[Unit]
Description=Firewall
Requires=network-online.target
After=network-online.target

[Service]
User=root
Type=oneshot
RemainAfterExit=yes
ExecStart=/etc/init.d/nom_de_votre_script_iptables(modifier_le_nom!) start
ExecStop=/etc/init.d/nom_de_votre_script_iptables(modifier_le_nom!) stop

[Install]
WantedBy=multi-user.target