I have tried to setup port forwarding from my zentyal server (port 5000) to my synology NAS (port 5000) too. Here is the result in terms of iptables rules :
Chain fredirects (1 references) target prot opt source destination faccept tcp -- 0.0.0.0/0 10.0.0.8 state NEW tcp dpt:5000 faccept udp -- 0.0.0.0/0 10.0.0.8 state NEW udp dpt:5000
Now, when I try this with telnet from another server hosted somewhere else on the internet, telnet can't contact my server... any ideas ? How may I do other tests ?