Linux Advanced Routing & Traffic Control HOWTO
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# 12.2. The `route` classifier

This classifier filters based on the results of the routing tables. When a packet that is traversing through the classes reaches one that is marked with the "route" filter, it splits the packets up based on information in the routing table.

    # tc filter add dev eth1 parent 1:0 protocol ip prio 100 route

Here we add a route classifier onto the parent node 1:0 with priority 100. When a packet reaches this node (which, since it is the root, will happen immediately) it will consult the routing table and if one matches will send it to the given class and give it a priority of 100. Then, to finally kick it into action, you add the appropriate routing entry:

The trick here is to define 'realm' based on either destination or source. The way to do it is like this:

    # ip route add Host/Network via Gateway dev Device realm RealmNumber

For instance, we can define our destination network 192.168.10.0 with a realm number 10:

    # ip route add 192.168.10.0/24 via 192.168.10.1 dev eth1 realm 10

When adding route filters, we can use realm numbers to represent the networks or hosts and specify how the routes match the filters.

    # tc filter add dev eth1 parent 1:0 protocol ip prio 100 \
      route to 10 classid 1:10

The above rule says packets going to the network 192.168.10.0 match class id 1:10.

Route filter can also be used to match source routes. For example, there is a subnetwork attached to the Linux router on eth2.

    # ip route add 192.168.2.0/24 dev eth2 realm 2
    # tc filter add dev eth1 parent 1:0 protocol ip prio 100 \
      route from 2 classid 1:2

Here the filter specifies that packets from the subnetwork 192.168.2.0 (realm 2) will match class id 1:2.

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