Adding To The Simple Port Scanner Written in C

/*
    TCP port scanner
    Written and Maintained by Michael Craze

TODO:
    make it use threads..
    make it check UDP
    make it scan ip address ranges for public use

    ip address ranges for private use:
    Class   Networks
    A   10.0.0.0 through 10.255.255.255
    B   172.16.0.0 through 172.31.0.0
    C   192.168.0.0 through 192.168.255.0
*/

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

typedef struct{
    char *ip;
    int sport;
    int eport;
} Host;

void usage(char *a){
    fprintf(stderr,"Invalid invocation of %s\n",a);
    printf("Usage: %s   \n",a);
    exit(1);
}

/* Main programs starts*/
int main(int argc, char **argv){
    int sd;
    int port;
    int rval;
    int checkip;
    int pcount = 0;

    /* going to use these ints to iterate
       through public IP classes to find all
       open servers to automate attacks.
    */
    /*
       int A,B,C,D;
    */

    /*
        char *message="shell";
    */
    /*
        char response[1024];
    */

    Host *h;
    struct hostent *hostaddr;
    struct sockaddr_in servaddr;

    if (argc < 4 ){
        usage(argv[0]);
    }

    h = (Host *)malloc(sizeof(Host));
    if(h == NULL){
        fprintf(stderr,"couldn't allocate memory for %s\n",argv[1]);
        exit(1);
    }
    if(sscanf(argv[1],"%d",&checkip) != 1){
        fprintf(stderr,"%s was not a valid ip address\n",argv[1]);
        usage(argv[0]);
    }
    if(sscanf(argv[2],"%d",&h->sport) != 1){
        fprintf(stderr,"%s was not a valid integer\n",argv[2]);
        usage(argv[0]);
    }
    if(sscanf(argv[3],"%d",&h->eport) != 1){
        fprintf(stderr,"%s was not a valid integer\n",argv[3]);
        usage(argv[0]);
    }

    h->ip = strdup(argv[1]);
    printf("Scanning host: %s on ports %d thru %d\n", h->ip,h->sport,h->eport);


    /* Start scanning ports */
    for (port = h->sport; port <= h->eport; port++){
        /* creating the tcp socket */
        sd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
        if (sd == -1){
            perror("Socket()\n");
            return (errno);
        }
        memset( &servaddr, 0, sizeof(servaddr));
        servaddr.sin_family = AF_INET;
        servaddr.sin_port = htons(port);
        hostaddr = gethostbyname(h->ip);

        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("  %-7d %-10s\n",port,"is open");
            pcount++;
        }
        close(sd);
    }
    if(pcount == 0){
        printf("No ports in range %d-%d are open on host %s\n",h->sport,h->eport,h->ip);
    }
    else{
        printf("%d ports in range %d-%d are open on host %s\n",pcount,h->sport,h->eport,h->ip);
    }

    free(h);
    return 0;
}