On one of my systems at work good ol' netstat shows multicast group membership information, and ss is missing a couple. I wonder why. For example:
[root@myhost ~]# netstat -gn | egrep "Inter|239.192"
Interface RefCnt Group
em4.204 1 239.192.33.183
em1.16 2 239.192.35.1
em1.16 2 239.192.12.98
em1.16 1 239.192.32.1
[root@myhost ~]# ss -apu | egrep "State|239.192"
State Recv-Q Send-Q Local Address:Port Peer Address:Port
UNCONN 0 0 239.192.35.1:12965 *:*
UNCONN 0 0 239.192.12.98:12965 *:*
UNCONN 0 0 239.192.35.1:12965 *:*
UNCONN 0 0 239.192.12.98:12965 *:*
Notice that ss shows only those groups with a RefCnt of 2.
Technically, ip maddr show is the replacement for netstat -gn but it doesn't include the RefCnt, and its output is more cumbersome. Also, we're interested in the more detailed output of ss which can include the PID of the listening processes (not seen here because no processes are actually currently listening to the multicast, as show by netstat -ulpn:
[root@myhost ~]# netstat -ulpn | egrep "Proto|239.192"
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
udp 0 0 239.192.35.1:12965 0.0.0.0:* -
udp 4480 0 239.192.12.98:12965 0.0.0.0:* -
udp 0 0 239.192.35.1:12965 0.0.0.0:* -
udp 0 0 239.192.12.98:12965 0.0.0.0:* -
