Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Best Current Practice                       A. Sullivan
Expires: October 16, 2015                                            Dyn
                                                             K. Fujiwara
                                                          April 14, 2015

                            DNS Terminology


   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

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
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   Drafts is at

   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 16, 2015.

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

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

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   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

   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) 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.

2.  Names

   Domain 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) -- 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 name would include every label, including the final, zero-
   length label of the root zone: such a name would be written
   "" (note the terminating dot).  But because every
   name eventually shares the common root, names are often written
   relative to the root (such as "") and are still called
   "fully qualified".
   This term first appeared in [RFC1206].

   Host name -- This term and its equivalent, "hostname", have been
   widely used but are not defined in [RFC1034], [RFC1035], [RFC1123],
   or [RFC2181].  The DNS was originally deployed into the Host Tables
   environment as outlined in [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

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   "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 -- 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 zones, and there are significant policy issues around their

   ccTLD -- A TLD that is allocated to a country.  Historically, these
   were two-letter TLDs, and were allocated to countries using the two-
   letter code from the ISO 3166-1 alpha-2 standard [ISO3166].  In
   recent years, there have been allocations of TLDs that conform to
   IDNA2008 ([RFC5890], [RFC5891], [RFC5892], [RFC5893], and [RFC5894]);
   these are still treated as ccTLDs for policy purposes.

   gTLD -- A "generic" TLD is a TLD that is not a ccTLD, and is not one
   of the small number of historical TLDs such as .int and .arpa.  There
   is no precise definition for which TLDs that are not ccTLDs are

   Public suffix -- A domain under which subdomains can be registered,
   and on which HTTP cookies ([RFC6265]) should not be set.  For
   example, at the time this document is published, is
   considered a public suffix, but .au is not.  Note that this term 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", and the authoritative answer bit
   is often called "the AA flag" or "the AA bit".

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   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,
   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 indicates that the queried domain
   name exists for the given class, but the resource record type being
   queried for does not exist.  A NODATA response is a combination of an
   RCODE of 0 (NOERROR) and an Answer section that is empty.  In
   addition, NODATA responses from authoritative servers have the
   authoritative answer (AA bit) set to 1 and include 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 no
   records of the given type".  The term "NXRRSET" is becoming more
   common as a synonym for NODATA.

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

4.  Resource Records

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

   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 extension
   mechanisms for DNS.  The extension mechanism, defined in [RFC6891],
   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 -- 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.

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

   SOA field names -- DNS documents, including the definitions here,
   often refer to the fields in the RDATA 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,
   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 -- 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

   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].

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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

   [[ 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 -- 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.  (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 -- 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 of [RFC1123].

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

   Recursive mode -- A resolution mode of a server that receives DNS
   queries and either responds to those queries from a local cache or
   sends queries to other servers in order to get the final answers to
   the original queries.  Section 2.3 of [RFC1034] describes this as
   "The first server pursues the query for the client at another
   server".  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".

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   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".

   Full-service resolver -- Section of [RFC1123] defines this
   term to mean a resolver that acts in recursive mode with a cache, as
   well as other 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 -- 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 -- 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 Additional section.

   Primary servers and secondary servers -- These are synonyms for
   "master server" and "slave server", which were the terms used in the
   early DNS RFCs, and defined below.  The current common usage has
   shifted to "primary" and "secondary".

   Slave server -- An authoritative server which uses zone transfer to
   retrieve the zone.  (Quoted from [RFC1996], section 2.1) [RFC2182]
   describes slave servers in detail.

   Master -- Any authoritative server configured to be the source of
   zone transfer for one or more slave servers.  (Quoted from [RFC1996],
   section 2.1) [RFC2136] defines "master" as "an authoritative server
   configured to be the source of AXFR or IXFR data for one or more
   slave servers".

   Primary master -- The primary master is named in the zone's SOA MNAME
   field and optionally by an NS resource record.  (Quoted from

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   [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 -- 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) A stealth server 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 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, and the
   IXFR ("Incremental Transfer") mechanism to copy only parts of the
   zone that have changed.  Many systems use non-standard methods for
   zone transfer outside the DNS protocol.

   Forwarder -- A system that receives a DNS query, possibly changes the
   query, sends on the resulting query (usually to a recursive
   resolver), receives the response, possibly changes the response, and
   sends the resulting response to the source of the query (usually a
   stub resolver).  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."

   [RFC5625] does not give a specific definition for DNS forwarder, but
   describes in detail what features they need to support.  The protocol
   interfaces for DNS forwarders are exactly the same as those for
   recursive resolvers (for interactions with DNS stubs) and as those
   for stub resolvers (for interactions with recursive resolvers).
   Forwarders are also sometimes called "DNS forwarders".  They are
   sometimes also called "DNS proxies", but that term has not yet been
   defined (even in [RFC5625]).

