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reduction of brute force login attempts via SSH through iptables --hashlimit
From: Jay Libove <libove () felines org>
Date: Mon, 27 Feb 2006 20:23:14 -0500 (EST)

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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