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Technical Reports |
| Version | 15.1.0 |
| Editors | Mark Davis ([email protected]), Michel Suignard ([email protected]) |
| Date | 2023-09-05 |
| This Version | https://www.unicode.org/reports/tr46/tr46-31.html |
| Previous Version | https://www.unicode.org/reports/tr46/tr46-29.html |
| Latest Version | https://www.unicode.org/reports/tr46/ |
| Latest Proposed Update | https://www.unicode.org/reports/tr46/proposed.html |
| Revision | 31 |
Client software, such as browsers and emailers, faced a difficult transition from the version of international domain names approved in 2003 (IDNA2003), to the revision approved in 2010 (IDNA2008). The specification in this document has been providing a mechanism that minimizes the impact of this transition for client software, allowing client software to access domains that are valid under either system.
The specification provides two main features: One is a comprehensive mapping to support current user expectations for casing and other variants of domain names. Such a mapping is allowed by IDNA2008. The second is a compatibility mechanism that supports the existing domain names that were allowed under IDNA2003. This second feature was intended to improve client behavior during the transition period.
This document has been reviewed by Unicode members and other interested parties, and has been approved for publication by the Unicode Consortium. This is a stable document and may be used as reference material or cited as a normative reference by other specifications.
A Unicode Technical Standard (UTS) is an independent specification. Conformance to the Unicode Standard does not imply conformance to any UTS.
Please submit corrigenda and other comments with the online reporting form [Feedback]. Related information that is useful in understanding this document is found in the References. For the latest version of the Unicode Standard, see [Unicode]. For a list of current Unicode Technical Reports, see [Reports]. For more information about versions of the Unicode Standard, see [Versions].
One of the great strengths of domain names is universality. The URL http://Apple.com goes to Apple's website from anywhere in the world, using any browser. The email address [email protected] can be used to send email to an editor of this specification from anywhere in the world, using any emailer.
Initially, domain names were restricted to ASCII characters. This was a significant burden on people using other characters. Suppose, for example, that the domain name system had been invented by Greeks, and one could only use Greek characters in URLs. Rather than apple.com, one would have to write something like αππλε.κομ. An English speaker would not only have to be acquainted with Greek characters, but would also have to pick those Greek letters that would correspond to the desired English letters. One would have to guess at the spelling of particular words, because there are not exact matches between scripts.
Most of the world’s population faced this situation until recently, because their languages use non-ASCII characters. A system was introduced in 2003 for internationalized domain names (IDN). This system is called Internationalizing Domain Names for Applications, or IDNA2003 for short. This mechanism supports IDNs by means of a client software transformation into a format known as Punycode. A revision of IDNA was approved in 2010 (IDNA2008). This revision has a number of incompatibilities with IDNA2003.
The incompatibilities forced implementers of client software, such as browsers and emailers, to face difficult choices during the transition period as registries shifted from IDNA2003 to IDNA2008. This document specifies a mechanism that has minimized the impact of this transition for client software, allowing client software to access domains that are valid under either system.
The specification provides two main features. The first is a comprehensive mapping to support current user expectations for casing and other variants of domain names. Such a mapping is allowed by IDNA2008. The second feature is a compatibility mechanism that supports the existing domain names that were allowed under IDNA2003. This second feature is intended to improve client behavior during the transition period. This specification contains both normative and informative material. Only the conformance clauses and the text that they directly or indirectly reference are considered normative.
The series of RFCs collectively known as IDNA2003 [IDNA2003] allows domain names to contain non-ASCII Unicode characters, which includes not only the characters needed for Latin-script languages other than English (such as Å, Ħ, or Þ), but also different scripts, such as Greek, Cyrillic, Tamil, or Korean. An internationalized domain name such as Bücher.de can then be used in an "internationalized" URL, called an IRI, such as http://Bücher.de#titel.
The IDNA mechanism for allowing non-ASCII Unicode characters in domain names involves applying the following steps to each label in the domain name that contains Unicode characters:
For example, typing the IRI http://Bücher.de into the address bar of any modern browser goes to a corresponding site, even though the "ü" is not an ASCII character. This works because the IDN in that IRI resolves to the Punycode string which is actually stored by the DNS for that site. Similarly, when a browser interprets a web page containing a link such as <a href="http://Bücher.de">, the appropriate site is reached. (In this document, phrases such as "a browser interprets" refer to domain names parsed out of IRIs entered in an address bar as well as to those contained in links internal to HTML text.)
In the case of IDN Bücher.de, the Punycode value actually used for the domain names on the wire is xn--bcher-kva.de. The Punycode version is also typically transformed back into Unicode form for display. The resulting display string will be a string which has already been mapped according to the IDNA2003 rules. This example results in a display string for the IRI that has been casefolded to lowercase:
http://Bücher.de → http://xn--bcher-kva.de → http://bücher.de
A major limitation of IDNA2003 is its restriction to the repertoire of characters in Unicode 3.2, which means that some modern languages are excluded or not fully supported. Furthermore, within the constraints of IDNA2003, there is no simple way to extend the repertoire. IDNA2003 also does not make it clear to users of registries exactly which string they are registering for a domain name (between Bücher.de and bücher.de, for example).
In early 2010, a new version of IDNA was approved. Like IDNA2003, this version consists of a collection of RFCs and is called IDNA2008 [IDNA2008]. IDNA2008 is intended to solve the major problems in IDNA2003. It extends the valid repertoire of characters in domain names, and establishes an automatic process for updating to future versions of the Unicode Standard. Furthermore, it defines the concept of a valid domain name clearly, so that registrants understand exactly what domain name string is being registered.
