IPv4 firewall setup

ip4

*filter

# Default policy is to drop all traffic
-P INPUT DROP
-P FORWARD DROP
-P OUTPUT DROP

# Allow all loopback traffic
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT

# Allow ping.
-A INPUT -p icmp -m state --state NEW --icmp-type 8 -j ACCEPT

# Allow incoming SSH, HTTP and HTTPS traffic
-A INPUT -i eth0 -p tcp -m multiport --dports 22,80,443 -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -o eth0 -p tcp -m multiport --sports 22,80,443 -m state --state ESTABLISHED -j ACCEPT

# Allow inbound traffic from established connections.
# This includes ICMP error returns.
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Log what was incoming but denied (optional but useful).
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables_INPUT_denied: " --log-level 7

# Allow outgoing SSH, HTTP and HTTPS traffic
# This is useful because we won't be able to download and install 
# NPM packages otherwise and use git over SSH
-A OUTPUT -o eth0 -p tcp -m multiport --dports 22,80,443 -m state --state NEW,ESTABLISHED -j ACCEPT
-A INPUT -i eth0 -p tcp -m multiport --sports 22,80,443 -m state --state ESTABLISHED -j ACCEPT

# Allow dns lookup
-A OUTPUT -p udp -o eth0 --dport 53 -j ACCEPT
-A INPUT -p udp -i eth0 --sport 53 -j ACCEPT

# Set rate limits for DOS attack prevention (optional)
# The rates here greatly depend on your application
-A INPUT -p tcp -m multiport --dports 80,443 -m limit --limit 250/minute --limit-burst 1000 -j ACCEPT

# Log any traffic which was sent to you
# for forwarding (optional but useful).
-A FORWARD -m limit --limit 5/min -j LOG --log-prefix "iptables_FORWARD_denied: " --log-level 7

COMMIT