plugin/forward: Return original message on truncation (#1674)

With this change the original truncated message returned by requested
server is returned to the client, instead of returning an empty dummy
message with only the truncation bit set.

plugin/forward/connect.go

@@ -66,7 +66,7 @@ func (p *Proxy) connect(ctx context.Context, state request.Request, forceTCP, me
 		if err == io.EOF && cached {
 			return nil, errCachedClosed
 		}
-		return nil, err
+		return ret, err
 	}
 
 	p.updateRtt(time.Since(reqTime))

plugin/forward/truncated_test.go

@@ -49,6 +49,9 @@ func TestLookupTruncated(t *testing.T) {
 	if !resp.Truncated {
 		t.Error("Expected to receive reply with TC bit set, but didn't")
 	}
+	if len(resp.Answer) != 1 {
+		t.Error("Expected to receive original reply, but answer is missing")
+	}
 
 	resp, err = f.Lookup(state, "example.org.", dns.TypeA)
 	if err != nil {
@@ -102,6 +105,9 @@ func TestForwardTruncated(t *testing.T) {
 	if !resp.Truncated {
 		t.Error("Expected to receive reply with TC bit set, but didn't")
 	}
+	if len(resp.Answer) != 1 {
+		t.Error("Expected to receive original reply, but answer is missing")
+	}
 
 	resp, err = f.Forward(state)
 	if err != nil {