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if_bridge - network bridge device
To compile this driver into the kernel, place the following line in your
kernel configuration file:
Alternatively, to load the driver as a module at boot time, place the
following line in loader.conf(5):
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).
The if_bridge interface randomly chooses a link (MAC) address in the
range reserved for locally adminstered addresses when it is created. The
address can be changed by assigning the desired link address using
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
All the bridged member interfaces need to be up in order to pass network
traffic. These can be enabled using ifconfig(8) or
ifconfig_〈interface〉="up" in rc.conf(5).
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 TXCSUM capability is disabled for any interface added to the bridge,
and it is restored when the interface is removed again.
The bridge supports “monitor mode”, where the packets are discarded after
bpf(4) processing, and are not processed or forwarded further. This can
be used to multiplex the input of two or more interfaces into a single
bpf(4) stream. This is useful for reconstructing the traffic for network
taps that transmit the RX/TX signals out through two separate interfaces.
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. The filtering behaviour can be controlled using
net.link.bridge.pfil_onlyip Controls the handling of non-IP packets
which are not passed to pfil(9). Set to 1
to only allow IP packets to pass (subject to
firewall rules), set to 0 to unconditionally
pass all non-IP Ethernet frames.
net.link.bridge.pfil_member Set to 1 to enable filtering on the incoming
and outgoing member interfaces, set to 0 to
net.link.bridge.pfil_bridge Set to 1 to enable filtering on the bridge
interface, set to 0 to disable it.
net.link.bridge.ipfw 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
net.link.bridge.ipfw_arp Set to 1 to enable layer2 ARP filtering with
ipfirewall(4), set to 0 to disable it.
Requires ipfw to be enabled.
ARP and REVARP packets are forwarded without being filtered and others
that are not IP nor IPv6 packets are not forwarded when pfil_onlyip 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 following when placed in the file /etc/rc.conf will cause a bridge
called “bridge0” to be created, and will add the interfaces “ath0” 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 ath0 addm fxp0 up"
For the bridge to forward packets all member interfaces and the bridge
need to be up. The above example would also require:
ifconfig_ath0="up ssid my_ap mode 11g mediaopt hostap"
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
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 be used as a regular host interface at the same time as
bridging between its member ports. In this example, the bridge connects
em0 and em1, and will receive its IP address through DHCP:
ifconfig_bridge0="addm em0 addm em1 DHCP"
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 18.104.22.168 22.214.171.124 up
ifconfig bridge0 create
ifconfig bridge0 addm fxp0 addm gif0 up
gif(4), ipf(4), ipfw(4), pf(4), ifconfig(8)
The if_bridge driver first appeared in FreeBSD 6.0.
The bridge driver was originally written by Jason L. Wright
〈email@example.com〉 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 〈firstname.lastname@example.org〉.
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.
Only wireless interfaces in hostap mode can be bridged due to the 802.11
framing format, bridging a wireless client is not supported yet.