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Internationalized domain name

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Internationalized domain name

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An internationalized domain name (IDN) is an Internet domain name that (potentially) contains non-ASCII characters. Such domain names could contain letters with diacritics, as required by many European languages, or characters from non-Latin scripts such as Arabic or Chinese. However, the standard for domain names does not allow such characters, and much work has gone into finding a way around this, either by changing the standard, or by agreeing on a way to convert internationalized domain names into standard ASCII domain names while preserving the stability of the domain name system.

IDN has, by the standards of the Internet, a long history; it was originally proposed in 1996 (by M. Duerst) and implemented in 1998 (by T.W.Tan et al). After much debate and many competing proposals, a system called Internationalizing Domain Names in Applications (IDNA) was adopted as the chosen standard, and is currently, as of 2005, in the process of being rolled out.

In IDNA, the term internationalized domain name means specifically any domain name consisting only of labels to which the IDNA ToASCII algorithm can be successfully applied. (For the meaning of 'label' and 'ToASCII', see the section ToASCII and ToUnicode below.)

Internationalizing domain names in applications

Internationalizing Domain Names in Applications (IDNA) is a mechanism defined in 2003 for handling internationalized domain names containing non-ASCII characters. Such domain names could not be handled by the existing DNS and name resolver infrastructure. Rather than redesigning the existing DNS infrastructure, it was decided that non-ASCII domain names should be converted to a suitable ASCII-based form by web browsers and other user applications; IDNA specifies how this conversion is to be done.

IDNA was designed for maximum backward compatibility with the existing DNS system, which was designed for use with names using only a subset of the ASCII character set.

An IDNA-enabled application is able to convert between the restricted-ASCII and non-ASCII representations of a domain, using the ASCII form in cases where it is needed (such as for DNS lookup), but being able to present the more readable non-ASCII form to users. Applications that do not support IDNA will not be able to handle domain names with non-ASCII characters, but will still be able to access such domains if given the (usually rather cryptic) ASCII equivalent.

ICANN issued guidelines for the use of IDNA in June 2003, and it was already possible to register .jp domains using this system in July 2003. Several other top-level domain registries started accepting registrations in March 2004.

Mozilla 1.4, Netscape 7.1, Opera 7.11 and Safari are among the first applications to support IDNA. A browser plugin is available for Internet Explorer 6 to provide IDN support. Internet Explorer 7.0 and Windows Vista's URL APIs provide native support for IDN [1].

ToASCII and ToUnicode

The conversions between ASCII and non-ASCII forms of a domain name are accomplished by algorithms called ToASCII and ToUnicode. These algorithms are not applied to the domain name as a whole, but rather to individual labels. For example, if the domain name is, then the labels are www, example and com, and ToASCII or ToUnicode would be applied to each of these three separately.

The details of these two algorithms are complex, and are specified in the RFCs linked at the end of this article. The following gives an overview of their behaviour.

ToASCII leaves unchanged any ASCII label, but will fail if the label is unsuitable for DNS. If given a label containing at least one non-ASCII character, ToASCII will apply the Nameprep algorithm (which converts the label to lowercase and performs other normalization) and will then translate the result to ASCII using Punycode before prepending the 4-character string "xn--". This 4-character string is called the ACE prefix, where ACE means ASCII Compatible Encoding, and is used to distinguish Punycode-encoded labels from ordinary ASCII labels. Note that the ToASCII algorithm can fail in a number of ways; for example, the final string could exceed the 63-character limit for the DNS. A label on which ToASCII fails cannot be used in an internationalized domain name.

ToUnicode reverses the action of ToASCII, stripping off the ACE prefix and applying the Punycode decode algorithm. It does not reverse the Nameprep processing, since that is merely a normalization and is by nature irreversible. Unlike ToASCII, ToUnicode always succeeds, because it simply returns the original string if decoding would fail. In particular, this means that ToUnicode has no effect on a string that does not begin with the ACE prefix.

Example of IDNA encoding

As an example of how IDNA works, suppose the domain to be encoded is (Bcher is German for books, and .ch is the country domain for Switzerland). This has two labels, Bcher and ch. The second label is pure ASCII, and so is left unchanged. The first label is processed by Nameprep to give bcher, and then by Punycode to give bcher-kva, and then has xn-- prepended to give xn--bcher-kva. The final domain suitable for use with the DNS is therefore

ASCII Spoofing and squatting concerns

Because IDN allows websites to use full Unicode names, it also makes it much easier to create a spoofed web site that looks exactly like another, including domain name and security certificate, but in fact is controlled by someone attempting to steal private information. These spoofing attacks potentially open users up to phishing attacks.

