controlfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
setsockopt(controlfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
setsockopt(controlfd, SOL_IP, IP_PKTINFO, &on, sizeof(on)); // recvfromto
controlfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
setsockopt(controlfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
setsockopt(controlfd, SOL_IP, IP_PKTINFO, &on, sizeof(on)); // recvfromto