lwip/napt: Added api to enable/disable napt based on lwip netif.

src/core/ipv4/ip4_napt.c

@@ -205,6 +205,37 @@ ip_napt_enable_no(u8_t number, int enable)
   }
 }
 
+int
+ip_napt_enable_netif(struct netif *netif, int enable)
+{
+  struct netif *each_netif;
+
+  if (!netif_is_up(netif)){
+    return 0;
+  }
+
+  if (!netif->napt && enable) {
+    /* Enable napt */
+    netif->napt = 1;
+    ip_napt_init(IP_NAPT_MAX, IP_PORTMAP_MAX);
+
+  } else if (netif->napt && !enable) {
+    /* Disable napt */
+    netif->napt = 0;
+
+    NETIF_FOREACH(each_netif) {
+      if (each_netif->napt) {
+        /* napt used on another interface, no need for cleanup */
+        return 1;
+      }
+    }
+    ip_napt_deinit();
+  }
+  /* Enabling/disabling napt a second time is a no operation */
+
+  return 1;
+}
+
 /* adjusts checksum in a packet
 - chksum points to the chksum in the packet
 - optr points to the old data in the packet (before)

src/include/lwip/lwip_napt.h

@@ -67,6 +67,7 @@ extern "C" {
 #define IP_NAPT_PORT_RANGE_START 49152
 #define IP_NAPT_PORT_RANGE_END   61439
 
+
 /**
  * Enable/Disable NAPT for a specified interface.
  *
@@ -87,6 +88,20 @@ void
 ip_napt_enable_no(u8_t number, int enable);
 
 
+/**
+ * Enable/Disable NAPT for a specified interface.
+ *
+ * @param netif interface handle
+ * @param enable non-zero to enable NAPT, or 0 to disable.
+ *
+ * @return
+ *         - 0: Failure
+ *         - 1: Success
+ */
+int
+ip_napt_enable_netif(struct netif *netif, int enable);
+
+
 /**
  * Register port mapping on the external interface to internal interface.
  * When the same port mapping is registered again, the old mapping is overwritten.