It sounds like dyndns is like NOIP which can be checked out at
NOIP LINK . I have never had used either of them myself but its a way to get a static IP if you have a dynamic one. You can still let people in with dynamic IPs but its a pain having to keep changing them. I am not sure if you can come up with some kind of script as it is you have to reboot your server whenever you add a new IP or change an existing one before the change takes effect.