plugin/header: Add support for query modification (#5548) (#5556)

plugin/header/README.md

@@ -2,22 +2,24 @@
 
 ## Name
 
-*header* - modifies the header for responses.
+*header* - modifies the header for queries and responses.
 
 ## Description
 
-*header* ensures that the flags are in the desired state for responses. The modifications are made transparently for
-the client.
+*header* ensures that the flags are in the desired state for queries and responses.
+The modifications are made transparently for the client and subsequent plugins.
 
 ## Syntax
 
 ~~~
 header {
-    ACTION FLAGS...
-    ACTION FLAGS...
+    [SELECTOR] ACTION FLAGS...
+    [SELECTOR] ACTION FLAGS...
 }
 ~~~
 
+* **SELECTOR** defines if the action should be applied on `query` or `response`. In future CoreDNS version the selector will be mandatory. For backwards compatibility the action will be applied on `response` if the selector is undefined.
+
 * **ACTION** defines the state for DNS message header flags. Actions are evaluated in the order they are defined so last one has the
   most precedence. Allowed values are:
     * `set`
@@ -34,7 +36,7 @@ Make sure recursive available `ra` flag is set in all the responses:
 ~~~ corefile
 . {
     header {
-        set ra
+        response set ra
     }
 }
 ~~~
@@ -44,8 +46,18 @@ Make sure "recursion available" `ra` and "authoritative answer" `aa` flags are s
 ~~~ corefile
 . {
     header {
-        set ra aa
-        clear rd
+        response set ra aa
+        response clear rd
+    }
+}
+~~~
+
+Make sure "recursion desired" `rd` is set for all subsequent plugins::
+
+~~~ corefile
+. {
+    header {
+        query set rd
     }
 }
 ~~~