diff --git a/nix/nixos-module/container/upstream.nix b/nix/nixos-module/container/upstream.nix
index 314a50d..31c707d 100644
--- a/nix/nixos-module/container/upstream.nix
+++ b/nix/nixos-module/container/upstream.nix
@@ -26,6 +26,7 @@ in
 
   networking.nat = lib.optionalAttrs (firstUpstreamInterface != null) {
     enable = true;
+    internalInterfaces = [ "core" ];
     externalInterface = firstUpstreamInterface;
     forwardPorts = config.site.hosts.${hostName}.forwardedPorts;
   };