idn

idn [OPTION]... [HOST]
idn {--punycode-decode | --punycode-encode} [STRING]

-a, --to-ascii
    Converts input domain names to their Punycode (ASCII) representation.

-d, --to-unicode
    Converts input domain names from Punycode (ASCII) to their Unicode representation.

-h, --help
    Displays a help message and exits.

-v, --version
    Shows version information and exits.

-u, --us-ascii
    Restricts input to only US-ASCII characters, returning an error for non-ASCII.

-N, --no-lookup
    Treats input as a literal domain name string without attempting DNS lookups.

--punycode-decode
    Decodes the given string from Punycode to Unicode.

--punycode-encode
    Encodes the given string from Unicode to Punycode.

--idna2003
    Uses the IDNA2003 standard for conversions. Note: Default may vary by library version.

--idna2008
    Uses the IDNA2008 standard for conversions (often the default for modern versions).

--uts46
    Uses the Unicode Technical Standard 46 (UTS46) standard for conversions (often the default for modern versions).

--validate
    Validates the input according to the chosen IDNA standard without performing conversion.

--strict
    Enables strict validation checks, often used with --validate.

--nfkc
    Applies Unicode Normalization Form KC (NFKC) to the input before processing.

The idn command is a powerful utility for handling Internationalized Domain Names (IDN). It primarily converts domain names between their standard Unicode representation and their ASCII Compatible Encoding (ACE), also known as Punycode. This conversion is crucial because the traditional Domain Name System (DNS) infrastructure is designed to handle only ASCII characters. IDN allows users to register and access domain names using characters from various languages and scripts (e.g., Arabic, Chinese, Cyrillic).

idn supports both IDNA2003 and the newer IDNA2008/UTS46 standards, ensuring compatibility with modern internet standards. It's an essential tool for developers and administrators working with globalized web content, enabling them to process, validate, and convert domain names in a format suitable for DNS resolution while presenting them in a user-friendly, localized form.

The behavior of idn can differ slightly between libidn (older) and libidn2 (newer) versions, particularly concerning the default IDNA standard (IDNA2003 vs. IDNA2008/UTS46). Users should be aware of which library their idn command is linked against to ensure consistent results, especially for edge cases or specific character sets. Proper handling of input character encoding (e.g., UTF-8) is also crucial.

Punycode is a specific encoding syntax used to represent Unicode characters in the ASCII-compatible Domain Name System (DNS). It allows IDNs to be stored and processed by the existing DNS infrastructure while still being readable in their original scripts by applications.

These are different versions and specifications for how Internationalized Domain Names should be processed and converted. IDNA2003 was the first widely adopted standard. IDNA2008 and UTS46 (Unicode Technical Standard 46) are newer revisions that address security concerns, ambiguities, and offer more comprehensive character handling, ensuring greater interoperability and consistency across the internet.

The concept of Internationalized Domain Names (IDN) was developed to allow people worldwide to use domain names in their native languages. The idn command emerged as part of the GNU Libidn project, providing a robust implementation of the IDNA (Internationalized Domain Names in Applications) standard. The initial standard, IDNA2003, was followed by IDNA2008 and UTS46, which addressed some limitations and ambiguities. The libidn2 library, and thus newer versions of the idn command, provides support for these updated standards, reflecting ongoing efforts to make the internet more accessible globally.

host(1), dig(1), nslookup(1)