Network Working Group J. Klensin
Internet-Draft March 9, 2009
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: September 10, 2009
Internationalized Domain Names for Applications (IDNA): Definitions and
Document Framework
draft-ietf-idnabis-defs-08.txt
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Abstract
This document is one of a collection that, together, describe the
protocol and usage context for a revision of Internationalized Domain
Names for Applications (IDNA), superseding the earlier version. It
describes the document collection and provides definitions and other
material that are common to the set.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. IDNA2008 . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1. Audiences . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2. Normative Language . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 5
1.3. Roadmap of IDNA2008 Documents . . . . . . . . . . . . . . 5
2. Definitions and Terminology . . . . . . . . . . . . . . . . . 6
2.1. Characters and Character Sets . . . . . . . . . . . . . . 6
2.2. DNS-related Terminology . . . . . . . . . . . . . . . . . 6
2.3. Terminology Specific to IDNA . . . . . . . . . . . . . . . 7
2.3.1. LDH-label . . . . . . . . . . . . . . . . . . . . . . 7
2.3.2. Terms for IDN Label Codings . . . . . . . . . . . . . 10
2.3.2.1. IDNA-valid strings, A-label, and U-label . . . . . 10
2.3.2.2. NR-LDH-label and Internationalized Label . . . . . 13
2.3.2.3. Internationalized Domain Name . . . . . . . . . . 13
2.3.2.4. Label Equivalence . . . . . . . . . . . . . . . . 13
2.3.2.5. ACE Prefix . . . . . . . . . . . . . . . . . . . . 14
2.3.2.6. Domain Name Slot . . . . . . . . . . . . . . . . . 14
2.3.3. Strings Proposed to be Used or Looked Up as Labels . . 15
2.3.4. Order of Characters in Labels . . . . . . . . . . . . 15
2.3.5. Punycode is an Algorithm, not a Name or Adjective . . 15
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
4. Security Considerations . . . . . . . . . . . . . . . . . . . 16
4.1. General Issues . . . . . . . . . . . . . . . . . . . . . . 16
4.2. Local Character Set Issues . . . . . . . . . . . . . . . . 16
4.3. Visually Similar Characters . . . . . . . . . . . . . . . 17
4.4. IDNA Lookup, Registration, and the Base DNS
Specifications . . . . . . . . . . . . . . . . . . . . . . 17
4.5. Legacy IDN Label Strings . . . . . . . . . . . . . . . . . 18
4.6. Security Differences from IDNA2003 . . . . . . . . . . . . 18
4.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 19
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Normative References . . . . . . . . . . . . . . . . . . . 19
6.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 22
A.1. Version -00 . . . . . . . . . . . . . . . . . . . . . . . 22
A.2. Version -01 . . . . . . . . . . . . . . . . . . . . . . . 22
A.3. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 22
A.4. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 22
A.5. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 23
A.6. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 23
A.7. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 23
A.8. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 23
A.9. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
1.1. IDNA2008
This document is one of a collection that, together, describe the
protocol and usage context for a revision of Internationalized Domain
Names for Applications (IDNA) that was largely completed in 2008,
known within the series and elsewhere as IDNA2008. The series
replaces an earlier version of IDNA, described in [RFC3490] and
[RFC3491]. It continues to use the Punycode algorithm [RFC3492] and
ACE (ASCII-compatible encoding) prefix from that earlier version.
The document collection is described in Section 1.3. As indicated
there, this document provides definitions and other material that are
common to the set.
1.1.1. Audiences
While many IETF specifications are directed exclusively to protocol
implementers, the character of IDNA requires that it be understood
and properly used by those whose responsibilities include
o Making decisions about what names are permitted in DNS zone files
o About policies related to names and naming, and
o About the handling of domain name strings in files and systems,
even with no immediate intention of looking them up.
This document and those concerned with the protocol definition, rules
for handling strings that include characters written right-to-left,
and the actual list of characters and categories will be of primary
interest to protocol implementers. This document and the one
containing explanatory material will be of primary interest to
others, although they may have to fill in some details by reference
to other documents in the set.
This document and the associated ones are written from the
perspective of an IDNA-aware user, application, or implementation.
While they may reiterate fundamental DNS rules and requirements for
the convenience of the reader, they make no attempt to be
comprehensive about DNS principles and should not be considered as a
substitute for a thorough understanding of the DNS protocols and
specifications.
