// Authors: Rajesh Balla, Wei Cheng, Corinne Rhodes, Saifee Suterwala.

// Date Created: May 2, 2000

// Last Modified: Sept 15, 2000

//

// Description: This is a program that implements the construction of a TCP packet. // It takes a source address, source port, destination address, destination port

// number, sequence number, acknowledgement number, the values for the SYN, ACK,

// FIN, and RST flags.

#include <stdio.h> // for printf() and fprintf()

#include <stdlib.h> // for atoi()

#include <signal.h>

#include <string.h> // for memset()

#include <unistd.h> // or close()

#include <netdb.h>

#include <netinet/in.h>

#include <sys/socket.h> // for socket(), connect(), send(), and recv()

#include <arpa/inet.h> // for socketaddr_in and inet_addr()

#include <netinet/ip.h> // for ip struct

#include <netinet/tcp.h>// for tcp struct

char myBUFFER[80];

void send_packet(unsigned int, unsigned short, unsigned int, unsigned short);

unsigned short in_cksum(unsigned short *, int);

unsigned int host2ip(char *);

void DieWithError (char *errorMessage); /* error handling function*/

main(int argc, char **argv)

{

unsigned int srchost;

unsigned int dsthost;

unsigned short dstport;

unsigned short srcport;

//check for the minimum number of arguements

if(argc < 5 || argc >= 6)

{

fprintf(stderr, "Usage: %s <srchost> <srcport> <dsthost> <dstport>\n", argv[0]);

exit(0);

}

srchost= host2ip(argv[1]);

srcport = atoi(argv[2]);

dsthost = host2ip(argv[3]);

dstport = atoi(argv[4]);

if(dstport == 0) dstport = 80;

send_packet(srchost, srcport, dsthost, dstport);

}

// Creates the IP and TCP packet, opens the socket, calculate IP checksum,

void send_packet(unsigned int source_addr, unsigned short srcport, unsigned int dest_addr, unsigned short dest_port)

{

struct send_tcp /* structure for the TCP/IP packet to send */

{

struct iphdr ip;

struct tcphdr tcp;

} send_tcp;

struct pseudo_header /* for calculating the checksum */

{

unsigned int source_address;

unsigned int dest_address;

unsigned char placeholder;

unsigned char protocol;

unsigned short tcp_length;

struct tcphdr tcp;

} pseudo_header;

int i;

int tcp_socket; /* raw socket to send our TCP packet through */

struct sockaddr_in sin;

int sinlen;

char ans = 'n';

char temp;

memset (&send_tcp, 0, sizeof(send_tcp));

printf("\n Do you want to enter the sequence number (y/n)?:");

fflush(stdin);

ans = getchar();

if(ans =='y' || ans == 'Y')

{

printf("\n Enter the sequence number: ");

scanf("%u",& send_tcp.tcp.seq);

}

else

{

printf("\n Using default sequence number = %d \n", getpid());

send_tcp.tcp.seq = getpid();

}

ans ='n';

printf("\n Do you want to enter the Acknowledge Number (y/n)?:");

getchar();

ans = getchar();

if(ans =='y' || ans == 'Y')

{

printf("\n Enter the Acknowledge Number: ");

scanf("%u", & send_tcp.tcp.ack_seq);

}

else

{

printf("\n Using default Acknowledge number = %d\n", 0);

send_tcp.tcp.ack_seq = 0;

}

ans ='n';

printf("\n Do you want to set the SYN Flag (y/n)?:");

getchar();

ans = getchar();

if(ans =='y' || ans == 'Y')

{

printf("\n Setting the SYN flag\n");

send_tcp.tcp.syn = 1;

}

else

{

printf("\n Resetting the SYN flag\n");

send_tcp.tcp.syn = 0;

}

ans ='n';

printf("\n Do you want to set the ACK Flag (y/n)?:");

getchar();

ans = getchar();

if(ans =='y' || ans == 'Y')

{

printf("\n Setting the ACK flag\n");

send_tcp.tcp.ack = 1;

}

else

{

printf("\n Resetting the ACK flag\n");

send_tcp.tcp.ack = 0;

}

ans ='n';

printf("\n Do you want to set the FIN Flag (y/n)?:");

getchar();

ans = getchar();

if(ans =='y' || ans == 'Y')

{

printf("\n Setting the FIN flag\n");

send_tcp.tcp.fin = 1;

}

else

{

printf("\n Resetting the FIN flag\n");

send_tcp.tcp.fin = 0;

