tc/netem and iptables: Delay traffic only from specified ip and port

Question

I have a Ubuntu Router with 2 network interfaces. eth0 is the private network and eth1 is the internet. What I want is the following: All packages (mostly udp) comming from a specified private ip (e.g. 192.168.0.4) to a given destination port (e.g. 5200) should be delayed by 500ms.

I tried several things including this, this and this. The only result is, no delay is added.

If possible I want to decide using iptables which packages should be delayed.

Here are some of the errors I got:

root@ubuntu:~# tc qdisc del dev eth1 root
root@ubuntu:~# tc qdisc ls dev eth1
qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
root@ubuntu:~# tc qdisc add dev eth1 root handle 1: htb default 20
root@ubuntu:~# tc class add dev eth1 parent 1: classid 1:1 pfifo_fast
Error: Qdisc "pfifo_fast" is classless.
root@ubuntu:~# tc class add dev eth1 parent 1: classid 1:1 prio
Error: Qdisc "prio" is classless.
root@ubuntu:~# tc class add dev eth1 parent 1: classid 1:10 netem delay 500ms
Error: Qdisc "netem" is classless.
root@ubuntu:~# tc class add dev eth1 parent 1:10 netem delay 500ms
Error: Qdisc "netem" is classless.

What am I doing wrong?

Answer 1

you are trying to apply 'netem' to a class, not a qdisc.

Basic idea is that you have to create the root qdisc, (as you have done) apply a class to that, which can have child classes, these child classes must have the same traffic method as the parent, (in this case htb). those leaf classes then may have a qdisc applied to them, and it is there that you would specify netem (or pfifo_fast, or other classful or classless forms) and its settings.

(or, if you want to be global about it, you can apply netem to the root qdisc without using a class, (hence '"netem" is classless'))

root qdisc --> inner (parent) class --> leaf (child) class --> qdisc

here's some old tc doc that I found pretty clear, though netem was not apparently an option when that was written or updated.