Transparent Filtering Bridge


The Transparent Filtering Bridge is not compatible with Traffic Shaping. Do not enable the traffic shaper when using the filtering bridge.


A transparent firewall can be used to filter traffic without creating different subnets. This application is called filtering bridge as it acts as a bridge connection two interfaces and applies filtering rules on top of this.

For more information on Filtering Bridged on FreeBSD, see filtering-bridges


  • For this howto we need a basic installation of OPNsense with factory defaults as a starting point.

  • And an appliance with 2 physical interfaces.


To create this howto version OPNsense 15.7.11 has been used. Some screenshots maybe outdated, but setting should apply up to at least 17.1.6. If you use a different version some options can be different.


The Menu System of the User Interface has been updated with sub items. Where tabs are shown in screenshots, these are now likely visible as submenu.

Configuration in 10 easy steps


During the configuration you will be asked to “Apply” your changes several times, however this may affect the current connection. So don’t apply anything until completely finished! You need to Save your changes for each step.

1. Disable Outbound NAT rule generation

To disable outbound NAT, go to Firewall ‣ NAT ‣ Outbound and select “Disable Outbound NAT rule generation”.

Filtering Bridge Step 1.png

2. Change system tuneables

Enable filtering bridge by changing from default to 1 in System ‣ Settings ‣ System Tuneables.

Filtering Bridge Step 2.png

And disable filtering on member interfaces by changing from default to 0 in System ‣ Settings ‣ System Tuneables.

Filtering Bridge Step2a.png

3. Create the bridge

Create a bridge of LAN and WAN, go to Interfaces ‣ Other Types ‣ Bridge. Add Select LAN and WAN.

Filtering Bridge Step 3a.png

Filtering Bridge Step 3b.png

4. Assign a management IP/Interface

To be able to configure and manage the filtering bridge (OPNsense) afterwards, we will need to assign a new interface to the bridge and setup an IP address.

Go to Interfaces ‣ Assign ‣ Available network port, select the bridge from the list and hit +.

Filtering Bridge Step 4.png

Now Add an IP address to the interface that you would like to use to manage the bridge. Go to Interfaces ‣ [OPT1], enable the interface and fill-in the ip/netmask.

5. Disable Block private networks & bogon

For the WAN interface we nee to disable blocking of private networks & bogus IPs.

Go to Interfaces ‣ [WAN] and unselect Block private networks and Block bogon networks.

Filtering Bridge Step 5.png

6. Disable the DHCP server on LAN

To disable the DHCP server on LAN go to Services ‣ DHCPv4 ‣ [LAN] and unselect enable.

Filtering Bridge Step 6.png

7. Add Allow rules

After configuring the bridge the rules on member interfaces (WAN/LAN) will be ignored. So you can skip this step.

Add the allow rules for all traffic on each of the three interfaces (WAN/LAN/OPT1).

This step is to ensure we have a full transparent bridge without any filtering taking place. You can setup the correct rules when you have confirmed the bridge to work properly.

Go to Firewall ‣ Rules and add a rule per interface to allow all traffic of any type.

Filtering Bridge Step 7.png

8. Disable Default Anti Lockout Rule

After configuring the bridge the rules on member interfaces (WAN/LAN) will be ignored. So you can skip this step.

As we now have setup allow rules for each interface we can safely remove the Anti Lockout rule on LAN

Go to Firewall ‣ Settings ‣ Admin Access: Anti-lockout and select this option to disable

9. Set LAN and WAN interface type to ‘none’

Now remove the IP subnets in use for LAN and WAN by changing the interface type to none. Go to Interfaces ‣ [LAN] and Interfaces ‣ [WAN] to do so.

Filtering Bridge Step 9.png

10. Now apply the changes

If you followed each step, then you can now apply the changes. The Firewall is now converted to a filtering bridge.

Done.. ready to set your own filtering rules

Now you can create the correct firewall/filter rules and apply them. To acces the firewall you need to use the IP adress you configured for the OPT1 Interface.


Rules need to be configured on the bridge. Rules on member interfaces will be ignored!


Don’t forget to make sure your PC/Laptop is configured with an IP adress that falls within the IP range of the OPT1 subnet!