Independent Submission                                        E. Lewis
Internet-Draft                                                   ICANN
Expires: July 30, 2016                          Date: January 30, 2016
Intended Status: unknown

                            Domain Names


This document states a definition of Domain Name beyond the use of
the term within the Domain Name System.  The document includes a
survey of the diverse ways Domain Names have been interpreted within
various protocols over time.  The purpose of this is to give a solid
foundation for work on Domain Names across all protocols making use
of Domain Names.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF).  Note that other groups may also distribute
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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 July 30, 2016.

Copyright Notice

Copyright (c) 2016 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

0.  NOTE TO RFC EDITOR AND REVIEWERS  . . . . . . . . . . . . . .   1
1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   1
2.  Reaching a definition of a Domain Name  . . . . . . . . . . .   1
3.  Subtypes of Domain Names  . . . . . . . . . . . . . . . . . .   1
4.  Interoperability Considerations   . . . . . . . . . . . . . .   1
5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   1
6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   1
7.  Security Considerations . . . . . . . . . . . . . . . . . . .   1
8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   1
9.  Authors' Addresses  . . . . . . . . . . . . . . . . . . . . .   1


A suitable venue for discussion of this document is the dnsop working
group.  Private comments may also be directed at the editor.

This document will be presented to the Dispatch WG as it relates
to the use of identifiers across protocols, despite perceptions
that the DNS defines Domain Names.

This section (and sub-sections) **probably** should be removed
prior to publication.

0.1 Backstory

Editorial note - I'm not sure whether 0.1 and 0.2 will remain in
the document.  Despite the "probably should" above.

Why bother to define Domain Names now, three decades after the
earliest mentions in RFC documents?  There are many examples of
names as identifiers in existence, a lot of running code.  There is
a large industry built on management of names as well, a lot of
financial investment made.  Would not a definition appearing now be
merely an academic exercise or worse, a disruption to what seems to
be a reliable system?

The tipping point related to addressing the lack of a definition is
the call to apply a special meaning to certain Domain Names, with
special meaning "not managed via the DNS."  The DNS, whose history
and explanation follow, has been created to manage Domain Names and
over time evolved to become the defining force behind Domain Names.
The emergence of other name management systems, in the sense of
"permissionless innovation", have called into question whether DNS
is the defining force for Domain Names.

There are good reasons to call this into question.  The DNS
definition makes some simplifyng assumptions about Domain Names,
carving out some functionality that the newer name management systems
wish to explore.  To foster this innovation, better definitions,
boundaries and even an architectural review of Domain Names is

The beneficiaries of such work include both the developers of
software that makes use of names and identifiers.  A single piece of
code code be used in different nameing environments if the code can
determine how to process a name.  With code reusable across different
environments, another beneficiary are those exploring new means of
identifying objects.

0.2 Goal

The work ahead has the ingredients of a "clarification" - a loose
or poorly worded initial definition, multiple diverse valid
interpretations in use, and a need to converge on a more precise
definition which may cause some issues with backwards compatibility.
Before knowing what a precise definition means, the goal ought to be

Already proposed, including a definition in this document, are
accurate but purely conceptual (or theoretical or mathematical)
definitions that, on the one hand, are pure to the idea but not
helpful to developing code.  Walking from the land of theory into
what is helpful means adding elements to the definition, for better
or worse.

What should the definition address?  There are many environments for
names and identifiers, what is the scope of Domain Names?  How can
one tell if a protocol, service, or application handles identifiers
according to the definition of domain names or not?  Can an
application deviate "slightly" from the definition or is it
all-or-nothing?  Purporting to support Domain Names may appear to
be more of a branding issue then a techinical one, that is, merely
a name and not a clear distinction.

