Sometimes you need to make sure some client with dynamic IP will never be blocked by lfd.
However if you put the FQDN of the dyndns used by the client in csf.dyndns then you also allow this client to bypass the firewall completely, which is not the desired behaviour.
I suggest creating a new file, something like csf.dyndnsignore, that will have the behaviour of csf.ignore, but for dynamic dns.