/**********************************************************************
** This is a real simple half opened connection scanner I
** put together. It scans ports 0 to 1024 for listening
** processes, and writes to stdout the ports that are 
** listening.
**
** halflife@saturn.net
**
** NOTE: You have to define MY_IP as your ip. If you have
** a dynamic ip, this is gonna bite :-).
*********************************************************************/

/* define the following as your ip address */
#define MY_IP		"193.62.1.250"

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <unistd.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <linux/ip.h>
#include <linux/tcp.h>

int syn_timeout = 0;

unsigned short in_cksum(unsigned short *, int);
int scan_port(unsigned short, unsigned int, unsigned int);
void alarm_handler(int);

void alarm_handler(int s)
{
   alarm(0);
   syn_timeout = 1;
}

int scan_port(unsigned short port, unsigned int src_addr, unsigned int dst_addr)
{
   struct tcphdr send_tcp;
   struct recv_tcp
   {
      struct iphdr ip;
      struct tcphdr tcp;
      unsigned char blah[65535];
   }recv_tcp;
   struct pseudo_header
   {
      unsigned int source_address;
      unsigned int dest_address;
      unsigned char placeholder;
      unsigned char protocol;
      unsigned short tcp_length;
      struct tcphdr tcp;
   }pseudo_header;
   int tcp_socket;
   struct sockaddr_in sin;
   int sinlen;
   static int blah = 0;
   
   blah++;
   send_tcp.source = getpid() + blah;
   send_tcp.dest = htons(port);
   send_tcp.seq = getpid() + blah;
   send_tcp.ack_seq = 0;
   send_tcp.res1 = 0;
   send_tcp.doff = 5;
   send_tcp.res2 = 0;
   send_tcp.fin = 0;
   send_tcp.syn = 1;
   send_tcp.rst = 0;
   send_tcp.psh = 0;
   send_tcp.ack = 0;
   send_tcp.urg = 0;
   send_tcp.window = htons(512);
   send_tcp.check = 0;
   send_tcp.urg_ptr = 0;               
   pseudo_header.source_address = src_addr;
   pseudo_header.dest_address = dst_addr;
   pseudo_header.placeholder = 0;
   pseudo_header.protocol = IPPROTO_TCP;
   pseudo_header.tcp_length = htons(20);
   bcopy(&send_tcp, &pseudo_header.tcp, 20);
   send_tcp.check = in_cksum((unsigned short *)&pseudo_header, 32);
   sin.sin_family = AF_INET;
   sin.sin_port = htons(port);
   sin.sin_addr.s_addr = dst_addr;
   sinlen=sizeof(sin);
   signal(SIGALRM, alarm_handler);
   tcp_socket = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
   if(tcp_socket < 0)
   {
      fprintf(stderr, "couldnt open raw socket\n");
      exit(1);
   }
   sendto(tcp_socket, &send_tcp, 20, 0, (struct sockaddr *)&sin, sinlen);
   syn_timeout = 0;
   alarm(10);
   while(1)
   {
      read(tcp_socket, (struct recv_tcp *)&recv_tcp, 65535);
      if(syn_timeout == 1) {close(tcp_socket);syn_timeout=0;return -1;}
      if(recv_tcp.tcp.dest == (getpid() + blah))
      {
         alarm(0);
         close(tcp_socket);
         if(recv_tcp.tcp.rst == 1) return 0;
         else return 1;
      }
   }
}

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);
}

main(int argc, char **argv)
{   
   unsigned short i;
   if(argc < 2)
   {
      fprintf(stderr, "%s target_ip\n", argv[0]);
      exit(0);
   }
   if(geteuid() != 0)
   {
      fprintf(stderr, "this program requires root\n");
      exit(0);
   }
   printf("Scanning %s\n", argv[1]);
   for(i=0;i < 1025;i++)
   {
      if(scan_port(i, inet_addr(MY_IP), inet_addr(argv[1]))==1)
         printf("Port %d active\n", i);
   }
}