You can forward 1024 - 5000, like was suggested, but that sorta defeats the purpose of a firewall. A better idea would be to reduce the number of ports. To do this, go to alt+o DCC | options, then there is a "DCC Ports:" group, enter something like 5000 and 5005. That way you only have to expose 5 ports to the outside world for DCC, rather than ~4000. Then simply setup port forwarding for 5000 - 5005.