[LARTC] Does -j TOS actually do anything?

[Thomas Worthington <lartc@tww.cx>]

Very simple setup: I have several machines, one of which (192.168.0.198) 
is used exclusively as a vnc client to a remote site. I want it to get 
priority over guff like email and web etc.

tc qdisc add dev eth1 root handle 1: prio
tc qdisc add dev eth1 parent 1:1 handle 10: pfifo
tc qdisc add dev eth1 parent 1:2 handle 20: pfifo
tc qdisc add dev eth1 parent 1:3 handle 30: pfifo

iptables -t mangle -I OUTPUT -p icmp -s 192.168.0.198 -j TOS --set-tos 16

Effect: zero. All the vnc traffic from x.198 goes into 1:2.

I've tried all the tables (PREROUTE etc) to no avail. I even tried 
applying the TOS change to ALL traffice bound for eth1 (the outside line) 
and still saw no change in the queuing despite tcpdump showing that the 
tos bits were being set.

Why does this not work? It seems as if the priority is being set before 
iptables' mangle rules are applied.

Thomas Worthington

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