diff --git a/modules/netcup-dns.nix b/modules/netcup-dns.nix
index 2052f20..6ec0c40 100644
--- a/modules/netcup-dns.nix
+++ b/modules/netcup-dns.nix
@@ -64,7 +64,19 @@ in
       User = "netcup-dns";
       Nice = 19;
       IOSchedulingClass = "idle";
-      ExecStart = "${pkgs.python3.withPackages my-python-packages}/bin/netcup-dns";
+
+      # Create directory `/run/netcup-dns`.
+      # `netcup-dns` uses it for caching.
+      # For systemd to create this directory automatically, `PermissionsStartOnly` is required: https://unix.stackexchange.com/questions/354583/how-to-automatically-create-a-runtime-folder-with-a-systemd-service-or-tmpfiles#comment628290_354583
+      RuntimeDirectoryMode = "0755";
+      RuntimeDirectory = "netcup-dns";
+      PermissionsStartOnly = true;
+      # Since we use `/run/netcup-dns` for caching between subsequent runs of `netcup-dns`, it should be kept and not deleted.
+      # Man page section `RuntimeDirectoryPreserve`:
+      #   If set to yes, then the directories are not removed when the service is stopped. Note that since the runtime directory /run/ is a mount point of "tmpfs", then for system services the directories specified in RuntimeDirectory= are removed when the system is rebooted.
+      RuntimeDirectoryPreserve = true;
+
+      ExecStart = "${pkgs.python3.withPackages my-python-packages}/bin/netcup-dns --cache-directory /run/netcup-dns";
     };
   };
 }