well 1st off the server has to be running to have the port open. Secondly, if you
have a firewall up that's doing some blocking, you will need what are called
iptables rules. For your case 1 rule will do just fine:
iptables -A INPUT -p udp --destination-port 12300 -j ACCEPT
just typing that in a console will only be good up to the next time you reboot your machine. For now i guess you could just drop that line in rc.local ( /etc/rc.local) and it will run every time you reboot. But you will really want to create a nice set of firewall rules. Look into getting firestarter. It has a GUI approach to the whole thing that makes it easier if you are new
to the game.
iptables is the name of the program, -A means Append a new rule, INPUT is the name of the table, -p means "match the following protoocl" the --destination-port....self explanitory (can be abbreviated --dport) -j means "jump to" and ACCEPT means, pass it on to the recieving application if t there is one.
if the last part said DROP then the packet would get drop kicked off the machine.
iptables is VERY VERY flexible, you can use it to do routing rules and all
that good stuff too. I suggest you get to know them, especially if you plan
on using your Linux box to host servers of any kind.
Welcome to Linux and LQ, you will like it here as there are many helpful
people in this forum (i like to think im one of em