documentation of Rcode in hearder

src/message/header.rs

@@ -39,6 +39,34 @@ pub struct Header {
     z: u8,
 
     /// Response Code
+    /// 
+    /// [RFC 1035]: https://datatracker.ietf.org/doc/html/rfc1035#section-4.1.1
+    /// 
+    /// 0               No error condition
+    ///
+    /// 1               Format error - The name server was
+    ///                 unable to interpret the query.
+    ///
+    /// 2               Server failure - The name server was
+    ///                 unable to process this query due to a
+    ///                 problem with the name server.
+    ///
+    /// 3               Name Error - Meaningful only for
+    ///                 responses from an authoritative name
+    ///                 server, this code signifies that the
+    ///                 domain name referenced in the query does
+    ///                 not exist.
+    ///
+    /// 4               Not Implemented - The name server does
+    ///                 not support the requested kind of query.
+    ///
+    /// 5               Refused - The name server refuses to
+    ///                 perform the specified operation for
+    ///                 policy reasons.  For example, a name
+    ///                 server may not wish to provide the
+    ///                 information to the particular requester,
+    ///                 or a name server may not wish to perform
+    ///                 a particular operation.
     rcode: u8,
 
     /// Counters