Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
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 setting
     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 desired.

     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
     filter 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 \

     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 11.0-PRERELEASE         April 29, 2006         FreeBSD 11.0-PRERELEASE


Want to link to this manual page? Use this URL:

home | help