Internet Engineering Task Force (IETF) J. Klensin
Request for Comments: 5890 August 2010
Obsoletes: 3490
Category: Standards Track
ISSN: 2070-1721
Internationalized Domain Names for Applications (IDNA):
Definitions and Document Framework
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5890.
Klensin Standards Track [Page 1]RFC 5890 IDNA Definitions August 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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RFC 5890 IDNA Definitions August 2010
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. IDNA2008 . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1. Audiences . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2. Normative Language . . . . . . . . . . . . . . . . . . 5
1.2. Road Map 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 . . . . . . . . . . . . . 11
2.3.2.1. IDNA-valid strings, A-label, and U-label . . . . . 11
2.3.2.2. NR-LDH Label . . . . . . . . . . . . . . . . . . . 13
2.3.2.3. Internationalized Domain Name and
Internationalized Label . . . . . . . . . . . . . 13
2.3.2.4. Label Equivalence . . . . . . . . . . . . . . . . 14
2.3.2.5. ACE Prefix . . . . . . . . . . . . . . . . . . . . 14
2.3.2.6. Domain Name Slot . . . . . . . . . . . . . . . . . 14
2.3.3. Order of Characters in Labels . . . . . . . . . . . . 15
2.3.4. 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. U-label Lengths . . . . . . . . . . . . . . . . . . . . . 16
4.3. Local Character Set Issues . . . . . . . . . . . . . . . . 17
4.4. Visually Similar Characters . . . . . . . . . . . . . . . 17
4.5. IDNA Lookup, Registration, and the Base DNS
Specifications . . . . . . . . . . . . . . . . . . . . . . 18
4.6. Legacy IDN Label Strings . . . . . . . . . . . . . . . . . 18
4.7. Security Differences from IDNA2003 . . . . . . . . . . . . 19
4.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 20
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Normative References . . . . . . . . . . . . . . . . . . . 20
6.2. Informative References . . . . . . . . . . . . . . . . . . 21
Klensin Standards Track [Page 3]RFC 5890 IDNA Definitions August 2010
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 [RFC3490] [RFC3491]. For
convenience, that version of IDNA is referred to in these documents
as "IDNA2003". The newer version continues to use the Punycode
algorithm [RFC3492] and ACE (ASCII-compatible encoding) prefix from
that earlier version. The document collection is described in
Section 1.2. 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 making
decisions about:
o what names are permitted in DNS zone files,
o policies related to names and naming, and
o the handling of domain name strings in files and systems, even
with no immediate intention of looking them up.
This document and those documents 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.
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1.1.2. Normative Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Road Map 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, RFC 5894 [RFC5894], 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 (IDNs). It is referred to
informally in other documents in the set as "Rationale". It is
not normative.
o A document, RFC 5891 [RFC5891], 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, RFC 5893 [RFC5893], that specifies special rules
(Bidi) for labels that contain characters that are written from
right to left.
o A specification, RFC 5892 [RFC5892], 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 5.2 [Unicode52] 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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o A document [IDNA2008-Mapping] that discusses the issue of mapping
characters into other characters and that provides guidance for
doing so when that is appropriate. That document, referred to
informally as "Mapping", provides advice; it is not a required
part of IDNA.
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 [Unicode52] is a coded character set containing somewhat over
100,000 characters assigned to code points as of version 5.2. 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 the 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 and 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 The Unicode Standard
[Unicode52]).
2.2. DNS-Related Terminology
When discussing the DNS, this document generally assumes the
terminology used in the DNS specifications [RFC1034] [RFC1035] as
subsequently modified [RFC1123] [RFC2181]. 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".
This is similar to common contemporary usage of that term in other
contexts. Consequently, any DNS zone administration is described as
a "registry", and the terms "registry" and "zone administrator" are
used interchangeably, regardless of the actual administrative
arrangements or level in the DNS tree. More details about that
relationship are included in the Rationale document.
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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" but
is used specifically in this document to refer to the set of naming
rules described in Section 2.3.1 below.
