TCP Connect Port Scanner with Linux Sockets (BSD)

An article by Binary Tides    No Comments

TCP Connect Port Scanner works by trying to establish a connection with every port that is being scanned. If a connectio is established then the port is open otherwise closed.

The steps are simple :

1. Create a socket
2. Run a Loop to connect with each port on the remote system ; if connection established then port open otherwise closed.

Code :

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

int main(int argc , char **argv)
{
 struct hostent *host;
 int err, i , net ,start , end;
 char hostname[100];
 struct sockaddr_in sa;
 //Get the hostname to scan
 printf("Enter hostname or IP : ");
 gets(hostname);
 //Get start port number
 printf("\nEnter start port number : ");
 scanf("%d" , &start);
 //Get end port number
 printf("\nEnter end port number : ");
 scanf("%d" , &end);

 //Initialise the sockaddr_in structure
 strncpy((char*)&sa , "" , sizeof sa);
 sa.sin_family = AF_INET;

 if(isdigit(hostname[0]))
 {
  printf("Doing inet_addr...");
  sa.sin_addr.s_addr = inet_addr(hostname);
  printf("Done\n");
 }
 else if((host = gethostbyname(hostname))!=0)
 {
  printf("Doing gethostbyname...");
  strncpy((char*)&sa.sin_addr , (char*)host->h_addr , sizeof sa.sin_addr);
  printf("Done\n");
 }
 else
 {
  herror(hostname);
  exit(2);
 }
 //Start the port scan loop
 printf("Starting the portscan loop : \n");
 for(i=start ; i<=end ; i++)
 {
  //Fill in the port number
  sa.sin_port = htons(i);
  //Create a socket of type internet
  net = socket(AF_INET , SOCK_STREAM , 0);
  //Check whether socket created fine or not
  if(net < 0)
  {
   perror("\nSocket");
   exit(1);
  }
  //Connect using that socket and sockaddr structure
  err = connect(net , (struct sockaddr*)&sa , sizeof sa);

  if(err<0)
  {
   printf("%s %-5d %s\r" , hostname , i, strerror(errno));
   fflush(stdout);
  }
  else
  {
   printf("%s %-5d accepted. \n",  hostname , i);
   //Now shutdown the read and write operations on this socket
   if(shutdown(net , SHUT_RDWR) < 0)
   {
    //Print error with error message mapped from err_no
    perror("\nShutdown");
    exit(1);
   }
  }
  close(net);
 }
 printf("\r");
 fflush(stdout);
 return(0);
}

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