firewall – Does port 8333 have to be opened as TCP_OUT or TCP_IN?
Port 8333 is simply related for inbound connections. Opening it permits different nodes to start out speaking together with your node.
That stated, a Bitcoin node can absolutely take part within the community even in case you do not permit inbound connections (to port 8333). It’s going to use outgoing connections, which begin from random greater unprivileged ports and connect with port 8333 of one other node. For those who can permit incoming connections that is good for the well being of the Bitcoin community, in any other case outbound connections are functionally equal.
Bitcoin Core may also supply a JSON-RPC interface on port 8332, so in case you want that, act accordingly.