233 lines
6.1 KiB
Groff
233 lines
6.1 KiB
Groff
.\"
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.\" $FreeBSD$
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.\"
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.Dd February 15, 2002
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.Dt BRIDGE 4
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.Os
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.Sh NAME
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.Nm bridge
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.Nd bridging support
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.Sh SYNOPSIS
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.Cd "options BRIDGE"
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.Sh DESCRIPTION
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.Fx
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supports bridging on Ethernet-type interfaces, including VLANs.
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Bridging support can be either compiled into the kernel, or loaded
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at runtime as a kernel module.
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.Pp
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A single
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.Fx
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host can do bridging on independent sets of interfaces,
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which are called
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.Dq clusters .
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Each cluster connects a set of interfaces, and is
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identified by a
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.Dq cluster-ID
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which is a number in the range 1..65535.
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A cluster in fact is very similar to what commercial switches call
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a
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.Dq VLAN .
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Note however that there is no relation whatsoever
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between the cluster-ID and the IEEE 802.1q VLAN-ID which appears
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in the header of packets transmitted on the wire.
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In fact, in most cases there is no relation between the
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so-called
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.Dq "VLAN identifier"
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used in most commercial switches, and
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the IEEE 802.1q VLAN-ID.
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.Pp
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By putting both physical and logical
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.Pq Xr vlan 4
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interfaces in the same cluster, a
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.Fx
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box can also implement what in commercial terms is called a
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.Dq trunk
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interface.
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This means that packets
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coming from one of the interfaces in a cluster
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will appear on the wire of the
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.Dq parent
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interface of any VLAN interface in a cluster,
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with the proper VLAN tag.
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Similarly, packets
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coming from a parent interface of any VLAN interface in a cluster
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will have the VLAN tag stripped,
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and will be forwarded to other interfaces in a cluster.
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See the
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.Sx EXAMPLES
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section for more details.
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.Pp
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Runtime operation of the
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.Nm
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is controlled by several
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.Xr sysctl 8
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variables, as follows.
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.Bl -tag -width indent
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.It Va net.link.ether.bridge
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set to
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.Li 1
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to enable bridging, set to
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.Li 0
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to disable it.
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.It Va net.link.ether.bridge_ipfw
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set to
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.Li 1
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to enable
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.Xr ipfw 8
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filtering on bridged packets.
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Note that
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.Xr ipfw 8
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rules only apply
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to IP packets.
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Non-IP packets are accepted by default.
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See the
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.Sx BUGS
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section and the
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.Xr ipfw 8
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manpage for more details on the interaction of bridging
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and the firewall.
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.It Va net.link.ether.bridge_cfg
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contains a list of interfaces on which bridging is to be performed.
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Interfaces are separated by spaces, commas or tabs.
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Each interface
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can be optionally followed by a colon and an integer indicating the
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cluster it belongs to (defaults to 1 if the cluster-ID is missing), e.g.\&
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.Dq Li "dc0:1,dc1,vlan0:3 dc2:3"
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will put
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.Li dc0
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and
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.Li dc1
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in cluster number 1, and
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.Li vlan0
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and
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.Li dc2
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in cluster
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number 3.
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See the
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.Sx EXAMPLES
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section for more examples.
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.Pp
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The list of interfaces is rescanned every time the list is
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modified, bridging is enabled, or new interfaces are created or
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destroyed.
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Interfaces that are in the list but cannot be used
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for bridging (because they are non-existing, or not Ethernet or VLAN)
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are not used and a warning message is generated.
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.El
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.Pp
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Bridging requires interfaces to be put in promiscuous mode,
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and transmit packets with Ethernet source addresses.
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Some interfaces (e.g.\&
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.Xr wi 4 )
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do not support this functionality.
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Also, bridging is not compatible with interfaces which
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use hardware loopback, because there is no way to tell locally
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generated packets from externally generated ones.
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.Sh EXAMPLES
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A simple bridge configuration with three interfaces in the same
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cluster can be set as follows.
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No cluster-ID is specified here, which
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will cause the interfaces to appear as part of cluster #1.
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.Pp
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.Dl "sysctl net.link.ether.bridge_cfg=dc0,dc1,fxp1"
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.Pp
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If you do not know what actual interfaces will be present on
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your system, you can just put all existing interfaces in the
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configuration, as follows:
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.Pp
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.Dl sysctl net.link.ether.bridge_cfg="`ifconfig -l`"
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.Pp
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This will result in a space-separated list of interfaces.
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Out of the list, only Ethernet and VLAN interfaces will be
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used for bridging, whereas for others the kernel will produce
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a warning message.
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.Pp
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More complex configurations can be used to create multiple
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clusters, e.g.\&
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.Pp
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.Dl "sysctl net.link.ether.bridge_cfg=dc0:3,dc1:3,fxp0:4,fxp1:4"
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.Pp
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will create two completely independent clusters.
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.Pp
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Finally, interesting configurations involve VLANs and parent interfaces.
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As an example, the following configuration will use interface
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.Li dc0
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as a
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.Dq trunk
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interface, and pass packets
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for 802.1q VLANs 10 and 20 to physical interfaces
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.Li dc1
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and
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.Li dc2 ,
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respectively:
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.Bd -literal -offset indent
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sysctl net.link.ether.bridge_cfg=vlan0:34,dc1:34,vlan1:56,dc2:56
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ifconfig vlan0 vlan 10 vlandev dc0
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ifconfig vlan1 vlan 20 vlandev dc0
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.Ed
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.Pp
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Note how there is no relation between the 802.1q VLAN identifiers
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(10 and 20) and the cluster-ID's (34 and 56) used in
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the
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.Va bridge_cfg
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variable.
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.Pp
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Note also that the trunk interface
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does not even appear in the
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.Va bridge_cfg ,
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as VLAN tag insertion/removal
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is performed by the
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.Xr vlan 4
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devices.
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When using VLAN devices, care must be taken by not creating loops
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between these devices and their parent interfaces.
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.Sh BUGS
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Care must be taken not to construct loops in the
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.Nm
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topology.
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The kernel supports only a primitive form of loop detection, by disabling
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some interfaces when a loop is detected.
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No support for a daemon running the
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spanning tree algorithm is currently provided.
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.Pp
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With bridging active, interfaces are in promiscuous mode,
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thus causing some load on the system to receive and filter
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out undesired traffic.
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.Pp
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When passing bridged packets to
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.Xr ipfw 8 ,
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remember that only IP packets are passed to the firewall, while
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other packets are silently accepted.
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Also remember that bridged packets are accepted after the
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first pass through the firewall irrespective of the setting
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of the sysctl variable
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.Va net.inet.ip.fw.one_pass ,
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and that some
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.Xr ipfw 8
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actions such as
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.Cm divert
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do not apply to bridged packets.
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It might be useful to have a rule of the form
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.Pp
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.Dl "skipto 20000 ip from any to any bridged"
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.Pp
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near the beginning of your ruleset to implement specific rulesets
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for bridged packets.
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.Sh FILES
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.Bl -tag -width ".Pa /boot/kernel/bridge.ko" -compact
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.It Pa /boot/kernel/bridge.ko
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.Nm
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loadable module.
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.El
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.Sh SEE ALSO
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.Xr ip 4 ,
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.Xr ng_bridge 4 ,
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.Xr vlan 4 ,
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.Xr ipfw 8 ,
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.Xr sysctl 8
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.Sh HISTORY
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Bridging was introduced in
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.Fx 2.2.8
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by
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.An Luigi Rizzo Aq luigi@iet.unipi.it .
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