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     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



     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

     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 interfaces 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 feature is known to break some L2 protocols, for example PPPoE that is
     provided 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 TOE, TSO, TXCSUM and TXCSUM6 capabilities on all interfaces added to the bridge are
     disabled if any of the interfaces doesn't support/enable them.  The LRO capability is always
     disabled.  All the capabilities are restored when the interface is removed from bridge.
     Changing capabilities in run time may cause NIC reinit and the 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


     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.  Bridging multiple
     interfaces change the zone configuration because multiple links are merged to each other and
     form a new single link while the member 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 conterintuitive or undesirable
     behavior in some edge cases when both of the if_bridge interface and one of the member
     interface 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 before the interface is added.

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

     Note that ACCEPT_RTADV and AUTO_LINKLOCAL interface flag are not enabled by default on
     if_bridge interface 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 Protocol (STP).  Spanning Tree is used
     to detect and remove loops in a network topology.

     RSTP provides faster spanning tree convergence than legacy STP, the protocol will exchange
     information with neighbouring switches to quickly transition to forwarding without creating

     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 physical 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 currently sharing the MAC address of the parent physical interface).  It is not
     possible 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 future.

     The previous paragraph is best illustrated with the following pictures.  Let

        the MAC address of the incoming packet's destination is nn:nn:nn:nn:nn:nn,

        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 xx:xx:xx:xx:xx:xx,

        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 packets based on their interface name, one should be aware
     of this implication.  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


     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 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 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

           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 page.


     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.