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Old 06-21-2010, 03:52 AM   #1
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Registered: Nov 2008
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Problem with DSCP tagging of DHCP Discovery packets

Hi all,
can you please help me to solve my problem?
I need to tag outgoing dhcp packets with certain dscp value. I use following iptables command:

iptables -t mangle -A POSTROUTING -p udp -o eth0 -j DSCP --set-dscp 13

This rule is working perfectly if I have common udp traffic (etc with iperf). But if my pc sends DHCP Discovery, the packet has still dscp=0x04 (tos=0x10) - is it a default value?
During DCHP renew, when pc sends DHCP Request, outgoing packet has correct dscp value (13 with this case). It's a problem for broadcast packets only, like DCHP Discovery, but I don't know why it happens and what's wrong.

I tried to put also very simple rule to FORWARD, OUTPUT and POSTROUTING chains of mangle table but DHCP Discovery packet has still dscp = 0x04

itpables -t mangle -A OUTPUT -p udp -j DSCP --set-dscp 13
itpables -t mangle -A FORWARD -p udp -j DSCP --set-dscp 13
itpables -t mangle -A POSTROUTING -p udp -j DSCP --set-dscp 13

I'm using Lenny 2.6.26-2-686 (standard distribution) with iptables v1.4.2 and my pc has only one wan interface (eth0).

Thank you for any help


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