How to use beginMulticast

[Phil1pp]

Does anybode have a sample project on how to correctly use Multicast on a bk7231n?
I am trying to port a project from ESP32 to bk7231n but I can't get it to work.

On ESP32 I can see the multicast traffic in Wireshark after calling beginMulticast, but on bk7231n there is no traffic at all and result is 0.

WiFiUDP _udp;
IPAddress mcastaddr = IPAddress(224, 0, 23, 12);
uint8_t result = _udp.beginMulticast(mcastaddr, 3671);

log:

lwip_socket(PF_INET, SOCK_DGRAM, 0) = 0
lwip_setsockopt(0, SOL_SOCKET, optname=0x4, ..) -> on
lwip_bind(0, addr=   0.   0.   0.   0 port=3671)
udp_bind(ipaddr =    0.   0.   0.   0, port = 3671)
udp_bind: bound to    0.   0.   0.   0, port 3671)
lwip_bind(0) succeeded
E [     28.174] could not join igmp: 125

[Phil1pp]

After I spent countless hours on debugging, I found the issue.
It seems that the flag "NETIF_FLAG_IGMP" is not automatically assigned to the WiFi interface and therefore the multicast fails.

In the LwIPmDNS.cpp I found the following:

if ((netif->flags & NETIF_FLAG_IGMP) == 0) {
netif->flags |= NETIF_FLAG_IGMP;

So, even though I don't need mDNS, I just called MDNS.begin(HOSTNAME) before using multicast
After that beginMulticast worked