plugin/file: Fix response for qtype=DS (#1300)

DS is peculiar and needs some special cases to do the right things.
For a direct query return the DS, but if we're hitting below the
delegation return the NS+DS in a delegation response.

plugin/file/lookup.go

@@ -127,6 +127,15 @@ func (z *Zone) Lookup(state request.Request, qname string) ([]dns.RR, []dns.RR,
 
 		// If we see NS records, it means the name as been delegated, and we should return the delegation.
 		if nsrrs := elem.Types(dns.TypeNS); nsrrs != nil {
+
+			// If the query is specifically for DS and the qname matches the delegated name, we should
+			// return the DS in the answer section and leave the rest empty, i.e. just continue the loop
+			// and continue searching.
+			if qtype == dns.TypeDS && elem.Name() == qname {
+				i++
+				continue
+			}
+
 			glue := z.Glue(nsrrs, do)
 			if do {
 				dss := z.typeFromElem(elem, dns.TypeDS, do)