Streaming pcap to a dummy interface

Posted at 10:09 pm in Uncategorized

In an earlier article, I described how to stream captured network traffic to a remote host for IDS analysis with Snort. Mikrotik units can stream captured traffic elsewhere using the TaZmen Sniffer Protocol (TZSP). tcpdump and Wireshark natively decode this protocol, but unfortunately it doesn’t seem to be supported by any other of the large amount of useful network analysis tool.

The main usage for streaming the network capture is feeding it to the Snort IDS, and for this single purpose (and since Snort can read from STDIN) piping the traffic directly from tzsp2pcap to Snort works very well. However, now and again I need to look at the network traffic without having to detach Snort from the live stream.

This was solved by making the network traffic stream available over a dummy interface with the brilliant tool tcpreplay, to which I can attach any tool that understands pcap. These are the required incantations:

/sbin/modprobe dummy
/sbin/ip link set name eth10 dev dummy0
/sbin/ifconfig eth10
/usr/bin/screen -dm -S tzsp2pcap \
bash -c "/usr/local/sbin/tzsp2pcap -f | \
/usr/bin/tcpreplay --topspeed -i eth10 -"
/etc/init.d/snort start # configured to listen to eth10

The tzsp2pcap process must be run in a screen. Involving bash is required because of the pipe. With tcpreplay --topspeed is useful to avoid delays in the feed.

At this stage, I can point any network tool at eth10 without disrupting Snort.

Written by bjorn on October 5th, 2015

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