dns/dnsmessage: don't crash with nil resource body

Change-Id: I51969f70d4fc69829fd5a8bcd8a34b3be15b9db0 Reviewed-on: https://go-review.googlesource.com/46930 Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: cixtor

dns/dnsmessage/message.go

@@ -280,6 +280,9 @@ type ResourceBody interface {
 }
 
 func (r *Resource) pack(msg []byte, compression map[string]int) ([]byte, error) {
+if r.Body == nil {
+return msg, &nestedError{"Resource", errors.New("nil resource body")}
+}
 oldMsg := msg
 r.Header.Type = r.Body.realType()
 msg, length, err := r.Header.pack(msg, compression)

dns/dnsmessage/message_test.go

@@ -534,6 +534,45 @@ func TestBuilder(t *testing.T) {
 }
 }
 
+func TestResourcePack(t *testing.T) {
+for _, m := range []Message{
+{
+Questions: []Question{
+{
+Name: mustNewName("."),
+Type: TypeAAAA,
+Class: ClassINET,
+},
+},
+Answers: []Resource{{ResourceHeader{}, nil}},
+},
+{
+Questions: []Question{
+{
+Name: mustNewName("."),
+Type: TypeAAAA,
+Class: ClassINET,
+},
+},
+Authorities: []Resource{{ResourceHeader{}, (*NSResource)(nil)}},
+},
+{
+Questions: []Question{
+{
+Name: mustNewName("."),
+Type: TypeA,
+Class: ClassINET,
+},
+},
+Additionals: []Resource{{ResourceHeader{}, nil}},
+},
+} {
+if _, err := m.Pack(); err == nil {
+t.Errorf("should fail: %v", m)
+}
+}
+}
+
 func BenchmarkParsing(b *testing.B) {
 b.ReportAllocs()