C / C++ / MFC

#include <winsock2.h>
#include <windows.h>
#include <ws2tcpip.h>
#include <stdio.h>
#include <conio.h>

#pragma comment(lib, "ws2_32.lib")
#pragma pack(1)

struct tcpheader {
	unsigned short int th_sport;
	unsigned short int th_dport;
	unsigned int th_seq;
	unsigned int th_ack;
	unsigned char th_x2:4, th_off:4;
	unsigned char th_flags;
	unsigned short int th_win;
	unsigned short int th_sum;
	unsigned short int th_urp;
}; /* total tcp header length: 20 bytes (=160 bits) */

struct ipheader {
	unsigned char ip_hl:4, ip_v:4; /* this means that each member is 4 bits */
	unsigned char ip_tos;
	unsigned short int ip_len;
	unsigned short int ip_id;
	unsigned short int ip_off;
	unsigned char ip_ttl;
	unsigned char ip_p;
	unsigned short int ip_sum;
	unsigned int ip_src;
	unsigned int ip_dst;
}; /* total ip header length: 20 bytes (=160 bits) */

// IP/TCP/UDP Checksum Function

USHORT checksum(USHORT *buffer, int size)
{
	unsigned long cksum=0;
	while (size > 1)
	{
		cksum += *buffer++;
		size -= sizeof(USHORT); 
	}
	if (size)
	{
		cksum += *(UCHAR*)buffer; 
	}
	cksum = (cksum >> 16) + (cksum & 0xffff);
	cksum += (cksum >>16); 
	return (USHORT)(~cksum); 
}

#define PORT 25

int main (void)
{

	WSADATA wsd;
	char datagram[4096];
	int bOpt = 1;

	if (WSAStartup(MAKEWORD(2,2), &wsd) != 0)
	{
		printf("WSAStartup() failed: %d\n", GetLastError());
		return -1;
	}

	// Create a raw socket

	SOCKET s = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);
	if (s == INVALID_SOCKET)
	{
		printf("WSASocket() failed: %d\n", WSAGetLastError());
		return -1;
	}

	struct ipheader *iph = (struct ipheader *) datagram;
	struct tcpheader *tcph = (struct tcpheader *) (datagram + sizeof (struct ipheader));
	struct sockaddr_in sin;

	sin.sin_family = AF_INET;
	sin.sin_port = htons (PORT);
	sin.sin_addr.s_addr = inet_addr ("2.3.4.5");

	memset (datagram, 0, 4096); /* zero out the buffer */

	iph->ip_hl = 5;
	iph->ip_v = 4;
	iph->ip_tos = 0;
	iph->ip_len = sizeof (struct ipheader) + sizeof (struct tcpheader);
	iph->ip_id = 1;
	iph->ip_off = 0;
	iph->ip_ttl = 255;
	iph->ip_p = 6;
	iph->ip_sum = 0;
	iph->ip_src = inet_addr ("1.2.3.4");
	iph->ip_dst = sin.sin_addr.s_addr;

	tcph->th_sport = htons (1234);
	tcph->th_dport = htons (PORT);
	tcph->th_seq = rand();
	tcph->th_ack = 0;
	tcph->th_x2 = 0;
	tcph->th_off = 5;
	tcph->th_flags = 2; // SYN
	tcph->th_win = htons(65535);
	tcph->th_sum = 0;
	tcph->th_urp = 0;

	iph->ip_sum = checksum((unsigned short *)&iph, sizeof(ipheader) >> 1);

	if (setsockopt(s, IPPROTO_IP, IP_HDRINCL, (char *)&bOpt, sizeof(bOpt)) == SOCKET_ERROR)
	{
		printf("setsockopt(IP_HDRINCL) failed: %d\n", WSAGetLastError());
		return -1;
	}

	printf("Started sending of raw data. Hit any key to quit\n");

	while (!kbhit())
	{
		// Send The Packet

		if (sendto(s, datagram, 40, 0, (SOCKADDR *)&sin, sizeof(sin)) == SOCKET_ERROR)
		{
			printf("sendto() failed: %d\n", WSAGetLastError());
			return -1;
		}
		printf(".");
		Sleep(1000);
		tcph->th_seq++;
	}

	return 0;
}

#pragma pack(push,1)<br />
struct<br />
{<br />
  BYTE x;<br />
  BYTE y;<br />
};<br />
#pragma pack(pop)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br />
Peter Weyzen<br />
Staff Engineer<br />
Santa Cruz Networks