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bridge: Automatically manage port promiscuous mode.
There exist configurations where the administrator or another management entity has the foreknowledge of all the mac addresses of end systems that are being bridged together. In these environments, the administrator can statically configure known addresses in the bridge FDB and disable flooding and learning on ports. This makes it possible to turn off promiscuous mode on the interfaces connected to the bridge. Here is why disabling flooding and learning allows us to control promiscuity: Consider port X. All traffic coming into this port from outside the bridge (ingress) will be either forwarded through other ports of the bridge (egress) or dropped. Forwarding (egress) is defined by FDB entries and by flooding in the event that no FDB entry exists. In the event that flooding is disabled, only FDB entries define the egress. Once learning is disabled, only static FDB entries provided by a management entity define the egress. If we provide information from these static FDBs to the ingress port X, then we'll be able to accept all traffic that can be successfully forwarded and drop all the other traffic sooner without spending CPU cycles to process it. Another way to define the above is as following equations: ingress = egress + drop expanding egress ingress = static FDB + learned FDB + flooding + drop disabling flooding and learning we a left with ingress = static FDB + drop By adding addresses from the static FDB entries to the MAC address filter of an ingress port X, we fully define what the bridge can process without dropping and can thus turn off promiscuous mode, thus dropping packets sooner. There have been suggestions that we may want to allow learning and update the filters with learned addresses as well. This would require mac-level authentication similar to 802.1x to prevent attacks against the hw filters as they are limited resource. Additionally, if the user places the bridge device in promiscuous mode, all ports are placed in promiscuous mode regardless of the changes to flooding and learning. Since the above functionality depends on full static configuration, we have also require that vlan filtering be enabled to take advantage of this. The reason is that the bridge has to be able to receive and process VLAN-tagged frames and the there are only 2 ways to accomplish this right now: promiscuous mode or vlan filtering. Suggested-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Vlad Yasevich <vyasevic@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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