Adds NAPT and port mapping functionality to esp8266 lwip

esp8266 can handle 2 network interfaces (Wifi client and softAP)
simultaneously in the WIFI_AP_STA mode.
Currently IP_FORWARD is implemented in lwip to forward packets
between 2 interfaces, but static routing must be setup manually.
This patch adds IP_NAPT config option to enable esp8266 to use
NAPT technique on forwarding packets from softAP clients to the
parent WiFi AP.

To enable NAPT on the interface, ip_napt_enable() must be called
with esp8266's softAP interface address. For example:

  extern "C" void ip_napt_enable(unsigned long addr, int enable);
  ip_napt_enable(IPAddress(192,168,4,1), 1);

This patch also provides port mappings to forward packets from
the parent WiFi network to softAP network. To register and
unregister port mappings, following functions can be used:

  void ip_portmap_add(byte proto, unsigned long maddr, unsigned short mport,
                      unsigned long daddr, unsigned short dport);
  bool ip_portmap_remove(byte proto, unsigned short mport);

For exmaple:

  #define IP_PROTO_TCP     6
  #define IP_PROTO_UDP     17
...
  ip_portmap_add(IP_PROTO_TCP, WiFi.localIP(), 8080,
                 IPAddress(192,168,4,3), 80);
  ip_portmap_add(IP_PROTO_UDP, WiFi.localIP(), 12345,
                 IPAddress(192,168,4,2), 12345);

These map TCP port 8080 on WiFi client address to 192.168.4.3:80, and
UDP port 12345 to 192.168.4.2:12345. To remove the first mapping:

  ip_portmap_remove(IP_PROTO_TCP, 8080);

To apply this change, SDK needs to be recompiled with this patch:
  $ cd tools/sdk/lwip/src
  $ make && make release  #=> intalled to ../../lib/liblwip_gcc.a

Note that this patch is experimental and is provided as is.

Author: NeoCat

tools/sdk/lwip/include/lwip/netif.h

@@ -228,6 +228,9 @@ struct netif {
   u16_t loop_cnt_current;
 #endif /* LWIP_LOOPBACK_MAX_PBUFS */
 #endif /* ENABLE_LOOPBACK */
+#if IP_NAPT
+  u8_t napt;
+#endif
 };
 
 #if LWIP_SNMP

tools/sdk/lwip/include/lwip/opt.h

@@ -501,7 +501,11 @@
  * interface, define this to 0.
  */
 #ifndef IP_FORWARD
-#define IP_FORWARD                      0
+#define IP_FORWARD                      1
+#endif
+
+#ifndef IP_NAPT
+#define IP_NAPT                         1
 #endif
 
 /**

tools/sdk/lwip/include/lwipopts.h

@@ -505,7 +505,11 @@
  * interface, define this to 0.
  */
 #ifndef IP_FORWARD
-#define IP_FORWARD                      0
+#define IP_FORWARD                      1
+#endif
+
+#ifndef IP_NAPT
+#define IP_NAPT                         1
 #endif
 
 /**

tools/sdk/lwip/src/core/init.c

@@ -322,4 +322,8 @@ lwip_init(void)
 #if LWIP_TIMERS
   sys_timeouts_init();
 #endif /* LWIP_TIMERS */
+
+#if IP_NAPT
+  ip_napt_init();
+#endif /* IP_NAPT */
 }