1.1.2. Normative Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Discussion Forum
[[ RFC Editor: please remove this section. ]]
IDNA2008 is being discussed in the IETF "idnabis" Working Group and
on the mailing list idna-update@alvestrand.no
1.3. Roadmap of IDNA2008 Documents
IDNA2008 consists of the following documents:
o This document, containing definitions and other material that are
needed for understanding other documents in the set. It is
referred to informally in other documents in the set as "Defs" or
"Definitions".
o A document [IDNA2008-Rationale] that provides an overview of the
protocol and associated tables together with explanatory material
and some rationale for the decisions that led to IDNA2008. That
document also contains advice for registry operations and those
who use internationalized domain names. It is referred to
informally in other documents in the set as "Rationale". It is
not normative.
o A document [IDNA2008-Protocol] that describes the core IDNA2008
protocol and its operations. In combination with the "Bidi"
document described immediately below, it explicitly updates and
replaces RFC 3490. It is referred to informally in other
documents in the set as "Protocol".
o A document [IDNA2008-Bidi] that specifies special rules ("Bidi")
for labels that contain characters that are written from right to
left.
o A specification [IDNA2008-Tables] of the categories and rules that
identify the code points allowed in a label written in native
character form (defined more specifically as a "U-label" in
Section 2.3.2.1 below), based on Unicode 5.1 [Unicode51] code
point assignments and additional rules unique to IDNA2008. The
Unicode-based rules are expected to be stable across Unicode
updates and hence independent of Unicode versions. That
specification obsoletes RFC 3941 and IDN use of the tables to
which it refers. It is referred to informally in other documents
in the set as "Tables".
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2. Definitions and Terminology
2.1. Characters and Character Sets
A code point is an integer value in the codespace of a coded
character set. In Unicode, these are integers from 0 to 0x10FFFF.
Unicode [Unicode51] is a coded character set with about 100,000
characters assigned to code points as of version 5.1. A single
Unicode code point is denoted in these documents by "U+" followed by
four to six hexadecimal digits, while a range of Unicode code points
is denoted by two four to six digit hexadecimal numbers separated by
"..", with no prefixes.
ASCII means US-ASCII [ASCII], a coded character set containing 128
characters associated with code points in the range 0000..007F.
Unicode is a superset of ASCII and may be thought of as a
generalization of it; it includes all the ASCII characters and
associates them with equivalent code points.
"Letters" are, informally, generalizations from the ASCII and common-
sense understanding of that term, i.e., characters that are used to
write text that are not digits, symbols, or punctuation. Formally,
they are characters with a Unicode General Category value starting in
"L" (see Section 4.5 of [Unicode51]).
2.2. DNS-related Terminology
When discussing the DNS, this document generally assumes the
terminology used in the DNS specifications [RFC1034] [RFC1035]. The
term "lookup" is used to describe the combination of operations
performed by the IDNA2008 protocol and those actually performed by a
DNS resolver. The process of placing an entry into the DNS is
referred to as "registration", similar to common contemporary usage
in other contexts. Consequently, any DNS zone administration is
described as a "registry", regardless of the actual administrative
arrangements or level in the DNS tree. More detail about that
relationship is included in the "Rationale" document.
The term "LDH code point" is defined in this document to refer to the
code points associated with ASCII letters (Unicode code points
0041..005A and 0061..007A), digits (0030..0039), and the hyphen-minus
(U+002D). "LDH" is an abbreviation for "letters, digits, hyphen".
The base DNS specifications [RFC1034] [RFC1035] discuss "domain
names" and "host names", but many people use the terms
interchangeably, as do sections of these specifications. Lack of
clarity about that terminology has contributed to confusion about
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intent in some cases. These documents generally use the term "domain
name". When they refer to, e.g., host name syntax restrictions, they
explicitly cite the relevant defining documents. The remaining
definitions in this subsection are essentially a review: if there is
any perceived difference between those definitions and the
definitions in the base DNS documents or those cited below, the
definitions in the other documents take precedence.
A label is an individual component of a domain name. Labels are
usually shown separated by dots; for example, the domain name
"www.example.com" is composed of three labels: "www", "example", and
"com". (The zero-length root label described in RFC 1123 [RFC1123],
which can be explicit as in "www.example.com." or implicit as in
"www.example.com", is not considered in this specification.) IDNA
extends the set of usable characters in labels that are treated as
text (as distinct from the binary string labels discussed in RFC 1035
and RFC 2181 [RFC2181] and the bitstring ones described in RFC 2673
[RFC2673]). For the rest of this document and in the related ones,
the term "label" is shorthand for "text label", and "every label"
means "every text label".
2.3. Terminology Specific to IDNA
This section defines some terminology to reduce dependence on terms
and definitions that have been problematic in the past. The
relationships among these definitions are illustrated in Figure 1 and
Figure 2.