Processing in IDNA2008 is identical to IDNA2003 for many common domain names. Both IDNA2003 and IDNA2008 transform a Unicode domain name in an IRI (like http://öbb.at) to the Punycode version (like http://xn--bb-eka.at). However, IDNA2008 does not maintain strict backward compatibility with IDNA2003. The main differences are:
For more details, see Section 7, IDNA Comparison.
The differences between IDNA2008 and IDNA2003 may cause interoperability and security problems. They affect extremely common characters, such as all uppercase characters, all halfwidth or fullwidth characters (commonly used in Japan, China, and Korea), and certain other characters like the German eszett (U+00DF ß LATIN SMALL LETTER SHARP S) and Greek final sigma (U+03C2 ς GREEK SMALL LETTER FINAL SIGMA).
IDNA2003 requires a mapping phase, which maps ÖBB.at to öbb.at, for example. Mapping typically involves mapping uppercase characters to their lowercase pairs, but it also involves other types of mappings between equivalent characters, such as mapping halfwidth katakana characters to normal katakana characters in Japanese. The mapping phase in IDNA2003 was included to match the insensitivity of ASCII domain names. Users are accustomed to having both CNN.com and cnn.com work identically. They expect domain names with accents to have the same casing behavior, so that ÖBB.at is the same as öbb.at. There are variations similar to case differences in other scripts. The IDNA2003 mapping is based on data specified in the Unicode Standard, Version 3.2; this mapping was later formalized as the Unicode property [NFKC_Casefold].
Note that case-folding generates a stable form of a string that erases functional case-differences. It is not the same as lowercasing. In particular, the lowercase Cherokee characters added in Unicode Version 8.0 are case-folded to their uppercase counterparts.
IDNA2008 does not require a mapping phase, but does permit one (called "Local Mapping" or "Custom Mapping"). For more information on the permitted mappings, see the Protocol document of [IDNA2008], Section 4.2, Permitted Character and Label Validation and Section 5.2, Conversion to Unicode.
The UTS #46 specification defines a mapping consistent with the normative requirements of the IDNA2008 protocol, and which is as compatible as possible with IDNA2003. For client software, this provides behavior that is the most consistent with user expectations about the handling of domain names with existing data—namely, that domain names will map consistently both on clients supporting IDNA2003 and on clients supporting IDNA2008 with the UTS #46 mapping.
There are a few situations where the use of IDNA2008 without compatibility mapping will result in the resolution of IDNs to different IP addresses from in IDNA2003, unless the registry or registrant takes special action. This affects a very small number of characters, but because these characters are very common in particular languages, a significant number of domain names in those languages are affected. This set of characters is referred to as "Deviations" and is shown in Table 1, Deviation Characters, illustrated in the context of IRIs.
Table 1. Deviation Characters
| Char | Example | IDNA2003 Result | IDNA2008 Result |
|---|---|---|---|
| ß 00DF |
href="http://faß.de" | http://fass.de → http://fass.de |
http://faß.de → http://xn--fa-hia.de |
| ς 03C2 |
href="http://βόλος.com" | http://βόλοσ.com → http://xn--nxasmq6b.com |
http://βόλος.com → http://xn--nxasmm1c.com |
| ZWJ 200D |
href="http://ශ්රී.com" | http://ශ්රී.com
→ http://xn--10cl1a0b.com |
http://ශ්රී.com
→ http://xn--10cl1a0b660p.com |
| ZWNJ 200C |
href="http://نامهای.com" | http://نامهای.com
→ http://xn--mgba3gch31f.com |
http://نامهای.com
→ http://xn--mgba3gch31f060k.com |
For more information on the rationale for the occurrence of these Deviations in IDNA2008, see the [IDN FAQ].
The differences in interpretation of Deviation characters result in potential for security exploits. Consider a scenario involving http://www.sparkasse-gießen.de, a German IRI containing an IDN for "Gießen Savings and Loan".
Alice ends up at the phishing site, supplies her bank password, and her money is stolen. While the .DE registar (DENIC) might have a policy about bundling all of the variants of ß together (so that they all have the same owner) it is not required of registries. It is unlikely that all registries will have and enforce such a bundling policy in all such cases.
There are two Deviations of particular concern. IDNA2008 allows the joiner characters (ZWJ and ZWNJ) in labels. By contrast, these are removed by the mapping in IDNA2003. When used in the intended contexts in particular scripts, the joiner characters produce a noticeable change in displayed text. However, when used between any other characters in those scripts, or in any other scripts, they are invisible. For example, when used between the Latin characters "a" and "b" there is no visible different: the sequence "a<ZWJ>b" looks just like "ab".
Because of the visual confusability introduced by the joiner characters, IDNA2008 provides a special category for them called CONTEXTJ, and only permits CONTEXTJ characters in limited contexts: certain sequences of Arabic or Indic characters. However, applications that perform IDNA2008 lookup are not required to check for these contexts, so overall security is dependent on registries having correct implementations. Moreover, the IDNA2008 context restrictions do not catch most cases where distinct domain names have visually confusable appearances because of ZWJ and ZWNJ.
To satisfy user expectations for mapping, and provide maximal compatibility with IDNA2003, this document specifies a mapping for use with IDNA2008. In addition, to transition more smoothly to IDNA2008, this document provides a Unicode algorithm for a standardized processing that allows conformant implementations to minimize the security and interoperability problems caused by the differences between IDNA2003 and IDNA2008. This Unicode IDNA Compatibility Processing is structured according to IDNA2003 principles, but extends those principles to Unicode 5.2 and later. It also incorporates the repertoire extensions provided by IDNA2008.
