variable nmap duration

[Daniel Rozsnyó <daniel () rozsnyo com>]

Hello,

  I am working on a FTP indexing service for our local college network.
The on/off line hosts and available servers are discovered by stealth
scan to port 21, per building subnet (4C, x10 buildings) which runs
again and again. Exact command:

/usr/local/bin/nmap -sS -P0 -p $tcpPort -n -T Normal -iL ./nmap.in -oX -

  The problem is, that sometimes the scan runs for relatively long time,
like 50 to 400 (sometimes over 1000) seconds. In 98%, it takes only 3 to
7 seconds (to scan the 4C). The distribution of long runs is totally
random during the whole day and during the summer holiday the server
mostly idles, so I see no reason for such a lags..

  Any idea why does this happen? Or have you a better idea how to map
hosts within guaranteed time? (OS is FreeBSD 5.4, nmap 3.93)

Thanks,
  Daniel

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