plugin/file: return SOA in authority for negative CNAME target answers (#8226)

plugin/file/lookup.go

@@ -315,6 +315,16 @@ func (a Apex) ns(do bool) []dns.RR {
 	return a.NS
 }
 
+// authority returns the records for the authority section of a response with
+// the given result: the SOA for negative answers (NXDOMAIN/NODATA), as
+// required by RFC 2308, and the NS records otherwise.
+func (z *Zone) authority(do bool, result Result) []dns.RR {
+	if result == NameError || result == NoData {
+		return z.soa(do)
+	}
+	return z.ns(do)
+}
+
 // externalLookup adds signatures and tries to resolve CNAMEs that point to external names.
 func (z *Zone) externalLookup(ctx context.Context, state request.Request, elem *tree.Elem, rrs []dns.RR) ([]dns.RR, []dns.RR, []dns.RR, Result) {
 	qtype := state.QType()
@@ -331,7 +341,7 @@ func (z *Zone) externalLookup(ctx context.Context, state request.Request, elem *
 	if elem == nil || (qtype == dns.TypeNS || qtype == dns.TypeSOA && targetName == z.origin) {
 		lookupRRs, result := z.doLookup(ctx, state, targetName, qtype)
 		rrs = append(rrs, lookupRRs...)
-		return rrs, z.ns(do), nil, result
+		return rrs, z.authority(do, result), nil, result
 	}
 
 	i := 0
@@ -351,7 +361,7 @@ Redo:
 		if elem == nil || (qtype == dns.TypeNS || qtype == dns.TypeSOA && targetName == z.origin) {
 			lookupRRs, result := z.doLookup(ctx, state, targetName, qtype)
 			rrs = append(rrs, lookupRRs...)
-			return rrs, z.ns(do), nil, result
+			return rrs, z.authority(do, result), nil, result
 		}
 
 		i++

test/file_dangling_cname_test.go

@@ -0,0 +1,58 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+// Zone with a CNAME whose target does not exist in the zone.
+const exampleOrgDanglingCNAME = `; example.org test file
+$TTL 3600
+@       IN  SOA   sns.dns.icann.org. noc.dns.icann.org. 2015082541 7200 3600 1209600 3600
+@       IN  NS    b.iana-servers.net.
+name    IN  CNAME name2.example.org.
+`
+
+// A negative (NXDOMAIN) answer produced by following a CNAME to a
+// non-existent in-zone name must carry the zone SOA in the authority
+// section, per RFC 2308, just like querying that name directly.
+// See https://github.com/coredns/coredns/issues/6385.
+func TestZoneDanglingCNAMENegativeAuthority(t *testing.T) {
+	t.Parallel()
+
+	zoneFile, rm, err := test.TempFile(".", exampleOrgDanglingCNAME)
+	if err != nil {
+		t.Fatalf("Failed to create zone: %s", err)
+	}
+	defer rm()
+
+	corefile := `example.org:0 {
+		file ` + zoneFile + `
+	}`
+
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	m := new(dns.Msg)
+	m.SetQuestion("name.example.org.", dns.TypeA)
+	resp, err := dns.Exchange(m, udp)
+	if err != nil {
+		t.Fatalf("Expected to receive reply, but didn't: %s", err)
+	}
+
+	if resp.Rcode != dns.RcodeNameError {
+		t.Errorf("Expected NXDOMAIN, got %s", dns.RcodeToString[resp.Rcode])
+	}
+	if len(resp.Ns) != 1 {
+		t.Fatalf("Expected 1 record in authority section, got %d: %v", len(resp.Ns), resp.Ns)
+	}
+	if _, ok := resp.Ns[0].(*dns.SOA); !ok {
+		t.Errorf("Expected SOA in authority section, got %T: %v", resp.Ns[0], resp.Ns[0])
+	}
+}