Doug's suggestion (inserting underscores) is both good and bad for
the same reason. Underscores are illegal in DNS names, hence a good
idea. But, if they're illegal, the test wouldn't work either.
Yes, I know that most resolvers/DNS servers handle them just fine,
but it's probably a bad idea to explicitly mandate a practise in
violation of existing RFC.
All 256 octet values are legal in DNS names (go read the DNS RFCs).
Use of octets outside a relatively restricted set is discouraged in DNS
names which name hosts (the exact set varies depending on which
reference you read). Some protocols break in the presence of some of
the discouraged octets (ssh, for example, has no way to represent a dot
which is part of a name component rather than being a component
separator, though it doesn't use DNS names in the protocol for much).
Others break implementations even if they don't break protocols (0x00,
for example, will break a whole lot of implementations). And there are
a few protocols, perhaps most notably SMTP, which actually specify
restrictions on the octets that may appear in DNS names used with them.
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