, including nameserver
      records of delegated sub-zones (NS records), address records that
      accompany those NS records (A, AAAA, etc), and any other stray
      data that might
   appear".  (Definition from [RFC2181], appear" ([RFC2181], section 5.4.1) This definition
   in [RFC2181] 5.4.1).  Although glue
      is sometimes used today with this wider than definition in mind, the one from [RFC1034], and bases
      context surrounding the [RFC2181] definition on suggests it is
      intended to apply to the contents use of a zone file.  Some implementers might
   only be thinking about the earlier definition when they see rules
   about glue records.

   In-bailiwick - 1. within the document itself
      and not necessarily beyond.

   In-bailiwick:

      (a) An adjective to describe a name server the name of which is
      either subordinate to or (rarely) the same as the zone origin.
      In-bailiwick name servers require glue in their parent zone.
   2.

      (b) Data for which the server is either authoritative, or else
      authoritative for an ancestor of the owner name.  This sense of
      the term normally is used when discussing the relevancy of glue
      records in a response.  For example, the server for the parent
      zone example.com might reply with glue records for
      ns.child.example.com.  Because the child.example.com zone is a
      descendant of the example.com zone, the glue records are in-bailiwick.

   Out-of-bailiwick - in-
      bailiwick.

   Out-of-bailiwick:  The antonym of in-bailiwick.

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

   Root zone -- zone:  The zone whose origin is the zero-length label.  Also
      sometimes called "the DNS root".

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

   Delegation-centric zone -- zone:  A zone which consists mostly of delegations
      to child zones.  This term is used in contrast to a zone which
      might have some delegations to child zones, but also has many data
      resource records for the zone itself and/or for child zones.

   Wildcard --  The
      term is used in [RFC4956] and [RFC5155], but is not defined there.

   Wildcard:  [RFC1034] defined "wildcard", but in a way that turned out
      to be confusing to implementers.  For an extended discussion of
      wildcards, including clearer definitions, see [RFC4592].

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

   Fast flux DNS:  This occurs when a domain is bound in DNS -- using A mechanism where a large number of hosts rapidly
   update the address
      records to multiple IP addresses, each of which has a zone, very short
      Time-to-Live (TTL) value associated with it.  This means that the
      domain resolves to varying IP addresses over a short period of
      time.  (Quoted from [RFC6561], section 1.1.5) It is often to
      deliver malware.  Because the addresses change so rapidly, it is
      difficult to definitively find all the hosts.  It should be noted
      that the technique also works with AAAA records, but such use is
      not frequently observed on the Internet as of this writing.

7.  Registration Model

   Registry --

   Registry:  The administrative operation of a zone that allows
      registration of names within that zone.

   Registrant --  People often use this
      term to refer only to those organizations that perform
      registration in large delegation-centric zones (such as TLDs); but
      formally, whoever decides what data goes into a zone is the
      registry for that zone.

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

   Registrar --

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

   EPP --

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

   WHOIS:  A protocol specified in [RFC3912], often used for querying
      registry databases.  WHOIS data is frequently used to associate
      registration data (such as zone management contacts) with domain
      names.

8.  General DNSSEC

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

   DNSSEC-aware and DNSSEC-unaware -- DNSSEC-unaware:  Section 2 of [RFC4033] defines many
      types of resolvers and validators.  In specific, the terms "non-
      validating security-aware stub resolver", "non-validating stub
      resolver", "security-aware name server", "security-aware recursive
      name server", "security-aware resolver", "security-aware stub
      resolver", and "security-oblivious 'anything'" are all defined.
      (Note that the term "validating resolver", which is used in some
      places in those documents, is nevertheless not defined in that
      section.)

   Signed zone -- zone:  A zone whose RRsets are signed and that contains
      properly constructed DNSKEY, Resource Record Signature (RRSIG),
      Next Secure (NSEC), and (optionally) DS records.  (Quoted from
      [RFC4033], section 2) 2.)  It has been noted in other contexts that
      the zone itself is not really signed, but all the relevant RRsets
      in the zone are signed.  Nevertheless, if a zone that should be
      signed contains any RRsets that are not signed (or opted out),
      those RRsets will be treated as bogus, so the whole zone needs to
      be handled in some way.  It should also be noted that, since the
      publication of [RFC6840], NSEC records are no longer required for
      signed zones: a signed zone might include NSEC3 records instead.

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

   NSEC --

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

      The NSEC resource record lists two separate things: the next owner
      name (in the canonical ordering of the zone) that contains
      authoritative data or a delegation point NS RRset, and the set of
      RR types present at the NSEC RR's owner name.  (Quoted from
      Section 4 of 4034)
   NSEC3 --

   NSEC3:  The NSEC3 resource record is quite different than the NSEC
      resource record.  Like the NSEC record, the NSEC3 record also
      provides authenticated denial of existence; however, NSEC3 records
      mitigates against zone enumeration and support Opt-Out.  NSEC3
      resource records are defined in [RFC5155].

