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IF_BRIDGE(4)		 BSD 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 lines 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 ex-
     ample, 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	administered addresses when it is created.
     This address is guaranteed	to be unique only across all if_bridge inter-
     faces on the local	machine.  Thus you can theoretically have two bridges
     on	the different machines with the	same link addresses.  The address can
     be	changed	by assigning the desired link address using ifconfig(8).

     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 an-
     other.  Multicast and broadcast packets are always	forwarded to all in-
     terfaces 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.

     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.

     The if_bridge driver implements the Rapid Spanning	Tree Protocol (RSTP or
     802.1w) with backwards compatibility with the legacy Spanning Tree	Proto-
     col (STP).	 Spanning Tree is used to detect and remove loops in a network

     RSTP provides faster spanning tree	convergence than legacy	STP, the pro-
     tocol will	exchange information with neighbouring switches	to quickly
     transition	to forwarding without creating loops.

     The code will default to RSTP mode	but will downgrade any port connected
     to	a legacy STP network so	is fully backward compatible.  A bridge	can be
     forced to operate in STP mode without rapid state transitions via the
     proto command in ifconfig(8).

     The bridge	can log	STP port changes to syslog(3) by enabling the variable using sysctl(8).

     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 additionally filter on the phys-
				  ical interface for locally destined packets.
				  Set to 0 to disable this feature.	  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.  Re-
				  quires 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.

     The packets originating from the bridging host will be seen by the	filter
     on	the interface that is looked up	in the routing table.

     The packets destined to the bridging host will be seen by the filter on
     the interface with	the MAC	address	equal to the packet's destination MAC.
     There are situations when some of the bridge members are sharing the same
     MAC address (for example the vlan(4) interfaces: they are currenly	shar-
     ing the MAC address of the	parent physical	interface).  It	is not possi-
     ble to distinguish	between	these interfaces using their MAC address, ex-
     cluding the case when the packet's	destination MAC	address	is equal to
     the MAC address of	the interface on which the packet was entered to the
     system.  In this case the filter will see the incoming packet on this in-
     terface.  In all other cases the interface	seen by	the packet filter is
     chosen from the list of bridge members with the same MAC address and the
     result strongly depends on	the member addition sequence and the actual
     implementation of if_bridge.  It is not recommended to rely on the	order
     chosen by the current if_bridge implementation: it	can be changed in the

     The previous paragraph is best illustrated	with the following pictures.

     o	 the MAC address of the	incoming packet's destination is

     o	 the interface on which	packet entered the system is ifX,

     o	 ifX MAC address is xx:xx:xx:xx:xx:xx,

     o	 there are possibly other bridge members with the same MAC address

     o	 the bridge has	more than one interface	that are sharing the same MAC
	 address yy:yy:yy:yy:yy:yy; we will call them vlanY1, vlanY2, etc.

     Then if the MAC address nn:nn:nn:nn:nn:nn is equal	to the
     xx:xx:xx:xx:xx:xx then the	filter will see	the packet on the interface
     ifX no matter if there are	any other bridge members carrying the same MAC
     address.  But if the MAC address nn:nn:nn:nn:nn:nn	is equal to the
     yy:yy:yy:yy:yy:yy then the	interface that will be seen by the filter is
     one of the	vlanYn.	 It is not possible to predict the name	of the actual
     interface without the knowledge of	the system state and the if_bridge im-
     plementation details.

     This problem arises for any bridge	members	that are sharing the same MAC
     address, not only to the vlan(4) ones: they we taken just as the example
     of	such situation.	 So if one wants the filter the	locally	destined pack-
     ets based on their	interface name,	one should be aware of this implica-
     tion.  The	described situation will appear	at least on the	filtering
     bridges that are doing IP-forwarding; in some of such cases it is better
     to	assign the IP address only to the if_bridge interface and not to the
     bridge members.  Enabling will let	you do
     the additional filtering on the physical interface.

     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 configu-
     ration could be used to implement a simple	802.11-to-Ethernet bridge (as-
     suming 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 Rapid 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 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 <>.

     Rapid Spanning Tree Protocol (RSTP) support was added by Andrew Thompson

     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.

BSD			      September	17, 2007			   BSD


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