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I want my application to create TCP SYN packet and send on network. I did not set IP_HDRINCL using setsockopt because I want the kernel to fill the IP Header.

I tried

  1. using a byte buffer to include TCP Header + Payload
  2. using a byte buffer to include IP Header + TCP Header + Payload.

Both above methods returned sendto error -1 and errno is set to 88 Following is the code I used 

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <errno.h>

int main()
{
    int sockfd;

    if(sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_TCP) < 0)
    {
        printf("sokcet functin failed : \n");
        exit (-1);
    }

    char packet[512];

    struct sockaddr_in remote; // remote address

    struct iphdr *ip = (struct iphdr *) packet;
    struct tcphdr *tcp = (struct tcphdr *) packet + sizeof(struct iphdr);


    /*
    struct tcphdr *tcp = (struct tcphdr *) packet; // tcp header
    */
    remote.sin_family = AF_INET; // family
    remote.sin_addr.s_addr = inet_addr("192.168.0.57"); // destination ip
    remote.sin_port = htons(atoi("3868")); // destination port

    memset(packet, 0, 512); // set packet to 0

    tcp->source = htons(atoi("6668")); // source port
    tcp->dest = htons(atoi("3868")); // destination port
    tcp->seq = htons(random()); // inital sequence number
    tcp->ack_seq = htons(0); // acknowledgement number
    tcp->ack = 0; // acknowledgement flag
    tcp->syn = 1; // synchronize flag
    tcp->rst = 0; // reset flag
    tcp->psh = 0; // push flag
    tcp->fin = 0; // finish flag
    tcp->urg = 0; // urgent flag
    tcp->check = 0; // tcp checksum
    tcp->doff = 5; // data offset

    int err;
    if((err = sendto(sockfd, packet, sizeof(struct iphdr), 0, (struct sockaddr *)&remote, sizeof(struct sockaddr))) < 0)
    { // send packet
        printf("Error: Can't send packet : %d %d !\n\n", err, errno);
        return -1;
    }

Why i am getting the sendto error -1 and errno 88?

Also, I want to know how to determine the length of data that is to be used in second argument in sendto function. If I hard code it to size of packet byte buffer i.e. 512 bytes, is it wrong ?

Subbu
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    "Can anyone help me in knowing the reason behind sendto error ?" No, but you can help yourself. Check the `errno`, per documentation, to determine the cause of the problem. Make sure you present it in [a human readable form](http://stackoverflow.com/q/1546882/1329652)! – Kuba hasn't forgotten Monica Mar 29 '16 at 18:20
  • edited my question – Subbu Mar 30 '16 at 06:01
  • The "errno 88" is not helpful. `sendto` returning -1 simply indicates that it failed. If you follow the documentation on your platform, you'll note that you then have to examine `errno` to check the cause of the error, and you can use `strerror` to translate "errno 88" to English. Do that. – Kuba hasn't forgotten Monica Mar 30 '16 at 12:18
  • strerror(errno) resulted in segmentation fault. – Subbu Mar 30 '16 at 14:12

2 Answers2

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Here I am not giving the solution but you can check the meaning of your error and can correct your error by and also I can not comment because I haven't much reputation.. I just want to help...

int err = sendto(sockfd, packet, sizeof(struct iphdr), 0, (struct sockaddr *)&remote, sizeof(struct sockaddr)));
int saved_error = errno;  // Save errno flag that is set by sendto.
if(err < 0) {
    fprintf(stderr, "Send Error: %s\n", strerror(saved_error));
}

I think it may help...But this is not the answer...This will give the meaning of error so that you can correct your error..

Shiv
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For the SOCK_RAW you can use AF_ATMPVC: Access to raw ATM PVCs

In your code:

if((sockfd = socket(AF_ATMPVC, SOCK_RAW, IPPROTO_TCP)) < 0)

I hope it might be helpful

Eraniehu
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