The base DNS specifications [RFC1034] [RFC1035] discuss "domain
names" and "hostnames", but many people use the terms
interchangeably, as do sections of these specifications. Lack of
clarity about that terminology has contributed to confusion about
intent in some cases. These documents generally use the term "domain
name". When they refer to, e.g., hostname 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 complete name convention using a trailing dot 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 bitstring ones
[RFC2673]), but only in certain contexts. The different contexts for
different sets of usable characters are outlined in the next section.
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", including the expanded context.
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. In the first of those figures, the parenthesized numbers
refer to the notes below the figure.
2.3.1. LDH Label
This is the classical label form used, albeit with some additional
restrictions, in hostnames [RFC0952]. Its syntax is identical to
that described as the "preferred name syntax" in Section 3.5 of RFC
1034 [RFC1034] as modified by RFC 1123 [RFC1123]. Briefly, it is a
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RFC 5890 IDNA Definitions August 2010
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
applications. That subset consists of the class of labels that begin
with the prefix "xn--" (case independent), but otherwise conform to
the rules for LDH labels. That subset is called "XN-labels" in this
set of documents. XN-labels are further divided into those whose
remaining characters (after the "xn--") are valid output of the
Punycode algorithm [RFC3492] and those that are not (see below). The
XN-labels that are valid Punycode output 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 octets in length, the portion of an XN-label derived from the
Punycode algorithm is limited to no more than 59 ASCII characters.
Non-Reserved LDH labels are the set of valid LDH labels that do not
have "--" in the third and fourth positions.
A consequence of the restrictions on valid characters in the native
Unicode character form (see U-labels) turns out to be that mixed-case
annotation, of the sort outlined in Appendix A of RFC 3492 [RFC3492],
is never useful. Therefore, since a valid A-label is the result of
Punycode encoding of a U-label, A-labels should be produced only in
lowercase, despite matching other (mixed-case or uppercase) potential
labels in the DNS.
Some strings that are prefixed with "xn--" to form labels may not be
the output of the Punycode algorithm, may fail the other tests
outlined below, or may 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
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RFC 5890 IDNA Definitions August 2010
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.
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RFC 5890 IDNA Definitions August 2010
ASCII Label
__________________________________________________________________
| |
| ____________________ LDH Label (1) (4) ________________ |
| | ___________________________________ | |
| | |IDN Reserved LDH Labels | | |
| | | ("??--") or R-LDH Labels | _______________ | |
| | | _______________________________ | |NON-RESERVED | | |
| | | | XN-labels | | | LDH Labels | | |
| | | | _____________ ___________ | | | (NR-LDH | | |
| | | | | A-labels | | Fake (3) || | | labels) | | |
| | | | | "xn--"(2) | | A-labels || | |_____________| | |
| | | | |___________| |__________|| | | |
| | | |_____________________________| | | |
| | |_________________________________| | |
| |_______________________________________________________| |
| |
| _____________NON-LDH label________ |
| | ______________________ | |
| | | Underscore labels | | |
| | | e.g., _tcp | | |
| | |____________________| | |
| | | Labels with leading| | |
| | | or trailing | | |
| | | hyphens "-abcd" | | |
| | | or "xyz-" | | |
| | | or "-uvw-" | | |
| | |____________________| | |
| | | Labels with other | | |
| | | non-LDH ASCII chars| | |
| | | e.g., #$%_ | | |
| | |____________________| | |
| |________________________________| |
|________________________________________________________________|
(1) ASCII letters (uppercase and lowercase), digits,
hyphen. Hyphen may not appear in first or last
position. No more than 63 octets.
(2) Note that the string following "xn--" must
be the valid output of the Punycode algorithm
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
applications that are not IDNA-aware.
Figure 1: IDNA and Related DNS Terminology Space -- ASCII Labels
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__________________________
| Non-ASCII |
| |
| ___________________ |
| | U-label (5) | |
| |_________________| |
| | | |
| | Binary Label | |
| | (including | |
| | high bit on) | |
| |_________________| |
| | | |
| | Bit String | |
| | Label | |
| |_________________| |
|________________________|
(5) To applications that are not IDNA-aware, 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, the three types of valid labels are
"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. These
documents make specific reference to the form appropriate to any
context in which the distinction is important.
