Re: Will capturing packets with tcpdump/tshark affect traffic processing?

[Guy Harris <guy () alum mit edu>]

On Aug 9, 2016, at 9:39 AM, Rayne <hjazz6 () ymail com> 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.
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