Working with large PCAPs

Recently I helped troubleshoot a networking issue with a 1Gbps link that was being fully saturated. In the end we configured a mirror port and dumped the traffic to analyse it. We ended up with >1000, 100MB files covering a half hour window. I was not sure what to do with such a quantity of data ~100GB and in the end we looked at a few caps and found the issue. However I could not help but feel that because most of the caps only covered a few seconds that I was not really looking at the full picture.

After the fact I made some discoveries…


There is a tool called mergecap that can concatenate pcap files, however it complains when given >~1000 files. So in order to get one big file I had to run it a few times. First merging 100 files into large files and then merging the result into the final large file.

$ for n in `seq 0 9`; do echo $n; mergecap -w badport_l00$n.cap badport_00$n*; done
$ for n in `seq 0 9`; do echo $n; mergecap -w badport_l01$n.cap badport_01$n*; done
$ mergecap -w barport.cap badport_l*.cap

Memory required

I didn’t try opening the large 100G file with wireshark I went straight for tshark. Howeve it exhausted all memory and eventually the OOM killer kicked in. After adding more memory to my virtual machine I found that for a 109G pcap that 72G memory is required. Indicating that a potential rule of thumb is that to open or process a pcap of size n one requires two thirds the size of n’s memory available.


I was not really interested in viewing all packets, I wanted to get a feel for the entire data set. The two main commands to do this are:

tshark -q -r badport.cap -z io,phs
tshark -q -r badport.cap -z endpoints,ip | head -50

These turn on quiet mode, read from the badport.cap file and print either the Protocol Hierarchy Statistics or IPv4 Endpoints statistics.

Each of these took around 30 min to complete, which means that best case between the copying the pcaps to the host, merging the pcaps and running the tshark statistics its best case time is around 2 hours.


I think I’ve concluded that next time I should aim to concatenate down to around 10G max, as these files are a lot easier two work with (eg, 2 minutes to complete above statistics). Also The above 2 hour process found the same issue that looking at a single 100M cap did.

Computer enthusiast and sysadmin