idn

idn [OPTION]... [NAME]...

--help
    Display help and exit

--version
    Output version information and exit

--quiet
    Omit progress indicator

--verbose
    Display more processing information

--locale=LOCALE
    Set locale for output encoding (default: UTF-8)

--allow-unsigned
    Allow unsigned Nameprep profiles

--nfc
    Normalize to NFC before processing

--nfd
    Normalize to NFD before processing

--punycode-encode
    Encode NAME to Punycode

--punycode-decode
    Decode NAME from Punycode

--uscore
    Treat underscores as valid labels

The idn command handles Internationalized Domain Names (IDNs) by converting between Unicode/local encoding and their ASCII-Compatible Encoding (ACE) form using Punycode. IDNs allow non-ASCII characters in domain names (e.g., café.com or bücher.example), but DNS requires ASCII, so Punycode encodes them as xn-- prefixed strings.

idn prepares input with Nameprep (normalizing and mapping Unicode), then applies IDNA encoding/decoding. By default, it encodes if input has non-ASCII, decodes if Punycode-like.

Key uses: validating IDNs for tools like browsers or resolvers; scripting domain checks; troubleshooting DNS with international characters. It supports locale-specific display and NFC/NFD normalization. Installed via libidn or idnkit packages.

Note: Primarily for IDNA 2003 spec; modern apps favor IDNA 2008 via idn2. Output assumes UTF-8 unless locale set.

Deprecated in favor of idn2(1) (IDNA2008); may fail on newer IDNA rules. Assumes UTF-8 input/output; specify --locale for others. Not for TLD validation.

idn münchen.de
→ münchen.de → xn--mnchen-3ya.de

idn --punycode-decode xn--bcher-kva.de
→ bücher.de

0 on success, 1 on invalid input/encoding error, 2 on usage error.

Developed as part of Idnkit by JPNIC (2000s) for IDNA2003; integrated into libidn. Superseded by GNU libidn2/idn2 for IDNA2008 compliance since ~2017.

idn2(1), host(1), dig(1), nslookup(8)