2.3.1. LDH-label
This is the classical label form used in host names [RFC0952] and
described as the preferred form in RFC 1035 [RFC1035]. It is a
string consisting of ASCII letters, digits, and the hyphen with the
further restriction that the hyphen cannot appear at the beginning or
end of the string. Like all DNS labels, its total length must not
exceed 63 octets.
LDH-labels include the specialized labels used by IDNA (described as
"A-labels" below) and some additional restricted forms (also
described below).
To facilitate clear description, two new subsets of LDH-labels are
created by the introduction of IDNA. These are called Reserved LDH
labels (R-LDH labels) and Non-Reserved LDH labels (NR-LDH labels).
Reserved LDH labels, known as "tagged domain names" in some other
contexts, have the property that they contain "--" in the third and
fourth characters but which otherwise conform to LDH-label rules.
Only a subset of the R-LDH labels can be used in IDNA-aware
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applications. That subset consists of the class of labels that begin
with the prefix "xn--" (case independent), but otherwise conform to
the LDH-label rules. That subset is called "XN-labels" in this set
of documents. XN-labels are further divided in those whose remaining
characters (after the "xn--") are valid output of the Punycode
algorithm RFC 3492 [RFC3492]. Such labels are known as "A-labels" if
they also meet the other criteria for IDNA-validity described below.
Because LDH-labels (and, indeed, any DNS label) must not be more than
63 characters in length, the Punycode-derived portion of XN-labels is
limited to no more than 59 characters. Non-reserved LDH labels are
the set of valid LDH labels that do not have "--" in the third and
fourth positions.
Some labels that are prefixed with "xn--" may not be the output of
the Punycode algorithm, or may fail the other tests outlined below or
violate other IDNA restrictions and thus are also not valid IDNA-
labels. They are called FAKE-A Labels for convenience.-
Labels within the class of R-LDH labels that are not prefixed with
"xn--" are also not valid IDNA-labels. To allow for future use of
mechanisms similar to IDNA, those labels MUST NOT be processed as
ordinary LDH-labels by IDNA-conforming programs and SHOULD NOT be
mixed with IDNA-labels in the same zone.
These distinctions among possible LDH labels are only of significance
for software that is "IDNA-aware" or for future extensions that use
extensions based on the same "prefix and encoding" model. For IDNA-
aware systems, the valid label types are: A-labels, U-labels and NR-
LDH labels.
IDNA-labels come in two flavors: An ACE-encoded form and a Unicode
(native character) form. These are referred to as A-labels and
U-labels respectively and are described in detail in the next
section.
[[anchor10: Note in Draft: Figure 1, including all of the notes and
annotation text, is 60-odd lines long, much longer than the 49 text-
content lines permitted by RFCs. Consequently, the xml processor is
breaking it across page boundaries. Once we have agreement that it
is right, I'll try to reformat it again, possibly removing the notes
to text blocks, to make it all fit on a page. Doing that before
things solidify is not just a waste of time but a menace to future
editing. --JcK (Ed.)]]
The figure on this page illustrates the relationships among some of
the terms defined above. The parenthesized numbers refer to the
notes below the figure.
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ASCII-LABEL
----------------------------------------------------------------
| |
| LDH-LABEL (1) (4) |
| _______________________________________________________ |
| | | |
| | | |
| | __________________________________ | |
| | |IDN Reserved LDH Labels | | |
| | | ("??--") or R-LDH LABELS | ______________ | |
| | | | |NON-RESERVED | | |
| | | ------------------------------- | | LDH LABELS | | |
| | | | XN LABELS | | | (NR-LDH- | | |
| | | | _____________ ___________ | | | labels) | | |
| | | | | | | || | |NR-LDH LABELS| | |
| | | | | A-labels | | Fake (3) || | | | | |
| | | | | "xn--"(2) | | A-labels || | |_____________| | |
| | | | |___________| |__________|| | | |
| | | |_____________________________| | | |
| | |_________________________________| | |
| |_______________________________________________________| |
| |
| NON-LDH-LABEL |
| _________________________________ |
| | | |
| | ---------------------- | |
| | | Underscore labels | | |
| | | e.g. _tcp | | |
| | |--------------------| | |
| | | leading | | |
| | | or trailing | | |
| | | hyphens "-abcd" | | |
| | | or "xyz-" | | |
| | | or "-uvw-" | | |
| | |--------------------| | |
| | | Other non-LDH | | |
| | | ASCII chars | | |
| | | e.g. #$%_ | | |
| | ---------------------- | |
| |--------------------------------| |
| |
|________________________________________________________________|
(1) ASCII letters (upper and lower case), digits,
hyphen. Hyphen may not appear in first or last
position. Less than 63 characters.
