Mailinglist Archive: opensuse-security (232 mails)

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Re: [suse-security] Under DDoS Attack
  • From: media Formel4 <info@xxxxxxxxxx>
  • Date: Thu, 27 Oct 2005 17:24:07 +0200
  • Message-id: <4360F117.6060206@xxxxxxxxxx>
Markus Roth schrieb:
media Formel4 wrote:

Question is:

- Is it possible with spoofed IP numbers to establish connections to
port 80? As far as I know you should get stuck after "SYN".
I'm asking that, because tracing back the IPs in question I find very often unrouted areas and non-reachable (but maybe firewalled) IPs.


i would say no (else the school was pretty useless ;-)

"There are more things in heaven and earth, Horatio, then are drempt of
in your philosophy."

Maybe you can send a spoofed SYN followed by a (or a dozen?) spoofed ACK where you "guess" the correct seq_num/ack_num? I'm not sure if this is not possible...



- How can I secure this server and/or stop this attack?


this attack is very mean and it succeeds almost always (even if you just do it from a single attacking machine).
i would do a search on google, there are definitively others who were under the same sort of attack.

All Ideas they produce is something like "Change the IP" - which is IMHO not a good solution, because not everything on that server is hostname driven...


just some thoughts about how it could be possible to protect (at least a bit). maybe it's possible to let netfilters connection tracking do the work for you. if you got it installed on your machine just enable it (by writing a simple rule, something like "iptables -A INPUT -p tcp --dport 80 --state NEW,ESTABLISHED -j ACCEPT") and then set the size of the connection table to some small number (check google how to do it). the idea behind it is, that i assume (i didn't try it or investigated in it!!) that the connection tracking will always drop the connection that was the longest non active and so the connections that send something should be kept alive and the "just open" sessions would be dropped. if you set the number to 100 or something, the backend httpd process should be protected (maybe). but take care that connection tracking doesn't lock you out as it is used on all connection (not just the one you write a rule for)

That might be worth a thought. Right now I've got a script running checking the web server and when MaxClients is reached for more then 20 seconds, all IPs are collected and every IP that was more then 5 times in that collection get blocked. I've got now a list of more then 4700 IPs blocked and the attack is still going on...

good luck

Thanks, guess I need it...

Ralf Koch

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