How can I block an outbound port for one device?

[RainCaster]

I have a NAS box that has updated itself last week, and ever since it has been connecting to a HUGE number of random IPs all over the world, none of which have WhoIs info... Seems suspicious, so I want to block that NAS from doing so. The connections are always from port 51413.
I know the fixed IP of the NAS, but I cannot find an entry anywhere for the "Client Port". How can I do this?

[dwasserman]

firewall app

[NuX]

adding to dwasserman post

if you need further assistance
something like this should work.
Screenshot 2020-11-30 195923.png

[sky-knight]

Source port is possible in firewall rules, but not without a UI hack because it's almost never legitimately used... so it was removed from the UI to save support tickets.

I personally would be using the above rule to prevent all Internet access from that device entirely, and be looking to manually flash the firmware on it. Also, what NAS is it? And did you enable any cloud services? Or have any install on it automatically?

[Armshouse]

Could that port have something to do with Bittorrent? So if you have an app running on the NAS that does that, that's how it's communicating with trackers etc... I have the same port show up on an Ubuntu server running the Transmission daemon.