network bridge device
To compile this driver into the kernel, place the following line in your kernel
Alternatively, to load the driver as a module at boot time, place the following
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.
interface is created at
runtime using interface cloning. This is most easily done with the
command or using the
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
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
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
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
All the bridged member interfaces need to be up in order to pass network
traffic. These can be enabled using
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
processing, and are not processed or forwarded further. This can be used to
multiplex the input of two or more interfaces into a single
stream. This is useful for reconstructing the traffic for network taps that
transmit the RX/TX signals out through two separate interfaces.
address family on bridge interfaces. The
variable configures an IPv6 link-local address on
or in a more explicit manner:
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
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
interface and one of the member
interface have an IPv6 address and applications use both of them.
To prevent this situation,
whether a link-local scoped IPv6 address is configured on a member interface
to be added and the
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
interface flag are not enabled by
interface even when
is set to
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 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
The bridge can log STP port changes to
by enabling the net.link.bridge.log_stp
Packet filtering can be used with any firewall package that hooks in via the
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
- Controls the handling of non-IP packets which are not passed 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
- 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
to disable this feature.
- Set to
1 to enable layer2 filtering with
0 to disable it. This needs to be enabled
support. When ipfw is enabled,
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
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
is enabled. IPFW can filter
Ethernet types using
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
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
. It is not
recommended to rely on the order chosen by the current
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
- the interface on which packet entered the system is
ifX MAC address is
- there are possibly other bridge members with the same MAC address
- the bridge has more than one interface that are sharing the same MAC
yy:yy:yy:yy:yy:yy; we will call
Then if the MAC address
equal to the
filter will see the packet on the interface
no matter if there are any other bridge
members carrying the same MAC address. But if the MAC address
is equal to the
then the interface that
will be seen by the filter is one of the
. It is not possible to predict the
name of the actual interface without the knowledge of the system state and the
This problem arises for any bridge members that are sharing the same MAC
address, not only to the
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
interface and not to the bridge
will let you
do the additional filtering on the physical interface.
The following when placed in the file
will cause a bridge called
” to be created, and will add
the interfaces “
” 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:
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
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
to provide an encrypted connection. Create a
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 126.96.36.199 188.8.131.52 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
driver first appeared in
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
driver has been
heavily modified from the original version by
Jason R. Thorpe
Rapid Spanning Tree Protocol (RSTP) support was added by
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.