Where the transitional processing is not needed, UTS #46 can be used purely as a preprocessing (local mapping) for IDNA2008 by claiming conformance specifically to Conformance Clause C3.
By using this Compatibility Processing, a domain name such as ÖBB.at will be mapped to the valid domain name öbb.at, thus matching user expectation for case behavior in domain names. For transitional use, the Compatibility Processing also allows domain names containing symbols and punctuation that were valid in IDNA2003, such as √.com (which has an associated web page). Such domain names containing symbols will gradually disappear as registries shift to IDNA2008.
Implementations may also restrict or flag (in a UI) domain names that include symbols and punctuation. For more information, see Unicode Technical Report # 36, Unicode Security Considerations [UTR36].
Using the Unicode IDNA Compatibility Processing to transform an IDN into a form suitable for DNS lookup is similar to the tactic of "try IDNA2008 then try IDNA2003". However, this approach avoids a potentially problematic dual lookup. It allows browsers and other clients, such as search engines, to have a single processing step, without the burden of maintaining two different implementations and multiple tables. It accounts for a number of edge cases that would cause problems, and provides a stable definition with predictable results.
The Unicode IDNA Compatibility Processing also provides alternate mappings for the Deviation characters. This facilitates the transition from IDNA2003 to IDNA2008. It is up to the registries to decide how to handle the transition, for example, by either bundling or blocking the Deviation characters that they support. In practice, for the deviation characters, the transition is complete. All major implementations have switched to nontransitional processing of the four deviation characters.
The term "registries" includes far more than top-level registries, such as for .de or .com. For example, .blogspot.com has more domain names registered than most top-level registries. There may be different policies in place for a registry and any of its subregistries. Thus millions of registries need to be considered in a transition strategy, not just hundreds.
In lookup software, transitions may be fine-grained: for example, it may be possible to transition to IDNA2008 rules regarding Deviations for .subdomain.com at a given point but not for .com, or vice versa. If .tld bundles or blocks the Deviation characters, then clients could transition Deviations for .tld, but not for (say) .subdomain.tld. Moreover, client software with a UI, such as the address bar in a browser, could provide more options for the transition. A full discussion of such transition strategies is outside of the scope of this document.
During the interim, authors of documents, such as HTML documents, can unambiguously refer to the IDNA2008 interpretation of characters by explicitly using the Punycode form of the domain name label.
There are two slightly different compatibility mechanisms for domain names during a transition and afterward. UTS #46 therefore specifies two specific types of processing: Transitional Processing (Conformance Clause C1) and Nontransitional Processing (Conformance Clause C2). The only difference between them is the handling of the four Deviation characters.
Summarized briefly, UTS #46 builds upon IDNA2008 in three areas:
For a demonstration of differences between IDNA2003, IDNA2008, and the Unicode IDNA Compatibility Processing, see the [DemoIDN]. For more detail on the differences, see Section 7, IDNA Comparison. UTS #46 does not change any of the terms defined in IDNA2008, such as A-Label or U-Label.
Neither the Unicode IDNA Compatibility Processing nor IDNA2008 address security problems associated with confusables (the so-called "paypal.com" problem). IDNA2008 disallows certain symbols and punctuation characters that can be used for spoofing, such as spoofs of the slash character ("/"). However, these are an extremely small fraction of the confusable characters used for spoofing. Moreover, confusable characters themselves account for a small proportion of phishing problems: most are cases like "secure-wellsfargo.com". For more information, see [Bortzmeyer] and the [IDN FAQ]. It is strongly recommended that Unicode Technical Report #36, Unicode Security Considerations [UTR36] and Unicode Technical Standard #39, Unicode Security Mechanisms [UTS39] be consulted for information on dealing with confusables, both for client software and registries. In particular, [UTS39] provides information that can be used to drastically reduce the number of confusables when dealing with international domain names, much beyond what IDNA2008 does. See also the [DemoConf].
IDNA2003 applications customarily display the processed string to the user. This improves security by reducing the opportunity for visual confusability. Thus, for example, the URL http://googIe.com (with a capital I in place of the L) is revealed as http://googie.com.
This specification is primarily targeted at applications doing lookup of IDNs. There is, however, one strong recommendation for registries: do not allow the registration of labels that are invalid according to Nontransitional Processing, and do use bundling or blocking for labels containing confusable characters.
These tactics can be described as follows:
Note: Some implementations outside Unicode use different terminology for these strategies. In particular, in the ICANN Root Zone Label Generation Rules [RZLGR5], the term allocatable variant of X is used for labels that can be bundled with X, and the term blocked variant is used for a mutually exclusive label.
The label that is actually registered and inserted into a registry has always been processed. For example, xn--bcher-kva corresponds to bücher. However, it may be useful for a registry to also ask for "unprocessed" labels, such as Bücher, as part of the registration process, so that they are aware of the registrant's intent. However, such unprocessed labels must be handled carefully:
Sets of code points are defined using properties and the syntax of Unicode Technical Standard #18, Unicode Regular Expressions [UTS18]. For example, the set of combining marks is represented by the syntax \p{gc=M} . Additionally, the "+" indicates the addition of elements to a set, for clarity.
