Ок, залез в RFC:
Параметр domain принимает значения из FQDN.
Который описан там же рядом: Fully-qualified host name (FQHN) means either the fully-qualified domain name (FQDN) of a host (i.e., a completely specified domain name ending in a top-level domain such as .com or .uk), or the
numeric Internet Protocol (IP) address of a host.
