rules

*nat
:PREROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
COMMIT

*filter
# Set default chain policies
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]

# Allows all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -d 127.0.0.0/8 -j REJECT

# Drop any tcp packet that does not start a connection with a syn flag.
-A INPUT -p tcp ! --syn -m state --state NEW -j DROP

# Accepts all established inbound connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Drop any invalid packet that could not be identified.
-A INPUT -m state --state INVALID -j DROP

# Blacklist (examples)
#-A INPUT -i eth0 -s 198.51.100.1 -p tcp --dport 22 -j DROP # Example IP from RFC 5737
#-A INPUT -i eth0 -s 203.0.113.34 -j DROP # Example IP from RFC 5737

# Allow incoming FTP
#-A INPUT -i eth0 -p tcp --dport 21 -j ACCEPT

# Allow incoming SSH
-A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT

# Allow incoming HTTP
#-A INPUT -i eth0 -p tcp --dport 80 -j ACCEPT

# Allow incoming HTTPS
#-A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT

# Allow ping from inside to outside
-A OUTPUT -o eth0 -p icmp --icmp-type echo-request -j ACCEPT
-A INPUT -i eth0 -p icmp --icmp-type echo-reply -m limit --limit 2/s -j ACCEPT

# Allow ping from outside to inside
-A INPUT -i eth0 -p icmp --icmp-type echo-request -m limit --limit 2/s -j ACCEPT
-A OUTPUT -o eth0 -p icmp --icmp-type echo-reply -j ACCEPT

# Allow NTP
#-A INPUT -i eth0 -p udp --dport 123 -j ACCEPT
#-A OUTPUT -o eth0 -p udp --sport 123 -j ACCEPT

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

# Log dropped packets
-N LOGGING
-A INPUT -j LOGGING
-A LOGGING -p tcp -m limit --limit 5/min --limit-burst 10 -j LOG --log-prefix "iptables: [denied TCP] " --log-level 7
-A LOGGING -p udp -m limit --limit 5/min --limit-burst 10 -j LOG --log-prefix "iptables: [denied UDP] " --log-level 7
-A LOGGING -p icmp -m limit --limit 5/min --limit-burst 10 -j LOG --log-prefix "iptables: [denied ICMP] " --log-level 7

# Reject all other inbound - default deny unless explicitly allowed policy:
-A INPUT -j REJECT

# Allow all other outbound traffic
-A OUTPUT -j ACCEPT

# Reject forwarded traffic, explicitely (despite chain policy)
-A FORWARD -j REJECT

COMMIT