"Getting network stats on a Linux machine is not particularly
difficult. With tools like sar, Iperf, and vnStat available for
nearly every distribution, command-line aficionados can get the
low-down on their network with just a few useful commands.
"Take, for instance, vnStat, a popular network traffic logger.
vnStat is different from a network sniffer like Wireshark since it
polls the network interface stats going to and from the kernel.
Wireshark, on the other hand, actually monitors packets coming in
and out of your machine.
"As useful as vnStat is, it is still predominantly a
command-line application, which can make it tricky to visualize
what's going on with your machine's network interface. But there
does exist a useful PHP-based front-end for vnStat that can
generate useful reports from vnStat logs."