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Why does nmap send multiple probes to the same port?
From: chok () chokmah org
Date: Wed, 11 Jan 2006 17:19:35 -0500

I am having trouble with nmap sending multiple probes to 
the same port to the same target. Because of this, a scan 
of all ports takes many days instead of an hour or two. I 
am using nmap version 3.81 on debian 3.1 (sarge).

Here is what the scan looks like:

# nmap -sT -P0 -p 1-65535 --packet_trace 192.0.2.1

Starting nmap 3.81 ( http://www.insecure.org/nmap/ ) at 
2006-01-06 09:47 CST
CONN (0.3610s) TCP localhost > 192.0.2.1:21 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:636 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:25 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:1723 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:389 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:53 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:256 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:443 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:3389 => Operation 
now in progress
CONN (0.3610s) TCP localhost > 192.0.2.1:22 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:22 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:3389 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:443 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:256 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:53 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:389 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:1723 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:636 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:113 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:23 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:554 => Operation 
now in progress
CONN (1.5020s) TCP localhost > 192.0.2.1:80 => Operation 
now in progress
CONN (1.6020s) TCP localhost > 192.0.2.1:23 => Operation 
now in progress
CONN (1.6020s) TCP localhost > 192.0.2.1:554 => Operation 
now in progress
CONN (2.7020s) TCP localhost > 192.0.2.1:554 => Operation 
now in progress
CONN (2.7020s) TCP localhost > 192.0.2.1:23 => Operation 
now in progress
CONN (3.8120s) TCP localhost > 192.0.2.1:23 => Operation 
now in progress
CONN (3.8430s) TCP localhost > 192.0.2.1:113 => Operation 
now in progress
CONN (3.9520s) TCP localhost > 192.0.2.1:113 => Operation 
now in progress
CONN (4.0510s) TCP localhost > 192.0.2.1:113 => Operation 
now in progress
CONN (5.1620s) TCP localhost > 192.0.2.1:113 => Operation 
now in progress
CONN (5.1920s) TCP localhost > 192.0.2.1:636 => Operation 
now in progress
CONN (5.2920s) TCP localhost > 192.0.2.1:636 => Operation 
now in progress
CONN (5.3920s) TCP localhost > 192.0.2.1:636 => Operation 
now in progress
CONN (6.5020s) TCP localhost > 192.0.2.1:636 => Operation 
now in progress
CONN (6.5320s) TCP localhost > 192.0.2.1:1723 => Operation 
now in progress
CONN (6.6320s) TCP localhost > 192.0.2.1:1723 => Operation 
now in progress
CONN (6.7320s) TCP localhost > 192.0.2.1:1723 => Operation 
now in progress
CONN (6.8320s) TCP localhost > 192.0.2.1:1723 => Operation 
now in progress

The IP address has been changed for privacy.

I have seen as many as seven probes per port after running 
for a while.

This scan is running over the Internet and the box running 
nmap has a public IP and is directly connected to the 
Internet without any kind of filtering on my end. The 
target end goes through a PIX and ports 25 and 80 are 
PATted through to a server on the inside and packets to 
all other ports should be dropped.

I have looked at the packets on the wire and for every 
port except for 25 and 80 there is no response.

I suppose this is a feature and nmap is doing this on 
purpose, but I don't understand why. Does anyone know why 
it is sending multiple probes like this and how I can get 
it to just send a single probe for each port?

chok



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