Provides methods to convert internationalized domain names (IDNs) between a normal Unicode representation and an ASCII Compatible Encoding (ACE) representation. Internationalized domain names can use characters from the entire range of Unicode, while traditional domain names are restricted to ASCII characters. ACE is an encoding of Unicode strings that uses only ASCII characters and can be used with software (such as the Domain Name System) that only understands traditional domain names.

Internationalized domain names are defined in RFC 3490. RFC 3490 defines two operations: ToASCII and ToUnicode. These 2 operations employ Nameprep algorithm, which is a profile of Stringprep, and Punycode algorithm to convert domain name string back and forth.

The behavior of aforementioned conversion process can be adjusted by various flags:

If the ALLOW_UNASSIGNED flag is used, the domain name string to be converted can contain code points that are unassigned in Unicode 3.2, which is the Unicode version on which IDN conversion is based. If the flag is not used, the presence of such unassigned code points is treated as an error.
If the USE_STD3_ASCII_RULES flag is used, ASCII strings are checked against RFC 1122 and RFC 1123. It is an error if they don't meet the requirements.

These flags can be logically OR'ed together.

The security consideration is important with respect to internationalization domain name support. For example, English domain names may be homographed - maliciously misspelled by substitution of non-Latin letters. Unicode Technical Report #36 discusses security issues of IDN support as well as possible solutions. Applications are responsible for taking adequate security measures when using international domain names.

Since: 1.6