diff --git a/modules/c3d2.nix b/modules/c3d2.nix
index 2d6e7122..bb2b1191 100644
--- a/modules/c3d2.nix
+++ b/modules/c3d2.nix
@@ -395,6 +395,11 @@ in
 
       nix = {
         settings = {
+          builders-use-substitutes = true;
+          connect-timeout = 20;
+          experimental-features = "nix-command flakes";
+          fallback = true;
+          # don't self feed hydra
           trusted-public-keys = lib.mkIf (config.networking.hostName != "hydra") [
             (builtins.readFile ../hosts/hydra/cache-pub.key)
           ];
@@ -418,10 +423,6 @@ in
             url = "https://gitea.c3d2.de/C3D2/nix-config.git";
           };
         };
-        extraOptions = ''
-          experimental-features = nix-command flakes
-          builders-use-substitutes = true
-        '';
       };
 
       services.openssh = {
diff --git a/modules/microvm-host.nix b/modules/microvm-host.nix
index 447bc573..8f66f3a4 100644
--- a/modules/microvm-host.nix
+++ b/modules/microvm-host.nix
@@ -59,13 +59,11 @@
     };
 
     nix.settings = {
+      min-free = 128000000;
+      max-free = 1000000000;
       # fetch github-prebuilt microvm-kernels
-      substituters = [
-        "https://microvm.cachix.org"
-      ];
-      trusted-public-keys = [
-        "microvm.cachix.org-1:oXnBc6hRE3eX5rSYdRyMYXnfzcCxC7yKPTbZXALsqys="
-      ];
+      substituters = [ "https://microvm.cachix.org" ];
+      trusted-public-keys = [ "microvm.cachix.org-1:oXnBc6hRE3eX5rSYdRyMYXnfzcCxC7yKPTbZXALsqys=" ];
     };
 
     environment.systemPackages = [ (