A basic starting template for iptables firewall rules.

firewall.sh

#!/bin/sh

###
# IPv4
###
# Flush and remove chains
/sbin/iptables -F
/sbin/iptables -X

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

# Allow existing connections
/sbin/iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

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

# Allow ICMP
/sbin/iptables -A INPUT -p icmp -j ACCEPT

# Goodies and baddies chains
/sbin/iptables -N GOODIES
/sbin/iptables -N BADDIES

# Filter through GOODIES whitelist
/sbin/iptables -A INPUT -j GOODIES

# Filter through BADDIES blacklist
/sbin/iptables -A INPUT -j BADDIES

# Ratelimit SSH to 3 per minute
/sbin/iptables -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 60 --hitcount 3 -j DROP
/sbin/iptables -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
/sbin/iptables -A INPUT -p tcp --dport 22 -m state --state NEW -j ACCEPT

# Web server
# /sbin/iptables -A INPUT -p tcp --dport 80 -j ACCEPT
# /sbin/iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# NTP server
# /sbin/iptables -A INPUT -p udp --dport 123 -j ACCEPT

# Mosh
# /sbin/iptables -A INPUT -p udp --dport 60000:61000 -j ACCEPT

###
# IPv6
###
# Flush and remove chains
/sbin/ip6tables -F
/sbin/ip6tables -X

# Set default policies
/sbin/ip6tables --policy INPUT DROP
/sbin/ip6tables --policy OUTPUT ACCEPT
/sbin/ip6tables --policy FORWARD DROP

# Allow existing connections
/sbin/ip6tables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow local loopback interface traffic
/sbin/ip6tables -A INPUT -i lo -j ACCEPT
/sbin/ip6tables -A OUTPUT -o lo -j ACCEPT

# Allow ICMP
/sbin/ip6tables -A INPUT -p icmpv6 -j ACCEPT

# Goodies and baddies chains
/sbin/ip6tables -N GOODIES
/sbin/ip6tables -N BADDIES

# Filter through GOODIES whitelist
/sbin/ip6tables -A INPUT -j GOODIES

# Filter through BADDIES blacklist
/sbin/ip6tables -A INPUT -j BADDIES

# Ratelimit SSH to 3 per minute
/sbin/ip6tables -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 60 --hitcount 3 -j DROP
/sbin/ip6tables -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
/sbin/ip6tables -A INPUT -p tcp --dport 22 -m state --state NEW -j ACCEPT

# Web server
# /sbin/ip6tables -A INPUT -p tcp --dport 80 -j ACCEPT
# /sbin/ip6tables -A INPUT -p tcp --dport 443 -j ACCEPT

# NTP server
# /sbin/ip6tables -A INPUT -p udp --dport 123 -j ACCEPT

# Mosh
# /sbin/ip6tables -A INPUT -p udp --dport 60000:61000 -j ACCEPT

/sbin/iptables-save > /etc/iptables.up.rules
/sbin/ip6tables-save > /etc/ip6tables.up.rules

cat <<EOF > /etc/network/if-pre-up.d/iptables
#!/bin/sh
/sbin/iptables-restore < /etc/iptables.up.rules
/sbin/ip6tables-restore < /etc/ip6tables.up.rules
EOF
chmod +x /etc/network/if-pre-up.d/iptables

cat <<EOF > /etc/network/if-post-down.d/iptables
#!/bin/sh
/sbin/iptables-save > /etc/iptables.up.rules
/sbin/ip6tables-save > /etc/ip6tables.up.rules
EOF
chmod +x /etc/network/if-post-down.d/iptables