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IF_BRIDGE(4)           FreeBSD Kernel Interfaces Manual           IF_BRIDGE(4)

     if_bridge -- network bridge device

     To compile this driver into the kernel, place the following line in your
     kernel configuration file:

           device if_bridge

     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

     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
     sysctl(8):  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.  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.     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 fil-
     ter 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 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
     <> 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 <>.

     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.

FreeBSD 6.2                     October 8, 2006                    FreeBSD 6.2


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