HowToLimitSSHScans

Add this to the very top of the iptables file

# ssh scanning block
-A RH-FW-1-INPUT -p tcp -m state --state NEW --dport 22 -m recent --name sshattack --set
-A RH-FW-1-INPUT -p tcp --dport 22 -m state --state NEW -m recent --name sshattack --rcheck --seconds 30 --hitcount 3 -j LOG --log-prefix "SSH REJECT: "
-A RH-FW-1-INPUT -p tcp --dport 22 -m state --state NEW -m recent --name sshattack --rcheck --seconds 30 --hitcount 3 -j REJECT --reject-with tcp-reset

so that you get

# Firewall configuration manually crafted for XXX
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:RH-FW-1-INPUT - [0:0]
:LOCAL-INPUT - [0:0]
-A INPUT -j RH-FW-1-INPUT
-A FORWARD -j RH-FW-1-INPUT
-A RH-FW-1-INPUT -i lo -j ACCEPT
# ssh scanning block
-A RH-FW-1-INPUT -p tcp -m state --state NEW --dport 22 -m recent --name sshattack --set
-A RH-FW-1-INPUT -p tcp --dport 22 -m state --state NEW -m recent --name sshattack --rcheck --seconds 30 --hitcount 3 -j LOG --log-prefix "SSH REJECT: "
-A RH-FW-1-INPUT -p tcp --dport 22 -m state --state NEW -m recent --name sshattack --rcheck --seconds 30 --hitcount 3 -j REJECT --reject-with tcp-reset
# related goes through
-A RH-FW-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
# services that may go through
...

Sources

• http://www.teaparty.net/technotes/ssh-rate-limiting.html