My TCP Port Scanner in C

This is a discussion on My TCP Port Scanner in C within the Networking/Device Communication forums, part of the General Programming Boards category; This is a simple port scanner coded in c. It uses a simple socket and a for loop. The port ...

  1. #1
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    Talking My TCP Port Scanner in C

    This is a simple port scanner coded in c. It uses a simple socket and a for loop. The port scanner uses TCP Connect to check whether the port is opened or closed.

    This is for beginners who are trying to grasp simple sockets in C.

    By the way this is for linux platform you can easily compile this on win32 using cygwin.

    Code:
    /* A TCP port scanner created by billy www.softhardware.co.uk*/
    
    #include <stdio.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <netdb.h>
    #include <stdlib.h>
    #include <errno.h>
    
    
    /* Main programs starts*/
    int main(int argc, char **argv)
    {
       int   sd;         //socket descriptor
       int    port;         //port number
       int   start;         //start port
       int    end;         //end port
       int    rval;         //socket descriptor for connect   
       char    responce[1024];      //to receive data
       char   *message="shell";       //data to send
       struct hostent *hostaddr;   //To be used for IPaddress
       struct sockaddr_in servaddr;   //socket structure
       
       if (argc < 4 )
       {
          printf("------Created By www.Softhardware.co.uk-----------\n");
          printf("--------------------------------------------------\n");
          printf("Usage: ./tscan <IPaddress> <Start Port> <End Port>\n");
          printf("--------------------------------------------------\n");
          return (EINVAL);
       }
       start = atoi(argv[2]);
       end   = atoi(argv[3]);
       for (port=start; port<=end; port++)
       {
    
             //portno is ascii to int second argument     
    
       sd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP); //created the tcp socket
       if (sd == -1)
       {
         perror("Socket()\n");
         return (errno);
       }   
    
       memset( &servaddr, 0, sizeof(servaddr));
    
       servaddr.sin_family = AF_INET;
       servaddr.sin_port = htons(port); //set the portno
       
       hostaddr = gethostbyname( argv[1] ); //get the ip 1st argument
       
       memcpy(&servaddr.sin_addr, hostaddr->h_addr, hostaddr->h_length);
         
       //below connects to the specified ip in hostaddr
       
     
    
       rval = connect(sd, (struct sockaddr *) &servaddr, sizeof(servaddr));
       if (rval == -1)
       {
       printf("Port %d is closed\n", port);
       close(sd);
       }
       else
       printf("Port %d is open\n",port);
       
       close(sd);         //socket descriptor
       }
       
    }

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    Some thoughts:
    • Why do you have close(sd) inside both the if and else blocks at the end of your program?
    • Your indentation could be better.
    • I don't like atoi() due to its lack of error checking, instead preferring strtol() or sscanf(), but that's just me.
    • "responce" is spelled "response".


    BTW, there is a network programming forum, where this should perhaps have been posted.
    dwk

    Seek and ye shall find. quaere et invenies.

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  3. #3
    C++ Witch laserlight's Avatar
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    Moved to Networking/Device Communication forum.
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  4. #4
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    What if theres a daemon running an UDP connection?

    and you shouldn't put the socket() into the for loop, because each time another port is opened, and it's more needless overhead

    oh and before I forget, control reaches end of non-void function
    you should add an return(0); to the end

    aside from those, I think your example might suit well for someone who's new to socket programming
    It's quite simple, and short, which makes it clear.
    But, like dwks already said, make over your identation for better readability.
    Someone who's new might not recognise, where the loops/nestings start and end

    If you're planning on expanding the portscanner, you could add a parser for /etc/services, to look if a port needs to be probed for TCP, UDP or both.

  5. #5
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    To make an effective port scanner you would need to make use of multiple threads. In each thread it creates a socket, sets it to NON-blocking mode, calls the connect function (which should return straight away because of non-blocking) and then use the select function with a timeval of about 10 seconds to check wether it connected or not.

  6. #6
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    Is it possible that all ports (1000-16000) are closed on 127.0.0.1?

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