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Old 05-11-2004, 09:51 AM   #1
Registered: Apr 2002
Location: New York, USA
Distribution: Redhat 7.2, 9.0 Slackware 9.1
Posts: 428

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Arrow Iptables and tcpdump question

When you are running tcpdump does it see the network traffic before iptables is run or after?

I have a backup server that doesn't do anything during the daytime and I also have a Intrusion Detection server(IDS), sometimes I see some oddball traffic coming through the IDS server but there is so much info that I cannot just look at that, I wanted to use iptables to redirect the traffic to my backup machine and then use tcpdump to just look at the packets I am redirecting.

Where the problem comes in is I have constent traffic from router protocols and netbios queries and such that I am having trouble just seeing what traffic is being redirected. Is there a way I can just block all the traffic going to the backup machine except for whats comming from the IDS server? When I block the traffic in iptables it seems that tcpdump picks it up before it is blocked by iptables.
Old 05-11-2004, 08:01 PM   #2
Registered: Oct 2003
Distribution: Slackware
Posts: 388

Rep: Reputation: 30
Tcpdump shows the packets as they are in the cable.

You can give tcpdump rules to select what to show/not show.

You could use an alternative sniffer such as ethereal.


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