(2) Note that the string following "xn--" must
be the valid output of the Punycode algorithm
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and must be convertible into valid U-label form.
(3) Note that a Fake A-Label has a prefix "xn--"
but the remainder of the label is NOT the valid
output of the Punycode algorithm.
(4) LDH-LABEL subtypes are indistinguishable to
IDNA-unaware applications.
Figure 1: IDNA and Related DNS Terminology Space -- ASCII labels
__________________________
| Non-ASCII |
| |
| ___________________ |
| | U-label (5) | |
| |_________________| |
| | | |
| | Binary Label | |
| | (including | |
| | high bit on) | |
| |_________________| |
| | | |
| | Bit String | |
| | Label | |
| |_________________| |
|________________________|
(5) To IDNA-unaware applications, U-labels are
indistinguishable from Binary ones.
Figure 2: Non-ASCII labels
2.3.2. Terms for IDN Label Codings
2.3.2.1. IDNA-valid strings, A-label, and U-label
For IDNA-aware applications, valid labels include "A-labels",
"U-labels", and "NR-LDH-labels", each of which is defined below. The
relationships among them are illustrated in Figure 1 and Figure 2.
o A string is "IDNA-valid" if it meets all of the requirements of
these specifications for an IDNA label. IDNA-valid strings may
appear in either of the two forms, defined immediately below, or
may be drawn from the NR-LDH-label subset. IDNA-valid strings
must also conform to all basic DNS requirements for labels and
must, in particular, be less than 63 characters long. These
documents make specific reference to the form appropriate to any
context in which the distinction is important.
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[[anchor12: Note in draft: the text above about conformity to DNS
requirements is somewhat redundant with the paragraph following
the bulleted list. Tentatively keeping both but, if anyone wants
to argue for getting rid of one, this is the time.]]
o An "A-label" is the ASCII-Compatible Encoding (ACE, see
Section 2.3.2.5) form of an IDNA-valid string. It must be a
complete label: IDNA is defined for labels, not for parts of them
and not for complete domain names. This means, by definition,
that every A-label will begin with the IDNA ACE prefix, "xn--"
(see Section 2.3.2.5), followed by a string that is a valid output
of the Punycode algorithm [RFC3492] and hence a maximum of 59
ASCII characters in length. The prefix and string together must
conform to all requirements for a label that can be stored in the
DNS including conformance to the rules for the preferred form
described in RFC 1034, RFC 1035, and RFC 1123. A string meeting
the above requirements is still not an A-label unless it can be
decoded into a U-label.
o A "U-label" is an IDNA-valid string of Unicode characters, in
normalization form NFC and including at least one non-ASCII
character, expressed in a standard Unicode Encoding Form (in an
Internet transmission context this will normally be UTF-8). It is
also subject to the constraints about permitted characters that
are specified in the Protocol and Tables documents, the Bidi
constraints in that document if it contains any character from
scripts that are written right to left, and the symmetry
constraint described immediately below. Conversions between
U-labels and A-labels are performed according to the "Punycode"
specification [RFC3492], adding or removing the ACE prefix as
needed.
[[anchor13: Note in draft: Insert section number references to
Protocol and maybe Tables and Bidi above before handoff to RFC
Editor and warn them to make sure they stay consistent.]]
To be valid, U-labels and A-labels must obey an important symmetry
constraint. While that constraint may be tested in any of several
ways, an A-label must be capable of being produced by conversion from
a U-label and a U-label must be capable of being produced by
conversion from an A-label. Among other things, this implies that
both U-labels and A-labels must be strings in Unicode NFC
[Unicode-UAX15] normalized form. These strings MUST contain only
characters specified elsewhere in this document series, and only in
the contexts indicated as appropriate.
Any rules or conventions that apply to DNS labels in general, such as
rules about lengths of strings, apply to whichever of the U-label or
A-label would be more restrictive. For the U-label, constraints
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imposed by existing protocols and their presentation forms make the
length restriction apply to the length in octets of the UTF-8 form of
those labels (which will always be greater than or equal to the
length in code points). The exception to this, of course, is that
the restriction to ASCII characters does not apply to the U-label.
For context, IDNA-unaware applications treat all LDH-labels as valid
for appearance in DNS zone files and queries. IDNA-aware
applications permit only A-labels and NR-LDH-labels to appear in zone
files and queries. U-labels can appear, along with the other two, in
presentation and user interface forms and in selected protocols other
than those of the DNS itself.
