I am trying to make my own Multicast routing daemon and I am trying to differentiate between local applications (not on router but call to router using IGMP messages) and resident applications (program with multicast socket on router). In the system call to add IP_ADD_MEMBERSHIP you give the Linux Kernel an interface address and a group address. Then the multicast routing socket needs to determine what is the incoming (iif) and outgoing (oif) interfaces with system calls such as MRT_ADD_MFC and MRT_DEL_MFC.

To demonstrate this my multicast routing daemon (MRD) must subscribe to the group address "224.0.0.2" in order to listen to leave requests. Would the iif and oif need to be the same? If not what should the proper configuration be?