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Re: Why does nmap send multiple probes to the same port?
From: Casey Williams <Lists () aviditysoftware com>
Date: Wed, 11 Jan 2006 23:11:23 -0600

On 04:37 Thu 01/12/06 Jan    , Andreas Ericsson wrote:
Casey Williams wrote:
I've noticed the same thing with -sS and -P0 which I did't think used 
connect().  I've actually been wondering about this recently myself 
because I've been writing my own lightweight port scanning engine using 
libnet to inject SYN probes.  I've noticed that if I loop over the range 
of IP addresses and immediately send the probe, I won't always get a reply 
from hosts that I know should have replied.  However, if I sleep() for 1 
millisecond between probe sends, it *will* send  the reply.  However, this 
1 ms sleep() can add up over many hosts/ports, and I can't seem to sleep() 
for a shorter period of time, (even nanosleep() doesn't seem to work for 

***pure conjecture ahead***

So I started wondering how nmap was able to be so reliable and yet SO fast 
(kudos!).  Then I realized that it may be sending these "retry" probes to 
hosts that it doesn't know the status of and that it didn't get a reply 

I've been reading the nmap source and stepping through it, but I haven't 
verified that this is correct.  Am I on the right track?  If not, would 
this tactic be unrecommended?

You're missing the point a bit. The error messages means that nmap 
didn't send those probes. It tried to, but the system told it there was 
already a connection attempt in progress.

I'm slightly off topic since I'm talking about SYN scans instead of connect() scans, and I'm not 
certain how your reply would apply to these.  (If it does apply to SYN scans, I apologize, I'm fairly 
new to this level of TCP/IP programming and I suppose I need to read more :))

In my case, when I try "nmap -sS -P0..." and I sniff the traffic that gets generated from that scan, 
I've noticed more than one probe gets sent to the same port on some of the hosts under certain 
circumstances.  I shouldn't see these "extra" probes in the packet capture if NMap didn't actually 
send them should I?  Is this the expected behavior, or am I imagining things? :)

-Casey Williams

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