diff --git a/lib/config-win32.h b/lib/config-win32.h
index f1276d3d1..bc2dedf54 100644
--- a/lib/config-win32.h
+++ b/lib/config-win32.h
@@ -506,6 +506,24 @@
 #  define USE_WIN32_SMALL_FILES
 #endif
 
+/* ---------------------------------------------------------------- */
+/*                       DNS RESOLVER SPECIALTY                     */
+/* ---------------------------------------------------------------- */
+
+/*
+ * Undefine both USE_ARES and USE_THREADS_WIN32 for synchronous DNS
+ */
+
+/* Define USE_ARES to enable c-ares asynchronous DNS lookups */
+/* #define USE_ARES 1 */
+
+/* Define USE_THREADS_WIN32 to enable threaded asynchronous DNS lookups */
+#define USE_THREADS_WIN32 1
+
+#if defined(USE_ARES) && defined(USE_THREADS_WIN32)
+#  error "Only one DNS lookup specialty may be defined at most"
+#endif
+
 /* ---------------------------------------------------------------- */
 /*                           LDAP SUPPORT                           */
 /* ---------------------------------------------------------------- */