}

ans ='n';

printf("\n Do you want to set the RESET Flag (y/n)?:");

getchar();

ans = getchar();

if(ans =='y' || ans == 'Y')

{

printf("\n Setting the RESET flag\n");

send_tcp.tcp.rst = 1;

}

else

{

printf("\n Resetting the RESET flag\n\n");

send_tcp.tcp.rst = 0;

}

ans ='n';

/* form ip packet */

send_tcp.ip.ihl = 5;

send_tcp.ip.version = 4;

send_tcp.ip.tos = 0;

send_tcp.ip.tot_len = htons(40);

send_tcp.ip.id = getpid();

send_tcp.ip.frag_off = 0;

send_tcp.ip.ttl = 255;

send_tcp.ip.protocol = IPPROTO_TCP;

send_tcp.ip.check = 0;

send_tcp.ip.saddr = source_addr;

send_tcp.ip.daddr = dest_addr;

/* form tcp packet */

send_tcp.tcp.source = htons(srcport);

send_tcp.tcp.dest = htons(dest_port);

send_tcp.tcp.seq = htonl(send_tcp.tcp.seq);

send_tcp.tcp.ack_seq = htonl(send_tcp.tcp.ack_seq);

send_tcp.tcp.res1 = 0;

send_tcp.tcp.doff = 5;

send_tcp.tcp.psh = 0;

send_tcp.tcp.urg = 0;

send_tcp.tcp.res2 = 0;

send_tcp.tcp.window = htons(1024);

send_tcp.tcp.check = 0;

send_tcp.tcp.urg_ptr = 0;

/* setup the sin struct */

sin.sin_family = AF_INET;

sin.sin_port = send_tcp.tcp.source;

sin.sin_addr.s_addr = send_tcp.ip.daddr;

/* (try to) open the socket */

tcp_socket = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);

if(tcp_socket < 0)

{

perror("socket");

exit(1);

}

/* calculate the ip checksum */

send_tcp.ip.check = in_cksum((unsigned short *)&send_tcp.ip, 20);

/* set the pseudo header fields */

pseudo_header.source_address = send_tcp.ip.saddr;

pseudo_header.dest_address = send_tcp.ip.daddr;

pseudo_header.placeholder = 0;

pseudo_header.protocol = IPPROTO_TCP;

pseudo_header.tcp_length = htons(20);

bcopy((char *)&send_tcp.tcp, (char *)&pseudo_header.tcp, 20);

send_tcp.tcp.check = in_cksum((unsigned short *)&pseudo_header, 32);

sinlen = sizeof(sin);

printf("\n Hit a key to continue");

getchar();

getchar();

sendto(tcp_socket, &send_tcp, sizeof(send_tcp), 0, (struct sockaddr *)&sin, sinlen);

printf("\n***** Sent TCP packet *****\n");

getchar();

printf("\n Hit any key to terminate!\n");

getchar();

close(tcp_socket);

}

// finds the TCP checksum

unsigned short in_cksum(unsigned short *ptr, int nbytes)

{

register long sum; /* assumes long == 32 bits */

u_short oddbyte;

register u_short answer; /* assumes u_short == 16 bits */

/*

* Our algorithm is simple, using a 32-bit accumulator (sum),

* we add sequential 16-bit words to it, and at the end, fold back

* all the carry bits from the top 16 bits into the lower 16 bits.

*/

sum = 0;

while (nbytes > 1) {

sum += *ptr++;

nbytes -= 2;

}

/* mop up an odd byte, if necessary */

if (nbytes == 1) {

oddbyte = 0; /* make sure top half is zero */

*((u_char *) &oddbyte) = *(u_char *)ptr; /* one byte only */

sum += oddbyte;

}

/*

* Add back carry outs from top 16 bits to low 16 bits.

*/

sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */

sum += (sum >> 16); /* add carry */

answer = ~sum; /* ones-complement, then truncate to 16 bits */

return(answer);

}

// converts the char string IP address to valid internet address

unsigned int host2ip(char *hostname)

{

static struct in_addr i;

struct hostent *h;

i.s_addr = inet_addr(hostname);

if(i.s_addr == -1)

{

h = gethostbyname(hostname);

if(h == NULL)

{

fprintf(stderr, "cant find %s!\n", hostname);

exit(0);

}

bcopy(h->h_addr, (char *)&i.s_addr, h->h_length);

}

return i.s_addr;

}