What are A-label and U-label in domain names

Question

I am working on extra SSL requirement of hostname check. Want to know what A-label and U-label in DN. Thanks

Answer 1

Here are the definitions of the ICANN :

A-label

The ASCII-compatible encoded (ACE) representation of an internationalized domain name, i.e. how it is transmitted internally within the DNS protocol. A-labels always commence with the prefix “xn--”. Contrast with U-label.

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U-label

The Unicode representation of an internationalized domain name, i.e. how it is shown to the end-user. Contrast with A-label.

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The links

• Glossary A
• Glossary U

Answer 2

In more simplistic terms:

The 'version' that is solely for technical purposes is the xn-- 'version' /aka the punycode (A*-label). The 'version' that is readable to a user of whichever language the idn is in is the unicode (u label)

*ASCII - American Standard Code for Information Interchange

Answer 3

DNS uses its own unique encoding for Unicode characters. For example:

www.á.com: How the DNS name may appear in, say, a web page

www.xn--1ca.com: What the browser converts the name to when sending it as a DNS request "over the wire"

The reason why DNS uses a complicated special encoding for non-ASCII Unicode characters instead of simply using UTF-8 is because RFC1035 section 2.3.1 says that names can only have letters, numbers, and the '-' (hyphen) character. I felt, a decade ago, that it would have been possible to add real UTF-8 support to DNS, but there was too much anxiety that doing so would break unspecified legacy DNS routers, appliances, etc.