Selfhosted

Half off-topic, sorry: if you have some spare time on the weekend, you might want to take a look at nftables. AFAIK iptables is also just using nftables under the hood, so you are basically using a deprecated technology.

nftables is so much nicer to work with. In the end I have my custom rules (which are much saner to define than in iptables) in /etc/nftables.conf, then I have a very simple systemd unit:

[Unit]
Description=Restore nftables firewall rules
Before=network-pre.target

[Service]
Type=oneshot
ExecStart=/usr/sbin/nft -f /etc/nftables.conf
ExecStop=/usr/sbin/nft flush table inet filter
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

and finally if I push updates via ansible I simply replace the file and run nft -f /etc/nftables.conf (via ansible; on-change event).

Edit: oh and as an example how the actual rules file looks like:

#!/usr/bin/nft -f

add table inet filter
flush table inet filter

table inet filter {
  chain input {
    type filter hook input priority 0;

    # allow established/related connections
    ct state {established, related} accept

    # early drop of invalid connections
    ct state invalid drop

    # allow from loopback
    iifname lo accept

    # allow icmp
    ip protocol icmp accept
    ip6 nexthdr icmpv6 accept

    # core services
    tcp dport {80, 443} accept comment "allow http(s)"
    udp dport 443 accept comment "allow http3"

    # everything else
    reject with icmpx type port-unreachable
  }

}

and with that I have my ipv4+6 firewall that allows pings and http