Why did you have to open a port for torrents?

Question

linux

Home network behind NAT. Previously, I used a ZyXEL Keenetic router and μTorrent on a Windows machine quietly downloaded and distributed without any router settings. Also, when you turn on the built-in torrent client on the ZyXEL itself, everything was distributed normally.
Now I use an Arch Linux computer as a router. The transmission daemon is up on it. And now he can't give away anything. Download only. I had to open port 51413 to the outside.
The iptables configuration was carried out according to the manual in the official Arch-wiki Simple stateful firewall (Russian) .
Why did you have to open a port on a Linux machine? Does she have stricter blocking rules than the "iron" router?
What steps should I take to secure my open port network?

Answer 1

μTorrent probably opened the port itself using upnp.