struct ip_mreqn is Linux extension to classic BSD multicast API. It
has extra fields allowing to specify the interface index explicitly.
We have it declared since r170613 and IP_MULTICAST_IF supports it.
In Linux the structure is used both for IP_MULTICAST_IF and for
The problem is that thirdy party programs see structure declaration
and assume that system supports ip_mreqn for IP_ADD_MEMBERSHIP.