In this document, a label is a substring of a domain name. That substring is bounded on both sides by either the start or the end of the string, or any of the following characters, called label-separators:
Many people use the terms "domain names" and "host names" interchangeably. This document follows [RFC3490] in use of the term "domain name".
A Bidi domain name is a domain name containing at least one character with Bidi_Class R, AL, or AN. See [IDNA2008] RFC 5893, Section 1.4.
The requirements for conformance on implementations of the Unicode IDNA Compatibility Processing algorithm are stated in the following clauses. An implementation can claim conformance to any or all of these clauses independently.
C1 (deprecated). Given a version of Unicode and a Unicode String, a conformant implementation of Transitional Processing shall replicate the results given by applying the Transitional Processing algorithm specified by Section 4, Processing.
C2. Given a version of Unicode and a Unicode String, a conformant implementation of Nontransitional Processing shall replicate the results given by applying the Nontransitional Processing algorithm specified by Section 4, Processing.
C3. Given a version of Unicode and a Unicode String, a conformant implementation of Preprocessing for IDNA2008 shall replicate the results specified by Section 4.4, Preprocessing for IDNA2008.
These specifications are logical ones, designed to be straightforward to describe. An actual implementation is free to use different methods as long the result is the same as that specified by the logical algorithm.
Any conformant implementation may also have tighter validity criteria than those imposed by Section 4.1, Validity Criteria. For example, an application could disallow or warn of domain name labels with certain characteristics, such as:
For more information, see Unicode Technical Report #36, Unicode Security Considerations [UTR36] and Unicode Technical Standard #39, Unicode Security Mechanisms [UTS39].
IDNA2003 provides for a flag, UseSTD3ASCIIRules, that allows for implementations to choose whether or not to abide by the rules in [STD3]. These rules exclude ASCII characters outside the set consisting of A-Z, a-z, 0-9, and U+002D ( - ) HYPHEN-MINUS. For example, some browsers also allow characters such as U+005F ( _ ) LOW LINE (underbar) in domain names, and thus use UseSTD3ASCIIRules=false, plus their own validity checks for the other ASCII characters.
While UseSTD3ASCIIRules=true is strongly recommended, Section 5, IDNA Mapping Table provides data to allow implementations to support UseSTD3ASCIIRules=false for compatibility with IDNA2003 implementations where necessary. The mapping table does this: providing the Status values and Mapping values for both UseSTD3ASCIIRules=true and UseSTD3ASCIIRules=false. Implementations that use UseSTD3ASCIIRules=false will need to apply their own validation to the mapped values as indicated in Section 4.1, Validity Criteria.
The input to Unicode IDNA Compatibility Processing is a prospective domain_name string expressed in Unicode, and a choice of Transitional or Nontransitional Processing. The domain name consists of a sequence of labels with dot separators, such as "Bücher.de". For more information about the composition of a URL, see Section 3.5 of [STD13].
Main Processing Steps
The following steps, performed in order, successively alter the input domain_name string and then output it as a converted Unicode string, plus a flag to indicate whether there was an error. Even if an error occurs, the conversion of the string is performed as much as is possible.
Input
Any input domain_name string that does not record an error has been successfully processed according to this specification. Conversely, if an input domain_name string causes an error, then the processing of the input domain_name string fails. Determining what to do with error input is up to the caller, and not in the scope of this document. The processing is idempotent—reapplying the processing to the output will make no further changes. For examples, see Table 2, Examples of Transitional Processing.
Implementations may make further modifications to the resulting Unicode string when showing it to the user. For example, it is recommended that disallowed characters be replaced by a U+FFFD to make them visible to the user. Similarly, labels that fail processing during step 4 may be marked by the insertion of a U+FFFD or other visual device.
With either Transitional or Nontransitional Processing, sources already in Punycode are validated without mapping. In particular, Punycode containing Deviation characters, such as href="xn--fu-hia.de" (for fuß.de) is not remapped. This provides a mechanism allowing explicit use of Deviation characters even during a transition period.
Each of the following criteria must be satisfied for a non-empty label:
The first 6 criteria are from [IDNA2008], except for the fourth criterion. Criterion #2 in particular is meant to allow for future label extensions beyond just xn--, such as for future versions of IDNA. Some implementations appear to consider such extentions unlikely, and allow labels such as "r3---sn-apo3qvuoxuxbt-j5pe".
Any particular application may have tighter validity criteria, as discussed in Section 3, Conformance.
If UseSTD3ASCIIRules=false, then the validity tests
for ASCII characters are not provided by the table Status values, but
are implementation-dependent. For example, if an implementation
allows the characters
[\u002Da-zA-Z0-9]
and also the underbar ( _ ), then it needs to use the table values
for UseSTD3ASCIIRules=false, and test for any other
ASCII characters as part of its validity criteria. These ASCII
characters may have resulted from a mapping: for example, a
U+005F ( _ ) LOW LINE (underbar) may have originally been a
U+FF3F ( ＿ ) FULLWIDTH LOW LINE.
There are currently no non-ASCII characters with the Status value disallowed_STD3_valid.
In addition, the label should meet the requirements for right-to-left characters specified in the Right-to-Left Scripts document of [IDNA2008], and for the CONTEXTJ requirements in the Protocol document of [IDNA2008]. It is strongly recommended that Unicode Technical Report #36, Unicode Security Considerations [UTR36] and Unicode Technical Standard #39, Unicode Security Mechanisms [UTS39] be consulted for information on dealing with confusables, and for characters that should be excluded from identifiers. Note that the recommended exclusions are a superset of those in [IDNA2008].
