All the time you are logging in you have a new IP-address. In this case the ssh should warn you every time that the authenticity can't be established (you are unknown). I don't know exactly if you can really switch off the ask for a password in this case. no this is not true, with dyndns you have a unique (not changing) hostname. if the ip-address changes, ssh automatically adds the ip address to the known_hosts file, but it doesn't handle it as a new address (since the host name is the same) I'm using the same thing without problems, but can't look at it at the moment to help ...
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