man/man4/bridge.4

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.\" Written by Jason R. Thorpe for Wasabi Systems, Inc.
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.Dd April 10, 2023
.Dt IF_BRIDGE 4
.Os
.Sh NAME
.Nm if_bridge
.Nd network bridge device
.Sh SYNOPSIS
To compile this driver into the kernel,
place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "device if_bridge"
.Ed
.Pp
Alternatively, to load the driver as a
module at boot time, place the following lines in
.Xr loader.conf 5 :
.Bd -literal -offset indent
if_bridge_load="YES"
bridgestp_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
driver creates a logical link between two or more IEEE 802 networks
that use the same (or
.Dq "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.
.Pp
Each
.Nm
interface is created at runtime using interface cloning.
This is
most easily done with the
.Xr ifconfig 8
.Cm create
command or using the
.Va cloned_interfaces
variable in
.Xr rc.conf 5 .
.Pp
The
.Nm
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
.Em only
across all
.Nm
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
.Xr ifconfig 8 .
.Pp
If
.Xr sysctl 8
node
.Va 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
.Xr ng_pppoe 4
and
.Xr ppp 8 .
Currently this feature is considered as experimental and is turned off
by default.
.Pp
A bridge can be used to provide several services, such as a simple
802.11-to-Ethernet bridge for wireless hosts, or traffic isolation.
.Pp
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.
.Pp
By default the bridge logs MAC address port flapping to
.Xr syslog 3 .
This behavior can be disabled by setting the
.Xr sysctl 8
variable
.Va net.link.bridge.log_mac_flap
to
.Li 0 .
.Pp
All the bridged member interfaces need to be up in order to pass network traffic.
These can be enabled using
.Xr ifconfig 8
or
.Va ifconfig_ Ns Ao Ar interface Ac Ns Li ="up"
in
.Xr rc.conf 5 .
.Pp
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.
.Pp
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.
.Pp
The bridge supports
.Dq monitor mode ,
where the packets are discarded after
.Xr 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
.Xr bpf 4
stream.
This is useful for reconstructing the traffic for network taps
that transmit the RX/TX signals out through two separate interfaces.
.Sh IPV6 SUPPORT
.Nm
supports the
.Li AF_INET6
address family on bridge interfaces.
The following
.Xr rc.conf 5
variable configures an IPv6 link-local address on
.Li bridge0
interface:
.Bd -literal -offset indent
ifconfig_bridge0_ipv6="up"
.Ed
.Pp
or in a more explicit manner:
.Bd -literal -offset indent
ifconfig_bridge0_ipv6="inet6 auto_linklocal"
.Ed
.Pp
However, the
.Li 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
.Nm
interface has another single,
aggregated link-local scope zone at the same time.
This situation is clearly against the description
.Qq 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
.Nm
interface and one of the member interfaces, have an IPv6 address
and applications use both of them.
.Pp
To prevent this situation,
.Nm
checks whether a link-local scoped IPv6 address is configured on
a member interface to be added and the
.Nm
interface.
When the
.Nm
interface has IPv6 addresses,
IPv6 addresses on the member interface will be automatically removed
before the interface is added.
.Pp
This behavior can be disabled by setting
.Xr sysctl 8
variable
.Va net.link.bridge.allow_llz_overlap
to
.Li 1 .
.Pp
Note that
.Li ACCEPT_RTADV
and
.Li AUTO_LINKLOCAL
interface flags are not enabled by default on
.Nm
interfaces even when
.Va net.inet6.ip6.accept_rtadv
and/or
.Va net.inet6.ip6.auto_linklocal
is set to
.Li 1 .
.Sh SPANNING TREE
The
.Nm
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.
.Pp
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.
.Pp
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
.Va proto
command in
.Xr ifconfig 8 .
.Pp
The bridge can log STP port changes to
.Xr syslog 3
by setting the
.Va net.link.bridge.log_stp
node using
.Xr sysctl 8 .
.Sh PACKET FILTERING
Packet filtering can be used with any firewall package that hooks in via the
.Xr 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
.Xr sysctl 8 :
.Bl -tag -width ".Va net.link.bridge.pfil_onlyip"
.It Va net.link.bridge.pfil_onlyip
Controls the handling of non-IP packets which are not passed to
.Xr pfil 9 .
Set to
.Li 1
to only allow IP packets to pass (subject to firewall rules), set to
.Li 0
to unconditionally pass all non-IP Ethernet frames.
.It Va net.link.bridge.pfil_member
Set to
.Li 1
to enable filtering on the incoming and outgoing member interfaces, set
to
.Li 0
to disable it.
.It Va net.link.bridge.pfil_bridge
Set to
.Li 1
to enable filtering on the bridge interface, set
to
.Li 0
to disable it.
.It Va net.link.bridge.pfil_local_phys
Set to
.Li 1
to additionally filter on the physical interface for locally destined packets.
Set to
.Li 0
to disable this feature.
.It Va net.link.bridge.ipfw
Set to
.Li 1
to enable layer2 filtering with
.Xr ipfirewall 4 ,
set to
.Li 0
to disable it.
This needs to be enabled for
.Xr dummynet 4
support.
When
.Va ipfw
is enabled,
.Va pfil_bridge
and
.Va pfil_member
will be disabled so that IPFW
is not run twice; these can be re-enabled if desired.