[[anchor14: Is it clear enough that this paragraph is a non-normative
summary, rather than an attempt to specify behavior of non-IDNA-aware
applications???]]
Specifically, for IDNA-aware applications, the three allowed
categories are A-label, U-label, and NR-LDH-label. Of the reserved
LDH labels (R-LDH-labels) only A-labels are valid for IDNA use.
In the operations of [IDNA2008-Protocol] strings are processed that
appear to be A-labels or U-labels --i.e., they appear as input to
operations or in other contexts where A-labels or U-labels would be
expected and are, respectively, ASCII strings starting in "xn--"
(case independent) or strings that contain one or more non-ASCII
characters-- but that are in the process of validation rather than
having been demonstrated to conform to all of the conditions outlined
above. These strings may be referred to as "unvalidated",
"putative", or "apparent", or as being "in the form of" one of the
labels types to indicate that they have not been verified to meet the
specified conformance requirements.
Unvalidated A-labels are known only to be XN Labels, while Fake
A-labels have been demonstrated to fail some of the A-label tests.
Similarly, unvalidated U-labels are simply Non-ASCII labels that may
or may not meet the requirements for U-labels.
[[anchor15: Note in Draft: The two paragraphs immediately above are
part of yet another attempt to make sure that the use of the
terminology is precise and accurate without introducing enough
finely-graduated terms to create their own source of confusion. If
the WG likes the approach (or dislikes it less than the
alternatives), we should review whether it would be helpful to reduce
the number of different circumlocutions used to describe these label
forms. --JcK, Ed.]]
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2.3.2.2. NR-LDH-label and Internationalized Label
These specifications use the term "NR-LDH-label" strictly to refer to
an all-ASCII label that obeys the preferred syntax (often known as
"hostname" (from RFC 952 [RFC0952]) or "LDH") conventions and that is
neither an IDN nor a label form reserved by IDNA (R-LDH-label). It
should be stressed that an A-label obeys the "hostname" rules and is
sometimes described as "LDH-conformant".
2.3.2.3. Internationalized Domain Name
An "internationalized domain name" (IDN) is a domain name that may
contain any mixture of NR-LDH-labels, A-labels, or U-labels. Just as
has been the case with ASCII names, some DNS zone administrators may
impose restrictions, beyond those imposed by DNS or IDNA, on the
characters or strings that may be registered as labels in their
zones. Because of the diversity of characters that can be used in a
U-label and the confusion they might cause, such restrictions are
mandatory for IDN registries and zones even though the particular
restrictions are not part of these specifications. Because these
restrictions, commonly known as "registry restrictions", only affect
what can be registered and not lookup processing, they have no effect
on the syntax or semantics of DNS protocol messages; a query for a
name that matches no records will yield the same response regardless
of the reason why it is not in the zone. Clients issuing queries or
interpreting responses cannot be assumed to have any knowledge of
zone-specific restrictions or conventions. See the section on
registration policy in [IDNA2008-Rationale] for additional
discussion.
"Internationalized label" is used when a term is needed to refer to a
single label of an IDN, i.e., one that might be any of an NR-LDH-
label, A-label, or U-label. There are some standardized DNS label
formats, such as the "underscore labels" used for service location
(SRV) records [RFC2782], that do not fall into any of the three
categories and hence are not internationalized labels.
2.3.2.4. Label Equivalence
In IDNA, equivalence of labels is defined in terms of the A-labels.
If the A-labels are equal in a case-independent comparison, then the
labels are considered equivalent, no matter how they are represented.
Because of the isomorphism of A-labels and U-labels in IDNA2008, it
is possible to compare U-labels directly; see [IDNA2008-Protocol] for
details. Traditional LDH labels already have a notion of
equivalence: within that list of characters, upper case and lower
case are considered equivalent. The IDNA notion of equivalence is an
extension of that older notion but, because there is no mapping, the
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only equivalents are:
o Exact (bit-string identity) matches between a pair of U-labels.
o Exact (bit-string identity) matches between a pair of A-labels
o Equivalence between a U-label and an A-label determined by
translating the U-label form into an A-label form and then testing
for an exact match between the A-labels.
[[anchor18: Note in Draft: The above text was rewritten in -07 to
make it exactly correct for the "no mapping" model. If we start
doing any mapping, we will probably need to revert to the IDNA2003
rules in which only comparison among A-labels can be used to
determine equivalence. The earlier statement about treating
equivalent labels as the same has also been removed. It didn't quite
make sense.]]
2.3.2.5. ACE Prefix
The "ACE prefix" is defined in this document to be a string of ASCII
characters "xn--" that appears at the beginning of every A-label.
