Network Working Group                                           E. Lewis
Internet-Draft                                                     ICANN
Intended status: Informational                            August 6, 2019
Expires: February 7, 2020

                      RFC Origins of Domain Names


   Is the concept of Domain Names owned by the DNS protocol or does the
   DNS protocol exist to support the concept of Domain Names?  This
   question has become pertinent in light of proposals to use Domain
   Names in protocols in ways incompatible with the DNS protocol and the
   operational environment built to run the protocol.

   This document is intended to help answer this question by presenting
   a look into the recorded history of relevant Requests for Comments.
   This document comprises the research and views of the author and has
   benefited from review and input from many IETF experts, but it does
   not represent the consensus opinion of the IETF.

Status of This Memo

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   This Internet-Draft will expire on February 7, 2020.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Goal  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Early RFCs  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Emergence of Domain Names . . . . . . . . . . . . . . . . . .   6
     3.1.  The Term "Domain Name" Itself . . . . . . . . . . . . . .   7
     3.2.  The Term "Resolve"  . . . . . . . . . . . . . . . . . . .   8
     3.3.  Where Does It Start?  . . . . . . . . . . . . . . . . . .   9
   4.  Dialects, So To Speak, of Domain Names  . . . . . . . . . . .  10
     4.1.  Domain Names in the DNS . . . . . . . . . . . . . . . . .  10
     4.2.  Host Names  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  URI Authority and Domain Names  . . . . . . . . . . . . .  12
     4.4.  Internet Protocol Address Literals  . . . . . . . . . . .  12
     4.5.  Internationalized Domain Names in Applications  . . . . .  12
     4.6.  Restricted for DNS Registration . . . . . . . . . . . . .  13
     4.7.  Tor Network Names . . . . . . . . . . . . . . . . . . . .  13
     4.8.  X.509 . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.9.  Multicast DNS . . . . . . . . . . . . . . . . . . . . . .  14
     4.10. /etc/hosts  . . . . . . . . . . . . . . . . . . . . . . .  14
     4.11. Other Protocols . . . . . . . . . . . . . . . . . . . . .  14
     4.12. Other Others  . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Interoperability Considerations . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   9.  Informational References  . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Which came first, the concept of Domain Names or the DNS?  This
   question is at the heart of whether or how Domain Names are put to
   use in ways avoiding the DNS protocol.

   The discussion leading to "The '.onion' Special-Use Domain Name"
   [RFC7686], a document designating "onion" as a top-level domain in
   the Special Use Domain Names registry (see "Special Use Domain Names"
   [RFC6761]), opened the question of how to treat Domain Names that
   were designed to be used external to the DNS.  The history of Domain

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   Names and the DNS had become intertwined over time to the point that
   what is essentially a case of permission-less innovation led to
   contentious discussions on the IETF's DNS Operations (DNSOP) working
   group mail list and an interim meeting of the DNSOP working

   A portion of the discussion centered around a seeming conflict among
   processes to register Domain Names, such as the process launched from
   "Memorandum of Understanding Concerning the Technical Work of the
   Internet Assigned Numbers Authority" [RFC2860], for registering a
   name in the global, public DNS and the process for registering a name
   in the Special Use Domain Names registry.  The latter process is
   documented in "Special Use Domain Names" cited in the previous

   To help establish a way forward, a look backward is thought to be a
   good start.  A document search, sticking to RFC documents, reveals
   evidence of discussions on Domain Names prior to the DNS, with the
   DNS protocol's base documents indicating that the DNS is based on
   some simplifying assumptions, implying there is a larger concept in

   To help bolster the idea that Domain Names came first, a look at how
   other protocols have treated identifying names, how Domain Names are
   put to use, how what a name is further restricted for the protocol's
   needs.  From this it has become apparent that the concept of Domain
   Names has drifted over time, which leads to some uncertainty when it
   comes to looking forward.

   During reviews of this document, documented studies of other
   difficulties resulting from the uncertainty have surfaced.  "IAB
   Thoughts on Encodings for Internationalized Domain Names" [RFC6055]
   documents issues related to converting human-readable forms of Domain
   Names in forms useful to automated applications when there is no
   clear architecture or precise definition of how to handle Domain
   Names.  "Issues in Identifier Comparison for Security Purposes"
   [RFC6943] documents issues related to the same conversion as related
   to evaluating security policies.  The presence of these studies
   suggests a need to examine the architecture of naming and

   The most glaring omission discovered by the document survey is a
   definitive foundation for Domain Names.  There are abstract
   descriptions of the concept that come close to qualifying as a
   definition.  The descriptions though are too loose to be something
   that can be tested objectively, frustrating discussions when it comes
   to innovations in the use of Domain Names.

