Like tcpdump, tcpflow gives you the ability to view the text contents of network packets in real time. However, tcpdump is more suited to capturing packets and protocol information. tcpflow is more suited toward viewing the actual data flow between two hosts. tcpdump displays output to the console by default, whereas tcpflow writes output to a file by default, and you must use the -c option if you want to see the tcpflow  output on the console.

Other than this, much of the syntax of tcpflow is similar to that of tcpdump. An example of using tcpflow is as follows:

tcpflow – i fxp0 -c host 192.168.1.3 and port 80

Such a command would capture packets on the fxp0 interface with a source or destination of 192.168.1.3 port 80. Unlike tcpdump, tcpflow is not part of the pfSense's default installation.