man/man4/bridge.4

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.Dd February 22, 2011
.Dt BRIDGE 4
.Os
.Sh NAME
.Nm bridge
.Nd network bridge device
.Sh SYNOPSIS
.Cd "pseudo-device 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
To use
.Nm ,
the administrator must first create the interface and configure
the bridge parameters.
The bridge is created using the
.Xr ifconfig 8
.Cm create
subcommand.
See the
.Xr ifconfig 8
manual page for further information on configuring bridges.
.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 bridge operates in a safe mode by default, setting the MAC source in
the link header on outgoing packets to the outgoing interface MAC.
This reduces the chance that the layer-2 switching in your switches
will become confused.
.Pp
The bridge supports various special features via
.Cm link
options.
.Bl -tag -width indent
.It Cm link0
The link0 option enables transparent bridging mode.
The bridge will make every effort to retain the ethernet header
when forwarding packets between interfaces, making the bridging
function work more like a hardware bridge device.
.It Cm link1
The link1 option enables keepalive transmission and automatically
places a member into a special blocked mode if no keepalive reception
occurs.
If either sides of the link uses this option then both sides must use
this option.
This option is impemented by sending CFG updates on the hello interval
to the remote.
The link is considered lost after 10 intervals (typically 20 seconds).
.It Cm link2
The link2 option enables channel bonding (see also ifbondweight).
All member interfaces with the same mac address are considered to
be in a bonding group.
When something like
.Xr tap 4
is used, you can manually control or copy the mac to create bonding groups.
When interface bonding is enabled normally blocked interfaces belonging
to the same bonding group as an active forwarding interface will be
changed to the bonding state.
Both sides of link the member represents must operate in bonding mode
for this to work, otherwise the remote end may decide to throw away
half your packets.
.El
.Pp
If your network becomes glitchy, with long pauses in tcp sessions, then
transparent bridging mode is likely the cause.  This mode should only be
used when you are bridging networks with devices that do MAC-based security
or firewalling (for example, the supremely braindead at&t uverse router),
or which impose severe limitations on MAC:IP assignments.
.Pp
If member interfaces constantly enter a 'blocked (link1)' state then the
other end of those interfaces is not implementing the link1 keepalive.
Both sides must implement the keepalive.
.Pp
If you get an enormous amount of packet loss and are using link2-based
bonding, then the other side of those member interfaces are probably
not implementing link2-based bonding.
.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
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, this behaviour can be controlled using
.Xr sysctl 8 :
Set
.Va net.link.bridge.pfil_member
to
.Li 1
to enable filtering on the incoming and outgoing member interfaces
and set
.Va net.link.bridge.pfil_bridge
to
.Li 1
to enable filtering on the bridge interface.
.Pp
ARP and REVARP packets are forwarded without being filtered and others
that are not IP nor IPv6 packets are not forwarded when filtering is
enabled.
.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.
.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.
.Sh EXTRA FEATURES
.Dx
implements two additional features to make spanning tree operation more
resilient.
.Pp
Specifying
.Cm link0
on the bridge interface places the bridge in transparent bridging mode.
The bridge will make every attempt to retain the original source MAC in
the ethernet link header.
.Pp
Specifying
.Cm link1
on the bridge interface forces the bridge to generate a 802.11d CFG
message on every hello interval for all interfaces participating
in the STP protocol.
Normally CFG messages are only generated by the root bridge interface
or during topology changes.
In addition the bridge code expects to receive 802.11d frames from
all interface participating in the STP protocol.
.Pp
An interface which fails to receive a 802.11d frame within 10 times
the hello interval (usually 20 seconds) automatically goes into
l1blocking mode, which can be observed in the ifconfig output for
the bridge.  This removes the interface from consideration and the
bridge code automatically routes around it.
.Pp
Using
.Cm link0
and
.Cm link1
together between two
.Dx
boxes allows you to maintain multiple parallel vpns between those
boxes via different networks (if you happen to be on more than one
with internet access).
Use separate openvpn instances and tap devices for each vpn link
to accomplish this, placing them in the same bridge interface on
the two endpoints.
The tap devices do not need any IP configuration when bridged and
can be assigned the same ether MAC (in fact they have to be
if you want the failover to work nicely).
.Sh SEE ALSO
.Xr pf 4 ,
.Xr ifconfig 8
.Sh HISTORY
The
.Nm
driver first appeared in
.Ox 2.5
and found its way into
.Dx 1.3 .
Transparent bridging (link0) was added in
.Dx 2.9
in 2011.
.Sh AUTHORS
.An -nosplit
The
.Nm
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 .
.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.