The operation corresponding to ToASCII of [RFC3490] is defined by the following steps:
Input
Processing
Implementations are advised to apply additional tests to these labels, such as those described in Unicode Technical Report #36, Unicode Security Considerations [UTR36] and Unicode Technical Standard #39, Unicode Security Mechanisms [UTS39], and take appropriate actions. For example, a label with mixed scripts or confusables may be called out in the UI. Note that the use of Punycode to signal problems may be counter-productive, as described in [UTR36].
The operation corresponding to ToUnicode of [RFC3490] is defined by the following steps:
Input
Processing
Implementations are advised to apply additional tests to these labels, such as those described in Unicode Technical Report #36, Unicode Security Considerations [UTR36] and Unicode Technical Standard #39, Unicode Security Mechanisms [UTS39], and take appropriate actions. For example, a label with mixed scripts or confusables may be called out in the UI. Note that the use of Punycode to signal problems may be counter-productive, as described in [UTR36].
The table specified in Section 5, IDNA Mapping Table may also be used for a pure preprocessing step for IDNA2008, mapping a Unicode string for input directly to the algorithm specified in IDNA2008.
Preprocessing for IDNA2008 is specified as follows:
Apply the Section 4.3, ToUnicode processing to the Unicode string.
Note that this preprocessing allows some characters that are invalid according to IDNA2008. However, the IDNA2008 processing will catch those characters. For example, a Unicode string containing a character listed as DISALLOWED in IDNA2008, such as U+2665 (♥) BLACK HEART SUIT, will pass the preprocessing step without an error, but subsequent application of the IDNA2008 processing will fail with an error, indicating that the string is not a valid IDN according to IDNA2008.
A number of optimizations can be applied to the Unicode IDNA Compatibility Processing. These optimizations can improve performance, reduce table size, make use of existing NFKC transform mechanisms, and so on. For example:
Note that the input domain_name string for the Unicode IDNA
Compatibility Processing must have had all escaped Unicode code
points converted to Unicode code points. For example,
U+5341
( 十 ) CJK UNIFIED IDEOGRAPH-5341 could have been escaped as any of
the following:
Examples are shown in Table 2, Examples of Processing:
Table 2. Examples of Processing
| Input | Map | Normalize | Convert | Validate | Comment |
|---|---|---|---|---|---|
| Bloß.de | bloss.de | = | n/a | ok | Transitional (deprecated): maps uppercase and sharp s |
| bloß.de | = | n/a | ok | Nontransitional: maps uppercase | |
| BLOẞ.de | bloß.de | = | n/a | ok | Maps uppercase |
| xn--blo-7ka.de | = | = | bloß.de | ok | Punycode is not mapped, so ß never changes (whether transitional or not). |
| u¨.com | = | ü.com | n/a | ok | Normalize changes u + umlaut to ü |
| xn--tda.com | = | = | ü.com | ok | Punycode xn--tda changes to ü |
| xn--u-ccb.com | = | = | u¨.com | error | Punycode is not mapped, but is validated. Because u + umlaut is not NFC, it fails. |
| a⒈com | error | error | error | error | The character "⒈" is disallowed, because it would produce a dot when mapped. |
| xn--a-ecp.ru | xn--a-ecp.ru | = | a⒈.ru | error | Punycode xn--a-ecp = a⒈, which fails validation. |
| xn--0.pt | xn--0.pt | = | error | error | Punycode xn--0 is invalid. |
| 日本語。ＪＰ | 日本語.jp | = | n/a | ok | Fullwidth characters are remapped, including 。 |
| ☕.us | = | = | n/a | ok | Post-Unicode 3.2 characters are allowed. |
For each code point in Unicode, the IDNA Mapping Table provides one of the following Status values:
If this Status value is mapped, disallowed_STD3_mapped or deviation, the table also supplies a mapping value for that code point.
A table is provided for each version of Unicode starting with Unicode 5.1, in versioned directories under [IDNA-Table]. Each table for a version of the Unicode Standard will always be backward compatible with previous versions of the table: only characters with the Status value disallowed may change in Status or Mapping value, with the following exception:
Unicode 15.1 also changed the Status of three conditionally-disallowed characters, which is not an exception:
Unlike the IDNA2008 table, this table is designed to be applied to the entire domain name, not just to individual labels. That design provides for the IDNA2003 handling of label separators. In particular, the table is constructed to forbid problematic characters such as U+2488 ( ⒈ ) DIGIT ONE FULL STOP, whose decompositions contain a "dot".
The Unicode IDNA Compatibility Processing is based on the Unicode character mapping property [NFKC_Casefold]. Section 6, Mapping Table Derivation describes the derivation of these tables. Like derived properties in the Unicode Character Database, the description of the derivation is informative. Only the data in IDNA Mapping Table is normative for the application of this specification.
The files use a semicolon-delimited format similar to those in the Unicode Character Database [UAX44]. The field values are listed in Table 2b, Data File Fields:
Table 2b. Data File Fields
| Num | Field | Description |
|---|---|---|
| 0 | Code point(s) | Hex value or range of values. |
| 1 | Status | valid, ignored, mapped, deviation, disallowed, disallowed_STD3_valid, or disallowed_STD3_mapped |
| 2 | Mapping | Hex value(s). Only present if the Status is ignored, mapped, deviation, or disallowed_STD3_mapped. |
| 3 | IDNA2008 Status | There are two values: NV8 and XV8. NV8 is only present if the Status is valid but the character is excluded by IDNA2008 from all domain names for all versions of Unicode. XV8 is present when the character is excluded by IDNA2008 for the current version of Unicode. These are not normative values. |
Example:
0000..002C ; disallowed # NULL..COMMA
002D ; valid # HYPHEN-MINUS
...
