nixos/dhparams: Add a defaultBitSize option

This allows to set the default bit size for all the Diffie-Hellman
parameters defined in security.dhparams.params and it's particularly
useful so that we can set it to a very low value in tests (so it doesn't
take ages to generate).

Regardless for the use in testing, this also has an impact in production
systems if the owner wants to set all of them to a different size than
2048, they don't need to set it individually for every params that are
set.

I've added a subtest to the "dhparams" NixOS test to ensure this is
working properly.

Signed-off-by: aszlig <aszlig@nix.build>

nixos/modules/security/dhparams.nix

@@ -4,13 +4,15 @@ let
   inherit (lib) mkOption types;
   cfg = config.security.dhparams;
 
-  paramsSubmodule = { name, config, ... }: {
+  bitType = types.addCheck types.int (b: b >= 16) // {
-    options.bits = mkOption {
-      type = types.addCheck types.int (b: b >= 16) // {
     name = "bits";
     description = "integer of at least 16 bits";
   };
-      default = 2048;
+
+  paramsSubmodule = { name, config, ... }: {
+    options.bits = mkOption {
+      type = bitType;
+      default = cfg.defaultBitSize;
       description = ''
         The bit size for the prime that is used during a Diffie-Hellman
         key exchange.
@@ -70,6 +72,11 @@ in {
           existing ones won't be cleaned up. Of course this only applies if
           <option>security.dhparams.stateful</option> is
           <literal>true</literal>.</para></warning>
+
+          <note><title>For module implementers:</title><para>It's recommended
+          to not set a specific bit size here, so that users can easily
+          override this by setting
+          <option>security.dhparams.defaultBitSize</option>.</para></note>
         '';
       };
 
@@ -89,6 +96,16 @@ in {
         '';
       };
 
+      defaultBitSize = mkOption {
+        type = bitType;
+        default = 2048;
+        description = ''
+          This allows to override the default bit size for all of the
+          Diffie-Hellman parameters set in
+          <option>security.dhparams.params</option>.
+        '';
+      };
+
       path = mkOption {
         type = types.str;
         default = "/var/lib/dhparams";

nixos/tests/dhparams.nix

@@ -54,6 +54,13 @@ in import ./make-test.nix {
     security.dhparams.params.bar2.bits = 19;
   };
 
+  nodes.generation5 = {
+    imports = [ common ];
+    security.dhparams.defaultBitSize = 30;
+    security.dhparams.params.foo3 = {};
+    security.dhparams.params.bar3 = {};
+  };
+
   testScript = { nodes, ... }: let
     getParamPath = gen: name: let
       node = "generation${toString gen}";
@@ -126,5 +133,12 @@ in import ./make-test.nix {
         'expr match ${getParamPath 4 "bar2"} ${builtins.storeDir}',
       );
     };
+
+    ${switchToGeneration 5}
+
+    subtest "check whether defaultBitSize works as intended", sub {
+      ${assertParamBits 5 "foo3" 30}
+      ${assertParamBits 5 "bar3" 30}
+    };
   '';
 }