What is an ideal way to get the number of packets per second on a Linux box?
Would I use tcpdump or are there better methods?
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if you need an exact number then yes, I'd use that. but if it's roughly then you might want to look into MRTG - Tobi Oetiker's MRTG - The Multi Router Traffic Grapher.
What do you need? Long term monitoring of packet statistics? Graphing? Further analysis?
If you just want to look at packet rates on a box, use iptraf.