diff --git a/tests/clamav.nix b/tests/clamav.nix
index 7c81b2d..f53de4c 100644
--- a/tests/clamav.nix
+++ b/tests/clamav.nix
@@ -34,6 +34,7 @@ import <nixpkgs/nixos/tests/make-test.nix> {
         {
             imports = [
                 ../default.nix
+                ./lib/config.nix
             ];
 
             virtualisation.memorySize = 1500;
@@ -103,6 +104,10 @@ import <nixpkgs/nixos/tests/make-test.nix> {
           exec grep '${clientIP}' "$@"
         '';
       in {
+        imports = [
+            ./lib/config.nix
+        ];
+
         environment.systemPackages = with pkgs; [
           fetchmail msmtp procmail findutils grep-ip
         ];
diff --git a/tests/extern.nix b/tests/extern.nix
index 0816d91..4b5b91f 100644
--- a/tests/extern.nix
+++ b/tests/extern.nix
@@ -21,6 +21,7 @@ import <nixpkgs/nixos/tests/make-test.nix> {
         {
             imports = [
                 ../default.nix
+                ./lib/config.nix
             ];
 
             services.rsyslogd = {
@@ -134,6 +135,9 @@ import <nixpkgs/nixos/tests/make-test.nix> {
             imap.close()
         '';
       in {
+        imports = [
+            ./lib/config.nix
+        ];
         environment.systemPackages = with pkgs; [
           fetchmail msmtp procmail findutils grep-ip check-mail-id test-imap-spam test-imap-ham
         ];
diff --git a/tests/intern.nix b/tests/intern.nix
index ce50123..5abf379 100644
--- a/tests/intern.nix
+++ b/tests/intern.nix
@@ -21,6 +21,7 @@ import <nixpkgs/nixos/tests/make-test.nix> {
     {
         imports = [
             ./../default.nix
+            ./lib/config.nix
         ];
 
         mailserver = {
diff --git a/tests/lib/config.nix b/tests/lib/config.nix
new file mode 100644
index 0000000..1d56ad1
--- /dev/null
+++ b/tests/lib/config.nix
@@ -0,0 +1,3 @@
+{
+    security.dhparams.defaultBitSize = 16; # really low for quicker tests
+}