0.3 Benchmark for revising this document

Given the numbering, there will come a time to re-organize the draft,
add in sensible paging, etc., or abandon it altogether.  The point
at which this document ought to "molt" into a more mature document
could be when - there is a clearer sense of necessity for the work,
a vague notion of a goal, and once the "deliverables" from the
document are clearer.  E.g., what is the definition, what is the
scope of the definition, and possible how we apply rules for
resolving names (is it DNS?  is it not DNS?).

1. Introduction

Two or three decades into the history of Domain Names, a popular
notion has taken hold that Domains Names were defined and specified
STD 13, the definition of the Domain Name System (DNS).  The
definitions within RFC 1034 and RFC 1035 have become the
apparently-authoritative source for discussions on what is a Domain

The truth is, RFC 1034 and RFC 1035 do not define Domain Names, those
documents define only how Domain Names are used and processed in the
DNS.  But the way in which those RFCs are written seem to lend to the

Throughout this document the term "Domain Names" is capitalized to
emphasize the concept of the names and DNS is used to describe the
protocol and algorithms described in STD 13, including any applicable
updates, related standards track documents and experimental track

The term domain is a generic term.  And there are many naming
systems in existence.  The use of the term Domain Names in this
document refers to the roughly-defined set of protocols and their
applications use of a naming structure that is prevalent amongst
many protocols defined in IETF RFC documents.

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 sufficent to avoid hand-waving arguments, recognizing the dimishing
return in trying to build a complete roster of uses of Domain Names.
If there are omissions that ought to be included, please send
references for the use case to the author (while this is an Internet
Draft, that is).

1.1 Deconstructing RFC 1034's Introduction

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."

Which 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 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 DNS as it is known today did not invent Domain Names (work on the
Simple Mail Transfer Protocol did) and, for what it's worth, wasn't
the first attempt at an Internet naming system (described in RFC 819
"The Domain Naming Convention for Internet User Applications").

One important phrase to keep in mind is:

"To simplify implementations,"

which appears in both RFC 1034 and RFC 1035 as well as their
predecessors RFC 882 and RFC 883.  This gives credence to the notion
that Domain Names exist beyond the DNS.

1.2 Purpose of the Document

This document has a goal of establishing a definition of Domain Names
for the purpose of continuing efforts to innovate in naming systems.
The path by which this document aims to meet the goal is to describe
in more detail the original definition of Domain Names (which never
seems to have been documented in an RFC) and then to illustrate how
Domain Names have been interpreted in the DNS, SMTP, X.509 Certificate
(PKIX), URNs and other environments.

1.3 Why Write This Now?

RFC 6761 defines "Special Use Domain Names" which has engendered much
confusion of the role of Domain Names with respect to the DNS,
particularly Top-Level Domains, which have come to have special

One of the outcomes from the recent discussion on RFC 6761 [this
section is written informally] is an assumption that client software,
that is an application receiving a string that looks like a name
without any supplimental context, will be able to determine how
to treat the string within the appropriate naming context.  Within
the familiar world of domain names today, certain top-level names
are recognized as special (such as those whose last label is local).
To date there are a limited number of special names, as the number
scales up, there will be more work for client software developers.
(See section 4.1 of this document.)

Note that whether or not the last label takes on this role is
suggested here solely as an example.  Whether it does or not is a
something for "a solution" for which we don't even have the starting
point nailed down.  Hence, it isn't clear 4.1 should be in this

Given a lack of an explicit starting point, meaning a clear definition
of what Domain Name means, this document is striving to provide a

2. Reaching a definition of a Domain Name

Domain Names emerged from the need to build a hierarchy around the
growing number of identified hosts exchanging email.  RFC 788, "SIMPLE
MAIL TRANSFER PROTOCOL", 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."

2.1 Historical Perspective

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.

RFC 799 "Internet Name Domains" 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, domain referred to principally an administrative
domain, such as the initial organizations involved in networks at the

RFC 801 "NCP/TCP TRANSITION PLAN" 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."

RFC 805 "Computer Mail Meeting Notes" 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."

