nixos/nsd: Allow to configure root zone

When trying to run NSD to serve the root zone, one gets the following
error message:

error: illegal name: '.'

This is because the name of the zone is used as the derivation name for
building the zone file. However, Nix doesn't allow derivation names
starting with a period.

So whenever the zone is "." now, the file name generated is "root"
instead of ".".

I also added an assertion that makes sure the user sets
services.nsd.rootServer, otherwise NSD will fail at runtime because it
prevents serving the root zone without an explicit compile-time option.

Tested this by adding a root zone to the "nsd" NixOS VM test.

Signed-off-by: aszlig <[e-mail]>
Cc: @hrdinka, @qknight

nixos/modules/services/networking/nsd.nix

···
     zoneStats = length (collect (x: (x.zoneStats or null) != null) cfg.zones) > 0;
   };
 
+  mkZoneFileName = name: if name == "." then "root" else name;
 
   nsdEnv = pkgs.buildEnv {
     name = "nsd-env";
···
   };
 
   writeZoneData = name: text: pkgs.writeTextFile {
-    inherit name text;
-    destination = "/zones/${name}";
+    name = "nsd-zone-${mkZoneFileName name}";
+    inherit text;
+    destination = "/zones/${mkZoneFileName name}";
   };
 
 
···
   zoneConfigFile = name: zone: ''
     zone:
       name:         "${name}"
-      zonefile:     "${stateDir}/zones/${name}"
+      zonefile:     "${stateDir}/zones/${mkZoneFileName name}"
       ${maybeString "outgoing-interface: " zone.outgoingInterface}
     ${forEach     "  rrl-whitelist: "      zone.rrlWhitelist}
       ${maybeString "zonestats: "          zone.zoneStats}
···
   };
 
   config = mkIf cfg.enable {
+
+    assertions = singleton {
+      assertion = zoneConfigs ? "." -> cfg.rootServer;
+      message = "You have a root zone configured. If this is really what you "
+              + "want, please enable 'services.nsd.rootServer'.";
+    };
 
     environment.systemPackages = [ nsdPkg ];
 

nixos/tests/nsd.nix

···
         { address = "dead:beef::1"; prefixLength = 64; }
       ];
       services.nsd.enable = true;
+      services.nsd.rootServer = true;
       services.nsd.interfaces = lib.mkForce [];
       services.nsd.zones."example.com.".data = ''
         @ SOA ns.example.com noc.example.com 666 7200 3600 1209600 3600
···
         @ SOA ns.example.com noc.example.com 666 7200 3600 1209600 3600
         @ A 9.8.7.6
         @ AAAA fedc::bbaa
+      '';
+      services.nsd.zones.".".data = ''
+        @ SOA ns.example.com noc.example.com 666 7200 3600 1209600 3600
+        root A 1.8.7.4
+        root AAAA acbd::4
       '';
     };
   };
···
 
         assertHost($_, "a", "deleg.example.com", qr/address 9.8.7.6$/);
         assertHost($_, "aaaa", "deleg.example.com", qr/address fedc::bbaa$/);
+
+        assertHost($_, "a", "root", qr/address 1.8.7.4$/);
+        assertHost($_, "aaaa", "root", qr/address acbd::4$/);
       };
     }
   '';