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   In reviews of this document, an important thought has been expressed.
   During the era when the early RFC documents were prepared, many
   considerations now deemed important were not considered, discussed,
   examined.  This lack of attention should be taken simply as the the
   limits of the problem space perceived at the time and not as an
   intentional non-statement about questions now under consideration.
   The fact that the history is presented here does not imply that
   history will contain the answers and guide the way forward, the
   history as presented is only a starting point.

   This document is a literature search covering the RFC series and
   makes a case for clarifications to be made.  There are obvious
   continuations to this work, such as the earlier Internet Engineering
   Notes series (IEN), other published works, and interviews with
   participants from the early days.  This document is intended to help
   answer this question of whether the concept of Domain Names is owned
   by the DNS protocol or does the DNS protocol exist to support the
   concept of Domain Names.  It does this by presenting a look into the
   recorded history of relevant Requests for Comments.  This document
   comprises the research and views of the author and has benefited from
   review and input from many IETF experts, but it does not represent
   the consensus opinion of the IETF.

1.1.  Goal

   To establish a solid foundation accommodating an installed base and
   permission-less innovation, having a clear definition of Domain Names
   would be great.  This document, however, does not attempt to achieve
   a definition.  This document's goal has settled into compiling a
   narrative on the history, within perhaps artificial bounds (the RFC
   series), and declaring that there is a need to clarify Domain Names.

   In this document are criteria for performing a clarification,
   recognizing from experience in preparing "The Role of Wildcards in
   the Domain Name System" [RFC4592] and "DNS Zone Transfer Protocol
   (AXFR)" [RFC5936] that clarifications may have adverse impacts on
   deployed software, thus entering into a clarifications activity is
   not to be taken without considerations.

   There are a few deviations from the strict rule of relying on the RFC
   series.  First is the research into the term "resolve" and then
   further additions during late reviews of this document.  The
   experience of these deviations illustrates the need to expand the
   literature search beyond the RFC series and to include other
   publications and recollections.

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1.2.  Terminology

   Throughout this document (except in document quotations) the term
   "Domain Names" is capitalized to emphasize the concept of the names
   and "the DNS" is used to describe the protocol and algorithms
   described in STD 13, including any applicable updates, related
   standards track documents and experimental track documents.  The
   words "DNS domain names" refers to the definition of Domain Names
   within the DNS (as well as, for example, "Tor domain names" referring
   to the definition of Domain Names within the Tor system).

   The term "domain" is a generic term, having many dictionary
   definitions.  There are many naming systems in existence, many
   unrelated to the Internet.  The use of the term Domain Names in this
   document refers to a roughly-defined set of protocols defined in IETF
   RFC documents and their applications' use of a somewhat common,
   interoperating, naming structure.  Lacking a formal, documented
   definition for Domain Names, which is why this document exists, it is
   hard to avoid a hand-waving reference.

2.  Early RFCs

   Two or three decades into the history of Domain Names, a popular
   notion has taken hold that Domains Names were defined and specified
   in the definition of the Domain Name System (DNS).  There are two
   documents that form the basic definition of the DNS, "Domain names -
   concepts and facilities" [RFC1034] and "Domain names - implementation
   and specification" [RFC1035] referred to as RFC 1034 and RFC 1035,
   respectively.  (Note that there is another pair of Request for
   Comments documents with the same titles [RFC0882] [RFC0883] that
   precede RFC 1034 and RFC 1035, declared obsolete in favor of the
   newer documents.)  Together RFC 1034 and RFC 1035 form STD 13, a full
   standard cataloged by the RFC Editor.  The definitions of the DNS'
   version of Domain Names within RFC 1034 and RFC 1035 have become the
   apparently-authoritative source for discussions on what is a Domain

   The truth is, the documents comprising STD 13 do not define Domain
   Names, the documents define only how Domain Names are used and
   processed in the DNS.  However, the way in which those RFC documents
   read seem to lend to the confusion.

   RFC 1034, section 2 begins with this text:

   "This RFC introduces domain style names, their use for Internet mail
   and host address support, and the protocols and servers used to
   implement domain name facilities."

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   That text seems to indicate that RFC 1034 is the origin of Domain
   Names.  Immediately following is section 2.1, entitled "The history
   of domain names" which includes the following text.  (The text
   differs from the original presentation only in wrapping of text to
   fit current formatting rules.)

