ND6: reply NS without LL opt

* This issue should be fixed for the upstream
* Reference to the RFC-4861#section-4.3 source Link-Layer address
  SHOULD be included in unicast solicitations. But there might be
  some NS messages which do not contain source link-layer address option.
* NA will be replied to those NS messages which do not contain
  source link layer option.

src/core/ipv6/nd6.c

@@ -505,7 +505,16 @@ nd6_input(struct pbuf *p, struct netif *inp)
       /* Sender is trying to resolve our address. */
       /* Verify that they included their own link-layer address. */
       if (lladdr_opt == NULL) {
-        /* Not a valid message. */
+        /* Per RFC4861#section-4.3, source Link-Layer address SHOULD be included in unicast solicitations.
+         * But, there might be some NSs which do not contain source link-layer address option.
+         * For LwIP implement, this kind of NS will be ignored. This issue should be fixed.
+         * Although this Neighbor Solicitation (NS) will be ignored in the subsequent process,
+         * a Neighbor Advertisement (NA) is still sent in response. */
+#if ESP_LWIP && LWIP_FORCE_ROUTER_FORWARDING
+        nd6_send_na(inp, &target_address, ND6_FLAG_ROUTER | ND6_FLAG_SOLICITED | ND6_FLAG_OVERRIDE);
+#else
+        nd6_send_na(inp, &target_address, ND6_FLAG_SOLICITED | ND6_FLAG_OVERRIDE);
+#endif
         pbuf_free(p);
         ND6_STATS_INC(nd6.proterr);
         ND6_STATS_INC(nd6.drop);

src/include/lwip/opt.h

@@ -2424,7 +2424,7 @@
 #endif
 
 /**
- * when LWIP_FORCE_ROUTER_FORWARDING is enbaled in lwip, the router flag in NA packet will always
+ * when LWIP_FORCE_ROUTER_FORWARDING is enabled in lwip, the router flag in NA packet will always
  * set to 1, otherwise, never set router flag for NA packets.
  */
 #if !defined LWIP_FORCE_ROUTER_FORWARDING || defined __DOXYGEN__