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Re: nmap OS detection providing different results: --osscan-guess
From: David Fifield <david () bamsoftware com>
Date: Sat, 2 Apr 2011 09:23:07 -0700

On Sat, Apr 02, 2011 at 08:23:47AM -0700, David Fifield wrote:
On Sat, Apr 02, 2011 at 09:33:45AM -0400, Ryan Giobbi wrote:
Hello,

When running against non-Windows hosts (AIX), I've noticed that nmap's
OS detection (nmap -O) doesn't provide the same results all of the
time. About 1/5 scans nmap fails to find the remote hosts when run
repeatedly.

Is this expected? Would pasting the OS signature that returns when the
current ones fail into nmap-os-db and submitting to nmap.org be a
reasonable workaround?

Yes, please submit the fingerprint you get, it's the only way for the
database to improve. It's not a workaround, it's the way the process is
supposed to work.

I can explain why this happens sometimes. Some of the fingerprint fields
are ranges. When a new fingerprint is added, we start the ranges pretty
narrow, so as to avoid overlapping with other fingerprints.

Another thing I should mention is that if you just want results, you
should use the --osscan-guess option. When a match fails for a reason
like this, it is still usually a 98% or 99% match. --osscan-guess will
show you the match instead of a fingerprint.

Why doesn't Nmap just show you both (the closest match and the
fingerprint)? Thew reason is that we fear that people will read the
closest match and just paste it into the submit form. Then an incorrect
fingerprint will just get reinforced.

The downside of using --osscan-guess is that it doesn't help grow the
database. (And potentially make --osscan-guess unnecessary for that
target in the future.)

David Fifield
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