net-firewall/iptables/files/systemd/iptables-store.service

[Unit]
Description=Store iptables firewall rules
Before=shutdown.target
DefaultDependencies=No

[Service]
Type=oneshot
ExecStart=/bin/sh -c "/sbin/iptables-save --counters > /var/lib/iptables/rules-save"

[Install]
WantedBy=shutdown.target