vnStat

Select the interfaces to monitor, then use vnStat’s daily, monthly or finer-grained reports to inspect accumulated traffic. Month boundaries can follow a billing period, and retained history persists across reboots. Date filters, JSON/XML output and optional PNG images make the totals usable outside the terminal. The project reads kernel interface statistics rather than sniffing packets, so its reports describe interface totals without identifying payload contents or the responsible application. Retention duration is configurable. The Linux/BSD project is GPL-licensed and supports operation without root permissions when installed and configured accordingly.