ip6: add a hook for ip6 source address selection

src/core/ipv6/ip6.c

@@ -290,6 +290,14 @@ ip6_select_source_address(struct netif *netif, const ip6_addr_t *dest)
   u8_t best_pref = 0;
   u8_t best_bits = 0;
 
+  best_addr = NULL;
+
+#ifdef LWIP_HOOK_IP6_SELECT_SRC_ADDR
+  if ((best_addr = LWIP_HOOK_IP6_SELECT_SRC_ADDR(netif, dest)) != NULL) {
+    return best_addr;
+  }
+#endif
+
   /* Start by determining the scope of the given destination address. These
    * tests are hopefully (roughly) in order of likeliness to match. */
   if (ip6_addr_isglobal(dest)) {
@@ -307,8 +315,6 @@ ip6_select_source_address(struct netif *netif, const ip6_addr_t *dest)
     dest_scope = IP6_MULTICAST_SCOPE_GLOBAL;
   }
 
-  best_addr = NULL;
-
   for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
     /* Consider only valid (= preferred and deprecated) addresses. */
     if (!ip6_addr_isvalid(netif_ip6_addr_state(netif, i))) {

src/include/lwip/opt.h

@@ -3027,6 +3027,28 @@
 #define LWIP_HOOK_ND6_GET_GW(netif, dest)
 #endif
 
+/**
+ * LWIP_HOOK_IP6_SELECT_SRC_ADDR(netif, dest):
+ * Called from ip6_select_source_address() (IPv6)
+ * Signature:\code{.c}
+ *   const ip6_addr_t *my_hook(struct netif *netif, const ip6_addr_t *dest);
+ * \endcode
+ * Arguments:
+ * - netif: the netif used for selecting
+ * - dest: the destination IPv6 address
+ * Return values:
+ * - the preferred source IPv6 address of the specified destination IPv6 address
+ * - NULL, in which case none source address is preferred by the hook, lwip
+ *         will determine a source address based RFC 6724.
+ *
+ * The returned address MUST be on the specified netif!
+ * This function is meant to implement advanced IPv6 source address selection with
+ * LWIP_HOOK_IP6_SELECT_SRC_ADDR().
+*/
+#ifdef __DOXYGEN__
+#define LWIP_HOOK_IP6_SELECT_SRC_ADDR(netif, dest)
+#endif
+
 /**
  * LWIP_HOOK_VLAN_CHECK(netif, eth_hdr, vlan_hdr):
  * Called from ethernet_input() if VLAN support is enabled