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fprobe Only Collects One-Way Data

Created by dave. Last edited by dave, 11 years and 21 days ago. Viewed 4,540 times. #7
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(7 November 2012)


Running fprobe-1.1, feeding nfsen 1.3.6 using nfdump 1.6.6 on CentOS-5. The data is being collected on a port-mirrored port from a HP-5412zl switch. The mirrored port is the LAN side of my firewall.

My issue is that nfsen is only showing traffic which flows in one direction (from the internet into my LAN). However if I do a tcpdump on the eth that fprobe is listening to, it sees all traffic (inbound and outbound).

Does anyone have any idea why my flows are not being collected in both directions?


I saw this issue once before. (>>Link) I had a 3Com 4900 which had a port with multiple VLANs sent to a port-mirror. Fprobe correctly collected all traffic in both directions from all VLANs involved. When that switch was replaced with a Dell 3448, I only saw a single direction (in that case, outbound-only) collected. I got around this issue by defining VLAN interfaces and attaching individual fprobe processes to each VLAN interface.

However in this case I'm not VLAN'd on this port at all. So I don't understand what is going wrong.


(10 November 2012):

So I messed around with tcpdump some more to see what was going on and I had some interesting results.

If I did my tcpdump as so:

# tcpdump -ni eth1 dst net $NET and not src net $NET
… I got the inbound traffic that I expected. However, flipping the src and dst around, ie:
# tcpdump -ni eth1 src net $NET and not dst net $NET
...yielded nothing.

If, however, I specified VLAN tagging:

# tcpdump -ni eth1 vlan and src net $NET and not dst net $NET
… I got the outbound traffic.

So clearly something weird is happening either on my network card or my switch. I'm tempted to think the former, because when I had this plugged into a different computer, the fprobe collection worked. The port-mirror has been unchanged. (Unfortunately I can't go back in time and see what was different about the old computer since it physically died.)

The man page for fprobe suggests I should be able to say -f "ip or vlan" to collect both vlan and non-vlan traffic, but fprobe barfs on this.

(13 November 2012):

So I found out that since the fprobe only collects one way, I have to have two fprobe instances:

/usr/local/sbin/fprobe -fvlan -K18 -i eth1 -e 120 -q 10240 $TARGET:9999
/usr/local/sbin/fprobe -fip -i eth1 -e 120 -q 10240 $TARGET:9998
The first one collects traffic heading out of my network, and the second one collects traffic coming in. Note the -K18 parameter, without which the -fvlan fprobe won't collect anything (see the fprobe man page, EXAMPLES section).

I further refine this by creating a live nfsen profile that has appropriate src net and not dst net (etc) rules, and using that live profile as a basis for analysis.

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