Originally Posted by Thecus - Ivan 
Dear testflash,
The N0204 uses port range 2106 -2706 for BT download.
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Official Thecus Support Representative
When I used sniffer to setup N0204 BT, I'd found the ports like 6967, 6963 etc...
I wrote ports 6881~6999 from esupport page 116 to reverse NAT and see no more rejected sessions (incl. ports 2xxx) from outside in router logs.
But BT download still doesn't work. N0204 drops all incoming sessions: syn-ack-small piece of data from peer-rst by nas.