#include <stdio.h>
#include <unistd.h>      // Defines implementation characteristics identified by POSIX standard.
#include <sys/types.h>      // data types
#include <sys/socket.h>      // Internet Protocol family
#include <netdb.h>       // definitions for network database operations
#include <netinet/in.h>    // Internet Protocol family
#include <time.h>      // standard time functions

#define START_PORT 1
#define END_PORT 100  // define constants

/*
Structures used, included for informational/testing purposes only

struct sockaddr_in
   short int          sin_family   // Address family
   unsigned short int sin_port   // Port number
   struct in_addr     sin_addr     // Internet address
   unsigned char      sin_zero[8]    // Same size as struct sockaddr

struct hostent
   char     *h_name          // name of the host
   char     **h_aliases      // A pointer to an array of pointers to alternative host names
        int      h_addrtype       // Address type being returned
   int      h_length         // The length of address in bytes
   char     **h_addr_list    // network addresses for the host, terminated by a null pointer
   
   

struct servent
         char    *s_name;        // official service name
         char    **s_aliases;    // alias list
         int     s_port;         // port number
         char    *s_proto;       // protocol to use
*/
 

int main(int argc, char **argv) {
     
    // declare data types   
    int socket_fd, port, start_port, end_port, open_ports, closed;
    time_t time1, time2;
    struct hostent *host;
    struct sockaddr_in target;
    struct servent* sp;
    float diff;
   
    // initiate variables
    open_ports = 0; closed = 0;
   
    // check for command line arguements
    if (argc<2) exit(printf("usage: %s <hostname>\n", argv[0]));

    // get hostname or exit with error
    host = gethostbyname(argv[1]);
    if (!host) exit(printf("Error looking up hostname.\n"));

    // specify address family
    target.sin_family = AF_INET;
   
    target.sin_addr.s_addr = *(long *)(host->h_addr);
   
    // get intitial clock time
    time1 = time(NULL);

    printf("scanning ports %d - %d on %s (%s)\n\n",START_PORT, END_PORT, inet_ntoa(*((struct in_addr *)host->h_addr)), host->h_name);

    printf("port \t state \t service\n");
   
    // loop through all ports specified
    for (port=START_PORT; port <= END_PORT; port++) {

       // convert ip to network based order
       target.sin_port = htons(port);
   
          // here we create the socket that returns a file desriptor
   // AF_INET = address family
   // SOCK_STREAM - basic TCP socket reliable, two-way, connection-based byte streams.
   // 0 - set protocol type
        socket_fd =socket(AF_INET,SOCK_STREAM,0);
        if (socket_fd < 0) exit(printf("Error creating socket.\n"));

   // check port is open
        if (!connect(socket_fd,(struct sockaddr *)&target, sizeof(target))) {
           
       // increment no of open ports
       open_ports++;
      
       // getservbyport tries to convert port to service name. checks /etc/services by default
       sp = getservbyport(htons(port),"tcp");
      
       // output
       printf("%i/tcp \t open \t %s\n", port,sp->s_name);
   }
   
   // if port closed, increment closed count
   else closed++;
      
   // clean up / close socket
        if (close(socket_fd)) exit(printf("error closing socket.\n"));
   
    } // end for loop
   
    // get final time
    time2 = time(NULL);
       
    // calculate time difference
    diff = difftime(time2,time1);
   
    // check if there are no open ports
    if(open_ports < 1) printf("No open ports found");   
   
    // (int)diff may return 0 if scanning localhost - yes its so fast ;)
    printf("\nScan completed in %d seconds, %d ports open, %d closed\n", (int)diff, open_ports, closed);
   
    return 0;
}

/*
 This is a simple port scanner written in c, it will basically do the same thing as nmap -sT <host>
 which is the default TCP connect scan. I do not recommend using it as it is quite slow, its purpose is
 to show you some basic socket usage in c.
*/