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

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Date: Wed Mar  1 18:49:09 2006
From: linux-fan at onda.com.br (Giancarlo Razzolini)
Subject: reduction of brute force login attempts via
	SSH	through iptables --hashlimit

GroundZero Security wrote:
> Well i had a few minutes time, so i updated the script a bit.
> 
> I did not use lastb though, as it wouldnt work (read the manpage.....)
> Anyhow, maybe someone found it usefull so here is v.0.2 :
> 
> http://www.groundzero-security.com/code/bruteforce-block.sh
> 
> Any suggestions are welcome, insults and flames can be sent to /dev/null
> 
> -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
> 
> Key fingerprint = A93E 41F8 7E82 5F2C 3E76  41F1 4BCF 3096 6992 8CB8
> 
> -----BEGIN PGP PUBLIC KEY BLOCK-----
> Version: GnuPG v1.0.6 (GNU/Linux)
> Comment: For info see http://www.gnupg.org
> 
> mQGiBEFX440RBADGTKOgZR9Y9VA/cfNLWTIN/OmXe9l6UZJ6pY8Hqcv6DFE//Kt9
> UfQMU470i+I7SvIHZN066Kl4ts4r90sLxXrE4r5VQCLTsJM68cliatrM8MbbZZs+
> xf3ldelZrHNvHkXDk4I/n3O56F9M6tZ/S71AIj++raIbFX57fn8Z8NNOnwCgwDr6
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> NANZBwADBwf8CrjVKiXSzyhUsdH1es1KQCZ/zH6PvPzdxqYuGuVVMzgaJeeOMS2G
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> koy4p1QAoIaYw3VxA0/mixUsMO4R13sXIL/pAJ9zodR+A9+bLqCRlVusG8JhItv1
> Ow==
> =E0o1
> -----END PGP PUBLIC KEY BLOCK-----
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> _______________________________________________
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> 
2 good solutions to this problem are:

1) put sshd to listen on a port that nmap or other port scanner doesn't
scan by default (> 60000, as example).
2) Completely disable password authentication and leave only public key
authentication enabled. Much more secure, as there is no password,
except the one used to unlock the secret key, on the client's machine,
which is never sent on the wire.

Whit these 2 measures, you avoid a lot, if not eliminate completely,
brute force attacks. And using other methods as --hashlimit from
iptables and other methods from other firewall solutions, or even a port
knocking system, may solve this problem better, then running log
analyzers that detects these attempts and blocks the attackers in "real
time".

My 2 cents,

-- 
Giancarlo Razzolini
Linux User 172199
Moleque Sem Conteudo Numero #002
Slackware Current
Snike Tecnologia em Inform?tica
4386 2A6F FFD4 4D5F 5842  6EA0 7ABE BBAB 9C0E 6B85

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