diff --git a/modules/testing/test-instrumentation.nix b/modules/testing/test-instrumentation.nix
index 5e62c698f274..dd61e68d9d15 100644
--- a/modules/testing/test-instrumentation.nix
+++ b/modules/testing/test-instrumentation.nix
@@ -80,6 +80,10 @@ in
     # serial port).
     services.syslogd.extraConfig = "*.*,kern.none /dev/ttyS0";
 
+    # Prevent tests from accessing the Internet.
+    networking.defaultGateway = mkOverride 50 {} "";
+    networking.nameservers = mkOverride 50 {} [ ];
+
   };
 
 }