igmp: leave groups in 224.0.0.0/24 to flooding

The switch keeps flooding 224.0.0.0/24 with IGMP on, but a report for a
group in that block still made an entry for it, and the entry limited
the group to the ports that reported it. A host announcing mDNS on
224.0.0.251 thereby cut mDNS off from every other port. Such reports
are now ignored, as RFC 4541 asks.

doc/igmp.md

@@ -85,6 +85,9 @@ for frames to 01:00:5e:xx:xx:xx and returns 0 when the frame is not a trapped IG
 so that it is passed on to the IP stack. IGMPv1/v2 reports (0x12, 0x16) join the group given
 in the IGMP header, IGMPv2 leaves (0x17) leave it, IGMPv3 reports use the first group record
 (type 4 joins, type 3 leaves). Trapped IGMP frames arrive without a VLAN tag.
+Reports for groups in 224.0.0.0/24 are ignored: the switch floods that block
+even with IGMP on, and an entry for a group such as 224.0.0.251 (mDNS) would
+otherwise limit it to the ports that reported it, as RFC 4541 warns against.
 
 `igmp_show()` prints out the IGMP configuration on the CLI.
 

rtl837x_igmp.c

@@ -250,6 +250,8 @@ uint8_t igmp_packet_handler(void) __banked
 #ifdef DEBUG
 	print_string("IGMP join "); print_byte(igmp_join); write_char('\n');
 #endif
+	if (uip_buf[igmp_grp] == IGMP_LOCAL_NET_0 && !uip_buf[igmp_grp + 1] && !uip_buf[igmp_grp + 2])
+		return 1;
 
 #ifdef IPMC_USES_L3MC
 	memset((__xdata uint8_t *)&entry, 0, sizeof(struct ipmc_table_entry));

rtl837x_igmp.h

@@ -10,6 +10,7 @@
 #define IGMP_V3_REPORT		0x22
 #define IGMP_V3_TO_INCLUDE	3
 #define IGMP_V3_TO_EXCLUDE	4
+#define IGMP_LOCAL_NET_0	224
 
 void igmp_setup(void) __banked;
 void igmp_enable(void) __banked;