middleware/caching (#360)

Add the rcode to the cached item and use this when we synthesize the
answer again. We could also infer the rcode from the reassembled
message, but this seems easier and is only an integer.

Also set the autoritative bit to 0 for all from-cache answers.

Fixes 357

middleware/cache/item.go

@@ -8,6 +8,7 @@ import (
 )
 
 type item struct {
+	Rcode              int
 	Authoritative      bool
 	AuthenticatedData  bool
 	RecursionAvailable bool
@@ -21,6 +22,7 @@ type item struct {
 
 func newItem(m *dns.Msg, d time.Duration) *item {
 	i := new(item)
+	i.Rcode = m.Rcode
 	i.Authoritative = m.Authoritative
 	i.AuthenticatedData = m.AuthenticatedData
 	i.RecursionAvailable = m.RecursionAvailable
@@ -45,12 +47,15 @@ func newItem(m *dns.Msg, d time.Duration) *item {
 }
 
 // toMsg turns i into a message, it tailers the reply to m.
+// The Autoritative bit is always set to 0, because the answer is from the cache.
 func (i *item) toMsg(m *dns.Msg) *dns.Msg {
 	m1 := new(dns.Msg)
 	m1.SetReply(m)
-	m1.Authoritative = i.Authoritative
+
+	m1.Authoritative = false
 	m1.AuthenticatedData = i.AuthenticatedData
 	m1.RecursionAvailable = i.RecursionAvailable
+	m1.Rcode = i.Rcode
 	m1.Compress = true
 
 	m1.Answer = i.Answer