While persuing the OpenBSD ports collection a few weeks ago, I came across the ifstat utility. This nifty utility allows you to view bandwidth totals for each interface in a server, and at specific intervals. Here is a sample run showing the bandwidth in and out of the sis0 and sis1 Ethernet interfaces, and the total bandwidth in and out of the system:
$ ifstat -TAb 5
sis0 sis1 Total
Kbps in Kbps out Kbps in Kbps out Kbps in Kbps out
129.96 4.37 3.91 131.71 133.87 136.09
130.48 5.43 4.77 131.98 135.25 137.41
132.21 4.24 3.60 133.71 135.81 137.95
This is a nifty utility, and one I plan to add to my stock OpenBSD builds!