Is Underscore Valid In Domain Name. For recommendations about how to name resources see Recommended naming and tagging conventions. Plus underscores arent allowed to be used in domain names.
In DNS host names are the name fields of A or MX records or the data fields of the SOA and NS records. Thus there are many DNS entries that are not hostnames. Underscores are only able to be used in subfolders.
0-9 and dashes - or a combination all of these.
Underscores in Domain Names While it may be perfectly valid by RFC 2181 section 11 to have underscores in a domain name you will not find any domain type ie TLD ccTLD or gLTD that supports registering domains with underscores in you are therefore restricted to hyphens as an separator. Users may have difficulty recognizing the business unit that an acronym represents. Objects are Pythons abstraction for data. All certificates containing an underscore character in any dNSName entry and having a validity period of more than 30 days MUST be revoked prior to January 15 2019.