Comment 19 for bug 144431

"The DNS itself places only one restriction on the particular labels that can be used to identify resource
records. That one restriction relates to the length of the label and the full name. [...] Those restrictions
aside, any binary string whatever can be used as the label of any resource record."
  -- RFC 2181, section 11

RFC3696, section 2 verifies this: "Any characters, or combination of bits (as octets), are permitted in
DNS names." Then it describes how the old ARPANET rules worked. But we moved beyond those
rules a long time ago. Just look at the international domain names.