IF_BRIDGE(4) FreeBSD Kernel Interfaces Manual IF_BRIDGE(4)
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).
When it is created, the if_bridge interface gets assigned a link (MAC)
address in the range of universally administered addresses reserved for
the FreeBSD Foundation by hashing the host UUID, jail name, and the
interface name. If this fails, a random, locally administered address is
generated instead. 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 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 a non-zero value, the
newly created bridge will inherit the MAC address from its first member
instead of choosing a random link-level address. This will provide more
predictable bridge MAC addresses 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). Currently 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, or 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.
By default the bridge logs MAC address port flapping to syslog(3). This
behavior can be disabled by setting the sysctl(8) variable
net.link.bridge.log_mac_flap to 0.
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 will have their MTU changed to match. If
the MTU of a bridge is changed after its creation, the MTU of all member
interfaces is also changed to match.
The TOE, TSO, TXCSUM and TXCSUM6 capabilities on all interfaces added to
the bridge are disabled if any of the interfaces do not support/enable
them. The LRO capability is always disabled. All the capabilities are
restored when the interface is removed from the bridge. Changing
capabilities at run-time may cause NIC reinit and a link flap.
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.
To allow the host to communicate with bridge members, IP addresses should
be assigned to the if_bridge interface itself, not to the bridge's member
interfaces. Attempting to assign an IP address to a bridge member
interface, or add a member interface with an assigned IP address to a
bridge, will return an EINVAL ("Invalid argument") error. For
compatibility with older releases where this was permitted, setting the
sysctl(8) variable net.link.bridge.member_ifaddrs to 1 will permit this
configuration. This sysctl variable will be removed in FreeBSD 16.0.
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="inet6 auto_linklocal"
However, the AF_INET6 address family has a concept of scope zone.
Bridging multiple interfaces changes 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 counterintuitive or undesirable behavior
in some edge cases when both, the if_bridge interface and one of the
member interfaces, 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 flags are not enabled
by default on if_bridge interfaces 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 setting the
net.link.bridge.log_stp node using sysctl(8).
VLAN SUPPORT
The if_bridge driver has full support for virtual LANs (VLANs). The
bridge implements independent VLAN learning, i.e. MAC addresses are
learned on a per-VLAN basis, and the same MAC address may be learned on
multiple interfaces on different VLANs. Incoming frames with an 802.1Q
tag will be assigned to the appropriate VLAN.
Traffic sent to or from the host is not assigned to a VLAN by default.
To allow the host to communicate on a VLAN, configure a vlan(4) interface
on the bridge and (if necessary) assign IP addresses there.
By default no access control is enabled, so any interface may participate
in any VLAN.
VLAN filtering may be enabled on a bridge using the ifconfig(8)
vlanfilter option. When VLAN filtering is enabled, an interface may only
send and receive frames based on its configured VLAN access list.
The interface's untagged VLAN ID may be configured using the ifconfig(8)
untagged option. If an untagged VLAN ID is configured, incoming frames
will be assigned to that VLAN, and the interface may receive outgoing
untagged frames in that VLAN.
The tagged VLAN access list may be configured using the tagged, +tagged
and -tagged options to ifconfig(8). An interface may send and receive
tagged frames for any VLAN in its access list.
The bridge will automatically insert or remove 802.1q tags as needed,
based on the interface configuration, when forwarding frames between
interfaces. This tag processing is only done for interfaces with VLAN
filtering enabled.
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 behavior 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 since it may change in the
future.
The previous paragraph is best illustrated with the following pictures.
Let
o the MAC address of the incoming packet's destination is
nn:nn:nn:nn:nn:nn,
o the interface on which packet entered the system is ifX,
o ifX MAC address is xx:xx:xx:xx:xx:xx,
o there are possibly other bridge members with the same MAC address
xx:xx:xx:xx:xx:xx,
o 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.
If the MAC address nn:nn:nn:nn:nn:nn is equal to xx:xx:xx:xx:xx:xx the
filter will see the packet on 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 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 were taken just as an example
of such a situation. So if one wants to 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.
NETMAP
netmap(4) applications may open a bridge interface in emulated mode. The
netmap application will receive all packets which arrive from member
interfaces. In particular, packets which would otherwise be forwarded to
another member interface will be received by the netmap application.
When the netmap(4) application transmits a packet to the host stack via
the bridge interface, if_bridge receive it and attempts to determine its
`source' interface by looking up the source MAC address in the
interface's learning tables. Packets for which no matching source
interface is found are dropped and the input error counter is
incremented. If a matching source interface is found, if_bridge treats
the packet as though it was received from the corresponding interface and
handles it normally without passing the packet back to netmap(4).
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
SEE ALSO
gif(4), ipf(4), ipfw(4), netmap(4), pf(4), vlan(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, which can be configured with the same MTU
size as the bridge device.
FreeBSD 15.1-STABLE-HBSD July 28, 2025 IF_BRIDGE(4)