Linux – Outgoing IP Packet Capture and Logging with iptables

iptableslinuxnetworkingsniffing

My goal is to use ipset lists in iptables to log outbound traffic to certain IP addresses. I intend to monitor an entire network passively.

I have port mirroring enabled and the port mirrored traffic is broadcasting to a server with two network adapters. Eth0 is dedicated to system management and Eth1 operates in promiscuous mode and is dedicated to capturing the traffic passed to it. Is it possible to use iptables to log outgoing traffic on the network that is being monitored? Thank you.

Best Answer

Have you considered using tcpdump rather than iptables? Something like:

tcpdump -w /var/log/packets -i eth1

Would dump all packets visible on eth1 to the file /var/log/packets. You could later on analyze the file using tcpdump, wireshark, or a variety of other tools.

Using iptables, you typically log packets using the LOG target, like this:

iptables -A INPUT -i eth1 -j LOG

I'm not sure whether or not iptables is the right tool for this job, because (a) I don't know off the top of my head how it will operate with the interface in promiscuous mode, and (b) given high packet rates this form of logging can have a substantial i/o impact on your system.

I think you're better off with the tcpdump model. The -G flag to tcpdump will cause it to rotate the capture file periodically, which you'll want to do if you're capturing for an extended period. So something like:

tcpdump -G 3600 -w /var/log/packets-%Y-%m-%d-%H

This would get you files of the form:

/var/log/packets-2013-06-20-10
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