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
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DNS including conformance to the rules for LDH labels
(Section 2.3.1). If and only if a string meeting the above
requirements can be decoded into a U-label is it an A-label.
o A "U-label" is an IDNA-valid string of Unicode characters, in
Normalization Form C (NFC) and including at least one non-ASCII
character, expressed in a standard Unicode Encoding Form (such as
UTF-8). It is also subject to the constraints about permitted
characters that are specified in Section 4.2 of the Protocol
document and the rules in the Sections 2 and 3 of the Tables
document, 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.
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 A1 must be capable of being produced by conversion
from a U-label U1, and that U-label U1 must be capable of being
produced by conversion from A-label A1. 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 apply to
whichever of the U-label or A-label would be more restrictive. There
are two exceptions to this principle. First, the restriction to
ASCII characters does not apply to the U-label. Second, expansion of
the A-label form to a U-label may produce strings that are much
longer than the normal 63 octet DNS limit (potentially up to 252
characters) due to the compression efficiency of the Punycode
algorithm. Such extended-length U-labels are valid from the
standpoint of IDNA, but caution should be exercised as shorter limits
may be imposed by some applications.
For context, applications that are not IDNA-aware treat all LDH
labels as valid for appearance in DNS zone files and queries and some
of them may permit additional types of labels (i.e., not impose the
LDH restriction). 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 protocols that use IDNA forms but that do not involve
the DNS itself.
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Specifically, for IDNA-aware applications and contexts, 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.
Strings that appear to be A-labels or U-labels are processed in
various operations of the Protocol document [RFC5891]. Those strings
are not yet demonstrably conformant with the conditions outlined
above because they are in the process of validation. Such strings
may be referred to as "unvalidated", "putative", or "apparent", or as
being "in the form of" one of the label 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.
2.3.2.2. NR-LDH Label
These specifications use the term "NR-LDH label" strictly to refer to
an all-ASCII label that obeys the LDH label syntax discussed in
Section 2.3.1 and that is neither an IDN nor a label form reserved by
IDNA (R-LDH label). It should be stressed that all A-labels obey the
"hostname" [RFC0952] rules other than the length restriction in those
rules.
2.3.2.3. Internationalized Domain Name and Internationalized Label
An "internationalized domain name" (IDN) is a domain name that
contains at least one A-label or U-label, but that otherwise 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 (the issue is
discussed in more detail in Section 4.3 of the Protocol document
[RFC5891]. 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
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RFC 5890 IDNA Definitions August 2010
assumed to have any knowledge of zone-specific restrictions or
conventions. See the section on registration policy in the Rationale
document [RFC5894] 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 the Protocol document
[RFC5891] for details. Traditional LDH labels already have a notion
of equivalence: within that list of characters, uppercase and
lowercase are considered equivalent. The IDNA notion of equivalence
is an extension of that older notion but, because the protocol does
not specify any mandatory mapping and only those isomorphic forms are
considered, the only equivalents are:
o Exact (bit-string identity) matches between a pair of U-labels.
o Matches between a pair of A-labels, using normal DNS
case-insensitive matching rules.
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 a match between the A-labels using normal DNS case-insensitive
matching rules.
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;
Klensin Standards Track [Page 14]RFC 5890 IDNA Definitions August 2010
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 "<IMG>" 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 "IDNA-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).
Name slots that are not IDNA-aware obviously include 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. 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.
2.3.4. 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 RFC 3492 [RFC3492]. The terms defined above
are preferred as much more clear than the term "Punycode string".
Klensin Standards Track [Page 15]RFC 5890 IDNA Definitions August 2010
3. IANA Considerations
IANA actions for this version of IDNA (IDNA2008) are specified in the
Tables document [RFC5892]. An overview of the relationships among
the various IANA registries appears in the Rationale document
[RFC5894]. 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 4.6), 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 this might
raise and take appropriate measures. The context for this issue is
discussed in more detail in the Rationale document [RFC5894].
In addition to the Security Considerations material that appears in
this document, the Bidi document [RFC5893] contains a discussion of
security issues specific to labels containing characters from scripts
that are normally written right to left.
