# 1. Delete all existing rules
iptables -F
# 2. Set the default chain policy
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 DRO
# 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. Only allow SSH from a specific 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
# 7. Multiple ports (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, but only on 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 to access the external network from the internal 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 outgoing 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 a specific network to rsync into the machine
#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. Only allow connections to MySQL 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 Attacks
iptables -A INPUT -p tcp --dport 80 -m limit --limit 25/minute --limit-burst 100 -j ACCEPT
# 24. Set port 422 to forward to port 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 discarded 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