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

Question

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!

1000ml 1000ml · Accepted Answer · 2015-10-22T09:31:45

Using IPPROTO_RAW implicitly enables the option IP_HDRINCL.
In your call to setsockopt() you disable IP_HDRINCL again because you set on = 0.
Try removing the setsockopt() or setting on = 1.

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

1 Answers