   Continuing the quote from RFC 1034:

   "The result was several ideas about name spaces and their management
   [IEN-116, RFC-799, RFC-819, RFC-830].  The proposals varied, but a
   common thread was the idea of a hierarchical name space, with the
   hierarchy roughly corresponding to organizational structure, and
   names using "." as the character to mark the boundary between
   hierarchy levels.  A design using a distributed database and
   generalized resources was described in [RFC-882, RFC-883].  Based on
   experience with several implementations, the system evolved into the
   scheme described in this memo."

   The only reference included in that text not otherwise mentioned in
   this document is to "INTERNET NAME SERVER", identified as IEN-

   The DNS as it is known today did not invent Domain Names.  Work on
   the Simple Mail Transfer Protocol (SMTP) preceding work on the DNS
   mentions Domain Names, and even SMTP too was not the origin of the
   concept.  The DNS is not even the first attempt at an Internet naming
   system, see "The Domain Naming Convention for Internet User
   Applications" [RFC0819] and "A Distributed System for Internet Name
   Service" [RFC0830].

   One important phrase to keep in mind is:

   "To simplify implementations,"

   which appears in both of the RFC 1034 and RFC 1035 documents, as well
   as their predecessor pair RFC 882 and RFC 883.  This gives credence
   to the notion that Domain Names exist beyond the DNS, in that the
   text following the phrase is meant to limit an existing definition or
   concept as opposed to introducing a new idea.

3.  Emergence of Domain Names

   The first effort taken, in preparation for writing this document, was
   to scan for the earliest use of the term "domain name" or "name
   domain".  This work is detailed in the following section, but, as
   noted in private email by reviews of early versions of the document,
   gave the impression that Domain Names were somehow a by-product of
   the effort to develop electronic mail.  To challenge the notion that

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   email begat Domain Names, a search through RFC documents for the use
   of the term resolve as it applies to Domain Names was also done.

3.1.  The Term "Domain Name" Itself

   Domain Names emerged from the need to build a hierarchy around the
   growing number of identified hosts exchanging email.  "SIMPLE MAIL
   TRANSFER PROTOCOL" [RFC0788], explains, in its section 3.7:

   "At some not too distant future time it might be necessary to
   expand the mailbox format to include a region or name domain
   identifier.  There is quite a bit of discussion on this at
   present, and is likely that SMTP will be revised in the future to
   take into account naming domains."

   Knowing the origins of a concept helps setting the correct boundaries
   for discussion.  The past isn't meant to restrict the future but
   meant to help provide a context, include forgotten ideas, and help
   identify rational for scope creep.

   "Internet Name Domains" [RFC0799] has (arguably) the first formation
   of what is a Domain Name:

   "In its most general form, a standard internet mailbox name has
   the syntax

                     <user>.<host>@<domain> ,

   where <user> is the name of a user known at the host <host> in the
   name domain <domain>."

   Prior to this, the term "domain" referred to principally an
   administrative domain, such as the initial organizations involved in
   networks at the time.

   "NCP/TCP TRANSITION PLAN" [RFC0801] contains this, indicating the
   passage from the host tables:

   "It might be advantageous to do away with the host name table and
   use a Name Server instead, or to keep a relatively small table as
   a cache of recently used host names."

   "Computer Mail Meeting Notes" [RFC0805] contains this:

   "The conclusion in this area was that the current "user@host" mailbox
   identifier should be extended to 'user@host.domain' where 'domain'
   could be a hierarchy of domains."

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   "The Domain Naming Convention for Internet User Applications"
   [RFC0819] contains this:

   "A decision has recently been reached to
   replace the simple name field, "<host>", by a composite name field,
   '<domain>' "

   A Domain Name began to take on its current form:

   "Internet Convention:  Fred@F.ISI.ARPA"

   In addition, "simple name" is defined as what we now call a label,
   and a "complete (fully qualified) name" is defined as "concatenation
   of the simple names of the domain structure tree nodes starting with
   its own name and ending with the top level node name".  Noticeably
   absent is a terminating dot or any mention or representation of a

   "The Domain Naming Convention for Internet User Applications" (RFC
   819) also defines ARPA as a top-level name (as opposed to top-level
   domain name).  This is an early mention of the role of top-level
   names.  Additionally, the use of "."  [RFC0020][ANSIX34] as a
   separating character is mentioned.

   This walk thru history relies solely on the record left behind inside
   RFC documents.  The precise chain of events is likely slightly
   different and nuanced.  The point of the exercise is to show that
   Domain Names are a concept the emerged over time, spawned the DNS
   with its Domain Names, a definition of host names derived from the
   host tables, and was heavily influenced by SMTP as the driving
   application.  The definition of the FTP protocol, originally defined
   in "FILE TRANSFER PROTOCOL" [RFC0959], never mentions hosts, domains
   or host names.  No formal definition of Domain Names has been written
   and recorded.

