feat: add support for DKIM private key files

This gives people an option to declaratively manage these secrets,
rather than having them generated.

default.nix

@@ -794,6 +794,27 @@ in
       '';
     };
 
+    dkimPrivateKeyFiles = mkOption {
+      type = types.nullOr (types.attrsOf types.path);
+      default = null;
+      example = {
+        "mail.example.com" = "/run/secrets/dkim/mail.example.com.mail.key";
+      };
+      description = ''
+        Paths to opendkim private keys generated with `opendkim-genkey`,
+        indexed by domain name.
+        If `null`, then the keys are auto generated.
+        If not `null`, then there must be an entry for every domain in
+        {option}`config.mailserver.domains`.
+
+        >>> TODO: explain/assert how this interacts with `dkimKeyBits`. would
+        this be cleaner if we had an explicit "generate dkim keys" option that
+        defaults to true, and perhaps we move the generation option (just
+        `dkimKeyBits` right now) under an optional `generateOpts` section? this
+        is not backward compatible, though <<<
+      '';
+    };
+
     dkimKeyDirectory = mkOption {
       type = types.path;
       default = "/var/dkim";

mail-server/opendkim.nix

@@ -23,14 +23,26 @@ let
   dkimUser = config.services.opendkim.user;
   dkimGroup = config.services.opendkim.group;
 
-  createDomainDkimCert = dom:
+  createOrLinkDkimCerts = dom:
     let
       dkim_key = "${cfg.dkimKeyDirectory}/${dom}.${cfg.dkimSelector}.key";
       dkim_txt = "${cfg.dkimKeyDirectory}/${dom}.${cfg.dkimSelector}.txt";
     in
+      if cfg.dkimPrivateKeyFiles != null then
+        let
+          dkimPrivateKeyFile = cfg.dkimPrivateKeyFiles.${dom};
+        in
         ''
-          if [ ! -f "${dkim_key}" ]
-          then
+          if [ ! -f "${dkimPrivateKeyFile}" ]; then
+            echo "DKIM keyfile does not exist: ${dkimPrivateKeyFile}"
+            exit 1
+          fi
+
+          ln -sf "${dkimPrivateKeyFile}" "${dkim_key}"
+        ''
+      else
+        ''
+          if [ ! -f "${dkim_key}" ]; then
               ${pkgs.opendkim}/bin/opendkim-genkey -s "${cfg.dkimSelector}" \
                                                    -d "${dom}" \
                                                    --bits="${toString cfg.dkimKeyBits}" \
@@ -41,7 +53,7 @@ let
               echo "Generated key for domain ${dom} selector ${cfg.dkimSelector}"
           fi
         '';
-  createAllCerts = lib.concatStringsSep "\n" (map createDomainDkimCert cfg.domains);
+  createOrLinkAllCerts = lib.concatStringsSep "\n" (map createOrLinkDkimCerts cfg.domains);
 
   keyTable = pkgs.writeText "opendkim-KeyTable"
     (lib.concatStringsSep "\n" (lib.flip map cfg.domains
@@ -76,7 +88,7 @@ in
         postfix.extraGroups = [ "${dkimGroup}" ];
       };
       systemd.services.opendkim = {
-        preStart = lib.mkForce createAllCerts;
+        preStart = lib.mkForce createOrLinkAllCerts;
         serviceConfig = {
           ExecStart = lib.mkForce "${pkgs.opendkim}/bin/opendkim ${escapeShellArgs args}";
           PermissionsStartOnly = lib.mkForce false;