What rule should be added to the firewall?

Question

System administration

There is a LAN_CLI and LAN_SRV interface. It is necessary to configure the firewall on the router so that LAN_CLI clients can connect to LAN_SRV clients, but not vice versa. How to set up a firewall?

Answer 1

Deny new connections in one direction, but allow packets to pass through already open connections. Google on connection state.

Answer 2

I would say it's better to google for the word stateful inspection (SPI).
allowing rule with SPI to the LAN_SRV side, prohibiting in the opposite direction.