A python library to extract TCP sessions from PCAPs.


TCPSession is a native Python library that extracts out session data sent over a TCP connection from both sides from a pcap. It's faster than firing up tshark -z "follow,tcp,ascii,# or Wireshark on the pcap and doing follow TCP stream in other words. I handles all the cases of TCP protocal, like - discarding the retransmissions, carrying out the assembly of segments at TCP layer, handling out-of-order delivery and then extracting out the data that is actually delivered to application layer by TCP layer on both the sides.

If we open a pcap with TCP packets in the Wireshark, and do follow stream on a TCP session, we get actual payload delivered to the application layer; this Wireshark UI feature is present in Wireshark command line utility (tshark) with command - tshark -r your.pcap -Y " == && tcp.port == 7201" -z "follow,tcp,ascii,1". Goal is to do this session data extraction faster than creating a tshark process to extract the payload.

Another option do this is using the binary tcpflow, available at As tcpflow, is written in C++, it's faster than this library for obvious reasons - Python can't match the speed of C++, but this library extracts data in more useful formats than tcpflow, for e.g., tcpflow only extracts data from both sides and combines them and store it in two files, the order in which that data was exchanged between peers is not preserved. This library extracts data as tcpflow, in addition it also stores the data where order of transfer is maintained. Additionally, it also saves the data in both hex and ASCII format.

Before writing this library, my research to this simple (actually pretty complex) job included looking into tools like, scapy, dpkt and wireshark libraries, but none of them provided a way to do the job I described earlier. Scapy can do it with the support of wireshark, but inside the hood it fires up the tshark (or Wireshrk) binary, which is not any different than what I mentioned earlier - firing wireshark binary and passing it a pcap to work on.

When I started working on this small problem (it's not small, it's actually implementation of whole TCP and IP stack but here we have pcap at our hands with data from both the side), I did not realize the scope of problem that how big of a problem this is, and that's why I guess there is no library available in Python which does such kind of job. So, hopefully this library will be helpful if you are looking for something in Python.

Another motivation for this project was to figure out the intricate details of TCP/IP stack and how to implement them. I got to know of libnids/pynids library after I had completed this project, it does what I needed in C/Python, though I have not verified it fully but from the project description it seems like it could do the job, one caveat for libnids is that it has not been updated since 2010. See - and Also, it's an IPS engine, which means one needs to do plumbing around the libnids to gather data and play pcaps, which doesn't seem like it's going to be faster because replaying a pcap will also need to spawn a process.

On correctness of this library, when I tested this project I wrote all the test cases from scratch, in which I tried to cover all the edge cases of TCP protocol. I didn't test IP layer, because we are dealing with pcaps here and in the end data will go to TCP layer and if it handles the data in right way, it will be able to extract correct payload. After I found out about libnids, which lists various test cases that needs to be handled correctly at TCP layer, see, I verfied that the cases mentioned in the TCP section, have been covered with my test cases. Test case F from libnids tests is not tested, and it doesn't need to because we have no obligation to deliver the data to application, we just need to extract it.


Python3, pip, wireshark (tshark), and tcpflow.

Clone the repository then install the requirements with pip install -r requirements.txt.

To install the library do python3 install.

If you want to use the script then you will need to install wireshark (tshark) and tcpflow as well.

To simply extract sessions from a pcap once TCPSession is in your python path:

import TCPSessions
pcap = "your.pcap"
tcpsessions = TCPSessions(pcap)

Example of usage could also be found in script, which also generates tcpsessions_from_pcap.log file. It also has option of doing performance testing. Additionally, it provides an option to extract JS from TCP sessions.

This library requires Python3. Before using the script to extract sessions from pcap, please install the requirements with command pip install -r requirements.txt. Few example of usage of the script are following:

Single pcap session extraction:
mkdir /tmp/tcpsessions_from_pcap && python3 -p data/big-920-sessions.pcap -o /tmp/tcpsessions_from_pcap

Check number of JSON file, one for each session, created:

ls /tmp/tcpsessions_from_pcap/tcpsession/big-920-sessions/*.json | wc -l

Open the input pcap in wireshark: Wireshark data/big-920-sessions.pcap&

Use the filter: eq 920 should result in zero packets but 919 will give you a session, which shows pcap has 920 sessions.

Check if our JS output is correct: Wireshark -R 'tcp.payload matches "<script"' data/big-920-sessions.pcap can show different session containing JS data.

By default verifies the result against tcpflow, but there is also an option to use wireshark to that job as well, eg below, don't wait for it to finish it because wireshark is going to take long for 920 sessions:

python3 -p data/big-920-sessions.pcap -o /tmp/tcpsessions_from_pcap -w

In some cases output would different from wireshark because it doesn't care about if there are any irregularity in TCP protocol, it just considers every handshake is correct and in some cases assumes it. See e.g. below:

python3 -p data/failed_200.1.71.197_80- -o /tmp/tcpsessions_from_pcap -w

To see where does output of this library didn't match against wireshark or tcpflow use:

grep "wrong" tcpsessions_from_pcap.log

and compare the same pcap against wireshark with command:

Wireshark data/failed_200.1.71.197_80-, you will notice that session whose checksum didn't match didn't have proper TCP handshake.

Session extraction from multiple pcaps in a directory

Use the on the directory containing files with .pcap extension as below, -r is for recursively and -c is to create tar of final output:

python3 -i data/ -o /tmp/tcpsessions_from_pcap -r -c

analyze the output with below commands:

ls -lah /tmp/tcpsessions_from_pcap/

find /tmp/tcpsessions_from_pcap -name "*.js" | wc -l

find /tmp/tcpsessions_from_pcap -name "*.json" | wc -l


Tests are in the test directory. To run all the tests do pytest test from the source code directory.

To see the code coverage:

coverage run -m pytest test
coverage report -m tcpsession/

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