Wireshark-users: Re: [Wireshark-users] Will capturing packets with tcpdump/tshark affect traffic
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From: Guy Harris <guy@xxxxxxxxxxxx>
Date: Tue, 9 Aug 2016 12:05:47 -0700
On Aug 9, 2016, at 9:39 AM, Rayne <hjazz6@xxxxxxxxx> wrote:

> 1) Wouldn't using a capture filter add more load to the processing, since the capturing program now also has to decode the packets?

A capture filter doesn't do much decoding; it's compiled into a program in a pseudo-machine language for an accumulator-based processor:

	http://www.tcpdump.org/papers/bpf-usenix93.pdf

and that is either interpreted in a module in the kernel or translated to machine code and executed in the kernel.  If the program rejects the packet, the packet's data is not copied to a capture buffer in the kernel, and thus not copied up to the program doing the capture; the CPU time saved not doing that more than outweighs the small amount of CPU time spent interpreting or running a capture filter program.

> 2) Does tcpdump use less CPU than tshark?

Yes.