Difference between revisions of "Secure SSH against brute force attacks"

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Revision as of 13:25, 28 August 2013

With more and more people installing Amahi we have more and more Amahi's exposed to the internet when people start opening up ports in their routers to be able to access their servers remotely. Since SSH is used by many this is often also exposed outwards. This opens up for SSH brute force attacks (which can be both fast and effective). To bring some kind of default protection to the platform against this I purpose that we include the following iptable rules by default

[root@dahome ~]# iptables --list
Chain INPUT (policy ACCEPT)
 target     prot opt source               destination
 ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:ssh state NEW recent: SET name: SSH side: source mask: 255.255.255.255
 LOG        tcp  --  anywhere             anywhere             tcp dpt:ssh recent: UPDATE seconds: 60 hit_count: 4 TTL-Match name: SSH side: source mask: 255.255.255.255 LOG level  warning prefix "SSH_brute_force " 
 DROP       tcp  --  anywhere             anywhere             tcp dpt:ssh recent: UPDATE seconds: 60 hit_count: 4 TTL-Match name: SSH side: source mask: 255.255.255.255
Chain FORWARD (policy ACCEPT)
 target     prot opt source               destination
Chain OUTPUT (policy ACCEPT)
 target     prot opt source               destination

This will allow a couple of failed SSH login attempts and then add a 60 second cool down. Effectively killing any interest in brute forcing the server in question. Another tip is to disable root ssh access if you don't really need it since that is one of the most common user names used when brute forcing.