Thread: Problem with Altenative Client Application.

Hello!Thanks for your time reading this.I was doing a Client/Server app programming
the server had following routines:

Code:

socket();
bind();
listen();
connect();

client:

Code:

socket();
connect();

Using Wireshark 5 packets where sniffed:
Then I wanted to make my own client to trigger the same number of packets as before

my program is like:

Code:

socket(PF_INET,SOCK_RAW,IPPROTO_TCP);
setsockopt();//tell the kernel not to bother about inserting its ip header
CreateIPHeader();
CreateTCPHeader();//with SYN flag set
sendto();//with  address and port of my server

After running the program several times the server doesn't send back SYN-ACK packet.Help Please!

My platform is:Ubuntu Jaunty 9.04 kernel v2.6.28
Thanks!

If you want help, how about providing some actual code instead of a few lines of gibberish?

Originally Posted by CommonTater

If you want help, how about providing some actual code instead of a few lines of gibberish?

OK ,Here is the code:

Code:

#include<stdio.h>
#include<sys/socket.h>
#include<stdlib.h>
#include<features.h>
#include<linux/if_packet.h>
#include<linux/if_ether.h>
#include<errno.h>
#include<sys/ioctl.h>
#include<net/if.h>
#include<net/ethernet.h>
#include<linux/ip.h>
#include<linux/tcp.h>
#include<string.h>
#include<arpa/inet.h>
#include<sys/time.h>
unsigned short		
checksum (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;
}
#define P 80
int 
main (void)
{
  int s = socket (PF_INET, SOCK_RAW,IPPROTO_TCP);	/* open raw socket */
 unsigned char*packet;	
  struct iphdr *iph = (struct iphdr *) malloc(20);
  struct tcphdr *tcph = (struct tcphdr *) malloc(20);
  packet=(unsigned char*)malloc(sizeof(iph)+sizeof(tcph)+sizeof(struct ethhdr));

  struct sockaddr_in sin;
		

  sin.sin_family = PF_INET;
  sin.sin_port = htons (P);
  sin.sin_addr.s_addr = inet_addr ("1.1.1.1");
  memset(&sin.sin_zero,'\0',8);
    int one = 1;
    const int *val = &one;
    if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
    perror("Warning: Cannot set HDRINCL!\n");
  
  iph->ihl = 5;
  iph->version = 4;
  iph->tos = 0;
  iph->tot_len = htons(sizeof (struct iphdr) + sizeof (struct tcphdr));	
  iph->id = htonl (54321);	
  iph->frag_off = 0;
  iph->ttl = 255;
  iph->protocol =IPPROTO_TCP;
  iph->check = 0;		
  iph->saddr = inet_addr ("1.1.1.2");//address of my client pc
  iph->daddr = sin.sin_addr.s_addr;

  /*TCP Header*/
  tcph->source = htons (12983);
  tcph->dest = htons (P);
  tcph->seq = random ();
  tcph->ack = 0;
  tcph->res1 = 0;
  tcph->doff = 5;		
  tcph->syn =1;	/* initial connection request */
  tcph->window= htonl (65535);	
  tcph->check = 0;
  tcph->urg_ptr= 0;

  iph->check = checksum ((unsigned short *) packet, ntohs(iph->tot_len)>> 1);
  memcpy(packet,iph,20);
  memcpy((packet+20),tcph,20);
  while (1)
    { 
      if (sendto (s,		/* our socket */
		  packet,	
		 40,	
		  0,		
		  (struct sockaddr *) &sin,	
		  sizeof (sin)) < 0)		
	perror ("sendto()\n");
      else
	printf ("Sent!");
	sleep(2);
    }
free(iph);
free(tcph);
free(packet);
  return 0;
}

I've removed two routines

Code:

(CreateIPHeader() & CreateTCPHeader())

seems fine though!!??

I don't get why you need to make your own TCP header when sockets will do that for you automatically...
Also you never call connect() ...

Here Tater,According to RFC 793 ,For a TCP connection to initiated a process referred as 3-way handshake is involved where by a host A sends a SYN packet to host B,host B sends a SYN-ACK packet back to host A,host A sends an ACK packet to host B ,after that they can exchange data.I've been taking Networking Programming tutorials lately.Am trying to see the standard work from my own work.The Client have just

Code:

socket() and connect()

i know the kernel does the job of adding the headers for this client and its the same header as mine as I view it from Wireshark.Thanks!

Originally Posted by lvandaz

Here Tater,According to RFC 793 ,For a TCP connection to initiated a process referred as 3-way handshake is involved where by a host A sends a SYN packet to host B,host B sends a SYN-ACK packet back to host A,host A sends an ACK packet to host B ,after that they can exchange data.I've been taking Networking Programming tutorials lately.Am trying to see the standard work from my own work.The Client have just

Code:

socket() and connect()

i know the kernel does the job of adding the headers for this client and its the same header as mine as I view it from Wireshark.Thanks!

Ok so you write your own header... then the kernel adds it's own header... it's not seeing an empty syn package it's seeing a packet from an unconnected machine and rejecting it....

You need to stop fooling with headers and use connect()

//Tater have u done this before??I don't know if u understood my code coz i don't thinks your trying to help here kernel adds its own headers???
do u know what this part of the code does ?

Code:

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

Just try to guide not block!Thanks

Ok, let me summarize my point in the form of an old adage...

"Why reinvent the wheel when you can use the one you already have?"

Sockets includes all necessary header features as an integral part of itself, inserting your own (faulty?) headers is obviously not helping you.