4.2. U-label Lengths
Labels associated with the DNS have traditionally been limited to 63
octets by the general restrictions in RFC 1035 and by the need to
treat them as a six-bit string length followed by the string in
actual calls to the DNS. That format is used in some other
applications and, in general, that representations of domain names as
dot-separated labels and as length-string pairs have been treated as
interchangeable. Because A-labels (the form actually used in the
DNS) are potentially much more compressed than UTF-8 (and UTF-8 is,
in general, more compressed that UTF-16 or UTF-32), U-labels that
obey all of the relevant symmetry (and other) constraints of these
documents may be quite a bit longer, potentially up to 252 characters
(Unicode code points). A fully-qualified domain name containing
several such labels can obviously also exceed the nominal 255 octet
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RFC 5890 IDNA Definitions August 2010
limit for such names. Application authors using U-labels must exert
due caution to avoid buffer overflow and truncation errors and
attacks in contexts where shorter strings are expected.
4.3. 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.
4.4. Visually Similar Characters
To help prevent confusion between characters that are visually
similar (sometimes called "confusables"), 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 Chinese characters
with Traditional ones that have Simplified forms, or to distinguish
zero and one from uppercase "O" and lowercase "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.
If multiple characters appear in a label and the label consists only
of characters in one script, individual characters that might be
confused with others if compared separately may be unambiguous and
non-confusing. On the other hand, that observation makes labels
containing characters from more than one script (often called "mixed-
script labels") even more risky -- users will tend to see what they
expect to see and context is a powerful reinforcement to perception.
At the same time, while the risks associated with mixed-script labels
are clear, simply prohibiting them will not eliminate problems,
especially where closely related scripts are involved. For example,
there are many strings that are entirely in Greek or Cyrillic scripts
that can be confused with each other or with Latin script strings.
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
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eliminate it. Some specific suggestions about identification and
handling of confusable characters appear in a Unicode Consortium
publication [Unicode-UTR36].
4.5. IDNA Lookup, Registration, and the Base DNS Specifications
The Protocol specification [RFC5891] describes procedures for
registering and looking up labels that are not compatible with the
preferred syntax described in the base DNS specifications (see
Section 2.3.1) 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 hostnames 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 the
Protocol document [RFC5891]. 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
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.
4.6. Legacy IDN Label Strings
The URI Standard [RFC3986] and a number of application specifications
(e.g., SMTP [RFC5321] and HTTP [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 in some contexts by
these specifications). Despite that prohibition, there are a
significant number of files and databases on the Internet in which
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RFC 5890 IDNA Definitions August 2010
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 the Rationale
document [RFC5894]. 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.
4.7. 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 the Rationale document [RFC5894] 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.
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.
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4.8. 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 early draft versions of the Rationale document
[RFC5894]. See the section of this name and the one entitled
"Contributors", in it.
Specific textual suggestions after the extraction process came from
Vint Cerf, Lisa Dusseault, Bill McQuillan, Andrew Sullivan, and Ken
Whistler. Other changes were made in response to more general
comments, lists of concerns or specific errors from participants in
the Working Group and other observers, including Lyman Chapin, James
Mitchell, Subramanian Moonesamy, and Dan Winship.
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.
[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.
Klensin Standards Track [Page 20]RFC 5890 IDNA Definitions August 2010
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms, Revision 31",
September 2009,
<http://www.unicode.org/reports/tr15/tr15-31.html>.
[Unicode52] The Unicode Consortium. The Unicode Standard, Version
5.2.0, defined by: "The Unicode Standard, Version
5.2.0", (Mountain View, CA: The Unicode Consortium,
2009. ISBN 978-1-936213-00-9).
<http://www.unicode.org/versions/Unicode5.2.0/>.
6.2. Informative References
[IDNA2008-Mapping]
Resnick, P. and P. Hoffman, "Mapping Characters in
Internationalized Domain Names for Applications (IDNA)",
Work in Progress, April 2010.
[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.
Klensin Standards Track [Page 21]RFC 5890 IDNA Definitions August 2010
[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.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
Internationalized Domain Names for Applications (IDNA)",
RFC 5893, August 2010.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[Unicode-UTR36]
The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations, Revision 7", July 2008,
<http://www.unicode.org/reports/tr36/tr36-7.html>.
Klensin Standards Track [Page 22]RFC 5890 IDNA Definitions August 2010
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
Klensin Standards Track [Page 23]
