fw - fwmark traffic control filter
tc filter ... fw [ classid CLASSID ] [ action ACTION_SPEC ]
the fw filter allows to classify packets based on a previously set fwmark by iptables. If it is identical to the filter’s handle, the filter matches. iptables allows to mark single packets with the MARK target, or whole connections using CONNMARK. The benefit of using this filter instead of doing the heavy-lifting with tc itself is that on one hand it might be convenient to keep packet filtering and classification in one place, possibly having to match a packet just once, and on the other users familiar with iptables but not tc will have a less hard time adding QoS to their setups.
Push matching packets to the class identified by CLASSID.
Apply an action from the generic actions framework on matching packets.
Take e.g. the following tc filter statement:
tc filter add ... handle 6 fw classid 1:1
will match if the packet’s fwmark value is 6. This is a sample iptables statement marking packets coming in on eth0:
iptables -t mangle -A PREROUTING -i eth0 -j MARK --set-mark 6