How to create port scanner using C programming language

How to create port scanner using C programming language

There are lot of port scanners available in the internet nmap is one of the good port scanner but what if you want to make your own port scanner and you don’t know how to do it here is the C-code to create port scanner.
In the last article we have covered on how to create port scanner in python
Well we thought C-Programming has the fastest compiler at current performance rating of the compilers while the python programming is interpreted and partial of the code is executed using the JIT (Just In time Compiler) but we kinda need standlone compiler to complete the operation hence we are doing this article.
The current port scanner works on by trying to establish a connection with every port that is to be scanned and make sure the port is open or closed it is not stealthy but it can do the job.
It involves in a three way hand shake between 2-hosts hence it consumes maximum time to finish the job.

Local system ----> sends tcp syn packet -----> Remote system
Local system <---- replies with a syn+ack packet <----- Remote system
Local system ----> sends ack packet -----> Remote system

There are different scenarios of the port scanning called the “tcp syn port scanning” which established full 3 way hand shake.
Here is the program to implement the TCP connect

/*
    Port scanner code in c
*/
#include "stdio.h"
#include "sys/socket.h"
#include "errno.h"
#include "netdb.h"
#include "string.h"
#include "stdlib.h"
 
int main(int argc , char **argv)
{
    struct hostent *host;
    int err, i , sock ,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("Enter end port number : ");
    scanf("%d" , &end);
 
    //Initialise the sockaddr_in structure
    strncpy((char*)&sa , "" , sizeof sa);
    sa.sin_family = AF_INET;
     
    //direct ip address, use it
    if(isdigit(hostname[0]))
    {
        printf("Doing inet_addr...");
        sa.sin_addr.s_addr = inet_addr(hostname);
        printf("Done\n");
    }
    //Resolve hostname to ip address
    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
        sock = socket(AF_INET , SOCK_STREAM , 0);
         
        //Check whether socket created fine or not
        if(sock < 0) 
        {
            perror("\nSocket");
            exit(1);
        }
        //Connect using that socket and sockaddr structure
        err = connect(sock , (struct sockaddr*)&sa , sizeof sa);
         
        //not connected
        if( err < 0 )
        {
            //printf("%s %-5d %s\r" , hostname , i, strerror(errno));
            fflush(stdout);
        }
        //connected
        else
        {
            printf("%-5d open\n",  i);
        }
        close(sock);
    }
     
    printf("\r");
    fflush(stdout);
    return(0);
}

Run the program
First compile the program using gcc. Its simple.
# gcc portscanner.c
Now run the program and provide the necessary input
# ./a.out
Enter hostname or IP : google.com
Enter start port number : 75
Enter end port number : 85
Doing gethostbyname…Done
Starting the portscan loop :
80 open
#
So the above program scanned ports 75 to 85 on google.com and found only port 80 to be open, which is the webserver port.
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