Improve Border Router packet forwarding logic

[Cristib05]

There are some cases when OpenThread opens a sockets and doesn't choose as default it's internal interface, this leading to usage of platform UDP module which will then send back the packet to the OpenThread interface. In this case, the packet should not be treated as originated from backbone interface.

Backbone router multicast listener callback functionality is improved. A route with a prefix length of 128 is set
and a multicast address is added for each listener registration. OpenThread interface joins that multicast address group.

Enabled forwarding capabilities for Backbone interface. A border router should be able to perform default packet forwarding for destination addresses with a multicast scope greater than admin-local. In order to achieve this, multicast routes have been added to those addreses. [https://datatracker.ietf.org/doc/rfc7346/]

For Border Router application, ip6_addr_cb is not installed. otIp6SubscribeMulticastAddress call would re-register an IPV6 multicast address which might have been registered by an OpenThread node using ipmadd add command and even if that node performed ipmaddr del, the address was still present in multicast listener table. This also led to a missing MLDv2 message with that specific multicast IPV6 address.

[jukkar]

This looks like overkill as you could create IPv6 address dynamically.
So something like

int mcast_groups[] = {
	/** Admin-Local scope multicast address */
	4,
	/** Site-Local scope multicast address */
	5,
	/** Organization-Local scope multicast address */
	8,
	/** Global scope multicast address */
	e,
};

ARRAY_FOR_EACH(mcast_groups, i) {
	struct in6_addr addr;

	net_ipv6_addr_create(&addr, (0xff << 8) | i, 0, 0, 0, 0, 0, 0, 1);

...
}

[Cristib05]

Updated. Thanks!

[pdgendt]

Add a trailing comma for nicer formatting

Suggested change

	static int mcast_group_idx[] = {/** Admin-Local scope multicast address */
	0x04,
	/** Site-Local scope multicast address */
	0x05,
	/** Organization-Local scope multicast address */
	0x08,
	/** Global scope multicast address */
	0x0e
	};
	static int mcast_group_idx[] = {
	/** Admin-Local scope multicast address */
	0x04,
	/** Site-Local scope multicast address */
	0x05,
	/** Organization-Local scope multicast address */
	0x08,
	/** Global scope multicast address */
	0x0e,
	};

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud