diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index 516fdd8d33fd..5575e9577029 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -248,7 +248,7 @@ rec {
   /* Apply fold functions to values grouped by key.
 
      Example:
-       foldAttrs (n: a: [n] ++ a) [] [{ a = 2; } { a = 3; }]
+       foldAttrs (item: acc: [item] ++ acc) [] [{ a = 2; } { a = 3; }]
        => { a = [ 2 3 ]; }
   */
   foldAttrs = op: nul:
diff --git a/pkgs/top-level/config.nix b/pkgs/top-level/config.nix
index 853e6c76ab54..ad06227e74f1 100644
--- a/pkgs/top-level/config.nix
+++ b/pkgs/top-level/config.nix
@@ -1,6 +1,6 @@
 # This file defines the structure of the `config` nixpkgs option.
 
-{ lib, ... }:
+{ config, lib, ... }:
 
 with lib;
 
@@ -33,6 +33,11 @@ let
 
     /* Config options */
 
+    warnUndeclaredOptions = mkOption {
+      description = "Whether to warn when <literal>config</literal> contains an unrecognized attribute.";
+      default = false;
+    };
+
     doCheckByDefault = mkMassRebuild {
       feature = "run <literal>checkPhase</literal> by default";
     };
@@ -122,6 +127,20 @@ let
 
 in {
 
+  freeformType =
+    let t = lib.types.attrsOf lib.types.raw;
+    in t // {
+      merge = loc: defs:
+        let r = t.merge loc defs;
+        in r // { _undeclared = r; };
+    };
+
   inherit options;
 
+  config = {
+    warnings = lib.optionals config.warnUndeclaredOptions (
+      lib.mapAttrsToList (k: v: "undeclared Nixpkgs option set: config.${k}") config._undeclared
+    );
+  };
+
 }
diff --git a/pkgs/top-level/default.nix b/pkgs/top-level/default.nix
index e95a0db78c3c..b32aabc3a1fd 100644
--- a/pkgs/top-level/default.nix
+++ b/pkgs/top-level/default.nix
@@ -79,15 +79,13 @@ in let
       ./config.nix
       ({ options, ... }: {
         _file = "nixpkgs.config";
-        # filter-out known options, FIXME: remove this eventually
-        config = builtins.intersectAttrs options config1;
+        config = config1;
       })
     ];
   };
 
   # take all the rest as-is
-  config = lib.showWarnings configEval.config.warnings
-    (config1 // builtins.removeAttrs configEval.config [ "_module" ]);
+  config = lib.showWarnings configEval.config.warnings configEval.config;
 
   # A few packages make a new package set to draw their dependencies from.
   # (Currently to get a cross tool chain, or forced-i686 package.) Rather than