Don't let interfaces get IPv6 addresses if networking.enableIPv6 is false

modules/config/networking.nix

@@ -3,7 +3,9 @@
 {config, pkgs, ...}:
 
 with pkgs.lib;
+
 let
+
   cfg = config.networking;
 
   options = {
@@ -43,7 +45,9 @@ in
         source = pkgs.writeText "hosts"
           ''
             127.0.0.1 localhost
-            ::1 localhost
+            ${optionalString cfg.enableIPv6 ''
+              ::1 localhost
+            ''}
             ${cfg.extraHosts}
           '';
         target = "hosts";

modules/tasks/network-interfaces.nix

@@ -278,6 +278,9 @@ in
                 '')}
                 EOF
 
+                # Disable or enable IPv6.
+                echo ${if cfg.enableIPv6 then "0" else "1"} > /proc/sys/net/ipv6/conf/all/disable_ipv6
+
                 # Set the default gateway.
                 ${optionalString (cfg.defaultGateway != "") ''
                   # FIXME: get rid of "|| true" (necessary to make it idempotent).