From owner-freebsd-security  Fri Nov 16  9:42:41 2001
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Date: Fri, 16 Nov 2001 23:46:18 -0500 (EST)
From: PaZt <pazt@razor.tuxtendo.nl>
To: xmen koh <xmenkoh@yahoo.com>
Cc: <freebsd-security@FreeBSD.ORG>
Subject: Re: How to stop DoS Attack??
In-Reply-To: <20011116063032.77688.qmail@web20904.mail.yahoo.com>
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On Thu, 15 Nov 2001, xmen koh wrote:

> 
> Dear security expert,
> 
> Recently I got a DoS on my web server. Does anyone
> know how to 
> stop a DoS attack and prevent it from happen again?
> Some help
> will be appreciated to explain the below TCPDump which
> I got
> during the attack. 
> 
> Here are some of my finding during the attack...
> 
> 1) There are a lot of connection from different IP at
> the same
>    time, which tries to overload the system using HTTP
> by non-
>    stop requests of URLs. Below is the TCPDUMP.
> 
> 2) The URLs requested as logged by the web server log
> seems to 
>    be quite valid, indicating that the attacker has
> studied the 
>    victim web site and many of these requests are for
> graphic
>    files, indicating a planned intention to clog up
> the server 
>    & network.
> 
> 3) Many source IPs, suspected all to be forged.
> 
> 4) A very close time between each packet timestamp
> indicates a
>    script is used for this attack.
> 
> 

Well, a small DDoS could be stopped by a simple firewall, yet a complex or 
very big DDoS attack is rather hard to stop, maybe even impossible.

If the DDoS isnt all too wisely set up, snort (/usr/ports/security/snort) 
could find the ips where the 
dos is coming from, wich would make you able to send abuse mails or 
whatever.

That's about all i can think of right now,

Grtz,

PaZt


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