RFC 819 "The Domain Naming Convention for Internet User Applications"
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

RFC 819 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

This walk through history relies solely on the record left behind
inside RFCs.  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 RFC 959 "FILE TRANSFER PROTOCOL", never mentions hosts, domains or
host names.  But no formal definition of Domain Names has been
written and recorded.

2.2 Newly Stated Definition of "Domain Name"

Looking through the early documents, and using the experience of the
past decades, this new definition of Domain Names is stated:

A Domain Name is a sequence of labels concatenated by a designated
separating character.  The Domain Name Space is organized in a strict
hierarchical manner with a recognized root Domain Name.  The
organization follows the rules of tree structure as defined by the
field of graph theory in mathematics [Diestel].

Each label represents a node in a conceptual tree.  The sequence of
labels is concatenated from the deepest node in the tree up to the
root node.  "Fully qualified" refers to a sequence that ends with the
root node.

When considering a fully qualifed name, the first label of the name
is the name of the deepest node in the tree, the last label is the
name of the node is the root.  The top-level label, top-level name,
or top-level domain is the label just before the root (or last)

Excluded from the definition is the appearance or representation of
the labels, the designated separator character's representation, the
ordering of the sequence in appearance, such as left-to-right or
right-to-left, nor the written script nor encoding.  The definition
is purely conceptual.

In RFC 819 "Simple Mail Transfer Protocol", the designated separating
character is the dot ('.') as represented in the ASCII [RFC 20]
[ANSIX34] character set.  This is the earliest application definition
of how it represents Domain Names.

2.2.1 Definition from Lyman Chapin

Included here is an emailed definition from Lyman Chapin, appearing
in the archives of  The definition is in-line
with the one in 2.2 except that it refers to a finite name space due
to length restrictions.

"In graph-theoretic terms, the domain name space constitutes a
labelled directed rooted tree in which the syntax of the label
associated with each vertex other than the unlabelled root is
defined by RFCs 1035, 1123, and 2181. The term "nth level domain
name label" refers to a member of the set of all vertices for which
the path to the root contains n edges. For n=1 the term most often
used is "top level domain name label" or simply "top level domain"
(TLD). A fully qualified domain name is a sequence of labels that
represents a path from the root to a leaf vertex of the domain name
space. The shorter term "domain name" is not formally defined; in
common usage it may be the shorthand equivalent of "fully qualified
domain name" (FQDN) or refer to any non-empty subset of the sequence
of labels formally identified by a fully qualified domain name.

"In this formulation, the term "domain name space" refers to the
complete graph consisting of all possible vertices and edges - not
just those with which a specific meaning has been associated (what
we might call "allocated" labels). It is a finite graph because the
length of the longest possible FQDN is finite. At any point in time,
there is another labelled directed rooted tree - a sub-graph of the
domain name space - containing only vertices that represent
allocated labels."

2.3 Limitation

There are many ways to build a name space, Domain Names are just one
example.  Domain Names are intended to build a name space that can
scale tremendously as opposed to a name space for closed cluster of
involved objects.  Domain Names are used across many protocols
defined inside and outside the IETF and have been defined to
interoperate across implementations and protocols.  This does not
make Domain Names an official or required standard despite the name
space's widespread use.

2.4 Is This a Domain Name?

In the vein of questions like "but is it art?" as to whether an
object is art worthy of display in a museum, one can question
whether any string with a dot in it is a Domain name.  For example,
is this multi-sentence paragraph a domain name?  It has
characters and dots in it.

The important question is not whether a string is an example of a
Domain Name based on its appearence.  The use of a string is what
makes it a Domain Name.  A path name of a file with an extension
looks like a Domain Name with the extension separated by a dot, if one
allows the directory seperating character (a '/' perhaps) as a legal
member of a label.  Within an OS, this is a file/path name.  In a
protocol it might be used as if it were a domain name.

