full-disclosure - reduction of brute force login attempts via SSH through iptables --hashlimit

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Message-ID: <002d01c63ca7$bd71bab0$0100a8c0@nuclearwinter>
Date: Tue Feb 28 20:43:50 2006
From: fd at g-0.org (GroundZero Security)
Subject: reduction of brute force login attempts via SSH
	through iptables --hashlimit

Hello,

i made a small bash script last year to block those bruteforce attempts automatically via the firewall.
In case someone is interested, i released it on our website. Someone may have a use for it :-)
http://www.groundzero-security.com/code/bruteforce-block.sh
Have a nice day everyone!

-sk


GroundZero Security Research and Software Development
http://www.groundzero-security.com

Wir widersprechen der Nutzung oder ?bermittlung unserer Daten
f?r Werbezwecke oder f?r die Markt- oder Meinungsforschung (? 28 Abs. 4 BDSG).

pub  1024D/69928CB8 2004-09-27 Stefan Klaas <sk@...undzero-security.com>
sub  2048g/2A3C7800 2004-09-27

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----- Original Message ----- 
From: "Jay Libove" <libove@...ines.org>
To: <full-disclosure@...ts.grok.org.uk>
Sent: Tuesday, February 28, 2006 2:23 AM
Subject: [Full-disclosure] reduction of brute force login attempts via SSH through iptables --hashlimit


> Quite some time back, I posted a question here about brute force login
> attempts through SSH which had recently become a noticeable annoyance.
> There was some discussion here on the list, someone suggested using
> hashlimit, and I think the issue of brute force attempts through SSH has
> become just one more part of the background noise of the Internet.
>
> I finally got back around to looking at this on my system, and I figured
> out why my first attempts at using the hashlimit functionality in iptables
> had not worked.  Hopefully late is better than never, so I present it here
> to anyone else who was as stupid and/or lazy as I was :) so that it took
> me this long to get back to work on it and get it right.
>
> Here is an iptables command to allow inbound SSH with a quite low limit on
> the number of connections which may arrive from a specific IP address in a
> short period of time. Combined with the default setting of OpenSSH which
> drops a connection after just a few failed login attempts, this has
> reduced the number of failed logins I am seeing in my nightly logwatch
> output from thousands to about ten per day. Since this use of hashlimit
> filters on source IP address, it does not create a denial of service
> against legitimate SSH connections, unless someone spoofs a very large
> range of source addresses and can somehow get those connections to
> actually open instead of just consume partly open TCP sessions.  In such a
> case, other defenses are needed anyway.
>
> # iptables --table filter -A INPUT --protocol tcp --source 0/0 \
> --destination-port ssh -m hashlimit --hashlimit 2/minute \
> --hashlimit-burst 3 --hashlimit-mode srcip --hashlimit-name ssh \
> -m state --state NEW --jump ACCEPT
>
> The stupid thing I did the first time I tried to set this up months ago
> was to put a command like the above in, and forget to take out the
> original iptables command allowing connections to the SSH port.
> hashlimit is a limiter on an iptables rule.  Having one rule with a
> hashlimit in it, and a second matching rule with no hashlimit, just
> results in all connections being accepted without limit.
>
> Of course, the same thing would work to reduce brute force speeds on
> telnet, FTP, &etc by changing the destination port argument.
>
>
> Please direct all flames to /dev/null, all cash contributions to /dev/me
> :) and all constructive comments and enhancement suggestions back to the
> list.
>
> Cheers!
> -Jay
> _______________________________________________
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