Not receiving SYN/ACK after sending SYN using RAW socket - c

Question

I am sending TCP SYN packets (with no payload) to a webserver in the same network. I am using sniffex.c
for capturing the packets .
The problem is that after I send a SYN packet, I do not receive the SYN/ACK packet from server.
In sniffex.c:
I have used my LAN IP as the source ip.
I have set the filter as "tcp" .
I am sending to port 80
When i print the fields of the sent packet ,after i capture it using sniffex , all fields are printed correctly, hence i assume that the structure of the sent packet is such that the server can understand it.
When I connect to the webserver using browser, the SYN/ACK is received successfully.
Another related query: how do I set the filter such that I get packets relating to this conversation (b/w my pc and webserver) only
I am using UBUNTU 14.04
EDIT: The c file with which I am trying to send the packet
#define __USE_BSD /* use bsd'ish ip header */
#include <sys/socket.h> /* these headers are for a Linux system, but */
#include <netinet/in.h> /* the names on other systems are easy to guess.. */
#include <netinet/ip.h>
#define __FAVOR_BSD /* use bsd'ish tcp header */
#include <netinet/tcp.h>
#include <unistd.h>
#include<stdio.h>
#include<stdlib.h>
#include<memory.h>
#include<errno.h>
#include<sys/socket.h>
#include<sys/types.h>
#define P 80 /* lets flood the sendmail port */
unsigned short /* this function generates header checksums */
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 ~sum;
}
int
main (void)
{
int s = socket (AF_INET, SOCK_RAW, IPPROTO_TCP);
printf("s=%d\n",s); /* 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 ("xxx.xxx.xxx.xxx");
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 ("xxx.xxx.xxx.xxx");/* SYN's can be blindly spoofed */
iph->ip_dst.s_addr = sin.sin_addr.s_addr;
tcph->th_sport = htons (2000); /* 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 = 5;
tcph->th_off = 5; /* 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 */
/* lets do it the ugly way.. */
int one = 1;
// const int *val = &one;
if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, &one, sizeof (one)) < 0)
printf ("Warning: Cannot set HDRINCL!\terrno = %d\n",errno);
// while (1)
// {
if (sendto (s, /* our socket */
datagram, /* the buffer containing headers and data */
iph->ip_len, /* 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() */
printf ("error\n");
else
printf ("SUCCESS\n\n\n\n");
//}
char buffer[8192];
memset (buffer, 0, 8192);
int n;
//while(n=read (s, buffer, 8192) > 0)
//{
//printf("n=%d\n",n);
//printf ("Caught tcp packet: %s\n", buffer);
//memset (buffer, 0, 8192);
//}
return 0;
}

Answer 1

[Summary of chat session]
In addition to the iph->ip_off issue, you may need to compute the TCP checksum yourself (your O/S may not do it for you). Useful info is here: http://www.tcpipguide.com/free/t_TCPChecksumCalculationandtheTCPPseudoHeader-2.htm and http://www.netfor2.com/tcpsum.htm
Also tcph->th_seq = htonl(23456); may be useful.