Re: Counter-intuitive handling of --min-parallelism argument without --max-parallelism

[David Fifield <david () bamsoftware com>]

On Sun, Jul 10, 2011 at 01:02:47PM -0500, Chris Woodbury wrote:
> I have been using the --min-parallelism argument for some time now to
> speed up my scans, but I just recently discovered that it does not
> have the effect I thought it did. My intention was to use
> --min-parallelism to set a floor on the scan's parallelism but to let
> it go higher dynamically. However, I discovered that using
> --min-parallelism without --max-parallelism does not have this effect,
> and that instead it sets the maximum to be the same as the minimum,
> potentially slowing down scans on fast networks. This behavior is not
> noted in the Nmap documentation and seems counter-intuitive to me.
>
> The reason for this is lines 482-483 of NmapOps.cc. The
> NmapOps::min_parallelism and NmapOps::max_parallelism variables are
> initially set to 0, which NmapOps.h defines as meaning their value
> "has not been set." If no values for these are provided on the command
> line, the various scan engines see the zeroes and use default values
> instead. However, if only the minimum is provided on the command line,
> when NmapOps.cc does a check on line 482 to see whether
> min_parallelism > max_parallelism, it sets max_parallelism to be the
> same as min_parallelism. This changes its meaning from "has not been
> set" to 'has been set to X' and the engines' defaults are no longer
> used.
>
> I can think of a few solutions off the top of my head:
> 1) Move the min > max checks into the scan engines - Users would be
> able to set a minimum while still using the default maximum. This is
> my favorite and seems to best align with my intuition of what
> --min-parallelism and --max-parallelism should be.

Thanks for noticing this. Couldn't option #1 be done much more simply
with something along the lines of

   /* Prevent performance values from getting out of whack */
-  if (min_parallelism > max_parallelism)
+  if (max_parallelism != 0 && min_parallelism > max_parallelism)
     max_parallelism = min_parallelism;

David Fifield
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