Tweaks algorithm so it's safe with an empty list and adds a PQ test.

command/agent/dns.go

@@ -487,22 +487,20 @@ func (d *DNSServer) formatNodeRecord(node *structs.Node, addr, qName string, qTy
 	return records
 }
 
-// trimAnswers makes sure a UDP response is not longer than allowed by RFC 1035
+// trimAnswers makes sure a UDP response is not longer than allowed by RFC 1035.
+// We first enforce an arbitrary limit, and then make sure the response doesn't
+// exceed 512 bytes.
 func trimAnswers(resp *dns.Msg) (trimmed bool) {
 	numAnswers := len(resp.Answer)
 
+	// This cuts UDP responses to a useful but limited number of responses.
 	if numAnswers > maxServiceResponses {
 		resp.Answer = resp.Answer[:maxServiceResponses]
 	}
 
-	// Check that the response isn't more than 512 bytes
-	for respBytes := resp.Len(); respBytes > 512; respBytes = resp.Len() {
+	// This enforces the hard limit of 512 bytes per the RFC.
+	for len(resp.Answer) > 0 && resp.Len() > 512 {
 		resp.Answer = resp.Answer[:len(resp.Answer)-1]
-
-		if len(resp.Answer) == 0 {
-			// We've done all we can
-			break
-		}
 	}
 
 	return len(resp.Answer) < numAnswers

command/agent/dns_test.go

@@ -1991,8 +1991,33 @@ func TestDNS_ServiceLookup_LargeResponses(t *testing.T) {
 		}
 	}
 
+	// Register an equivalent prepared query.
+	{
+		args := &structs.PreparedQueryRequest{
+			Datacenter: "dc1",
+			Op:         structs.PreparedQueryCreate,
+			Query: &structs.PreparedQuery{
+				Name: longServiceName,
+				Service: structs.ServiceQuery{
+					Service: longServiceName,
+					Tags:    []string{"master"},
+				},
+			},
+		}
+		var id string
+		if err := srv.agent.RPC("PreparedQuery.Apply", args, &id); err != nil {
+			t.Fatalf("err: %v", err)
+		}
+	}
+
+	// Look up the service directly and via prepared query.
+	questions := []string{
+		"_" + longServiceName + "._master.service.consul.",
+		longServiceName + ".query.consul.",
+	}
+	for _, question := range questions {
 		m := new(dns.Msg)
-	m.SetQuestion("_"+longServiceName+"._master.service.consul.", dns.TypeSRV)
+		m.SetQuestion(question, dns.TypeSRV)
 
 		c := new(dns.Client)
 		addr, _ := srv.agent.config.ClientListener("", srv.agent.config.Ports.DNS)
@@ -2016,6 +2041,7 @@ func TestDNS_ServiceLookup_LargeResponses(t *testing.T) {
 			t.Fatalf("should have truncate bit")
 		}
 	}
+}
 
 func TestDNS_ServiceLookup_MaxResponses(t *testing.T) {
 	dir, srv := makeDNSServer(t)