3. Subtypes of Domain Names

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

3.1 Domain Names as Restricted for DNS

The DNS protocol place size restrictions on Domain Names and defines
rules for matching domain names, treating sets of Domain Names as
equivalent to each other.  (This matching refers to treating upper
case and lower case 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 Domain Names 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
Domain Name hierarchy.

DNS defines two formats for domain names.  One 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 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

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 coins 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.

DNS domain names could appear to be the same as address literals, such
as "" or "0:0:0:0:0:FFFF::".  Such DNS domain
names are not used for two reasons.  Applications expecting a
Domain Name (as a comment line parameter as an example) would
opt to treat the string as an address literal and would therefore
not look for the string in the DNS domain name space.  The management
model of the DNS would prefer to aggregate (as in routing) addresses
belonging together in the same zone, resulting in labels appearing in
reverse order.  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" [RFC 3596], section 2.5.

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.

3.2 Host Names

Work on the definition of a host name began well before the issuance
of the STD 13 documents defining DNS.  The rules for the Preferred
Syntax in RFC 1034 conform to the host name rules outlined in RFC
952.  The host name definition was presented again in RFC 1123
"Requirements for Internet Hosts -- Application and Support" (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
RFC 952 for specifics.) RFC 1945 "Hypertext Transfer Protocol --
HTTP/1.0", Section 3.2.2 "http URL" specifically references section
2.1 of RFC 1123.  The reference is explicit.

RFC 5321 " Simple Mail Transfer Protocol" 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."

3.3 URI Authority and Domain Names

In RFC 3986, also known as STD 66, "Uniform Resource Identifier
(URI): Generic Syntax", 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 concept other than the DNS.  There's no
discussion on the rationale but this enables the reuse of code
parsing and marshalling the host subcomponent between different
Domain Name environments.

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.

3.4 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.  In some protocols, these domain names are
specified as being preceded by a "#" (find this and cite) or encased
in square brackets "[" and "]" (SMTP mentioned already).  In the DNS,
as previously described in section 3.1, they are represented
according to appropriate conventions.

3.5 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.

RFC 5890 "Internationalized Domain Names for Applications (IDNA):
Definitions and Document Framework", with associated other documents,
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 sub
sets 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.

Open question - how closely is IDNA tied to DNS domain names as
opposed to Domain Names?  "IAB Thoughts on Encodings for
Internationalized Domain Names" [RFC 6055], discusses the
representations of IDNs.

3.6 Restricted for DNS Registration

RFC 4290 "Suggested Practices for Registration of Internationalized
Domain Names (IDN)" presents reasons why registration of DNS domain
names is restricted, in the context of IDN.  (That RFC refers to an
older form than IDNA2008, but the concepts still apply.)  This is yet
another convention related to DNS domain names, excluding names that
would lead to undesirable outcomes.

3.7 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
network 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.

3.8 X.509

In RFC 5280 "Internet X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", section "Subject
Alternative Name" a dNSName is defined to be a host name, with the
further restriction that the name " " cannot be used.

3.9 Multicast DNS

Multicast DNS uses a name space ending with ".local." as described in
RFC 6762, "Multicast DNS".  The rules for Multicast DNS domain names
differ from DNS domain names.  Multicast DNS domain names are encoded
as Net-Unicode as defined in RFC5198 " Unicode Format for Network
Interchange" 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 is not used.

3.10 /etc/hosts

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

RFC 3493, "Basic Socket Interface Extensions for IPv6", addresses
this in Section 6, further documentation can be found as part of
The Open Group Base Specificatoins Issue 7 [IEEE1003.1] and Microsoft
Winsock Functions [WINSOCK].

3.11 Other Protocols

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

SSH [RFC 4251 "The Secure Shell (SSH) Protocol Architecture"] 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.

FTP [RFC 959 " FILE TRANSFER PROTOCOL (FTP)"] 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 (DNS vs.  MulticastDNS).

