Yesterday, I wanted to gather bandwith usage statistics on my FreeBSD pf(4) based firewall in order to graph incoming and outgoing bandwidth utilization.
pfctl(8) provides the '-s info' flag, which can provide statistics on a single interface at a time. The interface can be chosen with the either the "loginterface" directive in pf.conf, or by using the DIOCSETSTATUSIF ioctl. However, I needed statics for all the network interfaces, not just one.
Fortunately, pf(4) also provides the DIOCIGETIFACES ioctl, which allows me to gather packet and byte statistics on all interfaces at once. This article will provide an introduction to using the pf(4) ioctl interface to gather network interface traffic statistics. Full example source code can be downloaded here. Note that PF does not maintain statistics on traffic that does not pass through PF. If you don't use PF, all the counters will be zero.