Multicast packets not encrypted

Hi!

I am using an encrypted connection with two dedicated devices using freeradius and hostapd with eap encryption - respectively wpa_supplicant. Everything is working fine, except transmission of multicast packets.

I put an independant device into monitor mode and - as to be expected - I can only see encrypted packets. Apart from multicast packets - those I can read in plain.

How can this be?

On the machines using the wireless encryption this routing table is present:

:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.0.0.0        0.0.0.0         255.0.0.0       U     0      0        0 wlan0
224.0.0.0       0.0.0.0         240.0.0.0       U     0      0        0 wlan0

Once I issue a

ping 224.0.0.1

I can read the ping packets in plain on the machine with the wireless card in monitor mode. Apart from that, the machine, that should receive the multicast packet, does not "see" the multicast packets (nothing to see in tcpdump of the respective receiver device, that is part of the encrypted connection).

Any suggestions?

Best regards,
Dennis