Topic:Open/redirect necessary ports on your modem/router
if your server is behind a router/modem in your network, you need to Open/redirect necessary ports on your modem/router:
these ports need to be opened/redirected :
port 53 should be redirected both TCP/UDP, while for others, TCP is enaugh.
this is especially important if your server has a private internal ip address such as 192.168.1.100
On most routers/modems, you may find a NAT section or, so called "Virtual Server" section.
your port redirection should be like:
outside_ip:20 -> 192.168.1.100:20