Internationalized domain name
A domain name containing non-ASCII characters, such as accented or non-Latin letters, stored in DNS in an ASCII form beginning with xn--.
What it means
DNS itself carries ASCII, so IDNA (RFC 5890 and related) converts each Unicode label (a U-label) into an ASCII A-label using Punycode, prefixed with xn--. bücher.example becomes xn--bcher-kva.example. DKIM requires d= and s= values in A-label form.
IDNs let people use their own scripts, and they also enable homograph attacks, where letters from another script look identical to Latin ones. Showing the xn-- form of a sender's domain is one way to spot that.
Internationalized email addresses (non-ASCII local parts) are a separate extension, SMTPUTF8, which not every server supports.
Example
bücher.example -> xn--bcher-kva.exampleRelated terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5890: Internationalized Domain Names for Applications (IDNA): Definitions and Document Frameworkrfc-editor.org
- RFC 6376: DomainKeys Identified Mail (DKIM) Signatures, section 3.5rfc-editor.org
- Unicode Technical Report #36: Unicode Security Considerationsunicode.org
- RFC 6531: SMTP Extension for Internationalized Emailrfc-editor.org
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