0041 ; mapped ; 0061 # LATIN CAPITAL LETTER A ... 00A1..00A7 ; valid ; ; NV8 # INVERTED EXCLAMATION MARK..SECTION SIGN
00AD ; ignored # SOFT HYPHEN ... 00DF ; deviation ; 0073 0073 # LATIN SMALL LETTER SHARP S
...
19DA ; valid ; ; XV8 # 5.2 NEW TAI LUE THAM DIGIT ONE
...
The following describes the derivation of the mapping table. This description has nothing to do with the actual mapping of labels in Section 4, Processing. Instead, this section describes the derivation of the table in Section 5, IDNA Mapping Table. That table is then normatively used for mapping in Section 4, Processing.
The derivation is described as a series of steps. Step 1 defines a base mapping; Steps 2, 3, and 4 define three sets of characters. Step 5 will modify the base mapping or the sets of characters as needed to maintain backward compatiblity. The mapping and sets are all used in Step 6 to produce the mapping and Status values for the table. Step 7 removes characters whose mappings contain characters that are not valid. Each numbered step may have substeps: for example, Step 1 consists of Steps 1.1 through 1.2.
The computation is done twice, once with UseSTD3ASCIIRules=true, and once with UseSTD3ASCIIRules=false. Code points that are disallowed with UseSTD3ASCIIRules=true, but valid or mapped with UseSTD3ASCIIRules=false, are given the special Status values disallowed_STD3_valid and disallowed_STD3_mapped.
If a Unicode property changes in a future version in a way that would affect backward compatibility, a corresponding clause will be added to Step 5 to maintain compatibility. For more information on compatibility, see Section 5, IDNA Mapping Table.
This step specifies a base mapping, which is a mapping from each Unicode code point to sequences of zero or more code points. The value resulting from mapping a particular code point C is called the base mapping value of C. The base mapping value for C may be identical to C.
Unicode 6.3 adds Bidi_Control characters that were not present in Unicode 3.2. To preserve the intent of IDNA2003 in disallowing Bidi_Control characters rather than just ignoring them, Step 1.1.b was added. This step causes Step 6.3 to disallow all Bidi_Control characters.
Step 1.1.b only affects 5 new characters added in Unicode 6.3. It would also impact any new Bidi_Control characters in future versions of the standard.
Step 1.1.c (added in Unicode 15.1) maps the capital sharp s (ẞ) to the small sharp s (ß) rather than to ss because all major implementations have adopted nontransitional processing, which does not map ß to ss as in NFKC_Casefold.
The base valid set is defined by the sequential list of additions and subtractions in Table 3, Base Valid Set. This definition is based on the principles of IDNA2003. When applied to the repertoire of Unicode 3.2 characters, this produces a set which is closely aligned with IDNA2003.
Table 3. Base Valid Set
| Formal Set Notation | Description |
|---|---|
\P{Changes_When_NFKC_Casefolded} |
Start with characters that are equal to their [NFKC_Casefold] value. This criterion
excludes uppercase letters, for example, as well as characters that
are unstable under NFKC normalization, and default ignorable code
points.
Note that according to Perl/Java syntax, \P means the inverse of \p, so these are the characters that do not change when individually mapped according to [NFKC_Casefold]. |
+ \u00DF |
Add LATIN SMALL LETTER SHARP S (ß). |
- \p{c} - \p{z} |
Remove Unassigned, Controls, Private Use, Format, Surrogate, and Whitespace. |
-
\p{Block=Ideographic_Description_Characters} |
Remove ideographic description characters. |
- \u31EF |
Remove IDEOGRAPHIC DESCRIPTION CHARACTER SUBTRACTION. This is an ideographic description character that was added in Unicode 15.1 outside the now-filled Ideographic_Description_Characters block. |
- \p{ascii} + [\u002Da-zA-Z0-9] |
If UseSTD3ASCIIRules = True: Remove disallowed ASCII; '-' is valid. |
+ \p{ascii} - [\u002E] |
If UseSTD3ASCIIRules = False: Add all ASCII except for "." |
Form the base exclusion set in the following way:
For example, for Unicode 5.2 and 6.0, the base exclusion set consists of the list that follows. The subheads (like "Case Changes") are informational, and do not represent the principle for excluding the characters listed under them.
Characters that have a different mapping in IDNA2003 (Step 3.2 above)
Characters that are disallowed in IDNA2003 (Step 3.3 above)
This is the set of characters that deviate between IDNA2003 and IDNA2008.
This set is currently empty. Adjustments to the above sets or base mapping will be made in this section if the steps would cause an already existing character to change Status or mapping under a future version of Unicode, so that backward compatibility is maintained.
For each code point:
After processing all code points in previous steps:
For example, for Unicode 15.1, the set of characters set to disallowed in Step 7 consists of the following:
Note: Characters such as U+2488 ( ⒈ ) DIGIT ONE FULL STOP are disallowed by Step 6.3.
Note: In Unicode versions 15.0 and earlier, with UseSTD3ASCIIRules = True three additional characters were disallowed in this step: U+2260 (≠), U+226E (≮), and U+226F (≯). This was based on their canonical decompositions (NFD) containing characters that are not valid under that setting; that test was unnecessary for IDNA processing.