   Note: Concurrent with the writing of this document, the Domain Name
   Systems Operations working group is documenting a definition for
   "Domain Names".  The first edition of "DNS Terminology" [RFC7719] has
   a recitation of the original definition from STD 13, the successor
   edition (still in preparation) has a new, further reaching

3.2.  The Term "Resolve"

   In looking for other early mentions of Domain Names, a look for the
   use of the term "resolve" or "resolution" was conducted, reading
   through early (arbitrarily defined as pre-1000) RFC documents.  The
   term "resolve" appears numerous times, but in many different

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   contexts.  "Resolve" has many meanings, consulting a dictionary, such
   as Merriam Webster's dictionary [MWDICT], none which seem to match
   the use associated with domain names.  For example, a committee can
   resolve to solve a certain question.  This use of "resolve" occurred
   numerous times in early RFC documents unrelated to Domain Names.

   In "Proposed Official Standard for the Format of ARPA Network
   Messages" [RFC0724] the term resolve was used in the sense of mapping
   an identifier into an address or something actionable.  A section on
   Semantics (C), Address fields (1.), General (a.), bullet 1 states:

   "<path>s are used to refer to a location,  on  the  ARPANET,
   containing  a  stored  address  list.   The <phrase> should
   contain text which the referenced host  can  resolve  to  a
   file.   This  standard  is  not  a protocol and so does not
   prescribe HOW data  is  to  be  retrieved  from  the  file."

   Private email to the (reachable) authors of the document pointed to
   the use of "resolve" stemming from work on programming languages and
   compiler theory.  In that field of work, variables are associated
   with machine addresses when linking code.  There are formal papers
   including "A Theory of Name Resolution" [TONR15] using the term and
   the term resolution is used in the field of "Automated Reasoning"

   The exercise of determining how the term "resolve" came to be part of
   Domain Names and DNS shows that there are influences, topics, terms
   and concepts from technologies preceding Domain Name and DNS that can
   be researched to help establish a foundation from which to build.
   There is more work to do here.

3.3.  Where Does It Start?

   Without a definitive introduction to Domain Names it is hard to know
   how far back in documented history to search for references to the
   concept.  Chasing "domain" and variations has not necessarily found
   the beginning, chasing "resolution" and variations also has not
   necessarily found the beginning.  During later reviews of this
   document, a significant early document has been identified for
   inclusion, an IEN entitled "A Note On Inter-Network Naming,
   Addressing, and Routing" by John F.  Shoch.  That document is tagged
   as IEN-19 [IEN019].

   The note introduces the difference between names, addresses, and
   routes.  The term "domain" is used to scope a name space, giving
   examples from telephony and networking.  But there still is no formal
   definition of Domain Names nor any solution path towards Domain Names
   as they are commonly known today.

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   A relatively more modern document (15 years later), entitled "On the
   Naming and Binding of Network Destinations" [RFC1498], refers to IEN-
   19, extending the discussion on naming to divide into four categories
   of objects.  This document illustrates the continuing conceptual work
   covering naming as opposed to further developing the solution known
   as Domain Names.

4.  Dialects, So To Speak, of Domain Names

   Subtypes of Domain Names have come to be defined for different
   protocols, evolving and sometimes building on previous definitions.

4.1.  Domain Names in the DNS

   The DNS protocol defines a subset of Domain Names that referred to as
   DNS domain names.  The DNS places size restrictions on Domain Names
   and defines rules for matching DNS domain names, treating sets of DNS
   domain names as equivalent to each other.  (This matching refers to
   treating upper case and lowercase ASCII letters as equivalent.)  The
   DNS defines the format used to transmit the names across the network
   as well as rules for displaying them inside text zone files.  The DNS
   creates the notion that names are assigned by an authority per zone.

   Placing size restrictions on a DNS domain name is significant in
   reducing the overall population of names that can be represented in
   the DNS.  The matching rules have the effect of creating (to use a
   term from graph theory) cliques, distorting the tree-nature of the
   Domain Name graph.  A clique is a completely connected sub-graph
   implying cyclic paths, a tree is a graph that is acyclic.  In sum,
   the treatment of ASCII (and only ASCII) cases as equivalent is a
   distortion of the DNS domain name hierarchy.

