For a testing setup I have, I need to monitor the throughput through a "router"* at regular intervals of around 5 seconds or less (sub-second intervals would be very nice, but not required). Ideally, I would be able to generate a file which contained both the number of bytes and packets seen during each interval. I will eventually be generating a time-series of throughput from this data.

On a previous setup using an older version of FreeBSD, there was a tool called "bpfmon" which gave me this information. However, I need to do this under a modern version of Linux (namely, Ubuntu 11.04).

I have looked at both iptraf and iftop, but these do not appear to provide the resolution I need, nor do they seem to easily allow scraping the data I need. I understand iptables statistics may be able to give me what I'm after, but the examples I've seen of this seem to rely on repeatedly reading and resetting traffic counters, which seems like it could give inaccurate as read/reset is not an atomic operation. I already capture a tcpdump trace of the traffic I'm interested in on the link I want to monitor, so I am open to approaches which simply parse that.

I feel like this must be a common problem though, so I am hoping there will be a standard "best practice" tool for accomplishing this.

*I say "router" in quotes because I am really talking about a machine with two bridged NICs through which all the traffic I'm interested in passes.