"ACE" stands for "ASCII-Compatible Encoding".
2.3.2.6. Domain Name Slot
A "domain name slot" is defined in this document to be a protocol
element or a function argument or a return value (and so on)
explicitly designated for carrying a domain name. Examples of domain
name slots include: the QNAME field of a DNS query; the name argument
of the gethostbyname() or getaddrinfo() standard C library functions;
the part of an email address following the at-sign (@) in the
parameter to the SMTP MAIL or RCPT commands or the "From:" field of
an email message header; and the host portion of the URI in the src
attribute of an HTML ![]()
tag. A string that has the syntax of a
domain name but that appears in general text is not in a domain name
slot. For example, a domain name appearing in the plain text body of
an email message is not occupying a domain name slot.
An "IDN-aware domain name slot" is defined for this set of documents
to be a domain name slot explicitly designated for carrying an
internationalized domain name as defined in this document. The
designation may be static (for example, in the specification of the
protocol or interface) or dynamic (for example, as a result of
negotiation in an interactive session).
An "IDN-unaware domain name slot" is defined for this set of
documents to be any domain name slot that is not an IDN-aware domain
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name slot. Obviously, this includes any domain name slot whose
specification predates IDNA. Note that the requirements of some
protocols that use the DNS for data storage prevent the use of IDNs.
For example, the format required for the underscore labels used by
the service location protocol[RFC2782] precludes representation of a
non-ASCII label in the DNS using A-labels because those SRV-related
labels must start with underscores. Of course, non-ASCII IDN labels
may be part of a domain name that also includes underscore labels.
2.3.3. Strings Proposed to be Used or Looked Up as Labels
Strings are encountered at many places in these specifications that
are expected to be processed as labels of particular types but that
are not yet fully validated to conform to the requirements for the
particular type of label in question. If XYZ is a type of label
(e.g., "A" for A-label or "U" for a U-label), then the term "putative
XYZ-label" is used to refer to such a string before it is fully
validated or tested.
Similarly, terms similar to "a string in the form of an XYZ-label"
are used to refer to a string that appears to obey the syntax for an
XYZ-label on superficial examination. Specifically, a string that
would comply with the LDH syntax except that some characters are non-
ASCII is considered to be in the form of a U-label and one that
starts in "xn--" and is otherwise all-ASCII is considered to be in
the form of an A-label.
2.3.4. Order of Characters in Labels
Because IDN labels may contain characters that are read, and
preferentially displayed, from right to left, there is a potential
ambiguity about which character in a label is "first". For the
purposes of these specifications, labels are considered, and
characters numbered, strictly in the order in which they appear "on
the wire". That order is equivalent to the leftmost character being
treated as first in a label that is read left-to-right and to the
rightmost character being first in a label that is read right-to-
left. The "Bidi" specification contains additional discussion of the
conditions that influence reading order.
2.3.5. Punycode is an Algorithm, not a Name or Adjective
There has been some confusion about whether a "Punycode string" does
or does not include the ACE prefix and about whether it is required
that such strings could have been the output of the ToASCII operation
(see RFC 3490, Section 4 [RFC3490]). This specification discourages
the use of the term "Punycode" to describe anything but the encoding
method and algorithm of [RFC3492]. The terms defined above are
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preferred as much more clear than the term "Punycode string".
3. IANA Considerations
Actions for IANA are specified in other documents in this series
[IDNA2008-Protocol] [IDNA2008-Tables]. An overview of the
relationships among the various IANA registries appears in
[IDNA2008-Rationale]. This document does not specify any actions for
IANA.
4. Security Considerations
4.1. General Issues
Security on the Internet partly relies on the DNS. Thus, any change
to the characteristics of the DNS can change the security of much of
the Internet.
Domain names are used by users to identify and connect to Internet
hosts and other network resources. The security of the Internet is
compromised if a user entering a single internationalized name is
connected to different servers based on different interpretations of
the internationalized domain name. In addition to characters that
are permitted by IDNA2003 and its mapping conventions (See
Section 4.5), the current specification changes the interpretation of
a few characters that were mapped to others in the earlier version;
zone administrators should be aware of the problems that might raise
and take appropriate measures. The context for this issue is
discussed in more detail in [IDNA2008-Rationale]).
In addition to the Security Considerations material that appears in
this document, [IDNA2008-Bidi] contains a discussion of security
issues specific to labels containing characters from scripts that are
normally written right to left.
4.2. Local Character Set Issues
When systems use local character sets other than ASCII and Unicode,
these specifications leave the problem of converting between the
local character set and Unicode up to the application or local
system. If different applications (or different versions of one
application) implement different rules for conversions among coded
character sets, they could interpret the same name differently and
contact different servers. This problem is not solved by security
protocols, such as Transport Layer Security (TLS) [RFC5246], that do
not take local character sets into account.
