diff --git a/include/curl/curl.h b/include/curl/curl.h
index b35ac95553..eb06022501 100644
--- a/include/curl/curl.h
+++ b/include/curl/curl.h
@@ -1846,7 +1846,8 @@ typedef enum {
   /* allow GSSAPI credential delegation */
   CURLOPT(CURLOPT_GSSAPI_DELEGATION, CURLOPTTYPE_VALUES, 210),
 
-  /* Set the name servers to use for DNS resolution */
+  /* Set the name servers to use for DNS resolution.
+   * Only supported by the c-ares DNS backend */
   CURLOPT(CURLOPT_DNS_SERVERS, CURLOPTTYPE_STRINGPOINT, 211),
 
   /* Time-out accept operations (currently for FTP only) after this amount