Normally each interface on the pfSense® firewall represents its own broadcast domain with a unique IP subnet. In some circumstances it is desirable or necessary to combine multiple interfaces onto a single broadcast domain, where two ports on the firewall will act as if they are on the same switch, except traffic between the interfaces can be controlled with firewall rules. Typically this is done so multiple interfaces will act as though they are on the same flat network using the same IP subnet and so that clients all share broadcast and multicast traffic.
Certain applications and devices rely on broadcasts to function, but these are found more commonly in home environments than corporate environments. For a practical discussion, see Bridging and wireless.
For services running on the firewall, bridging can be problematic. Features such as limiters, Captive Portal, and transparent proxies require special configuration and handling to work on bridged networks. Specifically, the bridge itself must be assigned and the only interface on the bridge with an IP address must be the assigned bridge. Also, in order for these functions to work, the IP address on the bridge must be the address used by clients as their gateway. These issues are discussed more in-depth in Bridging interoperability.
Types of Bridges¶
There are two distinct types of bridges: Internal bridges and Internal/external bridges. Internal bridges connect two local interfaces such as two LAN interfaces or a LAN interface and a wireless interface. Internal/external bridges connect a LAN to a WAN resulting in what is commonly called a “transparent firewall”.
With an internal type bridge, ports on the firewall are linked such that they behave similar to switch ports, though with the ability to filter traffic on the ports or bridge and with much lower performance than a switch. The firewall itself is still visible to the local connected clients and acts as their gateway, and perhaps DNS and DHCP server. Clients on the bridged segments may not even know there is a firewall between them.
This type of configuration is commonly chosen by administrators to isolate and control a portion of the network, such as a wireless segment, or to make use of additional ports on the firewall in lieu of a proper switch where installing a switch would be impractical. Though it is not recommended, this type of bridge can also be used to join two remote networks over certain types of VPN connections.
The Hangouts Archive contains a video which includes practical examples of internal bridges: May 2015 Hangout on Wireless Access Points.
An Internal/External type bridge, also known as a “transparent firewall”, is used to insert a firewall between two segments without altering the other devices. Most commonly this is used to bridge a WAN to an internal network so that the WAN subnet may be used “inside” the firewall, or internally between local segments as an in-line filter. Another common use is for devices behind the firewall to obtain IP addresses via DHCP from an upstream server on the WAN.
In a transparent firewall configuration the firewall does not receive the traffic directly or act as a gateway, it merely inspects the traffic as it passes through the firewall.
Devices on the internal side of this bridge must continue to use the upstream gateway as their own gateway. Do not set any IP address on the firewall as a gateway for devices on a transparent bridge.
NAT is not possible with this style of bridge because NAT requires the traffic to be addressed to the firewall’s MAC address directly in order to take effect. Since the firewall is not the gateway, this does not happen. As such, rules to capture traffic such as those used by a transparent proxy do not function.
Bridging and Layer 2 Loops¶
When bridging, care must be taken to avoid layer 2 loops, or a switch configuration must be in place that handles loops. A layer 2 loop is when, either directly or indirectly, the switch has a connection back to itself. If a firewall running pfSense has interfaces bridged together, and two interfaces are plugged into the same switch on the same VLAN, a layer 2 loop has been created. Connecting two patch cables between two switches also does this.
Managed switches employ Spanning Tree Protocol (STP) to handle situations like this, because it is often desirable to have multiple links between switches, and the network shouldn’t be exposed to complete meltdown by someone plugging one network port into another network port. STP is not enabled by default on all managed switches, and is almost never available with unmanaged switches. Without STP, the result of a layer 2 loop is frames on the network will circle endlessly and the network will completely cease to function until the loop is removed. Check the switch configuration to ensure the feature is enabled and properly configured.
pfSense enables STP on bridge interfaces to help with loops, but it can still lead to unexpected situations. For instance, one of the bridge ports would shut itself down to stop the loop, which could cause traffic to stop flowing unexpectedly or bypass the firewall entirely.
In a nutshell, bridging has the potential to completely melt down the network unless anyone that plugs devices into the switch is careful.