Written by: mxpack3t
Disclosure: Copyright 2003 mxpack3t. This document is protected by 
international copyright laws. Unauthorized duplication in whole or 
in part is prohibited. In no way do I promote hacking for illegal 
actions. The knowledge you gain from this tutorial you are 
responsible for. The tutorial is for educational and security 
purpose only and nothing else.

Before you try this tutorial, you should have a basic understanding 
of the c programming language. Keep in mind that I am not an expert 
c programmer. This is just a basic port scanner. It works very good 
on one ip address. This is not meant to scan a whole c class. You 
should have a basic understanding of socket programming. You should 
read Beej's Guide to Network Programming tutorial. I have learned a 
lot from his tutorials. This port scanner is not very fast; it just 
gets the job done. This tutorial is so you can get an understanding 
of socket programming while writing something useful in your arsenal 
of tools. So here it goes.

/*
stdio.h = standard input output
netdb.h = network database
errno.h = system errors like perror and herror
netinet/in.h = internet address family
sys/types.h = data types
sys/socket.h = building socket so we do need it
*/

#include <stdio.h>
#include <netdb.h>
#include <errno.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <sys/socket.h>


/*
all c programmers should know what this is used for.
use this for command line arguments
*/

main(int argc, char *argv[])


/*
lets do some error checking for the user input.
also give a usage example for the command line.
*/

    {
    if(argc != 4) {
       printf("\nusage: mpscan target_ip start_port end_port\n");
       printf("\nexample: mpscan 192.168.1.100 1 2000\n");
    exit(1);
    }


/*
set the variables that will be used later

int sockfd is the socket file descriptor
int scan is used for the array
int sport is the starting port number
int pstop is the port number to stop at
struct hostent *target is a pointer to the target
struct hostent sockaddr_in mxpack3t will call elements of the 
socket address
*/

    int sockfd;
    int scan = 0;
    int sport = atoi(argv[2]);
    int pstop = atoi(argv[3]);
    struct hostent *target;
    struct sockaddr_in mxpack3t;


/*
do some error checking for the ip address.
herror will print out the error message
*/

    if((target = gethostbyname(argv[1])) == NULL)
       {
       herror("gethostbyname");
       exit(1);
       }


/*
this is your array for the ports
sport is the starting port number
<=pstop means less than or equal to the port to stop at
scan++ means go to the next port
*/

    for(scan = sport; scan <=pstop; scan++)
    {

/*
create a scocket and do some error checking
we use SOCK_STREAM for the port scanner
perror will print the error message if a socket can not be created
*/


    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
       {
       perror("socket");
       exit(1);
       }


/*
mxpack3t is what we will be storing information in.
.sin_family is the address family
.sin_port is the port numbers that are in the array scan htons means host to network short
.sin_addr this is the target address
*/

    mxpack3t.sin_family = AF_INET;
    mxpack3t.sin_port = htons(scan);
    mxpack3t.sin_addr = *((struct in_addr *)target->h_addr);


/* connect and check to see if the ports are open */

    if(connect(sockfd, (struct sockaddr *)&mxpack3t,
         sizeof(struct sockaddr)) == -1)


/* if the port is closed print closed */

    {
    printf("port %d\tclosed\n", scan);
    close(sockfd);
    }

/* if the port is not closed then we use the else
statement to print port is open.
*/
    else

       {
       printf("\t\t**** port %d OPEN ****\n", scan);
       close(sockfd);
       }
   }


/* exit the program */

   return 0;

}

I hope this tutorial has helped you gain a better understanding of 
how a port scanner is written. In order to completly understand this 
tutorial you need a basic understanding of the c language. Also you 
need to read the socket programmming tutorial from Beej's guide. I 
would like to thank shaun2k2 for helping me out when i was learning 
socket programming. Your posts helped me out.