feat: add support for DKIM private key files

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

README.md

@@ -22,7 +22,7 @@ SNM branch corresponding to your NixOS version.
 
 ## Features
 
-* [x] Continous Integration Testing
+* [x] Continuous Integration Testing
 * [x] Multiple Domains
 * Postfix
   * [x] SMTP on port 25
@@ -44,6 +44,7 @@ SNM branch corresponding to your NixOS version.
   * [x] Via ClamAV
 * DKIM Signing
   * [x] Via Rspamd
+  * [x] Allow passing DKIM signing keys
 * User Management
   * [x] Declarative user management
   * [x] Declarative password management
@@ -64,7 +65,6 @@ SNM branch corresponding to your NixOS version.
   * [ ] [Mobileconfig](https://support.apple.com/guide/profile-manager/distribute-profiles-manually-pmdbd71ebc9/mac)
 * DKIM Signing
   * [ ] Allow per domain selectors
-  * [ ] Allow passing DKIM signing keys
 * Improve the Forwarding Experience
   * [ ] Support [ARC](https://en.wikipedia.org/wiki/Authenticated_Received_Chain) signing with [Rspamd](https://rspamd.com/doc/modules/arc.html)
   * [ ] Support [SRS](https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme) with [postsrsd](https://github.com/roehling/postsrsd)

default.nix

@@ -794,6 +794,21 @@ in
       '';
     };
 
+    dkimDomainPrivateKeyFiles = 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 set, then there must be an entry for every domain in
+        {option}`config.mailserver.domains`.
+      '';
+    };
+
     dkimKeyDirectory = mkOption {
       type = types.path;
       default = "/var/dkim";
@@ -816,8 +831,8 @@ in
     };
 
     dkimKeyBits = mkOption {
-        type = types.int;
-        default = 1024;
+        type = types.nullOr types.int;
+        default = if cfg.dkimDomainPrivateKeyFiles == null then 1024 else null;
         description = ''
             How many bits in generated DKIM keys. RFC6376 advises minimum 1024-bit keys.
 

mail-server/assertions.nix

@@ -14,5 +14,28 @@
       assertion = config.mailserver.acmeCertificateName == config.mailserver.fqdn;
       message = "When the certificate scheme is not 'acme' (mailserver.certificateScheme != \"acme\"), it is not possible to define mailserver.acmeCertificateName";
     }
+  ] ++ (
+    let
+      sortedDomains = builtins.sort (a: b: a < b) config.mailserver.domains;
+      sortedDkimDomains = builtins.attrNames config.mailserver.dkimDomainPrivateKeyFiles;
+      prettyDomains = builtins.concatStringsSep ", " sortedDomains;
+      prettyDkimDomains = builtins.concatStringsSep ", " sortedDkimDomains;
+    in
+    lib.optionals (config.mailserver.enable && config.mailserver.dkimDomainPrivateKeyFiles != null && sortedDomains != sortedDkimDomains) [
+      {
+        assertion = config.mailserver.dkimKeyBits != null;
+        message = "When you bring your own DKIM private keys (mailserver.dkimDomainPrivateKeyFiles != null), the DKIM domains (${prettyDkimDomains}) must be identical to the mailserver.domains (${prettyDomains}).";
+      }
+    ]
+  ) ++ lib.optionals (config.mailserver.enable && config.mailserver.dkimDomainPrivateKeyFiles != null) [
+    {
+      assertion = config.mailserver.dkimKeyBits == null;
+      message = "When you bring your own DKIM private keys (mailserver.dkimDomainPrivateKeyFiles != null), you must not specify key generation options (mailserver.dkimKeyBits)";
+    }
+  ] ++ lib.optionals (config.mailserver.enable && config.mailserver.dkimDomainPrivateKeyFiles == null) [
+    {
+      assertion = config.mailserver.dkimKeyBits != null;
+      message = "When generating DKIM private keys (mailserver.dkimDomainPrivateKeyFiles = null), you must specify key generation options (mailserver.dkimKeyBits)";
+    }
   ];
 }

mail-server/rspamd.nix

@@ -26,22 +26,37 @@ let
   rspamdUser = config.services.rspamd.user;
   rspamdGroup = config.services.rspamd.group;
 
-  createDkimKeypair = domain: let
+  createOrLinkDkimCerts = domain: let
     privateKey = "${cfg.dkimKeyDirectory}/${domain}.${cfg.dkimSelector}.key";
     publicKey = "${cfg.dkimKeyDirectory}/${domain}.${cfg.dkimSelector}.txt";
-  in pkgs.writeShellScript "dkim-keygen-${domain}" ''
-    if [ ! -f "${privateKey}" ]
-    then
-      ${lib.getExe' pkgs.rspamd "rspamadm"} dkim_keygen \
-        --domain "${domain}" \
-        --selector "${cfg.dkimSelector}" \
-        --type "${cfg.dkimKeyType}" \
-        --bits ${toString cfg.dkimKeyBits} \
-        --privkey "${privateKey}" > "${publicKey}"
-      chmod 0644 "${publicKey}"
-      echo "Generated key for domain ${domain} and selector ${cfg.dkimSelector}"
-    fi
-  '';
+  in pkgs.writeShellScript "dkim-keygen-${domain}" (
+    if cfg.dkimDomainPrivateKeyFiles != null then
+      let
+        dkimPrivateKeyFile = cfg.dkimDomainPrivateKeyFiles.${domain};
+      in
+      ''
+        if [ ! -e "${dkimPrivateKeyFile}" ]; then
+          echo "DKIM keyfile does not exist: ${dkimPrivateKeyFile}"
+          exit 1
+        fi
+
+        ln -sf "${dkimPrivateKeyFile}" "${privateKey}"
+      ''
+    else
+      ''
+        if [ ! -f "${privateKey}" ]
+        then
+          ${lib.getExe' pkgs.rspamd "rspamadm"} dkim_keygen \
+            --domain "${domain}" \
+            --selector "${cfg.dkimSelector}" \
+            --type "${cfg.dkimKeyType}" \
+            --bits ${toString cfg.dkimKeyBits} \
+            --privkey "${privateKey}" > "${publicKey}"
+          chmod 0644 "${publicKey}"
+          echo "Generated key for domain ${domain} and selector ${cfg.dkimSelector}"
+        fi
+      ''
+  );
 in
 {
   config = with cfg; lib.mkIf enable {
@@ -165,7 +180,7 @@ in
           SupplementaryGroups = [ config.services.redis.servers.rspamd.group ];
         }
         (lib.optionalAttrs cfg.dkimSigning {
-          ExecStartPre = map createDkimKeypair cfg.domains;
+          ExecStartPre = map createOrLinkDkimCerts cfg.domains;
           ReadWritePaths = [ cfg.dkimKeyDirectory ];
         })
       ];