Unfortunately no one can be told what fun_plug is - you have to see it for yourself.
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I made an archive to setup ssh on a Dlink ShareCenter NAS using only the softwares available on the NAS, public key authentication and fun_plug scripts: http://pertusus.free.fr/pat_fun_plug.tar.gz
There is a README.txt in the archive that should hopefully explain everything.
It does not use the additional software coming with fonz fun_plug, but the sshd present on the NAS. I tested it only on a D-Link DNS-320 ShareCenter, but I think that it should work with minimal change on other systems provided they have a sshd program in them.
I only need sshd and rsync (and vi) so I didn't install anything new on the NAS, but I think it could also be used as a more secure first step of setting up fonz fun_plug to replace the telnetd step.
It is secure but somewhat similar with: