IF_BRIDGE(4) MidnightBSD Kernel Interfaces Manual IF_BRIDGE(4)


if_bridge — network bridge device


device if_bridge


The if_bridge driver creates a logical link between two or more IEEE 802 networks that use the same (or ‘‘similar enough’’) framing format. For example, it is possible to bridge Ethernet and 802.11 networks together, but it is not possible to bridge Ethernet and Token Ring together.

Each if_bridge interface is created at runtime using interface cloning. This is most easily done with the ifconfig(8) create command or using the cloned_interfaces variable in rc.conf(5).

A bridge can be used to provide several services, such as a simple 802.11-to-Ethernet bridge for wireless hosts, and traffic isolation.

A bridge works like a hub, forwarding traffic from one interface to another. Multicast and broadcast packets are always forwarded to all interfaces that are part of the bridge. For unicast traffic, the bridge learns which MAC addresses are associated with which interfaces and will forward the traffic selectively.

The if_bridge driver implements the IEEE 802.1D Spanning Tree protocol (STP). Spanning Tree is used to detect and remove loops in a network topology.

Packet filtering can be used with any firewall package that hooks in via the pfil(9) framework. When filtering is enabled, bridged packets will pass through the filter inbound on the originating interface, on the bridge interface and outbound on the appropriate interfaces. Either stage can be disabled, this behaviour can be controlled using sysctl(8):


Set to 1 to only allow IP packets to pass when packet filtering is enabled (subject to firewall rules), set to 0 to unconditionally pass all non-IP Ethernet frames.


Set to 1 to enable filtering on the incoming and outgoing member interfaces, set to 0 to disable it.


Set to 1 to enable filtering on the bridge interface, set to 0 to disable it.


Set to 1 to enable layer2 filtering with ipfirewall(4), set to 0 to disable it. This needs to be enabled for dummynet(4) support. When ipfw is enabled, pfil_bridge and pfil_member will be disabled so that IPFW is not run twice; these can be re-enabled if desired.

ARP and REVARP packets are forwarded without being filtered and others that are not IP nor IPv6 packets are not forwarded when pfil(9) filtering is enabled. IPFW can filter Ethernet types using mac-type so all packets are passed to the filter for processing.

Note that packets to and from the bridging host will be seen by the filter on the interface with the appropriate address configured as well as on the interface on which the packet arrives or departs.

The MTU of the first member interface to be added is used as the bridge MTU, all additional members are required to have exactly the same value.


The following when placed in the file /etc/rc.conf will cause a bridge called ‘‘bridge0’’ to be created, and will add the interfaces ‘‘wi0’’ and ‘‘fxp0’’ to the bridge, and then enable packet forwarding. Such a configuration could be used to implement a simple 802.11-to-Ethernet bridge (assuming the 802.11 interface is in ad-hoc mode).

ifconfig_bridge0="addm wi0 addm fxp0 up"

Consider a system with two 4-port Ethernet boards. The following will cause a bridge consisting of all 8 ports with Spanning Tree enabled to be created:

ifconfig bridge0 create
ifconfig bridge0 \
addm fxp0 stp fxp0 \
addm fxp1 stp fxp1 \
addm fxp2 stp fxp2 \
addm fxp3 stp fxp3 \
addm fxp4 stp fxp4 \
addm fxp5 stp fxp5 \
addm fxp6 stp fxp6 \
addm fxp7 stp fxp7 \

The bridge can tunnel Ethernet across an IP internet using the EtherIP protocol. This can be combined with ipsec(4) to provide an encrypted connection. Create a gif(4) interface and set the local and remote IP addresses for the tunnel, these are reversed on the remote bridge.

ifconfig gif0 create
ifconfig gif0 tunnel up
ifconfig bridge0 create
ifconfig bridge0 addm fxp0 addm gif0 up


ifconfig(8), gif(4), ipfw(4), ipf(4), pf(4)


The if_bridge driver first appeared in FreeBSD 6.0.


The bridge driver was originally written by Jason L. Wright 〈jason@thought.net〉 as part of an undergraduate independent study at the University of North Carolina at Greensboro.

This version of the if_bridge driver has been heavily modified from the original version by
Jason R. Thorpe 〈thorpej@wasabisystems.com〉.


The if_bridge driver currently supports only Ethernet and Ethernet-like (e.g., 802.11) network devices, with exactly the same interface MTU size as the bridge device.

MidnightBSD 0.3 June 10, 2005 MidnightBSD 0.3