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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.
IPV6 SUPPORT
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:
ifconfig_bridge0_ipv6="up"
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
net.link.bridge.allow_llz_overlap 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.
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 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
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.