   The DNS defines two representational formats for DNS domain names.
   One format is the "on-the-wire" format used inside messages, a flags-
   and-length octet followed by some count of octets for each label with
   the final length of 0 representing the root.  The other format is a
   version that can be rendered in printable ASCII characters, complete
   with a means to represent other characters via an escape sequence.
   This does not alter the Domain Name concept but has implications when
   it comes to interoperating with other protocol definitions of their
   domain name use.

   The DNS assumes that there is, in concept, a central authority
   creating names within the DNS management structure (called a zone).
   Although the DNS does not define how a central authority is
   implemented nor how it manages names, the names have to come from a
   single point to appear in a zone.  There are other means for claiming
   names, an example will be mentioned later.

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   DNS domain names allows for names to appear as address literals, such
   as "" or "0:0:0:0:0:FFFF::".  But such Domain Names
   are not used in the DNS for two reasons.  Applications expecting a
   Domain Name (as a comment line parameter as an example) could opt to
   treat the string as an address literal and therefore not look for the
   string in the DNS domain name space.  And, if addresses were stored
   using this representation, there would be no means to aggregate
   managed address ranges into zones.

   By reversing the order of the address components, DNS domain names
   can be aggregated (as in routing) into the same zone.  E.g., the
   network address would be represented by a DNS domain name
   as "" as described in RFC 1035.  For IPv6, the
   convention used is documented in "DNS Extensions to Support IP
   Version 6" [RFC3596], section 2.5.

   See also "Issues in Identifier Comparison for Security Purposes"
   [RFC6943] section 3.1, "Host Names", in particular, section 3.1.1 and
   3.1.2 on address literals, and section 4.1, "Conflation."

   DNS domain names have become the dominant definition of Domain Names
   due to the success (scale) of the DNS on the public Internet.  Many
   protocols interact with the DNS but instead of supporting the
   complete definition of DNS domain names the protocols rely on a
   subset more commonly called host names.

4.2.  Host Names

   Work on the definition of a host name began well before the issuance
   of the STD 13 documents defining the DNS.  The rules for the
   Preferred Syntax in RFC 1034 conform to the host name rules outlined
   in "DoD Internet host table specification" [RFC0952].  The host name
   definition was presented again in "Requirements for Internet Hosts --
   Application and Support" [RFC1123] (which is part of STD 3).  In
   section 2.1 of RFC 1123, one (of two mentions) definition of host
   name is presented, noting that the definition is a relaxation of what
   is in RFC 952.

   Host names are subsets of DNS domain names in the sense that the
   character set is limited.  In particular, only "let" (i.e.,
   presumably letters a-z), "digits" and "hyphen" can be used, with
   hyphen only internal to a label.  (This description is meant to be
   illustrative, not normative.  See the grammar presented on page 5 of
   "DoD Internet host table specification" for specifics.)  "Hypertext
   Transfer Protocol -- HTTP/1.0" [RFC1945], Section 3.2.2 "http URL"
   specifically references section 2.1 of RFC 1123.  The reference is

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   "Simple Mail Transfer Protocol" [RFC5321] refers to RFC 1035 for a
   definition of Domain Names but includes text close to what is in the
   previous paragraph, noting that Domain Names as used in SMTP refer to
   both hosts and to other entities.  RFC 5321 updates RFC 1123, but
   does not cite the latter for a definition of host names.  RFC 5321
   additionally requires brackets to surround address literals,
   referring to the use case as an "alternative to a domain name."

   See also "IAB Thoughts on Encodings for Internationalized Domain
   Names" [RFC6055], particularly section 3 entitled "Use of Non-ASCII
   in DNS" for more thoughts on host names.

4.3.  URI Authority and Domain Names

   In "Uniform Resource Identifier (URI): Generic Syntax" [RFC3986],
   also known as STD 66, mentions in its section 3.2.2 (page 20) that
   the host subcomponent of the URI Authority (section 3.2) "should
   conform to the DNS syntax".  This comes after discussion that the
   host subcomponent is not strongly tied to the DNS, i.e., names can be
   managed via a concept other than the DNS.  There's no discussion on
   the rationale but this enables the reuse of code parsing and
   marshaling the host subcomponent between different Domain Name

   This reinforces the notion that there's a need to understand how
   Domain Names interoperate amongst protocols and applications.  And
   reinforces the need to derive or make explicit a way for client
   software to know how to resolve a name, that is, convert a name into
   a network address.

4.4.  Internet Protocol Address Literals

   The above definition includes address literals such as for
   IPv4 and even IPv6 literals such as ::ffff:  Yes, these
   qualify as Domain Names.  The addresses might be encased in square
   brackets "[" and "]" (SMTP mentioned already).  In the DNS, as
   previously described in section 3.1, they are represented per
   appropriate conventions.

