plugin/dns64 : add support for DNS requests over IPv4 network (#4809)

plugin/dns64/README.md

@@ -27,11 +27,13 @@ Or use this slightly longer form with more options:
 dns64 [PREFIX] {
     [translate_all]
     prefix PREFIX
+    [allow_ipv4]
 }
 ~~~
 
 * `prefix` specifies any local IPv6 prefix to use, instead of the well known prefix (64:ff9b::/96)
 * `translate_all` translates all queries, including responses that have AAAA results.
+* `allow_ipv4` Allow translating queries if they come in over IPv4, default is IPv6 only translation.
 
 ## Examples
 
@@ -70,6 +72,19 @@ Enable translation even if an existing AAAA record is present.
 }
 ~~~
 
+Apply translation even to the requests which arrived over IPv4 network. Warning, the `allow_ipv4` feature will apply 
+translations to requests coming from dual-stack clients. This means that a request for a client that sends an `AAAA`
+that would normal result in an `NXDOMAIN` would get a translated result. 
+This may cause unwanted IPv6 dns64 traffic when a dualstack client would normally use the result of an `A` record request.
+
+~~~ corefile
+. {
+    dns64 {
+        allow_ipv4
+    }
+}
+~~~
+
 ## Metrics
 
 If monitoring is enabled (via the _prometheus_ plugin) then the following metrics are exported: