Network Working Group J. Klensin
Internet-Draft June 17, 2003
Expires: December 16, 2003
Registration Restrictions on Internationalized Domain Names -- An
Overview
draft-klensin-reg-guidelines-00.txt
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Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
IETF has introduced standards-track mechanisms to enable the use of
"internationalized", i.e., non-ASCII, names in the DNS and
applications that use it. This has led, in turn, to concerns that
characters with similar meanings or appearance could cause user
confusion and opportunities for deliberate deception and fraud. Part
of this problem can be addressed by limiting, on a per-zone (or
per-registry) basis, the specific characters that can be used to be a
subset of the list allowed by the standard and by creating
"reservations" of labels that might create confusion with those that
are permitted. The model for doing this for languages that use
characters that originated with Chinese has been extensively
developed in another document. This document discusses some of the
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issues in that design and relates them to considerations and
mechanisms that might be appropriate for other languages and scripts,
especially those involving alphabetic characters.
This document is intended to supply a basis for adapting methods
developed for Chinese, Japanese, and Korean to other languages and
scripts. If these adaptations are made carefully and with due
consideratio for local issues, the likelihood of problematic DNS
registrations with be significantly reduced.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The JET Guidelines Model . . . . . . . . . . . . . . . . . . . 3
3. Reserved Names and Label Packages . . . . . . . . . . . . . . 5
4. Languages and Scripts . . . . . . . . . . . . . . . . . . . . 5
5. Reservations and Exclusions . . . . . . . . . . . . . . . . . 7
5.1 Sequence Exclusions for Valid Characters . . . . . . . . . . . 7
5.2 Character Pairing Issues . . . . . . . . . . . . . . . . . . . 7
6. Some Implications of this Approach . . . . . . . . . . . . . . 7
7. Conclusions and Recommendations . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Introduction
Once work on the basic model for encoding non-ASCII strings in the
DNS with IDNA ([1], [2], [3]) was nearing completion, it became clear
that it would be desirable for registries to impose additional
restrictions on the names that could actually be registered (e.g.,
see [6]) as a means of reducing potential confusion among characters
that were similar in some way. These restrictions were, in many
respects, part of a long tradition. For example, while the original
DNS specifications [4] permitted any string of octets to be used in a
DNS label, they also recommended the use of a much more restricted
subset, one that was derived from the much older "hostname" rules [7]
and defined by the "LDH" (for "letter digit hyphen", the three
permitted types of characters) convention. Enforcement of those
restricted rules in registrations was the responsibility of the
registry or domain administrator. They were not embedded in the DNS
protocol itself, although some applications protocols, notably those
concerned with electronic mail, imposed and enforced similar rules.
For non-ASCII names (so-called "internationalized domain names" or
"IDNs"), the problem was more complicated than that which led to the
"LDH" (hostname) rules. In the earlier situation, all protocols,
hosts, and DNS zones used ASCII exclusively in practice, so the LDH
restriction could reasonably be applied uniformly across the
Internet. With the introduction of a very large character repertoire,
and different locations and languages considering different
characters important, the optimal registration restrictions became,
not a global matter, but ones that were different in different areas
and, hence, in different DNS zones.
To date, the best-developed system for handling registration
restrictions for IDNs is the JET Guidelines for Chinese, Japanese,
and Korean [5], the so-called "CJK" languages. That system is
limited to those languages and, in particular, to their common script
base. This document explores the principles behind those guidelines
and some of the issues that might arise in trying to adapt them to
alphabetic languages.
A terminology note: The term "confusion" is used very generically in
this document to cover the entire range from accidental user
misperception of the relationship between characters with some
characteristic in common (typically appearance, sound, or meaning) to
cybersquatting and [other] deliberate fraudulent attempts to exploit
those relationships.
2. The JET Guidelines Model
The JET Guidelines establish several new ideas for DNS registry
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management:
o "Reserved" names that do not appear in zone files
o "Packages" of names that are controlled, as a block, by a single
registrant
o Tables of permitted characters on a per-zone and per-language
basis, potentially with supplemental processing to impose
additional syntactic, semantic, or linguistic rules.
o Potential lists of "variant" characters, which are treated as
more-or-less equivalent to other characters.
In the JET Guideline model, a prospective registrant approaches the
registry for a zone (perhaps through an intermediate registrar) with
a label string --a proposed name to be registered-- and a list of
languages in which that name is to be interpreted. The languages are
defined according to the fairly high-resolution coding of [8] --
Chinese as used on the mainland of the People's Republic of China
("zh-cn") can, at registry option, be coded differently and
represented by a separate table compared to Chinese as used in Taiwan
("zh-tw").
The design of the JET Guideline took one important constraint as a
basis: IDNA was treated as a firm standard. A procedure that
modified some portion of the IDNA functions, or was a variant one
them, was considered a violation of those standards and should not be
encouraged (or, probably, even permitted).
Each registry is expected to construct (or obtain) a table for each
language it considers relevant and appropriate. These tables list,
for the particular zone, the characters permitted for that language.
If a character does not appear as a "valid code point" in that table,
than a name containing it cannot be registered. If multiple
languages are listed for the registration, then the character must
appear in the tables for each of those languages.
The tables may also contain columns that specify alternate or variant
forms of the valid character. If these variants appear, they are
used to synthesize labels that are alternatives to the original one.
These labels are all reserved and can be registered or "activated"
(placed into the DNS) only by the action or request of the original
registrant; some (the "preferred variant labels") are typically
registered automatically. The zone is expected to establish
appropriate policies for situations in which the variant forms of one
label conflict with already-reserved or already-registered labels.
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Most of these concepts were introduced because of concerns about
specific issues with CJK characters, beginning from the requirement
that the use of Simplified Chinese by some registrants and
Traditional Chinese by others not be permitted to create confusion or
opportunities for fraud. While they may be applicable to registry
tables contructed for alphabetic scripts, the transfer should be done
with care, since many analogies are not exact.
Some of the important issues are discussed in the sections that
follow.
3. Reserved Names and Label Packages
A basic assumption of the JET model is that, if the properties of
Unicode [9], [10] and IDNA cause two strings to appear similar enough
to cause confusion, either or both should be registered by the same
party or one of them should become unregisterable. The definition of
"appear similar enough" will differ for different cultures and
circumstances --and hence DNS zones-- but the principle is fairly
general. In the JET model, all of the "variant" strings are
identified, some are placed into the DNS automatically, and others
are simply reserved and can be activated, if at all, only be the
original registrant. Other zones might find other policies
appropriate. For example, a zone might conclude that having similar
strings registered in the DNS was undesirable. If so, the list of
variant labels would be used only to build a list of names that would
not be registerable.
4. Languages and Scripts
Conversations about scripts -- collections of characters associated
with particular languages -- are common when discussing character
sets and codes. But the boundaries between one script and another
are not well-defined. The Unicode Standard [9][10], for example,
does not define them at all, even though it is written in terms of
usually-related blocks of characters. The issue is complicated by
the common origin of most alphabetic scripts (Cf. [11]), with certain
character-symbols appearing in the scripts associated with multiple
languages, sometimes with very different sounds or meanings. This
differs from the CJK situation in which, if a character appears in
more than one of the relevant languages, it will almost always have
the same interpretation in each one and, at least for the subset of
characters that actually are ideographs, pronunciation is expected to
vary widely while meaning is preserved. At least in part because of
that similarity of meaning, it made sense in the JET case to permit a
registration to specfy multiple languages, to verify that the
characters in the label string were valid for each, and then to
generate variant labels using each language in turn. For many
alphabetic languages, it may make sense to prohibit the label string
submitted for registration from being associated with more than one
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language. Indeed, "one label, one language" has been suggested as an
important barrier against common sources of "look-alike" confusion.
For example, the imposition of that rule in a zone would prevent the
insertion of a few Greek or Cyrillic characters with shapes identical
to the Latin ones into what was otherwise a Latin-based string. For
a particular table, the list of valid characters may be thought of as
the script associated with the relevant language, with the
understanding that the table design does not prevent the same
character from appearing in the tables for multiple languages.