4.5.  Internationalized Domain Names in Applications

   The original uses of Domain Names (such as DNS domain names and host
   names) assumed the ASCII character set.  Specifically, making the
   labels case insensitive prohibited a straightforward use of any
   method of representation of non-ASCII characters.

   "Internationalized Domain Names for Applications (IDNA): Definitions
   and Document Framework" [RFC5890], with associated other documents,

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   defines IDNA2008 as a convention for handling non-ASCII characters in
   DNS domain names.  In figure 1 of that document, the sets of legal
   DNS domain name formats are defined.  Noted in the footnotes of the
   figure, applications unaware of IDNA2008 cannot distinguish the
   subsets defined by the document meaning this definition is not an
   alteration of Domain Names, but, like host names, yet another subset
   of DNS domain names.

4.6.  Restricted for DNS Registration

   "Suggested Practices for Registration of Internationalized Domain
   Names (IDN)" [RFC4290] presents reasons why DNS domain name
   registration is restricted in the context of IDN.  (That RFC refers
   to a obsoleted version of IDNA but the concepts still apply.)  This
   is yet another convention related to DNS domain names, which excludes
   names that fit the syntax but would lead to undesirable outcomes in

4.7.  Tor Network Names

   The Tor network is an activity organized by the Tor Project, Inc.,
   described on its main web page "

   One component of the Tor network name space are Domain Names ending
   in ".onion".  (There are other suffixes in use, but it isn't very
   clear how they are used, defined or whether they are active.)

   The way in which Domain Names are used in Tor is described in two web
   documents "Tor Rendezvous Specification" [RENDEV] and "Special
   Hostnames in Tor" [OHOST] available from the project's website.

   Syntactically, a Tor domain name fits within the DNS domain name
   definition but the manner of assignment is different in a manner
   incompatible with the DNS.  (Not better or worse, still significantly
   different.)  Tor domain names are derived from cryptographic keys and
   organized by distributed hash tables, instead of assigned by a
   central authority per zone.

4.8.  X.509

   "Internet X.509 Public Key Infrastructure Certificate and Certificate
   Revocation List (CRL) Profile" [RFC5280], section "Subject
   Alternative Name" a dNSName is defined to be a host name, with the
   further restriction that the name " " cannot be used.

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4.9.  Multicast DNS

   Multicast DNS uses a name space ending with ".local." as described in
   "Multicast DNS" [RFC6762].  The rules for Multicast DNS domain names
   differ from DNS domain names.  Multicast DNS domain names are encoded
   as Net-Unicode as defined in " Unicode Format for Network
   Interchange" [RFC5198] with the DNS domain name tradition of case
   folding the ASCII letters when matching names.  Appendix F of RFC
   6762 gives an explanation of why the punycode algorithm, defined in
   "Punycode: A Bootstring encoding of Unicode for Internationalized
   Domain Names in Applications (IDNA)" [RFC3492], is not used.

4.10.  /etc/hosts

   The precursor to the DNS, host tables, still exists in remnants in
   many operating systems.  There are library functions, used by
   applications to resolve Domain Names, that can return names of
   arbitrary length (meaning, for example, longer than what DNS domain
   names are defined to be).

   "Basic Socket Interface Extensions for IPv6" [RFC3493], addresses
   this in Section 6, further documentation can be found as part of "The
   Open Group Base Specifications Issue 7" [IEEE1003] and "Microsoft
   Winsock Functions" [WINSOCK].

4.11.  Other Protocols

   This section is used to list (some) other protocols that use Domain
   Names but in general do not impose any other restrictions that what
   has been mentioned above.

   SSH, documented in "The Secure Shell (SSH) Protocol Architecture"
   [RFC4251] uses host names, using the name when storing public keys of
   hosts.  SSH clients, not necessarily the protocol, illustrate how
   applications juggle the different forms of Domain Names.  SSH can be
   invoked to open a secure shell with a host via its DNS domain name/
   host name or it can be used to open a secure shell with a host via
   its Multicast DNS domain name.  Or, many others, including name of a
   purely local, per-user scope.  (Note that SSH does not distinguish
   between DNS domain names and Multicast DNS domain names in the
   protocol definition, the difference is handled in resolution
   libraries belonging to the computing platform.)

   FTP, defined in "FILE TRANSFER PROTOCOL (FTP)" [RFC0959], is silent
   on Domain Names but client implementations of the protocol behave as
   SSH clients, being un aware the differences between definitions of
   Domain Names.

