Add MDNS for ESP8266 (#153)

With ESP32 MDNS causes BLE to don't work properly

OpenMQTTGateway.ino

@@ -76,17 +76,23 @@ void callback(char*topic, byte* payload,unsigned int length);
   #include <WiFi.h>
   #include <ArduinoOTA.h>
   WiFiClient eClient;
+  #ifdef MDNS_SD
+    #include <ESPmDNS.h>
+  #endif
 #elif defined(ESP8266)
   #include <ESP8266WiFi.h>
   #include <ArduinoOTA.h>
   WiFiClient eClient;
+  #ifdef MDNS_SD
+    #include <ESP8266mDNS.h>
+  #endif
 #else
   #include <Ethernet.h>
   EthernetClient eClient;
 #endif
 
-// client parameters
-PubSubClient client(mqtt_server, mqtt_port, callback, eClient);
+// client link to pubsub mqtt
+PubSubClient client(eClient);
 
 //MQTT last attemps reconnection date
 unsigned long lastReconnectAttempt = 0;
@@ -188,6 +194,23 @@ void setup()
       else if (error == OTA_END_ERROR) trc(F("End Failed"));
     });
     ArduinoOTA.begin();
+
+   #ifdef MDNS_SD
+       trc(F("Connecting to MQTT by mDNS without mqtt hostname"));
+       connectMQTTmdns();
+   #else
+     #ifdef mqtt_server_name // if name is defined we define the mqtt server by its name
+       trc(F("Connecting to MQTT with mqtt hostname"));
+       IPAddress mqtt_server_ip;
+       WiFi.hostByName(mqtt_server_name, mqtt_server_ip);
+       client.setServer(mqtt_server_ip, mqtt_port);
+       trc(mqtt_server_ip.toString());
+     #else // if not by its IP adress
+       trc(F("Connecting to MQTT by IP adress"));
+       client.setServer(mqtt_server, mqtt_port);
+       trc(String(mqtt_server));
+     #endif
+   #endif
   #else
     //Launch serial for debugging purposes
     Serial.begin(SERIAL_BAUD);
@@ -205,6 +228,8 @@ void setup()
     digitalWrite(led_send, HIGH);
   #endif
 
+  client.setCallback(callback);
+  
   delay(1500);
 
   lastReconnectAttempt = 0;
@@ -253,7 +278,7 @@ void setup_wifi() {
   IPAddress subnet_adress(subnet);
   IPAddress dns_adress(Dns);
   WiFi.begin(wifi_ssid, wifi_password);
-  //WiFi.config(ip_adress,gateway_adress,subnet_adress); //Uncomment this line if you want to use advanced network config
+  //WiFi.config(ip_adress,gateway_adress,subnet_adress,dns_adress); //Uncomment this line if you want to use advanced network config
     
   trc(F("OpenMQTTGateway mac: "));
   Serial.println(WiFi.macAddress()); 
@@ -268,6 +293,39 @@ void setup_wifi() {
   
   trc(F("WiFi ok"));
 }
+
+#ifdef MDNS_SD
+  void connectMQTTmdns(){
+    if (!MDNS.begin("ESP_MQTT")) {
+        trc(F("Error setting up MDNS responder!"));
+        while(1){
+            delay(1000);
+        }
+    }
+    trc(F("Browsing for service MQTT "));
+    int n = MDNS.queryService("mqtt", "tcp");
+    if (n == 0) {
+        trc(F("no services found"));
+    }else {
+        trc(String(n));
+        trc(" service(s) found");
+        for (int i=0; i < n; ++i) {
+            // Print details for each service found
+            trc(String(i + 1));
+            trc(String(MDNS.hostname(i)));
+            trc(MDNS.IP(i).toString());
+            trc(String(MDNS.port(i)));
+        }
+        if (n==1) {
+          trc(F("One MQTT server found setting parameters"));
+          client.setServer(MDNS.IP(0), int(MDNS.port(0)));
+        }else{
+          trc(F("Several MQTT servers found, please deactivate mDNS and set your default server"));
+        }
+    }
+  }
+#endif
+
 #else
 void setup_ethernet() {
   Ethernet.begin(mac, ip); //Comment and uncomment the following line if you want to use advanced network config