Raw socket: unexpected IP header added when sending self-made IP/TCP packets

[esolve]

I want to use raw socket to send TCP packets which is a full IP packet(so the packet has IP header, TCP header and TCP payload, but has no ethernet header. The IP source and destination addresses are in a WLAN, 192.168.0.105 and 192.168.0.103), with the following codes


int on;
on = 0;
if ((sendfd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
perror("raw socket");
exit(1);
}
if (setsockopt(sendfd, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) < 0) {
perror("setsockopt");
exit(1);
}

nr_bytes = sendto(sendfd, packet, ip_len, 0, (struct sockaddr*)&client_addr, addr_len);



I use TCPdump to capture the sent-out packet and notice it has added an additional IP header to the IP packet, and the IP protocol number is 255（ip->ip_p is 255）. So it has two IP headers(with same pair of src and dst IP), which is unexpected.


what are the problems? thank you!