To determine which class shall process a packet, the so-called ‘classifier chain’ is called each time a choice needs to be made. This chain consists of all filters attached to the classful qdisc that needs to decide.
To reiterate the tree, which is not a tree:
root 1: | _1:1_ / | \ / | \ / | \ 10: 11: 12: / \ / \ 10:1 10:2 12:1 12:2
When enqueueing a packet, at each branch the filter chain is consulted for a relevant instruction. A typical setup might be to have a filter in 1:1 that directs a packet to 12: and a filter on 12: that sends the packet to 12:2.
You might also attach this latter rule to 1:1, but you can make efficiency gains by having more specific tests lower in the chain.
You can’t filter a packet ‘upwards’, by the way. Also, with HTB, you should attach all filters to the root!
And again – packets are only enqueued downwards! When they are dequeued, they go up again, where the interface lives. They do NOT fall off the end of the tree to the network adaptor!
As explained in the Classifier chapter, you can match on literally anything, using a very complicated syntax. To start, we will show how to do the obvious things, which luckily are quite easy.
Let’s say we have a PRIO qdisc called ’10:’ which contains three classes, and we want to assign all traffic from and to port 22 to the highest priority band, the filters would be:
# tc filter add dev eth0 protocol ip parent 10: prio 1 u32 match \ ip dport 22 0xffff flowid 10:1 # tc filter add dev eth0 protocol ip parent 10: prio 1 u32 match \ ip sport 80 0xffff flowid 10:1 # tc filter add dev eth0 protocol ip parent 10: prio 2 flowid 10:2
What does this say? It says: attach to eth0, node 10: a priority 1 u32 filter that matches on IP destination port 22 *exactly* and send it to band 10:1. And it then repeats the same for source port 80. The last command says that anything unmatched so far should go to band 10:2, the next-highest priority.
You need to add ‘eth0’, or whatever your interface is called, because each interface has a unique namespace of handles.
To select on an IP address, use this: