I want ask you if a need that I have is possible.
There are some firewalls that I can intercalate between the router and the LAN, in bridge mode, without changes in the router neither on LAN devices.
The doubt is: Can I do this with a router with OpenWRT and to filter trafic form/to LAN from/to Operator Router, by example, Parental Control, Layer 2 Firewall, etc...?
The scheme would be:
Origin Schema: Operator Router <---> LAN Switch
Final Schema: Operator Router <---> OpenWRT Router <---> LAN Switch
Without changing nothing in the Operator Router or in the LAN Devices.
It's often called "transparent firewall" or "bridged firewall"
Yes, it would be implemented kernel-level features than can be managed with OpenWrt and the appropriate kernel modules.
@Jeff, very thanks by your answer.
Can you tell me if there is a howto to configure the router in this mode with OpenWRT?
No, there isn't one I know of. It's a rather uncommon topology for home users.
You'd need to set up a management interface and then bridge the "outside" and "inside" interfaces to each other. That bridge would need to be packet filtered, either with something like ebtables ot nftables.
Iptables will filter bridges so long as the packet hits the kernel and you have enabled the bridge filtering sysctls. This means you can't have a hardware switch switching the packets directly. You need either 2 Ethernet connections, or one vlan per port on the hardware switch, then bridge all the vlans in a kernel bridge.