These attacks are not due to technical deficiencies in either the Unicode or IDNA specifications, but due to the fact that different characters in different languages can look the same, depending on the font used. For example, Unicode character U+0430, Cyrillic small letter a ("а"), can look identical to Unicode character U+0061, Latin small letter a, ("a") which is the lowercase "a" used in English. Characters that look alike in this way may be termed homonyms, homographs, or (less ambiguously) homoglyphs.

Although a computer may display visually identical or very similar glyphs for two different characters, these differences are still significant to the computer when locating web sites or validating certificates. The user assumes a one-to-one correspondence between the visual appearance of a name and the named entity, but when two names appear identical, this correspondence breaks down.

By contrast, with the old set of a to z, 0 to 9, and the hyphen, there is little in the way of homographs. l and 1 and 0 and o are the closest, and the combination "rn" looks similar to "m" in some fonts; however, most fonts make a noticeable visible distinction between them. Still, this means even in the worst case a site like Google would still only need to register 8 names to protect against the homograph attacks.

On December 2001, two Israeli researchers, Evgeniy Gabrilovich and Alex Gontmakher, published a paper titled "The Homograph Attack",[1] an attack that used Unicode URLs to spoof a website URL. To prove the feasibility of this kind of attack, the researchers successfully registered a variant of the domain name "" which incorporated Russian language characters.

In general, this kind of attack is known as a homograph spoofing attack. This problem was anticipated before IDN was introduced, and guidelines were issued to registries to try and avoid or reduce the problem -- for example, recommending that registries only accept the Latin alphabet and that of their own country, not all of Unicode. Unfortunately this advice was not followed by those in control of a number of major TLDs.

On February 7, 2005, Slashdot reported that this exploit was disclosed at the hacker conference Shmoocon with an example available at On browsers supporting IDNA, the URL "http://www.pа" (where the first a is replaced by a Cyrillic а) appears to lead to but instead lead to a spoofed PayPal web site that said "Meeow."

Internet Explorer 7 imposes restrictions on displaying non-ASCII domain names based on a user-defined list of allowed languages and provide an anti-phishing filter that checks suspicious Web sites against a remote database of known phishing sites.

Since Internet Explorer prior to version 7 does not support IDNs, it is not vulnerable to this kind of attacks. However, older versions of Internet Explorer can be made IDN-compatible by browser plug-ins some of which are vulnerable to the spoofing attacks. On July 9, 2005, the IDN-enabling plug-in Quero Toolbar 2.1.0 was released that implemented several anti-spoofing techniques like mixed-script detection and highlighting of characters belonging to different scripts.

On February 17, 2005, Mozilla developers announced that they would ship their next versions of their software with IDN support still enabled, but showing the punycode URLs instead, thus thwarting any attacks exploiting similarities between ASCII and non-ASCII letters (but not necessarily, for example, between Cyrillic and Greek letters, unless the user knows which Punycode URL corresponds to their chosen IDN URL) while still allowing people to access websites on an IDN domain. This is a change from the earlier plans to disable IDN entirely for the time being. [2]

Since then, both Mozilla and Opera have now announced that they will be using per-domain whitelists to selectively switch on IDN display for domain run by registries which are taking appropriate anti-spoofing precautions[3]. (See the article on homograph spoofing attacks for more details). As of September 9, 2005, the most recent version of Mozilla Firefox as well as the most recent Internet Explorer displays the spoofed Paypal URL as "", unsightly but clearly different from the actual

Safari's approach is to render problematic character sets as punycode. This can be changed by altering the settings in Safari's system preference files.

