From 365aea321109fe3ed1145e0954809cc88d168d43 Mon Sep 17 00:00:00 2001
From: Michele Spagnuolo <mikispag@gmail.com>
Date: Wed, 4 Oct 2023 16:52:44 +0200
Subject: [PATCH] Check for a successful answer before rewriting TTL.

---
 proxy/server.go | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/proxy/server.go b/proxy/server.go
index fca54d9..fe50865 100644
--- a/proxy/server.go
+++ b/proxy/server.go
@@ -223,11 +223,6 @@ func (s *Server) timer(ctx context.Context) {
 
 func (s *Server) forwardMessageAndCacheResponse(q *dns.Msg) (m *dns.Msg) {
 	m = s.forwardMessageAndGetResponse(q)
-	// Rewrite the TTL.
-	for _, a := range m.Answer {
-		// If the TTL provided upstream is smaller than `minTTL`, rewrite it.
-		a.Header().Ttl = uint32(max(a.Header().Ttl, uint32(s.minTTL)))
-	}
 	// Let's retry a few times if we can't resolve it at the first try.
 	for c := 0; m == nil && c < connectionsPerUpstream; c++ {
 		s.Log.Debugf("Retrying %q [%d/%d]...", q.Question, c+1, connectionsPerUpstream)
@@ -237,6 +232,13 @@ func (s *Server) forwardMessageAndCacheResponse(q *dns.Msg) (m *dns.Msg) {
 		s.Log.Infof("Giving up on %q after %d connection retries.", q.Question, connectionsPerUpstream)
 		return nil
 	}
+	if m.Answer != nil {
+		// Rewrite the TTL.
+		for _, a := range m.Answer {
+			// If the TTL provided upstream is smaller than `minTTL`, rewrite it.
+			a.Header().Ttl = uint32(max(a.Header().Ttl, uint32(s.minTTL)))
+		}
+	}
 	s.cache.put(q, m)
 	return m
 }