nixos-config/modules/authoritative-dns.nix

# SPDX-FileCopyrightText: 2023 Simon Bruder <simon@sbruder.de>
#
# SPDX-License-Identifier: AGPL-3.0-or-later

{ config, lib, pkgs, ... }:
let
  cfg = config.sbruder.knot;

  primaryHost = "vueko";
  secondaryHosts = [ "okarin" ];

  isPrimaryHost = config.networking.hostName == primaryHost;
  isSecondaryHost = lib.elem config.networking.hostName secondaryHosts;

  addresses = {
    vueko = [ "168.119.176.53" "2a01:4f8:c012:2f4::1" ];
    okarin = [ "82.165.242.252" "2001:8d8:1800:8627::1" ];
  };
in
{
  options = {
    sbruder.knot.generated-zones = lib.mkOption {
      type = lib.types.attrsOf lib.types.path;
      default = { };
      description = "List of zones generated by a nix expression";
    };
  };

  config = lib.mkIf (isPrimaryHost || isSecondaryHost) {
    services.knot = {
      enable = true;
      keyFiles = [
        # Managed in separate repository.
        # It includes all statically managed zones.
        # Even though it is not a key,
        # it needs to be included here
        # so the module disables configuration checks.
        "/var/lib/knot/static.conf"
      ];
      settings = lib.mkMerge [
        {
          server = {
            listen = map (address: "${address}@53") addresses.${config.networking.hostName};
            automatic-acl = true;
          };

          log = lib.singleton {
            target = "syslog";
            server = "info";
            control = "warning"; # otherwise stats gets logged every scrape
            zone = "info";
          };

          mod-stats = lib.singleton {
            id = "custom";
            edns-presence = true;
            flag-presence = true;
            query-size = true;
            query-type = true;
            reply-size = true;
          };

          remote = (lib.mapAttrsToList
            (host: hostAddresses: {
              id = host;
              address = hostAddresses;
            })
            addresses) ++ lib.optional isPrimaryHost {
            id = "inwx";
            # INWX only allows the specification of one primary DNS,
            # which limits the IP protocol usable for zone transfers to one.
            address = lib.singleton "185.181.104.96";
          };
        }
        (lib.mkIf isPrimaryHost {
          policy = lib.singleton {
            id = "default";
            nsec3 = true;
          };

          template = [
            {
              id = "default";
              storage = "/var/lib/knot/zones/";
              semantic-checks = true;
              # auto increment serial
              zonefile-sync = -1;
              zonefile-load = "difference-no-serial";
              journal-content = "all";
              # secondary
              notify = [ "inwx" ] ++ secondaryHosts;
              # dnssec
              dnssec-signing = true;
              dnssec-policy = "default";
              # stats
              module = "mod-stats/custom";
            }
            {
              id = "nix-generated";
              storage = "/var/lib/knot/nix-zones/";
              semantic-checks = true;
              # auto increment serial
              zonefile-sync = -1;
              zonefile-load = "difference-no-serial";
              journal-content = "all";
              # stats
              module = "mod-stats/custom";
            }
          ];

          zone = map
            (domain: {
              inherit domain;
              template = "nix-generated";
            })
            (lib.attrNames cfg.generated-zones);
        })
        (lib.mkIf isSecondaryHost {
          acl = lib.singleton {
            id = "primary_notify";
            address = lib.flatten addresses.${primaryHost};
            action = "notify";
          };

          template = lib.singleton {
            id = "default";
            master = [ primaryHost ];
            acl = [ "primary_notify" ];
            # stats
            module = "mod-stats/custom";
          };
        })
      ];
    };

    users.users.knot = {
      openssh.authorizedKeys.keys = config.sbruder.pubkeys.trustedKeys;
      shell = pkgs.bashInteractive;
    };

    systemd.tmpfiles.rules = [
      "f /var/lib/knot/static.conf 0644 knot knot - -"
    ] ++ (lib.optionals isPrimaryHost [
      "d /var/lib/knot/nix-zones 0755 knot knot - -"
    ]);

    systemd.services.knot-generated-zones = lib.mkIf isPrimaryHost {
      wantedBy = [ "knot.service" ];
      after = [ "knot.service" ];
      path = with pkgs; [ knot-dns ];
      script = ''
        set -euo pipefail
        ${lib.concatStringsSep "\n" (lib.mapAttrsToList (domain: zonefile: ''
          kzonecheck -o ${lib.escapeShellArg domain} ${lib.escapeShellArg zonefile}
          target=/var/lib/knot/nix-zones/${lib.escapeShellArg domain}.zone
          if [ -h "$target" ]; then
            pre_target="$(readlink "$target")"
          else
            pre_target="/invalid/path"
          fi
          ln -sf ${lib.escapeShellArg zonefile} "$target"
          if [ "$pre_target" != ${lib.escapeShellArg domain} ]; then
            echo -n "Zone for ${lib.escapeShellArg domain} changed, reloading… "
            knotc zone-reload ${lib.escapeShellArg domain}
          fi
        '') cfg.generated-zones)}
      '';
      restartTriggers = lib.attrValues cfg.generated-zones;
      serviceConfig = {
        Type = "oneshot";
        RemainAfterExit = true;
        User = "knot";

        CapabilityBoundingSet = ""; # clear
        LockPersonality = true;
        MemoryDenyWriteExecute = true;
        NoNewPrivileges = true;
        PrivateDevices = true;
        PrivateNetwork = true;
        PrivateTmp = true;
        PrivateUsers = true;
        ProtectClock = true;
        ProtectControlGroups = true;
        ProtectHome = true;
        ProtectHostname = true;
        ProtectKernelLogs = true;
        ProtectKernelModules = true;
        ProtectKernelTunables = true;
        ProtectProc = "invisible";
        ProtectSystem = true;
        RemoveIPC = true;
        RestrictAddressFamilies = [ "AF_UNIX" ]; # knot socket
        # this is not ideal, but I couldn’t find out how to get a bind mount of the knot socket to work otherwise
        RestrictNamespaces = [ true "~mnt" ];
        RestrictRealtime = true;
        RestrictSUIDSGID = true;
        SystemCallArchitectures = "native";
        SystemCallFilter = [ "@system-service" "~@privileged" "~@resources" ];
      };
    };

    networking.firewall = {
      allowedTCPPorts = [ 53 ];
      allowedUDPPorts = [ 53 ];
    };

    services.prometheus.exporters.knot = {
      enable = true;
      listenAddress = config.sbruder.wireguard.home.address;
    };

    assertions = [
      {
        assertion = isPrimaryHost -> (lib.hasAttr "vpn.sbruder.de" cfg.generated-zones);
        message = "The authoritative DNS module requires the server the wg-home wireguard server to run on the same host.";
      }
    ];
  };
}