diff --git a/ares/setup_once.h b/ares/setup_once.h
index 6daf9d8b77..41b42e9008 100644
--- a/ares/setup_once.h
+++ b/ares/setup_once.h
@@ -27,6 +27,13 @@
  ********************************************************************/
 
 
+/*
+ * Inclusion of common header files.
+ */
+
+#include <errno.h>
+
+
 /*
  * If we have the MSG_NOSIGNAL define, make sure we use
  * it as the fourth argument of function send()
@@ -219,10 +226,14 @@ typedef int sig_atomic_t;
 #define ETIMEDOUT        WSAETIMEDOUT
 #define ECONNREFUSED     WSAECONNREFUSED
 #define ELOOP            WSAELOOP
+#ifndef ENAMETOOLONG     /* possible previous definition in errno.h */
 #define ENAMETOOLONG     WSAENAMETOOLONG
+#endif
 #define EHOSTDOWN        WSAEHOSTDOWN
 #define EHOSTUNREACH     WSAEHOSTUNREACH
+#ifndef ENOTEMPTY        /* possible previous definition in errno.h */
 #define ENOTEMPTY        WSAENOTEMPTY
+#endif
 #define EPROCLIM         WSAEPROCLIM
 #define EUSERS           WSAEUSERS
 #define EDQUOT           WSAEDQUOT
diff --git a/lib/setup_once.h b/lib/setup_once.h
index 630cd05e38..2cf4ebea40 100644
--- a/lib/setup_once.h
+++ b/lib/setup_once.h
@@ -34,6 +34,13 @@
  ********************************************************************/
 
 
+/*
+ * Inclusion of common header files.
+ */
+
+#include <errno.h>
+
+
 /*
  * If we have the MSG_NOSIGNAL define, make sure we use
  * it as the fourth argument of function send()
@@ -226,10 +233,14 @@ typedef int sig_atomic_t;
 #define ETIMEDOUT        WSAETIMEDOUT
 #define ECONNREFUSED     WSAECONNREFUSED
 #define ELOOP            WSAELOOP
+#ifndef ENAMETOOLONG     /* possible previous definition in errno.h */
 #define ENAMETOOLONG     WSAENAMETOOLONG
+#endif
 #define EHOSTDOWN        WSAEHOSTDOWN
 #define EHOSTUNREACH     WSAEHOSTUNREACH
+#ifndef ENOTEMPTY        /* possible previous definition in errno.h */
 #define ENOTEMPTY        WSAENOTEMPTY
+#endif
 #define EPROCLIM         WSAEPROCLIM
 #define EUSERS           WSAEUSERS
 #define EDQUOT           WSAEDQUOT