1. Delete all existing rules
# iptables -F

2. Set default chain policies
# iptables -P INPUT DROP
# iptables -P FORWARD DROP
# iptables -P OUTPUT DROP

3. Block a specific ip-address
BLOCK_THIS_IP=”x.x.x.x”
# iptables -A INPUT -s “$BLOCK_THIS_IP” -j DROP

4. Allow ALL incoming SSH
# iptables -A INPUT -i eth0 -p tcp –dport 22 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 22 -m state –state ESTABLISHED -j ACCEPT

5. Allow incoming SSH only from a sepcific network
# iptables -A INPUT -i eth0 -p tcp -s 192.168.200.0/24 –dport 22 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 22 -m state –state ESTABLISHED -j ACCEPT

6. Allow incoming HTTP
# iptables -A INPUT -i eth0 -p tcp –dport 80 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 80 -m state –state ESTABLISHED -j ACCEPT

Allow incoming HTTPS
# iptables -A INPUT -i eth0 -p tcp –dport 443 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 443 -m state –state ESTABLISHED -j ACCEPT

7. MultiPorts (Allow incoming SSH, HTTP, and HTTPS)
# iptables -A INPUT -i eth0 -p tcp -m multiport –dports 22,80,443 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp -m multiport –sports 22,80,443 -m state –state ESTABLISHED -j ACCEPT

8. Allow outgoing SSH
# iptables -A OUTPUT -o eth0 -p tcp –dport 22 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A INPUT -i eth0 -p tcp –sport 22 -m state –state ESTABLISHED -j ACCEPT

9. Allow outgoing SSH only to a specific network
# iptables -A OUTPUT -o eth0 -p tcp -d 192.168.101.0/24 –dport 22 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A INPUT -i eth0 -p tcp –sport 22 -m state –state ESTABLISHED -j ACCEPT

10. Allow outgoing HTTPS
# iptables -A OUTPUT -o eth0 -p tcp –dport 443 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A INPUT -i eth0 -p tcp –sport 443 -m state –state ESTABLISHED -j ACCEPT

11. Load balance incoming HTTPS traffic
# iptables -A PREROUTING -i eth0 -p tcp –dport 443 -m state –state NEW -m nth –counter 0 –every 3 –packet 0 -j DNAT –to-destination 192.168.1.101:443
# iptables -A PREROUTING -i eth0 -p tcp –dport 443 -m state –state NEW -m nth –counter 0 –every 3 –packet 1 -j DNAT –to-destination 192.168.1.102:443
# iptables -A PREROUTING -i eth0 -p tcp –dport 443 -m state –state NEW -m nth –counter 0 –every 3 –packet 2 -j DNAT –to-destination 192.168.1.103:443

12. Ping from inside to outside
# iptables -A OUTPUT -p icmp –icmp-type echo-request -j ACCEPT
# iptables -A INPUT -p icmp –icmp-type echo-reply -j ACCEPT

13. Ping from outside to inside
# iptables -A INPUT -p icmp –icmp-type echo-request -j ACCEPT
# iptables -A OUTPUT -p icmp –icmp-type echo-reply -j ACCEPT

14. Allow loopback access
# iptables -A INPUT -i lo -j ACCEPT
# iptables -A OUTPUT -o lo -j ACCEPT

15. Allow packets from internal network to reach external network.
if eth1 is connected to external network (internet)
if eth0 is connected to internal network (192.168.1.x)
# iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT

16. Allow outbound DNS
# iptables -A OUTPUT -p udp -o eth0 –dport 53 -j ACCEPT
# iptables -A INPUT -p udp -i eth0 –sport 53 -j ACCEPT

17. Allow NIS Connections
rpcinfo -p | grep ypbind ; This port is 853 and 850
# iptables -A INPUT -p tcp –dport 111 -j ACCEPT
# iptables -A INPUT -p udp –dport 111 -j ACCEPT
# iptables -A INPUT -p tcp –dport 853 -j ACCEPT
# iptables -A INPUT -p udp –dport 853 -j ACCEPT
# iptables -A INPUT -p tcp –dport 850 -j ACCEPT
# iptables -A INPUT -p udp –dport 850 -j ACCEPT

18. Allow rsync from a specific network
# iptables -A INPUT -i eth0 -p tcp -s 192.168.101.0/24 –dport 873 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 873 -m state –state ESTABLISHED -j ACCEPT

19. Allow MySQL connection only from a specific network
# iptables -A INPUT -i eth0 -p tcp -s 192.168.200.0/24 –dport 3306 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 3306 -m state –state ESTABLISHED -j ACCEPT

20. Allow Sendmail or Postfix
# iptables -A INPUT -i eth0 -p tcp –dport 25 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 25 -m state –state ESTABLISHED -j ACCEPT

21. Allow IMAP and IMAPS
# iptables -A INPUT -i eth0 -p tcp –dport 143 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 143 -m state –state ESTABLISHED -j ACCEPT
# iptables -A INPUT -i eth0 -p tcp –dport 993 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 993 -m state –state ESTABLISHED -j ACCEPT

22. Allow POP3 and POP3S
# iptables -A INPUT -i eth0 -p tcp –dport 110 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 110 -m state –state ESTABLISHED -j ACCEPT
# iptables -A INPUT -i eth0 -p tcp –dport 995 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 995 -m state –state ESTABLISHED -j ACCEPT

23. Prevent DoS attack
# iptables -A INPUT -p tcp –dport 80 -m limit –limit 25/minute –limit-burst 100 -j ACCEPT

24. Port forwarding 422 to 22
# iptables -t nat -A PREROUTING -p tcp -d 192.168.102.37 –dport 422 -j DNAT –to 192.168.102.37:22
# iptables -A INPUT -i eth0 -p tcp –dport 422 -m state –state NEW,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -o eth0 -p tcp –sport 422 -m state –state ESTABLISHED -j ACCEPT

25. Log dropped packets
# iptables -N LOGGING
# iptables -A INPUT -j LOGGING
# iptables -A LOGGING -m limit –limit 2/min -j LOG –log-prefix “IPTables Packet Dropped: ” –log-level 7
# iptables -A LOGGING -j DROP