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   DHCP, defined in "Dynamic Host Configuration Protocol" [RFC2131],
   includes Domain Names in many DHCP options.  The use is described in
   many documents, starting with "DHCP Options and BOOTP Vendor
   Extensions" [RFC2132].  In "Dynamic Host Configuration Protocol
   (DHCP) Domain Search Option" [RFC3397] the encoding of Domain Names
   uses the on-the-wire format of DNS domain names.  In "The DHCP Client
   FQDN Option" [RFC4702] the same format is used.  "Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6)" [RFC8415] contains
   definitions related to DHCP designed for IPv6.  The significance of
   the DHCP protocols implementation of Domain Names is that, while most
   other protocols represent DNS domain names or host names in a human
   readable form, DHCP is using the machine-friendly format.

4.12.  Other Others

   If there is a use of Domain Names not listed here it is merely an
   omission.  The goal in this document is to provide a survey that is
   sufficient to avoid hand-waving arguments, recognizing the
   diminishing return building a complete roster of uses of Domain

5.  Interoperability Considerations

   Any single protocol can define a format for a conceptual Domain Name.
   Examples given above show that many protocols have done so.  From the
   examples, it is clear that the way in which protocols have
   interpreted Domain Names has varied, leading to, at least, user
   interfaces having to have built-in intelligence when handling names
   and, at worst, a growing confusion over how the Domain Name space is
   to be managed.

   When protocols having different formats and rules for Domain Names
   interact, software implementing the protocols translate one
   protocol's domain name format to another's format.  Even when the
   translation is straightforward, it is predictable that software will
   fail to handle this situation well.

   Often the clash of definitions impacts the design of a new protocol
   and/or an extension of a protocol.  For example, adding non-ASCII
   domain names has to be done with backwards compatibility with an
   installed base of ASCII-assuming code.  This clash can inhibit new
   uses of Domain Names.

   Search lists are a Domain Name mechanism studied in "SSAC Advisory on
   DNS 'Search List' Processing" [SSAC064].  (Note that the advisory's
   title labels search lists as a DNS mechanism although the idea of a
   search list spans many different naming schemes.)  One of the
   particular use cases related to this topic is the issuance of search

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   lists via DHCP and then used by any user-client protocol
   implementation.  This emphasizes an interoperability consideration
   for how Domain Names are treated in different protocols, not just
   among implementations of one protocol.

   The detection and handling of Fully Qualified Domain Names is an
   interoperability issue as well.  At issue is the significance of the
   terminating separation character in a printed version of a Domain
   Name.  Many clients, when passed a Domain Name as an identifier will
   add a dot at the end of the argument if the argument does not already
   end in a dot.  [TRAILDOT1] Some do this only after applying the
   aforementioned search list.  As mentioned in the SSAC document in the
   previous paragraph, inconsistency leads to unpredictable results.

   The Special Use Domain Names registry lists Domain Names that are to
   be treated in a manner inconsistent with the DNS normal processing
   rules.  This registry contains Domain Names regardless of whether the
   name is a DNS domain name and regardless whether the name is a top-
   level (domain) name or is positioned elsewhere in the tree structure.

   These are reasons this document is needed.  The reason for the
   confusion over what's a legal domain name stems from application-
   defined restrictions.  For example, using a one-label domain name
   ("dotless") for sending email is not a problem with the DNS nor the
   name in concept, but is a problem for mail implementations that
   expect more than one label.  (One-label names may be assumed to be in
   ARPA host table format.)  The "IAB Statement: Dotless Domains
   Considered Harmful" [IABSTMT] elaborates.

6.  IANA Considerations


7.  Security Considerations

   Nothing direct.  This document proposes a definition of the term
   "Domain Name" and surveys how it has been variously applied.  In some
   sense, loosely defined terms give rise to security hazards.  Beyond
   that, there is no impact of "security."

8.  Acknowledgements

   Comments or contributions from Andrew Sullivan, Paul Hoffman, George
   Michaelson, Kevin Darcy, Joe Abley, Jim Reid, Tony Finch, Robert
   Edmonds, hellekin, Stephane Bortzmeyer, Ray Bellis, Bob Harold, Alec
   Muffett, Stuart Cheshire, Dave Thaler, Niall O'Reilly, John Klensin,
   Dave Crocker, Ken Pogran, John Vittal, Lixia Zhang, Ralph Droms and a

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   growing list of others I am losing track of.  Not to imply

9.  Informational References

   [ANSIX34]  American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for
              Information Interchange, ANSI X3.4-1968", 1968.

