add dkimKeyBits configuration

default.nix

@@ -422,6 +422,19 @@ in
       '';
     };
 
+    dkimKeyBits = mkOption {
+        type = types.int;
+        default = 1024;
+        description = ''
+            How many bits in generated DKIM keys. RFC6376 advises minimum 1024-bit keys.
+
+            If you have already deployed a key with a different number of bits than specified
+            here, then you should use a different selector (dkimSelector). In order to get
+            this package to generate a key with the new number of bits, you will either have to
+            change the selector or delete the old key file.
+        '';
+    };
+
     debug = mkOption {
       type = types.bool;
       default = false;

mail-server/opendkim.nix

@@ -33,6 +33,7 @@ let
           then
               ${pkgs.opendkim}/bin/opendkim-genkey -s "${cfg.dkimSelector}" \
                                                    -d "${dom}" \
+                                                   --bits="${toString cfg.dkimKeyBits}" \
                                                    --directory="${cfg.dkimKeyDirectory}"
               mv "${cfg.dkimKeyDirectory}/${cfg.dkimSelector}.private" "${dkim_key}"
               mv "${cfg.dkimKeyDirectory}/${cfg.dkimSelector}.txt" "${dkim_txt}"

tests/extern.nix

@@ -38,6 +38,7 @@ import <nixpkgs/nixos/tests/make-test.nix> {
               fqdn = "mail.example.com";
               domains = [ "example.com" "example2.com" ];
               rewriteMessageId = true;
+              dkimKeyBits = 1535;
 
               loginAccounts = {
                   "user1@example.com" = {
@@ -321,6 +322,10 @@ import <nixpkgs/nixos/tests/make-test.nix> {
         $client->succeed("grep 'Received: from mail.example.com' ~/mail/*");
       };
 
+      subtest "dkim has user-specified size", sub {
+        $server->succeed("openssl rsa -in /var/dkim/example.com.mail.key -text -noout | grep 'Private-Key: (1535 bit)'");
+      };
+
       subtest "dkim singing, multiple domains", sub {
           $client->execute("rm ~/mail/*");
           # send email from user2 to user1