UDP_SRC_PORT
s4->sin_port = htons(UDP_SRC_PORT);
s6->sin6_port = htons(UDP_SRC_PORT);
cur += build_udp_header(cur, UDP_SRC_PORT, VN_PORT, pktlen);
cur += build_udp_packet(cur, UDP_DST_PORT, UDP_SRC_PORT, payload_len,