I am struggling with two executables that I have built, which should communicate over multicast. In my current setup they should communicate over the loopback device.
For this I have executed two commands before starting anything:

1. ifconfig lo multicast
2. route add -net 224.0.0.0 netmask 240.0.0.0 dev lo

My sending application does roughly this, in this order:

1. getaddrinfo
2. socket
3. setsockopt(IP_MULTICAST_LOOP)
4. connect

My listening application does roughly this, in this order:

1. getaddrinfo
2. socket
3. connect
4. setsockopt(IP_ADD_MEMBERSHIP)

When the two programs run, the wlan0 interface is also up besides lo.
My listening application receives nothing when I have the route towards 224.0.0.0 enabled, but tcpdump does . When I remove that route and restart the listening application everything works fine.

I wonder what steps I can take to debug this? It appears as if my sending application is always sending over the wlan0 interface.