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RE: TCP Syn Flooding
From: "Michael Parker" <mparker () rim net>
Date: Tue, 18 Feb 2003 12:49:05 -0500

Hi Anders - This was very interesting (useful) information...thanks :)  I wasn't aware of the residual traffic that 
could be generated after visiting a web site (and the resulting false positives).  In light of the few messages he did 
receive, it does make sense.


-----Original Message-----
From: Anders Reed Mohn [mailto:anders_rm () utepils com]
Sent: February 17, 2003 5:10 PM
To: Tim Laureska; security-basics
Subject: Re: TCP Syn Flooding

I received this message a few times yesterday after I installed the box:

Fri, 02/14/2003 20:35:01 - TCP connection dropped -
Source:, 80, WAN - Destination:, 20306, LAN -
'TCP:Syn Flooding' End of Log ----------

What should I make of this?

Not sure, Tim, but I'll make a guess.
Is there a website at that you've visited?

Now, the firewall will have reacted because this address sent one or more
SYN packets
that weren't expected. The target port for the SYN packet is a typical
client port,
and not a service, so it's probably not an attack of any sort.

This is something that all firewalls log tons of after you've visited a
I think the explanation is that when you _left_ the page, the
TCP-connections to
it were not closed. Thus, the remote server still thinks you are connected,
sends traffic to you. Your firewall, however, has already dropped the
and therefore thinks this is illegitimate traffic.

Anders :)

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