Guys, I have been trying to fix this but I think I need your help. I am newbie and I cant seem to find the issue. Sendto returns -1. For one, I don’t know how to make Sendto spit out more details about the error or what’s causing it to return -1. Im running Linux btw.

Here is the code:

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>

#define MAX_PACKET_SIZE 4096

u_int32_t src_addr, dst_addr;
u_int16_t src_port, dst_port;


unsigned short csum (unsigned short *buf, int nwords)
{
    unsigned long sum;
    for (sum = 0; nwords > 0; nwords--)
        sum += *buf++;
    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    return (unsigned short)(~sum);
}

void setup_ip_header(struct iphdr *iph)
{
    iph->saddr = src_addr;                          // SOURCE IP ADDRESS 
    iph->daddr = dst_addr;                          // TARGET IP ADDRESS

    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = htons(sizeof(struct iphdr) + sizeof(struct tcphdr));
    iph->id = htonl(54321); /*setup any arbitrary number for id */
    iph->frag_off = 0;
    iph->ttl = MAXTTL;
    iph->protocol = 6; /* upper layer protocol, TCP*/
    iph->check = 0;

}

void setup_tcp_header(struct tcphdr *tcph)
{
    tcph->source = htons(src_port);                 // SOURCE PORT
    tcph->dest = htons(dst_port);                   // TARGET PORT

    tcph->seq = random();
    tcph->ack_seq = 0;
    tcph->res2 = 0;
    tcph->doff = 5;
    tcph->syn = 1;
    tcph->window = htonl(65535);
    tcph->check = 0;
    tcph->urg_ptr = 0;
}



int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    struct sockaddr_in sin;                                         

    const char *src_addr_str = "192.168.1.50";
    src_addr = inet_addr(src_addr_str);
    src_port = 777;

    const char *dst_addr_str = "192.168.1.51";
    dst_addr = inet_addr(dst_addr_str);
    dst_port = 888;

    char datagram[MAX_PACKET_SIZE];
    struct iphdr *iph = (struct iphdr *)datagram;
    struct tcphdr *tcph = (struct tcphdr *)((u_int8_t *)iph + (5 * sizeof(u_int32_t)));
   
    char new_ip[sizeof "255.255.255.255"];


    int src_socket = socket(PF_INET, SOCK_RAW, IPPROTO_TCP);       

    if(src_socket < 0){
        fprintf(stderr, "Could not open raw socket.n");
        exit(-1);
    }

    /* a IP_HDRINCL call, to make sure that the kernel knows
    *     the header is included in the data, and doesn't insert
    *     its own header into the packet before our data
    */
    int tmp = 1;
    const int *val = &tmp;
    if(setsockopt(src_socket, IPPROTO_IP, IP_HDRINCL, val, sizeof (tmp)) < 0){
        fprintf(stderr, "Error: setsockopt() - Cannot set HDRINCL!n");
        exit(-1);
    }


    // SETUP DETAILS ABOUT THE TARGET MACHINE
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = dst_addr;
    sin.sin_port = htons(dst_port);

    // Clear the data
    memset(datagram, 0, MAX_PACKET_SIZE);

    // Set appropriate fields in headers
    setup_ip_header(iph);
    setup_tcp_header(tcph);

    iph->check = csum ((unsigned short *) datagram, iph->tot_len >> 1);


    for(;;){

        int ret = sendto(src_socket, datagram, iph->tot_len, 0, (struct sockaddr *) &sin, sizeof(sin));

        if(ret == -1)
        {
            fprintf(stderr, "sendto() error!!!.n");
        }
        else
        {
            fprintf(stdout, "Flooding %s at %u...n", dst_addr_str , dst_port);
        }

       
        sleep(1);

    }


    return 0;
}

Thank you so much in advance.

Regards,

X

I also noticed that the sizeof(sin) returns only 16, not sure if this is normal. Is that normal ?

How can I make the compiler spit out more info about an error when it encounters one ?