NAME¶
if_bridge —
network bridge
device
SYNOPSIS¶
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):
if_bridge_load="YES"
bridgestp_load="YES"
DESCRIPTION¶
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 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
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 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 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.
SPANNING TREE¶
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 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¶
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):
- 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 disable it.
- net.link.bridge.pfil_bridge
- Set to
1
to enable filtering on the
bridge interface, set to 0
to disable it.
- net.link.bridge.pfil_local_phys
- Set to
1
to additionally filter on
the physical 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
desired.
- 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 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
net.link.bridge.pfil_local_phys will let you do the
additional filtering on the physical interface.
EXAMPLES¶
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).
cloned_interfaces="bridge0"
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"
ifconfig_fxp0="up"
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 \
up
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:
cloned_interfaces="bridge0"
ifconfig_bridge0="addm em0 addm em1 DHCP"
ifconfig_em0="up"
ifconfig_em1="up"
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 1.2.3.4 5.6.7.8 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.
SEE ALSO¶
gif(4),
ipf(4),
ipfw(4),
pf(4),
ifconfig(8)
HISTORY¶
The
if_bridge driver first appeared in
FreeBSD 6.0.
AUTHORS¶
The
bridge driver was originally written by
Jason L. Wright ⟨jason@thought.net⟩ 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
⟨thorpej@wasabisystems.com⟩.
Rapid Spanning Tree Protocol (RSTP) support was added by
Andrew
Thompson ⟨thompsa@FreeBSD.org⟩.
BUGS¶
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.