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This also means that the hostname must not contain the domain name part anymore (i.e. must not be a FQDN). See RFC 1035 [0], "man 5 hostname", or the kernel documentation [1]. Note: For legacy reasons we also allow underscores inside of the label but this is not recommended and intentionally left undocumented. [0]: https://tools.ietf.org/html/rfc1035 [1]: https://www.kernel.org/doc/html/latest/admin-guide/sysctl/kernel.html#domainname-hostname Co-authored-by: zimbatm <zimbatm@zimbatm.com>