Unable to print TCP response using Raw Sockets

Question

I am using raw sockets to send datagrams as given in this tutorial (section III). However, I am unable to print the response properly (showing junk characters). Not sure what is wrong with the program. Anyone able to identify the error?

#define P 80        /* TCP port 80 - HTTP */

unsigned short /* this function generates header checksums */
csum(unsigned short *buf, int nwords)
{
    //checksum code here, omitting for stackoverflow question
}

int main()
{
    int s = socket(PF_INET, SOCK_RAW, IPPROTO_TCP); /* open raw socket */
    char datagram[4096]; /* this buffer will contain ip header, tcp header,
     and payload. we'll point an ip header structure
     at its beginning, and a tcp header structure after
     that to write the header values into it */
    struct ip *iph = (struct ip *) datagram;
    struct tcphdr *tcph = (struct tcphdr *) datagram + sizeof(struct ip);
    struct sockaddr_in sin;
    /* the sockaddr_in containing the dest. address is used
     in sendto() to determine the datagrams path */

    sin.sin_family = AF_INET;
    sin.sin_port = htons(P);/* you byte-order >1byte header values to network
     byte order (not needed on big endian machines) */
    sin.sin_addr.s_addr = inet_addr("74.125.224.72"); //google's ip address

    memset(datagram, 0, 4096); /* zero out the buffer */

    /* we'll now fill in the ip/tcp header values, see above for explanations */
    iph->ip_hl = 5;
    iph->ip_v = 4;
    iph->ip_tos = 0;
    iph->ip_len = sizeof(struct ip) + sizeof(struct tcphdr); /* no payload */
    iph->ip_id = htonl(54321); /* the value doesn't matter here */
    iph->ip_off = 0;
    iph->ip_ttl = 255;
    iph->ip_p = 6;
    iph->ip_sum = 0; /* set it to 0 before computing the actual checksum later */
    iph->ip_src.s_addr = inet_addr("1.2.3.4");/* SYN's can be blindly spoofed */
    iph->ip_dst.s_addr = sin.sin_addr.s_addr;
    tcph->th_sport = htons(1234); /* arbitrary port */
    tcph->th_dport = htons(P);
    tcph->th_seq = random();/* in a SYN packet, the sequence is a random */
    tcph->th_ack = 0;/* number, and the ack sequence is 0 in the 1st packet */
    tcph->th_x2 = 0;
    tcph->th_off = 0; /* first and only tcp segment */
    tcph->th_flags = TH_SYN; /* initial connection request */
    tcph->th_win = htonl(65535); /* maximum allowed window size */
    tcph->th_sum = 0;/* if you set a checksum to zero, your kernel's IP stack
     should fill in the correct checksum during transmission */
    tcph->th_urp = 0;

    iph->ip_sum = csum((unsigned short *) datagram, iph->ip_len >> 1);

    /* finally, it is very advisable to do 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 one = 1;
    const int *val = &one;
    if (setsockopt(s, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0)
        printf("Warning: Cannot set HDRINCL!\n");

    if (sendto(s,datagram,iph->ip_len,0,(struct sockaddr *) &sin, sizeof(sin)) < 0) 
        printf("error\n");
    else
        printf(".\n\n");

    sleep(2); // giving enough time to receive response

    char buffer[8192]; /* single packets are usually not bigger than 8192 bytes */
    memset(buffer, 0, 8192); /* zero out the buffer */
    while (recv(s, buffer, 8192, 0) > 0)
        printf("Caught tcp packet: %s\n", buffer + sizeof(struct iphdr) + sizeof(struct tcphdr));

    close(s);
    return 0;
}

The output I am getting is shown in this image: https://i.stack.imgur.com/4MfLg.jpg

Answer 1

Try changing:

printf("Caught tcp packet: %s\n", buffer + sizeof(struct iphdr) + sizeof(struct tcphdr));

to:

printf("Caught tcp packet: %s\n", buffer);

Answer 2

while (recv(s, buffer, 8192, 0) > 0)
    printf("Caught tcp packet: %s\n", buffer + sizeof(struct iphdr) + sizeof(struct tcphdr));

It is never correct not to store the result of recv() in a variable. You need to test it for zero and -1 and otherwise to use it as the length of the data received. Try this:

int count;
while ((count = recv(s, buffer, sizeof buffer, 0)) > 0)
    printf("Caught tcp packet: %.*s\n", count - sizeof(struct iphdr) - sizeof(struct tcphdr, buffer + sizeof(struct iphdr) + sizeof(struct tcphdr));
if (count < 0)
    perror("recv");

EDIT You seem to be trying to print the contents of a SYN-ACK packet. There aren't any.

Answer 3

You need to use the libpcap library to retrieve the packet from the link layer .