Force record TTL on alias records

Signed-off-by: Knut Ahlers <knut@ahlers.me>

alias.go

@@ -45,10 +45,14 @@ func (r *ResponseModifier) WriteMsg(res *dns.Msg) error {
 	}
 
 	// Find and delete CNAME record on that zone, storing the canonical name.
-	var cname string
+	var (
+		cname string
+		ttl   uint32
+	)
 	for i, rr := range res.Answer {
 		if rr.Header().Rrtype == dns.TypeCNAME && rr.Header().Name == zone {
 			cname = rr.(*dns.CNAME).Target
+			ttl = rr.(*dns.CNAME).Header().Ttl
 			// Remove the CNAME record
 			res.Answer = append(res.Answer[:i], res.Answer[i+1:]...)
 			break
@@ -59,6 +63,7 @@ func (r *ResponseModifier) WriteMsg(res *dns.Msg) error {
 	for _, rr := range res.Answer {
 		if rr.Header().Name == cname {
 			rr.Header().Name = zone
+			rr.Header().Ttl = ttl
 		}
 	}