{
  pkgs,
  notnft,
  config,
  ...
}: {
  services.ifstate.settings.namespaces.border = {
    routing.routes = [
      {
        to = "0.0.0.0/0";
        dev = "ppp-wan";
      }
    ];

    interfaces = [
      {
        name = "slan-vlan";
        link = {
          kind = "vlan";
          link = "slan";
          link_netns = null;
          vlan_id = 6;
          state = "up";
          mtu = config.bananapi.mtu.slan-vlan;
        };
      }

      # {
      #   name = "ppp-slan";
      #   link = {
      #     kind = "ppp";
      #     addresses = [
      #       "192.168.1.1/24"
      #     ]
      #   }
      # }

      {
        name = "wan-vlan";
        link = {
          kind = "vlan";
          link = "wan";
          link_netns = null;
          vlan_id = 6;
          state = "up";
          mtu = config.bananapi.mtu.wan-vlan;
        };
      }

      # {
      #   name = "ppp-wan";
      #   link = {
      #     kind = "dummy";
      #   };
      #   addresses = [
      #     "8.8.8.8/32"
      #   ];
      # }

      {
        name = "dmz";
        link = {
          kind = "veth";
          peer = "border";
          peer_netns = "dmz";
          state = "up";
        };
        addresses = [
          "10.0.0.1/24"
        ];
      }
    ];
  };

  networking.notnft.namespaces.border.rules =
    # ---
    with notnft.dsl;
    with payload;
    # ---
      ruleset {
        filter = add table {family = f: f.inet;} {
          port_dnat =
            add notnft.dsl.map {
              map = f: [f.ipv4_addr f.inet_service];
              type = f: [f.inet_proto f.ipv4_addr f.inet_service];
              flags = f: with f; [interval];
            } [
              [(concat ["udp" "86.80.70.193" 6666]) (concat ["10.0.0.2" 6666])]
              [(concat ["udp" "86.80.70.193" 500]) (concat ["10.0.0.2" 500])]
              [(concat ["udp" "86.80.70.193" 501]) (concat ["10.0.0.2" 501])]
              [(concat ["tcp" "192.168.1.1" 22]) (concat ["10.0.0.2" 22])]
            ];

          local_nets4 =
            add set {
              type = f: f.ipv4_addr;
              flags = f: with f; [interval];
            } [
              (cidr "10.0.0.0" 8)
              (cidr "172.16.0.0" 12)
              (cidr "192.168.0.0" 16)
            ];

          input =
            add chain {
              type = f: f.filter;
              hook = f: f.input;
              prio = -300;
              policy = f: f.drop;
            }
            [(is.eq meta.iifname "lo") accept]
            [
              (is.eq ip.saddr (set [
                (cidr "192.168.1.0" 25)
              ]))
              (is.eq ip.daddr (set [
                "192.168.1.1"
                "86.80.70.193"
              ]))
              (is.eq ip.protocol (f: f.icmp))
              accept
            ]
            [
              (is.eq ip.saddr (set [
                (cidr "10.0.0.0" 24)
                (cidr "10.1.0.0" 19)
              ]))
              (is.eq ip.daddr (set [
                "10.0.0.1"
                "86.80.70.193"
              ]))
              (is.eq ip.protocol (f: f.icmp))
              accept
            ]
            [
              (log {
                prefix = "[drop] border.input: ";
                queue-threshold = 1;
                group = 2;
              })
              drop
            ];

          output =
            add chain {
              type = f: f.filter;
              hook = f: f.output;
              prio = -300;
              policy = f: f.drop;
            }
            # accept related, established
            [
              (vmap ct.state {
                established = accept;
                related = accept;
              })
            ]
            [
              (is.eq ip.saddr (set [
                "192.168.1.1"
                "86.80.70.193"
              ]))
              (is.eq ip.daddr (set [
                (cidr "192.168.1.0" 25)
              ]))
              (is.eq ip.protocol (f: f.icmp))
              (is.eq icmp.type (f: f.echo-reply))
              accept
            ]
            [
              (is.eq ip.saddr (set [
                "10.0.0.1"
                "86.80.70.193"
              ]))
              (is.eq ip.daddr (set [
                (cidr "10.0.0.0" 24)
                (cidr "10.1.0.0" 19)
              ]))
              (is.eq ip.protocol (f: f.icmp))
              (is.eq icmp.type (f: f.echo-reply))
              accept
            ]
            [
              (log {
                prefix = "[drop] border.output: ";
                queue-threshold = 1;
                group = 2;
              })
              drop
            ];

