plugin/{forward,proxy}: check for truncated (#1644)

Check for trunacted in the lookup function as well and use the Match
function here as well.

plugin/forward/truncated.go

@@ -0,0 +1,25 @@
+package forward
+
+import "github.com/miekg/dns"
+
+// truncated looks at the error and if truncated return a nil errror
+// and a possible reconstructed dns message if that was nil.
+func truncated(ret *dns.Msg, err error) (*dns.Msg, error) {
+	// If you query for instance ANY isc.org; you get a truncated query back which miekg/dns fails to unpack
+	// because the RRs are not finished. The returned message can be useful or useless. Return the original
+	// query with some header bits set that they should retry with TCP.
+	if err != dns.ErrTruncated {
+		return ret, err
+	}
+
+	// We may or may not have something sensible... if not reassemble something to send to the client.
+	m := ret
+	if ret == nil {
+		m = new(dns.Msg)
+		m.SetReply(ret)
+		m.Truncated = true
+		m.Authoritative = true
+		m.Rcode = dns.RcodeSuccess
+	}
+	return m, nil
+}