add dnsIP() to ESP8266WiFi class

Author: martinayotte

hardware/esp8266com/esp8266/libraries/ESP8266WiFi/src/ESP8266WiFi.cpp

@@ -370,6 +370,12 @@ IPAddress ESP8266WiFiClass::gatewayIP()
     return IPAddress(ip.gw.addr);
 }
 
+IPAddress ESP8266WiFiClass::dnsIP()
+{
+    ip_addr_t dns_ip = dns_getserver(0);
+    return IPAddress(dns_ip.addr);
+}
+
 String ESP8266WiFiClass::SSID() const
 {
     static struct station_config conf;

hardware/esp8266com/esp8266/libraries/ESP8266WiFi/src/ESP8266WiFi.h

@@ -168,6 +168,13 @@ class ESP8266WiFiClass
      */
    IPAddress gatewayIP();
 
+    /*
+     * Get the DNS ip address.
+     *
+     * return: DNS ip address value
+     */
+   IPAddress dnsIP();
+
     /*
      * Return the current SSID associated with the network
      *