Again, an Internet search will reveal many different software flow sensors, but I recommend
softflowd, from http://www.mindrot.org/softflowd.html. Go there to identify the current version; if you can find a current package for your operating system, use it. If you cannot find a package, download the source code, and install the program by hand.
softflowd README file for instructions on how to build. Much like flow-tools,
softflowd has a
configure script. I prefer to install my add-on software in a different directory than the main system so I can easily keep it separate. Here I install
softflowd under /usr/local/softflowd:
This installs two programs ...