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IF_BRIDGE(4)	       FreeBSD 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

     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.

     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

     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 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.

     The bridge	may not	forward	fragments that have been reassembled by	a
     packet filter.  In	pf(4) fragment reassembly can be disabled in the scrub

FreeBSD	6.1			April 29, 2006			   FreeBSD 6.1


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