IF_BRIDGE(4) BSD Kernel Interfaces Manual IF_BRIDGE(4)NAMEif_bridge — network bridge device
To compile this driver into the kernel, place the following line in your
kernel configuration file:
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 net.link.bridge.inherit_mac 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
net.link.bridge.log_stp 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
net.link.bridge.pfil_onlyip 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.
net.link.bridge.pfil_member Set to 1 to enable filtering on the incoming
and outgoing member interfaces, set to 0 to
net.link.bridge.pfil_bridge 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.
net.link.bridge.ipfw 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
net.link.bridge.ipfw_arp 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
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 net.link.bridge.pfil_local_phys 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:
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 22.214.171.124 126.96.36.199 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
SEE ALSOgif(4), ipf(4), ipfw(4), pf(4), ifconfig(8)HISTORY
The if_bridge driver first appeared in FreeBSD 6.0.
The bridge driver was originally written by Jason L. Wright
⟨firstname.lastname@example.org⟩ 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 ⟨email@example.com⟩.
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