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tcpflow is a program that captures data transmitted as part of TCP connections (flows), and stores the data in a way that is convenient for protocol analysis and debugging. Each TCP flow is stored in its own file. Thus, the typical TCP flow will be stored in two files, one for each direction. tcpflow can also process stored ‘tcpdump’ packet flows.

tcpflow is similar to ‘tcpdump’, in that both process packets from the wire or from a stored file. It's also similar to WireShark, in that both allow analysis of network traffic. But unlike either tcpdump or WireShark, tcpflow reconstructs thousands (or millions) of TCP connections at a time and saves the results in ordinary files, making it easy to analyze the data with conventional tools.

tcpflow understands sequence numbers and will correctly reconstruct data streams regardless of retransmissions or out-of-order delivery.


tcpflow stores all captured data in files that have names of the form

where the contents of the above file would be data transmitted from host port 2345, to host port 45103.

Specify the -Fk , -Fm'' or-Fg''' options if you are likely to have more than 1000 connections; this will cause tcpflow to create subdirectories automatically. You can also use other options to create directors for each host or port.

Specify the -e netviz option to enable the network visualization layer, and make a pretty picture like this:



  • tcpflow does not understand IP fragments;
  • tcpflow does not understand 802.11 headers.


Jeremy Elson developed the first version of tcpflow in 1999 but stopped maintaining it in 2003. In 2006 Simson Garfinkel took over maintenance of the program and added:

  • support for VLANs
  • support for IPv6
  • DFXML output of the connections in a report.xml file.
  • Improved performance through the use of the C++ STL classes.
  • Support for continuous operation (tcpflow now purges out old flows).
  • Variable Filename specifications.
  • A plug-in architecture.

tcpflow is based on the LBL Packet Capture Library (available from LBL) and therefore supports the same rich filtering expressions that programs like ‘tcpdump’ support. It should compile under most popular versions of UNIX; see the INSTALL file for details.