History of IDN

  • 12/96: Martin Duerst's original Internet Draft proposing UTF5 (the first incarnation of what is known today as ACE) - UTF-5 was first defined by Martin Duerst at the University of Zrich in [4][5][6]
  • 03/98: Early Research on IDN at National University of Singapore (NUS), Center for Internet Research (formerly Internet Research and Development Unit - IRDU) led by Prof. Tan Tin Wee (IDN Project team - Lim Juay Kwang and Leong Kok Yong) and subsequently continued under a team at Bioinformatrix Pte. Ltd. (BIX Pte. Ltd.) - a NUS spin-off company led by Prof. S. Subbiah.
  • July 98: Geneva INET'98 conference with a BoF discussion on iDNS and APNG General Meeting and Working Group meeting.
  • 07/98: Asia Pacific Networking Group (APNG, now still in existence [7] and distinct from a gathering known as APSTAR [8]) iDNS Working Group formed. [9]
  • 10/98: James Seng was recruited to lead further IDN development at BIX Pte. Ltd. by Prof. S. Subbiah.
  • 02/99: iDNS Testbed launched by BIX Pte. Ltd. under the auspicies of APNG with participation from CNNIC, JPNIC, KRNIC, TWNIC, THNIC, HKNIC and SGNIC led by James Seng [10]
  • 02/99: Presentation of Report on IDN at Joint APNG-APTLD meeting, at APRICOT'99
  • 03/99: Endorsement of the IDN Report at APNG General Meeting 1 March 1999.
  • 06/99: Grant application by APNG jointly with the Centre for Internet Research (CIR), National University of Singapore, to the International Development Research Center (IDRC), a Canadian Government funded international organisation to work on IDN for IPv6. This APNG Project was funded under the Pan Asia R&D Grant administered on behalf of IDRC by the Canadian Committee on Occupational Health and Safety (CCOHS). Principal Investigator: Tan Tin Wee of National University of Singapore. [11]
  • 07/99 Tout, Walid R. (WALID Inc.) Filed IDNA patent application number US1999000358043 Method and system for internationalizing domain names. Published 2001-01-30 [12]
  • 07/99: [13]; Renewed 2000 [14] Internet Draft on UTF5 by James Seng, Martin Duerst and Tan Tin Wee.
  • 08/99: APTLD and APNG forms a working group to look into IDN issues chaired by Kilnam Chon. [15]
  • 10/99: BIX Pte. Ltd. and National University of Singapore together with New York Venture Capital investors, General Atlantic Partners, spun-off the IDN effort into 2 new Singapore companies - International Inc. and Pte. Ltd. that created the first commercial implementation of an IDN Solution for both domain names and IDN email addresses respectively.
  • 11/99: IETF IDN Birds-of-Feather in Washington was initiated by at the request of IETF officials.
  • 12/99: InternationalPte. Ltd. launched the first commercial IDN. It was in Taiwan and in Chinese characters under the top-level IDN TLD ".gongsi" (meaning loosely ".com") with endorsement by the Minister of Communications of Taiwan and some major Taiwanese ISPs with reports of over 200 000 names sold in a week in Taiwan, Hong Kong, Singapore, Malaysia, China, Australia and USA. Requires use of either plug-in or special DNS hacks.
  • Late 1999: Kilnam Chon initiates Task Force on IDNS which led to formation of MINC, the Multilingual Internet Names Consortium. [16]
  • 01/2000: IETF IDN Working Group formed chaired by James Seng and Marc Blanchet
  • 01/2000: The second ever commercial IDN launch was IDN TLDs in the Tamil Language, corresponding to .com, .net, .org, and .edu. These were launched in India with IT Ministry support by International. Requires use of either plug-in or special DNS hacks.
  • 02/2000: Multilingual Internet Names Consortium(MINC) Proposal BoF at IETF Adelaide. [17]
  • 03/2000: APRICOT 2000 Multilingual DNS session [18]
  • 04/2000: WALID Inc. (with IDNA patent pending application 6182148) started Registration & Resolving Multilingual Domain Names.
  • 05/2000: Interoperability Testing WG, MINC meeting. San Francisco, chaired by Bill Manning and Y.Yoneya 12 May 2000. [19]
  • 06/2000: Inaugural Launch of the Multilingual Internet Names Consortium (MINC) in Seoul [20] to drive the collaborative roll-out of IDN starting from the Asia Pacific. [21]
  • 07/2000: Joint Engineering TaskForce (JET) initiated in Yokohama to study technical issues led by JPNIC (K.Konishi)
  • 07/2000: Official Formation of CDNC Chinese Domain Name Consortium to resolve issues related to and to deploy Han Character domain names, founded by CNNIC, TWNIC, HKNIC and MONIC in May 2000. [22] [23]
  • 03/01: ICANN Board IDN Working Group formed
  • 07/01: Japanese Domain Name Association : JDNA Lauch Ceremony (July 13, 2001) in Tokyo, Japan.
  • 07/01: Urdu Internet Names System (July 28, 2001) in Islamabad, Pakistan, Organised Jointly by SDNP and MINC. [24]
  • 07/01: Presentation on IDN to the Committee Meeting of the Computer Science and Telecommunications Board, National Academies USA (JULY 11-13, 2001) at University of California School of Information Management and Systems, Berkeley, CA. [25]
  • 08/01: MINC presentation and outreach at the Asia Pacific Advanced Network annual conference, Penang, Malaysia 20th August 2001
  • 10/01: Joint MINC-CDNC Meeting in Beijing 18-20 October 2001
  • 11/01: ICANN IDN Committee formed
  • 12/01: Joint ITU-WIPO Symposium on Multilingual Domain Names organised in association with MINC, 6-7 Dec 2001, International Conference Center, Geneva.
  • 01/03: Free implementation of StringPrep, Punycode, and IDNA release in GNU Libidn.
  • 03/03: Publication of RFC 3454, RFC 3490, RFC 3491 and RFC 3492
  • 06/03: Publication of ICANN IDN Guidelines for registries Adopted by .cn, .info, .jp, .org, and .tw registries.
  • 05/04: Publication of RFC 3743, Joint Engineering Team (JET) Guidelines for Internationalized Domain Names (IDN) Registration and Administration for Chinese, Japanese, and Korean
  • 03/05: First Study Group 17 of ITU-T meeting on Internationalized Domain Names [26]
  • 05/05: .IN ccTLD (India) creates expert IDN Working Group to create solutions for 22 official languages
  • 04/06: ITU Study Group 17 meeting in Korea gave final approval to the Question on Internationalized Domain Names [27]
  • 06/06: Workshop on IDN at ICANN meeting at Marrakech, Morocco
  • 11/06: ICANN GNSO IDN Working Group created to discuss policy implications of IDN TLDs. Ram Mohan elected Chair of the IDN Working Group.
  • 12/06: ICANN meeting at So Paulo discusses status of lab tests of IDNs within the root.
  • 01/07: Tamil and Malayalam variant table work completed by India's C-DAC and Afilias
  • 03/07: ICANN GNSO IDN Working Group completes work, Ram Mohan presents report at ICANN Lisboa meeting. [28]

