Add destinationIP method to WiFiUDP

Might be useful to distinguish between normal and multicast packets arriving at the same port (#105)

Author: igrr

libraries/ESP8266WiFi/keywords.txt

@@ -47,6 +47,7 @@ beginPacket	KEYWORD2
 beginPacketMulticast	KEYWORD2
 endPacket	KEYWORD2
 parsePacket	KEYWORD2
+destinationIP	KEYWORD2
 remoteIP	KEYWORD2
 remotePort	KEYWORD2
 softAP	KEYWORD2

libraries/ESP8266WiFi/src/WiFiUdp.cpp

@@ -240,6 +240,17 @@ uint16_t WiFiUDP::remotePort()
     return _ctx->getRemotePort();
 }
 
+IPAddress WiFiUDP::destinationIP()
+{
+    IPAddress addr;
+
+    if (!_ctx)
+        return addr;
+
+    addr = _ctx->getDestAddress();
+    return addr;
+}
+
 uint16_t WiFiUDP::localPort()
 {
     if (!_ctx)

libraries/ESP8266WiFi/src/WiFiUdp.h

@@ -97,6 +97,9 @@ class WiFiUDP : public UDP {
   virtual IPAddress remoteIP();
   // Return the port of the host who sent the current incoming packet
   virtual uint16_t remotePort();
+  // Return the destination address for incoming packets,
+  // useful to distinguish multicast and ordinary packets
+  IPAddress destinationIP();
   // Return the local port for outgoing packets
   uint16_t localPort();
 

libraries/ESP8266WiFi/src/include/UdpContext.h

@@ -26,7 +26,8 @@ class UdpContext;
 extern "C" void esp_yield();
 extern "C" void esp_schedule();
 
-
+#define GET_IP_HDR(pb) reinterpret_cast<ip_hdr*>(((uint8_t*)((pb)->payload)) - UDP_HLEN - IP_HLEN);
+#define GET_UDP_HDR(pb) reinterpret_cast<udp_hdr*>(((uint8_t*)((pb)->payload)) - UDP_HLEN);
 class UdpContext
 {
 public:
@@ -122,7 +123,7 @@ class UdpContext
         if (!_rx_buf)
             return 0;
 
-        struct ip_hdr* iphdr = (struct ip_hdr*) (((uint8_t*)_rx_buf->payload) - UDP_HLEN - IP_HLEN);
+        ip_hdr* iphdr = GET_IP_HDR(_rx_buf);
         return iphdr->src.addr;
     }
 
@@ -131,10 +132,16 @@ class UdpContext
         if (!_rx_buf)
             return 0;
 
-        struct udp_hdr* udphdr = (struct udp_hdr*) (((uint8_t*)_rx_buf->payload) - UDP_HLEN);
+        udp_hdr* udphdr = GET_UDP_HDR(_rx_buf);
         return ntohs(udphdr->src);
     }
 
+    uint32_t getDestAddress()
+    {
+        ip_hdr* iphdr = GET_IP_HDR(_rx_buf);
+        return iphdr->dest.addr;
+    }
+
     uint16_t getLocalPort()
     {
         if (!_pcb)