   [DNSOP]    Woolf, S., "Interim DNSOP WG meeting on Special Use Names:
              some reading material", 2015,

   [IABSTMT]  Board, I. A., "IAB Statement: Dotless Domains Considered
              Harmful", 2013, <

              Group, T. I. A. T. O., "The Open Group Base Specifications
              Issue 7, IEEE Std 1003.1, 2013 Edition, Copyright
              2001-2013 The IEEE and The Open Group", 2013,

   [IEN019]   Shoch, J., "A note on Inter-Network Naming, Addressing,
              and Routing", IEN 19, January 1973,

   [IEN116]   Postel, J., "INTERNET NAME SERVER", IEN 116, August 1979,

   [MWDICT]   Merriam-Webster, Incorporated, "Merriam-Webster's Online
              Dictionary, 11th Edition (Merriam-Webster's Collegiate
              Dictionary)", 2003, <>.

   [OHOST]    Mathewson, N., "Special Hostnames in Tor", undated,

   [RENDEV]   Anonymous, "Tor Rendezvous Specification", undated,

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,

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   [RFC0724]  Crocker, D., Pogran, K., Vittal, J., and D. Henderson,
              "Proposed official standard for the format of ARPA Network
              messages", RFC 724, DOI 10.17487/RFC0724, May 1977,

   [RFC0788]  Postel, J., "Simple Mail Transfer Protocol", RFC 788,
              DOI 10.17487/RFC0788, November 1981,

   [RFC0799]  Mills, D., "Internet name domains", RFC 799,
              DOI 10.17487/RFC0799, September 1981,

   [RFC0801]  Postel, J., "NCP/TCP transition plan", RFC 801,
              DOI 10.17487/RFC0801, November 1981,

   [RFC0805]  Postel, J., "Computer mail meeting notes", RFC 805,
              DOI 10.17487/RFC0805, February 1982,

   [RFC0819]  Su, Z. and J. Postel, "The Domain Naming Convention for
              Internet User Applications", RFC 819,
              DOI 10.17487/RFC0819, August 1982,

   [RFC0830]  Su, Z., "Distributed system for Internet name service",
              RFC 830, DOI 10.17487/RFC0830, October 1982,

   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",
              RFC 882, DOI 10.17487/RFC0882, November 1983,

   [RFC0883]  Mockapetris, P., "Domain names: Implementation
              specification", RFC 883, DOI 10.17487/RFC0883, November
              1983, <>.

   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
              host table specification", RFC 952, DOI 10.17487/RFC0952,
              October 1985, <>.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985,

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   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,

   [RFC1498]  Saltzer, J., "On the Naming and Binding of Network
              Destinations", RFC 1498, DOI 10.17487/RFC1498, August
              1993, <>.

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945,
              DOI 10.17487/RFC1945, May 1996,

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,

   [RFC2860]  Carpenter, B., Baker, F., and M. Roberts, "Memorandum of
              Understanding Concerning the Technical Work of the
              Internet Assigned Numbers Authority", RFC 2860,
              DOI 10.17487/RFC2860, June 2000,

   [RFC3397]  Aboba, B. and S. Cheshire, "Dynamic Host Configuration
              Protocol (DHCP) Domain Search Option", RFC 3397,
              DOI 10.17487/RFC3397, November 2002,

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,

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   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, DOI 10.17487/RFC3493, February 2003,

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              DOI 10.17487/RFC3596, October 2003,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <>.

   [RFC4290]  Klensin, J., "Suggested Practices for Registration of
              Internationalized Domain Names (IDN)", RFC 4290,
              DOI 10.17487/RFC4290, December 2005,

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,

   [RFC4702]  Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
              Configuration Protocol (DHCP) Client Fully Qualified
              Domain Name (FQDN) Option", RFC 4702,
              DOI 10.17487/RFC4702, October 2006,

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,

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   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,

   [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
              Encodings for Internationalized Domain Names", RFC 6055,
              DOI 10.17487/RFC6055, February 2011,

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

   [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
              Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
              2013, <>.

   [RFC7686]  Appelbaum, J. and A. Muffett, "The ".onion" Special-Use
              Domain Name", RFC 7686, DOI 10.17487/RFC7686, October
              2015, <>.

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,

   [SSAC064]  Anonymous, "SSAC Advisory on DNS "Search List"
              Processing", 2014,

   [TONR15]   Wachsmuth, P. N. A. P. T. E. V. G., "A Theory of Name
              Resolution", last seen 2015, <

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              by), S. C. (. M., "Trailing Dots in Domain Names",

   [WIKIAR]   Anonymous, "Automated Reasoning", last edit 2016,

   [WINSOCK]  Microsoft, "getaddrinfo function", last seen 2017,

Author's Address

   Edward Lewis


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