freebsd/share/man/man4/if_bridge.4

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.Dd June 10, 2005
.Dt IF_BRIDGE 4
.Os
.Sh NAME
.Nm if_bridge
.Nd network bridge device
.Sh SYNOPSIS
.Cd "device if_bridge"
.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
A bridge can be used to provide several services, such as a simple
802.11-to-Ethernet bridge for wireless hosts, and traffic isolation.
.Pp
A bridge works like a hub, 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
The
.Nm
driver implements the IEEE 802.1D Spanning Tree protocol (STP).
Spanning Tree is used to detect and remove loops in a network topology.
.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 Va ="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 are required to have exactly the same value.
.Pp
The TXCSUM capability is disabled for any interface added to the bridge, and it
is restored when the interface is removed again.
.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 PACKET FILTERING
.Pp
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 behaviour 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.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.
.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
Note that packets to and from the bridging host will be seen by the
filter on the interface with the appropriate address configured as well
as on the interface on which the packet arrives or departs.
.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 ath0
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 ath0 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
ifconfig_ath0="up ssid my_ap mode 11g mediaopt hostap"
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 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 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 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 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 thorpej@wasabisystems.com .
.Sh BUGS
The
.Nm
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.
.Pp
The bridge implements version IEEE 802.1D 1998 of the Spanning Tree Protocol so
newer features in IEEE 802.1D 2004 such as RSTP are not available.