{ config, pkgs, lib, ... }:

with lib;

{
  options = {
    services.xl2tpd = {
      enable = mkEnableOption (lib.mdDoc "xl2tpd, the Layer 2 Tunnelling Protocol Daemon");

      serverIp = mkOption {
        type        = types.str;
        description = lib.mdDoc "The server-side IP address.";
        default     = "10.125.125.1";
      };

      clientIpRange = mkOption {
        type        = types.str;
        description = lib.mdDoc "The range from which client IPs are drawn.";
        default     = "10.125.125.2-11";
      };

      extraXl2tpOptions = mkOption {
        type        = types.lines;
        description = lib.mdDoc "Adds extra lines to the xl2tpd configuration file.";
        default     = "";
      };

      extraPppdOptions = mkOption {
        type        = types.lines;
        description = lib.mdDoc "Adds extra lines to the pppd options file.";
        default     = "";
        example     = ''
          ms-dns 8.8.8.8
          ms-dns 8.8.4.4
        '';
      };
    };
  };

  config = mkIf config.services.xl2tpd.enable {
    systemd.services.xl2tpd = let
      cfg = config.services.xl2tpd;

      # Config files from https://help.ubuntu.com/community/L2TPServer
      xl2tpd-conf = pkgs.writeText "xl2tpd.conf" ''
        [global]
        ipsec saref = no

        [lns default]
        local ip = ${cfg.serverIp}
        ip range = ${cfg.clientIpRange}
        pppoptfile = ${pppd-options}
        length bit = yes

        ; Extra
        ${cfg.extraXl2tpOptions}
      '';

      pppd-options = pkgs.writeText "ppp-options-xl2tpd.conf" ''
        refuse-pap
        refuse-chap
        refuse-mschap
        require-mschap-v2
        # require-mppe-128
        asyncmap 0
        auth
        crtscts
        idle 1800
        mtu 1200
        mru 1200
        lock
        hide-password
        local
        # debug
        name xl2tpd
        # proxyarp
        lcp-echo-interval 30
        lcp-echo-failure 4

        # Extra:
        ${cfg.extraPppdOptions}
      '';

      xl2tpd-ppp-wrapped = pkgs.stdenv.mkDerivation {
        name         = "xl2tpd-ppp-wrapped";
        phases       = [ "installPhase" ];
        nativeBuildInputs  = with pkgs; [ makeWrapper ];
        installPhase = ''
          mkdir -p $out/bin

          makeWrapper ${pkgs.ppp}/sbin/pppd $out/bin/pppd \
            --set LD_PRELOAD    "${pkgs.libredirect}/lib/libredirect.so" \
            --set NIX_REDIRECTS "/etc/ppp=/etc/xl2tpd/ppp"

          makeWrapper ${pkgs.xl2tpd}/bin/xl2tpd $out/bin/xl2tpd \
            --set LD_PRELOAD    "${pkgs.libredirect}/lib/libredirect.so" \
            --set NIX_REDIRECTS "${pkgs.ppp}/sbin/pppd=$out/bin/pppd"
        '';
      };
    in {
      description = "xl2tpd server";

      requires = [ "network-online.target" ];
      wantedBy = [ "multi-user.target" ];

      preStart = ''
        mkdir -p -m 700 /etc/xl2tpd

        pushd /etc/xl2tpd > /dev/null

        mkdir -p -m 700 ppp

        [ -f ppp/chap-secrets ] || cat > ppp/chap-secrets << EOF
        # Secrets for authentication using CHAP
        # client	server	secret		IP addresses
        #username	xl2tpd	password	*
        EOF

        chown root:root ppp/chap-secrets
        chmod 600 ppp/chap-secrets

        # The documentation says this file should be present but doesn't explain why and things work even if not there:
        [ -f l2tp-secrets ] || (echo -n "* * "; ${pkgs.apg}/bin/apg -n 1 -m 32 -x 32 -a 1 -M LCN) > l2tp-secrets
        chown root:root l2tp-secrets
        chmod 600 l2tp-secrets

        popd > /dev/null

        mkdir -p /run/xl2tpd
        chown root:root /run/xl2tpd
        chmod 700       /run/xl2tpd
      '';

      serviceConfig = {
        ExecStart = "${xl2tpd-ppp-wrapped}/bin/xl2tpd -D -c ${xl2tpd-conf} -s /etc/xl2tpd/l2tp-secrets -p /run/xl2tpd/pid -C /run/xl2tpd/control";
        KillMode  = "process";
        Restart   = "on-success";
        Type      = "simple";
        PIDFile   = "/run/xl2tpd/pid";
      };
    };
  };
}