plugin/cache: cap TTL on first answer (#1092)

Cache would let the first response through and would then cap subsequent
ones to whatever the cache duration was. This would lead to huge drops
in TTL values: 3600 -> 20 for instance, which is not only bad, but can
mess up your careful TTL planning business.

This PR fixes that and applies the cache duration to all replies. As a
bonus I could remove a time.Sleep() from the cache test and just check
for the cache duration as the TTL on the reply.

Fixes #1038

2 files changed, 14 insertions, 1 deletions

diff --git a/plugin/cache/cache.go b/plugin/cache/cache.go
index b37e527cf..5d9bd746b 100644
--- a/plugin/cache/cache.go
+++ b/plugin/cache/cache.go
@@ -117,6 +117,19 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 		return nil
 	}
 
+	// Apply capped TTL to this reply to avoid jarring TTL experience 1799 -> 8 (e.g.)
+	ttl := uint32(duration.Seconds())
+	for i := range res.Answer {
+		res.Answer[i].Header().Ttl = ttl
+	}
+	for i := range res.Ns {
+		res.Ns[i].Header().Ttl = ttl
+	}
+	for i := range res.Extra {
+		if res.Extra[i].Header().Rrtype != dns.TypeOPT {
+			res.Extra[i].Header().Ttl = ttl
+		}
+	}
 	return w.ResponseWriter.WriteMsg(res)
 }
 
diff --git a/plugin/cache/handler.go b/plugin/cache/handler.go
index ebd87d659..f3f2e675b 100644
--- a/plugin/cache/handler.go
+++ b/plugin/cache/handler.go
@@ -19,7 +19,7 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	qtype := state.QType()
 	zone := plugin.Zones(c.Zones).Matches(qname)
 	if zone == "" {
-		return c.Next.ServeDNS(ctx, w, r)
+		return plugin.NextOrFailure(c.Name(), c.Next, ctx, w, r)
 	}
 
 	do := state.Do() // TODO(): might need more from OPT record? Like the actual bufsize?