          forward =
            add chain {
              type = f: f.filter;
              hook = f: f.forward;
              prio = -300;
              policy = f: f.drop;
            }
            # accept related, established
            [
              (vmap ct.state {
                established = accept;
                related = accept;
                invalid = drop;
              })
            ]
            # allow forwarding traffic for the internet
            [
              (is.eq meta.iifname (set ["dmz" "ppp-slan"]))
              (is.eq meta.oifname "ppp-wan")
              accept
            ]
            # accept port forwarding from `wan`, `ppp-slan`, `ppp-wan` to `dmz`
            [
              (is.eq meta.iifname (set ["ppp-wan" "ppp-slan" "dmz"]))
              (is.eq meta.oifname "dmz")
              (is."in" ct.status "dnat")
              accept
            ]
            [
              (log {
                prefix = "[drop] border.forward: ";
                queue-threshold = 1;
                group = 2;
              })
              drop
            ];

          prerouting =
            add chain {
              type = f: f.nat;
              hook = f: f.prerouting;
              prio = -100;
              policy = f: f.accept;
            }
            [
              (dnat.ip {
                addr.map = {
                  key = concat [ip.protocol ip.daddr th.dport];
                  data = "@port_dnat";
                };
              })
            ];

          postrouting =
            add chain {
              type = f: f.nat;
              hook = f: f.postrouting;
              prio = -100;
              policy = f: f.accept;
            }
            [
              (is.eq meta.iifname "dmz")
              (is.eq meta.oifname "dmz")
              (is.eq
                (concat [ip.protocol th.dport])
                (set [
                  (concat ["udp" 500])
                  (concat ["udp" 501])
                  (concat ["udp" 6666])
                ]))
              (is.eq ip.saddr "10.0.0.2")
              (is.eq ip.daddr "10.0.0.2")
              masquerade
            ]
            [
              (is.eq meta.iifname "ppp-slan")
              (is.eq meta.oifname "ppp-slan")
              (is.eq
                (concat [ip.protocol th.dport])
                (set [
                  (concat ["udp" 500])
                  (concat ["udp" 6666])
                  (concat ["tcp" 2288])
                ]))
              (is.eq ip.saddr "192.168.1.2")
              (is.eq ip.daddr "192.168.1.2")
              masquerade
            ]
            [
              (is.eq meta.oifname "ppp-wan")
              masquerade
            ];
        };
      };

  services.pppoe-server.kpn = {
    interface = "slan-vlan";
    localAddress = "192.168.1.1";
    remoteAddressFile = pkgs.writeText "kpn-remote-address-file" ''
      192.168.1.2
    '';
    C = "195.190.228.154";
    pppdSettings = {
      ifname = ["ppp-slan"];
      mtu = [1500];
      mru = [1500];
    };
  };

  systemd.services.pppoe-server-kpn = {
    after = ["ifstate.service"];
    serviceConfig.NetworkNamespacePath = "/var/run/netns/border";
  };

  services.pppd = {
    enable = true;
    peers.kpn = {
      config = ''
        plugin ${pkgs.rp-pppoe}/etc/ppp/plugins/rp-pppoe.so
        nic-wan-vlan
        name "internet"
        password "internet"
        noauth
        hide-password
        debug
        +ipv6
        ipv6cp-accept-local
        noipdefault
        defaultroute
        defaultroute6
        persist
        maxfail 0
        holdoff 5
        mtu 1500
        mru 1500
        ifname ppp-wan
      '';
    };
  };

  systemd.services.pppd-kpn = {
    after = ["ifstate.service"];
    serviceConfig.NetworkNamespacePath = "/var/run/netns/border";
  };

  systemd.services.ulogd-border = {
    description = "Ulogd Daemon";
    wantedBy = ["multi-user.target"];
    wants = ["network-pre.target"];
    before = ["network-pre.target"];
    after = ["ifstate.service"];

    serviceConfig = let
      settingsFormat = pkgs.formats.ini {listsAsDuplicateKeys = true;};
      settingsFile = settingsFormat.generate "ulogd.conf" {
        # This one for logging to local file in emulated syslog format.
        global.stack = "log2:NFLOG,base1:BASE,ifi1:IFINDEX,ip2str1:IP2STR,print1:PRINTPKT,emu1:LOGEMU";

        log2.group = 2;

        emu1 = {
          file = "/var/log/nft_border_drop.log";
          sync = 1;
        };
      };
    in {
      NetworkNamespacePath = "/var/run/netns/border";
      ExecStart = "${pkgs.ulogd}/bin/ulogd -c ${settingsFile} --verbose --loglevel ${
        toString 5
      }";
      ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
    };
  };
}