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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 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	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 a non-zero value, the
     newly created bridge will inherit the MAC address from its	first member
     instead of	choosing a random link-level address.  This will provide more
     predictable bridge	MAC addresses without any additional configuration,
     but currently this	feature	is known to break some L2 protocols, for exam-
     ple PPPoE that is provided	by ng_pppoe(4) and ppp(8).  Currently 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, or traffic isolation.

     A bridge works like a switch, forwarding traffic from one interface to
     another.  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	MTU

     The TOE, TSO, TXCSUM and TXCSUM6 capabilities on all interfaces added to
     the bridge	are disabled if	any of the interfaces do not support/enable
     them.  The	LRO capability is always disabled.  All	the capabilities are
     restored when the interface is removed from the bridge.  Changing capa-
     bilities at run-time may cause NIC	reinit and a link flap.

     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.

     if_bridge supports	the AF_INET6 address family on bridge interfaces.  The
     following rc.conf(5) variable configures an IPv6 link-local address on
     bridge0 interface:


     or	in a more explicit manner:

	   ifconfig_bridge0_ipv6="inet6	auto_linklocal"

     However, the AF_INET6 address family has a	concept	of scope zone.	Bridg-
     ing multiple interfaces changes the zone configuration because multiple
     links are merged to each other and	form a new single link while the mem-
     ber interfaces still work individually.  This means each member interface
     still has a separate link-local scope zone	and the	if_bridge interface
     has another single, aggregated link-local scope zone at the same time.
     This situation is clearly against the description "zones of the same
     scope cannot overlap" in Section 5, RFC 4007.  Although it	works in most
     cases, it can cause some counterintuitive or undesirable behavior in some
     edge cases	when both, the if_bridge interface and one of the member in-
     terfaces, have an IPv6 address and	applications use both of them.

     To	prevent	this situation,	if_bridge checks whether a link-local scoped
     IPv6 address is configured	on a member interface to be added and the
     if_bridge interface.  When	the if_bridge interface	has IPv6 addresses,
     IPv6 addresses on the member interface will be automatically removed be-
     fore the interface	is added.

     This behavior can be disabled by setting sysctl(8)	variable to 1.

     Note that ACCEPT_RTADV and	AUTO_LINKLOCAL interface flags are not enabled
     by	default	on if_bridge interfaces	even when net.inet6.ip6.accept_rtadv
     and/or net.inet6.ip6.auto_linklocal is set	to 1.

     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 setting the node 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 behavior 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 currently 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 since it may change	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.

     If	the MAC	address	nn:nn:nn:nn:nn:nn is equal to xx:xx:xx:xx:xx:xx	the
     filter will see the packet	on interface ifX no matter if there are	any
     other bridge members carrying the same MAC	address.  But if the MAC ad-
     dress nn:nn:nn:nn:nn:nn is	equal to 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 were taken just as an example
     of	such a situation.  So if one wants to filter the locally destined
     packets based on their interface name, one	should be aware	of this	impli-
     cation.  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 (as-
     suming 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

     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.

FreeBSD	13.0		       November	17, 2021		  FreeBSD 13.0


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