   Full resolver -- This term is used in [RFC1035], but it is not
   defined there.  RFC 1123 defines a "full-service resolver" that may
   or may not be what was intended by "full resolver" in [RFC1035].  In
   the vernacular, a full-service resolver is usually one that would be
   suitable for use by a stub resolver.

   Consensual policy-implementing resolver -- A resolver that changes
   some answers it returns based on policy criteria, such as to prevent
   access to malware sites.  These policy criteria are agreed to by
   systems that query this resolver through some out of band mechanism

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   (such as finding out about the resolver from a web site and reading
   the policy).

   Non-consensual policy-implementing resolver -- A resolver that is not
   a consensual policy-implementing resolver that changes the answers it
   returns.  The difference between this and a consensual policy-
   implementing resolver is that users of this resolver are not expected
   to know that there is a policy to change the answers it returns.

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

   Open forwarder -- A DNS forwarder that accepts and forwards queries
   from any (or nearly any) stub resolver to a full resolver.

   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), and so on.  Views are often used to provide more names or
   different addresses to queries from "inside" a protected network than
   to those "outside" that network.  Views are not a standardized part
   of the DNS, but they are widely implemented in server software.

   Passive DNS -- A mechanism to collect large amounts of DNS data by
   storing queries and responses from recursive servers.  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 -- 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", 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 -- A unit of organization of authoritative data.  Zones can be
   automatically distributed to the name servers which provide redundant

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   service for the data in a zone.  (Quoted from [RFC1034], section

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

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

   Origin -- 1.  The domain name that appears at the top of a zone.  2.
   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
   "", then a master file line for "www" is in fact an entry
   for "".

   Zone 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 this term extensively, although never actually defines
   it.)  Section 4.2 of [RFC1034] uses "cuts" as "zone cut".

   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 -- 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 a zone cut NS resource records and their glue.  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

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   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.

   Glue records -- Resource records which are not part of the
   authoritative data [for a zone], and are address resource records for
   the servers [in a subzone].  These RRs are only necessary if the name
   server's name is "below" the cut, and are only used as part of a
   referral response.  (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], section 5.4.1) This definition
   in [RFC2181] is wider than the one from [RFC1034], and bases the
   definition on the contents of a zone file.  Some implementers might
   only be thinking about the earlier definition when they see rules
   about glue records.

   In-bailiwick - 1.  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.  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 might reply with glue records for
   Because the zone is a descendant of the
   zone, both glue records are in-bailiwick.

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

   Authoritative data -- All of the RRs attached to all of the nodes
   from the top node of the zone down to leaf nodes or nodes above cuts
   around the bottom edge of the zone.  (Quoted from 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 -- The zone whose origin is the zero-length label.  Also
   sometimes called "the DNS root".

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   Empty non-terminal -- A domain name that has no RRsets, but has
   descendants that have RRsets.  A typical example is in SRV records:
   in the name "", it is likely that
   "" has no RRsets, but that ""
   has (at least) an SRV RRset.

   Delegation-centric 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 -- [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 -- 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 -- A mechanism where a large number of hosts rapidly
   update the address records of a zone, often to deliver malware.
   Because the addresses change so rapidly, it is difficult to
   definitively find all the hosts.

7.  Registration Model

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

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

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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 -- 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

   Signed zone -- A zone whose RRsets are signed and that contains
   properly constructed DNSKEY, Resource Record Signature (RRSIG), Next
   Secure (NSEC), and (optionally) DS records.  (Quoted from [RFC4033],
   section 2) It has been noted in other contexts that the zone itself
   is not really signed, but all the relevant RRsets in the zone are
   signed.  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 -- 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 -- "The NSEC record allows a security-aware resolver to
   authenticate a negative reply for either name or type non-existence
   with the same mechanisms used to authenticate other DNS replies."
   (Quoted from Section 3.2 of [RFC4033]) 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

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

   Zone enumeration -- The practice of discovering the full content of a
   zone via successive queries.  (Quoted from Section 1.3 of [RFC5155])
   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 the management of a given zone
   implements procedures and controls at a high level.  (Quoted from
   [RFC6841], section 2)

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

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

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:

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   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

   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

   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:

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

   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

   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

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, and many others in
   the DNSOP Working Group have helped shape this document.

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13.  References

13.1.  Normative References

   [ISO3166]  International Organization for Standardization (ISO),
              "Country Codes - ISO 3166", February 2015,

   [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.

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   [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.

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

   [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

13.2.  Informative References

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

   [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.

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   [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


   Andrew Sullivan
   150 Dow St, Tower 2
   Manchester, NH  1604


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

   Phone: +81 3 5215 8451

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