On 07/10/2014 13:14, Oliver Lagni wrote:

Hello all,

I’m setting the DSCP on some traffic going out and getting in on my firewall.

With NFSEN I collect traffic from both segments, LAN and WAN Firewall sides.

On my firewall I set DSCP to 101110 for real-time traffic and I clearly see it on Nprobe server on both segments, as soon as I filter with TCPDump:

tcpdump -i eth2 -vvv -n ip and ip[1]=0xb8

0xb8 is 184 in HEX.. and I see this on eth2 (WAN) and eth3 (LAN):

14:21:23.236494 IP (*tos 0xb8*, ttl 126, id 4388, offset 0, flags [DF], proto TCP (6), length 450)

217.xx.xx.xx.47460 > 64.xx.xx.xx.https: Flags [P.], cksum 0x5af4 (correct), seq 949:1359, ack 84, win 256, length 410

But as soon as I filter on NFSEN with syntax Tos 184 or tos 0xb8 I don’t see anything.

Is there any reason? Can someone help me a bit on this?


I am not sure, but I think the tos value you filter with is the 3 most significant bits, so a value between 0-7

0 = 000xxxxxx
1 = 001xxxxxx
2 = 010xxxxxx
3 = 011xxxxxx
4 = 100xxxxxx
5 = 101xxxxxx
6 = 110xxxxxx
7 = 111xxxxxx

So "tos 1" filter matches your priority packets?

--
Regards,

Giles Coochey, CCNP, CCNA, CCNAS
NetSecSpec Ltd
+44 (0) 8444 780677
+44 (0) 7584 634135
http://www.coochey.net
http://www.netsecspec.co.uk
[email protected]

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