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Wireshark is the world's foremost network protocol analyzer, and is the de facto (and often de jure) standard across many industries and educational institutions. (Wireshark was known as Ethereal until June 09, 2006)
iftop does for network usage what top(1) does for CPU usage. It listens to network traffic on a named interface and displays a table of current bandwidth usage by pairs of hosts. Handy for answering the question "why is our ADSL link so slow?".
vnStat is a console-base network traffic monitor that keeps a log of daily and monthly network traffic for the selected interface. However, it isn't a packet sniffer. The traffic information is analyzed from the /proc -filesystem, so that vnStat can be used without root permissions.
netsniff-ng is is a free, performant Linux network analyzer and networking toolkit. The gain of performance is reached by zero-copy mechanisms, so that the kernel does not need to copy packets from kernelspace to userspace and vice versa. netsniff-ng consists of much more than only a network ... [More]