Fix MQTT rc -4

Fix MQTT rc -4 (#15809)

tasmota/tasmota_support/support_wifi.ino

@@ -740,6 +740,11 @@ bool WifiHostByName(const char* aHostname, IPAddress& aResult) {
   return true;
 }
 
+bool WifiDnsPresent(const char* aHostname) {
+  IPAddress aResult;
+  return WifiHostByName(aHostname, aResult);
+}
+
 void WifiPollNtp() {
   static uint8_t ntp_sync_minute = 0;
   static uint32_t ntp_run_time = 0;

tasmota/tasmota_xdrv_driver/xdrv_02_9_mqtt.ino

@@ -1038,6 +1038,19 @@ void MqttReconnect(void) {
 
   AddLog(LOG_LEVEL_INFO, PSTR(D_LOG_MQTT D_ATTEMPTING_CONNECTION));
 
+  if (MqttClient.connected()) { MqttClient.disconnect(); }
+
+  MqttSetClientTimeout();
+
+  MqttClient.setCallback(MqttDataHandler);
+
+  // Keep using hostname to solve rc -4 issues
+  if (!WifiDnsPresent(SettingsText(SET_MQTT_HOST))) {
+    MqttDisconnected(-5);  // MQTT_DNS_DISCONNECTED
+    return;
+  }
+  MqttClient.setServer(SettingsText(SET_MQTT_HOST), Settings->mqtt_port);
+
   if (2 == Mqtt.initial_connection_state) {  // Executed once just after power on and wifi is connected
     Mqtt.initial_connection_state = 1;
   }
@@ -1051,10 +1064,6 @@ void MqttReconnect(void) {
     mqtt_pwd = SettingsText(SET_MQTT_PWD);
   }
 
-  if (MqttClient.connected()) { MqttClient.disconnect(); }
-  MqttSetClientTimeout();
-  MqttClient.setCallback(MqttDataHandler);
-
 #ifdef USE_MQTT_TLS
   uint32_t mqtt_connect_time = millis();
   if (Mqtt.mqtt_tls) {
@@ -1099,17 +1108,8 @@ void MqttReconnect(void) {
     allow_all_fingerprints |= learn_fingerprint2;
     tlsClient->setPubKeyFingerprint(Settings->mqtt_fingerprint[0], Settings->mqtt_fingerprint[1], allow_all_fingerprints);
   }
-
-  MqttClient.setServer(SettingsText(SET_MQTT_HOST), Settings->mqtt_port);
 #else   // No USE_MQTT_TLS
   MqttClient.setClient(EspClient);
-
-  IPAddress mqtt_host_ip;
-  if (!WifiHostByName(SettingsText(SET_MQTT_HOST), mqtt_host_ip)) {
-    MqttDisconnected(-5);  // MQTT_DNS_DISCONNECTED
-    return;
-  }
-  MqttClient.setServer(mqtt_host_ip, Settings->mqtt_port);
 #endif  // USE_MQTT_TLS
 
   char stopic[TOPSZ];

tasmota/tasmota_xdrv_driver/xdrv_40_telegram.ino

@@ -105,8 +105,11 @@ String TelegramConnectToTelegram(const String &command) {
 
   if (!TelegramInit()) { return ""; }
 
-  String host = F("api.telegram.org");
   String response = "";
+  String host = F("api.telegram.org");
+  if (!WifiDnsPresent(host.c_str())) {
+    return response;
+  }
   uint32_t tls_connect_time = millis();
   if (telegramClient->connect(host.c_str(), 443)) {