bridge: Snoop Multicast Router Advertisements

When multiple multicast routers are present in a broadcast domain then
only one of them will be detectable via IGMP/MLD query snooping. The
multicast router with the lowest IP address will become the selected and
active querier while all other multicast routers will then refrain from
sending queries.

To detect such rather silent multicast routers, too, RFC4286
("Multicast Router Discovery") provides a standardized protocol to
detect multicast routers for multicast snooping switches.

This patch implements the necessary MRD Advertisement message parsing
and after successful processing adds such routers to the internal
multicast router list.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/igmp.h

@@ -93,6 +93,7 @@ struct igmpv3_query {
 #define IGMP_MTRACE_RESP		0x1e
 #define IGMP_MTRACE			0x1f
 
+#define IGMP_MRDISC_ADV			0x30	/* From RFC4286 */
 
 /*
  *	Use the BSD names for these for compatibility