Transparent Filtering Bridge: Cannot Access WebGUI on dual ethernet device if connected between modem and router

[korgano]

Important notices

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• [x] I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• [x] I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Describe the bug

When following OPNSense guidance for transparent filtering bridge on a dual ethernet miniPC, namely Step 4 of the documented guide, the webGUI is inaccessible from any device on the network when the OPNSense device is placed in the following configuration (as recommended by tutorials):

[Modem] <==> [OPNSense transparent filtering bridge] <==> [Router]

OPNSense also cannot connect to the internet to update or install plugins.

To Reproduce

Steps to reproduce the behavior:

1. Connect modem and router to the OPNSense host
2. Install OPNSense on miniPC with 2 ethernet connectors.
3. Set one ethernet jack to WAN.
4. Set other ethernet jack to LAN.
5. Create bridge interface with both ethernet connectors.
6. Set a static IPv4 address and subnet mask for the bridge interface.
7. Finish configuring the transparent filtering bridge as per guide.
8. Attempt to connect to webGUI address.
9. Address does not show up on router as a device and is inaccessible via other devices on network.

Expected behavior

Transparent filtering bridge is:

1. Functional, allowing devices on network to access the internet.
2. Visible to the router and other devices on the network.
3. Configurable via the webGUI on any device's browser.
4. Allowing OPNSense to update.

Describe alternatives you considered

Obtaining a USB-to-RJ45 ethernet adapter has been considered, but there seem to be issues with OPNSense recognizing such configurations as well, and generally only solves the webGUI access issue, as OPNSense cannot access the internet to update.

Additional context

This appears to be a common issue with OPNSense, with numerous Reddit and forum threads about this specific configuration or similar ones: https://forum.opnsense.org/index.php?topic=42645.0 https://www.reddit.com/r/opnsense/comments/1c8oo2p/comment/l2ve6yl/ https://www.reddit.com/r/opnsense/comments/1f608es/transparent_filtering_bridge_issues/ https://www.reddit.com/r/opnsense/comments/1c9ic3g/access_transparent_bridge_configuration_from_lan/ https://www.reddit.com/r/opnsense/comments/1ckqyjj/reaching_gui_after_placing_opnsense_between/ https://forum.opnsense.org/index.php?topic=40032.0