   Opt-out --

   Opt-out:  The Opt-Out Flag indicates whether this NSEC3 RR may cover
      unsigned delegations.  (Quoted from Section 3.1.2.1 of [RFC5155]) [RFC5155], section 3.1.2.1.)

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

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

   DNSSEC Practice Statement (DPS) -- A practices disclosure document
   that may support and be a supplemental document to the DNSSEC Policy
   (if such exists), and it states how
      successive queries for them, or matches the management contents of NSEC3
      records against such a given zone
   implements procedures and controls at a high level.  (Quoted from
   [RFC6841], section 2) dictionary.

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

   Zone signing key (ZSK) -- (ZSK):  DNSSEC keys that can be used to sign all the
      RRsets in a zone that require signatures, other than the apex
      DNSKEY RRset.  (Quoted from [RFC6781], Section section 3.1) Note that the
      roles KSK and ZSK are not mutually exclusive: a single key can be
      both KSK and ZSK at the same time.

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

   Secure Entry Point (SEP):  A flag in the DNSKEY RRdata that can be
      used to distinguish between keys that are intended to be used as
      the secure entry point into the zone when building chains of
      trust, i.e., they are (to be) pointed to by parental DS RRs or
      configured as a trust anchor.  (Quoted from [RFC6781], section
      3.2.3.)  Note that the SEP flag is only a hint, and its presence
      or absence may not be used to disqualify a given DNSKEY RR from
      use as a KSK or ZSK during validation.

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

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

9.  DNSSEC States

   A validating resolver can determine that a response is in one of four
   states: secure, insecure, bogus, or indeterminate.  These states are
   defined in [RFC4033] and [RFC4035], although the two definitions
   differ a bit.

   Section 5 of [RFC4033] says:

   A validating resolver can determine the following 4 states:

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

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

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

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

   Section 4.3 of [RFC4035] says:

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

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

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

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

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

10.  IANA Considerations

   This document has no effect on IANA registries.

11.  Security Considerations

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

12.  Acknowledgements

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

13.  References

13.1.  Normative References

   [ISO3166]  International Organization for Standardization (ISO),
              "Country Codes - ISO 3166", February 2015,
              <http://www.iso.org/iso/country_codes/country_codes>.

   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",
              RFC 882, November 1983.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC1206]  Malkin, G. and A. Marine, "FYI on Questions and Answers:
              Answers to commonly asked "new Internet user" questions",
              RFC 1206, February 1991.

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, August 1996.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2182]  Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection
              and Operation of Secondary DNS Servers", BCP 16, RFC 2182,
              July 1997.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, March 1998.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, July 2006.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, March 2008.

   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              STD 69, RFC 5730, August 2009.

   [RFC5936]  Lewis, E. and A. Hoenes, "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, June 2010.

   [RFC6561]  Livingood, J., Mody, N., and M. O'Reirdan,
              "Recommendations for the Remediation of Bots in ISP
              Networks", RFC 6561, March 2012.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, June 2012.

   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781, December
              2012.

   [RFC6840]  Weiler, S. and D. Blacka, "Clarifications and
              Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
              February 2013.

   [RFC6841]  Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
              Framework for DNSSEC Policies and DNSSEC Practice
              Statements", RFC 6841, January 2013.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.

   [RFC7344]  Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
              DNSSEC Delegation Trust Maintenance", RFC 7344, September
              2014.

13.2.  Informative References

   [DBOUND]   "DBOUND Working Group", 2015,
              <https://datatracker.ietf.org/wg/dbound/charter/>.

   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
              host table specification", RFC 952, October 1985.

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              August 1996.

   [RFC3912]  Daigle, L., "WHOIS Protocol Specification", RFC 3912,
              September 2004.

   [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
              RFC 4641, September 2006.

   [RFC4956]  Arends, R., Kosters, M., and D. Blacka, "DNS Security
              (DNSSEC) Opt-In", RFC 4956, July 2007.

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines", BCP
              152, RFC 5625, August 2009.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [RFC5892]  Faltstrom, P., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, August 2010.

   [RFC5893]  Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5893, August 2010.

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, August 2010.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

Authors' Addresses

   Paul Hoffman
   VPN Consortium
   127 Segre Place
   Santa Cruz, CA  95060
   USA

   Email: paul.hoffman@vpnc.org
   Andrew Sullivan
   Dyn
   150 Dow St, Tower 2
   Manchester, NH  1604
   USA

   Email: asullivan@dyn.com

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

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