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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
     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 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).

     If sysctl(8) node has non-zero value, newly
     created bridge will inherit MAC address from its first member instead of
     choosing random link-level address.  This will provide more predictable
     bridge MAC without any additional configuration, but currently this fea‐
     ture is known to break some L2 protocols, for example PPPoE that is pro‐
     vided by ng_pppoe(4) and ppp(8).  Now this feature is considered as
     experimental and is turned off by-default.

     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 switch, 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.

     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.
				  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.

     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,
     excluding 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
     interface.	 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.

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

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

     ·	 ifX MAC address is xx:xx:xx:xx:xx:xx,

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

     ·	 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
     implementation 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 “wlan0” 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
     (assuming the 802.11 interface is in ad-hoc mode).

	   ifconfig_bridge0="addm wlan0 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:

	   create_args_wlan0="wlanmode hostap"
	   ifconfig_wlan0="up ssid my_ap mode 11g"

     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

     Note that FreeBSD 6.1, 6.2, 6.3, 7.0, 7.1, and 7.2 have a bug in the
     EtherIP protocol.	For more details and workaround, see gif(4) manual

     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.

BSD				January 9, 2010				   BSD

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