fix(rewrite): truncated upstream response (#7277)

Forward information that a upstream response is truncated when rewriting
a CNAME. Otherwise, the cache plugin stores the truncated resonse,
making it impossible to receive the full response as a client via TCP.

Signed-off-by: Yannick Epstein <yannicke@spotify.com>

plugin/rewrite/cname_target.go

@@ -100,6 +100,9 @@ func (r *cnameTargetRuleWithReqState) RewriteResponse(res *dns.Msg, rr dns.RR) {
 					}
 				}
 				res.Answer = newAnswer
+				// if not propagated, the truncated response might get cached,
+				// and it will be impossible to resolve the full response
+				res.Truncated = upRes.Truncated
 			}
 		}
 	}