Indeed, this notion of a locally, and specifically, identified script
can be turned around: while the tables are referred to as "language
tables", they are associated with languages only insofar as thinking
about the character structure and word forms associated with a given
language helps to inform the construction of a table. A country like
Finland, for example, might use
o One table each for Finnish, Swedish, and English characters and
conventions, permitting a string to be registered in one, two, or
all three languages (although a three-language registration would
presumably prohibit any characters that did not appear in all
three languages).
o One table each, but with a "one label, one language" rule for the
zone.
o A combined table based on the observation that all three writing
systems were based on Roman characters and that the possibilities
for confusion that were of interest to registry would not be
reduced by "language" differentiation.
Regardless of what decisions were ade about those languages and
scripts, if they also decided to permit registrations of labels
containing Cyrillic characters, they might have a separate table for
them. That table might contain some Roman-derived characters (either
as "valid" or as variants) just as some CJK tables do. See also
Section 6, below.
It is also worth stressing, as the JET Guidelines do, that no tables
or systems of this type -- even if identified with languages as a
means of defining or describing those tables -- can assure linguistic
or even syntactic correctness of labels with regard to that language.
That level of assurance may not be possible without human
intervention or at least dictionary lookups of complete proposed
labels. It may even not be desirable to attempt that level of
correctness (see Section 6).
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Of course, if any language-based tests or constraints, including "one
label, one language", are to be applied to limit those sources of
confusion, each zone must have a table for each language in which it
expects to accept registrations; the notion of a single combined
table for the zone is simply unworkable. One could use a single
table for the zone if the intent were to impose only minimal
restrictions, e.g., to force alphabetic and numeric characters only
and exclude symbols and punctuation. That type of restriction might
be useful in eliminating some problems, such as those of unreadable
labels, but would be unlikely to be very helpful with, e.g.,
confusion caused by similar-looking characters.
5. Reservations and Exclusions
5.1 Sequence Exclusions for Valid Characters
The JET Guidelines are based on processing only single characters.
Any processing of pairs or longer sequences of characters are left to
what that document describes as "additional processing" -- procedures
specifically permitted by the Guildlines but defined by a registry in
addition to the variant table processing specified in the Guidelines
themselves. A different zone, with different needs, could use a
modified version of the table structure, or different types of
additional processing, to prohibit, as well as accept, particular
sequences of characters by marking them as invalid. Other
modifications or extensions might be designed to prevent certain
letters from appearing at the beginning or end of labels. The use of
regular expressions in the "valid characters" column might be one
way to implement these types of restrictions.
5.2 Character Pairing Issues
Some character pairings -- the use of a character form (glyph) in one
language and a different form with the same properties in a related
one -- closely approximate the issues with mapping between
Traditional and Simplified Chinese although the history is different.
For example, it might be useful to have "o" with a stroke (U+00F8) as
a variant for "o" with diaeresis above it (U+00F6) (and the
equivalent upper-case pair) in a Swedish table, and vice versa in a
Norwegian one, or to prohibit one of these characters entirely in
each table. Obviously, if the relevant language of registration is
unknown, this type of variant matching cannot be applied in any
sensible way.
6. Some Implications of this Approach
Historically, DNS labels were considered to be arbitrary identifier
strings, without any inherent meaning. Even in ASCII, there was no
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requirement that labels form words. Labels that could not possibly
represent words in any Romance or Germanic language have actually
been quite common. In general, in those languages, words contain at
least one vowel and do not have embedded numbers. The more one moves
toward "language"-based registry restrictions, the less it is going
to be possible to construct labels out of fanciful strings. Such
strings may make very good identifiers, while being terrible
candidates for "words". To take a trivial example using only ASCII
characters, "rtr32w", "rtr32x", and "rtr32z" might be very good DNS
labels for a particular zone and application, but would fail even the
most superficial of tests for valid Engish word forms given the
embedded digits and lack of vowels.
Consequently, registries applying the principles outlined in this
document should be careful not to apply more severe restrictions than
are reasonable and appropriate.