Table 4, IDNA Comparisons for Unicode 11.0 illustrates the differences between the three specifications in terms of valid character repertoire for Unicode 11.0. It omits the ASCII-repertoire code points, all code points unassigned in Unicode 11.0, as well as control characters, private-use characters, and surrogate code points. It also includes labels separators that are valid or mapped. The table separates the Unicode 3.2 characters from those encoded later, because they have a special status in IDNA2003. It also separates buckets where UTS #46 and IDNA2008 behave the same from those where they behave differently.
Each row in the table defines a bucket of code points that share a pattern of behavior across the three specifications. The columns provide the following information:
* This list of Valid characters for Unicode 4.0 and beyond is calculated as the union of characters with values CONTEXTJ, CONTEXTO, and PVALID under any version of Unicode from Version 5.2 or later. The union of valid characters over versions of Unicode is used for comparison because IDNA2008 does not guarantee backward compatibility over different versions of Unicode.
Table 4. IDNA Comparisons for Unicode 11.0
The table only includes counts up to Unicode 11.0. A detailed online listing of differences is found at [DemoIDNChars] and [DemoIDN]. The implications for confusability can be seen at [DemoConf].
Table 4, IDNA Comparisons for Unicode 11.0 can also be used to categorize implications for implementers.
If any characters are Mapped/Ignored in any specification—Rows (d), (e), (k)—then in the other specifications they are either Mapped/Ignored in precisely the same way, or they are Disallowed. This prevents domain names from being mapped differently on different browsers: either the characters map to the same result, or they do not work. Row (k) is unproblematic in this regard, assuming that registries follow one of the specifications, because characters like U+023A ( Ⱥ ) will not be valid in registered labels.
Note: The transition is complete in practice for the four problematic Deviations in Row (g). All major implementations treat them as Valid in UTS46, just like in IDNA2008.
This presumes that IDNA2008 implementations do not use custom, incompatible mappings: that is, that they do not take advantage of the fact that arbitrary mappings are allowed in IDNA2008, and choose a mapping that is incompatible with IDNA2003 or UTS #46. This pertains to any of Rows (e), (f), (j), (k). If custom mappings were used by any signficant client base, it would result in serious problems for security and interoperability. For more information, see the [IDN_FAQ].
With the exception of the above issues, implementation is straightforward:
A conformance testing file (IdnaTestV2.txt) is provided for each version of Unicode starting with Unicode 6.0, in versioned directories under [IDNA-Table]. It only provides test cases for UseSTD3ASCIIRules=true.
The test file is UTF-8, with certain characters escaped using the \uXXXX or \x{XXXX} convention for readability. The details are in the header of the test file.
To test for conformance to UTS #46, an implementation will perform the toUnicode, toAsciiN, and toAsciiT operations on the source string, then verify the resulting strings and relevant Status values. The details are in the header of the test file.
Implementations may be more strict than the default settings for UTS46. In particular, an implementation conformant to IDNA2008 would disallow the input for lines marked with NV8. Implementations need only record that there is an error: they need not reproduce the precise Status codes (after removing any ignored Status values).
The test format and file name changed in Version 11.0 so that it could express a variety of different combinations of input options that people needed. The new format allows the testing implementation to test for precisely the results of its combination of supported flags, by filtering out Status codes that correspond to an unsupported input flag. The value XV8 was also removed, since it was not very useful in practice.
The following illustrate the differences between the old and new format. The set of examples is not exhaustive, but shows how there is more information available for the same examples.
Old-format sample lines:
T; Faß.de; faß.de; fass.de N; Faß.de; faß.de; xn--fa-hia.de B; Bücher.de; bücher.de; xn--bcher-kva.de B; à\u05D0; [B5 B6]; [B5 B6] B; a。。b; [A4_2]; [A4_2]
New-format sample lines:
Faß.de; faß.de; []; xn--fa-hia.de; ; fass.de; Bücher.de; bücher.de; []; xn--bcher-kva.de; ; ; à\u05D0; àא; [B5 B6]; xn--0ca24w; ; ; a。。b; a..b; [A4_2]; a..b; ; ;
To facilitate comparison between versions of the Unicode Character Database and to highlight the implications for the addition of new characters and changes of character properties, the Unicode Technical Committee has prepared a collection of IDNA Derived Property data files. These data files are permanently posted at [IDNA-Derived].
For each version of the Unicode Standard starting with Unicode 6.1.0, the value of the enumerated IDNA2008_Category property is calculated and listed explicitly in a separate data file. This property matches the "IDNA Derived Property" as defined in RFC 5892 (see [IDNA2008]). The explicit listing is provided as a convenience for implementers. It is the result of performing the exact calculations defined in RFC 5892 concurrent with the release of each version of the Unicode Character Database.
RFC 5892 gives a list of code points for which the derivation is overridden by exceptional values. All known exceptions are applied when a data file is created, but exceptions added in future updates of the IDNA protocol are not applied retroactively.