DHCP [RFC 2131 "Dynamic Host Configuration Protocol"] includes domain
names in its Domain Search Option [RFC 3397 "Dynamic Host
Configuration Protocol (DHCP) Domain Search Option"].  The encoding of
Domain Names used is the on-the-wire format of the DNS, using
DNS-defined message compression.  DHCP handles Domain Names in other
options such as in RFC 4702 defined "The DHCP Client FQDN Option", in
the same format.  The significance of this is that most other
protocols represent DNS domain names or host names in a human readable
form, DHCP is using the machine-friendly format.

4. Interoperability Considerations

Any single protocol is able to define a format for a conceptual Domain
Name.  Examples given above show that many protocol 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, software often fails to handle error conditions well.
(Is there a citation for that?)

Often times 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 SAC064 "SSAC
Advisory on DNS 'Search List' Processing".   One of the particular
use cases related to this topic is the issuance of search 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 definition of a Fully Qualified Domain Name has two forms.  The
discussion over FQDN involved human-readable names.  The principle
question is whether to require the terminating dot or to assume it
when the end of an input string is hit.  Many protocol clients will
silently add a dot when a user types in a name to a command line.  But
some definitions, such as the one in SAC064 require the terminating
dot to be included before a name is considered to be fully qualified.

The Special Use Domain Names registry defined in RFC 6761 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 [RFC 819] 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.)

4.1(*) Use of Top-Level Domains as Protocol Identifiers

(Would like to have "dns" and "dns<digit>" added to the Special Use
Domain Names registry.  As well as the digits [address literals].)

(*) - I am not sure this belongs in this document.  The idea is what
one can select a name space by the top-level domain (label).  I
believe this would be scope creep for this document.  Leaving it
here for consideration.

5. Acknowledgements

Input received from Andrew Sullivan and Paul Hoffman.  Not to imply
endorsements of the text.

The definition in 2.2.1 was lifted from an email from Lyman Chapin.
The URL for that message is (combine the two lines):

Comments from George Michaelson, Kevin Darcy, Joe Abley, Jim Reid,
Tony Finch, Robert Edmonds, hellekin, Stephane Bortzmeyer, Ray Bellis,
Bob Harold, Alec Muffett, Stuart Cheshire and a growing list of
others I am losing track of.  Not to imply endorsement.

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. References

Many references are in-line throughout the text with titles to ease
comprehension of the prose.  All documents cited are listed here.
Whether there is a normative/informative split will depend what, if
any, track this document is processed.  For now, consider this a
reading list on the topic.

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

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

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

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

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

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

RFC 819    Postel, J., "Computer mail meeting notes", RFC 805, DOI
           10.17487/RFC0805, February 1982,

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

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

RFC 952    Mockapetris, P., "Domain names: Implementation
           specification", RFC 883, DOI 10.17487/RFC0883, November
           1983, <>.

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

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

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

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

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

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

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

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

RFC 3493   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,

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

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

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

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

RFC 4702   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, <>.

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

RFC 5280   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, <>.

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

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

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

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

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

Diestel    Diestel, R., "Graph Theory", Springer-Verlag, Heidelberg
           Graduate Texts in Mathematics, Volume 173 ISBN
           978-3-642-14278-9, July 2010 (2005, 2000, 1997),

SAC064     ICANN Security and Stability Committee, "SSAC Advisory on
           Search List Processing", SAC064, February 2014,

RENDEV     Anonymous, "Tor Rendezvous Specification", Undated,

OHOST      Nick Mathewson, "Special Hostnames in Tor", Undated,

IEEE1003   The Open Group Base Specifications Issue 7, IEEE Std
           1003.1, 2013 Edition, Copyright 2001-2013 The IEEE
           and The Open Group,

WINSOCK    URL only, <

9. Author's Address

   Edward Lewis
   801 17th Street NW
   Suite 401
   Washington DC, 20006 US

Lewis                  Expires July 30, 2016                [Page  1]