Monday, July 1, 2024

Automating tcpdump in Test Scripts

 It's not unusual for me to create a shell script to test networking software, and to have it automatically run tcpdump in the background while the test runs. Generally I do this "just in case something gets weird," so I usually don't pay much attention to the capture.

The other day I was testing my new "raw_send" tool, and my test script consisted of:

tcpdump -i $INTFC -w /tmp/raw_send.pcap &
sleep 0.2
./raw_send $INTFC \
sleep 0.2

Lo and behold, the tcpdump did NOT contain my packet! I won't embarrass myself by outlining the DAY AND A HALF I spent figuring out what was going on, so I'll just give the answer.

The tcpdump tool wants to be as efficient as possible, so it buffers the packets being written to the output file. This is important because a few large file writes are MUCH more efficient than many small file writes. When you kill the tcpdump (either with the "kill" command or with control-C), it does NOT flush out the current partial buffer. There was a small clue provided in the form of the following output:

tcpdump: listening on em1, link-type EN10MB (Ethernet), capture size 262144 bytes
0 packets captured
1 packets received by filter
0 packets dropped by kernel

I thought it was filtering out my packet for some reason. But no, the "0 packets captured" means that zero packets were written to the capture file ... because of buffering.

The solution? Add the option "--immediate-mode" to tcpdump:

tcpdump -i $INTFC -w /tmp/raw_send.pcap --immediate-mode &

Works fine.

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