# IPv6 on the PPPoE uplink: run networkd's DHCPv6 client on ppp0 to obtain a
# delegated prefix; each vlan-* interface (default.nix) carves a /64 out of it
# via DHCPPrefixDelegation and announces it to clients with SLAAC.
{
  config,
  lib,
  ...
}:
let
  cfg = config.cnx.router;
in
{
  config = lib.mkIf cfg.enable {
    systemd.network.networks."45-ppp0" = {
      matchConfig.Name = "ppp0";
      networkConfig = {
        DHCP = "ipv6";
        # pppd owns the v4 address/route on this link; don't let networkd
        # tear them down.
        KeepConfiguration = "static";
        # Default v6 route comes from the ISP's RA when they send one.
        IPv6AcceptRA = true;
      };
      # Many PPPoE ISPs never send an RA with the M flag; solicit regardless.
      dhcpV6Config.WithoutRA = "solicit";
      linkConfig.RequiredForOnline = "no";
    };

    boot.kernel.sysctl."net.ipv6.conf.all.forwarding" = lib.mkDefault 1;
  };
}