cat > /etc/iptables.rules <<EOF
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp --dport 22 -j ACCEPT
-A INPUT -p udp -m multiport --dports 200,400,600 -j ACCEPT
-A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -s 192.168.18.0/24 -j ACCEPT
COMMIT
*nat
:PREROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -s 192.168.18.0/24 -j SNAT --to-source x.x.x.x
COMMIT
EOF
cat > /etc/network/if-up.d/iptables <<EOF
#!/bin/sh
/sbin/iptables-restore < /etc/iptables.rules
EOF
chmod +x /etc/network/if-up.d/iptables