DNS registries known to have adopted IDNA

  • .ac: see details
  • .ae
  • .at: see details
  • .biz: NeuLevel/NeuStar supports Chinese, Danish, German, Icelandic, Japanese, Norwegian, Spanish, Swedish IDN in .biz, see details
  • .br: (May 9, 2005) for Portuguese names, see details
  • .cat: (February 14, 2006) for Catalan names, see details
  • .com: see details
  • .ch: (March 1, 2004)
  • .cl: (September 21, 2005), see details
  • .cn: see CNNIC's web site
  • .de: (March 1, 2004), see details
  • .dk: (January 1, 2004), (, , , , , , & ), see details
  • .fi: (September 1, 2005), see details
  • .gr: (July 4, 2005) for Greek names, see details
  • .hk: (March 8, 2007) for Chinese characters, see details
  • .hu
  • .info: (March 19, 2004) see details
  • .io: see details
  • .ir: see more details
  • .is: (July 1, 2004) see details
  • .jp: (July 2003), for Japanese characters (Kanji, hiragana & katakana)
  • .kr: (August 2003), for Korean characters
  • .li: (March 1, 2004)
  • .lt: (March 30, 2003), (ą, č, ę, ė, į, , ų, ū, ), see details
  • .lv: (2004), see details
  • .museum: (January 20, 2004), see details
  • .net: see details
  • .no: (February 9, 2004), see details
  • .nu: see details
  • .org: (January 18, 2005), see details
  • .pl: (September 11, 2003), see details
  • .pt: (July 1, 2005) for Portuguese characters
  • .se: (October 2003), for Swedish characters
  • .sh: see details
  • .tm: see details
  • .tr: (November 14, 2006), see website for details
  • .tw: Traditional Chinese characters, see TWNIC's site
  • .vn: Vietnamese, see character list

Non-IDNA or non-ICANN registries that support non-ASCII domain names

There are other registries that support non-ASCII domain names; a Singapore company called I-DNS, also proposes via an own registrar network generic domain name registrations in various languages, but the country codes at the end of the domain names are also transcripted into the same characters as the domain names. The company in Thailand supports .com registrations via its own modified DNS, ThaiURL.

Because these companies, and other organizations that offer modified DNS systems, do not subject themselves to ICANN's control, they must be regarded as alternate DNS roots. Domains registered with them will therefore not be supported by most Internet Service Providers, and as a result most users will not be able to look up such domains without manually configuring their computers to use the alternate DNS.

At ICANN's December meeting at So Paulo, IDNs were discussed in depth. ICANN has continued lab tests of IDNs within the root to implement the true IDN top level domains (IDN.IDN).


  1. ^

2. GNSO IDN Working Group Outcomes Paper:

External links

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This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

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