The fqdn is the host’s Fully Qualified Domain Name. Ie. The hostname with the domain that the host is in appended, this produces a resolvable DNS name. Eg. from inside the Tideway network you can see the mail server as just ‘mail’ (ie. ping mail) or as mail.tideway.com, but from outside it must be stated ‘fully-qualified’ as mail.tideway.com to be resolvable.

The dns_domain is from /etc/resolv.conf on the host and is the DNS domain to search in if none is specified. Ie. If I’m sitting on the Tideway network how does the DNS client on my machine know that when it gets a non-fully qualified name like ‘mail’ to turn it into ‘mail.tideway.com’. See man resolv.conf