IPTABLES¶

Statefull and Powerfull Firewall.

Enable for SSH Bruteforce Prevention¶

This is not a 100% proof setup as it does not block based on the result of authentication.
It just work based on connection.

Simple Rules¶

iptables -N BRTBLK
iptables -A INPUT -p tcp --dport 22 -m state --state NEW -j BRTBLK
iptables -A BRTBLK -m recent --set --name SSH
iptables -A BRTBLK -m recent --update --seconds 45 --hitcount 5 --name SSH -j DROP

Rules With Exceptions¶

Replace with your IP.
You can use multiple rules with diff. IP to allow exceptions.

iptables -N BRTBLK
iptables -A INPUT -p tcp --dport 22 -s <Exception> -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -m state --state NEW -j BRTBLK
iptables -A BRTBLK -m recent --set --name SSH
iptables -A BRTBLK -m recent --update --seconds 45 --hitcount 5 --name SSH -j DROP

Rules to Log Bloked IP¶

iptables -N BRTBLK
iptables -A INPUT -p tcp --dport 22 -m state --state NEW -j BRTBLK
iptables -A BRTBLK -m recent --set --name SSH
iptables -A BRTBLK -m recent --update --seconds 45 --hitcount 5 --name SSH -j LOG --log-level info --log-prefix "Bad IP : "
iptables -A BRTBLK -m recent --update --seconds 45 --hitcount 5 --name SSH -j DROP

Force SYN packets check¶

Make sure NEW incoming tcp connections are SYN packets; otherwise we need to drop them:

iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP

Force Fragments packets check¶

Packets with incoming fragments drop them. This attack result into Linux server panic such data loss.

iptables -A INPUT -f -j DROP

XMAS packets¶

Incoming malformed XMAS packets drop them:

iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP

Drop all NULL packets¶

Incoming malformed NULL packets:

iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP

xt_recent¶

In latest version ipt_recent replaced by xt_recent.
there is few change which could break your iptables rules.
In order to make ssh bruteforce protection working please use following rules.
this also effect Fedora 15 and latest iptables / kernel

:SSH - [0:0]
:BRTBLK - [0:0]


-A INPUT -p tcp -m multiport --dports 21,22 -m recent --update --seconds 8600 --name SSH_BAN --rsource -j DROP 
-A INPUT -p tcp -m multiport --dports 21,22 -m state --state NEW -j BRTBLK 

-A SSH -m limit --limit 5/min -j LOG --log-prefix "BAD IP: " --log-level 4 
-A SSH -m recent --set --name SSH_BAN --rsource -j DROP 

-A BRTBLK -m recent --set --name BRT --rsource 
-A BRTBLK -m recent --update --seconds 600 --hitcount 16 --name BRT --rsource -j SSH 
-A BRTBLK -m recent --update --seconds 60 --hitcount 4 --name BRT --rsource -j SSH 
-A BRTBLK -j ACCEPT

Now try to login to box 4 times quickly to test.
For more advanced Options please visit Th4tsG33ky