Raw socket not sending packets containing arbitrary data - c

Question

Take the following code example
https://gist.github.com/3825444
/*
Testing arbitrary raw ip packets
works only if datagram is filled with 0
filling with anything else will not send any packets, or atleast wireshark does not detect anything
this is strange
*/
#include<stdio.h>
#include<string.h> //memset
#include<sys/socket.h>
#include<stdlib.h> //for exit(0);
#include<errno.h> //For errno - the error number
#include<netinet/tcp.h> //Provides declarations for tcp header
#include<netinet/ip.h> //Provides declarations for ip header
int main (void)
{
//Create a raw socket
int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP);
if(s < 0)
{
perror("socket");
}
//Datagram to represent the packet
char datagram[4096] , source_ip[32];
struct sockaddr_in sin;
strcpy(source_ip , "192.168.1.2");
sin.sin_family = AF_INET;
sin.sin_port = htons(80);
sin.sin_addr.s_addr = inet_addr ("1.2.3.4");
memset (datagram, 2 , 4096); /* zero out the buffer */
//IP_HDRINCL to tell the kernel that headers are included in the packet
int one = 1;
const int *val = &one;
if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
{
printf ("Error setting IP_HDRINCL. Error number : %d . Error message : %s \n" , errno , strerror(errno));
exit(0);
}
//Uncommend the loop if you want to flood :)
while (1)
{
//Send the packet
if (sendto (s, /* our socket */
datagram, /* the buffer containing headers and data */
512, /* total length of our datagram */
0, /* routing flags, normally always 0 */
(struct sockaddr *) &sin, /* socket addr, just like in */
sizeof (sin)) < 0) /* a normal send() */
{
perror("sendto");
}
//Data send successfully
else
{
printf ("Packet Send \n");
}
}
return 0;
}
The above program does not generate any packets, or atleast wireshark will not detect any.
However if the datagram is filled with 0 by doing
memset (datagram, 0 , 4096); /* zero out the buffer */
then plenty of packets are generate and are detected by wireshark.
Why such a difference ?

Answer 1

You're putting garbage into the header. It's more remarkable that setting zeros succeeds than that setting 2's fails.