Raw Socket Programming using C

Hi everybody,
      i am stucked doing a client-server program using raw sockets. I have Red Hat 8.0 Linux installed in my m/c and here is my client module (server also has the same problem)

#define __USE_BSD
#include<sys/socket.h>
#include<stdio.h>
#include<netinet/in.h>
#include<netinet/ip.h>
#define __FAVOR_BSD
#include<netinet/tcp.h>
#include<arpa/inet.h>
#include<unistd.h>

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 ~sum;
}

int main()
{
 int sockfd=socket(PF_INET,SOCK_RAW,IPPROTO_IP);
 int len;
 char datagram[4096];
 struct ip *iph = (struct ip*)datagram;
 struct tcphdr *tcph=(struct tcphdr *)datagram + sizeof(struct ip);
 struct sockaddr_in address;
 int result;
 memset(datagram,0,4096);
 address.sin_family=AF_INET;
 address.sin_addr.s_addr=inet_addr("127.0.0.1");
 address.sin_port=htons(9738);
 len=sizeof(address);
 iph->ip_hl=5;
 iph->ip_v=4;
 iph->ip_tos=0;
 iph->ip_len=sizeof(struct ip)+sizeof(struct tcphdr);
 iph->ip_id=htonl(54321);
 iph->ip_off=0;
 iph->ip_ttl=255;
 iph->ip_p=6;
 iph->ip_sum=0;
 iph->ip_src.s_addr=inet_addr("127.0.0.1");
 iph->ip_dst.s_addr = address.sin_addr.s_addr;
 tcph->th_sport = htons (9738);      /* arbitrary port */
 tcph->th_dport = htons (1234);
 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);
 {
  int one=1;
  const int *val=&one;
  if(setsockopt(sockfd,IPPROTO_IP,IP_HDRINCL,val,sizeof(one))<0)
   printf("Warning: Cannot set HDRINCL!\n");
 }
 // connect the socket with the server's socket
 result=connect(sockfd,(struct sockaddr *)&address,len);
 if(result==-1){
  perror("oops: client");
  exit(1);
 }
 //sendto(sockfd,datagram,iph->ip_len,0,(struct sockaddr *)&address,sizeof(address));
 //recvfrom(sockfd,datagram,iph->ip_len,0,(struct sockaddr *)&address,&len);
 write(sockfd,datagram,4096);
 read(sockfd,datagram,4096);
 //printf("char from server = %c\n",datagram[4095]);
 close(sockfd);
 exit(0);
}

the problem is what i guess the kernel is not able to distinguish my packet and issuing the following message

Warning: Cannot set HDRINCL!
by capturing the error here.

{
  int one=1;
  const int *val=&one;
  if(setsockopt(sockfd,IPPROTO_IP,IP_HDRINCL,val,sizeof(one))<0)
   printf("Warning: Cannot set HDRINCL!\n");
 }

If i ommit this block of code then it is issuing "bad File descriptor". What is the actual problem and what may be the remedies.


Thanking in advance,
Avik.
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