nixos/tinydns: add module

with improvements suggested by Jörg Thalheim <joerg@thalheim.io>

nixos/modules/module-list.nix

@@ -518,6 +518,7 @@
   ./services/networking/tcpcrypt.nix
   ./services/networking/teamspeak3.nix
   ./services/networking/tinc.nix
+  ./services/networking/tinydns.nix
   ./services/networking/tftpd.nix
   ./services/networking/tox-bootstrapd.nix
   ./services/networking/toxvpn.nix

nixos/modules/services/networking/tinydns.nix

@@ -0,0 +1,53 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  ###### interface
+
+  options = {
+    services.tinydns = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = "Whether to run the tinydns dns server";
+      };
+
+      data = mkOption {
+        type = types.lines;
+        description = "The DNS data to serve, in the format described by tinydns-data(8)";
+      };
+
+      ip = mkOption {
+        default = "0.0.0.0";
+        type = types.str;
+        description = "IP address on which to listen for connections";
+      };
+    };
+  };
+
+  ###### implementation
+
+  config = mkIf config.services.tinydns.enable {
+    environment.systemPackages = [ pkgs.djbdns ];
+
+    users.extraUsers.tinydns = {};
+
+    systemd.services.tinydns = {
+      description = "djbdns tinydns server";
+      wantedBy = [ "multi-user.target" ];
+      path = with pkgs; [ daemontools djbdns ];
+      preStart = ''
+        rm -rf /var/lib/tinydns
+        tinydns-conf tinydns tinydns /var/lib/tinydns ${config.services.tinydns.ip}
+        cd /var/lib/tinydns/root/
+        ln -sf ${pkgs.writeText "tinydns-data" config.services.tinydns.data} data
+        tinydns-data
+      '';
+      script = ''
+        cd /var/lib/tinydns
+        exec ./run
+      '';
+    };
+  };
+}