7. Conclusions and Recommendations
Thinking about the implications of the use in DNS labels of the full
range of characters permitted by IDNA has led multiple groups to the
conclusion that some restrictions, on a per-registry or per-zone
basis, are needed to prevent many forms of user confusion about the
actual structure of a name or the word, phrase, or term that it
appears to spell out. It appears that the best way to approach such
restrictions involves drawing from the language and culture of the
community of registrants and users in the relevant zone: if
particular characters are likely to be unintelligible to both of
those groups, it is probably wise to not permit it to be used in
registrations. Registration restrictions can be carried much further
than restricting permitted characters to a selected Unicode subset.
The idea of a reserved "package" of related labels permits
probably-confusing combinations or sets of characters to be bound
together, under the control of a single registrant. While that
registrant might use the package in a way that confused his or her
own users, the possibility of turning potential confusion into a
hostile attack would be considerably reduced.
At the same time, excessive restrictions may make DNS identifiers
less useful for their original, intended, purpose: identifying
particular hosts and similar resources on the network in an orderly
way. Registries creating rules and policies about what can be
registered in particular zones -- whether those are based on the JET
Guidelines or the suggestions in this document-- should balance the
need for restrictions against the need for flexibility in
constructing identifiers.
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8. Security Considerations
Registration of labels in the DNS that contain essentially
unrestricted sequences of arbitrary Unicode characters may introduce
several opportunities for either attacks or simple confusion. Some
of these risks, such as confusion about which character, of several
that look alike), is actually intended, may be associated with the
presentation form of DNS names. Others may be linked to databases
associated with the DNS, e.g., with the difficulty of finding an
entry in a Whois file when it is not clear how to enter, or search
for, the characters that make up a name. This document discusses a
family of restrictions on the names that can be registered that can
be imposed on a DNS zone ("registry") and some possible tools for
implementing restrictions of that sort. No plausible set of
restrictions will eliminate all problems and sources of confusion:
for example, it has often been pointed out that the characters
digit-one ("1") and lower case L ("l") can easily be confused in some
fonts used to display ASCII. But, to the degree to which security
may be aided by sensible risk reduction, these techniques may be
helpful.
9. Acknowledgements
Discussions in the process of developing the JET Guidelines were
vital in developing this document and all of the JET participants are
consequently acknowledged. Attempts to explain some of the issues
there to, and feedback from, Vint Cerf, Wendy Rickard, and members of
the ICANN IDN Committee were also helpful in the thinking leading up
to this document.
An effort by Paul Hoffman to create a generic specification for
registration restrictions of this type helped to inspire this
document, which takes a somewhat different, more language-oriented,
approach.
The opinions expressed here are, of course, the sole responsibility
of the author. Some of those whose ideas are reflected in this
document may disagree with the conclusions the author has drawn from
them.
References
[1] Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 3490, March 2003.
[2] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile
for Internationalized Domain Names (IDN)", RFC 3491, March
2003.
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[3] Costello, A., "Punycode: A Bootstring encoding of Unicode for
Internationalized Domain Names in Applications (IDNA)", RFC
3492, March 2003.
[4] Mockapetris, P., "Domain names - implementation and
specification", RFC 1035, STD 13, November 1987.
[5] Seng, J., Ed. and J. Klensin, Ed., "International Domain Names
Registration and Administration Guidelines for Chinese,
Japanese, and Korean", draft-jseng-idn-admin-03.txt (work in
progress), June 2003.
[6] Internet Engineering Steering Group, IETF, "IESG Statement on
IDN", IESG Statement IDNstatement.txt, February 2003.
[7] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet host
table specification", RFC 952, October 1985.
[8] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.
[9] The Unicode Consortium, "The Unicode Standard--Version 3.0",
January 2000.
[10] The Unicode Consortium, "Unicode Standard Annex #28", March
2002.
[11] Drucker, J., "The Alphabetic Labyrinth: The Letters in History
and Imagination", 1995.
Author's Address
John C Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
USA
Phone: +1 617 491 5735
EMail: john-ietf@jck.com
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