iptables -t nat -F
nic #
 iptables -F
nic #
 iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination         
nic #
 iptables -I FORWARD -s 192.1.2.23 -p udp --sport 500 -j DROP
nic #
 echo done
done