nixos-demo/os/network/firewall.nix

{ ... }:

{
  networking.firewall.enable = true;
  networking.firewall.allowPing = true;
  networking.nftables.enable = false;
  networking.nftables.tables = {
    filter = {
      content = ''
        # Check out https://wiki.nftables.org/ for better documentation.
        # Table for both IPv4 and IPv6.
        chain input {
          type filter hook input priority 0;

          # accept any localhost traffic
          iifname lo accept

          # accept traffic originated from us
          ct state {established, related} accept

          # ICMP
          # routers may also want: mld-listener-query, nd-router-solicit
          ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, parameter-problem, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert } accept
          ip protocol icmp icmp type { destination-unreachable, router-advertisement, time-exceeded, parameter-problem } accept

          # allow "ping"
          ip6 nexthdr icmpv6 icmpv6 type echo-request accept
          ip protocol icmp icmp type echo-request accept

          # count and drop any other traffic
          counter drop
        }

        # Allow all outgoing connections.
        chain output {
          type filter hook output priority 0;
          accept
        }

        chain forward {
          type filter hook forward priority 0;
          accept
        }
      '';
      family = "inet";
    };
  };
}