.It Va net.link.bridge.ipfw_arp
Set to
.Li 1
to enable layer2 ARP filtering with
.Xr ipfirewall 4 ,
set to
.Li 0
to disable it.
Requires
.Va ipfw
to be enabled.
.El
.Pp
ARP and REVARP packets are forwarded without being filtered and others
that are not IP nor IPv6 packets are not forwarded when
.Va pfil_onlyip
is enabled.
IPFW can filter Ethernet types using
.Cm mac-type
so all packets are passed to
the filter for processing.
.Pp
The packets originating from the bridging host will be seen by
the filter on the interface that is looked up in the routing
table.
.Pp
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
.Xr 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
.Nm .
It is not recommended to rely on the order chosen by the current
.Nm
implementation since it may change in the future.
.Pp
The previous paragraph is best illustrated with the following
pictures.
Let
.Bl -bullet
.It
the MAC address of the incoming packet's destination is
.Nm nn:nn:nn:nn:nn:nn ,
.It
the interface on which packet entered the system is
.Nm ifX ,
.It
.Nm ifX
MAC address is
.Nm xx:xx:xx:xx:xx:xx ,
.It
there are possibly other bridge members with the same MAC address
.Nm xx:xx:xx:xx:xx:xx ,
.It
the bridge has more than one interface that are sharing the
same MAC address
.Nm yy:yy:yy:yy:yy:yy ;
we will call them
.Nm vlanY1 ,
.Nm vlanY2 ,
etc.
.El
.Pp
If the MAC address
.Nm nn:nn:nn:nn:nn:nn
is equal to
.Nm xx:xx:xx:xx:xx:xx
the filter will see the packet on interface
.Nm ifX
no matter if there are any other bridge members carrying the same
MAC address.
But if the MAC address
.Nm nn:nn:nn:nn:nn:nn
is equal to
.Nm yy:yy:yy:yy:yy:yy
then the interface that will be seen by the filter is one of the
.Nm vlanYn .
It is not possible to predict the name of the actual interface
without the knowledge of the system state and the
.Nm
implementation details.
.Pp
This problem arises for any bridge members that are sharing the same
MAC address, not only to the
.Xr 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
.Nm
interface and not to the bridge members.
Enabling
.Va net.link.bridge.pfil_local_phys
will let you do the additional filtering on the physical interface.
.Sh NETMAP
.Xr 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.
.Pp
When the
.Xr netmap 4
application transmits a packet to the host stack via the bridge interface,
.Nm
receive it and attempts to determine its
.Ql 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,
.Nm
treats the packet as though it was received from the corresponding interface
and handles it normally without passing the packet back to
.Xr netmap 4 .
.Sh EXAMPLES
The following when placed in the file
.Pa /etc/rc.conf
will cause a bridge called
.Dq Li bridge0
to be created, and will add the interfaces
.Dq Li wlan0
and
.Dq Li 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).
.Bd -literal -offset indent
cloned_interfaces="bridge0"
ifconfig_bridge0="addm wlan0 addm fxp0 up"
.Ed
.Pp
For the bridge to forward packets,
all member interfaces and the bridge need to be up.
The above example would also require:
.Bd -literal -offset indent
create_args_wlan0="wlanmode hostap"
ifconfig_wlan0="up ssid my_ap mode 11g"
ifconfig_fxp0="up"
.Ed
.Pp
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:
.Bd -literal -offset indent
ifconfig bridge0 create
ifconfig bridge0 \e
    addm fxp0 stp fxp0 \e
    addm fxp1 stp fxp1 \e
    addm fxp2 stp fxp2 \e
    addm fxp3 stp fxp3 \e
    addm fxp4 stp fxp4 \e
    addm fxp5 stp fxp5 \e
    addm fxp6 stp fxp6 \e
    addm fxp7 stp fxp7 \e
    up
.Ed
.Pp
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:
.Bd -literal -offset indent
cloned_interfaces="bridge0"
ifconfig_bridge0="addm em0 addm em1 DHCP"
ifconfig_em0="up"
ifconfig_em1="up"
.Ed
.Pp
The bridge can tunnel Ethernet across an IP internet using the EtherIP
protocol.
This can be combined with
.Xr ipsec 4
to provide an encrypted connection.
Create a
.Xr gif 4
interface and set the local and remote IP addresses for the
tunnel, these are reversed on the remote bridge.
.Bd -literal -offset indent
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
.Ed
.Sh SEE ALSO
.Xr gif 4 ,
.Xr ipf 4 ,
.Xr ipfw 4 ,
.Xr netmap 4 ,
.Xr pf 4 ,
.Xr ifconfig 8
.Sh HISTORY
The
.Nm
driver first appeared in
.Fx 6.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm bridge
driver was originally written by
.An Jason L. Wright Aq Mt jason@thought.net
as part of an undergraduate independent study at the University of
North Carolina at Greensboro.
.Pp
This version of the
.Nm
driver has been heavily modified from the original version by
.An Jason R. Thorpe Aq Mt thorpej@wasabisystems.com .
.Pp
Rapid Spanning Tree Protocol (RSTP) support was added by
.An Andrew Thompson Aq Mt thompsa@FreeBSD.org .
.Sh BUGS
The
.Nm
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.