iptables

### Create chains ###
iptables -N syn_flood
iptables -N LOGGING
iptables -N port-scanning

### Drop invalid packets ###
/sbin/iptables -t mangle -A PREROUTING -m conntrack --ctstate INVALID -j DROP

### Drop TCP packets that are new and are not SYN ###
/sbin/iptables -t mangle -A PREROUTING -p tcp ! --syn -m conntrack --ctstate NEW -j DROP

### Drop SYN packets with suspicious MSS value ###
/sbin/iptables -t mangle -A PREROUTING -p tcp -m conntrack --ctstate NEW -m tcpmss ! --mss 536:65535 -j DROP

### Drop fragments in all chains ###
iptables -t mangle -A PREROUTING -f -j DROP

### Limit connections per source IP ###
iptables -A INPUT -p tcp -m connlimit --connlimit-above 111 -j REJECT --reject-with tcp-reset

### Limit RST packets ###
iptables -A INPUT -p tcp --tcp-flags RST RST -m limit --limit 2/s --limit-burst 2 -j ACCEPT
iptables -A INPUT -p tcp --tcp-flags RST RST -j DROP

### Limit new TCP connections per second per source IP ###
iptables -A INPUT -p tcp -m conntrack --ctstate NEW -m limit --limit 60/s --limit-burst 20 -j ACCEPT
iptables -A INPUT -p tcp -m conntrack --ctstate NEW -j DROP

### SSH brute-force protection ###
iptables -A INPUT -p tcp --dport ssh -m conntrack --ctstate NEW -m recent --set
iptables -A INPUT -p tcp --dport ssh -m conntrack --ctstate NEW -m recent --update --seconds 60 --hitcount 10 -j LOGGING

### Protection against port scanning ###
iptables -A port-scanning -p tcp --tcp-flags SYN,ACK,FIN,RST RST -m limit --limit 1/s --limit-burst 2 -j LOGGING
iptables -A port-scanning -j DROP

### Protection against SYN flood ###
iptables -A INPUT -p tcp --syn -j syn_flood
iptables -A syn_flood -m limit --limit 1/s --limit-burst 3 -j LOGGING
iptables -A syn_flood -j DROP

### Logging section ###
iptables -A LOGGING -m limit --limit 2/min -j LOG --log-prefix "IPTables-Dropped: " --log-level 4
iptables -A LOGGING -j DROP;