TCP Connect Port Scanner Source Code in C with Winsock

An article by Binary Tides    1 Comment

TCP connect() scanning is the most basic form of TCP scanning. The program performs a connect() command on those ports of the target machine which are to be checked. If the port is open then the connect() command will succeed and a connection will be established. If the port is closed the connect() function would simply timeout in the connection attempt.

The simple steps would be :
1. Start a loop for the port number range to be scanned.
2. Create a Socket inside the loop.
3. Call the connect function using the socket and the port number to connect to the host.
4. If connect returns SOCKET_ERROR then the connection failed hence port closed, otherwise connection established and port open.

The following code does the same. It should be noted that it scans only TCP ports. For a linux version of the same code view this post.

Code :

/*
 TCP Connect portscanner with winsock
*/

#include<stdio.h>
#include<winsock2.h>
#pragma comment(lib, "ws2_32.lib"); //To link the winsock library  

int main(int argc, char **argv)
{
 WSADATA firstsock;
 SOCKET s;
 struct hostent *host;
 int err,i, startport , endport;
 struct sockaddr_in sa; //this stores the destination address
 char hostname[100];

 strncpy((char *)&sa,"",sizeof sa);
 sa.sin_family = AF_INET; //this line must be like this coz internet

 //Initialise winsock
 if (WSAStartup(MAKEWORD(2,0),&firstsock) != 0)  //CHECKS FOR WINSOCK VERSION 2.0
 {
  fprintf(stderr,"WSAStartup() failed"); //print formatted data specify stream and options
  exit(EXIT_FAILURE);        //or exit(1);
 } 

 printf("Enter hostname or ip to scan : ");
 gets(hostname);

 printf("Enter starting port : ");
 scanf("%d" , &startport);

 printf("Enter ending port : ");
 scanf("%d" , &endport);

 if(isdigit(hostname[0]))
 {
  printf("Doing inet_addr...");
  sa.sin_addr.s_addr = inet_addr(hostname); //get ip into s_addr
  printf("Done\n");
 }
 else if( (host=gethostbyname(hostname)) != 0)
 {
  printf("Doing gethostbyname()...");
  strncpy((char *)&sa.sin_addr , (char *)host->h_addr_list[0] , sizeof sa.sin_addr);
  printf("Done\n");
 }
 else
 {
    printf("Error resolving hostname");
       exit(EXIT_FAILURE);
 }

 //Start the portscan loop
 printf("Starting the scan loop...\n");
 for(i = startport ; i<= endport ; i++)
 {

  s = socket(AF_INET , SOCK_STREAM , 0); //make net a valid socket handle
  if(s < 0)  //if not a socket
  {
   perror("\nSocket creation failed");  // perror function prints an error message to stderr
   exit(EXIT_FAILURE);       //or exit(0);
  }

  sa.sin_port = htons(i);
  //connect to the server with that socket
  err = connect(s , (struct sockaddr *)&sa , sizeof sa);

  if(err == SOCKET_ERROR) //connection not accepted
  {
   printf("%s %-5d Winsock Error Code : %d\n" , hostname , i , WSAGetLastError());
   fflush(stdout);
  }
  else  //connection accepted
  {
   printf("%s %-5d accepted            \n" , hostname , i);
   if( shutdown( s ,SD_BOTH ) == SOCKET_ERROR )
   {
    perror("\nshutdown");// perror function prints an error message to stderr
    exit(EXIT_FAILURE);
   }
  }
  closesocket(s);   //closes the net socket
 }

 fflush(stdout); //clears the contents of a buffer or flushes a stream
 return(0);
}

The above can be compiled with vc++ 6.0 for example. Simply create a project and add this file to the project and click run.

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