Probably goes beyond the scope of this PR, but practically speaking DNS names can only be ASCII alphanumeric characters or the hyphen, plus the period to separate "words". (Technically, any 8-bit octet is valid, but most systems won't deal with those characters correctly.) There are plenty of other characters that a user could enter (other punctuation marks, UTF8, ...) that would result in invalid host names. If we're going to validate this field at all, it might be consistent to more thoroughly validate it.