net: bt: enable solicit node multicast group registration

6lowpan over BLE should work without solicit node multicast messages
according to RFC7668[1], but that requires Neighbor Solicitation with
Address Registration Option, which is currently not implemented in
either Zephyr or Linux. This is causing the router to fallback to normal
neighbor solicitation based discovery, but the NS frames are being
discarded in the host stack because the solicit node multicast groups
are not registered.

This drops the NET_L2_MULTICAST_SKIP_JOIN_SOLICIT_NODE as a workaround
and adds a TODO about it.

[1] https://tools.ietf.org/html/rfc7668#section-3.2.3

Signed-off-by: Fabio Baltieri <[email protected]>

Author: fabiobaltieri

subsys/net/l2/bluetooth/bluetooth.c

@@ -139,7 +139,10 @@ static int net_bt_enable(struct net_if *iface, bool state)
 
 static enum net_l2_flags net_bt_flags(struct net_if *iface)
 {
-	return NET_L2_MULTICAST | NET_L2_MULTICAST_SKIP_JOIN_SOLICIT_NODE;
+	/* TODO: add NET_L2_MULTICAST_SKIP_JOIN_SOLICIT_NODE once the stack
+	 * supports Address Registration Option for neighbor discovery.
+	 */
+	return NET_L2_MULTICAST;
 }
 
 NET_L2_INIT(BLUETOOTH_L2, net_bt_recv, net_bt_send,