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Re: nmap scanning techniques and execution privileges
From: Brahim Sakka <brahim.sakka () gmail com>
Date: Wed, 21 Dec 2011 19:57:27 +0100

2011/12/14, Guillaume Rousse <guillomovitch () gmail com>:
Hello list.

I'm using nmap to scan large IP ranges (B classes) this way:
nmap -sV -T5 -F -oX -

When advised that some of the servers were missing in the report, I read
the manual, and found than default scanning technique when run without
raw socket priveleges was to use TCP connect scan, instead of TCP SYN
scan, which is considered a better option. So I switched to root
privileges instead:
sudo nmap -sV -T5 -F -oX -

But now I discovered then some other servers were missing in the new
report. After investigating the issue, I found than raw socket
privileges allow to use ICMP to speed up the scan by skipping
non-responsing hosts, and the missing host were those not responding to
ICMP echo probes. So I attempted to disable this optimisation:
sudo nmap -sV -Pn -T5 -F -oX -

But now scanning a class B range needs more than 12 hours to complete :(

So, how does the first nmap command, run as standard user without ICMP
optimisation, complete in just one hour, while the same command run as
root is more than 10 times slower ?
You said it. The "enhanced" version of the command will scan every
single host, while the first/second one only scanned these hosts that
replied to ICMP requests. You do the math :-)

Also, if I can't have a magic set of options to discover all the running
servers in a single run, is there any way to merge results from two
different scans ?
Try importing both results in Zenmap.

--
BOFH excuse #353:

Second-system effect.
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