A pure Erlang IDNA implementation that folllow the RFC5891.

  • support IDNA 2008 and IDNA 2003.
  • label validation:
    • check NFC: Label must be in Normalization Form C
    • check hyphen: The Unicode string MUST NOT contain "--" (two consecutive hyphens) in the third and fourth character positions and MUST NOT start or end with a "-" (hyphen).
    • Leading Combining Marks: The Unicode string MUST NOT begin with a combining mark or combining character (see The Unicode Standard, Section 2.11 Unicode for an exact definition).
    • Contextual Rules: The Unicode string MUST NOT contain any characters whose validity is context-dependent, unless the validity is positively confirmed by a contextual rule. To check this, each code point identified as CONTEXTJ or CONTEXTO in the Tables document RFC5892 MUST have a non-null rule. If such a code point is missing a rule, the label is invalid. If the rule exists but the result of applying the rule is negative or inconclusive, the proposed label is invalid.
    • check BIDI: label contains any characters from scripts that are written from right to left, it MUST meet the Bidi criteria rfc5893


idna:encode/{1,2} and idna:decode/{1, 2} functions are used to encode or decode an Internationalized Domain Names using IDNA protocol.

Input can be mapped to unicode using uts46 by setting the uts46 flag to true (default is false). If transition from IDNA 2003 to IDNA 2008 is needed, the flag transitional can be set to true, (default is false). If conformance to STD3 is needed, the flag std3_rules can be set to true. (default is false).


1> idna:encode("日本語。JP", [uts46]).
2> idna:encode("日本語.JP", [uts46]).

Legacy support of IDNA 2003 is also available with to_ascii and to_unicode functions:

1> Domain = "www.詹姆斯.com".
2> Encoded =  idna:to_ascii("www.詹姆斯.com").
3> idna:to_unicode(Encoded).