I have a network that is setup for multicast traffic. My setup is the following

-Machine A : a server generates multicast traffic.
-Machine A : few clients subscribing to that multicast traffic
-Machine B : few clients subscribing to that multicast traffic

# Address I am using
IP  : 239.193.0.21
PORT: 20401

The clients in machine A , even if they join the group (I can see IGMP messages through wireshark), don't receive any data while (and this is the funny part) machine B,C and D receive everything.

I sorted that issue by completely disabling Linux firewall.

Before doing that, I enabled the multicast on the firwall ('reject all').

 iptables -A INPUT -m addrtype --src-type MULTICAST -j ACCEPT

My question is the following: what I can check in wireshark that can help me in spot such firewall issues in the futures? For TCP/IP I realize by using ping and looking at ICMP packets rejected.

What I can check/monitor for multicast?

I am using LInux/Red-Hat Enterprise 6.2