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Could you please provide a proof (e.g. and RFC section) that 7bit ASCII characters (like plain old ASCII space, code 0x20) that were previously illegal are still disallowed in IDNs?

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This question actually asks for a formal proof of the accepted answer in serverfault.com/questions/202262/… –  Yury Kartynnik Aug 1 '12 at 17:05

2 Answers 2

up vote 0 down vote accepted

How about RFC5894, section 3. http://tools.ietf.org/html/rfc5894#page-9

IDNA2008 adopts the inclusion model. A code point is assumed to be invalid for IDN use unless it is included as part of a Unicode property-based rule or, in rare cases, included individually by an exception.

And in Section 3.1:

IDNA2008 divides all possible Unicode code points into four categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED, and UNASSIGNED.

And now, RFC5892 sections 2 and 3 name the allowed code points. http://tools.ietf.org/html/rfc5892

Specifically:

2.1. allows "LetterDigits" note that doesn't say punctuation...

2.2. disallows "Unstable" things that are disputed or variable or not yet confirmed.

2.3. disallows "IgnorableProperties" includes Default_Ignorable_Code_Point, White_Space, & Noncharacter_Code_Point

2.4. disallows "IgnorableBlocks" including diacritical marks and formatting characters

2.5. specifically allows "LDH" which is the usual ASCII allowed by old-skool DNS within Unicode {002D, 0030..0039, 0061..007A}

2.6. specifies "Exceptions" which covers certain characters marked in Unicode as one of the disallowed classes that nevertheless are needed for one language or another

2.7. through 2.10. covers even more obscure cases and future proofing for things that change status as Unicode evolves.

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So LDH is the only ASCII subset that is allowed for IDNs, right? –  Yury Kartynnik Aug 2 '12 at 8:42
    
Best I can tell. You'd have to read the Unicode spec for each character to see what categories it falls into. Ugly. ;-) –  Mark Aug 2 '12 at 18:49

Could you please provide a proof (e.g. and RFC section) that 7bit ASCII characters (like plain old ASCII space, code 0x20) that were previously illegal are still disallowed in IDNs?

Quoting RFC 5564, Section 2.3.2:

The space character is strictly disallowed in domain names, as it is a control character. Instead, the hyphen (Al-sharta, i.e., u+02D) is proposed as a separator between Arabic words to avoid confusion that can take place if the words are typed without a separator.

It is acceptable to use the hyphen to separate between words within the same domain name label.

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The document you are citing is titled "Linguistic Guidelines for the Use of Arabic Language in Internet Domains", and it states that "This document is not an Internet Standards Track specification; it is published for informational purposes." As such, it doesn't seem to be a general IDN guideline. –  Yury Kartynnik Aug 1 '12 at 16:55
    
The second point is: space was only an example, what about an apostrophe or a question mark? –  Yury Kartynnik Aug 1 '12 at 16:57

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