Basic iptables firewall

firewall.sh

#!/bin/sh

# Drop ICMP echo-request messages sent to broadcast or multicast addresses
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

# Drop source routed packets
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route

# Enable TCP SYN cookie protection from SYN floods
echo 1 > /proc/sys/net/ipv4/tcp_syncookies

# Don't accept ICMP redirect messages
echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects

# Don't send ICMP redirect messages
echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects

# Enable source address spoofing protection
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter

# Log packets with impossible source addresses
echo 1 > /proc/sys/net/ipv4/conf/all/log_martians

# Flush all chains
/sbin/iptables --flush

# Allow unlimited traffic on the loopback interface
/sbin/iptables -A INPUT -i lo -j ACCEPT
/sbin/iptables -A OUTPUT -o lo -j ACCEPT

# Set default policies
/sbin/iptables --policy INPUT DROP
/sbin/iptables --policy OUTPUT DROP
/sbin/iptables --policy FORWARD DROP

# Previously initiated and accepted exchanges bypass rule checking
# Allow unlimited outbound traffic
/sbin/iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
/sbin/iptables -A OUTPUT -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

# Allow incoming TCP  - SSH & HTTP/HTTPS
/sbin/iptables -A INPUT -p tcp --dport 22 -m state --state NEW -j ACCEPT
/sbin/iptables -A INPUT -p tcp --dport 80 -j ACCEPT
/sbin/iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# Drop all other traffic
/sbin/iptables -A INPUT -j DROP

# Have these rules take effect when iptables is started
#/sbin/service iptables save