How to prevent SSH and FTP bruteforce attacks with iptables on Linux

Earlier I've blogged about how to prevent brute force attacks with fail2ban, denohosts and blockhosts , however there is easier way to secure against basic brute force attacks by not installing or configuring any external programs.
The way I'm talking about uses simple iptables rules to filter out brute force attacks.

Here is a small script to stop ssh and FTP invaders which try to initiate more than 3 consequential connections in 5 minutes time to port 22 or port 23:

SERVER_MAIN_IP='AAA.BBB.CCC.DDD';/sbin/iptables -N SSH_WHITELIST
/sbin/iptables -A INPUT -p tcp --dport 22 --syn -m recent --name sshbr --set
/sbin/iptables -A INPUT -p tcp --dport 22 --syn -j SSH_WHITELIST
/sbin/iptables -A INPUT -p tcp --dport 22 --syn -m recent --name sshbr
--update --rttl --hitcount 3 --seconds 300 -j REJECT --reject-with tcp-reset
/sbin/iptables -A SSH_WHITELIST -s $SERVER_MAIN_IP -p tcp --dport 22 --syn -m recent --rttl --remove

The only thinIf the rules are matched iptables filter rules will be added to the iptables CHAIN SSH_WHITELIST
In case if you want to add some more truested IPs add some more iptables rules, like:

ALLOW_IP='BBB.CCC.DDD.EEE';
/sbin/iptables -A SSH_WHITELIST -s $ALLOW_IP -p tcp --dport 22 --syn -m recent --rttl --remove

Each filtered IP that matches the rules will be filtered for 5 minutes, if 5 minutes is enough, the 300 value has to be increased.
 

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2 Responses to “How to prevent SSH and FTP bruteforce attacks with iptables on Linux”

  1. Thibs says:
    Firefox 8.0 Firefox 8.0 Windows 7 x64 Edition Windows 7 x64 Edition
    Mozilla/5.0 (Windows NT 6.1; WOW64; rv:8.0) Gecko/20100101 Firefox/8.0

    I think you could also do the same like this :

    iptables -A INPUT -m state –state NEW -p TCP –dport 22 –syn -m limit –limit 3/s –limit-burst 3 -j ACCEPT

    (limit 3/sec)

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    • admin says:
      Epiphany 2.30.6 Epiphany 2.30.6 Debian GNU/Linux x64 Debian GNU/Linux x64
      Mozilla/5.0 (X11; U; Linux x86_64; en-us) AppleWebKit/531.2+ (KHTML, like Gecko) Version/5.0 Safari/531.2+ Debian/squeeze (2.30.6-1) Epiphany/2.30.6

      Hi hi,

      Yo’re right thx

      best and Merry Christmas and happy receive of new year!

      best
      Georgi

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