#define _BSD_SOURCE
/* BSD compatibility */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <signal.h>
#include <netdb.h>
#include <net/if.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>


#define CISCO_AUTH 1999
#define RESPOND_TIME 10


struct pseudo {
	u_long saddr;
	u_long daddr;
	u_char zero;
	u_char protocol;
	u_short length;
};

/* diz piece is ripped off.. who carez */
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);
}

/* sendpack : ripped from another piece of mine. packet_body isn't used here.
 * (so length is not supposed to).
 */

int sendpack( int s, u_long srcaddr, u_short srcport, u_long dstaddr, u_short dstport,u_short th_flags, u_char *packet_body,u_long length) {

	u_char packet[sizeof(struct ip) + sizeof(struct pseudo) + sizeof(struct tcphdr)];
	struct sockaddr_in foo;
	struct in_addr srcinaddr,dstinaddr;
	struct ip *ip = (struct ip *) packet;
	struct pseudo *pseudo = (struct pseudo *) (packet + sizeof(struct ip));
	struct tcphdr *tcp = (struct tcphdr *) (packet + sizeof(struct ip) 
				+ sizeof(struct pseudo));
	bzero(packet, sizeof(packet));
	bzero(&foo,sizeof(foo));

	/* only BSD, linux has plain u_long declared */
	srcinaddr.s_addr = srcaddr;
	dstinaddr.s_addr = dstaddr;

/* building packets */
			pseudo->saddr = srcaddr;
			pseudo->daddr = dstaddr;
			pseudo->zero = 0;
			pseudo->protocol=IPPROTO_TCP;
			pseudo->length = htons(sizeof (struct tcphdr));
			ip->ip_v = 4;
			ip->ip_hl = 5;
			ip->ip_id = 1234;
			ip->ip_src = srcinaddr;
			ip->ip_dst = dstinaddr;
			ip->ip_p = IPPROTO_TCP;
			ip->ip_ttl = 40;
			ip->ip_off = 0;
			ip->ip_len = sizeof(struct ip) + sizeof(struct tcphdr)
						    + length;
			tcp->th_sport = htons(srcport);
			tcp->th_dport = htons(dstport);
			tcp->th_seq = htonl(rand());
			tcp->th_ack = htonl(rand());
			tcp->th_off=5;
			tcp->th_flags = th_flags;
			tcp->th_urp = 0;
			tcp->th_sum = in_cksum((u_short *) pseudo,
					sizeof(struct pseudo) +
					 sizeof(struct tcphdr));
			bcopy(tcp,pseudo,sizeof(struct tcphdr));
			foo.sin_family=AF_INET;
			foo.sin_addr.s_addr=dstaddr;
			sendto(s,packet,sizeof(struct ip) + 
					sizeof(struct tcphdr) + length, 0,
					(struct sockaddr *) &foo,sizeof(foo));

	return 0;
}

void usage(char *name) {
	fprintf(stderr,"Usage: %s local_IP remote_IP\n",name);
	exit(1);
}

u_long resolve_name(char *hostname) {
	struct hostent *host;
	u_long addr;
	if ((addr = inet_addr(hostname)) != -1) return addr;
	if ((host = gethostbyname(hostname)) == NULL) {
		fprintf(stderr,"Can not resolve name: %s\n",hostname);
		exit(1);
	}
	bcopy(host->h_addr,&addr,host->h_length);
	return addr;
}

int rawfd;

void sig_alarm(int sig) {
	fprintf(stderr,"No responce received. Packets might be blocked\n");
	close(rawfd);
	exit(1);
}

int main(argc,argv)
	int argc;
	char **argv;
{
	int rd,rsize;
	int one=1;
	int i;
	u_char buf[1024];
	struct sockaddr_in raddr;
	struct in_addr srcip,dstip;
	struct ip *ip;
	struct tcphdr *tcp;
	u_short srcport,dstport;

	if (argc!=3) usage(argv[0]);
	
	
	srcip.s_addr	=	resolve_name(argv[1]);
	srcport		=	rand();
	dstip.s_addr	=	resolve_name(argv[2]);
	dstport		=	CISCO_AUTH;

	if ((rawfd=socket(PF_INET,SOCK_RAW,IPPROTO_TCP))<0) {
		perror("RawSocket:");
		exit(1);
	}
	if (setsockopt(rawfd,IPPROTO_IP,IP_HDRINCL,&one,sizeof(one))<0) {
		perror("SetSockOpt:");
		close(rawfd);
		exit(1);
	}

fprintf(stderr,"Indent [%s]: ",inet_ntoa(dstip));
sendpack(rawfd,srcip.s_addr,srcport,dstip.s_addr,dstport,TH_SYN,NULL,0);

	signal(SIGALRM,sig_alarm);
	alarm(RESPOND_TIME);
		
for(;;) {
		if ((rd=recvfrom(rawfd,buf,1024,0,
				(struct sockaddr *)&raddr,&rsize))<0) break;

		ip=(struct ip *)buf;
		tcp=(struct tcphdr *)&buf[ip->ip_hl<<2];
		if(ip->ip_src.s_addr == dstip.s_addr || 
				tcp->th_sport == dstport) {
		if ((ip->ip_hl<<2)+(tcp->th_off<<2)==rd) 
					fprintf(stderr,"non-cisco device");
		else
			for(i=(ip->ip_hl<<2)+(tcp->th_off<<2);i<rd;i++) 
				fprintf(stderr,"%c",buf[i]);
		fprintf(stderr,"\n");
		break;
		}
	}
	close(rawfd);
	return 0;
}