Re: [LARTC] Does -j TOS actually do anything?

[Patrick McHardy <kaber@trash.net>]

Where did you get the information that setting TOS with
netfilter would affect prio classification ? From what
I can see in the source, prio classifies by skb->priority
which only inheirits a value from tos field during the
forwarding process. I can recall skb->priority also
beeing affected by setsockopt(IP_TOS) (besides SO_PRIORITY)
but not by simply setting the tos field in the ip header.

Best regards,
Patrick

BTW: the netfilter CLASSIFY target from 2.6 or netfiler
patch-o-matic can set skb->priority which is what you need.

Thomas Worthington wrote:

> 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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