[raw sockets, C] tcp header cut

darell · 10-08-2008, 12:03 PM

hi @all,

I want to send a tcp syn paket(and later answer to acks) via Raw Sockets in C.
My Problem is, that the tcp-header in my outgoing paket is cut.

The Sniffer detect this:

Code:

bogus tcp header length(0, must be at least 20)

Here is the code:

Code:

int send_tcp_syn()
{
    int sock, one = 1;

    if( (sock = socket(AF_INET, SOCK_RAW, IPPROTO_TCP)) == -1) {
        perror("building socket");
        return -1;
    }
    paket_size = sizeof(struct tcphdr) + sizeof(struct iphdr);
    paket = (unsigned char*) malloc(paket_size);
        
    struct iphdr *ip = (struct iphdr  *) paket;                        
    struct tcphdr *tcp = (struct tcphdr *) (paket + sizeof(struct iphdr));
    memset(paket, 0, paket_size);

/* set ip attributes */
    ip -> version    = 4;
    ip -> ihl    = 5;
    ip -> ttl    = 255;
    ip -> id    = htonl( random() );
    ip -> daddr       = inet_addr(HOST_IP);
    ip -> saddr    = inet_addr( get_random_ip() );
    ip -> protocol    = IPPROTO_TCP;
    ip -> tot_len     = paket_size;

/* tcp attributes */
    tcp -> source     = htons(HTTP_PORT);
    tcp -> dest         = htons(HTTP_PORT);
    tcp -> seq          = htonl(1000000000);
    tcp -> ack_seq  = htonl(1000000000);
    tcp -> syn          = 1;
    tcp -> window  = htons(1024);
    tcp -> check      = 0;

    host.sin_family     = AF_INET;
    host.sin_port        = tcp -> dest;
    host.sin_addr.s_addr     = ip  -> daddr;
    
    if( setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &one, 1) == -1) {    
        perror("setsockopt");
        return -2;
    }
    if( sendto(sock, paket, paket_size, 0, (struct sockaddr *)&host,  
                                           sizeof(host))     < 0 ) {
        perror("sendto");
        return -3;
    }
    return 0;
}

Thank you for helping me

estabroo · 10-09-2008, 08:55 AM

You need to set the data offset (tcp->doff) to the 4 byte rounded start of your data (basically tcp header size). It needs to be set even if its an empty packet.

darell · 10-09-2008, 11:04 AM

hi estabroo,

It work's;
Thank you very much, I posted the problem in a lot of boards and a long time no one was able to resolve it.

Next time I will post it here at first.

Darell