Programming

#include <sys/types.h>
#include <sys/socket.h>

#include <netinet/in.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <netdb.h>

#include <netinet/ip6.h>

int 
main()
{
  struct sockaddr_in6 sin6, sin6_accept;
  socklen_t sin6_len;
  int s0, s;
  int on;
  char hbuf[NI_MAXHOST];
  
  //ext header
  int rv;
  struct msghdr msg;
  struct cmsghdr *cmsg;
  int cmsglen = 0;
  
  char tstmsg[13] = "Hello World";
  
  struct iovec vec;
  
  struct ip6_dest dest;
  
  memset(&sin6, 0, sizeof(sin6));
  sin6.sin6_family = AF_INET6;
  sin6.sin6_len = sizeof(sin6);
  sin6.sin6_port = htons(5001);
  
  s0 = socket(AF_INET6, SOCK_STREAM, IPPROTO_TCP);
  on = 1;
  
  //setsockopt(s0, SOL_SOCKET, SO_RESUEADDR, &on, sizeof(on));
  
  setsockopt(s0, IPPROTO_IPV6, IPV6_V6ONLY, &on, sizeof(on));
  
  bind(s0, (const struct sockaddr *)&sin6, sizeof(sin6));
  listen(s0, 1);
  while(1) {
            sin6_len = sizeof(sin6_accept);
            s = accept(s0, (struct sockaddr *)&sin6_accept, &sin6_len);
            getnameinfo((struct sockaddr *)&sin6_accept, sin6_len,
                          hbuf, sizeof(hbuf), NULL, 0, NI_NUMERICHOST);            
            printf("accept a connection from %s\n", hbuf);
            
            
            memset(&dest, 0, sizeof(dest));
            
            /* Ancilliary Data  */
            msg.msg_name = &sin6;
            msg.msg_namelen = sizeof(sin6);
            
            msg.msg_iov = &vec;
            msg.msg_iovlen = 1;
            msg.msg_iov->iov_base = tstmsg;
            msg.msg_iov->iov_len = 13;
            
            
            cmsglen = CMSG_SPACE(sizeof(dest));
            
            cmsg = malloc(cmsglen);
            
            if(cmsg == NULL) {
              puts("cmsg malloc failed");
              exit(1);
            }          
            
            msg.msg_control = cmsg;
            msg.msg_controllen = cmsglen;
            
            cmsg = CMSG_FIRSTHDR(&msg);                        
            cmsg->cmsg_level = IPPROTO_IPV6;
            cmsg->cmsg_type = IPV6_DSTOPTS;
            cmsg->cmsg_len = CMSG_LEN(sizeof(dest));
            memcpy(CMSG_DATA(cmsg), &dest, sizeof(dest));
            
            setsockopt(s0, IPPROTO_IPV6, IPV6_DSTOPTS, &on, sizeof(int)); 
            
            //send message back
            rv = sendmsg( s0 , &msg, 0);          
              if(rv == -1)
               {
                 puts("Error returned from sendmsg");
               }
           }
    exit(0);
}