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4.3. Visually Similar Characters
To help prevent confusion between characters that are visually
similar, it is suggested that implementations provide visual
indications where a domain name contains multiple scripts, especially
when the scripts contain characters that are easily confused
visually, such as an omicron in Greek mixed with Latin text. Such
mechanisms can also be used to show when a name contains a mixture of
simplified and traditional Chinese characters, or to distinguish zero
and one from upper-case "O" and lower-case "L". DNS zone
administrators may impose restrictions (subject to the limitations
identified elsewhere in these documents) that try to minimize
characters that have similar appearance or similar interpretations.
It is worth noting that there are no comprehensive technical
solutions to the problems of confusable characters. One can reduce
the extent of the problems in various ways, but probably never
eliminate it. Some specific suggestions about identification and
handling of confusable characters appear in a Unicode Consortium
publication [Unicode-UTR36].
4.4. IDNA Lookup, Registration, and the Base DNS Specifications
The Protocol specification [IDNA2008-Protocol] describes procedures
for registering and looking up labels that are not compatible with
the preferred syntax described in the base DNS specifications (STD13
[RFC1034] [RFC1035] and Host Requirements [RFC1123]) because they
contain non-ASCII characters. These procedures depend on the use of
a special ASCII-compatible encoding form that contains only
characters permitted in host names by those earlier specifications.
The encoding used is Punycode [RFC3492]. No security issues such as
string length increases or new allowed values are introduced by the
encoding process or the use of these encoded values, apart from those
introduced by the ACE encoding itself.
Domain names (or portions of them) are sometimes compared against a
set of domains to be given special treatment if a match occurs, e.g.,
treated as more privileged than others or blocked in some way. In
such situations, it is especially important that the comparisons be
done properly, as specified in the Requirements section of
[IDNA2008-Protocol]. For labels already in ASCII form, the proper
comparison reduces to the same case-insensitive ASCII comparison that
has always been used for ASCII labels although IDNA-aware
applications are expected to look up only A-labels and NR-LDH-labels,
i.e., to avoid looking up R-LDH-labels that are not A-labels.
The introduction of IDNA meant that any existing labels that start
with the ACE prefix would be construed as A-labels, at least until
they failed one of the relevant tests, whether or not that was the
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intent of the zone administrator or registrant. There is no evidence
that this has caused any practical problems since RFC 3490 was
adopted, but the risk still exists in principle.
4.5. Legacy IDN Label Strings
The URI Standard [RFC3986] and a number of application specifications
(e.g., [RFC5321], [RFC2616]) do not permit non-ASCII labels in DNS
names used with those protocols, i.e., only the A-label form of IDNs
is permitted in those contexts. If only A-labels are used,
differences in interpretation between IDNA2003 and this version arise
only for characters whose interpretation have actually changed (e.g.,
characters, such as ZWJ and ZWNJ, that were mapped to nothing in
IDNA2003 and that are considered legitimate by these specifications).
Despite that prohibition, there are a significant number of files and
databases on the Internet in which domain name strings appear in
native-character form; a subset of those strings use native-character
labels that require IDNA2003 mapping to produce valid A-labels. The
treatment of such labels will vary by types of applications and
application-designer preference: in some situations, warnings to the
user or outright rejection may be appropriate; in others, it may be
preferable to attempt to apply the earlier mappings if lookup
strictly conformant to these specifications fails or even to do
lookups under both sets of rules. This general situation is
discussed in more detail in [IDNA2008-Rationale]. However, in the
absence of care by registries about how strings that could have
different interpretations under IDNA2003 and the current
specification are handled, it is possible that the differences could
be used as a component of name-matching or name-confusion attacks.
Such care is therefore appropriate.
4.6. Security Differences from IDNA2003
The registration and lookup models described in this set of documents
change the mechanisms available for lookup applications to determine
the validity of labels they encounter. In some respects, the ability
to test is strengthened. For example, putative labels that contain
unassigned code points will now be rejected, while IDNA2003 permitted
them (see [IDNA2008-Rationale] for a discussion of the reasons for
this). On the other hand, the protocol specification no longer
assumes that the application that looks up a name will be able to
determine, and apply, information about the protocol version used in
registration. In theory, that may increase risk since the
application will be able to do less pre-lookup validation. In
practice, the protection afforded by that test has been largely
illusory for reasons explained in RFC 4690 [RFC4690] and elsewhere in
these documents.