The format of these IDNA Derived Property data files is modeled closely on that specified in Appendix B.1 of RFC 5892, except that the comment section of each line is not truncated at column 72. For example, excerpted from RFC 5892:
007B..00B6 ; DISALLOWED # LEFT CURLY BRACKET..PILCROW SIGN
00B7 ; CONTEXTO # MIDDLE DOT
00B8..00DE ; DISALLOWED # CEDILLA..LATIN CAPITAL LETTER THORN
00DF..00F6 ; PVALID # LATIN SMALL LETTER SHARP S..LATIN SMALL LETT
Compare the same ranges excerpted from the data files:
007B..00B6 ; DISALLOWED # LEFT CURLY BRACKET..PILCROW SIGN
00B7 ; CONTEXTO # MIDDLE DOT
00B8..00DE ; DISALLOWED # CEDILLA..LATIN CAPITAL LETTER THORN
00DF..00F6 ; PVALID # LATIN SMALL LETTER SHARP S..LATIN SMALL LETTER O WITH DIAERESIS
This close match in format is designed to simplify scripted comparison between these IDNA Derived Property data files posted at unicode.org and other existing calculated listings based on RFC 5892 that have been posted at IANA or elsewhere.
Mark Davis and Michel Suignard authored the bulk of the text of this document, under direction from the Unicode Technical Committee. For their contributions of ideas or text to this specification, the editors thank Julie Allen, Matitiahu Allouche, Peter Constable, Craig Cummings, Martin Dürst, Peter Edberg, Asmus Freytag, Deborah Goldsmith, Laurentiu Iancu, Gervase Markham, Simon Montagu, Lisa Moore, Eric Muller, Simon Sapin, Murray Sargent, Markus Scherer, Jungshik Shin, Shawn Steele, Erik van der Poel, Chris Weber, and Ken Whistler. The specification builds upon [IDNA2008], developed in the IETF Idna-update working group, especially contributions from Matitiahu Allouche, Harald Alvestrand, Vint Cerf, Martin J. Dürst, Lisa Dusseault, Patrik Fältström, Paul Hoffman, Cary Karp, John Klensin, and Peter Resnick, and also upon [IDNA2003], authored by Marc Blanchet, Adam Costello, Patrik Fältström, and Paul Hoffman.
| [Bortzmeyer] | http://www.bortzmeyer.org/idn-et-phishing.html
The most interesting studies cited there (originally from Mike Beltzner of Mozilla) are:
|
| [DemoConf] | https://util.unicode.org/UnicodeJsps/confusables.jsp |
| [DemoIDN] | https://util.unicode.org/UnicodeJsps/idna.jsp |
| [DemoIDNChars] | https://util.unicode.org/UnicodeJsps/list-unicodeset.jsp?a=\p{age%3D3.2}-\p{cn}-\p{cs}-\p{co}&abb=on&g=uts46+idna+idna2008 |
| [IDNA2003] | The IDNA2003 specification is defined by a cluster of IETF RFCs: |
| [IDNA2008] | The IDNA2008 specification is defined by a
cluster of IETF RFCs:
|
| [IDNA-Derived] | https://www.unicode.org/Public/idna2008derived |
| [IDNA-Table] | https://www.unicode.org/Public/idna |
| [IDN-FAQ] | https://www.unicode.org/faq/idn.html |
| [NFKC_Casefold] | The Unicode property specified in [UAX44], and defined by the data in DerivedNormalizationProps.txt (search for "NFKC_Casefold"). |
| [RFC1034] | P. Mockapetris
"Domain names - concepts and facilities", RFC 1034, November 1987. https://www.rfc-editor.org/info/rfc1034 |
| [RFC3454] | P. Hoffman, M. Blanchet.
"Preparation of Internationalized Strings
("stringprep")", RFC 3454, December 2002. https://www.rfc-editor.org/info/rfc3454 |
| [RFC3490] | Faltstrom, P., Hoffman, P.
and A. Costello, "Internationalizing Domain Names in
Applications (IDNA)", RFC 3490, March 2003. https://www.rfc-editor.org/info/rfc3490 |
| [RFC3491] | Hoffman, P. and M. Blanchet,
"Nameprep: A Stringprep Profile for Internationalized Domain
Names (IDN)", RFC 3491, March 2003. https://www.rfc-editor.org/info/rfc3491 |
| [RFC3492] | Costello, A., "Punycode:
A Bootstring encoding of Unicode for Internationalized Domain Names
in Applications (IDNA)", RFC 3492, March 2003. https://www.rfc-editor.org/info/rfc3492 |
| [RZLGR5] | Integration Panel,
"Root Zone Label Generation Rules — LGR-5", 22 May 2022. https://www.icann.org/sites/default/files/lgr/rz-lgr-5-overview-26may22-en.pdf |
| [SafeBrowsing] | http://code.google.com/apis/safebrowsing/ |
| [Stability] | Unicode Consortium Stability
Policies https://www.unicode.org/policies/stability_policy.html |
| [STD3] | Braden, R.,
"Requirements for Internet Hosts -- Communication
Layers", STD 3, RFC 1122, and "Requirements for Internet
Hosts -- Application and Support", STD 3, RFC 1123, October
1989. https://www.rfc-editor.org/info/std3 |
| [STD13] | Mockapetris, P.,
"Domain names - concepts and facilities", STD 13, RFC
1034 and "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. https://www.rfc-editor.org/info/std13 |
| [UAX44] | UAX #44:Unicode
Character Database https://www.unicode.org/reports/tr44/ |
| [Unicode] | The Unicode Standard For the latest version, see: https://www.unicode.org/versions/latest/ |
| [UTR36] | UTR #36: Unicode
Security Considerations https://www.unicode.org/reports/tr36/ |
| [UTS18] | UTS #18: Unicode
Regular Expressions https://www.unicode.org/reports/tr18/ |
| [UTS39] | UTS #39: Unicode
Security Mechanisms https://www.unicode.org/reports/tr39/ |
The following summarizes modifications from the previous published version of this document.
Revision 31
Modifications for previous versions are listed in those respective versions.
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