refs：

http://askubuntu.com/questions/293356/how-to-open-a-particular-port-in-ubuntu

Run this on the commandline; it should solve issue: sudo iptables -A INPUT -m conntrack --ctstate NEW,RELATED,ESTABLISHED -j ACCEPT

To do a specific port:

sudo iptables -A INPUT -p <tcp OR udp> <--dport OR --sport> <port> -j ACCEPT

INPUT is the chain for incoming traffic. -p is protocol (either tcp or udp --dport or --sportspecify allowing for destination or source port. -j is "jump" and its where you ACCEPT, REJECT, orDROP the packet(s)

所以，開放5600埠接收廣播，結果：

sudo iptables -A INPUT -p udp -d 0/0 -s 0/0 --dport 5600 -j ACCEPT

開放8000埠接收tcp，結果：

sudo iptables -A INPUT -p tcp -d 0/0 -s 0/0 --dport  8000 -j ACCEPT

設定防火牆的規則，允許5600埠進入

 sudo ufw allow  5600

 sudo ufw allow  8000

檢視狀態：

sudo netstat -ntlup

nc 用法：

refs:

http://manpages.ubuntu.com/manpages/hardy/man1/nc_openbsd.1.html

CLIENT/SERVER MODEL

     It is quite simple to build a very basic client/server model using nc
 
.
     On one console, start nc listening on a specific port for a connection.
     For example:

           $ nc -l 1234

     nc is now listening on port 1234 for a connection.  On a second console
     (or a second machine), connect to the machine and port being listened on:

           $ nc 127.0.0.1 1234