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Any change to the Stringprep [RFC3454] procedure that is profiled and
used in IDNA2003, or, more broadly, the IETF's model of the use of
internationalized character strings in different protocols, creates
some risk of inadvertent changes to those protocols, invalidating
deployed applications or databases, and so on. But these
specifications do not change Stringprep at all; they merely bypass
it. Because these documents do not depend on Stringprep, the
question of upgrading other protocols that do have that dependency
can be left to experts on those protocols: the IDNA changes and
possible upgrades to security protocols or conventions are
independent issues.
4.7. Summary
No mechanism involving names or identifiers alone can protect against
a wide variety of security threats and attacks that are largely
independent of the naming or identification system. These attacks
include spoofed pages, DNS query trapping and diversion, and so on.
5. Acknowledgments
The initial version of this document was created largely by
extracting text from the "rationale" document [IDNA2008-Rationale].
See the section of this name, and the one entitled "Contributors", in
it.
Specific textual suggestions after the extraction process came from
Vint Cerf, Mark Davis, Bill McQuillan, Andrew Sullivan, and Ken
Whistler and other changes were made in response to more general
comments or lists of concerns from members of the Working Group.
6. References
6.1. Normative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", March 2008,
.
[Unicode51]
The Unicode Consortium, "The Unicode Standard, Version
5.1.0", 2008.
defined by: The Unicode Standard, Version 5.0, Boston, MA,
Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
Unicode 5.1.0
(http://www.unicode.org/versions/Unicode5.1.0/).
6.2. Informative References
[IDNA2008-Bidi]
Alvestrand, H. and C. Karp, "An updated IDNA criterion for
right to left scripts", November 2008, .
[IDNA2008-Protocol]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", February 2009, .
[IDNA2008-Rationale]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", February 2009, .
[IDNA2008-Tables]
Faltstrom, P., "The Unicode Code Points and IDNA",
Tables 2008, .
A version of this document is available in HTML format at
http://stupid.domain.name/idnabis/
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draft-ietf-idnabis-tables-02.html
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
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[Unicode-UTR36]
The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", July 2008,
.
Appendix A. Change Log
[[RFC Editor: Please remove this appendix]]
A.1. Version -00
This document was created by pulling selected material out of
draft-ietf-idnabis-rationale-03 ("Rationale") after a WG consensus
call indicated that the rearrangement was appropriate. Mark Davis
made the major contribution of getting the process started by
identifying particular sections to be moved, even though this draft
does not completely reflect his list.
For Version -00 only, each section is identified with the associated
former section of Rationale-03. Those sections were edited after
incorporation into this document, so "Formerly" should be interpreted
very loosely.
A.2. Version -01
o Typographical errors corrected and some sections slightly renamed
for clarity.
o Other adjustments made to synchronize with current versions of
"Rationale" and "Protocol".
A.3. Version -02
o All back pointers to section numbers in Rationale have been
removed.
o Some definitions clarified. Added one about string order.
o Usual small editorial tuning.
A.4. Version -03
o Additional fine tuning based on discussions during and immediately
before IETF 72.
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A.5. Version -04
o Corrections of text and improvement of definitions based on
discussions after -03 was released.
o Discussion of label comparisons tightened and made more consistent
with Protocol.
o Definitions of categories of labels supplemented with a picture
(Figure 1).
o Explicit text added (Section 2.3.3) to define strings that look
like A-labels or U-labels but are not.
A.6. Version -05
o Consolidated Security Considerations sections, moving material
from Protocol and Rationale here.
A.7. Version -06
o Added pointer to the discussion, in Rationale, of looking up
unassigned code points.
o Clarified relationship to base DNS specifications.
o Made several clarifications suggested by Mark Davis.
o Added a security considerations stub to more explicitly mention
issues with IDNA2003 labels (Section 4.5).
o Rewrote definitions and terminology using suggestions (and
considerable text and revised figures) from Vint Cerf. Relocated
the figures for easier accessibility.
o Small editorial corrections and new copyright material.
A.8. Version -07
o Modified Figure 1 to put an additional box around NR-LDH Labels
(per Andrew Sullivan) and rationalized spelling of "non-reserved".
o Added a temporary note about page breaks and the figures.
o Modified terminology slightly to mention "underscore labels" and
to revise the statements about equivalence.
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A.9. Version -08
o Corrected several typos, at least one of them confusing (NR-LDH-
Label instead or R-LDH-Label).
o Added new text to the discussion of U-labels and A-labels (end of
Section 2.3.2.1) to support the text in Protocol about not-yet-
validated strings. See the note there.
Author's Address
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john+ietf@jck.com
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