bach:~# more my-firewall-on.sh
#!/bin/sh
#courtesy of jrd - John Dennison

iptables -F -t nat; iptables -X -t nat;
iptables -F -t mangle; iptables -X -t mangle;
iptables -P INPUT ACCEPT; iptables -P OUTPUT ACCEPT
bach:~# ./my-firewall-on.sh
bach:~# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination